Structure-activity relationships for activation of adenylate cyclase by the diterpene forskolin and its derivatives

Forskolin (7 beta-acetoxy-8,13-epoxy-1 alpha, 6 beta, 9 alpha-trihydroxylabd-14-en-11-one), a diterpene from the Indian plant Coleus forskohlii, activates cyclic AMP generating systems in a number of mammalian tissues in a rapid and reversible fashion. Derivatives of forskolin have been tested for their ability to stimulate membrane adenylate cyclase from rat brain and rabbit heart, as well as cyclic AMP generation in guinea pig brain vesicular preparations, a model system for intact cells. Derivatives at the 6 beta- and 7 beta-hydroxy functions retain activity, but none have greater activity than that of forskolin. Reduction of the 11-keto function affords an active 11 beta-hydroxy derivative. Reduction of the 14,15-vinyl (alpha) substituent reduces activity, while epoxidation abolishes activity. Derivatization or lack of the 1 alpha- and 9 alpha-hydroxy functions results in a marked reduction in activity, emphasizing the importance of the alpha aspect of the molecule. However, the 1 alpha, 6 beta-di-O-acetyl derivative does retain activity. None of the inactive derivatives, which include the 14,15-epoxy, the 1,9-dideoxy, and the 1,6-diketo derivatives, antagonize the stimulatory effects of forskolin.