Inhibition of epidermal growth factor receptor-mediated signaling by "Combi-triazene" BJ2000, a new probe for Combi-Targeting postulates

Development of Dual Inhibitors Targeting Epidermal Growth Factor Receptor in Cancer Therapy

Epidermal growth factor receptor (EGFR) is of great significance in mediating cell signaling transduction and tumor behaviors. Currently, third-generation inhibitors of EGFR, especially osimertinib, are at the clinical frontier for the treatment of EGFR-mutant non-small-cell lung cancer (NSCLC). Regrettably, the rapidly developing drug resistance caused by EGFR mutations and the compensatory mechanism have largely limited their clinical efficacy. Given the synergistic effect between EGFR and other compensatory targets during tumorigenesis and tumor development, EGFR dual-target inhibitors are promising for their reduced risk of drug resistance, higher efficacy, lower dosage, and fewer adverse events than those of single-target inhibitors. Hence, we present the synergistic mechanism underlying the role of EGFR dual-target inhibitors against drug resistance, their structure-activity relationships, and their therapeutic potential. Most importantly, we emphasize the optimal target combinations and design strategies for EGFR dual-target inhibitors and provide some perspectives on new challenges and future directions in this field.

Rational Multitargeted Drug Design Strategy from the Perspective of a Medicinal Chemist

The development of multitarget-directed ligands (MTDLs) has become a widely focused research topic, but rational design remains as an enormous challenge. This paper reviews and discusses the design strategy of incorporating the second activity into an existing single-active ligand. If the binding sites of both targets share similar endogenous substrates, MTDLs can be designed by merging two lead compounds with similar functional groups. If the binding sites are large or adjacent to the solution, two key pharmacophores can be fused directly. If the binding regions are small and deep inside the proteins, the linked-pharmacophore strategy might be the only way. The added pharmacophores of second targets should not affect the binding mode of the original ones. Moreover, the inhibitory activities of the two targets need to be adjusted to achieve an optimal ratio.

Design and Mechanism of Action of a New Prototype of Combi-Molecule "Programed" to Release Bioactive Species at a pH Range Akin to That of the Tumor Microenvironment

The clinical use of cytotoxic agents is plagued by systemic toxicity. We report a novel approach that seeks to design a “combi-molecule” to behave as an alkylating agent on its own and to undergo acid-catalyzed conversion to two bioactive species at a pH range akin to that of a tumor microenvironment: an AL530 prototype was synthesized and we studied its ability to release a chlorambucil analogue (CBL-A) plus a potent mitogen-activated protein/extracellular signal-regulated kinase kinase (MEK) inhibitor (PD98059) at different pHs in buffered solutions, plasma and tumors. Its potency was compared in vitro with CBL+PD98059 (SRB assay) and in vivo in a xenograft model. Its target modulation was studied by western blotting and immunohistochemistry. AL530 released PD98059+CBL-A at mild acidic pH and in vitro was fivefold more potent than CBL and three-to-fivefold more potent than CBL+PD98059. In vivo it released high levels of PD98059 in tumors with a tumor/plasma ratio of five. It induced γ-H2AX phosphorylation and blocked pErk1,2, indirectly indicating its ability to damage DNA and modulate MEK. It induced significant tumor delay and less toxicity at unachievable doses for CBL and CBL+PD98059. We demonstrated the feasibility of a pH-labile combi-molecule capable of delivering high MEK inhibitor concentration in tumors, damaging DNA therein, and inducing tumor growth delay.

Subcellular distribution and mechanism of action of AL906, a novel and potent EGFR inhibitor rationally designed to be green fluorescent

To enhance the potency of EGFR inhibitors, we developed a novel strategy that seeks to conjugate EGFR to a bioactive moiety leading to a molecule termed "combi-molecule". In order to mimic the penetration of this type of molecules, based upon previously reported structure activity relationship studies, we designed a new molecule containing a quinazoline moiety tethered to a p-nitrobenzoxadiazole (NBD) moiety [molecular weight (MW) 700]. Despite its size, AL906 growth inhibitory activity was superior to that of the clinical drug gefitinib. Furthermore, AL906 retained significant EGFR inhibitory activity and good cellular penetration with abundant distribution in the perinuclear region of the cells. In an isogenic NIH3T3 transfected cell panel, it selectively inhibited the growth  of the NIH3T3-EGFR and HER2 transfectants. Confocal microscopy analysis revealed that it was capable of penetrating multilayer aggregates although to a lesser extent than FD105, a small inhibitor of EGFR inhibitor of the same class (MW 300). Its ability to inhibit EGFR auto-phosphorylation in monolayer culture was stronger than in the aggregates. The results suggest that our strategy did not negatively affect EGFR inhibitory potency, EGFR selectivity and growth inhibition. However, its molecular size may account for its decreased aggregate penetration when compared with a smaller EGFR inhibitor of the quinazoline class.

Anticancer Triazenes: from Bioprecursors to Hybrid Molecules

Triazenes are a very useful and diverse class of compounds that have been studied for their potential in the treatment of many tumors including brain tumor, leukemia and melanoma. Novel compounds of this class continue to be developed as either anticancer compounds or even with other therapeutic applications. This review focused on several types of triazenes from the simplest ones like 1,3-dialkyl-3-acyltriazenes to the more complex ones like combi-triazenes with an emphasis on how triazenes have been developed as effective antitumor agents.

A type I combi-targeting approach for the design of molecules with enhanced potency against BRCA1/2 mutant- and O6-methylguanine-DNA methyltransferase (mgmt)- expressing tumour cells

BACKGROUND

Mutations of the DNA repair proteins BRCA1/2 are synthetically lethal with the DNA repair enzyme poly(ADP-ribose) polymerase (PARP), which when inhibited, leads to cell death due to the absence of compensatory DNA repair mechanism. The potency of PARP inhibitors has now been clinically proven. However, disappointingly, acquired resistance mediated by the reactivation of wild type BRCA1/2 has been reported. In order to improve their efficacy, trials are ongoing to explore their combinations with temozolomide (TMZ). Here, in order to enhance potency in BRCA1/2-mutant cells, we report on the design of single molecules termed "combi-molecules" capable of not only inhibiting PARP but also damaging DNA like TMZ, which is known to induce a large number of DNA adducts. The majority of these lesions are processed through PARP-dependent base-excision repair machinery. Paradoxically, the least abundant lesion, the O6-methylguanine adduct is the most cytotoxic. Its repair by the O6-methylguanine DNA methyl transferase (MGMT) confers robust resistance to TMZ. Thus, we surmise that a combi-molecule designed to generate the same DNA adducts as TMZ, with an additional ability to block PARP, could induce BRCA1/2 mutant selective potency and a growth inhibitory profile independent of MGMT status.

