Accumulation and persistence of cyclophosphamide-induced sister chromatid exchange in murine peripheral blood lymphocytes.

Sister chromatid exchange (SCE) responses were evaluated in lipopolysaccharide-stimulated murine peripheral blood lymphocytes assayed at various times following single or multiple injections of cyclophosphamide (3 mg/kg). Following a single injection, SCE levels in cultured lymphocytes from blood sampled at 5 min, 20 min, 35 min, 1 hr, and 3 hr postexposure were 17.1 +/- 2.0 (S.D.), 19.9 +/- 3.0, 19.3 +/- 1.8, 21.6 +/- 2.4, and 20.6 +/- 2.3, respectively. The control base-line SCE frequency was 11.2 +/- 1.2. The rapid initial increase in SCEs is consistent with the rapid increase reported previously in circulating active metabolites in rats following cyclophosphamide treatment. In peripheral blood lymphocytes cultured at 1 and 24 hr after serial injections of cyclophosphamide (3.0 mg/kg) two, four, and six times (every other day), a dose-related accumulation of SCEs occurred. Accumulation of SCEs was also observed in lymphocytes cultured at 24 and 72 hr following 12 multiple injections (three times weekly) of cyclophosphamide (3 mg/kg) (24.8 +/- 1.5 and 17.6 +/- 1.4, respectively) as compared to the single-injection group assayed at 24 and 72 hr postexposure (16.0 +/- 2.4 and 12.9 +/- 1.4, respectively). In the 12-multiple-injection study, an initial rapid decline at 72 hr was followed by a gradual decrease in SCE levels (1 week, 16.9 +/- 1.3; 2 weeks, 15.2 +/- 0.7; and 4 weeks, 13.4 +/- 1.1) which returned to near base-line (11.2 +/- 0.9) levels at 8 weeks. In the 12-multiple-injection study, successful growth of parallel concanavalin A-stimulated cultures was achieved only at 1, 2, and 4 weeks postexposure. Elevated SCE frequencies were observed at these intervals (16.2 +/- 2.1; 16.7 +/- 0.9; 14.2 +/- 0.3, respectively) relative to base-line SCE levels in concanavalin A-stimulated cells (12.1 +/- 1.8). The observed accumulation of SCEs with repeated exposure and persistence of SCE-inducing lesions parallel human data reported previously. The maximum induced (total minus baseline) SCE levels (10.2) observed in cultured lipopolysaccharide-stimulated lymphocytes from blood sampled at 1 hr after a single 3.0-mg/kg injection of cyclophosphamide were comparable or slightly higher than those (7.1) produced by the same dose of cyclophosphamide in murine bone marrow cells labeled with BrdUrd in vivo. However, in contrast to lymphocytes, bone marrow and alveolar macrophage cells did not accumulate SCEs upon repeated cyclophosphamide treatments.