EFFECTS OF 9-AMINOCAMPTOTHECIN ON NEWLY SYNTHESIZED DNA IN PATIENT BONE-MARROW SAMPLES

9-Aminocamptothecin (9-AC) inhibited cell growth and DNA synthesis in HCT 116 human colon cancer cells in a concentration- and time-dependen t manner, Interference with nascent DNA chain elongation was monitored using pH step alkaline elution, After a 3-day 9-AC exposure, 38% (10 nM) and 53% (50 nM) of the total [H-3]DNA eluted with pH steps 11.3-11 .7, compared to 9% in control cells, Effects on nascent DNA integrity were also evaluated by fixed elution with pH 12.1 buffer, After a 3-da y exposure to 9-AC, 27% (10 nM) and 82.5% (50 nM) of the total [H-3]DN A eluted relative to control, Paired bone marrow samples were then obt ained in 10 patients before treatment and between 42 and 72 h of a con tinuous i.v. infusion of 9-AC (35-74 mu g/m(2)/h for 72 h), The mononu clear cells were incubated with [H-3]dThd for 2 or 4 h, and then analy zed using either pH step or fixed pH alkaline elution, respectively, I n seven patients receiving greater than or equal to 47 mu g/m(2)/h 9-A C, 4% +/- 1.5% (mean +/- SE) of the total [H-3]DNA eluted with pH step s less than or equal to 11.7 in the pretreatment samples compared to 1 3% +/- 3.6% during 9-AC (P = 0.037), An altered fixed pH elution profi le of nascent DNA was noted in two patients treated with 59 and 74 mu g/m(2)/h 9-AC compared to baseline, DNA synthesis was inhibited by 89% +/- 5% during infusion of greater than or equal to 59 mu g/m(2)/h 9-A C (n = 7), Since hematological toxicity is dose limiting on this 9-AC schedule, these cellular pharmacodynamic studies provide evidence of a DNA-directed cytotoxic effect of 9-AC in a sensitive host target tiss ue.