Antithrombotic Effects of the Novel Small-Molecule Factor XIa Inhibitor Milvexian in a Rabbit Arteriovenous Shunt Model of Venous Thrombosis

Background  Factor XIa (FXIa) is an emerging therapeutic target, and FXIa inhibition is a promising mechanism to improve therapeutic index over current anticoagulants. Milvexian (BMS-986177/JNJ-70033093) is an oral small-molecule FXIa inhibitor. Objective  Milvexian's antithrombotic efficacy was characterized in a rabbit arteriovenous (AV) shunt model of venous thrombosis and compared with the factor Xa inhibitor apixaban and the direct thrombin inhibitor dabigatran. Methods  The AV shunt model of thrombosis was conducted in anesthetized rabbits. Vehicle or drugs were administered as intravenous bolus plus a continuous infusion. Thrombus weight was the primary efficacy endpoint. Ex vivo activated partial thromboplastin time (aPTT), prothrombin time (PT), and thrombin time (TT) were measured as the pharmacodynamic responses. Results  Milvexian dose dependently reduced thrombus weights by 34.3 ± 7.9, 51.6 ± 6.8 ( p  < 0.01; n  = 5), and 66.9 ± 4.8% ( p  < 0.001; n  = 6) versus vehicle at 0.25 + 0.17, 1.0 + 0.67, and 4.0 ± 2.68 mg/kg bolus + mg/kg/h infusion, respectively. Ex vivo clotting data supported a dose-dependent prolongation of aPTT (with 1.54-, 2.23-, and 3.12-fold increases from baseline upon the AV shunt start), but no changes in PT and TT. Dose-dependent inhibition in thrombus weight and clotting assays was also demonstrated for both apixaban and dabigatran as the references for the model validation. Conclusion  Results demonstrate that milvexian is an effective anticoagulant for prevention of venous thrombosis in the rabbit model, which supports the utility of milvexian in venous thrombosis, as seen in the phase 2 clinical study.

Differential impact of milvexian, an investigational oral FXIa inhibitor vs apixaban and dabigatran on thrombin generation in human donor plasma in vitro

Introduction

All approved anticoagulants including the direct oral anticoagulants have been associated with an increased risk of bleeding in a dose-dependent manner when used either as monotherapy and/or on top of antiplatelet drugs. Literature reports have suggested that the extrinsic pathway of coagulation is important for haemostasis while the intrinsic pathway of coagulation along with thrombin feedback activation of FXI are primarily responsible for pathological thrombus formation and propagation.

Recent clinical trials with several FXI/FXIa inhibitors in development including milvexian (BMS-986177/ JNJ-70033093) have demonstrated robust antithrombic efficacy without any increase in bleeding compared with standard of care in the settings of total knee arthroplasty. Milvexian inhibits FXIa upstream in the intrinsic pathway of coagulation while approved anticoagulants including warfarin, heparin, bivalirudin, and DOACs inhibit FXa and/or thrombin, which are components of the final common pathway of coagulation that lies in the interface of the intrinsic and extrinsic pathways.

Purpose

To evaluate the impact of milvexian, apixaban and dabigatran on thrombin generation (TG) initiated by the intrinsic vs extrinsic pathway activators in human plasma in vitro.

Methods

Platelet-poor plasma prepared from citrated whole blood collected from consenting, healthy adult volunteers (n=6) was spiked with 0.1–10 μM milvexian, apixaban, or dabigatran. TG via the extrinsic pathway (5 pM tissue factor, PPP Reagent, Stago) and intrinsic pathway (1:100 dilution of STA®-PTT Automate) was measured using using the Thrombinoscope method (Stago) and included peak thrombin, lag time and the endogenous thrombin potential (ETP).

Results

All compounds tested demonstrated robust (&gt;70%) to complete inhibition of the ETP initiated by the intrinsic pathway activator (STA®-PTT Automate reagent, Fig. 1A). Apixaban and dabigatran also completely inhibited ETP initiated by the extrinsic pathway activator (PPP Reagent, Fig. 1B). In contrast, milvexian only had a modest inhibition (&lt;20%) of ETP when initiated by the extrinsic pathway activator.

Conclusions

By inhibiting the final common pathway of coagulation, apixaban and dabigatran inhibit TG initiated via both, intrinsic and extrinsic pathways, in a concentration-dependent manner. In contrast, by inhibiting FXIa upstream in the intrinsic pathway, milvexian predominantly inhibits TG initiated by the intrinsic pathway activator but only has a minimal impact on TG initiated by the extrinsic pathway that is deemed critical for haemostasis. These findings with milvexian are consistent with observations of TG in FXI immunodepleted plasma reported previously by our group (ISTH 2021). The potential haemostasis sparing effect of milvexian and perhaps of other FXI/FXIa inhibitors may explain in part the low rate of bleeding observed to date with this new class of experimental anticoagulant drugs in clinical development.

