Universidade de Lisboa

Faculdade de Farmácia

An update on anticancer triazene compounds

Ana Rita Custódio Santos

Mestrado Integrado em Ciências Farmacêuticas

2017

Universidade de Lisboa

Faculdade de Farmácia

An update on anticancer triazene compounds

Ana Rita Custódio Santos

Monografia de Mestrado Integrado em Ciências Farmacêuticas

apresentada à Universidade de Lisboa através da Faculdade de Farmácia

Orientador: Doutora Ana Paula Francisco, Professora auxiliar

2017

3

Abstract

Triazenes are a very useful and diverse class of compounds that have been studied,

principally, for their potential in the treatment of many tumors including brain,

leukemia and melanoma. Novel compounds of this class continue appearing in the

literature frequently as either anticancer compounds or even with other therapeutic

interest. This monography focused on several types of triazenes from the simplest ones

like 1,3-dialkyl-3-acyltriazenes to the more complex like combi-triazenes with

emphasis on the mechanisms of their antitumor action and how can they be developed

as effective antitumor agents. Although not all existing triazenes are addressed, those

chosen largely represent the class and the latest discoveries.

While aliphatic triazenes are very sensitive to proteolytic decomposition and all of

investigations are old, other triazenes reported have more clinical interest with good

results in recent investigations. In the case of 1,3-diaryltriazenes, two types of

molecules deserve special attention: alkyne analogues of diminazene aceturate (DMZ)

and 4-nitro-substituted 1,3-diaryltriazenes. Aryl morpholino triazenes are other class of

compounds recently investigated that could inhibit CYP1A1 and CYP1B1 at the

micromolar level and could also have similar therapeutic value as resveratrol in

preventing cancer. In the class of triazenoheterocycles, just one compound, was lately

studied and demonstrated biochemical selectivity for EGFR and HER-2 receptors and

significant reduction of cell growth in four human pancreatic cancer cell lines. Lastly,

in relation to combi-molecules and specifically to combi-triazenes, a quite number of

these molecules with dual targeting properties were developed and demonstrated

anticancer activities both in vitro an in vivo, in the recent two decades. The most recent

combi-triazene was designed to have a poly(ADP-ribose) polymerase (PARP) inhibitor.

Keywords: Triazene; Anticancer activity; 1,3-diaryltriazenes; Aryl morpholino

triazenes; Combi-triazenes

4

Resumo

Os triazenos são uma classe de compostos muito útil e diversificada e têm vindo a ser

estudados, principalmente, devido ao seu potencial terapêutico em muitos tumores

incluindo do cérebro, leucemias e melanoma. Frequentemente, novos compostos

pertencentes a esta classe aparecem na literatura quer com a sua típica atividade

anticancerígena ou mesmo com outros interesses terapêuticos. Esta monografia contém

uma revisão destes compostos desde os mais simples como os 1,3-dialquil-3-

aciltriazenos aos mais complexos como os combi-triazenos enfatizando os seus

mecanismos de ação e o seu desenvolvimento como agentes antitumorais efetivos.

Apesar de nem todos os tipos de triazenos existentes estarem contemplados nesta

monografia, os escolhidos representam amplamente a classe e as novas descobertas.

Enquanto que os triazenos alifáticos, pela sua estrutura, são muito sensíveis à

decomposição proteolítica e todas as investigações já são antigas, outros triazenos

possuem maior interesse clínico com bons resultados em estudos recentes. No caso dos

1,3-diariltriazenos duas moléculas merecem especial atenção: os análogos alcino do

aceturato de diminazeno (DMZ) e os 1,3-diariltriazenos substituídos em para com um

grupo nitro. Os aril morfolino triazenos são outra classe de compostos recentemente

investigada que consegue inibir os CYP1A1 e CYP1B1 apresentando um valor

terapêutico semelhante ao resveratrol na prevenção do cancro. Dentro dos triazenos

heterocíclicos, apenas um composto foi estudado ultimamente, demonstrando

seletividade bioquímica para os recetores EGFR e HER-2 e redução significativa do

crescimento celular em quatro tipos de células de cancro do pâncreas. Por fim, várias

moléculas quiméricas e especificamente os combi-triazenos, foram desenvolvidos nas

últimas duas décadas sendo que o mais recente foi delineado para conter um inibidor

da poli(ADP-ribose)polimerase (PARP).

Palavras-chave: Triazeno; Atividade anticancerígena; 1,3-diaryltriazenos; Aril

morfolino triazenos; Combi-triazenos

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Acknowlegments

Tanto a elaboração desta monografia como o longo percurso de 5 anos só foram

possíveis graças a um conjunto admirável de pessoas.

Um especial agradecimento à professora Doutora Ana Paula Francisco, orientadora

desta monografia, pelo sorriso e amabilidade, pela transmissão de conhecimentos

científicos e pela permanente disponibilidade.

Aos meus pais, avó e irmão pelos valores que sempre me transmitiram e pelo apoio

incondicional em todas as fases do curso.

Ao meu namorado, André, que desde o primeiro dia me motivou a fazer mais e melhor

com as suas sábias e amáveis palavras.

Às minhas 7 companheiras desta longa jornada pela amizade, pelos sorrisos e pela

interajuda.

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Abbreviations

ADP Adenosine diphosphate

AGT O6-alkylguanine-DNA alkyltransferase

AIC 5-aminoimidazole-4-carboxamide

ATP Adenosine triphosphate

BaP Benzo[a]pyrene

CBzM 1-(2-chloroethyl)-3-benzyl-3-(methylcarbamoyl)triazene

CMC 1-(2-chloroethyl)-3-methyl-3-carbethoxytriazene

CML Chronic myeloid leukemia

CMM 1-(2-chloroethyl)-3-methyl-3-(methylcarbamoyl)triazene

CR Cascade release

CYP Cytochrome P450

CYP1A1 Cytochrome P450 1A1

CYP1A2 Cytochrome P450 1A2

CYP1B1 Cytochrome P450 1B1

DMA 3-acetyl-1,3-dimethyltriazene

DMC 3-carbethoxy-1,3-dimethyltriazene

DMM 3-(N-methylcarbamoyl)-1,3-dimethyltriazene

DMZ Diminazene aceturate

DNA Deoxyribonucleic acid

dNu Deoxynucleotide

DTIC Dacarbazine

EGFR Epidermal growth factor receptor

EH Epoxide hydrolase

EMEA European Medicines Agency

ER Endoplasmic reticulum

7

FDA US Food and Drug Administration

HER-2 Human epidermal growth factor receptor 2

HMTIC 5-(3-hydroxymethyl-3-methyl-1-triazeno)imidazole-4-carboxamide

IC50 Half-maximal inhibitory concentration

M Molar concentration

MAP Mitogen-activated protein

MGMT O6-methylguanine-DNA methyltransferase

MRSA Methicillin resistant Staphylococcus aureus

MTIC 5-(3-methyl-1-triazeno)imidazole-4-carboxamide

nM nanomolar

O6-BG O6-benzylguanine

PARP poly(ADP-ribose) polymerase

PAHs Polycyclic aromatic hydrocarbons

Ph Philadelphia

RNA Ribonucleic acid

ROS Reactive oxygen species

SAPK/JNK Stress-activated protein kinase/c-Jun NH2-terminal kinase

TB Tuberculosis

TK Tirosine kinase

TMZ Temozolomide

XRCC1 X-ray repair cross complementing 1

8

Index: 1 Introduction .......................................................................................................... 11

1.1 Triazenes in anticancer chemotherapy ......................................................... 111.2 Dacarbazine .................................................................................................. 121.3 Temozolomide ............................................................................................. 131.4 Goals of monography ................................................................................... 15

2 Materials and methods ......................................................................................... 163 Prodrugs of aliphatic triazenes ............................................................................. 17

3.1 1,3-dialkyl-3-acyltriazenes ........................................................................... 173.1.1 Synthesis .............................................................................................. 183.1.2 Mechanism of action ............................................................................ 20

4 1,3 – diaryltriazenes ............................................................................................. 234.1 Synthesis ...................................................................................................... 234.2 Mechanism of action .................................................................................... 24

4.2.1 Diminazene aceturate (Berenil) and derivatives .................................. 244.2.2 4-nitro-substituted 1,3-diaryltriazenes ................................................. 27

4.3 Other therapeutic interests ........................................................................... 284.3.1 Anti-mycobacterial activity ................................................................. 284.3.2 Antibacterial activity against MRSA ................................................... 28

5 Aryl morpholino triazenes ................................................................................... 306 Triazenoheterocycles ........................................................................................... 33

6.1 Synthesis ...................................................................................................... 336.2 Properties ..................................................................................................... 336.3 2-Triazenoazaindoles and pancreatic cancer ............................................... 34

7 “Combi-triazenes” ............................................................................................... 367.1 Development of resistance and concept of hybrid drugs ............................. 367.2 EGFR TK inhibitor-linked DNA damaging agents ..................................... 37

7.2.1 First generation of combi-molecules ................................................... 377.2.2 Combi-molecules with improved properties ........................................ 39

7.2.2.1 ZRBA1 as a radiosensitizer ............................................................. 407.2.2.2 JDE52: a bistriazene combi-molecule ............................................. 40

