BACKGROUND The growth-inhibiting and differentiating effects of sodium phenylacetate against hematopoietic and solid tumor cell lines has aroused clinical interest in its use as an anticancer drug. In an earlier Phase I trial of phenylacetate aimed at maintaining serum drug concentrations in the range that proved active in vitro (> 250 micrograms/ml) for 2 consecutive weeks, infusion rates approached the maximum velocity of drug elimination and commonly resulted in drug accumulation and reversible dose-limiting neurologic toxicity. In this study, the authors described the nonlinear pharmacokinetics, metabolism, toxicity, and clinical activity of phenylacetate. METHODS The treatment regimen of this Phase I study was designed to expose patients intermittently to drug concentrations exceeding 250 micrograms/ml and to allow time for drug elimination to occur between doses to minimize accumulation. Sodium phenylacetate was administered as a 1-hour infusion twice daily (8 a.m., 5 p.m.) at two dose levels of 125 and 150 mg/kg for a 2-week period. Therapy was repeated at 4-week intervals for patients who did not experience dose-limiting toxicity or disease progression. RESULTS Eighteen patients (4 of whom previously were treated with phenylacetate by continuous intravenous infusion) received 27 cycles of therapy. Detailed pharmacokinetic studies for eight patients indicated that phenylacetate induced its own clearance by a factor of 27% in a 2-week period. Dose-limiting toxicity, consisting of reversible central nervous system depression, was observed for three patients at the second dose level. One patient with refractory malignant glioma had a partial response, and one with hormone-independent prostate cancer achieved a 50% decline in prostate specific antigen level, which was maintained for 1 month. CONCLUSIONS Phenylacetate administered at a dose of 125 mg/kg twice daily for 2 consecutive weeks is well tolerated. High grade gliomas and advanced prostate cancer are reasonable targets for Phase II clinical trials.