Funding Acknowledgement

Type of funding sources: Private company. Main funding source(s): Janssen Research & Development

Discovery of Potent and Orally Bioavailable Pyridine N-Oxide-Based Factor XIa Inhibitors through Exploiting Nonclassical Interactions

Activated factor XI (FXIa) inhibitors are promising novel anticoagulants with low bleeding risk compared with current anticoagulants. The discovery of potent FXIa inhibitors with good oral bioavailability has been challenging. Herein, we describe our discovery effort, utilizing nonclassical interactions to improve potency, cellular permeability, and oral bioavailability by enhancing the binding while reducing polar atoms. Beginning with literature-inspired pyridine N-oxide-based FXIa inhibitor 1, the imidazole linker was first replaced with a pyrazole moiety to establish a polar C-H···water hydrogen-bonding interaction. Then, structure-based drug design was employed to modify lead molecule 2d in the P1' and P2' regions, with substituents interacting with key residues through various nonclassical interactions. As a result, a potent FXIa inhibitor 3f (K_i = 0.17 nM) was discovered. This compound demonstrated oral bioavailability in preclinical species (rat 36.4%, dog 80.5%, and monkey 43.0%) and displayed a dose-dependent antithrombotic effect in a rabbit arteriovenous shunt model of thrombosis.

Comparison of the sensitivity of commercial aPTT tests in measurement of the pharmacodynamic response of milvexian, a novel FXIa inhibitor

Introduction

The activated partial thromboplastin time (aPTT) assay is a test routinely used to evaluate abnormalities or deficiencies in coagulation factors of the intrinsic and common pathways. The composition of the surface activator and phospholipids in the aPTT reagent is known to influence the response of heparin and direct FXa or thrombin inhibitors.

Milvexian (formerly referred to as BMS-986177/JNJ-70033093) is an investigational small-molecule Factor XIa (FXIa) inhibitor being studied for the prevention and treatment of major thrombotic conditions. Clinical pharmacology evaluation of milvexian includes aPTT as the primary pharmacodynamic assay, thus underscoring the need for a sensitive reagent to evaluate its anticoagulant activity.

Purpose

This study evaluated the sensitivity of six commercially available aPTT reagents in plasma from individual donors spiked with milvexian, at concentrations spanning the anticipated clinically relevant exposures.

Methods

Platelet-poor plasma (PPP) prepared from citrated whole blood collected from consenting, healthy adult volunteers (n=12) was spiked with vehicle (DMSO, 0.5% v/v) or 0.1–10 μM milvexian. The aPTT was measured using six commercially available aPTT reagent kits which included silica; kaolin, or ellagic acid as the contact activator, in combination with natural or synthetic phospholipids. The coagulation automated analysers used for testing were matched to the kit's manufacturer recommendations. All reagents and respective normal or abnormal controls were prepared as instructed by the manufacturer.

Results

Milvexian exhibited dose-dependent prolongation of aPTT with all reagents tested. Assays performed with aPTT reagents containing kaolin or ellagic acid demonstrated the highest sensitivity, as measured by the concentration that achieved a 2-fold aPTT prolongation (EC2x), and showed the widest dynamic range of response. Coefficient of variability of aPTT measurements in plasma from 12 individual donors was between 5.6–7.9% for Dade® Actin® FS, and 7.1–8% for STA®-C.K. Prest® 5.

Conclusions

Prolongation of aPTT in PPP spiked with milvexian exhibited a dose-dependent relationship, with statistically significant differences observed among reagents at milvexian concentrations above 0.3 μM. The highest sensitivity measured by changes in the ratio to baseline was obtained with aPTT reagents containing an activator/procoagulant phospholipid combination of ellagic acid + purified soy phosphatides or kaolin + cephalin, which performed similarly. Identification of the reagents with the best combination of sensitivity, precision, and dynamic range may help guide the selection of reagent for assessing milvexian activity.

Funding Acknowledgement

Type of funding sources: Private company. Main funding source(s): Janssen Research & Development, LLC

Translational PK/PD and model-informed development of JNJ-67842125, a Fab reversal agent for JNJ-64179375, a long-acting thrombin inhibitor

BACKGROUND AND PURPOSE

Antigen-binding fragment (F_ab ) reversal agents were developed to reverse, in bleeding emergency, the long-acting anticoagulant effect of JNJ-64179375 (JNJ-9375), a monoclonal antibody that binds exosite-1 on thrombin.

EXPERIMENTAL APPROACH

The pharmacokinetic and pharmacodynamic (PK/PD) activities of three reversal agents of varying in vitro binding affinities to JNJ-9375 were characterised in cynomolgus monkeys. The time course of JNJ-9375 anticoagulant activity and reversal effects of each agent were evaluated. A mechanism-based PK/PD model, which integrated free serum concentrations of reversal agent, total and free serum concentrations of JNJ-9375, and thrombin time, was developed to quantitatively relate JNJ-9375 neutralisation to reversal of induced thrombin time prolongation. Model-based allometric scale-up of the lead reversal agent and the PK/PD relationship of JNJ-9375 in healthy volunteers were utilised to predict clinical dosing regimens.

KEY RESULTS

Lowering of free JNJ-9375 by the reversal agents corresponded with reversal of thrombin time prolongation. Total JNJ-9375 displayed typical mAb clearance at 2.75 ml·day^-1 ·kg^-1 , whereas reversal agents cleared faster between 1400 and 2400 ml·day^-1 ·kg^-1 . The model-estimated in vivo K_D values for JNJ-9375 reversal agents were 9 nM (ICHB-256), 0.4 nM (ICHB-281) and 13.7 pM (ICHB-164), in rank-ordered agreement of their K_D values determined in vitro. The three reversal agents exhibited different neutralisation characteristics in vivo, governed primarily by their binding kinetics to JNJ-9375. The model predicted a priori free JNJ-9375 kinetics after dosing ICHB-164 (JNJ-67842125) and JNJ-9375 under a different regimen.

