Differential induction of apoptosis on human lymphoblastic leukemia Nalm-6 and Molt-4 cells by various antitumor drugs.

To investigate how chemotherapy agents interact with the leukemic cell death pathway, we examined apoptosis of human lymphoblastic leukemia cells (Nalm-6 and Molt-4) treated with various anticancer drugs (etoposide (VP-16), camptothecin (CPT), adriamycin (ADR), cytosine arabinoside (Ara-C), methotrexate (MTX), 6 mercaptopurine (6MP), cyclophosphamide (CPM), vincristine (VCR) and prednisolone (PRD)) by flow cytometric procedures. The proportion of apoptotic cells was estimated from the presence of cells with a fractional DNA content in the DNA histograms after the incubation of drug-treated cells with a DNA extraction buffer. Treatment with Ara-C, CPT, VP-16 and ADR resulted in rapid apoptosis with 40-60% apoptotic cells by 8 h. Treatment with MTX, VCR, 6MP and PRD induced no apparent apoptosis until 12 h, but further treatments with these drugs resulted in apoptosis with 50% (MTX), 20-30% (6MP and VCR) and 5-10% (PRD) apoptotic cells, respectively, at 24 h. CPM induced apoptosis with 10-20% apoptotic cells at 10(-6) M, but higher doses (> 10(-5) M) caused a rapid cell death by necrosis. The cell cycle position of apoptotic cells was assessed by the terminal deoxynucleotidyl transferase (TdT) assay of DNA strand breaks combined with DNA staining. MTX, Ara-C, CPT, VP-16 and ADR preferentially induced apoptosis in the S phase. PRD and 6MP induced apoptosis in the G1 phase and G1 + S phases, respectively. CPM showed no cell cycle phase specificity. These findings suggested that the susceptibility of cells to apoptosis was not the sole determinant of cellular sensitivity of cytotoxic drugs.