7.2.3 Cascade-release targeting combi-molecules ........................................ 417.2.4 Combi-molecule containing N-acetoxymethyl carbamate ................... 437.2.5 Synthesis of EGFR TK inhibitor-linked DNA damaging agents ......... 43

7.3 Bcr-Abl TK inhibitor-linked DNA-damaging agents .................................. 457.4 AGT inhibitor-linked DNA-damaging agents ............................................. 497.5 PARP inhibitor-linked DNA damaging agents ............................................ 50

8 Conclusion ........................................................................................................... 53Bibliography ................................................................................................................ 55

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Figure Index: Figure 1 - Triazene general structure. .......................................................................... 11Figure 2 - General synthetic routes to triazenes. (3) .................................................... 12Figure 3 - Structure of dacarbazine. ............................................................................. 12Figure 4 - Structure of temozolomide. ......................................................................... 13Figure 5 - Mechanism of action of DTIC and TMZ. (7) ............................................ 14Figure 6 - General structure of 1,3-dialkyl-3-acyltriazenes. ........................................ 17Figure 7 - Synthesis of 1-(2-Chloroethyl)-3-methyl-3-acyltriazenes. (22,23) ............. 19Figure 8 - Decomposition of 1-(2-chloroethyl)-3-acyl-3-methyltriazenes. (24) ......... 20Figure 9 - Proposed pathway for oxidative metabolism of CBzM. (26) ..................... 21Figure 10 - Structure of 1,3-diaryltriazenes. ................................................................ 23Figure 11 - Synthesis of 1,3-diaryltriazenes and their N-acyl derivatives. .................. 23Figure 12 - Diminazene aceturate structure and physicochemical properties. (30) ..... 24Figure 13 - Example of alkyne analogue of DMZ. ...................................................... 26Figure 14 - Sonogashira coupling. Adapted from (36). ............................................... 26Figure 15 - Structure of compound 11 and its rationale. ............................................. 27Figure 16 - 1,3-diaryltriazene with anti-mycobacterial activity. ................................. 28Figure 17 - Triazenide salts 13a-13d. Adapted from (31). .......................................... 29Figure 18 - BaP metabolization catalyzed by CYP. .................................................... 31Figure 19 - Resveratrol. ............................................................................................... 31Figure 20 - Compound 15. ........................................................................................... 32Figure 21 - Synthesis of Compound 15. Adapted from (43). ...................................... 32Figure 22 - Examples of triazenoheterocycles. ............................................................ 34Figure 23 – Compound 20. .............................................................................. 35Figure 24 - Degradation of SMA41 and BJ2000. Adapted from (60). ........................ 38Figure 25 - Action of SMA41 and BJ2000. Adapted from (60). ................................. 39Figure 26 - Hydrolysis and binary targeting of ZRBA1. (61) ..................................... 40Figure 27 - JDE52. (64) ............................................................................................... 41Figure 28 - Chemical decomposition and targets of RB24. Adapted from (65). ......... 42Figure 29 - RB107. ...................................................................................................... 42Figure 30 - ZRS1. ........................................................................................................ 43Figure 31 - Synthesis of SMA41, BJ2000 and ZRBA1. Adapted from (74). .............. 44Figure 32 - Synthesis of RB24 and RB107. Adapted from (66). ................................. 44Figure 33 - Synthesis of ZRS1. Adapted from (71). .................................................... 45Figure 34 - Imatinib and Dasatinib molecules. ............................................................ 46Figure 35 – ZRCM5 and its hydrolysis. Adapted from (65). ................................. 46Figure 36 - ZRF1. Adapted from (78). ........................................................................ 47Figure 37 - Synthesis of Compound 23. ...................................................................... 47Figure 38 - Synthesis of Compound 26. ...................................................................... 48Figure 39 - Synthesis of ZRMC5 and ZRF1. Adapted from (80). ............................... 48Figure 40 - Compound 27 and its hydrolysis. Adapted from (65) ............................... 49Figure 41 - Synthesis of Compound 27. Adapted from (82) ....................................... 50Figure 42 - Examples of first, second and third generation PARP inhibitors. ............. 51Figure 43 - EG22 and its hydrolysis. Adapted from (86) ............................................ 52Figure 44 - Synthesis of EG22. Adapted from (86) ..................................................... 52

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Table Index: Table 1 - Structures of some acyltriazenes. Adapted from. (14) ................................. 18Table 2 – Advantages of DMZ bind tightly to G-quadruplexes DNA. ....................... 26Table 3 - Extrahepatic organs where CYP1A1 and 1B1 are expressed. ...................... 30Table 4 - Host factors. .................................................................................................. 36

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1 Introduction

1.1 Triazenes in anticancer chemotherapy

Cancer is a multifaceted disease and represents one of the leading causes of mortality

in developed countries exceeded only by heart diseases. The treatment of cancer, in

particular metastases, is still one of the many challenges of medicine. (1)

Radiation therapy became a valuable tool after 1960 and, before that, just surgery was

mainly used. The beginning of the modern era of chemotherapy is directly related with

the discovery of nitrogen mustards as an effective treatment and with the necessity to

eradicate metastatic cancer that surgery or radiation could not. Drugs, biological

molecules and immune-mediated therapies have therefore become the focus for the

current efforts to cure cancer and the target-therapy revolution has arrived. The new

targets included growth factors, signalling molecules, cell-cycle proteins, modulators

of apoptosis and molecules that promoted angiogenesis. (2)

Triazenes are a very useful and diverse class of compounds that have been studied,

principally, for their anticancer potential in many tumors including brain, leukemia and

melanoma. They are straight-chain molecules that contain three contiguous nitrogen

atoms, in which N1 is double-bonded to N2, which is linked by a single bond to N3

(e.g. R1N=N–NR2R3 - Figure 1) and their mechanism of action is based on the

generation of an alkyldiazonium species that damages DNA at the O6 and N7 positions

of guanine. These group of compounds have an excellent pharmacokinetic, limited

toxicity and also have similar chemical, physical, antitumor and mutagenic properties.

(3–5)

Triazene are easily synthesized from available anilines or alkyl azides (Figure 2). In the

aniline way, the synthesis is performed with nitrite ion under acidic conditions to form

a diazonium salt, which reacts with a primary or secondary amine. To obtain triazenes

Figure 1 - Triazene general structure.

12

from alkyl azides, a reaction of an alkyl azide with the appropriate Grignard or alkyl

lithium reagent must occur. (3)

Triazene-derived compounds of high clinical interest are alkylating agents dacarbazine

(DTIC) and temozolomide (TMZ). (6)

1.2 Dacarbazine

DTIC, i.e. 5-(3,3-dimethyltriazeno)imidazole-4-carboxamide, is an imidazole-

carboxamide derivate that belongs to triazene class of 1-aryl-3,3-dialkyltriazenes and is

structurally related to purines. This compound was synthesized for the first time in 1959

as a result of a rational attempt to develop a drug able to interfere with purine

biosynthesis. Chemically, is composed of an imidazole ring fused with an amidic group

and with a mono unsaturated chain of three nitrogen atoms ending with two methyl

groups (Figure 3). (5,7,8) Dacarbazine has been in clinical use for the treatment of

malignant melanoma, soft-tissue sarcoma, and Hodgkin’s disease since 1970s, when

DTIC was approved in USA and in France. (3,5)

Although DTIC shows features similar to an intermediate metabolite of purine

biosynthesis, the principal mechanism of action of the agent does not allow to classify

this molecule as an antimetabolite. DTIC is a prodrug that requires metabolic activation,

Figure 3 - Structure of dacarbazine.

Figure 2 - General synthetic routes to triazenes. (3)

13

principally in the liver, by cytochrome P450 isoforms (CYP1A1, CYP1A2 and

CYP2E1) to give a hydroxymethylated compound (HMTIC). Then, HMTIC, by the loss

of formaldehyde, is converted into its monomethyl derivate (MTIC) which has a very

short half-time and decomposes spontaneously into an inactive derivative, 5-

aminoimidazole-4-carboxamide (AIC) and methyldiazonium cation, the alkylating

specie. This alkylating agent is responsible for producing methyl adducts in DNA. In

quantitative terms, the most frequent site of DNA alkylation is the N7 position of

guanine. Nevertheless, the alkylation on the O6 position is the principal responsible for

the cytotoxicity and the mutagenic effect of DTIC because it can generate an incorrect

base pairing with thymine (Figure 5). (5,7–9)

DTIC is administered by the intravenous route and the most common side effects are

nausea and vomiting, myelosuppression, cardiac and hepatic toxicity, immune

depression and mucocutaneous toxicity. (5,10,11)

1.3 Temozolomide

TMZ, i.e. 8-carbamoyl-3-methylimidazo[5,1-d]-1,2,3,5-tetrazin-4(3H)-one, is an

alkylanting agent of imidazotetrazine class that is structurally and functionally related

to DTIC. It was first synthesized in 1984 and, chemically, is composed by an imidazole

ring with an amidic group bound to C1 (imidazole-carboxamide) condensed with a

second tetrazinone ring system, that contains three adjacent nitrogen atoms (Figure 4).

