Differential Suppression of Thromboxane Biosynthesis by Indobufen and Aspirin in Patients With Unstable Angina

Background We have previously reported aspirin failure in suppressing enhanced thromboxane (TX) biosynthesis in a subset of episodes of platelet activation during the acute phase of unstable angina. The recent discovery of a second prostaglandin H synthase (PGHS-2), inducible in response to inflammatory or mitogenic stimuli, prompted us to reexamine TXA 2 biosynthesis in unstable angina as modified by two cyclooxygenase inhibitors differentially affecting PGHS-2 despite a comparable impact on platelet PGHS-1. Methods and Results We randomized 20 patients (15 men and 5 women aged 59±10 years) with unstable angina to short-term treatment with aspirin (320 mg/d) or indobufen (200 mg BID) and collected 6 to 18 consecutive urine samples. Urinary 11-dehydro-TXB 2 was extracted and measured by a previously validated radioimmunoassay as a reflection of in vivo TXA 2 biosynthesis. Metabolite excretion averaged 102 pg/mg creatinine (median value; n=76) in the aspirin group and 55 pg/mg creatinine (median value; n=99) in the indobufen group ( P <.001). There were 16 samples (21%) with 11-dehydro-TXB 2 excretion >200 pg/mg creatinine among patients treated with aspirin versus 6 such samples (6%) among those treated with indobufen ( P <.001). In vitro and ex vivo studies in healthy subjects demonstrated the capacity of indobufen to largely suppress monocyte PGHS-2 activity at therapeutic plasma concentrations. In contrast, aspirin could only inhibit monocyte PGHS-2 transiently at very high concentrations. Conclusions We conclude that in unstable angina, episodes of aspirin-insensitive TXA 2 biosynthesis may reflect extraplatelet sources, possibly expressing the inducible PGHS in response to a local inflammatory milieu, and a selective PGHS-2 inhibitor would be an ideal tool to test the clinical relevance of this novel pathway of arachidonic acid metabolism in this setting.