CONCLUSION AND IMPLICATIONS

The results enabled selection of JNJ-67842125 as the reversal agent for JNJ-9375.

Abstract 521: Pharmacological Profile of JNJ-64179375: a Novel, Long Acting Exosite-1 Thrombin Inhibitor

Background: Thrombin is a key serine protease involved in hemostasis and thrombosis by mediating the conversion of soluble fibrinogen to insoluble fibrin that forms the meshwork of a clot. In addition, it is the most potent activator of platelets. Hence thrombin inhibition has been a target for the development of anticoagulant drugs. Recently, an IgA that binds to the exosite 1 region of thrombin was identified in a patient that was profoundly anticoagulated but without any abnormal bleeding episodes over a prolonged follow-up. JNJ-64179375 (JNJ-9375) is a monoclonal antibody derivative of the IgA which by selectively inhibiting exosite 1 may allow for some of thrombin functions mediated by exosite 2 and the active site which might be important for hemostasis. JNJ-9375 is being developed as a new long-acting anti-coagulant with a potential for better efficacy with a reduced bleeding liability.

Objectives: To assess the in vitro effects of spiked JNJ-9375 on various clotting assays in human and animal plasma and to evaluate the antithrombotic activity in a rat A-V shunt model of thrombosis.

Methods and results: JNJ-9375 produced a concentration-dependent prolongation of thrombin time (TT), ecarin clotting time (ECT), prothrombin time (PT), and activated partial thromboplastin time (aPTT) when added to normal human plasma. The concentrations required to prolong clotting time by twofold (EC2Xs) were 4.4, 12.4, 202.7, and 172.6 μg/ml for TT, ECT, aPTT, and PT respectively. Therefore, TT was the most sensitive assay to show pharmacodynamic activity of JNJ-9375. Similar effects were observed in other species studied including monkey, rat, and mouse. In the rat A-V shunt model of thrombosis, JNJ-9375 dose dependently inhibited thrombus weight with an efficacious dose of 0.3 mg/kg, which was accompanied by prolongation of TT, ECT, PT, and aPTT from ex vivo plasma samples.

Conclusion: Results from these studies demonstrate that selective inhibition of exosite 1 on thrombin leads to robust anticoagulation and antithrombotic efficacy. JNJ-9375 is currently in early clinical development.

A novel approach to assess the spontaneous gastrointestinal bleeding risk of antithrombotic agents using Apc min/+ mice

Assessment of the bleeding risk of antithrombotic agents is usually performed in healthy animals with some form of vascular injury to peripheral organs to induce bleeding. However, bleeding observed in patients with currently marketed antithrombotic drugs is typically spontaneous in nature such as intracranial haemorrhage (ICH) and gastrointestinal (GI) bleeding, which happens most frequently on top of preexisting pathologies such as GI ulcerations and polyps. Apc min/+ mice are reported to develop multiple adenomas through the entire intestinal tract and display progressive anaemia. In this study, we evaluated the potential utility of Apc min/+ mice as a model for assessing spontaneous GI bleeding with antithrombotic agents. Apc min/+ mice exhibited progressive blood loss starting at the age of nine weeks. Despite the increase in bleeding, Apc min/+ mice were in a hypercoagulable state and displayed an age-dependent increase in thrombin generation and circulating fibrinogen as well as a significant decrease in clotting times. We evaluated the effect of warfarin, dabigatran etexilate, apixaban and clopidogrel in this model by administering them in diet or in the drinking water to mice for 1–4 weeks. All of these marketed drugs significantly increased GI bleeding in Apc min/+ mice, but not in wild-type mice. Although different exposure profiles of these antithrombotic agents make it challenging to compare the bleeding risk of compounds, our results indicate that the Apc min/+ mouse may be a sensitive preclinical model for assessing the spontaneous GI bleeding risk of novel antithrombotic agents.

Platelet transfusion reverses bleeding evoked by triple anti-platelet therapy including vorapaxar, a novel platelet thrombin receptor antagonist

Vorapaxar is a novel protease-activated receptor-1 (PAR-1) antagonist recently approved for the reduction of thrombotic cardiovascular events in patients with a history of myocardial infarction or with peripheral arterial disease. Patients who received vorapaxar in addition to standard of care antiplatelet therapy had an increased incidence of major bleeding events compared with placebo. To assess whether platelet transfusion can restore hemostasis in primates on triple antiplatelet therapy, template bleeding times were assessed concurrently in the buccal mucosa, finger pad, and distolateral tail of anesthetized cynomolgus macaques to evaluate bleeding with vorapaxar as either monotherapy or in combination with aspirin or aspirin and clopidogrel. Aspirin (5mg/kg, IV) or vorapaxar (1mg/kg, PO) alone had no significant effect on bleeding times in the three vascular beds examined. A modest (<2-fold) increase in bleeding time was achieved in the three beds with the dual combination of aspirin and vorapaxar. Major increases in bleeding time were achieved in the three beds with the triple combination of aspirin (5mg/kg, IV), vorapaxar (1mg/kg, PO), and clopidogrel (1mg/kg, PO). Transfusion of fresh human platelet rich plasma, but not platelet poor plasma, reversed the increase in bleeding time in the triple therapy group. Transfusion of human platelets may be a viable approach in situations requiring a rapid reversal of platelet function in individuals treated with triple anti-platelet therapy that includes vorapaxar.

