C6 modification of the pyridinone core of thrombin inhibitor L-374,087 as a means of enhancing its oral absorption

1 (L-374,087) is a potent, selective, efficacious, and orally bioavailable thrombin inhibitor that contains a core 3-amino-2-pyridinone moiety. Replacement of the C6 pyridinone methyl group of 1 by a propyl group gave 5 (L-375,052), which retained all the excellent properties of 1, and also yielded higher plasma levels after oral dosing in dogs and rats.

L-374,087, an efficacious, orally bioavailable, pyridinone acetamide thrombin inhibitor

Replacement of the amidinopiperidine P1 group of 3-benzylsulfonylamino-6-methyl-2-pyridinone acetamide thrombin inhibitor L-373,890 (2) with a mildly basic 5-linked 2-amino-6-methylpyridine results in an equipotent compound L-374,087 (5, Ki = 0.5 nM). Compound 5 is highly selective for thrombin over trypsin, is efficacious in the rat ferric chloride model of arterial thrombosis and is orally bioavailable in dogs and cynomolgus monkeys. The structural basis for the critical importance of both methyl groups in 5 was confirmed by X-ray crystallography.

Discovery and development of the novel potent orally active thrombin inhibitor N-(9-hydroxy-9-fluorenecarboxy)prolyl trans-4-aminocyclohexylmethyl amide (L-372,460): coapplication of structure-based design and rapid multiple analogue synthesis on solid support

Early studies in these laboratories of peptidomimetic structures containing a basic P1 moiety led to the highly potent and selective thrombin inhibitors 2 (Ki = 5.0 nM) and 3 (Ki = 0.1 nM). However, neither attains significant blood levels upon oral administration to rats and dogs. With the aim of improving pharmacokinetic properties via a more diverse database, we devised a resin-based route for the synthesis of analogues of these structures in which the P3 residue is replaced with a range of lipophilic carboxylic amides. Assembly proceeds from the common P2-P1 template 7 linked via an acid-labile carbamate to a polystyrene support. Application of the methodology in a repetitive fashion afforded several interesting analogues out of a collection of some 200 compounds. Among the most potent of the group, N-(9-hydroxy-9-fluorenecarboxy)-prolyl trans-4-aminocyclohexylmethyl amide (L-372,460 8, Ki = 1.5 nM), in addition to being fully efficacious in a rat model of arterial thrombosis at an infusion rate of 10 micrograms/kg/min, exhibits oral bioavailability of 74% in dogs, and oral bioavailability of 39% in monkeys with a serum half-life of just under 4 h. On the basis of its favorable biological properties, inhibitor 8 has been subject to further evaluation as a possible treatment for thrombogenic disorders.

Effects of pentobarbital on pharmacokinetics and pharmacodynamics of a potent fibrinogen receptor antagonist, L-734,217, in dogs

Effects of pentobarbital on pharmacokinetics and pharmacodynamics of L-734,217, a potent fibrinogen receptor antagonist, were studied in male dogs. L-734,217 was given intravenously at 0.01 mg kg-1, in a cross-over fashion, to conscious dogs or to dogs anesthetized with pentobarbital. Plasma concentrations of L-734,217 were measured using a radioimmunoassay and inhibitory effects on ex vivo platelet aggregation induced by ADP or collagen were determined. In pentobarbital-treated dogs, L-734,217 plasma concentrations during the first 3 h collection period were significantly higher than those in the control animals. Corresponding to the increased plasma levels, the mean ex vivo inhibitory effects on ADP- or collagen-induced platelet aggregation in dogs under anesthesia appeared greater than in those without the anesthetic treatment. Pharmacokinetic analysis revealed a modest, but significant (up to 40%) elevation in the area under the plasma concentration-time curve during 6 h of the drug administration, and a reduction in L-734,217 plasma clearance and volumes of distribution, in the anesthetized dogs. Analysis of pharmacodynamic data indicated that the EC50 and the Hill coefficient of the platelet aggregation response-plasma concentration curve were not altered by pentobarbital treatment. The results are in agreement with the findings that the administration of pentobarbital alone (in the absence of L-734,217) did not affect appreciably the ex vivo platelet aggregatory responses. In a separate group of dogs, L-734,217 was found to be metabolically stable, and was eliminated unchanged renally (64 +/- 4%) and hepatically (32 +/- 6%). In addition, L-734,217 did not bind substantially to canine plasma proteins or blood cellular components. It is possible that alterations of regional hemodynamics, reportedly mediated by pentobarbital, contributed to changes observed in the present study. That is, alterations occurred in L-734,217 elimination and distribution processes which resulted in an increase in drug plasma levels. Since pentobarbital anesthesia influenced only the pharmacokinetics, and not the pharmacodynamics, of L-734,217, the apparent increases in the inhibition of platelet aggregation responses observed following L-734,217 administration to the anesthetized dogs were probably sequential effects of the pharmacokinetic interactions.

