Susceptibility to genotoxic effects of niclosamide in human peripheral lymphocytes exposed in vitro and in vivo.

Niclosamide added in 2-h pulses to lymphocyte cultures induced a small clastogenic effect in one blood donor, while in two other blood donors it inhibited mitosis. In the presence of the 'S9' metabolic activation system, the antihelminthic drug exhibited a dose-related increase in clastogenicity in 2 of 4 blood samples. A weak dose-related increase in S.C.E. was observed only in the lymphocytes from one of these blood samples. From 5 patients treated with niclosamide, 3 showed an increase in chromosomal aberrations after treatment; in none of them was an induction of S.C.E. observed. These results suggest differences in lymphocyte susceptibility to the genotoxic effects of therapeutic drugs and also underline the need for evaluating chromosomal aberrations as well as SCE in any study of genotoxic substances.