Inhibition of platelet function with synthetic peptides designed to be high-affinity antagonists of fibrinogen binding to platelets

Fresh frozen plasma resuscitation attenuates platelet dysfunction compared with normal saline in a large animal model of multisystem trauma

BACKGROUND

Platelet dysfunction following trauma has been identified as an independent predictor of mortality. We hypothesized that fresh frozen plasma (FFP) resuscitation would attenuate platelet dysfunction compared with 0.9% normal saline (NS).

METHODS

Twelve swine were subjected to multisystem trauma (traumatic brain injury, liver injury, rib fracture, and soft tissue injury) with hemorrhagic shock (40% of estimated blood volume). Animals were left in shock (mean arterial pressure, 30-35 mm Hg) for 2 hours followed by resuscitation with three times shed volume NS (n = 6) or one times volume FFP (n = 6) and monitored for 6 hours. Platelet function was assessed by adenosine diphosphate (ADP)-induced platelet aggregation at baseline, after 2 hours of shock following resuscitation, and 6 hours after resuscitation. Fibrinogen levels and markers of platelet activation (transforming growth factor β [TGF-β], sP-Selectin, and CD40L) as well as endothelial injury (intercellular adhesion molecule 1 [ICAM-1], vascular cell adhesion molecule 1 [VCAM-1]) were also assayed. Thromboelastography was used to measure clotting activity.

RESULTS

ADP-induced platelet aggregation was significantly higher in the FFP group (46.3 U vs. 25.5 U, p < 0.01) following resuscitation. This was associated with higher fibrinogen levels (202 mg/dL vs. 80 mg/dL, p < 0.01) but lower endothelial activation (VCAM-1, 1.25 ng/mL vs. 3.87 ng/mL, p = 0.05). Other markers did not differ.After 6 hours of observation, ADP-induced platelet aggregation remained higher in the FFP group (53.8 U vs. 37.0 U, p = 0.03) as was fibrinogen levels (229 mg/dL vs. 153 mg/dL, p < 0.01). Endothelial activation was lower (ICAM-1, 21.0 ng/mL vs. 24.4 ng/mL, p = 0.05), whereas TGF-β levels were higher (2,138 pg/mL vs. 1,802 pg/mL, p = 0.03) in the FFP group. Other markers did not differ. Thromboelastography revealed increased clot strength in the FFP group at both postresuscitation time points.

CONCLUSION

Resuscitation with FFP resulted in an immediate and sustained improvement in platelet function and clot strength compared with high-volume NS resuscitation. This was associated with an increase in fibrinogen levels and an attenuation of endothelial activation.

Decoration of fibrinogen γ-chain peptide on adenosine diphosphate-encapsulated liposomes enhances binding of the liposomes to activated platelets

For the purpose of efficient hemostasis, we previously developed ADP-encapsulated liposomes modified with a dodecapeptide (HHLGGAKQAGDV, H12), H12-(ADP)Lipo. This liposome actually enhanced platelet aggregation in vitro, and showed significant hemostatic effect in vivo. Since fibrinogen (Fbg) is abundant in the bloodstream, it is unclear why this liposome binds platelets so efficiently, overcoming the competition with Fbg. Therefore, we investigated the relationship between H12 density on the liposome and the binding ability to platelets, and evaluated the inhibitory effect of Fbg on the binding of H12-(ADP)Lipo to platelets. As a result, the binding ability to platelets steeply increased depending on H12 density until it reached about 3×10(15) H12 molecules/m(2). The 50% inhibition concentration of Fbg on the binding of H12-(ADP)Lipo to platelets was about 25-fold over the concentration of H12 molecules on the liposome. Moreover, almost no inhibition by Fbg was observed at the physiological concentration of it. This result suggests that the ability of H12 to bind to GPIIb/IIIa increased overwhelmingly by the anchoring to the liposome that enabled the cooperative binding of H12 peptides to the platelets.

