Thymidylate Synthase Inhibitors

Folate analogues that inhibit thymidylate synthase (TS) selectively were developed based on TS and folate molecular structures and properties. The structure-activity relationship, preclinical and clinical development, and issues of potential importance in the future success of these TS inhibitors are reviewed herein. Properties of these new compounds depend mainly on the use of the reduced folate carrier (RFC) proteins for cellular entry and on intracellular polyglutamation, which augments TS inhibitory function and cellular retention. CB3717 [Zeneca (formerly ICI Pharmaceuticals), Macclesfield, United Kingdom], the first selective TS inhibitor developed, demonstrated antineoplastic activity in Phase I trials, but its development was abandoned due to nephrotoxicity. ZD1694 (Tomudex; Zeneca), a watersoluble, nonnephrotoxic quinazoline, demonstrated activity in colorectal, breast, and pancreatic cancer. Phase III trials of Tomudex in advanced colorectal cancer were completed recently. LY231514 (Eli Lilly Research Labs, Indianapolis, IN) uses the RFC and polyglutamation to exert its selective TS inhibitory action. Phase I trials of this compound have been completed. ZD9331 (Zeneca), currently in preclinical studies, was designed to obviate the use of the RFC for cellular entry. 1843U89 (Glaxo-Wellcome, Research Triangle Park, NC), currently in Phase I trial, does not require the RFC; polyglutamation of this potent TS inhibitor leads to its higher cellular retention without augmenting its TS inhibitory activity. Currently in clinical trials, AG337 (p.o. and i.v. forms) and AG331 (both by Agouron, La Jolla, CA) are lipophilic potent TS inhibitors with action independent of the RFC and polyglutamation. Multiple mechanisms through which cells may overcome TS inhibition have been described. Combining TS inhibitors with other agents that affect TS, interfere with TS gene and mRNA regulation, or inhibit salvage mechanisms of thymidylate depletion may potentially enable greater clinical utility of this class of compounds. Introduction TS 2 inhibitors are a new class of compounds that target TS, an enzyme in the folate metabolic pathways necessary for thymidylate synthesis, specifically. By inhibiting TS, these agents decrease de novo thymidylate synthesis, which is necessary for DNA synthesis and repair. Contrary to prior empiric approaches to anticancer drug discovery, the development of new TS inhibitors was based on the targeted and specific design of new molecules and relied on extensive knowledge of the structure, conformation, and properties of TS and the mode of action of folate analogues. These compounds are being tested currently in preclinical and clinical settings, and their role in clinical practice as single agents or in combination regimens remains to be defined. This review will provide a background of the mechanism of TS inhibition, the development and current status of folate analogues that inhibit TS selectively, the mechanisms of resistance to TS inhibition, and the current approaches to augment the efficacy of these new compounds.