Nonpeptide glycoprotein IIb/IIIa inhibitors: 14: oral antithrombotic efficacy of L-738,167 in a conscious canine model of coronary artery electrolytic injury

BACKGROUND

A conscious dog model of left circumflex coronary artery electrolytic injury was used to assess the oral antithrombotic efficacy of L-738,167, a potent nonpeptide antagonist of platelet GP IIb/IIIa. L-738,167 was administered either as a single oral pretreatment dose 2 hours before initiation of vessel injury or as two oral doses administered 24 hours apart, 12 hours before and after initiation of vessel injury.

METHODS AND RESULTS

In untreated controls, electrolytic coronary injury (50 microA, 3 hours) resulted in thrombotic occlusion and myocardial ischemia in 15 of 16 dogs, with 4 developing lethal arrhythmias. Significant reductions in thrombus mass and complete prevention of myocardial ischemia and infarction were achieved with a single 100- to 300-microg/kg dose of L-738,167 pretreatment and with two 100-microg/kg doses administered 12 hours before and after initiation of vessel injury. Delays and/or reductions in incidence of ischemia, thrombus mass, and infarct sizes also were achieved with 10- to 30-microg/kg pretreatment and with two 30-microg/kg doses administered 12 hours before and after initiation of vessel injury. None of the L-738,167-treated animals developed lethal arrhythmias. A single oral 100-microg/kg dose of L-738,167 achieved >90% inhibitions of ADP (extent)- and collagen (rate)-induced ex vivo platelet aggregation and fivefold to sixfold or greater elevations in bleeding time; a single oral 30-microg/kg dose of L-738,167 achieved sustained 40% to 70% inhibitions of ADP- and collagen-induced ex vivo platelet aggregation and modest twofold to threefold elevations in bleeding time. At 12 to 24 hours after single oral 30- and 100-microg/kg doses of L-738,167, a substantially greater L-738,167 concentration was associated with platelets than free in plasma.

CONCLUSIONS

These findings are indicative of potent and sustained oral antithrombotic efficacy and suggest that L-738,167 possesses potential for the oral management of chronic thrombotic occlusive disorders.

Non-peptide glycoprotein IIb/IIIa inhibitors. 17. Design and synthesis of orally active, long-acting non-peptide fibrinogen receptor antagonists

The synthesis and pharmacological evaluation of 5 (L-738, 167), a potent, selective non-peptide fibrinogen receptor antagonist is reported. Compound 5 inhibited the aggregation of human gel-filtered platelets with an IC50 value of 8 nM and was found to be > 33000-fold less effective at inhibiting the attachment of human endothelial cells to fibrinogen, fibronectin, and vitronectin than it was at inhibiting platelet aggregation. Ex vivo platelet aggregation was inhibited by > 85% 24 h after the oral administration of 5 to dogs at 100 micrograms/kg. The extended pharmacodynamic profile exhibited by 5 appears to be a consequence of its high-affinity binding to GPIIb/IIIa on circulating platelets and suggests that 5 is suitable for once-a-day dosing.

Nonpeptide glycoprotein IIb/IIIa inhibitors. 15. Antithrombotic efficacy of L-738,167, a long-acting GPIIb/IIIa antagonist, correlates with inhibition of adenosine diphosphate-induced platelet aggregation but not with bleeding time prolongation

The nonpeptide platelet glycoprotein IIb/IIIa antagonist, L-738, 167, was characterized in dog and nonhuman primate. In an anesthetized canine model of coronary artery electrolytic lesion, L-738,167 elicited dose-dependent (3, 4, 4.5 and 5 micrograms/kg i.v.) decreases in incidence of occlusion, reductions in thrombus mass and elevations in bleeding time. Antithrombotic efficacy correlated with inhibition of adenosine diphosphate-induced platelet aggregation but was dissociated from marked bleeding time elevation. Similarly, suppression of platelet-dependent cyclic flow reductions with L-738,167 in the canine coronary artery (5 micrograms/kg i.v.) and African green monkey carotid artery (10 micrograms/kg i.v.) correlated with inhibition of adenosine diphosphate-induced platelet aggregation but not with inhibition of thrombin-induced platelet aggregation or significant prolongation of bleeding time. In conscious dogs and sedated chimpanzees, single dose intravenous bolus (5-20 micrograms/kg) and oral (25-200 micrograms/kg) administration of L-738,167 exhibited long duration (> or = 8 hr) inhibition of ex vivo platelet aggregation. Once daily oral administration to conscious dogs (10-30 micrograms/kg/day for 15 days) and rhesus monkeys (200-250 micrograms/kg/day for 11 days) maintained significant but submaximal (50-90% inhibition) trough levels of inhibition of adenosine diphosphate-induced ex vivo platelet aggregation. Platelet sensitivity to adenosine diphosphate after multiple days of oral dosing in dogs was similar to pretreatment sensitivity. L-738,167 showed characteristics suitable for chronic oral therapy with a glycoprotein IIb/IIIa inhibitor.