METHODS

The hydrolysis of EG22 and its stabilized form ZSM02 was analyzed by HPLC and fluorescence spectroscopy. Growth inhibitory potency was determined by SRB assay. PARP inhibition was determined by an enzyme assay and DNA damage by the comet assay. Subcellular distribution was visualized by confocal microscopy.

RESULTS

Studies on EG22 showed that: (a) it inflicted anomalously higher levels of DNA damage than TMZ (b) it induced PARP inhibitory potency in the same range as ANI, a known PARP inhibitor (IC50 = 0.10 μM) (c) it showed strong potency in both BRCA1/2 wild type and mutated cells with 6-fold selectivity for the mutants and it was 65-303-fold more potent than TMZ and 4-63-fold than ANI alone and 3-47-fold than their corresponding equimolar combinations and (d) its potency was independent of MGMT expression.

CONCLUSION

The results in toto suggest that a combi-molecular approach directed at blocking PARP and damaging DNA can lead to single molecules with selective and enhanced potency against BRCA1/2 mutant and with activity independent of MGMT, the major predictive biomarker for resistance to TMZ.

The potential of combi-molecules with DNA-damaging function as anticancer agents

DNA-damaging agents, such as methylating agents, chloroethylating agents and platinum-based agents, have been extensively used as anticancer drugs. However, the side effects, high toxicity, lack of selectivity and resistance severely limit their clinical applications. In recent years, a strategy combining a DNA-damaging agent with a bioactive molecule (e.g., enzyme inhibitors) or carrier (e.g., steroid hormone and DNA intercalators) to produce a new 'combi-molecule' with improved efficacy or selectivity has been attempted to overcome these drawbacks. The combi-molecule simultaneously acts on two targets and is expected to possess better potency than the parent compounds. Many studies have shown DNA-damaging combi-molecules exhibiting excellent anticancer activity in vitro and in vivo. This review focuses on the development of combi-molecules, which possess increased DNA-damaging potency, anticancer efficacy and tumor selectivity and reduced side reactions than the parent compounds. The future opportunities and challenges in the discovery of combi-molecules were also discussed.

Target modulation by a kinase inhibitor engineered to induce a tandem blockade of the epidermal growth factor receptor (EGFR) and c-Src: the concept of type III combi-targeting

Cancer cells are characterized by a complex network of interrelated and compensatory signaling driven by multiple kinases that reduce their sensitivity to targeted therapy. Therefore, strategies directed at inhibiting two or more kinases are required to robustly block the growth of refractory tumour cells. Here we report on a novel strategy to promote sustained inhibition of two oncogenic kinases (Kin-1 and Kin-2) by designing a molecule K1-K2, termed "combi-molecule", to induce a tandem blockade of Kin-1 and Kin-2, as an intact structure and to be further hydrolyzed to two inhibitors K1 and K2 directed at Kin-1 and Kin-2, respectively. We chose to target EGFR (Kin-1) and c-Src (Kin-2), two tyrosine kinases known to synergize to promote tumour growth and progression. Variation of K1-K2 linkers led to AL776, our first optimized EGFR-c-Src targeting prototype. Here we showed that: (a) AL776 blocked EGFR and c-Src as an intact structure using an in vitro kinase assay (IC50 EGFR = 0.12 μM and IC50 c-Src = 3 nM), (b) it could release K1 (AL621, a nanomolar EGFR inhibitor) and K2 (dasatinib, a clinically approved Abl/c-Src inhibitor) by hydrolytic cleavage both in vitro and in vivo, (c) it could robustly inhibit phosphorylation of EGFR and c-Src (0.25-1 μM) in cells, (d) it induced 2-4 fold stronger growth inhibition than gefitinib or dasatinib and apoptosis at concentrations as low as 1 μM, and, (e) blocked motility and invasion at sub-micromolar doses in the highly invasive 4T1 and MDA-MB-231 cells. Despite its size (MW = 1032), AL776 blocked phosphorylation of EGFR and c-Src in 4T1 tumours in vivo. We now term this new targeting model consisting of designing a kinase inhibitor K1-K2 to target Kin-1 and Kin-2, and to further release two inhibitors K1 and K2 of the latter kinases, "type III combi-targeting".

Biological effects of AL622, a molecule rationally designed to release an EGFR and a c-Src kinase inhibitor

In breast cancer cells expressing c-Src and EGFR, a control of one of the two oncogenes over proliferation and invasion is observed, whereas in others, the synergistic interaction between them is required for tumor progression. With the purpose of developing molecules with the highest probability for blocking the adverse effects of these two oncogenes, we designed AL622, which contains a quinazoline head targeted to EGFR and a linker that bridges it to the PP2-like structure for targeting c-Src. In case the entire molecule would not be capable of blocking c-Src, we designed AL622 to hydrolyze to an intact c-Src-targeting PP2 molecule. After confirming its binary c-Src-EGFR targeting potency of AL622, we analyzed its potency in isogenic NIH3T3 cells transfected with EGFR and HER2 and human breast cancer cells known to be dominated by c-Src function. The results showed that in EGFR/HER-2-driven cells, it was more potent than PP2 and its activity was in the same range as the latter in more c-Src-driven cells. Its ability to block motility and invasion was comparable with that of PP2 and corresponding combinations, indicating that AL622 could be a better antitumor agent in cells where c-Src and/or EGFR play a role.