Selection and structure of ion-selective ligands for platelet integrin alpha IIb(beta) 3

Integrins contain a number of divalent cation binding sites that control ligand binding affinity. Ions such as Ca(2+) and Mg(2+) bind to distinct sites on integrin and can have opposing effects on ligand binding. These effects are presumably brought about by alterations of the shape of the ligand binding pocket. To gain insight into the nature of these structural differences, we probed the integrin ligand binding site with an RGD-based library of unparalleled complexity. A cysteine-constrained phage library containing six random amino acids and the RGD motif present in seven different registers was used to select for ligands that exhibit ion-selective binding to integrin alpha(IIb)beta(3). The library was used to select for peptides that bind to the integrin alpha(IIb)beta(3) preferentially in Ca(2+) versus Mg(2+). Peptides were identified which bound selectively in each ion. The Ca(2+)-selective peptides had a range of sequences, with the only obvious consensus involving a motif that had four cysteine residues bonded in a 1,4:2,3 arrangement. Interestingly though, the Mg(2+)-selective peptides exhibited a well defined consensus motif containing Cys-X-aromatic-L/G-R-G-D-hydrophobic-R-R/K-Cys. As a first step toward understanding the structural basis for this selectivity, solution NMR structures were obtained for representatives of both sets of peptides. All peptides formed turns, with the RGD motif at the apex. The Mg(2+)-selected peptides contained a unique basic patch that protrudes from the base of the turn.

Incorporation of an Asp-Ser sequence to form an RGDS-like motif in hirutonin: the effect on in vitro platelet function

We investigated the effect on in vitro platelet function of hirutonin, a modified hirutonin with an RGD-like motif, a pseudo-RGDS peptide and a linear RGDS peptide. Inhibition of expression of surface fibrinogen on ADP-activated platelets with 40 microM of the peptide was as follows: hirutonin 10+/-3%, modified chimeric peptide 26+/-5%, pseudo-RGDS 66+/-11% and linear RGDS 93+/-13%. Both hirutonin and the chimeric peptide significantly inhibited ADP-induced platelet activation as detected by CD62 expression. Unlike the RGDS and pseudo-RGDS controls, neither the chimeric peptide nor the parent hirutonin inhibited ADP-induced platelet aggregation even at 140 microM. The chimeric hirutonin peptide reduced ATP release from ADP-stimulated platelets by 40+/-4%. This inhibition was stronger than that caused by hirutonin (23+/-13%), but less than the RGDS (90+/-2%) and pseudo RGDS-peptides (59+/-11%). Primary platelet haemostasis was slightly but not significantly affected by the peptide at 40 and 80 microM. However, shear-induced platelet adhesion to vWF and especially subsequent aggregate formation was interrupted after the addition of the chimeric peptide. Similar results were obtained with hirutonin. This inhibition was not as marked as with the RGDS- and pseudo-RGDS peptides. Both the parent hirutonin and the chimeric peptide caused prolongation of the clinical coagulation assays aPTT and TT. In conclusion, the chimeric hirutonin peptide with introduction of the RGD motif retained its anticoagulant effect but had little formal disintegrin activity. Instead, it appeared to have novel anti-platelet effects that may be of therapeutic use.

PLATSAK, a potent antithrombotic and fibrinolytic protein, inhibits arterial and venous thrombosis in a baboon model

Antiplatelet-antithrombin-staphylokinase (PLATSAK) is a chimeric protein that was recombinantly produced in Escherichia coli cells. The protein was designed to target haemostasis at three different levels. It consists of staphylokinase for activation of fibrinolyis, the Arg-Gly-Asp sequence for the prevention of platelet aggregation, and an antithrombotic peptide for the inhibition of thrombin. The in vivo activity of PLATSAK was evaluated by assessing its effect on platelet deposition in a baboon model of arterial and venous thrombosis. Dacron vascular graft segments and expansion chambers, inserted as extensions into permanent femoral arteriovenous shunts, were used to simulate arterial and venous thrombosis, respectively. PLATSAK (3.68 mg/kg) was administered as a bolus 10 minutes before placement of the thrombogenic devices. Platelet deposition onto the graft surface and in the expansion chamber was imaged in real time with a scintillation camera as the deposition of 111In-labeled platelets. After 2 hours, platelet deposition in the graft segments and expansion chambers was inhibited by 50% and 85%, respectively, when compared to control studies. The activated partial thromboplastin time was lengthened to greater than 120 seconds. Interestingly, the level of fibrinogen degradation products in plasma did not increase after administration of PLATSAK. These results demonstrate that PLATSAK effectively inhibited platelet deposition in both arterial- and venous-type thrombosis in an animal model.

cDNA cloning of brevinase, a heterogeneous two-chain fibrinolytic enzyme from Agkistrodon blomhoffii brevicaudus snake venom, by serial hybridization-polymerase chain reaction

