Superiority of the neonatally thymectomized Lewis rat (NTLR) to monitor a clinical trial in lepromatous leprosy of the two regimens of rifampin and dapsone.

The ability of the neonatally thymectomized Lewis rat (NTLR) and the congenitally athymic (nude) rat systems to detect low numbers of viable Mycobacterium leprae in tissues from lepromatous leprosy patients undergoing short-course chemotherapy was compared with that of the commonly employed mouse foot pad assay. Fifteen previously untreated lepromatous patients were randomly assigned to treatment regimens of either a single initial 1500 mg dose of rifampin plus daily doses of 100 mg of dapsone, or weekly doses of 900 mg of rifampin plus daily doses of 100 mg of dapsone. Four skin biopsies from each patient taken sequentially up to one month after initiation of therapy were used as the source of the M. leprae inocula. Only 2 of 57 skin biopsies (2%) proved positive for viable M. leprae following direct inoculation into mouse foot pads. However, 30 of 58 patient biopsies (52%) provided positive for viable M. leprae following direct passage into NTLR foot pads or in subsequent mouse subpassage. In contrast, the nude rat was observed to be a poor monitor of such trials. Although not statistically significant, the regimen consisting of a single dose of rifampin plus daily dapsone resulted in a lower percentage of biopsies found to contain viable M. leprae at each of the four sampling intervals.