Temozolomide was approved by FDA and EMEA in 1999 and, because of its good

central nervous system distribution, was first used to treat both primary brain tumors

like glioblastoma multiforme and oligodendroglioma, and to radiosensitise these

tumors. Latter, TMZ proved its efficacy in the treatment of melanoma. (5,12)

Figure 4 - Structure of temozolomide.

14

In contrast to Dacarbazine, TMZ spontaneously hydrolyses to MTIC in aqueous

solution at physiological pH with a non-enzymatic conversion. Thereafter, MTIC

rapidly degrades to AIC and methyldiazonium cation. This process of conversion is

irreversible and pH-dependent (Figure 5). (5,7)

Due to the fact that TMZ does not require metabolic activation by the liver, it is also

active in the case of liver function impairment and has wide potential clinical

applications including its use for loco-regional therapy. In addition, the intact TMZ

molecule, but not Dacarbazine or MTIC, crosses easily the blood brain barrier, because

of its lipophilic character, and is then activated in the brain compartment. Moreover,

differently from DTIC, the drug can be successfully administered by both parenteral

and oral routes because it is optimally adsorbed by the intestinal tract (100%

bioavailability). Temozolomide is also associated with a low incidence of severe

adverse events and the most common side effect is myelosuppression. In contrast,

nausea and vomiting are limited (10-15%), whereas the same side effects are

remarkably severe and highly frequent in patients treated with DTIC. Today, because

of all these characteristics, TMZ is almost replacing Dacarbazine in the clinic.

(5,7,11,13)

Figure 5 - Mechanism of action of DTIC and TMZ. (7)

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1.4 Goals of monography

The focus of this monography is the fundamental physicochemical properties of some

triazenes, namely, prodrugs of aliphatic triazenes, 1,3-diaryltriazenes, aryl morpholino

triazenes, triazenes heterocycles and combi-triazenes with emphasis on the mechanisms

of their antitumor action and how can they be developed as effective antitumor agents.

In general, the synthesis of these compounds is always explained as well as their

advantages in anticancer therapy.

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2 Materials and methods

This monography was carried out through a comprehensive analysis of articles and

patents. The articles were searched in the following databases: b-on, ScienceDirect,

PubMed and Web of Science. The search of patents was performed in the United States

Patent and Trademark Office (USPTO).

The keywords used were “triazene”, “alkyltriazene”, “1,3-diaryltriazene”, “aryl

morpholino triazene”, “triazenopyrazoles”, “triazenoindazoles”, “triazenopyrroles”,

“triazenoindoles” and “combi triazenes”. All the references found during the

conduction of this research were studied in detail and were considered valid the full-

text articles which contain the keywords mentioned above in the title and/or abstract.

The references of the selected articles were also investigated for additional information.

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3 Prodrugs of aliphatic triazenes

Arylalkyltriazenes have been subject of study by chemists and biologists for many years

due to the fact that many members of this class shown mutagenic, carcinogenic and

antitumor properties. One of this triazenes, DTIC, as described above, has proven to be

therapeutically valuable. (14,15)

Although the first preparation of alkyltriazenes was reported by Dimroth more than a

century ago (16), this class of compounds remained essentially unstudied until the 80s.

These compounds, di- and trialkyltriazenes, are also DNA-alkylating agents but, in

contrast to the aryldialkyltriazenes, do not require metabolic activation due to their

instability. These aliphatic triazenes are very sensitive to proteolytic decomposition and

most of them are stable in aprotic media or as pure compounds but are rapidly

hydrolyzed in water. (3,14–18)

Alkyltriazenes have been useful model compounds for the study of DNA alkylation in

the absence of metabolic activation, however biological studies were complicated by

the fact that they are extremely unstable in aqueous solutions. Therefore, in an effort to

acquire greater chemical stability while retaining alkylating activity, the 1,3-dialkyl-3-

acyltriazenes1 were developed. (14,19)

3.1 1,3-dialkyl-3-acyltriazenes

1,3-dialkyl-3-acyltriazenes are another class of triazenes which show potent antitumor

activity and are more stable to proteolytic decomposition than the parent 1,3-

dimethyltriazene. The general structure is represented in the Figure 6 and structures of

some acyltriazenes are given in Table 1. (3,14,20)

1 The term 1,3-dialkyl-3-acyltriazenes refers to compounds with N3 directly attached to a carbonyl-containing a substituent and it is used for convenience to emphasize the chemical behavior resulting from the presence of the carbonyl moiety. It is noted that the substituents like carbethoxy or methylcarbamoyl are not acyl groups in the strictest usage of the term. (14)

Figure 6 - General structure of 1,3-dialkyl-3-acyltriazenes.

18

Table 1 - Structures of some acyltriazenes. Adapted from. (14)

Compound Abbreviation R R’ X

1a DMA CH3 CH3 CH3

1b DMC CH3 CH3 OCH2CH3

1c DMM CH3 CH3 NHCH3

1d CMC CH2CH2Cl CH3 OCH2CH3

1e CMM CH2CH2Cl CH3 NHCH3

1f CBzM CH2CH2Cl CH2C6H5 NHCH3

3.1.1 Synthesis

The simplest examples of acyltriazenes are those that, like DMA, DMC and DMM,

contain a 1,3-dialkyl-3-acyl structure. In the synthesis of these compounds, except for

DMM, the direct acylation of the triazene was very sluggish or would not proceed

because of low nucleophilicity of dialkyltriazenes. Therefore, in the case of DMM the

direct acylation is possible and 1,3-dimethyltriazene reacted smoothly and directly with

methyl isocyanate in pentane. The compounds DMA and DMC, in turn, are obtained,

respectively, from the reaction of an acid chloride or ethyl chloroformate with a

disubstituted triazene anion. The disubstituted anions are obtained by treating an alkyl

azide with the appropriate alkyl lithium or Grignard reagent followed by addition of

potassium hydride. All of these reactions produce the compounds with good yield.

(3,15,21)

Although the synthesis of simple 1,3-dialkyltriazenes and their acylated derivatives can

be accomplished in a straightforward manner by the reaction of alkyl azides with

alkyllithiums or Grignard agent, this route was not applicable to chloroethyl derivatives

like CMC, CMM and CBzM. It is not possible to prepare organometallic reagents

containing a heteroatom on the b-carbon because of immediate elimination and,

similarly, attempts to add organometallic reagents to 2-haloethyl azides also resulted in

elimination reactions. So, in order to prepare these chloroethyl derivatives a less direct

19

approach was adopted2 (Figure 7). The first compound, 2-azidoethanol 2, was prepared

by the reaction of 2-chloroethanol with sodium azide. Then, the reaction with tert-

butyldimethylsilyl3 chloride (TBDMS-Cl) catalyzed by imidazole, results in the

formation of the compound 3. The protected azide was then reacted with methyllithium

in diethyl ether solution, using the reverse addition technique (methyllithium added to

the azide) in order to minimize the possible reaction of the organometallic reagent with

the protecting group. If the objective was to prepare the compounds 5a and 5b, the

resulting product was converted, in diethyl ether to its anion by the reaction with slight

excess of potassium hydride, catalyzed by 18-crown-6 ether and then, the resulted

solution was treated with ethyl chloroformate to give compound 5a or with acetyl

chloride to give compound 5b. In the case of compound 5c, the formation of the anion

compound is not necessary. Finally, the deprotection of the siloxytriazenes was

accomplished with tetra-n-butylammonium fluoride in tetrahydrofuran solution and

then, the triazenes were converted smoothly to the chloroethyl derivates by reaction

with carbon tetrachloride and triphenylphosphine. (22,23)

2 This method can be applied to all chloroethyl derivates including CBzM with the necessary modifications. 3 The tert-butyldimethylsilyl (TBDMS) group is used widely for the protection of hydroxyl group in synthetic organic chemistry due to their ease installation and removal without affecting other functional groups. (88)

Figure 7 - Synthesis of 1-(2-Chloroethyl)-3-methyl-3-acyltriazenes. (22,23)

20

3.1.2 Mechanism of action

Acyltriazenes also decompose chemically to yield alkyldiazonium ions, but with more

complex mechanism. Generally, they exhibit acid-catalyzed decomposition at lower

pH, uncatalyzed decomposition at neutral pH, and base-catalyzed decomposition at

higher pH. (14,20) The mechanism of action of most of these compounds at acidic or

neutral pH involves N2–N3 heterolysis (Major Path A), leading to production of an

alkyldiazonium ion derived from the N1 alkyl group and an amide derived from the N3

portion of the molecule. The alkyldiazonium ion can either chloroethylate (Minor Path

D) or hydroxyethylate DNA (Major Path C). At basic pH, or in the presence of a

hydrolytic enzyme such as esterase, deacylation becomes the predominant pathway

leading to the prodution of two tautomeric dialkyltriazenes (Minor Path B), which can

then decompose to give either a chloroethylating or hydroxyethylating species, or a

methylating agent. The different ratios of methylation to chloroethylation or

hydroxyethylation depend on the triazene structure (Figure 8). (20,24,25)

Figure 8 - Decomposition of 1-(2-chloroethyl)-3-acyl-3-methyltriazenes. (24)

21

The biological activity of acyltriazenes and especially of acyltriazenes bearing a

carbethoxy or a carbamoyl substituent could not easily be explained by chemical

decomposition alone due to their high degree of chemical stability under physiological

conditions. Therefore, an important question concerning the ability of these compounds

to alkylate DNA in vivo was raised. A partial answer to this question came from findings

that in the presence of porcine liver esterase, the rate and extend of alkylation by CMC

were increased markedly and thus, it was postulated that CMC and similar compounds

would be potent alkylating agents in vivo, but via a prior enzymatic deacylation step.