Brevinase is a heterogeneous two-chain fibrinolytic enzyme, different from all of the known single-chain enzymes. A cDNA encoding brevinase was cloned from the venom gland of Agkistrodon blomhoffii brevicaudus by serial hybridization-PCR. Serial hybridization-PCR effectively amplified the complete cDNA of brevinase from the mixture of closely related transcripts. The cDNA sequence of 744 nucleotides was determined. The cDNA sequence included an open reading frame of 233 amino acids composed of an A chain (77 residues) and a B chain (156 residues). The deduced amino acid sequence included a potential N-glycosylation site (N54-X-S56), O-glycosylation site (Ser179), and RGD sequence. Brevinase included a unique Arg77 residue at the C-terminus of the A chain, distinguishing it from all of the compared homologous single-chain proteases. It could be assumed that a posttranslational cleavage site is located between Arg77 and Asn78. Based on the sequence similarity to those of the venom proteases, we could deduce that the critical catalytic residues are His40, Asn78, Asp85, and Ser179 and that the six potential disulfide bonds are Cys7-Cys138, Cys26-Cys41, Cys73-Cys231, Cys117-Cys185, Cys149-Cys164, and Cys175-Cys200. Despite the conservation of critical sequences, the phylogenetic tree showed that two-chain brevinase might be evolved separately from the homologous single-chain proteases.

Inhibitory effect of hexapeptide (RGRHGD) on platelet aggregation

The B chain of beta-bungarotoxin 1-6 sequence, RGRHGD, presents the highest local average hydrophilicity measured by Kyte and Doolittle modeling analysis. The RGRHGD holds parts of both RGD and KGD peptides, which have been reported as having high binding affinity to GPIIb-IIIa. The present study evaluates whether the synthesized hexapeptide, RGRHGD, has an antiplatelet effect and further elucidates the possible mechanisms of action. RGRHGD dose-dependently inhibited rabbit platelet aggregation and adenosine triphosphate release induced by arachidonic acid, collagen, platelet-activating factor, thrombin, or U46619 with the IC50 range of 82.7 to 510 microg/mL. The platelet thromboxane B2 formation induced by collagen or thrombin was also significantly decreased by RGRHGD, but there was no effect on arachidonic acid-induced thromboxane B2 formation. In addition, RGRHGD also inhibited the rise of intracellular calcium level stimulated by arachidonic acid, collagen, or thrombin in Fura 2-AM-loaded platelets. The adenosine 3',5'-cyclic monophosphate level of washed platelets was not affected by RGRHGD. In conclusion, these data indicate that the inhibitory effect of RGRHGD on platelet aggregation may be due to the attenuation of thromboxane A2 formation and intracellular calcium mobilization. In addition, this study may provide a useful method of finding potential therapeutic agents by using molecular modeling analysis.

Affinity of Fibrinogen Binding to Platelet Membrane Glycoprotein IIb/IIIa Increases with RGDS and gamma Chain Fibrinogen Peptide Hybrid

Arg-Gly-Asp (RGD)-containing peptides and the peptide unique to fibrinogen in the C-terminal domain of the gamma chain are important for fibrinogen binding to platelet membrane glycoprotein (GP) II b/III a. We synthesized a unique hybrid peptide of YRGDSPLGGAKQAGDV encompassing the RGD sequence (RGDS) with the gamma chain peptide (LGGAKQAGDV). Maximum binding to thrombin-stimulated platelets was achieved within 40 minutes with the YRGDS peptide and within 20 minutes with the hybrid peptide and native fibrinogen. The platelet binding sites were 58,600 molecules with the hybrid peptide and 52,400 molecules with YRGDS. These peptides inhibited fibrinogen binding to thrombin-stimulated platelets in a dose-dependent manner. The order of inhibitory potency of these peptides was as follows: fibrinogen models R: hybrid peptide > YRGDS > GQQHHLGGAKQAGDV (G15). The RGES peptide had no inhibitory activity. These three peptides inhibited binding of the anti-GP II b/III a monoclonal antibody (LJ-CP8) to platelets. The hybrid peptide showed the most potent inhibitory activity, with an IC50 of 48 µM. These results suggest that the linear combined RGDS and gamma chain peptide created with proline increases the affinity of binding to activated platelets.

