Cellular and molecular mechanisms of resistance to antifolate drugs: new analogues and approaches to overcome the resistance

A number of antifolate drugs, which inhibit the key enzymes in the 'thymidylate cycle', dihydrofolate reductase (DHFR) and thymidylate synthase (TS), have been developed as part of the search for analogues with superior antitumor efficacy to a 'classical' antifolate, methotrexate (MTX), and those which are active against the MTX-resistant tumor cells. Recent development of newer classes of antifolate drugs is based on the extensive understanding of the relationship between chemical structures and biological properties and of analogue interactions with target enzymes, transport proteins and folate metabolizing enzyme, folylpolyglutamate synthetase (FPGS). Tumor cells may develop resistance to an antifolate drug by virtue of, (1) amplified activity in its target enzyme, (2) impaired function of drug transport protein, e.g. reduced folate carrier (RFC), (3) induction of mutated target enzyme with low affinity for antifolate(s), and (4) defective polyglutamation of drug(s) in the cells. Recent studies have elucidated in part the molecular events involved in the resistance to antifolates. These include amplification and/or mutation of the gene encoding a target enzyme, reduced or altered gene expression of the RFC, and mutated expression of the FPGS gene. To overcome or circumvent the resistance mechanisms, new antifolates with diverse structures and different biological properties have been designed and developed for clinical use. Trimetrexate (TMQ), a lipophilic DHFR inhibitor which is not a substrate for RFC and FPGS, could overcome the MTX-resistance through impaired RFC and diminished polyglutamation, and partially through amplified DHFR. Selective inhibitors of TS with a folate structure such as raltitrexed could circumvent the resistance by virtue of DHFR overproduction, and this class of compounds which have higher substrate activities for FPGS than MTX may be of value for the treatment of myeloid leukemias in addition to lymphocytic malignancies resistant to conventional chemotherapy. Several strategies to overcome antifolate resistance by using gene therapy are currently under investigation. (C) 1997 Elsevier Science Ireland Ltd.