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

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.

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.

NBGNU: a hypoxia-activated tripartite combi-nitrosourea prodrug overcoming AGT-mediated chemoresistance

Aim: A hypoxia-activated combi-nitrosourea prodrug, N-(2-chloroethyl)-N'-2-(2-(4-nitrobenzylcarbamate)-O ⁶-benzyl-9-guanine)ethyl-N-nitrosourea (NBGNU), was synthesized and evaluated for its hypoxic selectivity and anticancer activity in vitro. Results: The prodrug was designed as a tripartite molecule consisting of a chloroethylnitrosourea pharmacophore to induce DNA interstrand crosslinks (ICLs) and an O ⁶-benzylguanine analog moiety masked by a 4-nitrobenzylcarbamate group to induce hypoxia-activated inhibition of O ⁶-alkylguanine-DNA alkyltransferase. NBGNU was tested for hypoxic selectivity, cytotoxicity and DNA ICLs ability. The reduction product amounts, cell death rates and DNA ICL levels induced by NBGNU under hypoxic conditions were all significantly higher than those induced by NBGNU under normoxic conditions. Conclusion: The tripartite combi-nitrosourea prodrug exhibits desirable tumor-hypoxia targeting ability and abolished chemoresistance compared with the conventional chloroethylnitrosoureas.

Molecular analysis of the dual targeting of the epidermal growth factor receptor and the O6-methylguanine-DNA methyltransferase with a double arm hybrid molecule

Disordered expression of the epidermal growth factor receptor (EGFR) has been associated with induction of DNA repair genes (e.g. XRCC1, ERCC1) and resistance to radiation and genotoxic drugs. However, our previous work showed that EGFR inhibition did not affect O⁶-methylguanine-DNA methyltransferase (MGMT)-mediated resistance. In order to block uncoupled events associated with EGFR and MGMT, we designed MR30, a single molecule termed “combi-molecule” that contains a quinazoline arm targeted to EGFR and an O⁶-benzylguanine (O⁶-BG) moiety to block MGMT. Molecular analysis of the mechanism of action of its two arms showed that: (a) it could block EGFR phosphorylation, (b) down-regulate the RAF-MAPK and the PI3K-AKT pathways, and (c) covalently modify MGMT through S-benzylation, as confirmed by MALDI analysis of a direct binding assay with isolated MGMT, (d) it induced a dose-dependent down-regulation of MGMT in lung and melanoma cells. The pleiotropic mechanism of action of MR30 culminated into strong growth inhibition (IC₅₀: 0.018-6.02 μM), with superior activity when compared with an equimolar combination of gefitinib (a clinical EGFR inhibitor) and O⁶-BG (a known MGMT inhibitor). Pulse exposure experiments were required to attenuate the contribution of EGFR inhibition to the strong potency of MR30, thereby allowing to achieve the dose level required to sensitize cells to temozolomide (TMZ). Indeed, MR30 significantly sensitized EGFR-MGMT co-expressing cells to TMZ (p<0.05-0.0001). The results in toto suggest that MR30 is the first prototype of agents that may be used against tumours addicted to EGFR and to sensitize resistant tumours co-expressing EGFR and MGMT to TMZ.

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.

Novel 4-arylaminoquinazoline derivatives with (E)-propen-1-yl moiety as potent EGFR inhibitors with enhanced antiproliferative activities against tumor cells

A series of novel 4-anilinoquinazoline derivatives with (E)-propen-1-yl moiety were designed, synthesized and evaluated for biological activities in vitro. Most compounds exhibited highly antiproliferative activities against all tested tumor cell lines including A431, A549, NCI-H1975 and SW480 cells. Especially, compound 6e not only presented strong antiproliferative activities against the tested four tumor cell lines (IC₅₀ of 1.35, 8.83, 5.53 and 6.08 μM, respectively) which expressed wild type or L858R/T790M double mutant epidermal growth factor receptor (EGFR), but also showed potent inhibitory activity against wild type EGFR (IC₅₀ = 20.72 nM). The result of molecular docking with EGFR suggested the binding mode of 6e was similar to gefitinib, but different from lapatinib. Additionally, western blot analysis showed that 6e inhibited the phosphorylation of EGFR and its downstream signaling proteins in lung cancer cells. The work could be very useful starting point for developing a new series of tyrosine kinase inhibitors targeting EGFR.

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.