The Concept of Divergent Targeting through the Activation and Inhibition of Receptors as a Novel Chemotherapeutic Strategy: Signaling Responses to Strong DNA-Reactive Combinatorial Mimicries

Recently, we reported the combination of multitargeted ErbB1 inhibitor-DNA damage combi-molecules with OCT in order to downregulate ErbB1 and activate SSTRs. Absence of translation to cell kill was believed to be partially due to insufficient ErbB1 blockage and DNA damage. In this study, we evaluated cell response to molecules that damage DNA more aggressively and induce stronger attenuation of ErbB1 phosphorylation. We used three cell lines expressing low levels (U87MG) or transfected to overexpress wildtype (U87/EGFR) or a variant (U87/EGFRvIII) of ErbB1. The results showed that Iressa ± HN2 and the combi-molecules, ZRBA4 and ZR2003, significantly blocked ErbB1 phosphorylation in U87MG cells. Addition of OCT significantly altered cell cycle distribution. Analysis of the DNA damage response pathway revealed strong upregulation of p53 by HN2 and the combi-molecules. Apoptosis was only induced by a 48 h exposure to HN2. All other treatments resulted in cell necrosis. This is in agreement with Akt-Bad pathway activation and survivin upregulation. Despite strong DNA damaging properties and downregulation of ErbB1 phosphorylation by these molecules, the strongest effect of SSTR activation was on cell cycle distribution. Therefore, any enhanced antiproliferative effects of combining ErbB1 inhibition with SSTR activation must be addressed in the context of cell cycle arrest.

The combi-targeting concept: mechanism of action of the pleiotropic combi-molecule RB24 and discovery of a novel cell signaling-based combination principle

RB24 (NSC 741279), a 3-methyltriazene termed "combi-molecule" designed to possess mixed epidermal growth factor receptor (EGFR) targeting and DNA methylating properties showed over a 100-fold greater antiproliferative activity than Temodal(®) (TEM), a 4-fold greater potency than gefitinib and a 5-fold stronger activity than an equi-effective combination of gefitinib+TEM against the O(6)-alkylguanine transferase (AGT)-proficient DU145 cell line that co-expresses EGFR. Investigation of the mechanisms underlying the unique potency of RB24 revealed that cell exposure to TEM was accompanied by activation of p38MAPK and concomitant elevation of the levels of X-ray repair cross-complementing group 1 (XRCC1) protein. Levels of phospho-p38MAPK and XRCC1 were increased by 2-fold in EGF-stimulated cells. In contrast, EGF-stimulation did not alter the status of these proteins in RB24-treated cells and this translated into a 2-fold lower level of XRCC1 when compared with those exposed to TEM+EGF. These effects correlated with significantly delayed DNA repair activity in combi-molecule-treated cells when compared with TEM-exposed ones. Further analysis demonstrated that in contrast to TEM, RB24 could block Bad phosphorylation at serine 136 in a dose-dependent manner and induced significantly higher levels of apoptosis than the former molecule. Tandem depletion of XRCC1 and Bad activation through alternative pathways using the MEK1 inhibitor, PD98059, led to substantial levels of apoptosis in RB24-treated cells. The results in toto indicate that the superior activity of the combi-molecule may be attributed to its ability to down-regulate DNA repair proteins such as XRCC1 and to alleviate anti-apoptotic signaling through blockade of EGFR-mediated signaling while inflicting high levels of DNA lesions to the cells.

Synthesis and antiproliferative activities against Hep-G2 of salicylanide derivatives: potent inhibitors of the epidermal growth factor receptor (EGFR) tyrosine kinase

A series of salicylanilide derivatives (compounds 1-32) were synthesised by reacting substituted salicylic acids and anilines. The chemical structures of these compounds were determined by (1)H-NMR, electrospray ionisation mass spectrometry (ESI-MS) and elemental analysis. The compounds were assayed for their antiproliferative activities against the Hep-G2 cell line by the 3-(4,5-dimethylthylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) method. Among the compounds tested, 22 and 28 showed the most favouable antiproliferative activities with 50% inhibitory concentration (IC(50)) values of 1.7 and 1.3 μM, respectively, which were comparable to the positive control of 5-fluorouracil (IC(50)=1.8 μM). A solid-phase ELISA assay was also performed to evaluate the ability of compounds 1-32 to inhibit the autophosphorylation of the epidermal growth factor receptor tyrosine kinase (EGFR TK). Docking simulations of 22 and 28 were carried out to illustrate the binding mode of the molecule into the EGFR active site, and the result suggested that both compounds 22 and 28 could bind the EGFR kinase well.

Receptor activation and inhibition in cellular response to chemotherapeutic combinational mimicries: the concept of divergent targeting

The antiproliferative effect of tandem somatostatin receptor (SSTR) activation, epidermal growth factor receptor (EGFR) inhibition, and induction of DNA damage was analyzed using octreotide (OCT), a SSTR agonist, the clinical DNA methylating agent temozolomide (TMZ), Iressa, an EGFR inhibitor, and dual EGFR-DNA targeting agents termed "combi-molecules". Using SSTR-expressing glioma cells harbouring low levels of EGFR (U87MG) or transfected to overexpress EGFR (U87/EGFR) or a variant (U87/EGFRvIII), we showed that Iressa, alone or in combination with the DNA damaging agent TMZ, and combi-molecules RA2 and RA5 inhibited EGF-induced phosphorylation of EGFR in U87MG and more moderately in U87/EGFR and U87/EGFRvIII transfected cells. This translated into equivalent levels of Erk 1/2 inhibition. Activation of SSTRs with OCT did not modulate the effects of the various treatments on Erk 1/2 phosphorylation. Likewise, SSTR activation did not alter TMZ- or DNA-damaging combi-molecules, RA2 and RA5, induced p53 activation nor upregulation. However, SSTR activation significantly shifted TMZ-, RA2- and RA5-induced cell-cycle arrest to earlier phases (i.e., G2/M to late S, late S to S, S to G1). Further analysis showed that apoptosis was not induced. This was in agreement with the fact that p53 activation did not induce Bax upregulation nor did EGFR inhibition promote Bad dephosphorylation. Moreover, enhancement of survivin, an anti-apoptotic protein, expression was observed. The results in toto suggest that the combination of SSTR activation with EGFR inhibition and DNA damage affects cell-cycle progression but a disconnection between the targeted signalling pathways in these brain tumour cells precludes synergistic cell-killing by the triple growth inhibitory events.

Synthesis and uptake of fluorescence-labeled Combi-molecules by P-glycoprotein-proficient and -deficient uterine sarcoma cells MES-SA and MES-SA/DX5

Here, we report on the first synthesis of fluorescent-labeled epidermal growth factor receptor-DNA targeting combi-molecules, and we studied the influence of P-glycoprotein status of human sarcoma MES-SA cells on their growth inhibitory effect and cellular uptake. The results showed that 6, bearing a longer spacer between the quinazoline ring and the dansyl group, was more stable and more cytotoxic than 4. In contrast to the latter, it induced significant levels of DNA damage in human tumor cells. Moreover, in contrast to doxorubicin, a drug known to be actively effluxed by P-gp, the more stable combi-molecule 6 induced almost identical levels of drug uptake and DNA damage in P-gp-proficient and -deficient cells. Likewise, in contrast to doxorubicin, 4 and 6 exerted equal levels of antiproliferative activity against the two cell types. The results in toto suggest that despite their size, the antiproliferative effects of 4 and 6 were independent of P-gp status of the cells.

