Irofulven (hydroxymethylacylfulvene, HMAF, MGI 114) is a novel agent with alkylating activity and a potent inducer of apoptosis. It is currently undergoing Phase II clinical trials for several tumor types, including hormone-refractory prostate cancer. Reduction of serum prostate-specific antigen (PSA) levels has been proposed as a generally useful endpoint for evaluating the antitumor efficacy of treatments for prostate cancer. However, the utility of PSA as a marker of tumor cell burden could be compromised, if drugs directly affected PSA secretion and/or expression. In these studies, we evaluated the effects of irofulven on PSA protein and mRNA levels during the course of treatment of prostate tumor cells in vitro. The rate of PSA secretion (normalized per equal cell number) by control and drug treated cells was similar, as determined by a solid phase, two-site immunoradiometric assay. Consistent with the lack of effect of irofulven on PSA protein level, the drug does not appear to affect the expression of PSA mRNA (on a per cell basis) as assessed by RT-PCR. Thus, changes in PSA secretion and expression appear to reflect irofulven-induced cell growth inhibition rather than reflecting a direct effect of the drug on PSA. These results suggest that PSA should be a reasonable marker of tumor burden in irofulven-treated prostate cancer patients.