Implications for improved high-dose methotrexate therapeutic effects in cultured human breast cancer and bone marrow cells.

The cytotoxicity of high-dose methotrexate (MTX), 10 and 100 microM, and 5-fluorouracil (5-FU) combinations is independent of sequence in human MDA-MB-436 breast carcinoma cells. The growth inhibitory effects of 10 and 100 microM MTX are 22.54+/-1.56% and 16.20+/-0.74%, respectively, of the control rate. When the MTX and 5-FU concentrations are 10 microM, antiproliferative effects of MTX 2 hr before 5-FU (MTX/5-FU) and 5-FU 2 h before MTX (5-FU/MTX) are 25.17+/-1.23% and 25.60+/-1.28% of the control rate, respectively. The percentage of control rates for 5-FU alone is 94.89+/-1.35%. The growth rates of MDA-MB-436 cells in 100 microM MTX and 10 microM 5-FU are 15.19+/-0.62% (MTX/5-FU) and 16.53+/-0.85% (5-FU/MTX) of the control rate. The growth of cancer cells in the presence of 5-FU alone is 93.82+/-1.69% of the control rate. A comparison of the cell-killing effects of MTX and the nonpolyglutamable antifolate trimetrexate (TMQ) alone and in combination with 5-FU was performed to indirectly explore the role of polyglutamylation in breast cancer and bone marrow cells. The comparisons were made in equitoxic concentrations (10 microM) of MTX and TMQ and the time of exposure was the same. The inhibitory effects of TMQ, TMQ/5-FU, and 5-FU/TMQ in breast cancer cells were identical, but significantly less than MTX, MTX/5-FU, and 5-FU/MTX. The interaction between TMQ and MTX, TMQ/5-FU and MTX/5-FU, and 5-FU/TMQ and 5-FU/MTX was quantitatively similar in bone marrow. (Significant protection occurred in bone marrow cells exposed to 5-FU/TMQ and 5-FU/MTX.) Because the effects of 5-FU/MTX and 5-FU/TMQ on bone marrow were the same, it is unlikely that polyglutamylation plays a significant role in the protective effects of 5-FU. However, the greater inhibitory effect of MTX or MTX and 5-FU combinations, when compared with TMQ or TMQ and 5-FU, suggests that polyglutamylation of MTX may contribute to the cytotoxicity of this antifolate to breast cancer cells. Hence, these studies suggest that a priming and nontoxic dose of 5-FU before high-dose MTX sustains MTX cytotoxicity in breast cancer and protects against MTX toxicity to bone marrow progenitor cells.