Subcellular distribution of a fluorescence-labeled combi-molecule designed to block epidermal growth factor receptor tyrosine kinase and damage DNA with a green fluorescent species

To monitor the subcellular distribution of mixed epidermal growth factor (EGF) receptor (EGFR)-DNA targeting drugs termed combi-molecules, we designed AL237, a fluorescent prototype, to degrade into a green fluorescent DNA damaging species and FD105, a blue fluorescent EGFR inhibitor. Here we showed that AL237 damaged DNA in the 12.5 to 50 mumol/L range. Despite its size, it blocked EGFR phosphorylation in an enzyme assay (IC(50) = 0.27 mumol/L) and in MDA-MB468 breast cancer cells in the same concentration range as for DNA damage. This translated into inhibition of extracellular signal-regulated kinase 1/2 or BAD phosphorylation and downregulation of DNA repair proteins (XRCC1, ERCC1). Having shown that AL237 was a balanced EGFR-DNA targeting molecule, it was used as an imaging probe to show that (a) green and blue colors were primarily colocalized in the perinuclear and partially in the nucleus in EGFR- or ErbB2-expressing cells, (b) the blue fluorescence associated with FD105, but not the green, was colocalized with anti-EGFR red-labeled antibody, (c) the green fluorescence of nuclei was significantly more intense in NIH 3T3 cells expressing EGFR or ErbB2 than in their wild-type counterparts (P < 0.05). Similarly, the growth inhibitory potency of AL237 was selectively stronger in the transfectants. In summary, the results suggest that AL237 diffuses into the cells and localizes abundantly in the perinuclear region and partially in the nucleus where it degrades into EGFR and DNA targeting species. This bystander-like effect translates into high levels of DNA damage in the nucleus. Sufficient quinazoline levels are released in the cells to block EGF-induced activation of downstream signaling. Mol Cancer Ther; 9(4); 869-82. (c)2010 AACR.

The combi-targeting concept: selective targeting of the epidermal growth factor receptor- and Her2-expressing cancer cells by the complex combi-molecule RB24

Within the context of a new tumor-targeting strategy termed "combi-targeting," we designed RB24 to inhibit epidermal growth factor receptor (EGFR) or Her2 phosphorylation and to further degrade upon hydrolysis to 4-(3'-bromophenylamino)-6-aminoquinazoline (RB10; another EGFR/Her2 inhibitor) plus a strong DNA-alkylating species. 6-(3-Acetoxymethyl-3-methyltriazenyl)-4-(3'-bromophenylamino)quinazoline (RB24) showed significant antiproliferative activity against human breast cancer cells, and transfection of one such cell line, MDA-MB-435, with ErbB1 or ErbB2 (Her2) dramatically enhanced cell death by apoptosis. RB24 was capable of releasing 2- to 3-fold higher levels of RB10 in the transfectants than in their wild-type counterparts. More importantly, RB10 was abundantly distributed in the perinuclear region of the cells, and its elevated levels in the ErbB transfectants were concomitant with increased levels of DNA lesions in the latter cells. This selectivity could be abolished by coincubation of the cells with exogenous RB10, suggesting that the entire combi-molecule may bind primarily to its cognate perinuclear sites before degradation. This localization may exert a bystander effect, allowing the alkylating species to be abundantly propagated into the nucleus. Cell response to this novel targeting mechanism was mediated by 1) activation of c-Jun NH(2)-terminal kinase in response to DNA damage and 2) down-regulation of Bad through blockade of EGFR tyrosine kinase activity: two events that cooperatively converged into enhancement of apoptosis in the oncogene-transfected cells.

Interaction of ionizing radiation and ZRBA1, a mixed EGFR/DNA-targeting molecule

ZRBA1 is a molecule termed 'combi-molecule' designed to induce DNA-alkylating lesions and to block epidermal growth factor receptor (EGFR) tyrosine kinase. Owing to its ability to downregulate the EGFR tyrosine kinase-mediated antiapoptotic signaling and DNA repair proteins, we inferred that it could significantly sensitize cells to ionizing radiation. Using the MDA-MB-468 human breast cancer cell line in which ZRBA1 has already been reported to induce significant EGFR/DNA-targeting potency, the results showed that: (i) concurrent administration of ZRBA1 and 4 Gy radiation led to a significant decrease in cell viability, (ii) the greater efficacy of the combination was sequential, being limited to conditions wherein the drug was administered concurrently with radiation or before radiation, and (iii) the efficacy enhancement of the combination was further confirmed by clonogenic assays from which a dose enhancement factor of 1.34 could be observed at survival fraction of 0.01. Flow cytometric analysis showed significant enhancement of cell cycle arrest in G2/M (P<0.046, irradiated cells vs. cells treated with ZRBA1 and radiation) and increased apoptosis when ZRBA1 was combined with radiation. Likewise, significant levels of double-strand breaks were observed for the combination, as determined by neutral comet assay (P<0.045, irradiated cells vs. cells treated with ZRBA1 and radiation). These results in toto suggest that the superior efficacy of the ZRBA1 plus radiation combination may be secondary to the ability of ZRBA1 to arrest the cells in G2/M, a cell cycle phase in which tumor cells are sensitive to radiation. Furthermore, the increased levels of DNA damage, combined with the concomitant downregulation of EGFR-mediated signaling by ZRBA1, may account for the significant levels of cell killing induced by the combination.

