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.