Preclinical pharmacology of the natural product anticancer agent bryostatin 1, an activator of protein kinase C

Bryostatin 1, a natural product anticancer agent isolated from a marine bryozoan, has been shown in tissue culture to activate protein kinase C. This agent has recently undergone Phase I testing in humans given either as a bolus i.v. injection or a continuous infusion. To understand how bryostatin 1 might be used best as an anticancer agent, a study of the pharmacokinetics, tissue distribution, metabolism, and elimination of bryostatin 1 in mice was undertaken, using [C26-³H]-labeled bryostatin 1. Following i.v. administration, the plasma disappearance curve for bryostatin 1 could be described by a two-compartment model, with half-lives of 1.05 and 22.97 h, respectively. In contrast, the plasma disappearance curve for bryostatin 1 administered i.p. was better described by a first order absorption one- compartment model, with an absorption half-life of 0.81 h and an elimination half-life of 28.76 h, respectively. The majority of radioactivity in plasma was associated with the intact drug for up to 24 h alter dosing. In the first 12 h after i.v administration, urinary excretion represented the major pathway of elimination, with 23.0 ± 1.9% (mean ± SD) of the administered dose excreted. Within 72 h after i.v. administration, approximately equal amounts of radioactivity (40%) were excreted in fetes compared to urine. Bryostatin 1 was widely distributed in many organs but concentrated in the lung, liver, gastrointestinal tract, and fatty tissue. The concentration in the gastrointestinal tract, along with the fecal excretion, suggests the possibility of enterohepatic circulation of this drug. In summary, this study demonstrates that bryostatin 1 is relatively stable in vivo, widely distributed but concentrated in some major tissues, and rapidly excreted first through urine and at later times through the feces. The data from this animal study should be useful in the design of future human trials with this anticancer drug.