Himbacine-derived thrombin receptor antagonists: c7-spirocyclic analogues of vorapaxar

We have synthesized several C7-spirocyclic analogues of vorapaxar and evaluated their in vitro activities against PAR-1 receptor. Some of these analogues showed activities and rat plasma levels comparable to vorapaxar. Compound 5c from this series showed excellent PAR-1 activity (K i = 5.1 nM). We also present a model of these spirocyclic compounds docked to the PAR-1 receptor based on the X-ray crystal structure of vorapaxar bound to PAR-1 receptor. This model explains some of the structure-activity relationships in this series.

Zucker Diabetic Fatty rats exhibit hypercoagulability and accelerated thrombus formation in the Arterio-Venous shunt model of thrombosis

INTRODUCTION

Diabetes is a significant risk factor for thrombosis. The present study aimed at assessing coagulability, platelet reactivity, and thrombogenicity of the diabetic female Zucker Diabetic Fatty (ZDF) rat model and its relevance in studying antithrombotic mechanisms.

MATERIALS AND METHODS

The basal coagulant state in ZDF rats was evaluated by clotting times, thromboelastography, and thrombin generation assay. A 14-day treatment with dapagliflozin in ZDF rats was pursued to investigate if glycemic control can improve coagulability. Thrombus formation in the Arterio-Venous (A-V) shunt model and the FeCl3-induced arterial thrombosis model was studied, with the antithrombotic effect of apixaban in the former model further investigated.

RESULTS

ZDF rats exhibited significantly shortened clotting times, enhanced thrombin generation, and decreased fibrinolysis at baseline. Effective glycemic control achieved with dapagliflozin did not improve any of these parameters. ZDF rats displayed accelerated thrombus formation and were amenable to apixaban treatment in the A-V shunt model albeit with less sensitivity than normal rats. ZDF rats exhibited less platelet aggregation in response to ADP, collagen and PAR-4, and attenuated thrombotic response in the FeCl3 model.

CONCLUSIONS

ZDF rats are at a chronic hypercoagulable and hypofibrinolytic state yet with compromised platelet reactivity. They display accelerated and attenuated thrombosis in the A-V shunt and FeCl3 model of thrombosis, respectively. Results shed new light on the pathophysiology of the ZDF rat model and illustrate its potential value in translational research on anticoagulant agents in diabetics. Caution needs to be exerted in utilizing this model in assessing antiplatelet mechanisms in diabetes-associated atherothrombosis.

Himbacine-derived thrombin receptor antagonists: c7-aminomethyl and c9a-hydroxy analogues of vorapaxar

We have synthesized several C7-aminomethyl analogues of vorapaxar that are potent PAR-1 antagonists. Many of these analogues showed excellent in vitro binding affinity and pharmacokinetics profile in rats. Compound 6a from this series showed excellent PAR-1 activity (K i = 5 nM). We have also synthesized a C9a-hydroxy analogue of vorapaxar, which showed very good PAR-1 affinity (K i = 19.5 nM) along with excellent rat pharmacokinetic profile and ex vivo efficacy in the cynomolgus monkey.

Inhibition of protease-activated receptor 1 ameliorates intestinal radiation mucositis in a preclinical rat model

PURPOSE

To determine, using a specific small-molecule inhibitor of protease-activated receptor 1 (PAR1) signaling, whether the beneficial effect of thrombin inhibition on radiation enteropathy development is due to inhibition of blood clotting or to cellular (PAR1-mediated) thrombin effects.

METHODS AND MATERIALS

Rats underwent fractionated X-irradiation (5 Gy×9) of a 4-cm small-bowel segment. Early radiation toxicity was evaluated in rats receiving PAR1 inhibitor (SCH602539, 0, 10, or 15 mg/kg/d) from 1 day before to 2 weeks after the end of irradiation. The effect of PAR1 inhibition on development of chronic intestinal radiation fibrosis was evaluated in animals receiving SCH602539 (0, 15, or 30 mg/kg/d) until 2 weeks after irradiation, or continuously until termination of the experiment 26 weeks after irradiation.

RESULTS

Blockade of PAR1 ameliorated early intestinal toxicity, with reduced overall intestinal radiation injury (P=.002), number of myeloperoxidase-positive (P=.03) and proliferating cell nuclear antigen-positive (P=.04) cells, and collagen III accumulation (P=.005). In contrast, there was no difference in delayed radiation enteropathy in either the 2- or 26-week administration groups.

CONCLUSION

Pharmacological blockade of PAR1 seems to reduce early radiation mucositis but does not affect the level of delayed intestinal radiation fibrosis. Early radiation enteropathy is related to activation of cellular thrombin receptors, whereas platelet activation or fibrin formation may play a greater role in the development of delayed toxicity. Because of the favorable side-effect profile, PAR1 blockade should be further explored as a method to ameliorate acute intestinal radiation toxicity in patients undergoing radiotherapy for cancer and to protect first responders and rescue personnel in radiologic/nuclear emergencies.

