Transport of Melphalan by Sensitive and Resistant Li 2i 0 Cells1

The in vitro transport of L-p-(di-2-Chloroethylamino)[14C]phe nylalanine by i.p. grown mouse L1210 leukemias has been studied in melphalan-sensitive and melphalan-resistant cell populations. As has been reported previously, uptake of the drug is a temperature-sensitive, partly sodium-dependent, ac tive process with cell:medium accumulation in excess of 5:1 at 370 and a process which is competitively inhibited by L-leucine. Efflux studies demonstrated that at least 75% of the intracel lular radioactivity was free to leave the cell. At low concentrations of melphalan (3 to 10 ,xM),the sensitive cells were characterized by an apparent Kmfor transport of 1 1 .1 ± 2.1 (S.E.) pM and an apparent Vmax of 25.6 ± 9.9 pmol/106 cells/mm while at high concentrations of melphalan (1 4 to 80 pM), the corresponding values were 90 ± 16 ,zM and 1 58 ± 1 5 pmol/1 06 cells/mm. These data are consistent with previous reports that there are at least two distinct carrier systems which transport melphalan. Transport studies on melphalan-resistant Li 210 cells re vealed both a lower velocity of drug uptake and a lower intracellular accumulation of the drug at equilibrium than in sensitive Li 210 cells. At high concentrations of melphalan (14 to 80 @LM), a kinetic analysis indicated that in the resistant cells there was a 3-fold increase in the apparent Kmfor transport with no appreciable change in the Vmax. At low concentrations of melphalan (3 to i 0 pM), there was a similar decrease in the transport of melphalan by the resistant cells as compared to sensitive cells. However, in the presence of 1 mM DL-f.@-2aminobicyclo[2,2, 1]heptane-2-carboxylic acid, a specific inhib itor of the lower-affinity L system, there was no significant difference between melphalan transport in resistant and sen sitive cells at the low concentration. These studies suggest that the nature of the alteration in the melphalan transport in the resistant Li 210 cell may be a specific mutation in the lower affinity L system rendering melphalan transport less efficient.