Merbarone: an antitumor agent entering clinical trials.

Merbarone was developed to clinical trial stage on the basis of its 'curative' activity against P388 and L1210 leukemias and moderate activity against B16 melanoma and M5076 sarcoma. Its activity appears to be schedule-dependent favoring a longer duration of administration. The mechanism of action of merbarone is not yet established but it does induce single strand breaks in DNA apparently without binding to DNA. The pharmacokinetic data in the dog indicate that clearance mechanisms may be saturable. Merbarone is hydroxylated at the 4' position in the rat, mouse and dog, and glucuronidated in the dog. Parent drug and the hydroxy metabolite are excreted in the urine. If saturable clearance mechanisms also pertain to man, this will mean that infusion rate (and therefore steady state concentrations reached) may be a significant factor in determining acute toxicity. Preclinical toxicology studies revealed that major target tissues are in the lymphoid organs, bone marrow, gastrointestinal tract and kidney. Some behavioral signs of reversible central nervous system toxicity were observed. Phase I trials have commenced using only a 5-day continuous intravenous infusion schedule based on the preclinical data. The pharmacokinetic information from these trials will be crucial for further clinical development of the compound, including selection of the optimal schedule(s) for phase II/III evaluation.