ORAL-CONTRACEPTIVES AND HORMONE REPLACEMENT THERAPY DO NOT INCREASE THE INCIDENCE OF ARTERIAL THROMBOSIS IN A NONHUMAN PRIMATE MODEL

Older oral contraceptive (OC) formulations containing high doses of po tent synthetic estrogens and progestins are associated with increased risk of thrombosis. To examine the effects of current low-dose OC and hormone replacement therapy (HRT) regimens on arterial thrombosis, pre menopausal and surgically postmenopausal cynomolgus monkeys were divid ed into four treatment groups. Premenopausal monkeys were given either no OCs or ethinyl estradiol and levonorgestrel as an OC at a dose equ ivalent to that currently given to women. Postmenopausal monkeys were given either to HRT or conjugated equine estrogens and medroxyprogeste rone as an HRT at a dose equivalent to that currently given to women. The monkeys were fed an atherogenic diet containing these treatments f or 27 to 30 months. At the end of this time, arterial thrombosis was e valuated with a standardized stenosis/injury procedure in the left car otid artery. Blood flow velocity was monitored for cyclic or permanent occlusive thrombosis, The current OC and HRT regimens did not increas e thr susceptibility of the artery wall to develop an occlusive thromb us following injury and stenosis. In fact, there was a reduction in th e incidence of thrombosis in the OC animals compared with untreated co ntrols. Increased amounts of atherosclerosis were associated with an i ncreased incidence of occlusive arterial thrombosis. Several selected coagulation parameters [von Willebrand factor, protein C, lipoprotein( a), and platelet aggregation] did not appear to be associated with eit her the amount of atherosclerosis or incidence of arterial thrombosis.