Abstract 264: Differential Profiles of Thrombin Inhibitors (Dabigatran, Hirudin, Bivalirudin and Heparin) in the Thrombin Generation Assay (TGA) and Thromboelastography (TEG) in Vitro

Thrombin is a central enzyme in haemostasis and thrombosis, and a proven target for anticoagulant therapies. Different classes of thrombin inhibitors, while exerting therapeutic benefits in most clinical trials, have different indications, dosing regimens, and bleeding complications. To gain more insight into the underlying mechanisms for their differential clinical profiles, we compared four marketed and representative agents, including dabigatran, hirudin, bivalirudin (direct thrombin inhibitors, DTIs), and heparin (an indirect thrombin inhibitor), in two in vitro spike-in assays with concentration titrations covering their therapeutic ranges. The two assays were the Thrombinoscope TGA with plasma, triggered by low tissue factor (1 nM TF), and TEG with whole blood, triggered by 1:8000 Recombiplastin (equivalent to low TF), with or without a threshold level of tPA to induce fibrinolysis. In TGA, the largest effect was prolongation of lag time, with the potency of the three DTIs rank-ordered as hirudin>dabigatran>bivalirudin; regarding peak, slope, and ETP, while complete inhibition was achieved with 1-2 μM dabigatran or hirudin, bivalirudin had no effect even at 4 μM, possibly due to its short half life in plasma. In TEG, the three DTIs prolonged clotting time (R) in the same rank order as TGA; for clot strength (MA), while all four agents reduced MA in synergy with tPA, only hirudin reduced MA without tPA, likely due to its highest potency. With tPA-induced fibrinolytic activity (Ly30), dabigatran and bivalirudin enhanced Ly30 (dabigatran>bivalirudin), but hirudin and heparin did not. This contrast might involve differential access to clot-bound thrombin. Heparin had a steep dose-response curve for both lag time in TGA and R in TEG, which is in line with its very narrow therapeutic index. All three DTIs, but not heparin, displayed the previously reported paradoxical increase in peak and slope in TGA in the low concentration range, suggesting this is indeed a class effect of DTI. In summary, our observations highlight the distinct features of each agent in thrombin generation, coagulation, and fibrinolysis. These results in combination with known clinical properties are informative on efforts to define the optimal profiles of new anticoagulants.

Discovery of SCH 900271, a Potent Nicotinic Acid Receptor Agonist for the Treatment of Dyslipidemia

Structure-guided optimization of a series of C-5 alkyl substituents led to the discovery of a potent nicotinic acid receptor agonist SCH 900271 (33) with an EC50 of 2 nM in the hu-GPR109a assay. Compound 33 demonstrated good oral bioavailability in all species. Compound 33 exhibited dose-dependent inhibition of plasma free fatty acid (FFA) with 50% FFA reduction at 1.0 mg/kg in fasted male beagle dogs. Compound 33 had no overt signs of flushing at doses up to 10 mg/kg with an improved therapeutic window to flushing as compared to nicotinic acid. Compound 33 was evaluated in human clinical trials.

Discovery of a potent nicotinic Acid receptor agonist for the treatment of dyslipidemia

Nicotinic acid has been used clinically for decades to control serum lipoproteins. Nicotinic acid lowers very low-density lipoprotein (VLDL)-cholesterol, low-density lipoprotein (LDL)-cholesterol, and lipoprotein-a (LPa), and it is also effective in raising high-density lipoprotein (HDL)-cholesterol. However, nicotinic acid has several side effects in clinical use. The most notable is intense cutaneous vasodilation "flushing" on the upper body and face. We discovered a pyranopyrimidinedione series to be nicotinic acid receptor agonists. A potent nicotinic acid receptor agonist from this series {5-(3-cyclopropylpropyl)-2-(difluoromethyl)-3H-pyrano[2,3-d]pyrimidine-4,7-dione}with reduced flushing side effect in dogs was identified.

SCH 602539, a protease-activated receptor-1 antagonist, inhibits thrombosis alone and in combination with cangrelor in a Folts model of arterial thrombosis in cynomolgus monkeys

OBJECTIVE

To determine the antithrombotic effects of SCH 602539, an analog of the selective protease-activated receptor (PAR)-1 antagonist vorapaxar (formerly SCH 530348) currently in advanced clinical development, and the P2Y(12) ADP receptor antagonist cangrelor, alone and in combination.

METHODS AND RESULTS

Multiple platelet activation pathways contribute to thrombosis. The effects of SCH 602539 and cangrelor alone and in combination on cyclic flow reductions were evaluated in a Folts model of thrombosis in cynomolgus monkeys. The effects of these treatments on ex vivo platelet aggregation and coagulation parameters were also monitored. Dose-dependent inhibition of cyclic flow reductions was observed after treatment with SCH 602539 alone and cangrelor alone (P<0.05 versus vehicle for the 2 highest concentrations of each agent). The combination of SCH 602539 and cangrelor was associated with synergistic antithrombotic effects (P<0.05 versus vehicle for all combinations tested). The 2 highest doses of SCH 602539 inhibited platelet aggregation in response to PAR-1-selective high-affinity thrombin receptor agonist peptide by greater than 80% but did not affect platelet aggregation induced by other agonists; also, they did not affect any coagulation parameters.

CONCLUSIONS

The combined inhibition of the PAR-1 and the P2Y(12) ADP platelet activation pathways had synergistic antithrombotic and antiplatelet effects. The addition of a PAR-1 antagonist to a P2Y(12) ADP receptor antagonist may provide incremental clinical benefits in patients with atherothrombotic disease, both in short- and long-term settings. These hypotheses need to be tested clinically.