Combi-targeting concept: an optimized single-molecule dual-targeting model for the treatment of chronic myelogenous leukemia

Blockade of Bcr-Abl by the inhibitor Imatinib has proven efficacious in the therapy of chronic myelogenous leukemia (CML). However resistance to the drug emerges at the advanced phases of the disease. Therefore, novel therapy models remained to be designed. We have developed a novel dual targeted agent termed "combi-molecule" designed to not only block Bcr-Abl but also damage DNA. ZRF1, the first optimized prototype of the approach, was "programmed" to degrade into another inhibitor ZRF0 plus a methyl diazonium species. It was approximately 2-fold stronger Abl tyrosine kinase inhibitor than Imatinib and a more potent DNA-damaging agent than Temodal. In the p53 wild-type Mo7p210 cells, the potency of ZRF1 was approximately 1,000-fold superior to that of the equieffective combinations of Imatinib plus Temodal. More importantly, its superior potency over Imatinib was more pronounced in Bcr-Abl-positive cells coexpressing wild-type p53. Studies to rationalize these results showed that, through its Bcr-Abl inhibitory function, it down-regulated p53. However, sufficient level of the latter protein was available for transactivating p21 and Bax, which are required for cell cycle arrest and apoptosis. The results suggest that, in p53 wild-type cells, apoptosis is induced not only through Bcr-Abl inhibition but also through the p53-controlled DNA-damaging pathway, leading to an additive effect that translates into enhanced cell death. The study conclusively showed that p53 is a major determinant for the cytotoxic advantages of the novel combi-molecular approach in CML, a disease in which 70% to 85% of all the cases express wild-type p53.

Optimization of novel combi-molecules: Identification of balanced and mixed bcr-abl/DNA targeting properties

Steps toward the identification of combi-molecules with strong abl tyrosine kinase (TK) inhibitory property and significant DNA damaging potential are described. The optimized combi-molecule 13a was shown to induce approximately twofold stronger abl TK inhibitory activity than Gleevec and high levels of DNA damage in chronic myelogenous leukemic cells.

The Combi-Targeting Concept: In vitro and In vivo Fragmentation of a Stable Combi-Nitrosourea Engineered to Interact with the Epidermal Growth Factor Receptor while Remaining DNA Reactive

PURPOSE

JDA58 (NSC 741282), a "combi-molecule" optimized in the context of the "combi-targeting concept," is a nitrosourea moiety tethered to an anilinoquinazoline. Here, we sought to show its binary epidermal growth factor receptor (EGFR)/DNA targeting property and to study its fragmentation in vitro and in vivo.

EXPERIMENTAL DESIGN

The fragmentation of JDA58 was detected in cells in vitro and in vivo by fluorescence microscopy and tandem mass spectrometry. EGFR phosphorylation and DNA damage were determined by Western blotting and comet assay, respectively. Tumor data were examined for statistical significance using the Student's t test.

RESULTS

JDA58 inhibited EGFR tyrosine kinase (IC(50), 0.2 micromol/L) and blocked EGFR phosphorylation in human DU145 prostate cancer cells. It induced significant levels of DNA damage in DU145 cells in vitro or in vivo and showed potent antiproliferative activity both in vitro and in a DU145 xenograft model. In cell-free medium, JDA58 was hydrolyzed to JDA35, a fluorescent amine that could be observed in tumor cells both in vitro and in vivo. In tumor cells in vitro or in vivo, or in plasma collected from mice, the denitrosated species JDA41 was the predominant metabolite. However, mass spectrometric analysis revealed detectable levels of the hydrolytic product JDA35 in tumor cells both in vitro and in vivo.

CONCLUSIONS

The results in toto suggest that growth inhibition in vitro and in vivo may be sustained by the intact combi-molecule plus JDA35 plus JDA41, three inhibitors of EGFR, and the concomitantly released DNA-damaging species. This leads to a model wherein a single molecule carries a complex multitargeted-multidrug combination.

DNA repair inhibitors in cancer treatment

Chemotherapy and radiation are two important modalities for cancer treatment. Many agents in clinical used have the ability to induce DNA damage, however they may be highly cytotoxic as a secondary effect. Different mechanisms are involved both, in detection and repair of DNA damage. The modulation of these pathways, has a great impact on clinical outcome and is frequently responsible of therapeutic resistance. Therefore, pharmacological inhibition of DNA damage repair pathways has been explored as a useful strategy to enhance chemo and radiosensitivity, thus it could be used for reversing drug resistance. Different agents have shown excellent results in preclinical studies in combination with radiation or chemotherapy. Early phase clinical trials are now being carried out using different DNA repair inhibitors targeting several enzymes such as PARP, DNA-PK or MGMT.

The Combi-Targeting Concept: Synthesis of Stable Nitrosoureas Designed to Inhibit the Epidermal Growth Factor Receptor (EGFR)

According to the "combi-targeting" concept, the EGFR tyrosine kinase (TK) inhibitory potency of compounds termed "combi-molecules" is critical for selective growth inhibition of tumor cells with disordered expression of EGFR or its closest family member erbB2. Here we report on the optimization of the EGFR TK inhibitory potency of the combi-molecules of the nitrosourea class by comparison with their aminoquinazoline and ureidoquinazoline precursors. This led to the discovery of a new structural parameter that influences their EGFR TK inhibitory potency, i.e., the torsion angle between the plane of the quinazoline ring and the ureido or the nitrosoureido moiety of the synthesized drugs. Compounds (3'-Cl and Br series) with small angles (0.5-3 degrees ) were generally stronger EGFR TK inhibitors than those with large angles (18-21 degrees ). This was further corroborated by ligand-receptor van der Waals interaction calculations that showed significant binding hindrance imposed by large torsion angles in the narrow ATP cleft of EGFR. Selective antiproliferative studies in a pair of mouse fibroblast NIH3T3 cells, one of which NIH3T3/neu being transfected with the erbB2 oncogene, showed that IC(50) values for inhibition of EGFR TK could be good predictors of their selective potency against the serum-stimulated growth of the erbB2-tranfected cell line (Pearson r = 0.8). On the basis of stability (t(1/2)), EGFR TK inhibitory potency (IC(50)), and selective erbB2 targeting, compound 23, a stable nitrosourea, was considered to have the structural requirements for further development.

The combi-targeting concept: evidence for the formation of a novel inhibitor in vivo

With the purpose of developing drugs that can block multiple targets in tumor cells, molecules termed combi-molecules or TZ-I have been designed to be hydrolyzed in vitro to TZ+I, where TZ is a DNA-damaging species and I is an inhibitor of the epidermal growth factor receptor (EGFR). Using HPLC and liquid chromatography-mass spectrometry (LC-MS), we investigated the mechanism of in vivo degradation of a prototype of one such combi-molecule, ZRBA1, which when administered i.p. rapidly degraded into FD105 (Cmax=50 micromol/l, after 30 min), a 6-aminoquinazoline that was N-acetylated to give FD105Ac (IAc) (Cmax=18 micromol/l, after 4 h). A similar rate of acetylation was observed when independently synthesized FD105 was administered i.p. More importantly, the EGFR binding affinity of IAc was 3-fold greater than that of I, indicating that the latter is converted in vivo into an even more potent EGFR inhibitor. The results in toto suggest that while in vitro TZ-I is only hydrolyzed to I+TZ, further acetylation of I in vivo leads to a third component--a highly potent EGFR inhibitor with a delayed Cmax.

