Analysis of 'early' thymidine/inosine protection as an adjunct to methotrexate therapy

The feasibility of 'early' thymidine and inosine protection of methotrexate (MTX) toxicity is evaluated in this paper. This approach is based on the proposition that the most vulnerable period for susceptible host cells to MTX toxicity is an interval following a pulse of MTX when the MTX monoglutamate level is high. In contrast, tumor cells that accumulate active MTX polyglutamyl derivatives are exposed to the cytotoxic effects of MTX, not only when the monoglutamate levels are high, but also over the much longer interval during which polyglutamyl derivatives are retained within these cells. The protecting agents employed, thymidine and inosine, circumvent the antipurine and antipyrimidine effects of MTX, but neither agent inhibits the transport or the polyglutamylation of MTX. Nucleoside protection given over 6 hours after MTX markedly decreased toxicity to normal BDF₁ mice at MTX doses up to 150 mg/kg/day for 3 consecutive days. The LD10 for unprotected mice was 14 mg/kg/day, while the LD10 for the protected mice was 114 mg/kg/day, a greater than eightford increase in the drug dose delivered. MTX with early nucleoside protection produced a 50% increase in median life span in tumor-bearing mice at doses of drug that were toxic to unprotected mice. Flow cytometric analyses indicated that consecutive daily pulses of MTX with early nucleoside protection alter the cell cycle distribution. By 24 hours after the last of three daily pulses of MTX, the G₁ component was maximal (80% of the cell population), with <2% of the cells in G₂M and 17% of the cells in S phase. This distribution persisted for 24 hours, after which a normal pattern emerged, a process that was accelerated by neither thymidine/inosine nor leucovorin. These studies support the concept that toxicity to the host is initiated largely in the intervals shortly after MTX administration. The data indicate that early nucleoside protection permits the repetitive administration of much higher doses of MTX than can be given with MTX alone. This approach may be useful not only in the treatment of human neoplasms but in the treatment of other disorders, such as rheumatoid arthritis, psoriatic arthritis, or psoriasis. Finally, this paper considers the potential advantages of early thymidine/inosine protection in comparison to leucovorin rescue with the administration of low or moderate doses of MTX.