Heterotricyclic himbacine analogs as potent, orally active thrombin receptor (protease activated receptor-1) antagonists

Pursuing our earlier efforts in the himbacine-based thrombin receptor antagonist area, we have synthesized a series of compounds that incorporate heteroatoms in the C-ring of the tricyclic motif. This effort has resulted in the identification of several potent heterocyclic analogs with excellent affinity for the thrombin receptor. Several of these compounds demonstrated robust inhibition of platelet aggregation in an ex vivo model in cynomolgus monkeys following oral administration. A detailed profile of 28b, a benchmark compound in this series, with a Ki of 4.3 nM, is presented.

Himbacine derived thrombin receptor (PAR-1) antagonists: SAR of the pyridine ring

The structure-activity relationship (SAR) of the vinyl pyridine region of himbacine derived thrombin receptor (PAR-1) antagonists is described. A 2-vinylpyridyl ring substituted with an aryl or a heteroaryl group at the 5-position showed the best overall PAR-1 affinity and pharmacokinetic properties. One of the newly discovered analogs bearing a 5-(3-pyridyl) substituent showed excellent PAR-1 affinity (Ki = 22 nM) and oral activity with reduced ClogP and improved off-target selectivity compared to an earlier development candidate.

Metabolism-Based Identification of a Potent Thrombin Receptor Antagonist

The metabolism of our prototypical thrombin receptor antagonist 1, Ki = 2.7 nM, was studied and three major metabolites (2, 4, and 5) were found. The structures of the metabolites were verified independently by synthesis. Compound 4 was shown to be a potent antagonist of the thrombin receptor with a Ki = 11 nM. Additionally, compound 4 showed a 3-fold improvement in potency with respect to 1 in an agonist-induced ex-vivo platelet aggregation assay in cynomolgus monkeys after oral administration; this activity was sustained with 60% inhibition observed at 24 h post-dose. Compound 4 was highly active in functional assays and showed excellent oral bioavailability in rats and monkeys. Compound 4 showed a superior rat enzyme induction profile relative to compound 1, allowing it to replace compound 1 as a development candidate.

Discovery and synthesis of a novel series of quinoline-based thrombin receptor (PAR-1) antagonists

The design, synthesis, and SAR studies of a structurally novel series of highly potent thrombin receptor (PAR-1) antagonists are described. Compound 30 is a highly potent thrombin receptor antagonist (IC(50)=6.3 nM), a related compound 36 showing efficacy in a monkey ex vivo study.

Discovery of potent orally active thrombin receptor (protease activated receptor 1) antagonists as novel antithrombotic agents

Structurally novel thrombin receptor (protease activated receptor 1, PAR-1) antagonists based on the natural product himbacine are described. The prototypical PAR-1 antagonist 55 showed a Ki of 2.7 nM in the binding assay, making it the most potent PAR-1 antagonist reported. 55 was highly active in several functional assays, showed excellent oral bioavailability in rat and monkey models, and showed complete inhibition of agonist-induced ex vivo platelet aggregation in cynomolgus monkeys after oral administration.

Optimization of purine based PDE1/PDE5 inhibitors to a potent and selective PDE5 inhibitor for the treatment of male ED

In search of a PDE5 inhibitor for erectile dysfunction, an SAR was developed from a PDE1/PDE5 purine series of leads, which had modest PDE5 potency and poor isozyme selectivity. A compound (41) with PDE5 inhibition and in vivo activity similar to sildenafil was discovered from this effort. In addition, purine 41 demonstrated superior overall PDE isozyme selectivity when compared to the approved PDE5 inhibitors sildenafil, vardenafil, and tadalafil, which may result in a more favorable side-effect profile.

SAR development of polycyclic guanine derivatives targeted to the discovery of a selective PDE5 inhibitor for treatment of erectile dysfunction

Development of structure-activity relationship of cyclic guanines I lead us to discovery of a potent and selective series of phosphodiesterase 5 inhibitors 52-59 (IC50=1.3-11.0 nM, PDE6/5=116-600).

Design and synthesis of xanthine analogues as potent and selective PDE5 inhibitors

We have discovered potent and selective xanthine PDE5 inhibitors. Compound 25 (PDE5 IC(50)=0.6 nM, PDE6/PDE5=101) demonstrated similar functional efficacy and PK profile to Sildenafil (PDE5 IC(50)=3.5 nM, PDE6/PDE5=7).

Quantitative screening and matrix effect studies of drug discovery compounds in monkey plasma using fast-gradient liquid chromatography/tandem mass spectrometry

A higher-throughput bioanalytical method based on fast-gradient (1 min run time) high-performance liquid chromatography (HPLC) coupled with tandem mass spectrometry (MS/MS) was developed for screen-type analyses of plasma samples from early drug discovery studies in support of exploratory pharmacodynamic studies. The HPLC system equipped with minibore column was interfaced with either atmospheric pressure chemical ionization (APCI) or electrospray (ESI) ionization techniques. The matrix ion suppression effect of both quantitative HPLC/MS/MS analyses was compared using the post-column infusion system. The use of the described methods provided advantages such as a shorter chromatographic region of ion suppression, less solvent consumption and shorter run times in comparison with standard analytical column HPLC/MS/MS methods. The analytical results obtained by both HPLC/MS/MS methods were in good agreement (within 15% of error) and displayed a good correlation with the pharmacodynamic outcome.