A new model of dual interacting ligand binding sites on integrin alphaIIbbeta3

The platelet integrin alphaIIbbeta3 mediates platelet aggregation and platelet adhesion. This integrin is the key to hemostasis and also to pathologic vascular occlusion. A key domain on alphaIIbbeta3 is the ligand binding site, which can bind to plasma fibrinogen and to a number of Arg-Gly-Asp (RGD)-type ligands. However, the nature and function of the ligand binding pocket on alphaIIbbeta3 remains controversial. Some studies suggest the presence of two ligand binding pockets, whereas other reports indicate a single binding pocket. Here we use surface plasmon resonance to show that alphaIIbbeta3 contains two distinct ligand binding pockets. One site binds to fibrinogen, and a separate site binds to RGD-type ligands. More importantly, however, the two ligand binding pockets are interactive. RGD-type ligands are capable of binding to alphaIIbbeta3 even when it is already occupied by fibrinogen. Once bound, RGD-type ligands induce the dissociation of fibrinogen from alphaIIbbeta3. This allosteric cross-talk has important implications for anti-platelet therapy because it suggests a novel approach for the dissolution of existing platelet thrombi.

A new short chain RGD-containing disintegrin, accutin, inhibits the common pathway of human platelet aggregation

A new short-chain disintegrin, accutin, was purified from the Formosan Agkistrodon acutus venom by using of gel filtration, ion exchanger and reverse phase HPLC. The homogeneous protein is a 47-residue polypeptide with a molecular mass of 5241 Da containing an Arg-Gly-Asp sequence and seven cysteinyl residues at positions highly homologous to other disintegrins. Accutin dose-dependently inhibited human platelet aggregation stimulated by ADP, collagen, thrombin or the thromboxane analogue U46619 in platelet suspension with IC50 values of 66-267 nM. It was also active in inhibiting platelet aggregation of platelet-rich plasma. However, accutin apparently did not affect the shape change caused by these agonists. Accutin also inhibited fibrinogen-induced aggregation of human elastase-treated platelets in a dose-dependent manner. Furthermore, accutin dose-dependently inhibited the binding reaction of fluorescein isothiocyanate (FITC)-conjugated arietin, a member of the disintegrin family, to human platelets. In addition, the binding of FITC-conjugated accutin to platelets was almost completely blocked by a monoclonal antibody, 7E3, raised against the platelet glycoprotein IIb/IIIa complex. On the other hand, accutin as well as other disintegrins, rhodostomin and arietin, exhibited an inhibitory effect on 7E3 binding toward platelets and endothelial cells in a dose-dependent manner. It is concluded that accutin, a new platelet aggregation inhibitor belonging to the short-chain disintegrin family, acts specifically on a binding epitope of GPIIb/IIIa overlapping with that of 7E3, leading to the blockade of fibrinogen binding to its receptor.

GPIIb/IIIa integrin antagonists with the new conformational restriction unit, trisubstituted beta-amino acid derivatives, and a substituted benzamidine structure

Ethyl N-[3-(2-fluoro-4-(thiazolidin-3-yl(imino)methyl)benzoyl)amino-2, 2-dimethylpentanoyl]piperidine-4-acetate 40 (NSL-96184) is a highly potent and orally active fibrinogen receptor antagonist, which is characterized by the presence of the trisubstituted beta-amino acid residue, 3-ethyl-2,2-dimethyl-beta-alanine. This compound was developed on the basis of the SAR study of N-[3-(N-4-amidinobenzoyl)amino-2, 2-dimethyl-3-phenylpropionyl]piperidine-4-acetic acid 1(NSL-95301) with the derivatization focused on the central trisubstituted beta-amino acid unit as well as the basic amidinobenzoyl unit, and the esterification of the carboxyl group for prodrug composition. Compound 1, which was reported in our previous study, was discovered by the application of combinatorial chemistry. The molecular modeling study suggests that the trisubstituted beta-amino acid unit is responsible for fixing the molecule to its active conformation. Compound 40 showed an excellent profile in the in vitro and in vivo studies for its human platelet aggregation inhibitory activity and oral availability in guinea pigs. This oral availability largely depends on the modification of the amidino group with a cyclic secondary amine, i.e., thiazolidine in 40. In in vivo studies, the onset of the antiplatelet action of 40 is very fast after oral administration, whereas its duration of action is relatively short. These results suggest that 40 has an excellent therapeutic potential, especially for antithrombotic treatment in the acute phase. 3-Substituted-2,2-dimethyl-beta-amino acid residues would serve as new and useful linear templates to restrict the conformational flexibility of peptidomimetics.

