Determinants of subacute response to clopidogrel: relative impact of CYP2C19 genotype and PGE1/adenylate cyclase signalling

Does high on-treatment platelet aggregability reflect poor individual response to clopidogrel?

INTRODUCTION

On-treatment platelet aggregability represents the major form of functional assessment for patients treated with P2Y₁₂ receptor antagonists, with "high" on-treatment platelet aggregability (HTPA) predicting thrombotic risk. However HTPA reflects a variable combination of pre-treatment hyperaggregability and poor response to P2Y₁₂ antagonists. We have previously shown that integrity of platelet adenylate cyclase/cAMP signaling, assessed with PGE₁, is a strong predictor of individual responses to clopidogrel. We therefore sought to determine the extent to which HTPA reflects impaired platelet responsiveness to clopidogrel.

METHODS

Using data from our previous investigations of acute and sub-acute effects of clopidogrel, we analyzed the relationship between on-treatment aggregability and acute/steady state responsiveness to clopidogrel, utilizing ADP, the thromboxane A₂ mimetic U46619, and thrombin receptor-activating peptide (TRAP) as pro-aggregants. The relationship between anti-aggregatory response to PGE₁ and both on-treatment and pre-treatment aggregability was also examined.

RESULTS AND CONCLUSIONS

With all 3 pro-aggregants, (1) response to clopidogrel after 4 h, as measured by ΔADP response, exhibits a strong inverse relationship with on-treatment aggregation, with a similarly inverse relationship between pre-treatment PGE₁ response and on-treatment aggregability; (2) there is a weaker inverse relationship between clopidogrel response and pre-treatment platelet aggregability, and a significant inverse relationship between pre-treatment PGE₁ response and pre-treatment platelet aggregability. Furthermore, pre-treatment PGE₁ response also predicts on-treatment platelet aggregability in response to ADP at steady state. Thus, HTPA largely represents clopidogrel resistance.

Mechanism of Nucleoside Triphosphate Diphosphohydrolase-1-Associated Imbalance in Adenosine Diphosphate Degradation, B-Cell Activation, and Related Injury During Acute Antibody-Mediated Rejection

OBJECTIVE

The objective of this study was to investigate the effect of nucleoside triphosphate diphosphohydrolase-1 (NTPDase1) during acute antibody-mediated rejection (AMR).

METHODS

NTPDase1 overexpression, NTPDase1 knockout, and wild-type nude mice skin graft models were used to induce acute AMR. NTPDase1 expression in B cells, NTPDase1 messenger RNA expression in skin grafts, extracellular adenosine diphosphate (ADP) concentration, B-cell volume and surface antigens expression, average platelet transport rate, and ultrastructure and apoptosis of skin graft cells were investigated.

RESULTS

During acute AMR in nude mice, higher NTPDase1 expression caused lower extracellular ADP concentration, smaller increase in B-cell volume, and major histocompatibility complex II surface antigen expression, suggesting a negative correlation between them; higher NTPDase1 expression also caused slower average platelet transport rate and less severe skin graft injury, suggesting a negative correlation between them. Pretreatment with high-dose exogenous NTPDase1 inhibited platelet activation and protected skin grafts, but it resulted in prolonged bleeding time (by 51.4%) and prolonged coagulation time (by 44.1%).

CONCLUSION

An NTPDase1-associated imbalance in extracellular ADP degradation may contribute to B-cell activation, platelet activation, and more severe skin graft injury in nude mice. Pretreatment with high-dose exogenous NTPDase1 effectively protected skin grafts in nude mice at 1 week, but it increased the risk of bleeding.

Prior chronic clopidogrel therapy is associated with increased adverse events and early stent thrombosis

Despite the growing use of clopidogrel, limited data exist regarding the prognostic significance of chronic clopidogrel therapy in patients sustaining acute coronary syndrome (ACS). Our aim was to determine whether patients sustaining ACS while on chronic clopidogrel therapy have a worse prognosis than clopidogrel-naïve patients. A total of 5,386 consecutive ACS patients were prospectively characterised and followed-up for 30 days. Of them, 680 (13 %) were treated with clopidogrel prior to the index ACS. Major adverse cardiovascular events (MACE) were defined as death, recurrent ACS, stroke and/or stent thrombosis. Compared with clopidogrel-naïve, chronic clopidogrel-treated patients were older (66 ± 12 vs 63 ± 13, respectively; p< 0.01), suffered more from diabetes mellitus, hypertension, dyslipidaemia, prior cardiovascular history, including prior myocardial infarction, revascularisation, coronary artery bypass graft and stroke (p< 0.01 for all), and were less likely to present with ST-elevation myocardial infarction (21 % vs 45 %; respectively; p < 0.001). Prior clopidogrel therapy was associated with a two-fold increase in in-hospital (1.6 °% vs 0.6, respectively; p =0.006) as well as 30-day stent thrombosis (2.2 % vs 1.0 %, respectively; p=0.007). MACE at 30 days was also higher among chronic clopidogrel-treated compared with clopidogrel-naïve patients [12.3 % vs 9.4 %, respectively; p< 0.01]. In multivariate log regression analysis chronic clopidogrel treatment was an independent predictor of stent thrombosis [OR=2.6 (95 %CI 1.2–5.6), p=0.001]. Patients sustaining ACS while on chronic clopidogrel treatment are at higher risk for in-hospital and 30-day adverse outcomes, including stent thrombosis.

New Developments in Platelet Cyclic Nucleotide Signalling: Therapeutic Implications

Altered platelet physiology may contribute to the emergence of thrombosis in patients with many forms of cardiovascular disease. Excess platelet activation may reflect increased stimulation of pro-aggregatory pathways. There is, however, increasing evidence that excessive platelet response, due to impaired efficacy of anti-aggregatory autacoids such as nitric oxide (NO) and prostacyclin (PGI₂), may be just as important. For example, diminished platelet response to NO has been documented in acute and chronic myocardial ischaemia, heart failure, aortic valve disease and in the presence of hyperglycaemia. This "NO resistance" has been shown to reflect both the scavenging of NO by reactive oxygen species and dysfunction of its intracellular "receptor", soluble guanylate cyclase. Importantly, these abnormalities of NO signalling are potentially reversible through judicious application of pharmacotherapy. The analogous condition of impaired PGI₂/adenylate cyclase (AC) signalling has received comparatively less attention to date. We have shown that platelet response to prostaglandin E₁ (PGE₁) is frequently impaired in patients with symptomatic myocardial ischaemia. Because the effects of ADP receptor antagonists such as clopidogrel and ticagrelor at the level of the P2Y₁₂ receptor are coupled with changes in activity of AC, impaired response to PGE₁ might imply both increased thrombotic risk and a reduced efficacy of anti-aggregatory drugs. Accordingly, patient response to treatment with clopidogrel is determined not only by variability of clopidogrel bio-activation, but also extensively by the integrity of platelet AC signalling. We here review these recent developments and their emerging therapeutic implications for thrombotic disorders.