Inhibition of platelet adhesion and aggregation by E4021, a type V phosphodiesterase inhibitor, in guinea pigs

E4021 (sodium 1-[6-chloro-4-(3, 4-methylenedioxybenzyl)-aminoquinazolin-2-yl]piperidine-4-ca rboxylate sesquihydrate) is a highly selective and potent inhibitor of type V phosphodiesterase(PDE5). The in vitro and in vivo effect of E4021 on platelet function was evaluated, using echistatin, a potent disintegrin, as a positive reference agent. E4021 inhibits aggregatory response to collagen in washed human platelets (IC50 = 5 microM, vs. 0.14 microM with echistatin). In the ex vivo-platelet aggregation assay using whole blood from treated guinea pigs, E4021 (9 mg/kg i.v.) showed a moderate inhibition (43%) against collagen (0.125 microg/ml), whereas echistatin (250 microg/kg i.v.) exerted a 88% inhibition. The absence of endothelium-derived factors (NO) may account for the moderate in vitro and ex vivo antiplatelet activity of E4021. In an in vivo model of reversible platelet aggregation elicited by collagen (100 microg/kg i.v.), both E4021 and echistatin attenuated the intrapulmonary platelet accumulation in guinea pigs (-36% and -44%, respectively). In addition, E4021 (9 mg/kg i.v.) and echistatin (250 microg/kg i.v.) caused a similar inhibition of platelet adhesion at sites of microfilament-induced vascular injury in guinea pigs (52% and 65%, respectively). The two agents in combination did not show additive effect, suggesting that E4021 inhibits platelet activation and impairs interactions of adhesion receptors with matrix proteins. E4021 caused a selective increase in cGMP concentrations in the platelets isolated from treated guinea pigs: cAMP was not affected. It is concluded that the antiplatelet activity of E4021 is mediated through cGMP mechanism by virtue of selective inhibition of PDE5 in the platelets.

Characterization of in vitro and in vivo platelet responses to thrombin and thrombin receptor-activating peptides in guinea pigs

Guinea pig platelets are similar to human platelets in their responsiveness to thrombin receptor-activating peptides and other agonists. Therefore, guinea pigs anesthetized with Inactin (90 mg/kg i.p.) were used to assess in vivo activities of thrombin and thrombin receptor-activating peptides (TRAPs) using 111 In-labeled platelets and a microcomputer-based system. The aggregatory responses are expressed as percent change for a 20 min period over basal radioactivity (AUC). Reversible accumulation of platelets occurred in the pulmonary microcirculation in response to stimuli. Human thrombin (50 and 100 U/kg i.v.) caused a dose-related platelet accumulation. Responses of similar magnitude were induced by SFLLRN (TRAP-(1-6)) and Ala-Phe(p-F)-Arg-Cha-HArg-Tyr-NH2 (high-affinity thrombin receptor-activating peptide, 0.03, 0.1 and 0.3 mg/kg i.v.). High-affinity thrombin receptor-activating peptide, a new synthetic oligopeptide agonist, is about 3-fold more potent than TRAP-(1-6), a wild-type sequence. Similarly, high-affinity thrombin receptor-activating peptide is about 4 times more potent than TRAP-(1-6) in the radioligand binding study using platelet membrane. By comparison, high-affinity thrombin receptor-activating peptide manifested an aggregatory activity (EC60 = 1.2 microM) about 15 times more potent than that of TRAP-(1-6)(EC60 = 18.6 microM) in washed guinea pig platelets. The intrapulmonary platelet aggregation in response to thrombin, TRAP-(1-6) and high-affinity thrombin receptor-activating peptide was characterized by long duration (approximately 30 min); a reduction in response (18-54%) tended to occur with repeated challenges, presumably due to desensitization and consumption. The response to thrombin (100 U/kg) was greatly inhibited by (D)-Phe-Pro-Arg-chloromethyl ketone (PPACK), a potent thrombin inhibitor (250 micrograms/kg + 6 micrograms/kg per min i.v. x 30): AUC, 150 +/- 552 vs. 7171 +/- 1052 in the control period (n = 8, P < 0.05). The response to high-affinity thrombin receptor-activating peptide (0.03 mg/kg), which acts on thrombin receptor directly, was not affected by PPACK. It is concluded that guinea pigs are an appropriate preparation for evaluation of in vivo activity of thrombin inhibitors as well as thrombin receptor agonists and antagonists.

Antiplatelet and antiproliferative effects of SCH 51866, a novel type 1 and type 5 phosphodiesterase inhibitor

SCH 51866 is a potent and selective PDE1 and PDE5 inhibitor. The antiplatelet, antiproliferative, and hemodynamic effects of SCH 51866 were compared with those of E4021, a highly selective PDE5 inhibitor. SCH 51866 inhibited PDE1 and PDE5 isozymes with a 50% inhibitory concentration (IC50) of 70 and 60 nM, respectively. SCH 51866 and E4021 inhibited washed human platelet aggregation induced by collagen with an IC50 of 10 and 4 microM, respectively, and attenuated (p < 0.05) the adhesion of 111indium-labeled platelets to the nylon filament-injured rat aorta. The doses of SCH 51866 and E4021 that inhibited platelet adhesion caused significant increases in platelet cyclic guanosine monophosphate (cGMP; p < 0.05). SCH 51866 (1-10 mg/kg, p.o. twice daily) but not E4021 (3-30 mg/kg, p.o twice daily) inhibited neointima formation in the carotid arteries of spontaneously hypertensive rats (SHRs) subjected to balloon angioplasty. Moreover, SCH 51866 (0.3-10 mg/kg, p.o.) elicited dose-dependent reduction in blood pressure in SHRs, whereas E4021 (3-30 mg/kg, p.o.) did not affect blood pressure in SHRs. In conclusion, the data suggest that inhibition of PDE1 and PDE5 isozymes by SCH 51866 exerts antiplatelet and vascular protective effects. In comparison, inhibition of PDE5 alone by E4021 exhibited antiplatelet effects without affecting neointima formation.

