Platelet aggregometry and receptor binding to predict the magnitude of antithrombotic and bleeding time effects of clopidogrel in rabbits

Milvexian, an orally bioavailable, small-molecule, reversible, direct inhibitor of factor XIa: In vitro studies and in vivo evaluation in experimental thrombosis in rabbits

BACKGROUND

Milvexian (BMS-986177/JNJ-70033093) is an orally bioavailable factor XIa (FXIa) inhibitor currently in phase 2 clinical trials.

OBJECTIVES

To evaluate in vitro properties and in vivo characteristics of milvexian.

METHODS

In vitro properties of milvexian were evaluated with coagulation and enzyme assays, and in vivo profiles were characterized with rabbit models of electrolytic-induced carotid arterial thrombosis and cuticle bleeding time (BT).

RESULTS

Milvexian is an active-site, reversible inhibitor of human and rabbit FXIa (Ki 0.11 and 0.38 nM, respectively). Milvexian increased activated partial thromboplastin time (APTT) without changing prothrombin time and potently prolonged plasma APTT in humans and rabbits. Milvexian did not alter platelet aggregation to ADP, arachidonic acid, or collagen. Milvexian was evaluated for in vivo prevention and treatment of thrombosis. For prevention, milvexian 0.063 + 0.04, 0.25 + 0.17, and 1 + 0.67 mg/kg+mg/kg/h preserved 32 ± 6*, 54 ± 10*, and 76 ± 5%* of carotid blood flow (CBF) and reduced thrombus weight by 15 ± 10*, 45 ± 2*, and 70 ± 4%*, respectively (*p < .05; n = 6/dose). For treatment, thrombosis was initiated for 15 min and CBF decreased to 40% of control. Seventy-five minutes after milvexian administration, CBF averaged 1 ± 0.3, 39 ± 10, and 66 ± 2%* in groups treated with vehicle and milvexian 0.25 + 0.17 and 1 + 0.67 mg/kg+mg/kg/h, respectively (*p < .05 vs. vehicle; n = 6/group). The combination of milvexian 1 + 0.67 mg/kg+mg/kg/h and aspirin 4 mg/kg/h intravenous did not increase BT versus aspirin monotherapy.

CONCLUSIONS

Milvexian is an effective antithrombotic agent with limited impact on hemostasis, even when combined with aspirin in rabbits. This study supports inhibition of FXIa with milvexian as a promising antithrombotic therapy with a wide therapeutic window.

Effect of clopidogrel in bone healing-experimental study in rabbits

BACKGROUND

Clopidogrel is a widely prescribed drug for prevention of myocardial infarction and stroke in patients at risk. It inhibits thrombus formation via inhibition of the P2Y12 purinergic receptor on platelets, which is important in their activation by ADP. However, the P2Y12 receptor has also been found to be expressed in both osteoblasts and osteoclasts. Accumulated evidence suggests that purinergic receptors regulate important functions of bone turnover. Previous studies on the effect of clopidogrel on bone metabolism indicated potential harmful effects, but their results remain conflicting. Thus, clopidogrel treatment may affect bone healing, but it has not yet been studied.

AIM

To evaluate if continuous perioperative clopidogrel treatment has any negative effect on bone healing in the rabbit calvarial defect model.

METHODS

Sixteen male white New Zealand rabbits were randomly assigned in two groups: One group received daily 3 mg/kg of clopidogrel per os and the other group received the vehicle alone for a week prior to the surgical procedures; the treatments were continued for another 6 wk postoperatively. The surgical procedures included generation of two circular calvarial defects 11 mm in diameter in every animal. After the 6-wk period of healing, postmortem radiographic and histomorphometric evaluation of the defects was performed.

RESULTS

Both the surgical procedures and the postoperative period were uneventful and well tolerated by all the animals, without any surgical wound dehiscence, signs of infection or other complication. New bone was formed either inwards from the defect margins or in the central portion of the defect as separated bony islets. While defect healing was still incomplete in both groups, the clopidogrel group had significantly improved radiographic healing scores. Moreover, the histomorphometric analysis showed that bone regeneration (%) was 28.07 ± 7.7 for the clopidogrel group and 19.47 ± 4.9 for the control group, showing a statistically significant difference between them (P = 0.018). Statistically significant difference was also found in the defect bridging (%), i.e. 72.17 ± 21.2 for the clopidogrel group and 41.17 ± 8.5 for the control group, respectively (P = 0.004), whereas there was no statistical difference in bone tissue density between the groups.

CONCLUSION

Our results indicate that maintenance of perioperative clopidogrel treatment does not negatively affect bone healing but rather promotes it. Further research is needed in order to find useful applications of this finding.

