Impaired immune response by isoniazid treatment during intravesical BCG administration in the guinea pig

At present, isoniazid (INH) is being used prophylactically to reduce the side effects of intravesical BCG therapy for superficial bladder cancer, although it is not clear whether or not this reduces the antitumor efficacy of BCG. In this study the impact of INH treatment on the immune response after repeated intravesical BCG administration was investigated in guinea pigs. INH was given during 3 days starting on the day prior to the BCG administration. It was found that the administration of INH severely impaired the immunological effects of BCG. The induction of mononuclear cell infiltration in the bladder wall was reduced. Enlargement of the regional lymph nodes (weight and number of cells), and increase of Major Histocompatibility Complex (MHC) class II expression on the lymph node cells, normally observed after intravesical BCG administration, were inhibited by INH. Systemic immunity, measured by the DTH reaction in the skin to PPD, was also diminished due to the combined treatment of BCG with INH. A five-fold increase of the dose of BCG did not overcome the effect of INH. INH probably did not exert a direct suppression of the immune system of the guinea pig as the DNCB skin reactivity was not influenced. Although INH concentrations in the urine were high at the onset of the instillation, in vitro experiments indicated that the effect of INH was probably not caused by killing of the BCG organisms shortly after application in the bladder. In conclusion, our data in guinea pigs suggest that the use of INH may impair the immune response to intravesical BCG. As this response may be important for the antitumor effect of BCG, urologists should be cautious with the prophylactic use of INH. The influence on the antitumor efficacy is now investigated in man.