The emerging role of DNA repair proteins as predictive, prognostic and therapeutic targets in cancer

Advanced cancer is the second leading cause of death in the western world. Chemotherapy and radiation are the two main treatment modalities currently available to improve patient outcomes, but treatment related toxicity and the emergence of resistance limit their effectiveness. Hence there is an urgent need to develop novel treatment strategies. Rapid advances in cancer biology have identified key pathways involved in the repair of DNA damage induced by chemotherapeutic agents and irradiation. Efficient DNA repair in the cancer cell is an important mechanism for therapeutic resistance. Up to 130 genes have been identified that are associated with human DNA repair. Several of these proteins are emerging as important predictive and prognostic factors in solid tumours. Inhibition of DNA repair has the potential to enhance the efficacy of currently available DNA damaging agents. In recent years, several promising drug targets have been identified and novel drugs synthesised that target specific DNA repair proteins. These agents have shown impressive anti-cancer effects in preclinical studies in combination with chemotherapy or irradiation. Their role in human cancer is now being investigated in early phase clinical trials in combination with chemotherapy. MGMT inhibitors, PARP inhibitors and methoxyamine are currently in early stages of clinical development. Innovative clinical trial designs are essential to evaluate the potential of DNA repair inhibitor in cancer therapy.

DNA repair inhibition: a selective tumour targeting strategy

Advanced cancer is a leading cause of death in the developed world. Chemotherapy and radiation are the two main treatment modalities currently available. The cytotoxicity of many of these agents is directly related to their propensity to induce DNA damage. However, the ability of cancer cells to recognize this damage and initiate DNA repair is an important mechanism for therapeutic resistance and has a negative impact upon therapeutic efficacy. Pharmacological inhibition of DNA repair, therefore, has the potential to enhance the cytotoxicity of a diverse range of anticancer agents. Moreover, the use of inhibitors of DNA repair or DNA damage signalling pathways appears to provide an exciting opportunity to target the genetic differences that exist between normal and tumour tissue.

Mechanism of action of a novel "combi-triazene" engineered to possess a polar functional group on the alkylating moiety: evidence for enhancement of potency

Previous studies showed that SMA41, a 3-methyltriazene termed "combi-molecules" possessing a dual epidermal growth factor receptor (EGFR)/DNA targeting properties induced potent antiproliferative activity against alkylating-agent-resistant cells expressing EGFR in vitro. However, despite its marked potency, its antitumour activity in vivo was significantly hampered by its poor hydrosolubility and the moderate reactivity of its alkylating moiety. To circumvent this problem, we designed the quinazolinotriazene ZRBA1 to contain a N,N-dimethylaminoethyl group grafted to the 3-position of the triazene chain where it could serve both as a water soluble and a more potent alkylating moiety. ZRBA1 exhibited five-fold stronger EGFR tyrosine kinase (TK) inhibitory activity (IC(50)=37nM) than SMA41, decomposed into a 6-amino-quinazoline FD105 (IC(50)=200nM) and preferentially blocked EGF- over platelet-derived growth factor (PDGF)-or serum-induced cell growth. ZRBA1 induced DNA damage, concomitantly blocked EGF-stimulated EGFR phosphorylation by a partially irreversible mechanism in MDA-MB-468 breast cancer cells, and induced partially irreversible antiproliferative activity. It also prevented EGFR-mediated MAP kinase activation and, in contrast to FD105 and SMA41, induced high levels of apoptosis. Furthermore, ZRBA1 showed significantly greater antitumor activity (p<0.05) than SMA41 in the human MDA-MB-468 breast cancer xenograft model. The results in toto indicate that the appendage of N,N-dimethylaminoethyl to combi-triazenes may be an alternative to the reduced hydrosolubility and also to the lack of potency of monofunctional combi-triazenes against resistant tumours.

Cytokinetics and mechanism of action of AKO4: a novel nitrogen mustard targeted to bcr-abl

The "combi-targeting" concept seeks to design molecules to not only block tyrosine kinase (TK) activity but also to induce DNA damage. Here we design AK04, a molecule that combines the pharmacophore chlorambucil with that of STI-571 (Gleevec). The results showed that although a less potent abl TK inhibitor than STI571, AK04 was capable of significantly blocking bcr-abl phosphorylation not only in a purified abl assay but also in the bcr-abl+ K562 cells. In contrast to STI571 and like chlorambucil, it induced a dose-dependent increase in DNA damage in these cells. More importantly, AK04 was 12-32-fold more potent than chlorambucil in all bcr-abl+ cells of our cell panel. In the isogenic human megakaryocytic Mo7e and Mo7/bcr-abl cells, AK04 selectively killed the bcr-abl transfectants. Flow cytometry revealed that despite being a five-fold less potent inhibitor of bcr-abl than STI-571, it induced a significant dose-dependent increase in levels of cell death by apoptosis in KU812 cells 24 h post-treatment. Under these conditions, chlorambucil did not induce any significant level of apoptosis. These results suggest that AK04 is a nitrogen mustard with binary bcr-abl/DNA targeting effects, a property that may account for its superior potency when compared with the classical mustard chlorambucil.

Synthesis of half-mustard combi-molecules with fluorescence properties: correlation with EGFR status

The synthesis of 6-(2-chloroethylamino)-4-anilinoquinazolines ZR2002 and ZR2003 designed to block EGFR tyrosine kinase and to damage genomic DNA is described. These compounds present fluorescence properties that permitted the quantitation of their subcellular uptake by flow cytometry. Fluorescence intensities increased with increasing levels of EGFR in a panel of isogenic and established cell lines.