Evidence for the presence of a proteinase-activated receptor distinct from the thrombin receptor in vascular endothelial cells

The thrombin receptor was the first cloned G protein-coupled receptor reported to be activated by proteolytic cleavage of its extracellular amino terminus. A second proteinase-activated receptor (PAR-2) was cloned recently and expressed in Xenopus laevis oocytes. PAR-2 was activated by trypsin and by a peptide (SLIGRL) derived from the new amino terminus. Since PAR-2 mRNA was detected in highly vascularized organs, we compared the physiological functions of the thrombin receptor and PAR-2 in vascular endothelium. Thrombin and trypsin both elicited endothelium-dependent relaxations in prostaglandin F2alpha (PGF2alpha)-contracted strips of porcine coronary artery. Whereas high doses of both thrombin or trypsin (10 U/mL) caused homologous desensitization, trypsin caused further relaxation of thrombin-desensitized tissues. Thrombin and PAR-2-derived peptides (SFLLRN and SLIGRL) both induced endothelium-dependent relaxations in PGF2alpha-contracted porcine coronary arteries. SFLLRN or SLIGRL (30 micronmol/L) also showed homologous desensitization but not cross desensitization. In the presence of the NO synthase inhibitor NG-monomethyl-L-arginine (1 mmol/L), both SFLLRN- and SLIGRL-induced relaxations were partially inhibited. SFLLRN elicited weak contraction in coronary arteries without endothelium, whereas SLIGRL had no effect. Intravenous injection of SFLLRN (1 mg/kg, bolus) into anesthetized rats elicited a transient depressor response followed by pronounced pressor response. In contrast, intravenous administration of SLIGRL (1 mg/kg, bolus) produced only a marked depressor response. Consistent with the in vivo data, SFLLRN contracted the endothelium-rubbed rat aortic rings and aggregated human platelets in vitro, whereas SLIGRL had no effect. The finding that both trypsin and SLIGRL induced endothelium-dependent relaxations indicates the presence of PAR-2 on endothelial cells. In addition, both trypsin and SLIGRL elicited relaxations in thrombin- or SFLLRN-desensitized tissue, suggesting that PAR-2 is distinct from thrombin receptor in vascular endothelium. The lack of PAR-2-mediated platelet aggregation or smooth muscle contraction suggested it might not share the pathogenic properties associated with the thrombin receptor in the vasculature.

In vivo and ex vivo effects of adenosine A1 and A2 receptor agonists on platelet aggregation in the rabbit

We have investigated both in vivo and ex vivo antiaggregatory activity of three adenosine receptor agonists in the anesthetized rabbit: the non-selective, 5'-N-ethyl-carboxamidoadenosine (NECA), the selective adenosine A1 receptor agonist, 2-chloro-N6-cyclopentyladenosine (CCPA) and the new selective A2 receptor agonist, 2-hexynyl-NECA. The drugs were administered by 30-min intravenous infusion at a dose reducing mean blood pressure by 40-50%. NECA and CCPA also markedly decreased heart rate. In ex vivo experiments, NECA (10 micrograms/kg) and 2-hexynyl-NECA (10 micrograms/kg) maximally inhibited adenosine 5'-diphosphate (ADP)-induced platelet aggregation at the end of drug infusion by 26.7 +/- 2.9% and 25.2 +/- 3.5%, respectively. In in vivo studies, the inhibition of platelet aggregation was evaluated using the technique based on selective accumulation of 111In-labeled platelets in pulmonary microcirculation upon challenge with ADP 100 micrograms/kg. NECA (10 micrograms/kg) and 2-hexynyl-NECA (10 micrograms/kg) decreased peak values for platelet accumulation by 35.3 +/- 6.9% and 52.5 +/- 5.9% and the area under curve values by 37.7 +/- 8.7% and 41.2 +/- 12.0%, respectively. In comparison, CCPA (100 micrograms/kg) did not affect platelet responses to ADP in either of the experimental models. Thus, the present study clearly demonstrates for the first time the in vivo antiplatelet activity of adenosine A2 receptor agonists, whereas the adenosine A1 receptor agonist was inactive, in consonance with the in vitro data.

Attenuation of ischemic acute renal failure by phosphoramidon in rats

The protective effects of phosphoramidon, a dual inhibitor of endothelin-converting enzyme and neutral endopeptidase (E.C. 24.11), on renal function in ischemic acute renal failure were investigated in anesthetized rats. Intravenous infusion of phosphoramidon (0.03 and 0.1 mg/kg per min) significantly suppressed tubular sodium wasting (measured by fractional excretion of sodium) and proteinuria in the postischemic kidney without modifying functional parameters in the contralateral normal kidney. Phosphoramidon (0.1 mg/kg/min) was associated with increased glomerular filtration in the ischemic kidney. In comparison, SCH 42354, a highly selective inhibitor of neutral endopeptidase at 0.3 mg/kg/min, did not inhibit endothelin-converting enzyme or afford renal protection. The data suggest that the protective action of phosphoramidon against ischemic acute renal failure is most likely mediated by inhibition of endothelin formation.