Non-peptide glycoprotein IIb/IIIa antagonists. 11. Design and in vivo evaluation of 3,4-dihydro-1 (1H)-isoquinolinone-based antagonists and ethyl ester prodrugs

The structure-activity relationship of a series of orally active glycoprotein IIb/IIIa antagonists containing a nitrogen heterocycle grafted onto a 3,4-dihydro-1 (1H)-isoquinolinone core is described. These compounds are structurally novel analogs of the progenitor compound 1 (L-734,217,[[3(R)-[2-(piperidin-4-yl)ethyl]-2-oxopiperidinyl ]acetyl]-3(R)- methyl-beta-alanine) in which the lactam chiral center has been removed. The 4-piperazinyl- and 4-piperidinyl-substituted 3,4-dihydro-1(1H)-isoquinolinones were found to be optimal for in vitro potency. In addition, substitution at the 3-position of the beta-amino acid enhanced potency with the 3-pyridyl and 3-ethynyl analogs being the most potent prepared. Attempts to improve the in vivo profile of these compounds focused on modification of the physical properties. Ester prodrugs were prepared to increase the lipophilicity and remove the zwitterionic nature of the antagonists. The prodrug approach, coupled with the arylpiperazine terminus (pKa = approximately 9.0), afforded moderately basic and relatively nonpolar compounds. The acid N-[[7-(piperazin-1-yl)-3,4-dihydro-1(1H)-oxoisoquinolin-2-yl ]acetyl]-3(S)- ethynyl-beta-alanine, 6d (L-767,679), is a potent fibrinogen receptor antagonist able to inhibit the ADP-induced aggregation of human gel-filtered platelets with an IC50 of 12 nM. Although 6d is orally active based on the results of an ex vivo dog assay at 0.3 mg/kg, the ethyl ester prodrug of this compound, 19 (L-767,685), is better absorbed at this dose than 6d. Upon oral dosing, the ester 19 is converted to 6d in vivo in dog with an estimated oral systemic availability of > 17% (0-8 h, AUC19po/AUC6div). In addition, studies in monkey at an oral dose of 1 mg/kg show that 19 affects the complete inhibition of the ex vivo platelet aggregation in response to ADP between 2 and 8 h postdose with the level of inhibition remaining at 40% at 12 h postdose. This level of activity was superior to that observed for 6d and 1 at the same dose. Using ex vivo ADP-induced aggregation data from rhesus monkey (n = 2, 0-8 h using the AUC19po/AUC6div), the estimated systemic oral availability of 6d when dosed as 19 is 32%.

Nonpeptide glycoprotein IIb/IIIa inhibitors. 8. Antiplatelet activity and oral antithrombotic efficacy of L-734,217

The antiplatelet activity of L-734,217, a nonpeptide platelet GPIIb/IIIa antagonist, was evaluated in the rat, guinea pig and dog. IC50 for inhibition of in vitro platelet aggregation for these species (agonists: adenosine diphosphate, collagen) were rat, 838,000 and > 1,100,000 nM; guinea pig, 124 and 156 nM; dog, 42 and 50 nM. In an in vivo rat/in vitro dog platelet aggregation assay, effective antiaggregatory plasma concentrations of L-734,217 were achieved after 8.0 to 16.0 mg/kg p.o. vs. 0.3 to 1.0 mg/kg i.v. to rats. Delays in platelet-dependent hemostatic plug formation in severed mesenteric arteries were observed after 2.0 to 5.0 mg/kg p.o. vs. 0.1 to 0.2 mg/kg i.v. to guinea pigs. Dose-dependent inhibitions of ex vivo platelet aggregation after 0.3 to 3.0 mg/kg p.o. and 0.03 to 0.3 mg/kg i.v. L-734,217 to conscious dogs yielded estimates of 8 to 16% oral bioavailability. The antiplatelet activity of 3.0 mg/kg p.o. L-734,217 in dogs was unaffected by dosage form or food. In a conscious dog model of left circumflex coronary artery electrolytic lesion, 3.0 mg/kg p.o. L-734,217 q4 to 8 hr reduced thrombus mass, prevented occlusive coronary artery thrombosis and reduced or prevented myocardial infarction and ventricular ectopy. In anesthetized dogs, a dissociation between inhibition of ex vivo platelet aggregation and template bleeding time prolongation was observed with i.v. L-734,217. The results of the coadministration of heparin, aspirin and L-734,217 to anesthetized dogs suggested a synergistic effect on template bleeding time with no effect on plasma L-734,217 concentrations. These findings indicate L-734,217 to be an important lead structure for the development of therapeutically useful oral antiplatelet agents.