Engineering 3-alkyltriazenes to block bcr-abl kinase: a novel strategy for the therapy of advanced bcr-abl expressing leukemias

Recently, within the framework of a new strategy termed "combi-targeting," we designed ZRCM5 to contain a 2-phenylaminopyrimidopyridine moiety targeted to bcr-abl kinase and a triazene tail capable of generating a methyldiazonium species upon hydrolysis. The ability of ZRCM5 to block tyrosine kinase activity was tested in a short 10 min exposure ELISA involving isolated bcr-abl kinase and Western blotting assays. The results showed that: (a) ZRCM5 was hydrolyzed with a half-life of 27 min in cell culture media, (b) it blocked bcr-abl autophosphorylation in promyeloblastic leukemia K562 cells in a dose-dependent manner (IC(50)=14.01 microM) and (c) it induced dose-dependent levels of DNA strand breaks. In contrast, temozolomide (TEM), a clinical DNA damaging triazene capable of generating, like ZRCM5, a methyldiazonium species, could neither block bcr-abl tyrosine kinase activity in isolated enzyme nor in whole cell autophosphorylation assays. In cells expressing varied levels of bcr-abl, ZRCM5 was consistently more potent than TEM. The significant potency of ZRCM5 against the leukemia cells was attributed to its ability to simultaneously to block bcr-abl and related DNA repair activity while inducing significant DNA lesions in bcr-abl expressing leukemia cells. Further studies are ongoing to increase the affinity of ZRCM5 with the purpose of further enhancing its potency in bcr-abl expressing cells.

Multiple mechanisms of action of ZR2002 in human breast cancer cells: a novel combi-molecule designed to block signaling mediated by the ERB family of oncogenes and to damage genomic DNA

The mechanism of action of ZR2002, a chimeric amino quinazoline designed to possess mixed EGFR tyrosine kinase (TK) inhibitory and DNA targeting properties, was compared to those of ZR01, a reversible inhibitor of the same class and PD168393, a known irreversible inhibitor of EGFR. ZR2002 exhibited 4-fold stronger EGFR TK inhibitory activity than its structural homologue ZR01 but was approximately 3-fold less active than the 6-acrylamidoquinazoline PD168393. It preferentially blocked EGF and TGFalpha-induced cell growth over PDGF and serum. It also inhibited signal transduction in heregulin-stimulated breast tumour cells, indicating that it does not only block EGFR but also its closely related erbB2 gene product. In contrast to its structural homologues, ZR2002 was capable of inducing significant levels of DNA strand breaks in MDA-MB-468 cells after a short 2 hr drug exposure at a concentration as low as 10 microM. Reversibility studies using whole cell autophosphorylation and growth assays in human breast cell lines showed that in contrast to its reversible inhibitor counterpart ZR01, ZR2002 induced irreversible inhibition of EGF-stimulated autophosphorylation in MDA-MB-468 cells and irreversible inhibition of cell growth. Moreover despite possessing a weaker binding affinity than PD168393, it induced a significantly more sustained antiproliferative effect than the latter after a pulse 2 hr exposure. More importantly, in contrast to ZR01 and PD168393, ZR2002 was capable of inducing significant levels of cell death by apoptosis in MDA-MB-468 cells. The results in toto suggest that the superior antiproliferative potency of ZR2002 may be due to its ability to induce a protracted blockade of receptor tyrosine kinase-mediated signaling while damaging cellular DNA, a combination of events that may trigger cell-killing by apoptosis.

Sustained antiproliferative mechanisms by RB24, a targeted precursor of multiple inhibitors of epidermal growth factor receptor and a DNA alkylating agent in the A431 epidermal carcinoma of the vulva cell line

Recently, with the purpose of enhancing the potency of epidermal growth factor receptor (EGFR)-based therapies, we designed a novel strategy termed ‘Cascade-release targeting’ that seeks to develop molecules capable of degrading to multiple tyrosine kinase (TK) inhibitors and highly reactive electrophiles, in a stepwise fashion. Here we report on the first prototype of this model, RB24, a masked methyltriazene, that in addition to being an inhibitor on its own was designed to degrade to RB14, ZR08, RB10+a DNA alkylating methyldiazonium species. The cascade degradation of RB24 requires the generation of two reactive electrophiles: (a) an iminium ion and (b) a methyldiazonium ion. Thus, we surmise that these species could alkylate the active site of EGFR, thereby irreversibly blocking its action and that DNA damage could be induced by the methyldiazonium. Using the EGFR-overexpressing human epidermoid carcinoma of the vulva cell line, A431, we demonstrate herein that (a) RB24 and its derived species (e.g. RB14, ZR08) irreversibly inhibit EGFR autophosphorylation, (b) RB24 induced significant levels of DNA strand breaks, (c) sustained inhibition of EGFR by RB24 was associated with blockade of MAPK activation and c-fos gene expression, (d) RB24 induced irreversible cell growth inhibition with a 100-fold greater potency than Temodal™, a clinical methyltriazene. The pronounced growth inhibitory potency of RB24 was attributed to its ability to simultaneously damage DNA and irreversibly block EGFR TK activity.

The Combi-Targeting Concept: Dissection of the Binary Mechanism of Action of the Combi-Triazene SMA41 in Vitro and Antitumor Activity in Vivo

We have previously reported the synthesis of SMA41, a unimolecular combination of an epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor (TKI) of the quinazoline class and a DNA-damaging monomethyltriazene termed "combimolecule". Hydrolysis of 1-[4-(m-tolylamino)-6-quinazolinyl]-3-methyltriazene (SMA41) gives rise to an intact TKI [6-amino-4-(3-methylanilino)quinazoline; SMA52] capable of inhibiting epidermal growth factor (EGF)-induced EGFR autophosphorylation and a DNA-targeting methyldiazonium species. Herein, we showed that SMA41 blocked EGF-induced EGFR autophosphorylation by an irreversible mechanism, suggesting that it may covalently damage the receptor in these cells. More importantly, this was associated with significant inhibition of mitogen-activated protein kinase activation in A431 cells. In cells treated with [14C]SMA41, radio-high-performance liquid chromatography detection of both N7- and O6-methylguanine revealed an almost complete repair of the O6-methylguanine lesions and a greater tolerance of the N7-methylguanine adducts 24 h post-treatment. In contrast to temozolomide (a cyclic triazene used in the clinic) and the reversible inhibitor SMA52, SMA41 induced significant cell cycle arrest in S, G2, and M phases 24 h after a 2-h drug exposure. Furthermore, in vivo studies demonstrated that SMA41 was well tolerated. At 200 mg/kg, it showed approximately 2-fold greater antiproliferative activity than SMA52 in A431 cells implanted in immunocompromised SCID mice. These results suggest that the binary targeting properties of SMA41 are associated with a binary cascade of events in the cells that seem to culminate into significant growth inhibition in vitro and in vivo.

