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.
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