Enma V. P. Espinosa

The Antidepressant 5-HT2A Receptor Antagonists Pizotifen and Cyproheptadine Inhibit Serotonin-Enhanced Platelet Function

There is considerable interest in defining new agents or targets for antithrombotic purposes. The 5-HT2A receptor is a G-protein coupled receptor (GPCR) expressed on many cell types, and a known therapeutic target for many disease states. This serotonin receptor is also known to regulate platelet function. Thus, in our FDA-approved drug repurposing efforts, we investigated the antiplatelet activity of cyproheptadine and pizotifen, two antidepressant 5-HT2A Receptor antagonists. Our results revealed that cyproheptadine and pizotifen reversed serotonin-enhanced ADP-induced platelet aggregation in vitro and ex vivo. And the inhibitory effects of these two agents were found to be similar to that of EMD 281014, a 5-HT2A Receptor antagonist under development. In separate experiments, our studies revealed that these 5-HT2A receptor antagonists have the capacity to reduce serotonin-enhanced ADP-induced elevation in intracellular calcium levels and tyrosine phosphorylation. Using flow cytometry, we also observed that cyproheptadine, pizotifen, and EMD 281014 inhibited serotonin-enhanced ADP-induced phosphatidylserine (PS) exposure, P-selectin expression, and glycoprotein IIb-IIIa activation. Furthermore, using a carotid artery thrombosis model, these agents prolonged the time for thrombotic occlusion in mice in vivo. Finally, the tail-bleeding time was investigated to assess the effect of cyproheptadine and pizotifen on hemostasis. Our findings indicated prolonged bleeding time in both cyproheptadine- and pizotifen-treated mice. Notably, the increases in occlusion and bleeding times associated with these two agents were comparable to that of EMD 281014, and to clopidogrel, a commonly used antiplatelet drug, again, in a fashion comparable to clopidogrel and EMD 281014. Collectively, our data indicate that the antidepressant 5-HT2A antagonists, cyproheptadine and pizotifen do exert antiplatelet and thromboprotective effects, but similar to clopidogrel and EMD 281014, their use may interfere with normal hemostasis.

Mouse transient receptor potential channel 6: role in hemostasis and thrombogenesis.

Although changes in the intracellular levels of calcium (Ca(2+)) are a central step in platelet activation, the underlying mechanism of Ca(2+) entry is still unclear. Previous studies have demonstrated that TRPC6, a member of the canonical transient receptor potential channel (TRPC) family is expressed in platelets in a significant amount, and is predominantly found on the plasma membrane. Based on these considerations, we hypothesized that TRPC6 plays a critical role in platelet function. To characterize the role of TRPC6 in platelet function in vivo, we employed a genetic approach, subjecting TRPC6 knockout mice to the tail bleeding time test and a carotid artery injury thrombosis model. We found that TRPC6-deficient animals displayed a prolonged bleeding time, and an increased time for occlusion of the injured carotid artery, compared to their wild-type littermates. Taken together, our data demonstrate for the first time, that TRPC6 deletion in mice results in defects in hemostasis and protection against thrombogenesis, suggesting a vital role in platelet function. Furthermore, TRPC6 may define a new therapeutic target for managing multiple thrombosis-based disorders.

Thromboxane A2 Receptor Biology and Function of a Peculiar Receptor that Remains Resistant for Therapeutic Targeting

While blood platelets express several G-protein-coupled receptors (GPCRs) that play pivotal roles in their activation, several diseases, for example thrombotic disorders, may develop if these receptors are inappropriately activated. Thus, these receptors have been the subject of investigations to design therapeutic interventions for managing multiple thrombosis-based disease states. One such GPCR, the thromboxane A2 receptor (TPR), remains resistant to such interventions. The present review provides a critical examination of the binding, structural biology, and signaling of TPRs. The review also provides a rationale for using principles of “drug rediscovery” as an alternative/viable approach for the therapeutic targeting of TPRs. To this end, it is noteworthy that many US Food and Drug Administration (FDA)–approved drugs have been found to selectively (and nonselectively) block TPR-mediated functional responses, for example platelet aggregation, as described in this review. Therefore, while none of the antagonists, thus far developed for targeting TPRs, have made it into clinical use, this peculiar receptor can be antagonized by a large number of drugs used for indications unrelated to thrombosis.

