Significant Improvement of Antithrombotic Responses to Clopidogrel by Use of a Novel Conjugate as Revealed in an Arterial Model of Thrombosis

Evaluation of the Efficacy and Safety of Antiplatelet Therapeutics in Rabbits

Hemostasis is a multifactorial process that involves vasoconstriction of blood vessels, activation of the coagulation cascade, and platelet aggregation. Inappropriate activation of hemostatic processes can result in thrombosis and tissue ischemia. In patients at risk for thrombotic events, antiplatelet therapeutic agents inhibit platelet activation, thereby reducing the incidence of pathologic clot formation. Platelets are activated by several endogenous chemical mediators, including adenosine diphosphate, thrombin, and thromboxane. These activation pathways serve as attractive drug targets. The protocols described in this article are designed to evaluate the preclinical efficacy and safety of novel antiplatelet therapeutics in rabbits. Here, we provide two protocols for blood collection, two for determining platelet activation, and one for assessing bleeding safety. Together, these protocols can be used to characterize the efficacy and safety of antiplatelet agents for hemostasis. © 2023 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Blood collection via the central ear artery Alternative Protocol 1: Blood collection via the jugular vein Basic Protocol 2: Platelet aggregation assessment via light transmission aggregometry Alternative Protocol 2: Platelet activation assessment via flow cytometry Basic Protocol 3: Determination of tongue bleeding time.

Pleiotropic effects of clopidogrel

Clopidogrel is a widely prescribed prodrug with anti-thrombotic activity through irreversible inhibition of the P2Y₁₂ receptor on platelets. It is FDA-approved for the clinical management of thrombotic diseases like unstable angina, myocardial infarction, stroke, and during percutaneous coronary interventions. Hepatic clopidogrel metabolism generates several distinct metabolites. Only one of these metabolites is responsible for inhibiting the platelet P2Y₁₂ receptor. Importantly, various non-hemostatic effects of clopidogrel therapy have been described. These non-hemostatic effects are perhaps unsurprising, as P2Y₁₂ receptor expression has been reported in multiple tissues, including osteoblasts, leukocytes, as well as vascular endothelium and smooth muscle. While the "inactive" metabolites have been commonly thought to be biologically inert, recent findings have uncovered P2Y₁₂ receptor-independent effects of clopidogrel treatment that may be mediated by understudied metabolites. In this review, we summarize both the P2Y₁₂ receptor-mediated and non-P2Y₁₂ receptor-mediated effects of clopidogrel and its metabolites in various tissues.

Clopidogrel treatment inhibits P2Y2-Mediated constriction in the rabbit middle cerebral artery

Clopidogrel is an effective purinergic 2Y12 receptor (P2Y12) antagonist used to prevent arterial thrombosis, but its use is associated with adverse bleeding. Clinical studies have demonstrated that clopidogrel users have an increased risk of cerebral microbleeds and intracerebral hemorrhage. Our previous studies suggest that non-platelet mechanisms mediate these adverse bleeding events; we hypothesize that clopidogrel or one of its metabolites interacts with blood vessels directly to cause bleeding. New Zealand white rabbits (1.9-2.7 kg) were treated orally with vehicle or clopidogrel (3 or 10 mg/kg) for three days. On the fourth day, the rabbits were anesthetized for blood collection and then euthanized. The brain was collected, and the middle cerebral arteries were isolated. We used light transmission aggregometry and pressure myography to elucidate the mechanisms of the off-target effects associated with clopidogrel treatment. We confirmed that inhibition of P2Y12 activation by clopidogrel inhibited ADP-induced platelet aggregation but had no impact on P2Y12-independent arachidonic acid- or collagen-induced platelet aggregation. Analysis of middle cerebral arteries from clopidogrel treated rabbits showed that clopidogrel did not affect P2Y4, P2Y6, and P2Y14 receptor-mediated contraction but attenuated the contractile response after P2Y2 receptor activation. Further analysis determined P2Y2-mediated constriction was endothelium-dependent. Vasoconstriction is a primary component of hemostasis, and impaired vasoconstriction can prolong bleeding. These results suggest clopidogrel inhibits the endothelial P2Y2 receptor in the middle cerebral artery, which provides a mechanistic explanation for the adverse cerebral bleeding associated with the drug.

Microglia enable mature perineuronal nets disassembly upon anesthetic ketamine exposure or 60-Hz light entrainment in the healthy brain

Perineuronal nets (PNNs), components of the extracellular matrix, preferentially coat parvalbumin-positive interneurons and constrain critical-period plasticity in the adult cerebral cortex. Current strategies to remove PNN are long-lasting, invasive, and trigger neuropsychiatric symptoms. Here, we apply repeated anesthetic ketamine as a method with minimal behavioral effect. We find that this paradigm strongly reduces PNN coating in the healthy adult brain and promotes juvenile-like plasticity. Microglia are critically involved in PNN loss because they engage with parvalbumin-positive neurons in their defined cortical layer. We identify external 60-Hz light-flickering entrainment to recapitulate microglia-mediated PNN removal. Importantly, 40-Hz frequency, which is known to remove amyloid plaques, does not induce PNN loss, suggesting microglia might functionally tune to distinct brain frequencies. Thus, our 60-Hz light-entrainment strategy provides an alternative form of PNN intervention in the healthy adult brain.

