Chemoprotectants for cancer chemotherapy

Maximal dosing of cytotoxic chemotherapy drugs is often limited by the development of severe nonmyelosuppressive toxicities. Numerous studies have demonstrated that sulfur-containing nucleophiles can antagonize the dose-limiting effects of alkylating agents on the genitourinary tract. Examples include the use of sodium thiosulfate to prevent cisplatin-induced renal tubular necrosis and the use of sulfhydryl-containing compounds like N-acetylcysteine and 2-mercaptoethanesulfonate (mesna) to block oxazophosphorine-induced bladder toxicity. Mesna does not block the antitumor action of oxazophosphorines due to its rapid formation of the inactive dimer dimesna in the bloodstream. The active monomer is selectively reduced from dimesna in renal tubule cells, thereby limiting the inactivation of toxins like acrolein to the genitourinary tract. Recent clinical trials suggest that oral mesna has adequate bioavailability (roughly 50% by urinary thiol measurements) to prevent urotoxicity in high-dose ifosfamide regimens. In addition, mesna is stable in aqueous oral formulations. This may facilitate more convenient oral mesna dosing in protocols using high-dose cyclophosphamide or ifosfamide. Whereas agents like mesna and sodium thiosulfate complex directly with activated (electrophilic) alkylator species, chemoprotectants for the anthracyclines appear to complex with metal cofactors like iron, which are required for the production of cardiotoxicity. Several ethylenediaminetetraacetic-like agents have been evaluated, and a water-soluble piperazinyl derivative, ICRF-187, is currently undergoing clinical evaluation in patients receiving large cumulative doxorubicin doses. An initial clinical trial suggests that ICRF-187 can prevent doxorubicin-induced cardiomyopathy. As with mesna, ICRF-187 does not block the myelosuppressive or the antitumor effects of doxorubicin. Overall, these studies show that site-selective chemoprotection is now feasible for at least two major classes of anticancer agents.