Nitric oxide is an important cellular mediator that plays a role in tumor growth and angiogenesis. The present study was conducted to evaluate whether camptothecin (CPT), a topoisomerase I inhibitor, exhibits antitumor activity through regulation the inducible nitric oxide synthase (iNOS) biosynthesis pathway. Experiment was performed on RAW 264.7 cells, a transformed macrophage-like cell line, stimulated with lipopolysaccharide (LPS) plus interferon-gamma (IFN-gamma). Incubation of RAW264.7 cells with CPT (0.1 to 10 microM) inhibited the LPS/IFN-gamma-induced nitrite accumulation in a concentration-dependent manner with an IC50 value of 0.59+/-0.07 microM. Treatment of cells with concentrations of CPT (< or =3 microM) that are not growth inhibitory or cytotoxic strongly inhibited their ability to express iNOS mRNA and iNOS protein, however, without a direct regulatory effect on iNOS activity. Time course analysis also revealed that CPT acted in a fashion similar to the transcription inhibitor actinomycin-D. Thus, the suppressant effects of CPT on LPS/IFN-gamma-stimulated NO production seemed to be mediated probably through inhibition of iNOS gene transcription. From this observation we propose that inhibition of NO biosynthesis by CPT may underlie, at least in part, the efficacy of this antitumor agent.