To determine the relation between apoptosis and cytotoxicity the number of apoptotic (AP) cells was measured in MOLT-4 and HL-60 cultures treated with adriamycin, etoposide, cisplatin and 1-B-D-arabinofuranosylcytosine (ara-C) in the dose range inducing 20-95% growth inhibition. DNA degradation in individual AP cells was identified with anti-ssDNA monoclonal antibody. There was a near linear relationship between the number of AP cells and drug dose. All drugs induced apoptosis at a comparable level of cytotoxicity. Higher chemosensitivity of MOLT-4 than HL-60 cells was in agreement with the higher number of AP cells. Synergistic cytotoxicity of drug combinations was predicted by apoptosis assay. The number of AP cells was a reliable indicator of chemosensitivity when various cell lines and different drugs were compared. Low doses of cyclohexemide (CHX) inhibited etoposide-induced apoptosis when cells were exposed to both drugs for 6 h. Treatment with CHX alone during longer period or at higher doses induced apoptosis. Inhibition or induction of apoptosis by CHX in the same cell line was determined by the dose of drugs and the time of treatment. MOLT-4/Adr subline developed by continuous exposure to adriamycin was 1.6 - 2.3 fold resistant to adriamycin, etoposide and ara-C by growth inhibition and apoptosis assays. Spontaneous apoptosis induced by medium depletion was decreased in resistant cells. The selection of cells with diminished apoptotic response at early stage of acquired resistance induced pleoitropic drug resistance.

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