However, during the course of the investigations, CMM exhibited much less toxicity

than its corresponding carbethoxytriazene analog, CMC. So, the great disparity in their

toxicity led to the hypothesis that their behaviour in vivo must be strikingly different.

Due to the fact that CMC and CMM differ only in the acyl substituent, studies were

consequently initiated to determine the metabolic pathway of the

(methylcarbamoyl)triazenes, focusing on CBzM as a model compound. (14,26,27)

Results indicated that degradation of the (methylcarbamoyl)triazenes was oxidative

rather than hydrolytic and was most likely catalysed by one or more isozymes of

cytochrome P450. Further results proposed that the (methylcarbamoyl)triazenes, CBzM

Figure 9 - Proposed pathway for oxidative metabolism of CBzM. (26)

1f

22

and CMM, are subject to oxidative metabolism at a minimum of two sites:

methylcarbamoyl and chloroethyl substituents as shown in Figure 9.

Hydroxylation of the methylcarbamoyl group may lead to demethylation, at least in the

case of CBzM, but does not directly result in breakdown of the triazene or the

generation of an obvious alkylating agent. In contrast, hydroxylation of the chloroethyl

group results in destruction of the triazene moiety, with formation of

chloroacetaldehyde, a substituted urea, and presumably molecular nitrogen.

Therefore, the last investigations indicated that the antitumor efficacy of the

(methylcarbamoyl)triazenes may be attributable to a unique mechanism that does not

involve hydrolytic deacylation or the formation of an alkyldiazonium ion. However, the

biological activity of these compounds may be explained with the formation of

chloroacetaldehyde, a known mutagenic DNA alkylating agent. (26)

23

4 1,3 – diaryltriazenes

Aryl triazenes are a class of compounds that have been studied over 130 years for their

interesting structural, anticancer, and reactivity properties. 1,3-diaryltriazenes belongs

to this class and chemically are constituted of two aromatic groups connected by a

triazene bridge (Figure 10). (28–30)

4.1 Synthesis

The general synthesis of 1,3-diaryltriazenes and N-acyl derivatives via diazonium

intermediates is represented in Figure 114. The first step, is done by treating the

appropriately substituted anilines with concentrated hydrochloric acid (HCl), sodium

nitrite (NaNO2) and saturated sodium acetate (C2H3NaO2) at low temperature. The

second step, consists in the addition of the corresponding amine, if the final compound

is not symmetrical. The last and optional step relative to acylation of the selected 1,3-

diaryltriazenes into N-acyl-1,3-diaryltriazenes, is achieved by using the appropriate

acyl chloride (ClCOR) in acetonitrile (CH3CN) or acetone (CH3COCH3) solution in the

presence of triethylamine (Et3N) as a base. (6,30–32)

4 R2-R5 represent the various substituents that can be added.

Figure 11 - Synthesis of 1,3-diaryltriazenes and their N-acyl derivatives.

8

Figure 10 - Structure of 1,3-diaryltriazenes.

24

4.2 Mechanism of action

4.2.1 Diminazene aceturate (Berenil) and derivatives

The well-known representative of 1,3-diaryltriazenes is diminazene aceturate

(BerenilÒ or DMZ), the salt of 1,3-bis(4-amidinophenyl) triazene (Figure 12). This

compound is an aromatic diamidine of synthetic origin that was developed more than

six decades ago. BerenilÒ has been in the market since 1955 for the control of

trypanosomiasis caused by several species of flagellated protozoa belonging to the

genus Trypanosoma that are responsible for a large number of infections in animals. In

addition to its trypanocidal activity, diminazene aceturate has also demonstrated

applicability in the treatment of animals infected with protozoa of the genus Babesia.

(6,30,33)

The capacity of diminazene aceturate to bind to DNA has been recognized very early.

(34) Because of its chemical structure that contains two identical cationic groups

(dicationic diamidine), this compound presents a great affinity for the sequences of

adenine-thymine base pairs. Therefore, the binding to DNA occurs via complexation

Figure 12 - Diminazene aceturate structure and physicochemical properties. (30)

9

25

into the minor groove of AT-rich5 domains of DNA double helices. DMZ can also bind

to RNA and to DNA duplexes, exhibiting characteristic properties of both intercalation

as well as minor groove binding. (6,30) Nevertheless, recently it was revealed that

diminazene actually binds to G-quadruplexes 1000 times better than DNA duplexes,

with dissociation constants approaching 1 nM. (35,36)

G-quadruplexes are secondary structures in both DNA and RNA that are emerging as

important regulatory elements that control diverse processes in the cell, ranging from

telomere maintenance, gene expression, translation, alternative splicing, RNA

metabolism and protein sequestration.

There are ~3 760 000 guanine-rich regions in the human genome, which have the

potential to form G-quadruplexes including those at the telomere end and promoter

regions. In animal chromosomes, the telomerase enzyme (which is up-regulated in

certain cancers) is responsible for maintaining the telomere length thereby rendering

cancer cells immortal. The telomere is G-rich and has been shown to be capable of

forming G-quadruplexes. In addition to telomeres, G-quadruplexes are present in the

promoter regions of a number of oncogenic genes such as c-myc, BCL-2, KRAS and c-

kit. Small molecules that stabilize the G-quadruplex structure have been shown to

inhibit the extension of the DNA substrate by telomerase and transcription of cancer-

related genes and hence these molecules have the potential to be used as anticancer

agents.

Nevertheless, DMZ does not have strong anticancer activity because the binding of this

compound to the minor groove of AT-rich DNA with a micromolar dissociation

constant would limit the targeting of G-quadruplexes. In fact, in the complex cellular

environment the concentration of duplex DNA is several orders of magnitude greater

than G-quadruplexes. Despite this, the advantages of DMZ listed in the Table 2, shows

that DMZ scaffold is a good starting point to develop potent G-quadruplexes ligands.

So, recently, it was discovered that alkyne analogues of DMZ that also bind to G-

quadruplexes have good anticancer properties against ovarian, prostate and triple

negative breast cancers. (35,36)

5 AT-rich are regions with high content of adenine and thymine residues. This repeated sequences are commonly present in the sites for DNA replication initiation in bacterial, archaeal, and eukaryotic replicons. (89)

26

Table 2 – Advantages of DMZ bind tightly to G-quadruplexes DNA.

1. Connections with other biomolecules are not probable because DMZ

does not readily form p-aggregates.

2. The amidine groups on DMZ improve aqueous solubility and it is an

important factor for drugs.

3. DMZ has a simple structure and could be readily diversified and

synthesized on a large scale.

4. The amidine group, which is protonated at physiological pH, also would

facilitate drug permeation across lipid membranes.

These compounds, monoamidine analogues that bear alkyne moieties are, unlike of

DMZ, selective G-quadruplex binders with good anticancer properties. This occurs due

to the fact that since DMZ requires both amidine groups to bind to duplex DNA,

removing one amidine group will reduce duplex DNA binding affinity. One example

of these compounds is shown in the Figure 13.

The synthesis of this compounds is shown in Figure 11. The corresponding amines, in

the case of the example of the figure above, alkyne-substituted aromatic amines, are

synthetized via Sonogashira coupling (Figure 14). (36)

Figure 13 - Example of alkyne analogue of DMZ.

Figure 14 - Sonogashira coupling. Adapted from (36).

10

27

4.2.2 4-nitro-substituted 1,3-diaryltriazenes

As mentioned before, diminazene aceturate has very low activity against tumor cells

(IC50>100 µM). Therefore, in order to improve the antitumor efficacy of

diarytltriazenes, it was demonstrated that inactive 1,3-diaryltriazenes can be modified

to highly cytotoxic compounds by the introduction of two nitro groups at the para

position of the benzene rings, and two additional electron-withdrawing groups (bromo,

chloro, trifluoromethyl, or fluoro substituent) at their ortho positions. These

compounds, 1,3-bis(4-nitrophenyl)triazenes, show cytotoxicity at very low

concentrations (IC50 0.22 to 12.8 µM). (6,37,38) he compound 11, 3-acetyl-1,3-bis(2-

chloro-4-nitrophenyl)-1-triazene, shows highly cytotoxic activity against different

tumor cell lines, including cisplatin-resistant laryngeal carcinoma cells. Notably, its

antiproliferative action is significantly higher against tumor cells than against normal

cells. The rationale behind the structure of this compound and the structure itself are

represented in the Figure 15.

The mechanism of action of this compound is not related with the binding to DNA,

unlike the alkyltriazenes (dacarbazine and temozolomide) or the diaryltiazene

derivative, Berenil. Instead, compound 11 induces the formation of reactive oxygen

species (ROS) and the endoplasmic reticulum (ER) stress response, resulting in

apoptosis. Moreover, recently, the cytotoxicity was also associated to the induction of

the SAPK/JNK signalling pathway. (6,38)

11

Figure 15 - Structure of compound 11 and its rationale.

