Bryostatin 1 activates and subsequently down-regulates
protein kinase C (PKC) in vitro and has potential use as an
immunomodulator and as an anti-
cancer agent. Despite extensive examination of its activities in vitro and anti-
tumor effects in vivo, previous studies have failed to document that
bryostatin 1 modulates total cellular PKC activity in
tumor or normal tissues when administered in vivo. After a single bolus injection of
bryostatin 1 (1.0 microgram) in normal C57BI/6 mice, blood was drawn at various intervals and assayed for
bryostatin 1 levels. In addition, spleens from
bryostatin-treated mice were harvested 10 min to 10 days
after treatment, weighed and analyzed for cell numbers, PKC activity and cell surface phenotypes.
Bryostatin 1 levels in plasma rose rapidly, reaching peak levels of 56.5 nM less than 1 min after injection, and then declined to undetectable levels by 1 h. A similar pattern was observed when
bryostatin 1 was incubated with
leukemia cells in vitro, raising the possibility that the rapid fall in plasma levels results from intracellular uptake and binding.
Bryostatin 1 induced marked depletion of total splenocyte PKC activity (as much as 69% relative to control values) at 24-96 h after
drug administration, but not at earlier times (i.e. 1 h). A single injection of
bryostatin 1 also induced expression of the T cell activation marker CD69, leading to positivity in 53% of cells at 3-24 h versus 11% in control mice, and resulted in marked
splenomegaly, associated with increased numbers of nucleated cells at 48-96 h. Together, these studies demonstrate that despite rapid disappearance of the
drug from plasma, a single i.v. dose of
bryostatin 1 exhibits significant and sustained effects on normal murine spleen cells, including early lymphocyte activation, prolonged depletion of PKC activity, splenocyte proliferation and
splenomegaly. These findings may have implications for attempts to understand the in vivo effects of
bryostatin 1 in normal host tissues.