Anti-thrombotic activity of RG13965, a novel platelet fibrinogen receptor antagonist

RG13965, a pseudotetrapeptide analogue of Arg-Gly-Asp (RGD), inhibited collagen-induced dog, monkey, human, hamster, mouse, and pig platelet aggregation in vitro with IC50 values of 3.7, 4.6, 6.3, 126, 136 and 1600 microM, respectively. RG13965 (3, 10, and 30 mg/kg, i.v.) decreased the incidence of collagen/epinephrine-induced thrombosis in mice from 90% in untreated animals to 63, 37, and 0%, respectively. In hamsters, RG13965 (10 and 30 mg/kg, i.v.) prolonged the time required for formation of a hemostatic plug in severed mesenteric arteries by 1.6- and 3.6-fold, respectively. In a canine model of repetitive platelet thrombus formation in the coronary artery, RG13965 (0.1, 0.3, and 1 mg/kg, i.v.) reversibly inhibited cyclic flow reductions (CFRs) and inhibited ADP-induced ex vivo platelet aggregation by 29, 57, and 77%, respectively. RG13965 (1 mg/kg) completely inhibited CFRs for at least 40 min. Platelet count was not altered at any dose and template bleeding time was prolonged modestly (1.8-fold) at only the highest dose. RG13965 dose-dependently and reversibly inhibited thrombus formation at doses which did not completely inhibit ex vivo platelet aggregation and only modestly prolonged template bleeding time.

Investigation of (Oxodioxolenyl)methyl carbamates as nonchiral bioreversible prodrug moieties for chiral amines

The preparation of (oxodioxolenyl)methyl carbamates and their evaluation as novel nonchiral prodrug moieties for chiral primary and secondary amino functional drugs are described. 4-(Carbamoylmethyl)-2-oxo-1,3-dioxolene derivatives of 3,4-dimethoxyphenethylamine with 5-methyl, 5-phenyl, and 5-anisyl substitution (5a, 5b, and 5c) on the dioxolenone ring were prepared as model amine prodrugs by a one step process involving displacement of p-nitrophenol from appropriately substituted ring opening of these carbamates led to a cascade reaction resulting in the rapid and quantitative regeneration of the parent amine drug. Aryl substitution did not significantly alter the hydrolysis rates of these dioxolenone carbamates in buffers at pH 1 and 7.4 or in rat plasma, although the hydrolysis rates of 5-phenyl- (1b) and 5-anisyl- 4-methyl-1,3-dioxol-4-en-2-one (1c) in pH 7.4 phosphate buffer were 2-3 fold faster than that of the 5-methyl-substituted analog (1a). Application of this prodrug strategy to the chiral fibrinogen receptor antagonist L-734,217 resulted in a prodrug that gave quantitative reconversion in rat and dog plasma in vitro and oral bioavailability of 23 +/- 6% in dogs for the parent drug.

Non-peptide fibrinogen receptor antagonists. 7. Design and synthesis of a potent, orally active fibrinogen receptor antagonist

The design, synthesis, and pharmacological evaluation of L-734,217, a potent, low-molecular weight, orally active fibrinogen receptor antagonist, is reported. A strategy for producing low-molecular weight inhibitors from the peptide c-[(Ac)CRGDC] A, previously reported from these laboratories, is outlined. This strategy combines a retrodesign analysis of the conformationally defined cyclic peptide A with stereochemical information present in the arginine-glycine-aspartic acid (RGD) tripeptide sequence, culminating with the discovery of L-734,217. L-734,217 inhibited the aggregation of human, dog, and chimpanzee platelets at concentrations below 100 nM and was found to be > 15000-fold less effective at inhibiting the attachment of human umbilical vein endothelial cells to fibrinogen, fibronectin, and vitronectin than it was at inhibiting the aggregation of platelets. L-734,217 showed significant ex vivo antiplatelet activity following oral administration in dogs and chimpanzees at doses of 1.0 and 2.0 mg/kg, respectively, and has been selected as a clinical candidate for development as an antithrombotic agent.

An antibody against the exosite of the cloned thrombin receptor inhibits experimental arterial thrombosis in the African green monkey

BACKGROUND

Thrombin inhibitors have been shown to be efficacious in animal models of thrombosis and in initial human clinical trials. It is unknown if their efficacy is due to their prevention of thrombin-mediated fibrin formation or to an inhibitory effect on thrombin-stimulated platelet activation. Appropriate tools to address this question have not been available. Therefore, to evaluate the role of the platelet thrombin receptor in intravascular thrombus formation, a polyclonal antibody was raised against a peptide derived from the thrombin-binding exosite region of the cloned human thrombin receptor. This antibody serves as a selective inhibitor of the thrombin receptor for in vivo evaluation.

