The effects of tamoxifen and its metabolites on platelet function and release of reactive oxygen intermediates.

Tamoxifen is effective in the prevention and treatment of breast cancer, but its use is associated with an increased risk of thrombosis. The mechanism for this effect is unknown. Reactive oxygen intermediates enhance platelet-dependent thrombosis, and in oncological studies, tamoxifen has been shown to increase production of reactive oxygen species. Therefore, the effects of tamoxifen and its bioactive metabolites on platelet activity and platelet reactive oxygen species were determined. Platelets were incubated with tamoxifen or the metabolites 4-hydroxy-tamoxifen (4-OH), N-desmethyl tamoxifen, or 4-hydroxy-N-desmethyl tamoxifen (endoxifen). Tamoxifen metabolites have been previously shown to possess enhanced bioactivity, and consistent with this observation, tamoxifen metabolites but not tamoxifen modestly increased platelet aggregation. These effects were similar with platelets isolated from male or female subjects. Platelet nitric oxide release or cGMP levels were not altered by incubation with tamoxifen or any of its metabolites. Incubation with tamoxifen metabolites increased stimulation-dependent platelet superoxide release [8.1 +/- 1.6 arbitrary units (a.u.) for control versus 15.2 +/- 3.5 a.u. for 4-OH; P < 0.01]. Coincubation with a superoxide dismutase mimetic eliminated the tamoxifen metabolite-induced enhancement of platelet aggregation. Corresponding to increased superoxide release, incubation with tamoxifen metabolites enhanced the functional activation of NADPH oxidase as determined by phosphorylation of its subunits p47(phox) and p67(phox). In summary, incubation of platelets with the active metabolites of tamoxifen increases stimulation-dependent superoxide release through a NADPH oxidase-dependent mechanism. This results in modest changes in platelet function and seems to be consistent with previous oncological studies demonstrating tamoxifen-dependent increase in reactive oxygen species generation.