Some HIV therapeutics in the nucleos(t)ide reverse transcriptase (N(t)RTI) inhibitor class (i.e. abacavir sulphate [ABC]) are reportedly associated with increased risk of cardiovascular events, such as myocardial infarction (MI). Increased MI risk is hypothesised to result from altered platelet reactivity in response to antiretroviral therapy. Thus, it is important to determine the impact of N(t)RTIs, including newly emerging therapeutics, such as tenofovir alafenamide (TAF), upon platelet aggregation as part of the process of determining their potential cardiovascular risk profile.
[i]In vitro[/i]: Platelets were isolated from healthy, HIV-negative, donors and aggregation assessed [i]in vitro[/i] using a 96-well plate assay. Platelets were pre-incubated with plasma C[sub]max[/sub] concentrations of N(t)RTIs and stimulated by platelet agonists (ADP, collagen, TRAP6 or thrombin). Platelet activation (granule release and integrin activation) was further assessed by multi-colour flow cytometry. [i]In vivo[/i]: Collagen-evoked radiolabelled platelet aggregation was monitored in mice following i.p. administration of N(t)RTIs.
Platelet aggregation in response to ADP, collagen or TRAP6 was unaffected by incubation with ABC, TAF or TDF (tenofovir disoproxil fumarate). Equivalent plasma concentrations of tenofovir (TFV) or carbovir triphosphate (Cbv-TP) also did not lead to altered platelet aggregation in vitro. Treatment with Cbv-TP, but not TFV, led to a reversal of NO-mediated inhibition of platelet aggregation. ABC significantly enhanced expression of platelet activation markers whereas TAF and TDF had no effect (see Table 1). These increases demonstrate increased degranulation, indicating enhanced platelet activation and potentially a pro-thrombotic impact, in the presence of ABC. In vivo studies showed normal platelet aggregation responses in the presence of TAF or TDF, whereas ABC potentiated aggregation, again indicative of a pro-thrombotic effect.
The reported increased MI risk in patients prescribed ABC may be driven by pharmacological modulation of the platelet activation response as well as interruption of NO-mediated inhibition of platelet aggregation, resulting in a greater propensity for agonist-induced platelet activation. Unlike earlier clinical studies, our observations are made in the absence of HIV infection, allowing assessment of direct pharmacological impacts of N(t)RIs on platelets.