A rabbit model of cerebral microembolic signals for translational research: preclinical validation for aspirin and clopidogrel

Essentials Microembolic signal (MES) is an independent predictor of stroke risk in patients. A rabbit model of cerebral microembolic signals was established. Therapeutic efficacy was demonstrated for aspirin and clopidogrel on microembolic signals. Potential translational value of this preclinical model of MES was demonstrated.

SUMMARY

Objectives Cerebral microembolic signals (MESs) detected by transcranial Doppler (TCD) ultrasound constitute an independent predictor of stroke risk and prognosis. The aim of this study was to develop a novel preclinical model of MESs to facilitate translational research. Methods A clinical TCD ultrasound machine was used to detect MESs in the cerebral circulation of New Zealand White rabbits. Technical feasibility was assessed for the measurement of MESs in the middle cerebral artery (MCA) by TCD. FeCl3 -induced carotid arterial thrombosis was optimized for the generation of endogenous microemboli. Ascending doses of two antithrombotic agents (aspirin and clopidogrel) were evaluated individually and in combination for their effects on both arterial thrombosis and MESs in a 30% FeCl3 -induced carotid arterial thrombosis model, along with ex vivo functional assays. Results Dose-dependent FeCl3 -induced arterial thrombosis studies showed that 30% FeCl3 resulted in the most consistent and reproducible MESs in the MCA (3.3 ± 0.7 MESs h(-1) ). Ascending-dose studies showed that the effective doses for 50% inhibition (ED50 ) of thrombus formation, based on integrated blood flow and thrombus weight, respectively, were 3.1 mg kg(-1) and 4.2 mg kg(-1) orally for aspirin, and 0.3 mg kg(-1) and 0.28 mg kg(-1) orally for clopidogrel. The ED50 values for MES incidence were 12.7 mg kg(-1) orally for aspirin, and 0.25 mg kg(-1) orally for clopidogrel. Dual treatment with aspirin (5 mg kg(-1) ) and clopidogel (0.3 mg kg(-1) ) resulted in significant reductions in cerebral MESs (P < 0.05) as compared with monotherapy with either agent. Conclusions Our study demonstrated the successful establishment of the MES model in rabbits, and it may provide translational value for MESs and ischemic stroke research.

rLOAD: does sex mediate the effect of acute antiplatelet loading on stroke outcome

BACKGROUND

Biologic sex can influence response to pharmacologic therapy. The purpose of this proof-of-concept study was to evaluate the medicating effects of estrogen in the efficacy of acute antiplatelet loading therapy on stroke outcome in the rabbit small clot embolic model.

METHODS

Female and male (20/group) New Zealand White rabbits were embolized to produce embolic stroke by injecting small blood clots into the middle cerebral artery via an internal carotid artery catheter. Two hours after embolization, rabbits were treated with standard dose antiplatelet loading (aspirin 10 mg/kg plus clopidogrel 10 mg/kg). Primary outcome measures were platelet inhibition, behavioral outcome P 50 (the weight of microclots (mg) that produces neurologic dysfunction in 50% of a group of animals), and effect of endogenous estrogen on outcome.

RESULTS

For the first time in a non-rodent model of stroke, it was found that higher endogenous estrogen levels resulted in significantly better behavioral outcome in female subjects (r s -0.70, p < 0.011). Platelet inhibition in response to collagen, arachidonic acid, and adenosine diphosphate (ADP) was not significantly different in females with higher vs. lower estrogen levels.

CONCLUSIONS

Behavioral outcomes are improved with females with higher endogenous estrogen levels treated with standard dose antiplatelet loading. This is the first non-rodent study to demonstrate that higher endogenous estrogen levels in female rabbits appear to be neuroprotective in ischemic stroke. This research supports the further study of the effect of endogenous estrogen levels on outcome with standard dose antiplatelet loading in stroke patients not eligible for revascularization therapies.

A Pilot Trial of Low-Dose Intravenous Abciximab and Unfractionated Heparin for Acute Ischemic Stroke: Translating GP IIb/IIIa Receptor Inhibition to Clinical Practice