METHODS AND RESULTS

The immune IgG (IgG 9600) inhibited thrombin-stimulated aggregation and secretion of human platelets. In contrast, it had no effect on platelet activation induced by other agonists including ADP, collagen, or the thrombin receptor-derived peptide SFLLR-NH2. IgG 9600 also inhibited thrombin-induced aggregation of African Green monkey (AGM) platelets. By Western blot analysis, the IgG identified a protein of approximately 64 kD in homogenates of both human and AGM platelets. The effect of thrombin receptor blockade by this antibody on arterial thrombosis was evaluated in an in vivo model of platelet-dependent cyclic flow reductions (CFRs) in the carotid artery of the AGM. The intravenous administration of IgG 9600 (10 mg/kg) abolished CFRs in three monkeys and reduced CFR frequency by 50% in a fourth monkey. Ex vivo platelet aggregation in response to up to 100 nmol/L thrombin was completely inhibited during the 120-minute postbolus observation period in all four animals. There was a twofold increase in bleeding time, which was not statistically different from baseline, and ex vivo clotting time (APTT) was not changed. The glycoprotein IIb/IIIa receptor antagonist MK-0852 and the thrombin inhibitor recombinant hirudin also demonstrated inhibitory effects on CFRs at doses that did not significantly prolong template bleeding time. Control IgG had no effect on CFRs, ex vivo platelet aggregation, bleeding time, or APTT.

CONCLUSIONS

These results demonstrate that blockade of the platelet thrombin receptor can prevent arterial thrombosis in this animal model without significantly altering hemostatic parameters and suggest that the thrombin receptor is an attractive antithrombotic target.

Nonpeptide glycoprotein IIb/IIIa inhibitors. 5. Antithrombotic effects of MK-0383

The antiaggregatory and antithrombotic actions of MK-0383, a low molecular weight, nonpeptide antagonist of the platelet glycoprotein IIb/IIIa, were evaluated in a variety of canine models. Inhibition of ex vivo platelet aggregation responses to ADP and collagen were observed after the acute sequential i.v. administrations of 10 to 500 micrograms/kg or 360-min continuous i.v. infusions of 1 to 10 micrograms/kg/min of MK-0383. Hemostatic function normalized within 30 (platelet response to collagen, template bleeding times) to 90 min (platelet response and sensitivity to ADP) after the termination of 360-min i.v. MK-0383 infusions, suggesting no protracted, direct effects on platelet function. With acute sequential i.v. administrations of MK-0383, platelet response to ADP was abolished without significant extension of bleeding time. In a model of platelet-dependent cyclic flow reductions in injured, stenosed left circumflex coronary artery, the bolus i.v. administrations of 300 and 1000 micrograms/kg of MK-0383 totally abolished cyclic flow reductions for periods of 18 +/- 1 and 37 +/- 5 min, respectively. In a model of electrically induced left circumflex coronary artery occlusive thrombosis, 10 micrograms/kg/min i.v. of MK-0383 initiated 15 min before electrical injury prevented occlusive thrombosis in three of six preparations despite continued electrical stimulation of the vessel for 300 min, delayed occlusion in three of six preparations (160.3 +/- 5.5 min) and reduced thrombus mass (5.1 +/- 1.3 mg), compared to the development of occlusive thrombosis in six of six saline-treated preparations (50.5 +/- 8.7 min; 19.1 +/- 3.0 mg). When administered as an adjunct to thrombolytic agents in the presence of background heparin for lysis of electrically induced left circumflex coronary artery occlusive thrombus, 10 micrograms/kg/min i.v. of MK-0383 initiated 15 min before tissue-type plasminogen activator or streptokinase increased the incidence of (tissue-type plasminogen activator: eight of nine MK-0383 vs. three of eight saline; streptokinase: eight of eight MK-0383 vs. two of eight saline) and accelerated reperfusion, and reduced the incidence of acute thrombotic reocclusion during continued MK-0383 infusion. These findings indicate significant antithrombotic potential for MK-0383 alone or as an adjunct to thrombolytic therapy in the treatment of coronary artery ischemic syndromes.