Clopidogrel Resistance in a Murine Model of Diet-Induced Obesity Is Mediated by the Interleukin-1 Receptor and Overcome With DT-678

OBJECTIVE

Clopidogrel is a commonly used P2Y₁₂ inhibitor to treat and prevent arterial thrombotic events. Clopidogrel is a prodrug that requires bioactivation by CYP (cytochrome P450) enzymes to exert antiplatelet activity. Diabetes mellitus is associated with an increased risk of ischemic events, and impaired ability to generate the active metabolite (AM) from clopidogrel. The objective of this study is to identify the mechanism of clopidogrel resistance in a murine model of diet-induced obesity (DIO). Approach and Results: C57BL/6J mice and IL-1R^-/- mice were given high-fat diet for 10 weeks to generate a murine model of diet-induced obesity. Platelet aggregation and carotid arterial thrombosis were assessed in response to clopidogrel treatment. Wild-type DIO mice exhibited resistance to antiplatelet and antithrombotic effects of clopidogrel that was associated with reduced hepatic expression of CYP genes and reduced generation of the AM. IL (Interleukin)-1 receptor-deficient DIO (IL1R^-/- DIO) mice showed no resistance to clopidogrel. Lack of resistance was accompanied by increased exposure of the clopidogrel AM. This resistance was also absent when wild-type DIO mice were treated with the conjugate of the clopidogrel AM, DT-678.

CONCLUSIONS

These findings indicate that antiplatelet effects of clopidogrel may be impaired in the setting of diabetes mellitus due to reduced prodrug bioactivation related to IL-1 receptor signaling. Therapeutic targeting of P2Y₁₂ in patients with diabetes mellitus using the conjugate of clopidogrel AM may lead to improved outcomes.

DT-678 inhibits platelet activation with lower tendency for bleeding compared to existing P2Y12 antagonists

The novel clopidogrel conjugate, DT-678, is an effective inhibitor of platelets and thrombosis in preclinical studies. However, a comparison of the bleeding risk with DT-678 and currently approved P2Y12 antagonists has yet to be determined. The objective of this study was to evaluate the bleeding tendency of animals treated with clopidogrel, ticagrelor, and DT-678. Ninety-one New Zealand white rabbits were randomized to one of 13 treatment groups (n = 7). Platelet activation was assessed by flow cytometry and light transmission aggregometry before and after the administration of various doses of DT-678, clopidogrel, and ticagrelor. Tongue template bleeding times were also measured before and after drug treatment. Treatment with P2Y12 receptor antagonists caused a dose-dependent reduction in markers of platelet activation (P-selectin and integrin αIIbβ3) and aggregation in response to adenosine diphosphate stimulation. At the same doses required for platelet inhibition, clopidogrel and ticagrelor significantly prolonged bleeding times, while DT-678 did not. DT-678 and the FDA-approved P2Y12 antagonists clopidogrel and ticagrelor are effective inhibitors of platelet activation and aggregation. However, unlike clopidogrel and ticagrelor, DT-678 did not prolong bleeding times at equally effective antiplatelet doses. The results suggest a more favorable benefit/risk ratio for DT-678 and potential utility as part of a dual antiplatelet therapy regimen.

Acute thrombus formation on phosphorilcholine surface modified flow diverters

Purpose

Thromboembolic complications remain a limitation of flow diverting stents. We hypothesize that phosphorilcholine surface modified flow diverters (Pipeline Flex with Shield Technology, sPED) would have less acute thrombus formation on the device surface compared with the classic Pipeline Embolization device (cPED).

Methods

Elastase-induced aneurysms were created in 40 rabbits and randomly assigned to receive cPED or sPED devices with and without dual antiplatelet therapy (DAPT) (four groups, n=10/group). Angioplasty was performed to enhance apposition and create intimal injury for a pro-thrombotic environment. Both before and after angioplasty, the flow diverter was imaged with intravascular optical coherence tomography. The outcome measure was the number of predefined segments along the implant relative to the location of the aneurysm with a minimum of 0 (no clot formation) and maximum of 3 (all segments with thrombus). Clot formation over the device at ostia of branch arteries was assessed as either present or absent.

Results

Following angioplasty, the number of flow diverter segments with clots was significantly associated with the flow diverter (p<0.0001), but not with DAPT (p=0.3872) or aneurysm neck size (p=0.8555). The incidence rate for clots with cPED was 1.72 times more than with sPED. The clots on the flow diverter at the location corresponding to side branch ostia was significantly lower with sPED than with cPED (OR 0.180; 95% CI 0.044 to 0.734; p=0.0168), but was not associated with DAPT (p=0.3198).

Conclusion

In the rabbit model, phosphorilcholine surface modified flow diverters are associated with less thrombus formation on the surface of the device.