Guanylhydrazones are cationic heteroaromatic drugs similar to the diamidines which are effective in the treatment of African trypanosomiasis and pneumocystosis. On the basis of their antitrypanosomal activity, different guanylhydrazones were selected for evaluation in a rat model of Pneumocystis carinii pneumonia. The most active compounds were the 2-(4'-formylphenyl)-1-methylimidazo-[1,2-a]pyridinium guanylhydrazones which, at a dose of 2 mg/kg/day, were about as effective as trimethoprim-sulfamethoxazole at a dose of 50 mg of trimethoprim per kg/day plus 250 mg of sulfamethoxazole per kg/day. The anti-P. carinii activity of these guanylhydrazone derivatives was found with parenteral but not with oral administration. The 1,3-arylene diketone bis(guanylhydrazones) were generally ineffective, although a triacetyl derivative showed some anti-P. carinii activity. Nitroimidazole guanylhydrazone derivatives were also ineffective. Attempts to improve the therapeutic efficacy of the different guanylhydrazones were limited by problems of toxicity. We conclude that some guanylhydrazone derivatives are potent anti-P. carinii drugs and that further studies should be pursued to develop safer compounds and investigate structure-activity relationships.Drugs currently licensed for the treatment of Pneumocystis carinii pneumonia in human immunodeficiency virus-infected patients are characterized by problems of efficacy and/or a high frequency of adverse reactions (18). Cationic aromatic compounds such as the diamidines, which were originally developed as antitrypanosomal drugs, are known to have potent anti-P. carinii properties (30, 32). The major limitation to the wider use of these agents is their toxicity, as evidenced by the fact that only one member of this class of compounds, pentamidine isethionate, has been licensed for use in this country (1,19). Research efforts to develop better diamidine derivatives have proceeded along two general lines. Most of the work has been devoted to synthesizing new compounds and examining their structure-activity relationships (6,15,26,27). Alternatively, our laboratory has explored existing classes of cationic drugs and found anti-P. carinii activity in the following: diamidines, carbanilides, aminoquinaldines, and phenanthridiniums (30, 32).Guanylhydrazones represent another class of quaternary heteroaromatic drugs which have shown trypanocidal activity. In a previous study, we found that methylglyoxal-bis(guanylhydrazone) (MGBG) had limited efficacy in an immunosuppressed-rat model of pneumocystosis (32). Over the past decade, several hundred guanylhydrazone derivatives have been synthesized and their structure-activity relationships have been studied in a mouse model of African trypanosomiasis (24,28,29). In the present study, we used this information to select and compare the activities of guanylhydrazone compounds in a standard immunosuppressed-rat model of P. carinii pneumonia. MATERIALS AND METHODS Drugs. The compounds used in this study and their structures are shown in Fig. 1. M...