Butyrate and phenylacetate as differentiating agents: practical problems and opportunities.

Differentiating agents, including butyrate, phenylacetate and several other agents, have long been known to alter abnormal or transformed cell lines in vitro to a more normal state including phenotype and function. The effect depends on prolonged exposure to a minimum concentration of the agent. In vivo studies of butyrate and analogues have been limited, largely due to rapid in vivo metabolism. A butyrate prodrug, the triglyceride tributyrin, shows great promise in achieving effective and prolonged serum levels when given orally to mice and rats, and has been recommended for human trial. In vitro, butyrate and its mono- and triglyceride have shown potent synergy with retinoic acid, suggesting a ten-fold reduction in serum level requirements. Other butyrate prodrugs have been prepared and studied; several sugar esters of butyrate show promise. Phenylacetate, a normal mammalian metabolite, is also a potent differentiating agent, but its clinical use is limited by its objectionable odor per se and in treated subjects. Phenylbutyrate, a prodrug of phenylacetate, is more acceptable and may have greater promise. The availability of effective prodrugs of effective differentiating agents, such as tributyrin and phenylbutyrate, creates many opportunities for possible therapeutic and chemopreventive applications, especially if synergy in vivo can be demonstrated with retinoids (e.g., retinoic acid) or deltanoids (e.g., active vitamin D analogues), confirming in vitro studies. Particular disease targets would include certain leukemias, thalassemia, and sickle cell anemia.