Sensitivity to melphalan as a function of transport activity and proliferative rate in BALB/c 3T3 fibroblasts

Melphalan transport by exponentially dividing BALB/c 3T3 mouse fibroblasts was approximately 3-fold greater at the steady-state than that observed with resting cells at drug concentrations ranging from 1 to 100 microM. Both the rate of incorporation of [3H]thymidine and the percentage of S-phase cells estimated by a fluorescent cell sorter provided evidence of higher DNA synthetic activity in logarithmic phase cells. Transport of melphalan by BALB/c 3T3 fibroblasts was mediated by the two amino acid transport systems, the DL-beta-2-aminobicyclo(2,2,1)heptane-2-carboxylic acid-sensitive sodium-independent system preferring leucine as substrate and the sodium-dependent system preferring alanine, serine, and cysteine as substrates. At low drug concentrations (about 1 microM) the sodium-dependent mechanism predominated, but at high concentrations of melphalan (100 microM) a shift occurred from the sodium-dependent to the sodium-independent system. Both transport systems were markedly affected by the proliferative state of the cells and became progressively less active the longer cells remained in the stationary phase. Stationary-phase cells were also less sensitive to the cytotoxic action of melphalan than exponentially dividing cells. Serum deprivation experiments showed that unidirectional melphalan influx decreased about 40% when fibroblasts previously in exponential growth were shifted from normal medium to serum-depleted medium 24 h prior to drug transport studies. Thus, serum deprivation and maintenance of cells in stationary phase growth had parallel effects on drug influx. These experiments emphasize the importance of the proliferative state of the cell as a determinant of melphalan transport and cytocidal activity.