Effects of amino acids on the transport and cytotoxicity of melphalan by human bone marrow cells and human tumor cells

Abstract

In human tumor cells freshly obtained from patients with breast cancer, ovarian cancer, or adenocarcinoma of unknown etiology and in normal human bone marrow cells, the cell-to-medium ratio (intracellular/extracellular concentration) in vitro of 5.42 microM melphalan rose rapidly to levels of 6-17 after 35 min at 37 degrees C in Dulbecco's phosphate-buffered saline containing bovine serum albumin and glucose. Only patient C (breast cancer) had received chemotherapy. In all cells studied, L amino acids (1 mM) such as leucine, glutamine, tyrosine, and methionine reduced the cell-to-medium ratio of melphalan at 3 and 35 min. There was a good correlation between the reduction of melphalan transport at 35 min in the heterogeneous nucleated bone marrow cell population by amino acids and their effect on melphalan cytotoxicity in the CFU-C system. Aminoisobutyric acid (A1B), a specific substrate of the A system of amino acid transport, at a concentration between 1 and 50 mM had no significant effect on melphalan uptake at 3 min in any of the human cells studied except those of patient C. At 35 min A1B (10 or 50 mM) significantly reduced the intracellular melphalan concentration in normal bone marrow cells and tumor cells from patients B and C. At 2 mM, 2-aminobicyclo-(2, 2,1)-heptane-2-carboxylic acid (BCH), a specific substrate of the L system of amino acid transport, reduced the cell-to-medium ratio to 70% of control at 3 and 35 min in human bone marrow cells. In tumor cells from patients A, B, D, and F, 2 mM BCH had no significant effect on melphalan uptake at 3 min; it slightly decreased uptake in tumor cells from patient C. At 35 min, 2 mM BCH significantly reduced melphalan transport in tumor cells from patients C and F only. The lack of a BCH-suppressible component to melphalan uptake into human tumor cells freshly obtained from previously untreated patients contrasts with the presence of this component in murine L1210 leukemia cells, murine P388 leukemia cells, and human tumor cell lines. This suggests that minor differences in melphalan transport may exist amongst species and also between human tumor cells which are freshly obtained and cell lines maintained in culture.