28

4.3 Other therapeutic interests

In addition to the already mentioned antitumor and antitrypanosomal activities of these

compounds other therapeutic interest have arisen, such as anti-mycobacterial (39) and

antibacterial (31) activities.

4.3.1 Anti-mycobacterial activity

Identification and development of novel compounds to target Mycobacterium

tuberculosis, the etiological agent of tuberculosis (TB), is still of utmost importance

mainly due to the rapid generation and spread of resistances. For this reason, several

substituted 1,3-diaryltriazenes were evaluated for its potential as anti-tubercular agent.

The compound 12 represented in the Figure 16 was selected with the best biological

properties (IC50 = 3.26 µM). This compound showed the ability to inhibit the growth of

a multi-drug resistant M. tuberculosis as well as the intracellular replication of M.

tuberculosis.

Despite these good results, it is unlikely that these compounds could find their

application as first line drugs. This occurs because the TB chemotherapy requires

intensive treatment for several months and the anti-tubercular potency of these

compounds was accompanied with an acute cytotoxicity, combined with the DNA

binding properties of the anti-trypanosomal drugs. However, with the increase in

incidence of multi-, extensively- and total-drug resistant TB, when other treatment

options are exhausted, this compound class might prove of use. (39)

4.3.2 Antibacterial activity against MRSA

Antibiotic resistance has become one of the most serious health care problems in the

world and there is an urgent need to develop new effective antibacterial agents that

circumvent the emergence of resistance.

The antimicrobial properties of 1,3-diaryltriazenes depended on the type of substituent

group attached to the two constituent benzene rings. Specifically, a trifluoromethyl

Figure 16 - 1,3-diaryltriazene with anti-mycobacterial activity.

12

29

group at ortho position, and a nitro substituent at para position relative to the triazene

was found to be critical to its activity. Thus, the triazenide salts represented in the Figure

17 showed high activity against one of the most clinically important bacterial species,

MRSA.

These triazenide salts were prepared from the corresponding diaryltriazene. The

reaction of the diaryltriazene with methyl (HCºCCO2CH3) or ethyl propiolate

(HCºCCO2C2H5) in the presence of trimethylamine (Et3N) yielded compounds 13a and

13b, whereas trituration with trimethylamine (Et3N) and potassium hydroxide (KOH)

gave triazenide salts 13c and 13d respectively.

The molecular basis of the triazene activity against S. aureus remained unclear and

appears to have more than one mechanism of action. Although, it seems that occurs a

modification of phospholipid metabolism and consequently the characteristics of the

staphylococcal cell membrane.

In addition, the selected compounds were found to be very effective against other gram-

positive bacteria, such as S. pneumoniae, B. subtilis, Vancomycin resistant E. faecalis,

and M. smegmatis. (31)

Figure 17 - Triazenide salts 13a-13d. Adapted from (31).

30

5 Aryl morpholino triazenes

Cytochromes P450 (CYP) constitute a large family of hemoproteins involved in the

detoxication of foreign compounds and the biosynthesis of endogenous compounds,

including steroid hormones, bile acids, and cholesterol. More than 21,000 CYPs have

been identified to date, and 18 families, including over 50 enzymes are found in

humans.

P450 families 1, 2, 3, and 4 contribute most extensively to the transformation of

xenobiotics to more polar metabolites that can be better excreted. The CYP1 subfamily

contains three members: CYP1A1, CYP1A2 and CYP1B1. Human CYP1B1 share 41

and 40% amino acid sequence homology with human CYP1A1 and CYP1A2,

respectively, while the latter two are 72% identical. CYP1A2 is expressed mainly in

liver, whereas CYP1A1 and 1B1 are expressed in many extrahepatic organs (Table 3).

(40,41)

Table 3 - Extrahepatic organs where CYP1A1 and 1B1 are expressed.

CYP1A1 Pancreas, thymus, uterus and small intestine

CYP1B1 Breast, prostate and uterus (manly)

Kidney, intestine, eye and brain (low levels)

Within CYP1 subfamily, CYP1A1 and CYP1B1 have been widely studied because they

are involved in the conversion of a large number of polycyclic aromatic hydrocarbons

(PAHs) into carcinogens. In fact, PAHs, found in tobacco smoke, automobile exhaust,

and charbroiled meat, can be oxidized by CYP1A1 and CYP1B1 into carcinogenic

epoxydiols through three enzyme-mediated reactions:

1. Oxidation of a double bond catalyzed by P450 enzymes to unstable arene

oxides;

2. Hydrolysis of the arene oxides by microsomal epoxide hydrolase (EH) to

dihydrodiols;

3. CYP-catalyzed oxidation at double bond adjacent to the diol function to

generate a high reactive vicinal diol-epoxide.

31

The formed epoxydiols are good electrophiles that react with the primary amino groups

in adenosine and guanosine to form 2’-deoxyguanosine and 2’-deoxyadenosine in

DNA. These DNA adducts cause misreplication, which lead to mutations in DNA that

can cause cancer. Benzo[a]pyrene (BaP) is an example of PAH and its metabolization

is given in the Figure 18. (42–44)

Because CYP1A1 and CYP1B1 oxidation of PAHs has been linked to tumor formation,

various compounds have been studied for their ability to inhibit these enzymes such as

stilbenes, naphthoquinone and anthraquinone derivatives, flavonoids, coumarin

derivatives, alkaloids, and other compounds. Many of these inhibitors share several

structural motifs: they are planar and contain one or more hydrophobic aromatic rings.

One example is resveratrol (Figure 19), a stilbene molecule, that is planar with two

aromatic rings and exhibit anti-cancer activity. (41,43)

Figure 18 - BaP metabolization catalyzed by CYP.

Figure 19 - Resveratrol.

14

32

Aryl morpholino triazenes have similar structural features and in addition contain a

triazene unit that could form p-p interactions with the enzyme active site amino acids.

Lee et al prepared several aryl morpholino triazenes and the compound 15, 4-[(E)-2-

(3,4,5-trimethoxyphenyl)diazenyl]-morpholine (Figure 20), was the only compound

that inhibited both CYP1A1 and CYP1B1 as well as or more so than resveratrol. Its

IC50 was 10 µM with CYP1A1 and 18 µM with CYP1B1 compared to resveratrol’s

IC50 values which were determined to be 34 µM with CYP1A1 and 55 µM with

CYP1B1. These values indicate that the trimethoxy groups, which are electron donating

and highly hydrophobic, provide a favorable interaction with the hydrophobic enzyme

active site.

The compound 15 was synthesized at 0ºC by combining a mixture of 3,4,5-

trimethoxyaniline and 6 M hydrochloric acid (HCl) with an aqueous solution of sodium

nitrite (NaNO2) to produce a diazonium salt that underwent coupling with morpholine

(Figure 21). (43)

In conclusion, aryl morpholino triazenes are a new class of compounds that inhibit

CYP1A1 and CYP1B1 at the micromolar level. (43)

Figure 20 - Compound 15.

15

Figure 21 - Synthesis of Compound 15. Adapted from (43).

33

6 Triazenoheterocycles

As mentioned above, a triazene derivative currently used in anticancer therapy is

dacarbazide. The potent antitumor activity shown by this compound led to the

development not only of aryl but also of heteroaryl derivatives, such as

triazenopyrazoles, -indazoles, -pyrroles and -indoles. (45,46)

6.1 Synthesis

The general synthesis of these compounds, like for other triazenes, was performed in

two steps. The first step consists in the formation of the diazonium salt from the

corresponding amine using an acid medium and aqueous sodium nitrite at low

temperature. The second and last step, involves the addition of the appropriate amine

in order to obtain the triazene compound. (46,47)

6.2 Properties

Triazene activity can be modulated by carrier structure, which influences

pharmacokinetics and/or compound stability. So, the nature of the heterocyclic portion

plays an important role and the activity increases as the rings become more electron-

rich, although at the expense of stability. (47–49) Consequently, triazenoindoles are

quite more active than triazenopyrroles and the same occur relatively to

triazenopyrroles and triazenoindazoles. All of these compounds showed cytotoxic

activity but, differently by Dacarbazine, did not follow a mechanism of action based on

the microsomal activation. (45,50)

Several examples of these compounds are shown in the Figure 22. The

triazenoindazoles (compound 17) showed more cytotoxic activity than the

corresponding pyrazole derivatives (compound 16). The compound of the type of

compound 17 with more activity was the diethyltriazenoindazole with chloro in the

position 5 (IC50 = 11.7 µM against human leukemia cell lines). (50) In accordance with

these results, 3-triazenopyrrole (compound 18), also showed cytotoxic activity against

erythroleukemia cells in the range of 1.1-3.1 µM. However, the benzocondensation on

this series that led to 3-triazenoindoles (compound 19) shown 20-40-fold more

activivity than the pyrrole derivatives against erythroleukemia and multidrug-resistant

cells with IC50 values of 0.053-0.080 µM and 0.10-0.14 µM, respectively. (46,49,51)

34

6.3 2-Triazenoazaindoles and pancreatic cancer

The majority of malignant tumors affecting the exocrine pancreas are histologically

defined as pancreatic ductal adenocarcinomas. This type of tumor rapidly grow and

metastasize representing one of the leading causes of cancer-related death in developed

countries.