Species variability in platelet and other cellular responsiveness to thrombin receptor-derived peptides

The aggregation of platelets from a variety of animal species in response to thrombin receptor-derived activating peptides was evaluated. A series of 14-(SFLLRNPNDKYEPF), 7-(SFLLRNP-NH2), 6-(SFLLRN-HN2) or 5-(SFLLR-NH2) residue peptides, the structures of which were based on the deduced amino acid sequence of the human thrombin receptor, promoted full aggregation of platelets in plasma from humans, African Green and Rhesus monkeys, baboons and guinea pigs at 4-50 microM depending on the peptide used. Platelets in plasma from rabbit, dog, pig, and hamster underwent a shape change but failed to aggregate in response to these peptides over 3 log units of peptide up to 800 microM, despite being fully responsive to human thrombin. However, because the receptor peptides induced shape change in the platelets from these non-aggregating species, they apparently can activate some of the intracellular signaling system(s) usually initiated by thrombin in these platelets. In contrast, platelets from rats did not undergo shape change or aggregate in response to the peptides. A 7-residue receptor-derived peptide based on the deduced amino acid sequence of the clone of the hamster thrombin receptor (SFFLRNP-N2) was nearly as efficacious as the corresponding human receptor-derived 7-residue peptide to promote aggregation of human platelets. However, the hamster peptide could not promote aggregation of hamster platelets in plasma at up to 800 microM peptide, while a shape change response was elicited. Platelets from rats, rabbits and pigs also did not aggregate in response to this peptide derived from the hamster thrombin receptor, but all species except the rat underwent a shape change.(ABSTRACT TRUNCATED AT 250 WORDS)

Mouse antithrombotic assay. Inhibition of platelet thromboembolism by disintegrins

The mouse antithrombotic assay represents a model of fatal pulmonary thromboembolism induced by intravenous injection of collagen and epinephrine. Mice were protected by low doses of two disintegrins, albolabrin (10 micrograms/mouse) and eristostatin (0.6 micrograms/mouse), whereas high doses of a thrombin inhibitor and an inhibitor of von Willebrand Factor binding to glycoprotein Ib were not effective. Injection of collagen and epinephrine resulted in the drop of platelet count and accumulation of platelet aggregates in the lung that appears to be the immediate cause of death. Albolabrin or eristostatin administration did not prevent the decrease of platelet count. Injection of albolabrin resulted in the formation of smaller and reversible platelet aggregates in the lungs and decreased accumulation of 51Cr-labeled platelets in the lung suggesting that this disintegrin decreases formation of platelet aggregates in vivo. We compared the effects of albolabrin and erisostatin on platelet aggregation, tail bleeding time, and survival of challenged animals. Eristostatin was about 5 times more potent in inhibiting platelet aggregation in vitro than albolabrin and 38 times more potent than albolabrin in protecting animals from sudden death. Both disintegrins, at the same doses (0.6-5 micrograms/mouse), caused similar dose-dependent prolongation of the bleeding time; however, only eristostatin exerted a protective effect. In conclusion, a) the mouse antithrombotic assay is a suitable model to screen and to evaluate the potency of platelet fibrinogen receptor antagonists in vivo; b) the results of the antithrombotic assay correlate better with the inhibition of platelet aggregation in vitro than with the prolongation of bleeding time.

Age-related alterations in the arterial microvasculature of skeletal muscle

This study investigated the possibility that the aging process results in alterations in the structure and/or functional reactivity of the microvessels that could contribute to increased resistance to blood flow in working skeletal muscle. Initially, latex casts were made of the cremaster muscle microvasculature in adult (12 mo) and senescent (24 mo) male Fischer 344 rats. Although the average diameter was not different between age groups, segmental length (distance between adjacent branches) increased significantly (3rd order) during aging. Additionally, in vivo experiments were performed to determine the response of the vessels to the topical application of norepinephrine and adenosine. There was no increase in vasoconstriction produced by norepinephrine; however, the vasodilation in response to adenosine declined dramatically (1st and 2nd order) with advancing age. It can be concluded that the increase in skeletal muscle vascular resistance during contraction in aged male rats could be explained by morphological changes and/or the diminished vasodilation elicited by adenosine.

Binding of glycoprotein IIIa-derived peptide 217-231 to fibrinogen and von Willebrand factors and its inhibition by platelet glycoprotein IIb/IIIa complex

