Heritable and environmental determinants of platelet response to acetylsalicylic acid/

Consistent levels of platelet inhibition are required to deliver effective antiplatelet therapy. Growing awareness that not all individuals respond comparably to aspirin has led to the concept of aspirin ‘resistance’ and prompted search for an means to confirm individual response. However, the term is misleading because there are many determinants of failure to respond to treatment. Aspirin’s primary pharmacological effect and that which is understood to prevent thrombosis is almost complete inactivation of platelet cyclooxygenase-1 and thereby inhibition of thromboxane biosynthesis. In this thesis I report findings from a study of patients with cardiovascular disease taking aspirin, in which I apply point-of-care and laboratory assays with varying specificity for cyclooxygenase inhibition. I demonstrate weak correlation between aspirin assays, some of which measure cyclooxygenase inhibition and others that detect platelet reactivity despite cyclooxygenase inactivation. I identify supernatant thromboxane generation as a sensitive assay of aspirin response. I also demonstrate that low dose enteric-coated aspirin preparations fail to deliver an adequate dose to many patients.

Heritable factors determine platelet reactivity to a greater extent that environmental factors and largely determine residual activity in the presence of cyclooxygenase inhibition. I demonstrate that haplotypic variation in the cyclooxygenase-1 gene and a common splice variant in the gene that encodes the serotonin transporter modulate COX inhibition. I also identify a platelet G-protein (33 subunit splice variant that influences epinephrine-induced platelet activation via a non-COX dependent pathway. I demonstrate that the same variant increased bleeding in patients treated with the oral glycoprotein Ilb/IIIa antagonist orbofiban in the OPUS-TIMI 16 trial.

Lastly I investigate the role of human growth arrest specific gene 6 in platelets. I take a molecular biology and in-silico analysis approach to characterising the gene and its transcriptional response elements. I also screen for genetic variants in its regulatory regions to assess their role in determining platelet reactivity.