Thrombolysis remains a mainstay in the treatment of ischemic stroke. While not usually considered in the spectrum of clot lysis, experimental data show that inhibition of the platelet glycoprotein (GP) IIb/IIIa receptor can reduce as well as reverse thrombus formation and improve microvascular flow in stroke models. However, a recent clinical trial of GP IIb/IIIa inhibition in stroke did not demonstrate clinical benefit and was associated with increased hemorrhage. Based on an understanding of the relationship between GP IIb/IIIa receptor inhibition, efficacy and hemorrhage, we hypothesized that a lower dose of abciximab would achieve a favorable range of platelet inhibition and potentially good clinical outcomes. Forty-four patients with suspected large vessel occlusion, who were not eligible for rt-PA were offered treatment with approximately 30% lower total dose of intravenous abciximab if within 6 h for anterior circulation or 24 h for posterior circulation stroke (later modified to 12 h). Concomitant anticoagulation, usually with unfractionated heparin was employed. The extent of platelet inhibition was measured in 21 patients. Hemorrhage rate and 90-day functional outcomes and mortality were obtained. A matching algorithm involving finding the nearest neighbor from individual subjects in the control arm of the NINDS rt-PA database was used to compare outcomes at similar baseline characteristics and gender. Mean platelet inhibition was 92.1 ± 6.7% vs inhibition reported with percutaneous coronary intervention (PCI) of 96 ± 10; p = 0.08. Successful matching to NINDS controls was accomplished: after outlier elimination, median and mean NIHSS of the abciximab subjects compared to NINDS controls was 16.5 vs 15.5 (p = 0.92) and 16.3 vs 16.0 (p = 0.86). Mean age was 67.2 vs 67.1 (p = 0.97). Mean glucose was 141 vs 142 (p = 0.92). There was one symptomatic hemorrhage; minor hemorrhages occurred in 9%. The percent of patients who achieved an mRS 0-2 or died in the treated vs matched NINDS control patients was 63 vs 38 (p = .02) and 23 vs 23 (p = 1.0). Our pilot results indicated that a lower dose of abciximab results in platelet inhibition similar to that achieved in the coronary vascular bed during PCI. Comparison to matched historical controls suggests that this lower dose in combination therapy may be safe and effective therapy for thrombotic stroke and a randomized trial is warranted.

Cilostazol, a phosphodiesterase inhibitor, attenuates photothrombotic focal ischemic brain injury in hypertensive rats

The aim of this study was to evaluate and compare the effects of anti-platelet agents with different modes of action (cilostazol, aspirin, and clopidogrel) on brain infarction produced by photothrombotic middle-cerebral-artery (MCA) occlusion in male, spontaneously hypertensive rats. Cerebral blood flow (CBF) was measured with laser-Doppler flowmetry in the penumbral cortex. Infarct size was evaluated 24 h after MCA occlusion. The effects of these drugs on infarct size were examined by pretreatment of rats undergoing MCA occlusion. Pretreatment with cilostazol (100 mg/kg) significantly reduced infarct size. In contrast, aspirin (10 mg/kg) and clopidogrel (3 mg/kg) failed to mitigate infarct size, regardless of their apparent inhibitory effects on platelet aggregation. Post-treatment with cilostazol also significantly attenuated the infarct size, associated with improved CBF in the penumbral region. In support of this effect, cilostazol increased nitric oxide (NO) production and prostaglandin-I(2) (PGI(2)) release in cultured human brain microvascular endothelial cells. Cilostazol-induced NO production and PGI(2) release were completely abolished by an NO synthase inhibitor and aspirin, respectively. These findings show that cilostazol reduced brain infarct size due to an improvement in penumbral CBF possibly in association with increased endothelial NO and PGI(2) production.

Arterial antithrombotic and bleeding time effects of apixaban, a direct factor Xa inhibitor, in combination with antiplatelet therapy in rabbits

BACKGROUND

Optimal treatment of arterial thrombosis may include a combination of antiplatelet and anticoagulant drugs. We evaluated apixaban, a direct and highly selective factor Xa inhibitor, in combination with clinically relevant doses of aspirin and/or clopidogrel for prevention of arterial thrombosis in rabbits.

METHODS

Studies were conducted in rabbit models of electrically induced carotid artery thrombosis and cuticle bleeding time (BT). Apixaban 0.04 and 0.3 mg kg(-1) h(-1) or aspirin 1 mg kg(-1) h(-1) was infused intravenous (i.v.) continuously from 1 h before artery injury or cuticle bleed until the end of the experiment. Clopidogrel at 3 mg kg(-1) was dosed orally once daily for three days, with the last dose given 2 h before injury.

RESULTS

Control thrombus weight and BT averaged 8.6 +/- 0.9 mg and 181 +/- 12 s, respectively (n = 6 per group). Effective doses of apixaban that reduced thrombus weight by 20 and 50% (ED(20) and ED(50)) were 0.04 and 0.3 mg kg(-1) h(-1) i.v., respectively. Addition of aspirin to apixaban ED(20) and ED(50) significantly reduced the thrombus weight from 7.4 +/- 0.5 to 5.3 +/- 0.3 and 3.6 +/- 0.3 mg, respectively, with no significant increases in BT (190 +/- 7 s vs.181 +/- 9 and 225 +/- 11 s, respectively). Addition of aspirin and apixaban (ED(20) dose) to clopidogrel produced a further significant reduction in thrombus weight from 5.3 +/- 0.3 to 0.7 +/- 0.1 mg. This combination of clopidogrel and aspirin with apixaban (ED(20) dose) produced a significant but moderate BT increase of 2.1 times control.

CONCLUSIONS

The combination of apixaban and aspirin or apixaban, aspirin and clopidogrel can reduce formation of occlusive arterial thrombosis without excessive increases in BT in rabbits.