Endothelium-dependent vasorelaxant and anti-aggregatory effect and mechanism of action of some antifibrinogen RGD (Arg-Gly-Asp-containing) peptides

Vasorelaxation caused by some antifibrinogen RGD (Arg-Gly-Asp-containing) peptides and their basic mechanism of action was studied on rabbit isolated thoracic aortic rings preconstricted with 0.25 microM phenylephrine. GRGDS (Gly-Arg-Gly-Asp-Ser-OH) and RGDV (Arg-Gly-Asp-Val-OH) caused dose-dependent relaxation. RGDS (Arg-Gly-Asp-Ser-OH) had a biphasic effect (a transient relaxation followed by a contraction) while GRGDS-[SE] (Gly-Arg-Gly-Asp-Ser(SO3)-OH) did not change the isometric tone of precontracted aortic preparations. GRGDS and RGDV exerted no relaxing effect on endothelium-denuded blood vessels suggesting that the vascular action of these peptides was entirely dependent on the presence of functionally intact endothelium. L-NG-Nitro-arginine (30 microM) attenuated the relaxation induced by GRGDS and abolished that induced by RGDV. All of the four RGD congeners inhibited ADP-induced aggregation of human platelets. These findings indicate that the relaxant effect of RGDV is mediated exclusively by the nitric oxide pathway, but GRGDS could cause, besides nitric oxide release, the release of another substance which is different from nitric oxide. Because the rank order of the vasorelaxant potencies of RGD peptides differed from that found for their anti-aggregatory activities, a vascular effector mechanism mediated by an RGD-recognizing structure other than the known glycoprotein IIb/IIIa-like RGD-binding site is suggested.

Discovery and structure--activity relationship studies of a novel and specific peptide motif, Pro-X-X-X-Asp-X, as a platelet fibrinogen receptor antagonist

A novel hexapeptide, H-Pro-Ser-Nva-Gly-Asp-Trp-OH 6, a specific antagonist of platelet fibrinogen receptor (GpIIb/IIIa), was discovered in a structure-activity relationship (SAR) study where the role of the N-terminal Pro moiety of an RGD-containing peptide, H-Pro-Ser-Arg-Gly-Asp-Trp-OH 1, which is a potent but not specific antagonist toward GpIIb/IIIa integrin, was investigated. This novel peptide 6 exhibits very high activity as a human platelet aggregation inhibitor (IC50 0.59 microM, human PRP/collagen) as well as marked specificity for GpIIb/IIIa. A series of substitutions at the third position (Nva residue) in this hexapeptide, focused on the conformational rigidity, led to compounds which are superior to the original novel peptide 6 with regard to anti-platelet activity. The peptides, H-Pro-Ser-Hyp-Gly-Asp-Trp-OH 17 and H-Pro-Ser-delta Pro-Gly-Asp-Trp-OH 18 with the 5-membered ring structure, which restricted the conformation of the peptide backbone at the third position, inhibited the aggregation of human platelets at submicromolar concentrations (IC50 0.39 and 0.30 microM, respectively). Further structure-activity relationship studies at each position of the peptide sequence suggest a novel motif sequence, Pro-X1-X2-X3-Asp-X4, for specific GpIIb/IIIa integrin recognition, in which the N-terminal free Pro residue and the Asp residue at the fifth position are essential to the activity. This motif sequence is summarized as follows: (1) a small amino acid such as Ser, Ala or Gly is preferable at X1 position; (2) X2 may be any amino acid, preferably a bulky amino acid such as Tle or a cyclic amino acid such as Pro; (3) X3 must be a small amino acid such as Gly; and (4) X4 is preferably an amino acid with an aromatic side chain.

Thrombosis and Hemostasis in Cardiology: Review of Pathophysiology and Clinical Practice (Part I)

The adverse consequences of thrombosis are per haps nowhere more evident than in clinical cardiology. Throm bosis and hemostasis are primary issues in the management of patients with atrial fibrillation, prosthetic heart valves, severe left ventricular dysfunction, and coronary artery disease. Clini cal trials have defined a crucial role for anticoagulation with warfarin in patients with atrial fibrillation to reduce the inci dence of stroke. Anticoagulation with warfarin and aspirin in combination offers significant protection from systemic emboli in patients with mechanical prosthetic valves, without a sub stantial increased risk of hemorrhage. The risk of systemic emboli may also be reduced by anticoagulation in patients with severe left ventricular dysfunction. Disturbance of the normal balance of hemostasis is a major factor in the pathophysiology of coronary artery disease. Antiplatelet therapy, antithrombin agents, anticoagulants, and fibrinolytic agents have been used to prevent and treat acute coronary thrombosis and to prevent reocclusion following thrombolysis and interventional therapy. Guidelines are presented for antithrombotic therapy in the prac tice of clinical cardiology.

