TUMOR SELECTIVE INACTIVATION OF THE REPAIR PROTEIN AGT

几种具有抗肿瘤活性的烷化剂前药靶向于 DNA.这些包括氯乙基化剂Cloretazine、1，2-双（甲基磺酰基）-1-（2-氯乙基）-2-[[（1-甲基磺酰基）-2-（2-甲基磺酰基）氨基]乙基]-3-甲基-苯并[d]咪唑。| （4-硝基苯基）乙氧基]羰基]肼（KS 119）和KS 119 W（KS 119的水溶性形式）， 卡莫司汀（BCNU）和洛莫司汀（CCNU）以及甲基化剂替莫唑胺（TMZ），甲基苄肼， 达卡巴嗪（DTIC）和链脲佐菌素。氯乙基化剂是最有效的，因为 DNA中鸟嘌呤的O-6位的烷基化导致形成1-^-脱氧胞苷基0 ^A/1- 在氯乙基化药物中，Cloretazine和KS 119是通过脱氧鸟苷基）乙烷（G-C）DNA交联， 大边缘最特异的为鸟嘌呤的O-6位。所有的氯乙基化和甲基化 试剂对修复蛋白O 6-烷基鸟嘌呤-DNA烷基转移酶（AGT）敏感， 将烷基和甲基基团从鸟嘌呤的0 -6位化学计量地转移至半胱氨酸145。 通过将鸟嘌呤O-6加合物从DNA螺旋中翻转到AGT中的结合口袋中， 分子。AGT的烷基化形式被蛋白酶体系统迅速降解，DNA被 恢复到其天然状态;该作用代表了肿瘤和宿主组织抗性的主要机制 氯雷他嗪、KS 119、BCNU和CCNU。O 6-苄基鸟嘌呤（O 6-BG）是已知的最有效的 AGT的抑制剂;该试剂与AGT反应在蛋白质的活性位点形成S-苄基半胱氨酸， 消耗AGT并增加肿瘤和宿主细胞对氯乙基化和 甲基化DNA中鸟嘌呤的O-6位置。已证明O 6-BG的相对无毒剂量 细胞系统和患者以耗尽肿瘤的AGT含量。这种作用使细胞系统敏感， 然而，由于AGT水平也被正常组织中的O 6-BG耗尽，因此80%的AGT水平被BCNU耗尽。 需要减少BCNU的剂量，导致BCNU的无效治疗剂量水平。 这些发现意味着相对于正常组织，选择性地消耗肿瘤组织中的AGT的方法， 组织需要避免AGT诱导的肿瘤对鸟嘌呤O-6烷化剂的抗性。到 为了实现这一点，我们建议使用实体瘤中存在的低氧肿瘤细胞部分，这是一个主要的 肿瘤易感性位点，以选择性地激活前药，从而产生AGT的有效抑制剂。主 总体目标是选择一种前药用于最终的临床开发，以与O-联用 6鸟嘌呤氯乙基化和甲基化试剂。所选的类似物必须选择性地耗尽AGT，或 优选地在实体瘤中，从而允许以接近完全治疗的连续组合使用。 使用的烷基化剂剂量不会增强骨髓抑制或对其他正常人的毒性 组织.

Several alkylating agent prodrugs with antitumor activity target the O-6 position of guanine residues in DNA. These include the chloroethylating agents Cloretazine, 1,2-bis(methylsulfonyl)-1-(2-chloroethyl)-2-[[(1-| (4-nitrophenyl)ethoxy]carbonyl]hydrazine (KS119) and KS119W (the water-soluble form of KS119), carmustine (BCNU) and lomustine (CCNU) and the methylating agents temozolomide (TMZ), procarbazine, dacarbazine (DTIC) and streptozocin. The chloroethylating agents are the most potent because the alkylation of the O-6 position of guanine in DNA leads to the formation of a l-^-deoxycytidinyO^A/1- deoxyguanosyl)ethane (G-C) DNA cross-link, Of the chloroethylating drugs, Cloretazine and KS119 are by a large margin the most specific for the O-6 position of guanine. All of the chloroethylating and methylating agents are susceptible to the repair protein O6-alkylguanine-DNA alkyltransferase (AGT) which stoichiometrically transfers alkyl and methyl groups from the O-6 position of guanine to cysteine 145 of the AGT molecule by flipping the guanine O-6 adduct out of the DNA helix into a binding pocket in the AGT molecule. The alkylated form of AGT is rapidly degraded by the proteasomal system and the DNA is restored to'its native state; this action represents the primary mechanism of tumor and host tissue resistance to Cloretazine, KS119, BCNU and CCNU. O6-Benzylguanine (O6-BG) is among the most potent known inhibitors of AGT; this agent reacts with AGT to form S-benzylcysteine in the active site of the protein, depleting AGT and increasing the sensitivity of both tumor and host cells to agents that chloroethylate and methylate the O-6 position of guanine in DNA. Relatively non-toxic doses of O6-BG have been shown in both cell systems and patients to deplete the AGT content of tumors. This action sensitizes cell systems and tumors in vivo to BCNU; however, since AGT levels are also depleted by O6-BG in normal tissues, an 80% reduction in the dosage of BCNU is required, leading to an ineffective therapeutic dosage level of BCNU. These findings imply that methodology that selectively depletes AGT in tumor tissue relative to normal tissues is required to circumvent AGT induced tumor resistance to guanine O-6 alkylating agents. To accomplish this we propose to use the hypoxic tumor cell fraction present in solid tumors, which is a major site of tumor vulnerability, to selectively activate prodrugs to generate potent inhibitors of AGT. The primary overall objective is the selection of a prodrug for eventual clinical development to use in combination with O- 6 guanine chloroethylating and methylating agents. The analog selected must deplete AGT selectively or preferentially in solid tumors, thereby permitting usage in sequential combination of close to full therapeutic dosage of the alkylating agent employed without enhanced myelosuppression or toxicity to other normal tissue.