Transport, Metabolism, and DNA Interaction of Melphalan in Lymphocytes from Patients with Chronic Lymphocytic Leukemia¹

We investigated the transport of [chloroethyl-¹⁴C]melphalan with lymphocytes from three groups of patients with chronic lymphocytic leukemia (untreated, treated sensitive, and treated resistant). There was no significant difference in the K_m or V_max of melphalan transport in lymphocytes from the three groups. In addition, there were no significant differences in intracellular melphalan levels after a 35-min incubation with 5.4 µm melphalan among the three groups. There was no evidence of intracellular metabolism of melphalan to dihydroxymelphalan except in lymphocytes from one treated sensitive patient. dl-2-Aminobicyclo[2,2,1]heptane-2-carboxylic acid, a specific analogue of the sodium-independent leucine-preferring amino acid transport system, inhibited the uptake of melphalan to a greater extent in lymphocytes from resistant patients than in those of untreated patients. Glutathione levels were not significantly different in lymphocytes from resistant patients as compared to those of untreated patients. The percentage of DNA cross-links as determined by an ethidium bromide fluorescence assay was 2-5-fold greater in lymphocytes from untreated patients than in those of resistant patients. These results suggest that resistance to the nitrogen mustards in patients with chronic lymphocytic leukemia is secondary to neither a transport defect nor alteration in intracellular melphalan levels but rather due to some other mechanism responsible for decreased DNA cross-links.