Based on previous reports in the literature and the high homology between platelet glycoprotein (GP) IIIa 217-231 and similar portions of other beta subunits of integrin receptors, we hypothesized that this region may participate in ligand binding. Using a polyclonal antibody against GPIIIa 217-231(YC), we tested the interaction of a synthetic peptide representing this region with fibrinogen (Fg), in the enzyme-linked immunosorbent assay (ELISA) system. Results show a calcium-independent, dose-related, direct interaction between GPIIIa 217-231(Y) and immobilized Fg. This peptide also bound to von Willebrand Factor (vWF) and fibronectin (Fn), but did not attach to a 50 kDa Fn fragment which is deficient in the cell attachment site. In addition, purified GPIIb/IIIa displaced GPIIIa 217-231(Y) from Fg and vWF. Binding of 125I-GPIIIa 217-231(Y) to Fg coated tubes was inhibited by soluble Fg and by the GPIIb/IIIa complex. We synthesized this peptide with several alterations; similar peptides with Pro-219 replaced with an Ala showed significantly reduced binding to Fg and vWF. The decreased binding of the peptides with Pro-219 substitutes suggests that the confirmation of GPIIIa 217-230 is important for its ability to bind to adhesive ligands. In conclusion, the amino acid residues between 217 and 231 of GPIIIa appear to be involved in ligand binding and Pro-219 probably plays a significant role in this interaction.

Platelet factor 4 efficiently reverses heparin anticoagulation in the rat without adverse effects of heparin-protamine complexes

BACKGROUND

It has been observed that the reversal of heparin anticoagulation in humans by protamine sulfate (PS) results in various adverse reactions including leukopenia, thrombocytopenia, activation of complement, increased vascular permeability, systemic hypotension, pulmonary vasoconstriction, and pulmonary edema. The purpose of this study was to compare the efficacy and effects of native platelet factor 4 (PF4) and recombinant platelet factor 4 (rPF4) with those of PS in heparin neutralization in vivo, using a rat model.

METHODS AND RESULTS

Sprague-Dawley rats were anesthetized with sodium pentobarbital, and the right femoral vein and carotid artery were cannulated. For determination of activated partial thromboplastin time, platelet count, white blood cell count, and complement titer, arterial blood samples were taken before and immediately after heparin (10 units/100 g) infusion and at several time points after the infusion of the neutralizing agent (PS, 0.1 mg/100 g; PF4, 0.5 mg/100 g). In separate groups of animals, mean arterial blood pressure was monitored throughout identical protocols and the lungs were prepared for histological examination. The anticoagulant activity of heparin was effectively reversed by all of the neutralizing agents (PS, PF4, and rPF4). Platelet count (48% of initial), white blood cell count (52% of initial), complement titer (60% of initial), and mean arterial pressure (20% decrease) decreased significantly in heparinized animals receiving PS but not in those receiving PF4 or rPF4. Lung interstitium appeared normal when heparin was followed by PF4; however, interstitial edema and hemorrhage were observed with heparin-PS.

CONCLUSIONS

These results suggest that PF4 efficiently reverses heparin anticoagulation in the rat without the adverse effects of heparin-protamine complexes. Therefore, rPF4 may be an appropriate substitute for PS in patients undergoing cardiovascular surgery and other procedures that require heparin anticoagulation.

Cyclosporin A augments human platelet sensitivity to aggregating agents by increasing fibrinogen receptor availability

Clinical use of cyclosporin A (CsA) has been associated with platelet hypersensitivity and an increased incidence of thrombotic and vasoactive events. The purpose of this study was (1) to confirm that CsA enhances platelet sensitivity to the soluble agonists, adenosine diphosphate (ADP) and epinephrine (EPI), and (2) to determine if this enhancement is mediated by alteration in the availability of platelet surface fibrinogen receptors, a final mediator of platelet activation. Mean log dose of ADP required to achieve complete second-wave platelet aggregation in vitro decreased from 1.90 to 1.49 microM (n = 19, paired t test, P less than 0.05) and 2.86 to 2.11 microM (n = 16, P less than 0.05) following a 15-min and 3-hr incubation in the absence (saline) and presence of CsA (1000 ng/ml), respectively. At the threshold dose of ADP, concurrent thromboxane B2 levels at 15 min were 245 +/- 44 ng/ml (n = 12, saline) and 265 +/- 54 ng/ml (n = 9, CsA; P greater than 0.05). At 3 hr respective levels were 333 +/- 57 and 442 +/- 81 ng/ml (P greater than 0.05). Similar results were obtained with EPI. The number of fibrinogen binding sites in response to 50 microM ADP was determined in washed platelets in the absence and presence of CsA by radioligand binding. In 6 of 7 volunteers, CsA increased fibrinogen receptors from 26,635 +/- 4841 to 35,925 +/- 7290 sites/platelet (means +/- SEM; P less than 0.05). No change in receptor affinity was noted. In conclusion, cyclosporine does augment platelet reactivity.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of the disulfide bond pattern in albolabrin, an RGD-containing peptide from the venom of Trimeresurus albolabris: significance for the expression of platelet aggregation inhibitory activity

