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

Molecular Analysis of the Superior Efficacy of a Dual Epidermal Growth Factor Receptor (EGFR)-DNA-Targeting Combi-Molecule in Comparison with Its Putative Prodrugs 6-Mono-Alkylamino- and 6,6-Dialkylaminoquinazoline in a Human Osteosarcoma Xenograft Model

Background: ZR2002 is a dual EGFR-DNA-targeting combi-molecule that carries a chloroethyl group at the six-position of the quinazoline ring designed to alkylate DNA. Despite its good pharmacokinetics, ZR2002 is metabolized in vivo into dechlorinated metabolites, losing the DNA-alkylating function required to damage DNA. To increase the DNA damage activity in tumor cells in vivo, we compared ZR2002 with two of its 6-N,N-disubstituted analogs: "JS61", with a nitrogen mustard function at the six-position of the quinazoline ring, and "JS84", with an N-methyl group. Methods: Tumor xenografts were performed with the human Saos-2 osteosarcoma cell line expressing EGFR. Mice were treated with ZR2002, JS84 or JS61, and the tumor burden was measured with a caliper and CT/PET imaging. Drug metabolism was analyzed with LC-MS. EGFR and ɣ-H2AX phosphorylation were quantified via Western blot analysis and immunohistochemistry. Results: In vivo analysis showed that significant tumor growth inhibition was only achieved when ZR2002 was administered in its naked form. The metabolic dealkylation of JS61 and JS84 did not release sufficient concentrations of ZR2002 for the intratumoral inhibition of P-EGFR or enhanced levels of P-H2AX. Conclusions: The results in toto suggest that intratumoral concentrations of intact ZR2002 are correlated with the highest inhibition of P-EGFR and induction of DNA damage in vivo. ZR2002 may well represent a good drug candidate for the treatment of EGFR-expressing osteosarcoma.

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.

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".

Positional isomerization of a non-cleavable combi-molecule dramatically altered tumor cell response profile

To potentiate the quinazoline-based inhibitor of the epidermal growth factor receptor (EGFR), a chloroethyl alkylating moiety was appended to its 6-position. This led to molecules with extremely strong EGFR inhibitory potency and anomalously strong DNA-damaging potential. To assess the role of the chloroethyl group on potency, we designed a molecule in which it is shifted to the 7-position where it would be less reactive and away from the cys773 of the EGFR ATP site. The results showed that (i) ZR2009 was 10-fold less potent than its positional isomer ZR2003 in EGFR tyrosine kinase inhibition, (ii) it consistently exhibited significantly weaker antiproliferative potency than ZR2003, (iii) in reversibility assays, while ZR2003 induced sustained inhibition of EGFR phosphorylation, ZR2009 inhibitory activity was partially reversed, and (iv) likewise, ZR2009 significantly lost its activity in short exposure growth inhibitory assays and induced lower levels of DNA damage than ZR2003. Molecular modeling suggested that while the chloroethylamino group in ZR2003 was at 3.5 Å away from Cys773, that of ZR2009 was at 6.3 Å. The results in toto suggest that, while the chloroethyl is a strong alkylating group, its appendage to the 6-position is optimal for DNA damage, sustained EGFR, and growth inhibition.

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.

"Combi-targeting" mitozolomide: conferring novel signaling inhibitory properties to an abandoned DNA alkylating agent in the treatment of advanced prostate cancer

PURPOSE

At the preclinical stage, mitozolomide (MTZ) showed exciting preclinical activity but failed later in clinical trial due to toxic side effects. We surmised that by targeting MTZ to epidermal growth factor receptor (EGFR), we may not only alter its toxicity profile, but also enhance its potency in EGFR-overexpressing tumors. To test this hypothesis, we designed JDF12, studied its mechanism of action in human prostate cancer (PCa) cells and determined its potency in vivo.

EXPERIMENTAL DESIGN

To analyze its mixed EGFR-DNA targeting potential, we performed an enzyme linked immunosorbent assay (ELISA) and western blotting analysis of EGFR phosphorylation in cells stimulated with EGF. DNA damage was analyzed using the comet assay, and apoptosis quantitated by annexin V binding assay. Growth inhibition in vitro was determined by the sulforhodamine B (SRB) assay and in vivo efficacy analyzed in male CD-1 nude mice.

RESULTS

The results showed that: Under physiological conditions, JDF12 was hydrolyzed to JDF04R and both agents were capable of inhibiting isolated EGFR tyrosine kinase (TK) and EGFR phosphorylation in EGF-stimulated cells. JDF12 significantly damaged DNA, induced apoptosis in DU145 cells and was up to 2-10-fold more potent than equieffective combinations of MTZ and JDF04R or Iressa in a panel that also included LNCaP and its EGFR and ErbB2 transfectants. In vivo, it induced significant antitumor activity in a DU145 xenograft model.

CONCLUSIONS

The results suggest that the superior cytotoxicity of JDF12 when compared with MTZ and JDF04R may be imputed to its potent EGFR-DNA targeting properties and confirm the ability of this novel strategy to confer EGFR targeting properties to a classical alkylator.

Synthesis and studies on three-compartment flavone-containing combi-molecules designed to target EGFR, DNA, and MEK

In order to induce a tandem targeting of EGFR, DNA, and MEK, we built complex combi-molecules containing an EGFR targeting quinazoline and an aminoethyltriazene moiety linking the entire molecule to PD98059. Two complex molecules were synthesized: one with a short aminoethyl spacer, AL232, and the other AL414 with a longer aminoethylaminoethyl spacer. AL414 was a more potent inhibitor of EGFR tyrosine kinase than AL232. Both combi-molecules blocked EGFR phosphorylation in whole cells and downregulated extracellular signaling-regulated kinases (ERK1,2). However, only AL414 was capable of inducing DNA damage. Thus, it was taken in vivo for metabolic analysis. The results showed that 3 h after injection, AL414 was hydrolyzed to an EGFR inhibitor FD105, which was further acetylated to FD105Ac, a more potent inhibitor of EGFR. The detected flavone derivative was PD98059 linked to the hydroxyalkyl moiety resulting from the decomposition of the alkyldiazonium species. Independent synthesis of the latter metabolite and further in vitro analysis showed that it was deprived of antiproliferative activity. The results in toto suggest that while AL414 is a three-compartment combi-molecule, only the EGFR and DNA targeting species can be released and the cleavage to the intact MEK inhibitor PD98059 was mitigated by the stability of the carbamate.