Rapid, radiometric in vitro assay for the evaluation of the anti-leprosy activity of clofazimine and its analogues

Abstract

The effect of clofazimine and 6 analogues (B 3691, B 3713, B 3640, B 3648, B 720 and B 749) on the viability of Mycobacterium leprae was tested in a rapid, radiometric, in vitro assay. Thirteen human and 1 armadillo-derived freshly-extracted M. leprae, maintained in peritoneal macrophages, incorporated 3H-thymidine to significant levels as compared to parallel cultures with heat-killed bacilli. Exposure of such cultures to clofazimine for 48 h showed significant inhibition of the radiolabel uptake without any adverse effects on the host macrophages. A sharp linear increase in inhibition was observable at concentrations from 1 to 10 ng/ml, with a plateau up to 40 ng/ml. Further increases of drug concentration up to 100 ng/ml showed marginal increase in the percentage inhibition of 3H-thymidine incorporation. The analogues tested showed levels of inhibition similar to that of clofazimine when left for 72 h and 15 days in M. leprae macrophage cultures. However, they were less effective than clofazimine when tested for the shorter duration of 48 h at the lower concentration of 5 ng/ml.