[Clinical pharmacology of anticancer agents [Part 5] Antimetabolites (2)]

III Purine antagonists Biosynthesis of guanine nucleotides has been reported to be up regulated in tumor cells. In guanine nucleotide synthesis, there are 2 rate-limiting enzymes, i.e. inosine monophosphate dehydrogenase for de novo synthesis and hypoxanthine guanine phosphoribosyltransferase for the salvage pathway. Therefore, agents acting on these 2 enzymes to inhibit guanine nucleotide synthesis could be expected to have a superior effect on tumor proliferation. The main antitumor agents belonging to this class are thiopurines [including 6-mercaptopurine (6 MP), 6-thioguanine (6 TG) and 6-thioinosine (TIR)], thiazofurin (TZF), and arabinofuranosylfluoroadenine (F-ara-A). In the activation of 6MP to its ribotide. PRPP is the rate limiting factor. After the ribotide is produced, it is metabolized to another active form by enzymes catalyzing purine nucleotide metabolism. The antitumor effect of TZF is enhanced by the combination of TZF with allopurinol, which increases the plasma hypoxanthine level and subsequently inhibits recovery of the reduced guanine nucleotide pool by TZF. F-ara-A induces DNA strand damage as well as inhibiting DNA synthesis and is expected to have a significant antitumor effect on slowly growing tumors. These agents are mainly effective for treating hematological malignancies. IV Antifolic agents Among the antifolates, methotrexate (MTX) is the most useful drug for both hematologic malignancies and solid tumors. MTX primarily inhibits one-carbon transfer through the inhibition of dihydrofolate reductase and thus blocks the biosynthesis of both purine and pyrimidine nucleotides. Formyl polyglutamate synthetase catalyzes folate to its polyglutamate, both the active and retention forms. It is also important as an activating enzyme as well as being a target of MTX. MTX directly inhibits thymidylate synthetase, which could be the main target during high-dose therapy. High-dose MTX therapy with leucovorin (LV) rescue is effective even for tumors which are resistant to conventional treatment. During clinical use, not only MTX levels but also those of its inactive metabolites [7-hydroxy-MTX and 2, 4-diamino-N10-methylpteroic acid(DAMPA)] should be monitored. High-dose MTX therapy with LV rescue requires precise monitoring and LV rescue should be continued until the MTX level falls below 5 x 10(-8) M. MTX is also known as the safest drug which can be directly administered to into the central nervous system. Many other antifolates are under development, among which trimetrexate might be the most promising. Studies on antimetabolites have developed side by side with research on nucleotide tumor cell metabolism, which has produced a number of the antitumor agents now available for cancer chemotherapy.(ABSTRACT TRUNCATED AT 400 WORDS)