Current therapeutic treatments for patients with advanced disease show only modest

effectiveness and are associated with considerable toxicity. Moreover, pancreatic

cancer shows a considerable drug resistance. Overexpression or aberrant activation of

members of the ErbB family of transmembrane tyrosine kinase growth factor receptors,

which includes EGFR (ErbB1) and HER-2 (ErbB2), occurs frequently and is associated

with multiple drug resistance and decreased patient survival.

In order to targeting these receptors, a novel low-molecular weight agent was

developed, i.e. ethyl 2-(3,3-dibenzyl 1-triazenyl)-1H-pyrido(2,3-c)pyrrolo-3-

carboxylate (Figure 23). Compound 20 induces decreased EGFR and HER-2

expression by blocking transcriptional genes and causes significant reduction of cell

growth and metabolic activity in four human pancreatic cancer cell lines.

The observed biochemical selectivity of the compound 20 for both receptors together

with its potent anti-proliferative effects makes the class of 2-triazenoazaindoles

Figure 22 - Examples of triazenoheterocycles.

16 17

18 19

35

attractive in order to produce novel chemotherapeutic agents for the treatment of

patients with pancreatic cancer. (52)

Figure 23 – Compound 20.

20

36

7 “Combi-triazenes”

7.1 Development of resistance and concept of hybrid drugs

One of the reasons for failure of chemotherapy is development of resistance. Failure of

a patient’s cancer to respond to a specific therapy can result from host factors listed in

the Table 4 and specific genetic or epigenetic alterations in the cancer cells. This genetic

and epigenetic heterogeneity in the face of the powerful selection imposed by potent

anticancer drugs result in an overgrowth of drug-resistant variants and the rapid

acquisition of resistance by many cancers.

Table 4 - Host factors.

1. Poor absorption or rapid metabolism or excretion of a drug, resulting in

low serum levels.

2. Poor tolerance to effects of a drug, especially elderly patients, resulting

in a need to reduce doses below optimal levels.

3.

Inability to deliver a drug to the site of a tumor, as could occur with

bulky tumors or with biological agents of high molecular mass and low

tissue penetration such as monoclonal antibodies and immunotoxins.

4. Various alterations in the host-tumor environment that affect response of

the tumor including local metabolism of a drug by non-tumor cells.

5.

Unusual features of the tumor blood supply that may affect transit time

of drugs within tumors and the way in which cells in a cancer interact

with each other and with interstitial cells.

Combinations of drugs that impact multiple targets simultaneously are the standard of

care in cancer treatment as they are better at controlling complex disease systems and,

more important, are less prone to drug resistance. In order to improve the efficiency of

using a two-drugs cocktail, one approach involves the use of so-called hybrid drugs,

which comprises the incorporation of two drugs in a single molecule with the intention

of exerting dual drug action. (53)

37

The first approach that has been used to design new anticancer hybrids is based on the

ability of a combination of haptophoric moieties on a new molecular structure to retain

their affinity and activity for the biological targets. This concept is achieved using two

strategies:

1. Merging of two haptophoric groups selected from two drugs exhibiting the same

cytocidal mechanism of action with the aim of improving activity, selectivity

and biopharmaceutical properties.

2. Merging of haptophoric groups from two drugs acting through different

mechanisms of action to improving the pharmacokinetic and

pharmacodynamics properties of the parent components as well as to synergize

their mechanisms of action in a single molecular entity structure.

The second approach combines two or several entire drugs in the same molecular

structure (combi-molecules) with similar or different mechanism of action. The

connection of the two molecular entities can be achieved using cleavable or non-

cleavable linkages. While the connection through non-cleavable linking arms is based

on the ability of the different molecules to retain their biological activity and their

specific and respective affinity for their biological targets, the approach using cleavable

bond is based on the release of two parental molecular structures under physiological

or the enzymatic conditions to improve pharmacokinetic and selectivity. (54)

7.2 EGFR TK inhibitor-linked DNA damaging agents

7.2.1 First generation of combi-molecules

Initially the feasibility of the combi-molecule strategy was proven by the first prototype

of combi-molecules SMA41 and BJ2000.

Solid tumors are often characterized by the expression of DNA repair enzymes, that

confer resistance to chemotherapy, and by the overexpression or dysfunction of proteins

directly implicated in mitogenic signaling. The expression of DNA repair enzymes,

such O6-alkylguanine-DNA alkyltransferase (AGT), significantly decreases the

chemosensitivity of these tumors to alkylating agents like triazene class. On the other

hand, the overexpression and dysregulation of tyrosine kinase (TK) is commonly

observed in a large number of cancers and is associated with aggressive tumor

progression, poor patient survival and more important reduced chemosensitivity. For

38

example, the epidermal growth factor receptor (EGFR) is often overexpressed in breast,

ovarian, and prostate tumors. (55–58)

The inhibitors of EGFR tyrosine kinase that belong to the quinazoline class act by

competitively binding to its ATP binding site, thereby blocking subsequent activation

of downstream signalling cascades, including mitogen-activated protein (MAP) kinase

and the transcription of genes associated with cell proliferation. Due to the fact that this

type of drugs required prolonged and repeated doses for the induction of sustained

antitumor activity, the combination of this drugs with classical cytotoxic agents has

become a common approach to increase the potency of EGFR-directed therapy. (58,59)

The “combi-triazenes” SMA41 and BJ2000 are both a chimeric and unimolecular

combination of two molecules associated with two major mechanisms of action: 4-

anilinoquinazoline that defines its ability to inhibit EGFR-mediated cell signalling and

3-methyl-1,2,3-triazene that masks its alkylating metabolite. These chimeric molecules

are able to target EGFR on its own and to degrade under physiological conditions to

give SMA52/FD105 (inhibitors of EGFR – I) and methyldiazonium (an alkylating

agent – TZ) as shown in the Figure 24. The cytotoxic contribution of the

methyldiazonium to DNA damage depend not only on the O6-alkylguanine adducts, but

also on the N7-methylguanine adducts what are important to minimize problems

associated with the presence of AGT. (57–60)

As depicted in the Figure 25, TZ-I can penetrate in EGFR-overexpressing cells by

passive diffusion and once inside the cells either directly bind to the EGFR ATP-

binding site to provide the TZ-I-EGFR complex (path 3) or degrade into a cytotoxic TZ

molecule plus an EGFR TK inhibitor I (path 2). More importantly, the TZ-I may

directly alkylate the EGFR, as outlined in path 4, wherein an inactivated (covalently

Figure 24 - Degradation of SMA41 and BJ2000. Adapted from (60).

39

modified) receptor (TZ-EGFR) may be formed, leaving an irreversibly inhibited

receptor. (59,60)

The only difference between SMA41 and BJ2000 is that the latter, with a less bulky

chloro substituent, has 2-fold stronger affinity than SMA41 and is capable of generating

an EGFR TK inhibitor with 5-fold stronger affinity than SMA52. (60)

7.2.2 Combi-molecules with improved properties

SMA41, the first prototype combi-molecule studied in vivo, showed a rather moderate

antitumor activity and this was imputed principally to:

1. Poor water solubility;

2. Decreased potency observed in AGT-expressing cells;

3. Very short half-life.

In an attempt to develop a better approach, a new prototype combi-triazene, ZRBA1,

was synthesized. This new molecule contain a 3’-chloro group in the quinazoline ring

because, as mentioned above, BJ2000, with chloro group showed superior potency, and

a N,N-dimethylaminoethyl group attached to the N3 of 1,2,3-triazene. The last group

improved hydrosolubility because its polar properties promotes aqueous solvatation

and, more importantly, induce N,N-dimethylaminoethylguanine adducts that are

perhaps less susceptible to AGT repair than those produced by SMA41. Moreover,

ZRBA1 demonstrated improved stability than SMA41 with longer half-life (108

Figure 25 - Action of SMA41 and BJ2000. Adapted from (60).

40

minutes compared with 30 minutes) maybe due to its ability to form intramolecular

hydrogen bonding that stabilizes the conjugated tautomer (Figure 26). (61)

7.2.2.1 ZRBA1 as a radiosensitizer

In locally advanced solid tumors, the combination of cytotoxic treatments such as

chemotherapy and radiation has been shown to improve local control, organ

preservation, and long-term survival.

ZRBA1 has radiosensitizer potential that may be secondary to its ability to arrest the

cells in G2/M, a cell cycle phase in which tumor cells are sensitive to radiation. The

combination of this molecule with radiation increased levels of DNA damage that

associated with the concomitant downregulation of EGFR-mediated signaling by

ZRBA1 contribute for significant levels of cell killing and can enhance tumor response.

