Significance of Drug Interactions With Rifampin in Pneumocystis carinii Pneumonia Prophylaxis

To the Editor.—  In a recent issue of the ARCHIVES,¹ an update on drug interactions with rifampin was presented. The authors mention the possible clinical importance of such interactions in the treatment of Pneumocystis carinii pneumonia (PCP) with dapsone.We share this concern and have recently cautioned about potential problems with the combination of dapsone and rifampin treatment in prophylaxis of PCP.² Dosage regimens used in prophylaxis are often lower than those used for treatment purposes, and pharmacokinetic interactions may, thus, have an even greater impact. Intermittent dosage regimens, a common practice in PCP prophylaxis with dapsone therapy, may be particularly problematic. Borcherding et al¹ indicate that interactions of rifampin with dapsone in the treatment of leprosy have been investigated and do not appear to be clinically important. This may be explained by the exquisite sensitivity of Mycobacterium leprae to dapsone (2.5 to 10 μg/L).The minimal