Third-hand Smoke: Impact on Hemostasis and Thrombogenesis.

Abstract: Cigarette smoking is a major risk factor for acute coronary thrombosis. In fact, both active/first-hand smoke and passive/second-hand smoke exposure are known to increase the risk of coronary thrombosis. Although recently a new risk has been identified and termed third-hand smoke (THS), which is the residual tobacco smoke contaminant that remains after a cigarette is extinguished, it remains to be determined whether it can also enhance the risk of thrombogenesis, much like first-hand smoke and second-hand smoke. Therefore, the present studies investigated the impact of THS exposure in the context of platelet biology and related disease states. It was found that THS-exposed mice exhibited an enhanced platelet aggregation and secretion responses as well as enhanced integrin GPIIb-IIIa activation. Furthermore, it was found that THS exposure shortens the tail bleeding time and the occlusion time in a model of thrombosis. Thus, our data demonstrate for the first time (at least in mice) that THS exposure increases the risk of thrombosis-based disease states, which is attributed, at least in part, to their hyperactive platelets.

Aspirin: Pharmacology and Clinical Applications

Antiplatelet therapy has been documented to reduce risks of cardiovascular disease after acute myocardial infarction, coronary artery bypass graft, and in chronic atrial fibrillation patients, amongst other risk factors. Conventional management of thrombosis-based disorders includes the use of heparin, oral anticoagulants, and the preferred antiplatelet agent aspirin. Interestingly, aspirin was not intended to be used as an antiplatelet agent; rather, after being repurposed, it has become one of the most widely prescribed antithrombotic drugs. To this end, there have been several milestones in the development of antiplatelet agents in the last few decades, such as adenosine diphosphate receptor inhibitors, phosphodiesterase inhibitors, and GPIIb/IIIa inhibitors. However, given some of the limitations of these therapies, aspirin continues to play a major role in the management of thrombotic and cardiovascular disorders and is expected to do so for years to come.

Characterization of a Novel Function-Blocking Antibody Targeted Against the Platelet P2Y1 Receptor

Objective— Platelet hyperactivity is associated with vascular disease and contributes to the genesis of thrombotic disorders. ADP plays an important role in platelet activation and activates platelets through 2 G-protein–coupled receptors, the Gq-coupled P2Y1 receptor (P2Y1R), and the Gi-coupled P2Y12 receptor. Although the involvement of the P2Y1R in thrombogenesis is well established, there are no antagonists that are currently available for clinical use. Approach and Results— Our goal is to determine whether a novel antibody targeting the ligand-binding domain, ie, second extracellular loop (EL2) of the P2Y1R (EL2Ab) could inhibit platelet function and protect against thrombogenesis. Our results revealed that the EL2Ab does indeed inhibit ADP-induced platelet aggregation, in a dose-dependent manner. Furthermore, EL2Ab was found to inhibit integrin GPIIb-IIIa activation, dense and &agr; granule secretion, and phosphatidylserine exposure. These inhibitory effects translated into protection against thrombus formation, as evident by a prolonged time for occlusion in a FeCl3-induced thrombosis model, but this was accompanied by a prolonged tail bleeding time. We also observed a dose-dependent displacement of the radiolabeled P2Y1R antagonist [3H]MRS2500 from its ligand-binding site by EL2Ab. Conclusions— Collectively, our findings demonstrate that EL2Ab binds to and exhibits P2Y1R-dependent function-blocking activity in the context of platelets. These results add further evidence for a role of the P2Y1R in thrombosis and validate the concept that targeting it is a relevant alternative or complement to current antiplatelet strategies.