(62,63)

7.2.2.2 JDE52: a bistriazene combi-molecule

JDE52, a bistriazene combi-molecule, was designed with the aim of improve the

potency of ZRBA1. Like is shown in the Figure 27, this bistriazene was programmed

Figure 26 - Hydrolysis and binary targeting of ZRBA1. (61)

41

to release two equivalents of EGFR TK inhibitor FD105 and a more cytotoxic

bifunctional DNA-damaging species. In fact, apoptosis was triggered by JDE52 at a

faster rate than ZRBA1 and led to higher levels of cell killing. This superior potency of

JDE52, when compared with ZRBA1, may be imputed to mechanisms associated with

the generation of higher intracellular concentrations of FD105 and to the induction of

DNA cross-links. (64,65)

7.2.3 Cascade-release targeting combi-molecules

In order to enhance the potency and stability of the combi-targeting molecule a novel

strategy termed “cascade release” (CR) was developed. This strategy seeks to mask the

combi-molecule into a prodrug planned to release the antitumor species by hydrolytic

activation.

RB24, a masked methyltriazene, was designed according with the premise that

acetoxymethyltriazenes are known to be hydrolyzed to the hydroxymethyltriazene

intermediate that rapidly degrades into the corresponding monoalkyltriazene and this

further heterolyses latter to an aromatic amine plus a DNA-damaging species. Thus,

RB24 degrade to RB14, ZR08, RB10 and a DNA alkylating methyldiazonium ion in a

Figure 27 - JDE52. (64)

42

progressive way, as shown in the Figure 28. In this cascade, in addition to

methyldiazonium ion, another reactive electrophile is formed: an iminium ion. This ion

alkylates the active site of EGFR, thereby irreversibly blocking its action.

RB24 induced significantly higher levels of DNA damage in EGFR transfected cells

than in wild type cells what indicate that RB24 could selectively target the cancer cells

overexpressing EGFR. Moreover, RB24 showed great antiproliferative activity that

could be imputed to the additional growth factor receptor TK irreversible inhibitory

property imprinted in the molecule that induced a depletion of DNA repair protein and

inactivation of anti-apoptotic signaling while high level of cytotoxic DNA lesions were

being inflicted. (65–69)

RB107 (Figure 29) is also a cascade release molecule with an acetoxymethyl function

that is hydrolyzed rapidly to generate BJ2000, a monoalkyltriazene that further

degrades to FD105 and DNA alkylating methylating species. This molecule has similar

properties to RB24. (70)

Figure 28 - Chemical decomposition and targets of RB24. Adapted from (65).

Figure 29 - RB107.

43

7.2.4 Combi-molecule containing N-acetoxymethyl carbamate

Previous strategies for stabilizing combi-triazenes were based on masking 1,2,3-

triazene chain with 3-acetoxymethylene group as in RB24 and RB107. While the latter

type of molecules were more stable than their parent triazenes, their half-lives were

only 5-10 minutes longer than the latter in cell culture medium. In order to further

increase the bioavailability of combi-molecules, a novel approach that seeks to mask

the monoalkyltriazene with carbamates was designed and the half-lives were prolonged

20-55 minutes. (70–72)

ZRS1 (Figure 30), that contains a more stable acetoxymethyl carbamate function, is a

second-generation derivative of RB107. This molecule, in vitro was extremely stable,

but it is rapidly metabolized in vivo perhaps due to intracellular or plasma esterases that

cleave the acetoxymethyl function. ZRS1 reached the plasma where rapidly releases

BJ2000, which in turn decomposed to FD105 and the methyldiazonium species. A

fraction of ZRS1 and BJ2000 may reach the tumors and decompose in situ. Plasma-

released FD105 was further metabolized in the liver to give its acetylated metabolite

FD105Ac that was further delivered to tumors. This FD105Ac metabolite was shown

to be more potent EGFR inhibitor than FD105 and it is significant over previous

methylating combi-molecules. (70)

7.2.5 Synthesis of EGFR TK inhibitor-linked DNA damaging agents

The synthesis of the EGRF TK-inhibitor-linked DNA damaging agents, including the

last mentioned RB24, RB107 and ZRS1, is done sequentially.

Firstly, the compounds SMA41, BJ2000 and ZRBA1 was synthesized as described in

the Figure 31. The first compound, 5-nitroanthranilonitrile was treated with a sulfuric

acid (H2SO4)/formic acid (CH2O2) mixture. The resulting quinazoline was heated with

phosphorus pentachloride (PCl5) to provide the chloro compound, which was treated

Figure 30 - ZRS1.

44

with substituted anilines (ArNH2). The nitro compounds, in the case of X=Br (RB10),

was reduced with Fe in ethanol, however in the other cases, was reduced by catalytic

hydrogenation. Lastly, the triazene moiety was formed using NOBF4 in acetonitrile

(CH3CN) followed by addition of the corresponding amine and neutralization with

triethylamine (Et3N). (73,74)

In order to synthesize compounds RB24 and RB107, the compounds RB10 and FD105

were also treated in acetonitrile (CH3CN) with NOBF4, but with the addition of a 10:1

mixture of formaldehyde (H2CO) / methylamine (MeNH2) and an alkalinization with

K2CO3. The resulting compound was treated with acetic anhydride (CH3CO)2O in

pyridine to obtain RB24 or RB107 (Figure 32). (66)

Figure 31 - Synthesis of SMA41, BJ2000 and ZRBA1. Adapted from (74).

Figure 32 - Synthesis of RB24 and RB107. Adapted from (66).

45

Finally, ZRS1 was obtained by treatment of BJ2000 with chloromethyl chloroformate

and pyridine in acetonitrile (CH3CN) at cold temperature to give ZRL4. Exchange of

the Cl atom using potassium iodide (KI) in dry acetone, gave the corresponding

iodomethyloxycarbonyl-methyltriazene, which was treated with silver acetate to obtain

ZRS1 (Figure 33). (71)

7.3 Bcr-Abl TK inhibitor-linked DNA-damaging agents

A reciprocal translocation between chromosomes 9 and 22 produces a Philadelphia (Ph)

chromosome which leads to the formation of the novel Bcr-Abl fusion gene. This fusion

gene leads to the production of an abnormal tyrosine kinase protein that activates

multiple downstream signalling pathways resulting in survival and proliferation of

chronic myeloid leukemia (CML) cells. In fact, Ph chromosome is present in the

leukemia cells of more than 95% patients with CML.

The blockade of the Bcr-Abl TK activity with Imatibin and Dasatinib (Figure 34), two

potent inhibitors approved by FDA for the treatment of CML, has been proven to

exhibit significant antitumor activity. However, the development of resistance and

relapses were observed and novel therapy models have been designed like combi-

molecules. TK Inhibitors-resistance mechanisms can include, for example, mutations

and amplification of Bcr-Abl genes or drug efflux mechanisms. (65,75–77)

Figure 33 - Synthesis of ZRS1. Adapted from (71).

46

ZRCM5, the first prototype of combi-molecule designed to release an Imatinib analog,

contain a 2-phenylaminopyrimidopyridine moiety targeted to Bcr-Abl kinase and a

triazene tail capable of generating a methyldiazonium species. ZRCM5 is a

hydroxymethyltriazene that does not require metabolic oxidation to generate the

cytotoxic species, since following loss of formaldehyde leads to the formation of

monoalkyltriazene, a hydrolabile species that generates the methyldiazonium cation

upon hydrolysis (Figure 35).

As expected, ZRCM5 was found to be approximately 74-fold more potent than

temozolomide, which might be due to its ability to simultaneously block Bcr-Abl and

related DNA repair activity, while inducing significant DNA lesions in Bcr-Abl

expressing leukemia cells. (4,65) However, although its capacity of inducing high levels

of DNA damage, exhibited only weak Bcr-Abl inhibitory activity what lead to the

Figure 34 - Imatinib and Dasatinib molecules.

Figure 35 – ZRCM5 and its hydrolysis. Adapted from (65).

47

design of a novel compound, ZRF1, that contain a trifluoromethyl benzamide moiety

for enhancing its Bcr-Abl potency. (78)

ZRF1 (Figure 36) was “programmed” to degrade into another inhibitor ZRF0 plus a

methyldiazonium species and demonstrated a 1.6-fold greater Bcr-Abl TK inhibitory

potency than Imatinib and 37-fold greater potency than ZRCM5. Molecular modelling

studies confirmed the importance of the hydrophobic interaction mediated by the CF3

group with the ATP-binding site of TK. (78)

More importantly, the superior potency of ZRF1 over Imatinib was more pronounced

in Bcr-Abl-positive cells coexpressing wild-type p53, which is available for

transactivating apoptosis protein p21 and Bax. Thus, ZRF1 is the first ever

multitargeted combi-molecule exerting a tandem targeting of Bcr-Abl mediated

antiapoptotic signaling and activation of the DNA damage response pathway. (65,78)

The synthesis of compounds ZRCM5 and ZRF1 is also done sequentially. Firstly, to

obtain the compound 23, the compound 21 was treated with N,N-dimethyl-formamide

dimethylacetal to form compound 22, which, in turn, was treated with substituted

phenylguanidine. The nitro group was reduced by catalytic hydrogenation (Figure 37).

(79)

Figure 36 - ZRF1. Adapted from (78).

21 22 23

Figure 37 - Synthesis of Compound 23.

48

Then, the commercially available compound 24 was hydrolyzed under acidic conditions

to give compound 25, which was chlorinated with SOCl2 to give compound 26 (Figure

38).

