In situ methotrexate polyglutamate formation in rat tissues.

Rats given a single i.p. injection of 10 mg/kg of methotrexate rapidly accumulate drug in several tissues, including liver, kidneys, testes, brain and red cells. Using a tissue extraction procedure which minimizes the breakdown of methotrexate polyglutamate derivatives and molecular sieve high-performance liquid chromatography, we examined the formation of methotrexate polyglutamates in these tissues throughout a period of 1 week after a single dose of drug. Higher polyglutamates (n greater than 3) were found in the brain, liver, kidneys and testes as early as 3 hr. The percentage of methotrexate polyglutamates increases slowly in the liver, kidney, testis and brain during the course of the week, concomitant with a decline of unconjugated tissue methotrexate and a low plasma concentration of drug. This is in contrast to the blood cells, which have little polyglutamate formation and a rapid half-life for methotrexate. The consistently high levels of methotrexate polyglutamate levels found in the liver, kidneys, testes and brain are in agreement with data showing little or no disappearance of methotrexate from these tissues for up to 26 days.