A novel antibody targeting the ligand binding domain of the thromboxane A2 receptor exhibits antithrombotic properties in vivo

In efforts to define new targets for antithrombotic purposes, there is interest in utilizing antibodies targeting ligand binding domains of platelet receptors. To this end, we have recently shown that an antibody (designated C-EL2Ab), which targets the C-terminus of the 2nd extracellular loop (C-EL2) of the thromboxane A(2) receptor (TPR), selectively blocks TPR-mediated platelet aggregation, under both in vitro and ex vivo experimental conditions. In the current studies we sought to determine whether C-EL2Ab exhibits in vivo antithrombotic activity, by employing a carotid artery injury thrombosis model. It was found that mice treated with C-EL2Ab, exhibited a significant increase in time for occlusion, when compared to controls such as normal rabbit IgG, or an antibody which targets a region separate from the ligand binding site (i.e., EL1). We next examined the effect of C-EL2Ab on hemostasis, and found no increase in tail bleeding times in C-EL2Ab treated mice, compared to the aforementioned controls. Collectively, these results clearly demonstrate that C-EL2Ab has anti-platelet/anti-thrombotic effects, and is devoid of increased bleeding risk. Moreover, the identification of a functionally active TPR sequence should significantly aid molecular modeling study predictions for organic derivatives which possess in vivo activity.

Characterization of the In Vivo Antiplatelet Activity of the Antihypertensive Agent Losartan

Objective: The purpose of this study is to investigate the potential in vivo antiplatelet and thromboprotective properties of the antihypertensive drug losartan in mice. Methods: Aggregometry studies were performed on platelets obtained from mice administered losartan for 5 days, via tail vein to examine the ex vivo effects (dose dependence) of this agent and to select an appropriate dose for the in vivo studies. Next, the tail bleeding time test and the time for occlusion in a carotid artery injury thrombosis model (ferric chloride) were also performed to assess the in vivo effects of losartan treatment. Results: These data indicate that the antihypertensive agent losartan exerts dose-dependent inhibition of the thromboxane receptor-mediated (U46619/agonist)-induced platelet aggregation (ex vivo), whereas it produced no detectable effects on aggregation triggered by adenosine diphosphate or the thrombin receptor activating peptide 4. Findings from the in vivo analysis revealed that tail bleeding time of losartan-treated mice was not different from vehicle-treated mice. On the other hand, in the carotid artery injury thrombosis model, it was found that the losartan-treated mice had significantly longer time for occlusion in comparison with those treated with vehicle control. Conclusions: These findings provide evidence that administration of the antihypertensive drug losartan into live mice produces thromboxane A2 receptor–specific antiplatelet effects. Furthermore, interestingly, this antiplatelet activity appears to translate into thromboprotective properties, without resulting in a bleeding phenotype. Consequently, aside from its potential use as an antithrombotic agent, losartan’s chemistry may provide a “blueprint” for designing or repurposing novel derivatives which may have the potential to serve as an antiplatelet and thromboprotective agents but are deprived of the usually concomitant bleeding adverse effects.

The C-terminal segment of the second extracellular loop of the thromboxane A2 receptor plays an important role in platelet aggregation.

There is considerable interest in discovering novel antiplatelet approaches with an enhanced safety profile. To this end, in our efforts to define new targets for antithrombotic activity, we investigated the utility of antibodies which recognize the ligand binding domains of the platelet thromboxane A(2) receptor (TPR). We hypothesized that an antibody (abbreviated as C-EL2Ab), which interacts with the C-terminus of the second extracellular loop (C-EL2; i.e., ligand binding domain) of TPR exhibits antagonistic activity. Our findings demonstrate that C-EL2Ab did indeed inhibit TPR-mediated platelet aggregation. However, it was devoid of any apparent effects on aggregation triggered by ADP or the thrombin receptor activating peptides 1 or 4. Furthermore, results from radiolabeled ligand binding studies indicate that C-EL2Ab competitively displaced the classical TPR antagonist [(3)H]SQ29,548 from its binding sites. On the other hand, control experiments indicated that normal rabbit IgG and an antibody which targets a TPR domain separate from those involved in ligand recognition, failed to inhibit aggregation in response to TPR activation. Collectively, these findings demonstrate that C-EL2 of TPR plays a critical role in platelet activation, and establish C-EL2Ab as a function blocking antibody. Furthermore, our data suggest a potential for the therapeutic application of C-EL2Ab, which may serve either as an alternative to, or a complement for current treatments. Finally, the identification of a functionally active TPR sequence should aid molecular modeling study predictions for organic derivatives which possess in vivo activity.