Finally, compound 23 was added to compound 26 in methylene chloride

(CH2Cl2)/pyridine at 0ºC to give the nitro compound, which was reduced with Fe in

ethanol to provide the amine compound. The triazene moiety was formed with

nitrosonium tetrafluoroborate (NOBF4) in acetonitrile (CH3CN) followed by the

addition of methylamine (MeNH2) or 1:30 mixture of methylamine (MeNH2) and

formaldehyde (H2CO) to obtain ZRF1 or ZRMC5, respectively. Then the solution was

neutralized with triethylamine (Et3N) (Figure 39). (80,81)

25 24 26

Figure 38 - Synthesis of Compound 26.

23 26

Figure 39 - Synthesis of ZRMC5 and ZRF1. Adapted from (80).

49

7.4 AGT inhibitor-linked DNA-damaging agents

The alkylation of O6-position of guanine in DNA is the major source of antitumor

activity of DNA alkylating agents, such as dacarbazine and temozolomide. However, a

DNA repair enzyme previously mentioned, O6-alkylguanine-DNA alkyltransferase

(AGT), also called O6-methylguanine-DNA methyltransferase (MGMT), can repair the

O6-lesion of guanine by transferring the O6-alkyl group to the active center at the

Cys145 residue and restore normal DNA. AGT is a suicide enzyme that is degraded

after accepting the lesion groups and its activity can only be recovered by its

resynthesis. (65,82–84)

Due to the fact that the increasing of AGT level correlates well with the enhancement

of tumor resistance to guanine O6-alkylating agents, a series of AGT inhibitors were

synthesized as adjuvants to improve the chemotherapeutic effects. O6-benzylguanine

(O6-BG) was the first potent AGT inhibitor that had entered clinical. O6-BG acts by

transferring its benzyl groups to active site of AGT inactivating the enzyme.

Unfortunately, the combination of O6-BG with guanine O6-alkylating agents only

exhibited limited response in clinical trials. (65,82,83)

To overcome this weakness and based on the concept of combi-molecule, a novel

approach containing the DNA methylating triazenes and the antiresistance agent O6-

BG in one molecule was designed. An important advantage of this strategy is that AGT-

depletion induced by O6-BG release and formation of the methylating species occur

simultaneously and in the same environment, ensuring optimal effect from DNA

methylation. The selected combi-molecule was compound 27 (Figure 40) because it has

an optimal half-life of approximately 23 min and was the most active (IC50 » 10 µM).

This superior activity is due to the AGT depletion by O6-BG released from the parent

molecule and its favorable penetration property. (65,82)

Figure 40 - Compound 27 and its hydrolysis. Adapted from (65)

27

50

The synthesis of compound 27 is done in various steps like is shown in the Figure 41.

Firstly, the compound 28 was treated with NaOBn in DABCO and DMSO to give

compound 29, to which was added the Boc group. Then, the nitration of compound 30

with tetrabutylammonium nitrate/trifluoroacetic anhydride (TBAN/TFAA) gave

compound 31. This last compound, was hydrogenated with H2/Pt-C and after reoxidated

with sodium periodate (NaIO4) to give compound 32. Finally, the condensation of

compound 32 with corresponding acylated hydrazine gave the compound 27. The Boc

group was removed with piperidine in acetonitrile (CH3CN). (82)

7.5 PARP inhibitor-linked DNA damaging agents

The past five years have seen significant development in the field of DNA repair

inhibitors. In this context, a cellular condition termed “synthetic lethality” is being

targeted for selective chemotherapeutic intervention against solid tumors of the breast,

ovary and pancreas. The concept of synthetic lethality applies to a situation where one

of two genes “A” or “B” is mutated and the functions of a non-mutated gene are

required to rescue the cells from dysfunction of the mutated one. Therefore, in cells in

which gene “A” is mutated, blockade or dysfunction of gene product “B”, leads to cell

dead.

A typical case of synthetic lethality is the mutation of the BRCA 1 or 2 genes, which

are tumor suppressor genes. Loss of BRCA1/2 functions impair DNA repair process and

Figure 41 - Synthesis of Compound 27. Adapted from (82)

27

28 29 30 31

32

51

an alternative to this deficiency is the expression of the PARP gene product commonly

referred to as poly(ADP-ribose) polymerase (PARP) that is responsible for the

recruitment of DNA polymerase and X-ray repair cross complementing 1 (XRCC1).

Since this is the only alternative for rescuing the cells in BRCA1 or 2 mutant cells,

blockade of PARP creates a synthetic lethality condition that ultimately leads to cell

dead. Thus, PARP inhibitors selectively kill tumor cells with disordered expression of

BRCA1/2.

The first generations of PARP inhibitors, including nicotinamide, 3-aminobenzamide

and 2-metlhylquinazolin-4-[3H]-one, were rather weak. While the later generations

(e.g. 5-aminoisoquinolinone and 4-amino-1,8-naphthalimide - ANI) were more potent,

it was not until 2014 that the first PARP inhibitor, olaparib, was approved for the

treatment of ovarian tumors characterized by BRCA1/2 mutations (Figure 42).

Disappointingly, clinical trials revealed that some patients become resistant to PARP

inhibitors and this is believed to be due to reactivation of BRCA1 and 2. Therefore,

strategies to improve the potency of PARP inhibitors are under evaluation and recently

Figure 42 - Examples of first, second and third generation PARP inhibitors.

52

a novel combi-molecule, EG22 (Figure 43), was designed to inhibit PARP (ANI) and

to release a DNA damaging species (methyldiazonium). (85,86)

The mechanism proposed was that EG22 can penetrate the cells and possibly the

nucleus where it may intercalate into DNA leads to an in situ generation of the DNA

damaging species. This may account for its ability to generate high levels of DNA

damage. Moreover, it is important to note that even in the cells expressing AGT, this

compound exhibit a strong potency and it was suggested that EG22 acting by a different

mechanism of action when compared with TMZ. Perhaps, tandem blockage of PARP

and induction of DNA damage allow to bypass the AGT-mediated resistance. (86) In

fact, the ability of PARP inhibitor to potentiate TMZ in tumor cells has already been

reported. (5,87)

EG22 was synthesized as described in the Figure 44. The 4-amino-1,8-naphthalimide

was treated with sodium nitrite (NaNO2) in trifluoroacetic acid at cold temperature to

give the diazonium salt. To this latter was added methylamine (MeNH2) to obtain the

final compound. The solution was subsequently neutralized with sodium bicarbonate.

(86)

Figure 43 - EG22 and its hydrolysis. Adapted from (86)

Figure 44 - Synthesis of EG22. Adapted from (86)

53

8 Conclusion

Triazenes represent a very useful and diverse class of antineoplastic compounds and a

large variety of malignancies refractory to conventional chemotherapy are potentially

sensitive to triazenes. In fact, this class of compounds continues appearing in the

literature frequently as either anticancer compounds or even with other therapeutic

interest like antibacterial activity.

While aliphatic triazenes are very sensitive to proteolytic decomposition and all of

investigations are old, other triazenes reported have more clinical interest with good

results in recent investigations.

In the case of 1,3-diaryltriazenes, two types of molecules deserve special attention:

alkyne analogues of DMZ and 4-nitro-substituted 1,3-diaryltriazenes. Alkyne

analogues of DMZ are easy to synthesize and are selective G-quadruplexes binders with

good anticancer properties. Future efforts will be directed at expanding the DMZ library

members and to increase the solubility profiles of the analogues. The selected

compound 11, the promise representative of 4-nitro-substituted 1,3-diaryltriazenes,

demonstrated high cytotoxicity against different tumor cell lines, but further studies will

be needed to understand all the mechanisms of this promising new anticancer

compound.

Aryl morpholino triazenes are other class of compounds recently investigated that could

inhibit CYP1A1 and CYP1B1 at the micromolar level and could also have therapeutic

value in preventing cancer. Relatively to this class, future work will involve designing

triazenes that will have enhanced therapeutic activity.

In the class of triazenoheterocycles, only compound 20, was lately studied. This

compound demonstrated biochemical selectivity for EGFR and HER-2 receptors and

significant reduction of cell growth in four human pancreatic cancer cell lines, however

additional studies are required to determine the precise molecular mechanisms

responsible for its anti-proliferative effects.

Lastly, in relation to combi-molecules and specifically to combi-triazenes, a quite

number of these molecules with dual targeting properties were developed and have

demonstrated anticancer activities both in vitro an in vivo, in the recent two decades.

However, great challenges are still existing: the activity of each component in the

54

combi-molecule must not be significantly reduced, the chemotherapeutic properties of

the parent compounds should not be changed and the entire combi-molecule must have

desirable biological properties like solubility, stability, selectivity, bioavailability and

efficacy in vivo. Thus, in the future, combi-triazenes can be focused on further

exploration of novel molecules targeting crucial processes in DNA repair, cell-cycle

control, drug metabolism and delivery and, moreover, the pharmacodynamics and

pharmacokinetic properties also should be optimized.

Despite being discovered a century ago triazenes continue to be innovative and potent

compounds with a wide range of therapeutic activities and great potential to become

useful drugs.

55

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