Production of a recombinant antithrombotic and fibrinolytic protein, PLATSAK, in Escherichia coli

The three main components involved in thrombosis and haemostasis are thrombin, platelets, and plasmin. Almost all inhibitors of thrombosis are focused either on the inhibition of thrombin or on the inhibition of platelets. We designed a construct using the fibrinolytic activity of staphylokinase, fused via a cleavable linker to an antithrombotic peptide of 29 amino acids. The peptide was designed to include three inhibitory regions: (1) the Arg-Gly-Asp (RGD) amino acid sequence to prevent fibrinogen binding to platelets; (2) a part of fibrinopeptide A, an inhibitor of thrombin; and (3) the tail of hirudin, a potent direct antithrombin. The amino acid sequence of the 29 amino acid peptide was reverse translated, and the gene was chemically synthesised and cloned into an expression vector as a 3' fusion to the staphylokinase gene. Gene expression was induced in E. coli Top 10 cells and the fusion protein, designated PLATSAK, was purified using metal affinity chromatography. The purified fusion protein significantly lengthened the activated partial thromboplastin time and thrombin time and inhibited the amidolytic activity of thrombin. The fibrinolytic activity was almost equal to that of recombinant staphylokinase as measured with a thrombelastograph. Platelet aggregation was not markedly inhibited by PLATSAK, probably due to the unfavourable three dimensional structure, with the Arg-Gly-Asp sequence buried inside. Our results confirm that it is feasible to design and produce a hybrid multifunctional protein that targets various components of the haemostatic process.

Intranasal absorption of the platelet glycoprotein IIb/IIIa receptor antagonist, DMP 755, and the effect of anesthesia on nasal bioavailability

Intranasal absorption and bioavailability of DMP 755, a peptidomimetic, platelet glycoprotein IIb/IIIa receptor antagonist, were examined in anesthetized and lightly sedated dogs. Nasal bioavailability was determined by measuring plasma concentrations relative to those after intravenous dosing. DMP 755 is an ester prodrug, and bioavailability reflects concentrations of the acid hydrolysis product. Nasal bioavailability in dogs anesthetized with pentobarbital was 85 +/- 4%, whereas in dogs anesthetized with the short-acting anesthetic propofol, bioavailability was 32 +/- 7%. Nasal bioavailability was greater than the reported oral bioavailability of DMP 755 in dogs, and was quite consistent. Because anesthesia affects nasal bioavailability, an effect that may depend on the absorption half-life of the test compound, a conscious or lightly sedated animal model is preferred.

A novel peptide motif for platelet fibrinogen receptor recognition

To develop a specific antagonist of platelet alphaIIbbeta3 using small linear peptides, we synthesized a series of hexapeptides that did not have an Arg-Gly-Asp (RGD) sequence and examined their anti-platelet activity and their specificity for alphaIIbbeta3. We found a novel motif sequence, Pro-X1-X2-X3-Asp-X4, where X1 to X4 were all L-form alpha-amino acids, which specifically inhibited aggregation of human platelets at submicromolar concentrations. The Pro residue at the N terminus was essential to the anti-platelet activity, and the acetylation of the imino group of this residue also resulted in the complete loss of the activity. The results of the binding assay using purified human platelet alphaIIbbeta3 and placental alphavbeta3 and those of the cell adhesion assay suggest that this motif peptide is highly specific for platelet alphaIIbbeta3 among other integrins. Flow cytometric studies using an fluorescein isothiocyanate-labeled RGD peptide showed that this motif peptide inhibited the binding of an RGD peptide to activated platelets, suggesting that it has the same inhibitory mode as RGD peptides. Conformational analysis of this motif peptide and an RGD-containing peptide suggests that the imino group of the Pro residue may substitute for the role of the guanidino group of the Arg residue of the RGD sequence.

Ligand binding to integrin alphaIIbbeta3 is dependent on a MIDAS-like domain in the beta3 subunit

