Multiple mechanisms of resistance to polyglutamatable and lipophilic antifolates in mammalian cells: Role of increased folylpolyglutamylation, expanded folate pools, and intralysosomal drug sequestration

Chinese hamster ovary pyr(R100) cells display more than 1000-fold resistanc e to pyrimethamine (Pyr), a lipophilic antifolate inhibitor of dihydrofolat e reductase. Pyr(R100) cells had wild-type DHFR activity, lost folate expor ter activity, and had a 4-fold increased activity of a low pH folic acid tr ansporter. Here we report on the marked alterations identified in Pyr(R100) cells compared with parental cells: 1) similar to 100-fold decreased folic acid growth requirement; 2) a 25-fold higher glucose growth requirement in Pyr-containing medium; 3) a 2.5- to 4.1-fold increase in folylpolyglutamat e synthetase activity; 4) a 3-fold increase in the accumulation of [H-3]fol ic acid and a 3-fold expansion of the intracellular folate pools; 5) a 4-fo ld increase in the activity of the lysosomal marker beta-hexoseaminidase, s uggesting an increased lysosome number/Pyr(R100) cell; and 6) a small reduc tion in the steady-state accumulation of [H-3]Pyr and no evidence of catabo lism or modification of cellular [H-3]Pyr. Consequently, Pyr(R100) cells we re markedly resistant to the lipophilic antifolates trimetrexate (40-fold) and AG377 (30-fold) and to the polyglutamatable antifolates 5,10-Dideaza-5, 6,7,8-tetrahydrofolic acid (DDATHF) (26-fold) and AG2034 (14-fold). Resista nce to these drugs was reversed in Pyr(R100) cells transferred into folate- depleted medium. In conclusion, these multiple resistance factors collectiv ely result in a prominent increase in folate accumulation, an expansion of the intracellular folylpolyglutamate pool, and abolishment of the cytotoxic activity of polyglutamatable and lipophilic antifolates. The role of incre ased lysosome number per cell in sequestration of hydrophobic weak base dru gs such as Pyr is also discussed as a novel mechanism of drug resistance.