Cross resistance of pleiotropically drug resistant P338 leukemia cells to the lipophilic antifolates trimetrexate and BW 301U

Several antifolate compounds were examined for their cytotoxic activity in a pleiotropically resistant P388 cell line (P388R). The sensitivity of P338R cells to methotrexate (MTX) and the lipophilic antifols, metoprine and methotrexate gamma-mono t-butyl ester (MTX-γ- t-butyl ester) were comparable with that activity observed in the parental cell line (P388S). P388R cells were, however, resistant to 2 other lipophilic antifols, trimetrexate (TMQ) and BW 301U. The degree of resistance to TMQ and BW 301U was 22-fold and 15-fold, respectively and could be partially overcome by the calcium channel blocker, verapamil (VER) or the detergent Tween 80. Transport studies showed that net accumulation of trimetrexate was markedly reduced in P388R cells resulting in a steady-state level which was 25% of the sensitive line. This impaired uptake was reversed by 5 μg/ml VER which increased the steady-state to a level comparable to P388S. P388R also exhibited a 50% reduction in the unindirectional influx rate, however, this defect could not be reversed by VER. The resistance of P388R cells to TMQ and BW 301U and their potentiation by VER extends pleiotropic resistance to yet another class of drugs which have important clinical implications.