Substitution of beta3 residue Asp119, Ser121, or Ser123 results in a loss of the ligand binding function of integrin alphaIIbbeta3. Homologous residues in other integrin beta subunits are similarly critical for ligand binding function. This DXSXS motif is also present in the I domain of certain integrin alpha subunits, where it constitutes a portion of the unique metal ion-dependent adhesion site (MIDAS). In this report, we have utilized the crystal structure of the recombinant alphaM I domain to produce a three-dimensional model of the homologous region in the integrin beta3 subunit. We performed mutagenesis of candidate amino acid residues predicted from this model to be involved in cation coordination and ligand binding. We report the identification of Asp217 and Glu220 as residues essential for the ligand binding function of alphaIIbbeta3. Alanine substitution of these residues did not affect receptor expression but abolished the binding of activation-dependent (PAC1) and -independent (OPG2) ligand mimetic antibodies. In our proposed model, beta3 Asp217 is analogous to a metal-coordinating residue in the alphaM MIDAS domain, while Glu220 does not correspond to a functional MIDAS domain residue. Substitution of the highly conserved beta3 residue Thr197 corresponding to a critical MIDAS metal-coordinating Thr residue did not affect ligand binding function, suggesting that this region of beta3 adopts a structure that is very similar to but not identical to that of the MIDAS domain. These data support a functional linkage between these two sequences and further define a common feature of ligand binding to integrins.

Platelet aggregation is inhibited by phycolectins

Lectins from four marine algal species were examined for interaction with human platelets. The lectin designated hypnin A, from the red alga Hypnea japonica, inhibited adenosine diphosphate (ADP)- or collagen-induced human platelet aggregation in a dose-dependent manner. Complete inhibition was observed at concentrations of 100 and 5 micrograms/ml of the lectin with ADP (2 microM) and collagen (0.2 microgram/ml)-induced platelet aggregation, respectively. At the inhibitory concentration of 0.5 to 100 micrograms/ml, the lectin did not induce aggregation of resting platelets. Lectins from the other three algal species also inhibited ADP-induced human platelet aggregation. These results indicate that the algal lectins are a new group of inhibitors and may be useful to study glycoconjugates on platelet membranes and to design novel platelet aggregation inhibitors.

Oral antiplatelet, antithrombotic efficacy of DMP 728, a novel platelet GPIIb/IIIa antagonist

BACKGROUND

Currently used antiplatelet drugs, including aspirin and ticlopidine, are effective against certain but not all of the many endogenous platelet activators. Because of their limited efficacy, a significant number of serious thromboembolic complications still occur, highlighting the need for a more effective therapy. DMP 728 has been characterized as a potent and specific platelet glycoprotein IIb/IIIa complex (GPIIb/IIIa) antagonist. The goals of the present study were to determine the oral antiplatelet and antithrombotic efficacies of DMP 728 in various arterial thrombosis models in dogs.

METHODS AND RESULTS

In conscious and anesthetized mongrel dogs, DMP 728 at 0.02 to 1.0 mg/kg PO in gelatin capsules produced dose-dependent antiplatelet effects in inhibiting ex vivo platelet aggregation induced by ADP and prolonging template bleeding time. DMP 728 effects on bleeding time prolongation could be reversed more rapidly than those on platelet aggregation inhibition. A maximal antiplatelet effect for DMP 728 was demonstrated at 1.0 mg/kg PO. DMP 728 demonstrated dose-dependent oral antiplatelet effects with an absolute oral bioavailability of 8% to 12% in dogs. Additionally, the antithrombotic efficacy of DMP 728 was examined after intravenous and oral administration at different doses in various models of arterial thrombosis. In the coronary artery Folts' model in dogs, DMP 728 demonstrated maximal antithrombotic efficacy at 0.01 mg/kg IV and < 0.6 mg/kg PO. Additionally, DMP 728 at 0.1 and 1.0 mg/kg IV or PO demonstrated 60% to 100% prevention of primary thrombosis (P < .01) in an electrolytically induced carotid artery thrombosis model in dogs.

CONCLUSIONS

These data suggest that DMP 728, a low-molecular-weight GPIIb/IIIa receptor antagonist, may have therapeutic potential as an oral antithrombotic agent in coronary and carotid artery thromboembolic disorders.

Direct demonstration of radiolabeled von Willebrand factor binding to platelet glycoprotein Ib and IIb-IIIa in the presence of shear stress

In this study it is demonstrated for the first time that shear stress induces the binding of exogenous von Willebrand factor (vWF) multimers to platelets. The vWF preparations used were: 125I-vWF purified from human cryoprecipitate (and including all vWF multimers present in normal plasma); and 35S-cysteine-vWF secreted by human umbilical vein endothelial cells (HUVECs) (and containing unusually large vWF forms, as well as all plasma-type vWF multimers). Direct shear-induced binding to washed platelets (300-360 x 10(3)/microliters) of radiolabeled vWF was maximum at 60-120 dynes/cm2 evaluated at 30 sec and was in extent about one-quarter of the binding stimulated by ristocetin after 3 min of incubation. The shear-induced binding of only a small percentage of added radiolabeled vWF was sufficient to initiate aggregation. Radiolabeled vWF attached to both glycoprotein (GP) Ib and GPIIb-IIIa receptors in the shear field, with complete inhibition of binding occurring with simultaneous blockade of both receptors. Binding was potentiated by ADP released from sheared platelets.

