Induction of mutant lymphocytes in cyclophosphamide- and chlorambucil-treated patients

Monitoring patients treated with single antineoplastic agents is aiding our understanding of what hazard these drugs pose in vivo. In this study, the frequency of mutant 6-thioguanine-resistant (TGR) peripheral blood lymphocytes was monitored before treatment and for ≤35 weeks after treatment of patients with cyclophosphamide (CP) or chlorambucil (CAB). The mean mutant frequency before treatment for six multiple sclerosis patients treated with high-dose CP was 2.53 × 10–5 and increased after treatment to 4.61 × 10–5 (P = 0.08, paired t-test). Using each patient as their own control, there were significant increases (each at P < 0.04) detectable within 2–4 weeks in four of the multiple sclerosis patients treated with CP. There was no increase in an untreated control monitored over the same period. In a patient receiving five sequential CP treatments at 1 month intervals, there were cumulative increases in the frequency of mutant cells. The mutant frequency increased from 0.31 × 10–5 before treatment to 3.64 × 10–5 after the final treatment and had decreased to 0.53 × 10–5 at 35 weeks after treatment. In one of two CAB-treated patients with indolent non-Hodgkin's lymphoma, there was a significant increase in mutant frequency (P < 0.03) after treatment. Freshly isolated peripheral blood lymphocytes treated with 4-hydroperoxy-CP in vitro demonstrate a dose-dependent increase in mutant frequency. The increment in mutant frequency observed in vivo is of the order expected from the in vitro experiments. Although this study demonstrates that single or multiple doses of a single antineoplastic agent are mutagenic in vivo for some patients, further studies are needed to determine the extent and mechanism of the inter-individual variations in mutagenic response.