Synthesis and Antitumor Activity Evaluation of a Novel Combi-nitrosourea Prodrug: BGCNU

Chloroethylnitrosoureas (CENUs) are an important type of alkylating agent employed in the clinical treatment of cancer. However, the anticancer efficacy of CENUs is greatly decreased by a DNA repairing enzyme, O⁶-alkylguanine-DNA alkyltransferase (AGT), by preventing the formation of interstrand cross-links (ICLs). In this study, a combi-nitrosourea prodrug, namely, N-(2-chloroethyl)-N'-2-(O⁶-benzyl-9-guanine)ethyl-N-nitrosourea (BGCNU), which possesses an O⁶-benzylguanine (O⁶-BG) derivative and CENU pharmacophores simultaneously, was synthesized and evaluated for its ability to induce ICLs. The target compound is markedly more cytotoxic in human glioma cells than the clinically used CENU chemotherapies ACNU, BCNU, and their respective combinations with O⁶-BG. In the AGT-proficient cells, significantly higher levels of DNA ICLs were observed in the groups treated by BGCNU than those by ACNU and BCNU, which indicated that the activity of AGT was effectively inhibited by the O⁶-BG derivatives released from BGCNU.

Synthesis and antitumor activity evaluation of a novel combi-nitrosourea prodrug: Designed to release a DNA cross-linking agent and an inhibitor of O(6)-alkylguanine-DNA alkyltransferase

The drug resistance of CENUs induced by O(6)-alkylguanine-DNA alkyltransferase (AGT), which repairs the O(6)-alkylated guanine and subsequently inhibits the formation of dG-dC cross-links, hinders the application of CENU chemotherapies. Therefore, the discovery of CENU analogs with AGT inhibiting activity is a promising approach leading to novel CENU chemotherapies with high therapeutic index. In this study, a new combi-nitrosourea prodrug 3-(3-(((2-amino-9H-purin-6-yl)oxy)methyl)benzyl)-1-(2-chloroethyl)-1-nitrosourea (6), designed to release a DNA cross-linking agent and an inhibitor of AGT, was synthesized and evaluated for its antitumor activity and ability to induce DNA interstrand cross-links (ICLs). The results indicated that 6 exhibited higher cytotoxicity against mer(+) glioma cells compared with ACNU, BCNU, and their respective combinations with O(6)-benzylguanine (O(6)-BG). Quantifications of dG-dC cross-links induced by 6 were performed using HPLC-ESI-MS/MS. Higher levels of dG-dC cross-link were observed in 6-treated human glioma SF763 cells (mer(+)), whereas lower levels of dG-dC cross-link were observed in 6-treated calf thymus DNA, when compared with the groups treated with BCNU and ACNU. The results suggested that the superiority of 6 might result from the AGT inhibitory moiety, which specifically functions in cells with AGT activity. Molecular docking studies indicated that five hydrogen bonds were formed between the O(6)-BG analogs released from 6 and the five residues in the active pocket of AGT, which provided a reasonable explanation for the higher AGT-inhibitory activity of 6 than O(6)-BG.

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.

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.

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.

Type II combi-molecules: design and binary targeting properties of the novel triazolinium-containing molecules JDD36 and JDE05

We recently designed molecules termed "type II combi-molecules" to block the epidermal growth factor receptor and to damage DNA without the requirement for hydrolytic cleavage. Here, we studied two such combi-molecules (JDD36 and JDE05), containing a novel quinazoline-linked chloroethyltriazolinium system. The epidermal growth factor receptor-targeting potential of these novel structures was studied by ELISA for isolated epidermal growth factor receptor and by Western blotting for whole-cell assays. DNA damage was analyzed using the single-cell microelectrophoresis comet assay. Antiproliferative effects were determined by the sulforhodamine B assay. JDD36 showed an IC50 of 0.6 micromol/l in the ELISA for epidermal growth factor receptor tyrosine kinase, a dose-dependent inhibition of epidermal growth factor receptor phosphorylation and significant levels of DNA damage in the human DU145 prostate cancer cell line. JDD36 was an overall 2- to 15-fold stronger antiproliferative agent than JDE05 that showed potent epidermal growth factor receptor inhibitory activity (IC50 epidermal growth factor receptor, 0.035 micromol/l) but weak DNA-damaging potential. In a panel of LNCaP erbB transfectants, in contrast to JDE05, JDD36 showed remarkable and selective potency against the LNCaPerbB2 transfectant. The results in toto suggest that the overall superior potency of JDD36 when compared with JDE05 may be imputed to its balanced binary epidermal growth factor receptor-DNA-targeting properties that may induce a tandem blockade of epidermal growth factor receptor-mediated mitogenic signaling while depleting alternative survival mechanism by damaging DNA.

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.

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.

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.