Inhibition of cell signaling by the combi-nitrosourea FD137 in the androgen independent DU145 prostate cancer cell line

BACKGROUND

FD137, a nitrosourea appended to a quinazoline ring, was designed to simultaneously block epidermal growth factor receptor (EGFR)-mediated signaling and damage genomic DNA in refractory EGF-dependent prostate tumors.

METHODS

The mixed inhibition of cell signaling and DNA damage by FD137 were determined by Western blotting, RT-PCR, flow cytometry, sulforhodamine B (SRB), and comet assay.

RESULTS

FD137 and its metabolite FD110 induced a dose-dependent increase in inhibition of EGF-stimulated EGFR autophosphorylation and this translated into blockade of c-fos gene expression in DU145 cells. FD137 induced significant levels of DNA damage and showed 150-fold greater anti-proliferative activity than BCNU, a classical nitrosourea. In contrast to BCNU, complete inhibition of EGF-induced cell transition to S-phase was observed at concentrations of FD137 as low as 3 microM.

CONCLUSION

FD137 could not only damage DNA, but also significantly block downstream EGFR-mediated signaling. The superior activity of FD137 may be imputable to the combined effect of its mixed EGFR/DNA targeting properties. This novel strategy may well represent a new approach to target nitrosoureas to EGFR-overexpressing carcinomas of the prostate.

The combi-targeting concept: intracellular fragmentation of the binary epidermal growth factor (EGFR)/DNA targeting “combi-triazene” SMA41

We have designed a novel tumor targeting strategy that consists of designing molecules termed "combi-molecules" or TZ-I to be masked forms of multiple antitumor agents. One such molecule SMA41, the TZ-I prototype, has been shown to target the epidermal growth factor receptor (EGFR) and to degrade under physiological conditions to give SMA52 (I) (an inhibitor of EGFR) and methyldiazonium (TZ) (a DNA alkylating species). While the antiproliferative advantages of this novel binary targeting strategy have now been demonstrated, the exact subcellular localization of the degradation products released from SMA41 remained elusive. Here we exploited the fluorescence properties of SMA52 to study its release from SMA41 and its subcellular distribution. Further, using 14C-labeled SMA41, we determined the distribution of the methydiazonium within subcellular macromolecules (DNA, RNA, protein). The results showed that SMA41 degraded to SMA52 in the carcinoma of the vulva cell line A431 with a half-life of 11min. The latter compound was primarily distributed in the perinuclear region. At equimolar concentrations, higher levels of SMA52 were observed when released from SM41 than when the cells were directly exposed to SMA52, indicating that the combi-molecular approach may offer a transport advantage to the released bioactive species. Radioactivity associated with SMA41 3-[ 14C]-methyl group was distributed throughout DNA, RNA, and protein, the latter macromolecule being the most alkylated. The results suggest that SMA41 (TZ-I) may diffuse into the cells, break down into two species: SMA52 (I) concentrated in the perinuclear region and methyldiazonium (TZ) that diffuses in all intracellular organelles and unspecifically alkylates RNA, protein, and nuclear DNA.

Synthesis of a Prodrug Designed To Release Multiple Inhibitors of the Epidermal Growth Factor Receptor Tyrosine Kinase and an Alkylating Agent: A Novel Tumor Targeting Concept

The synthesis of a novel acetoxymethyltriazene designed to be a prodrug of multiple inhibitors of the epidermal growth factor receptor (EGFR) and a methyldiazonium species is described. Studies with each of the expected metabolites demonstrated significant EGFR tyrosine kinase inhibitory activities and the released methyldiazonium was trapped with p-nitrobenzylpyridine. Their ability to damage genomic DNA in whole cells was demonstrated by using the single cell microelectrophoresis (comet) assay. The results suggest that this approach may well represent a novel drug combination strategy involving single molecules masking multiple bioactive agents.

Synthesis of pyrimidinopyridine-triazene conjugates targeted to abl tyrosine kinase

The synthesis and abl tyrosine kinase inhibitory activities of alkyltriazenes conjugated to phenylaminopyrimidines are described. Significant abl inhibitory activities were observed only when a benzamido spacer was inserted between the 1,2,3-triazene chain and the 2-phenyaminopyridopyrimidine moiety.

The Combi-Targeting Concept: Chemical Dissection of the Dual Targeting Properties of a Series of “Combi-Triazenes”

The combi-targeting concept postulates that a molecule termed a "combi-molecule" designed to interact with an oncoreceptor on its own and allowed to further degrade to another more stable inhibitor of the latter receptor + a DNA-damaging species should be more potent than the individual combination of the same inhibitor with a DNA-damaging agent in cells expressing the targeted receptor. Recently, using the epidermal growth factor receptor (EGFR) as a target, we demonstrated the feasibility of combi-molecules with dual EGFR/DNA-targeting properties and with the ability to degrade to another potent inhibitor of EGFR. However, despite a clear demonstration of their superior potency when compared with classical combinations in EGFR-expressing cells, the true contribution of each fragment of the combi-molecules to their overall antiproliferative activity remained elusive. Here, we report a structure-function approach whereby a series of quinazoline-based "combi-triazenes" were altered to either abrogate the affinity of the EGFR-targeting quinazoline head or to suppress the DNA-damaging property of the triazene tail. The results showed that (a) inactivation of the quinazoline head by appending an N-methylaniline group to its 4-position reduced EGFR tyrosine kinase (TK) inhibitory activity by ca. 200-fold and decreased the ability of the combi-molecule to block serum-induced growth stimulation in c-erbB2 transfected NIH3T3 cells by ca. 10-fold, (b) abrogation of the alkylating activity or the DNA-damaging potential of the triazene tail by forming 3,3-dimethyltriazenes did not suppress EGFR TK inhibitory affinity but decreased the antiproliferative activity in basal growth assays, and (c) the antiproliferative activities of the monoalkyltriazenes that possessed binary EGFR TK inhibitory and alkylating activities were superior to those of their monotargeted counterparts. The results in toto suggest that each component of the dual targeting property of combi-triazenes plays a critical role in their overall antiproliferative activity.