Albolabrin is a 73 amino acid peptide isolated from the venom of Trimeresurus albolabris. It contains an RGD sequence and 12 cysteines and is a potent inhibitor of both platelet aggregation and fibrinogen binding to the GPIIb/IIIa complex. This protein shows a high degree of analogy (primarily due to the alignment of all cysteines and the RGD) with a number of other viper venom proteins which inhibit cell adhesion and platelet aggregation and are referred to as disintegrins: rhodostomin, trigramin, flavoridin, applagin, elegantin, and batroxostatin. In this study, we found that the reduction and vinylpyridylethylation of albolabrin and flavoridin decreased their platelet aggregation inhibitory activity approximately 40-50 times. It can be postulated that the higher potency of native and reduced flavoridin as compared to albolabrin depends on the substitution of the Asp of albolabrin with a Phe at the C-terminal end of the RGD in flavoridin. The activity of a synthetic C-terminal peptide derived from flavoridin (residues 35-65) containing four cysteines was about 75 times lower than that of the original flavoridin. The substitution of a pair of cysteine residues with alanines in this peptide resulted in further loss of activity. In order to identify the disulfide bonds in albolabrin, the molecule was digested consecutively by trypsin and porcine pancreatic elastase. Peptides resulting from this digestion were isolated by reverse-phase HPLC and identified by amino acid composition and mass spectrometry.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of an antibody raised against reduced glycoprotein IIIa of human platelets

A polyclonal antibody against reduced and vinylpyridylethylated human glycoprotein IIIa was raised in rabbits. Its reactivity with reduced GPIIIa was about 500 times higher than that of the antibody against native GPIIIa. The lowest amounts of purified reduced and native GPIIIa recognized by the antibody against reduced GPIIIa were 25 and 400 ng, respectively. The antibody did not recognize native GPIIIa (about 1-2 micrograms) in platelet extracts and chymotryptic degradation products of GPIIIa. It inhibited ADP-induced platelet aggregation but it did not inhibit fibrinogen binding to ADP-stimulated platelets. Our experiments suggest that the antigenicity of GPIIIa (beta 3 integrin) depends on the conformation of the molecule determined by numerous S-S bridges between cysteine residues.

Disintegrins: a family of integrin inhibitory proteins from viper venoms

Disintegrins represent a new class of low molecular weight, RGD-containing, cysteine-rich peptides isolated from the venom of various snakes. They interact with the beta 1 and beta 3 families of integrins and their potency is at least 500-2000 times higher than short RGDX peptides. Analysis of the amino acid sequences of 14 different disintegrins suggests that the RGD sequence, in the spatial configuration determined by the appropriate pairing of the cysteine residues, functions as a cell recognition site. However, certain nonconserved amino acids appear to modify the activity of disintegrins, their specificity for various receptors, and their ability to compete specifically with various ligands.

Inhibition of platelet adhesion to surfaces of extracorporeal circuits by disintegrins. RGD-containing peptides from viper venoms

Previous studies indicate that exposure of fibrinogen receptors associated with glycoprotein IIb/IIIa complex contributes to platelet loss during cardiopulmonary bypass. Recently, we isolated a number of RGD (Arg-Gly-Asp)-containing, low molecular weight, cysteine-rich peptides from viper venoms. These peptides, which we propose to call "disintegrins," block platelet-fibrinogen interaction and platelet aggregation. We compared the effect of RGDS (Arg-Gly-Asp-Ser) and four disintegrins (echistatin, flavoridin, albolabrin, and bitistatin) on platelet behavior in a membrane oxygenator. During simulated extracorporeal circulation for 2 hours, platelet count decreased to about 30% of initial values. Addition of echistatin (60-200 nM), albolabrin (60-200 nM), bitistatin (60 nM), and flavoridin (45 nM) significantly inhibited platelet loss in the circuit. RGDS (33 microM) did not show any significant inhibitory effect. ADP-induced platelet aggregation was inhibited in samples of platelet-rich plasma taken from the circuits containing disintegrins. However, echistatin appeared to be a more potent inhibitor of platelet aggregation, whereas albolabrin and flavoridin interfered more selectively with platelet loss from the circuit. Echistatin prevented the accumulation of glycoprotein IIIa on the surface of the circuit. Echistatin (60-200 nM), flavoridin (45 nM), bitistatin (60 nM), and albolabrin (200 nM) significantly inhibited the loss of beta-thromboglobulin from platelets into circulating plasma. Electron microscopy studies demonstrated shape change but not degranulation in platelets circulating in the presence of 200 nM echistatin. On the other hand, this peptide (up to 1,000 nM) did not prevent loss of alpha granules and beta-thromboglobulin from thrombin-stimulated platelets, although it prevented their aggregation. In conclusion, disintegrins protect platelets in the circuit by preventing their adhesion to surfaces and, therefore, preventing fragmentation of adhered platelets under the shear stress of flowing blood. This study indicates that disintegrins may be potential candidates for platelet protection during cardiopulmonary bypass.