Staphylococcus aureus induces platelet aggregation via a fibrinogen-dependent mechanism which is independent of principal platelet glycoprotein IIb/IIIa fibrinogen-binding domains

Platelet aggregation by bacteria is felt to play an important role in the pathogenesis of infective endocarditis. However, the mechanisms involved in bacterium-induced platelet aggregation are not well-defined. In the present study, we examined the mechanisms by which Staphylococcus aureus causes rabbit platelet aggregation in vitro. In normal plasma, the kinetics of S. aureus-induced platelet aggregation were rapid and biphasic. The onset and magnitude of aggregation phase 1 varied with the bacterium-platelet ratio, with maximal aggregation observed at a ratio of 5:1. The onset of aggregation phase 2 was delayed in the presence of apyrase (an ADP hydrolase), suggesting that this later aggregation phase may be triggered by secreted ADP. The onset of aggregation phase 2 was delayed in the presence of prostaglandin I2-treated platelets, and this phase was absent when paraformaldehyde-fixed platelets were used, implicating platelet activation in this process. Platelet aggregation phase 2 was dependent on S. aureus viability and an intact bacterial cell wall, and it was mitigated by antibody directed against staphylococcal clumping factor (a fibrinogen-binding protein) and by the cyclooxygenase inhibitor indomethacin. Similarly, aggregation phase 2 was either delayed or absent in three distinct transposon-induced S. aureus mutants with reduced capacities to bind fibrinogen in vitro. In addition, a synthetic pentadecapeptide, corresponding to the staphylococcal binding domain in the C terminus of the fibrinogen delta-chain, blocked aggregation phase 2. However, phase 2 of aggregation was not inhibited by two synthetic peptides (alone or in combination) analogous to the two principal fibrinogen-binding domains on the platelet glycoprotein (GP) IIb/IIIa integrin receptor: (i) a recognition site on the IIIa molecule for the Arg-Gly-Asp (RGD) sequence of the fibrinogen alpha-chain and (ii) a recognition site on the IIb molecule for a dodecapeptide sequence of the fibrinogen delta-chain. This differs from ADP-induced platelet aggregation, which relies on an intact platelet GP IIb/IIIa receptor with an accessible RGD sequence and dodecapeptide recognition site for fibrinogen. Furthermore, a monoclonal antibody directed against the RGD recognition site on rabbit platelet GP IIb/IIIa receptors failed to inhibit rabbit platelet aggregation by S. aureus. Collectively, these data suggest that S. aureus-induced platelet aggregation requires bacterial binding to fibrinogen but is not principally dependent upon the two major fibrinogen-binding domains on the platelet GP IIb/IIIa integrin receptor, the RGD and dodecapeptide recognition sites.

Concept and progress in the development of RGD-containing peptide pharmaceuticals

The cell adhesion domain, arginine-glycine-aspartic acid (RGD), has been incorporated into synthetic peptides to perform either of two modes of drug action, antagonist or agonist. Short, conformationally constrained peptides have been developed as antagonists for the platelet membrane glycoprotein complex, the integrin alpha IIb beta 3, using cell-based and integrin-based assays. In combination with a comparative molecular modeling study, these results have helped identify common conformational elements in the pharmacophore of this class of molecules. Peptides are presented that are highly potent, integrin specific, and that possess reduced pharmacological side effects. Also presented is the development of a peptide that modifies, noncovalently, the surfaces of a wide variety of synthetic materials used in medical implants. The agonist activity of [corrected] this molecule is evident from its ability to stimulate cell attachment on these surfaces. This is shown to translate into an in vivo activity of faster and more complete tissue integration, and a reduction in foreign body response.

Multiple peptide synthesis on acid-labile handle derivatized polyethylene supports

Using the multipin peptide synthesis approach, a range of peptides with native amide and carboxylate C-termini were generated using an acid-labile approach. Polyethylene crowns grafted with hydroxyethylmethacrylate (HEMA) polymer were functionalized with either 4-hydroxymethylphenoxyacetic acid for the generation of peptide-carboxylate or p-[(R,S)-alpha-[1-(9H-fluoren-9-yl)methoxyformamido]-2,4-dim ethoxy- benzyl]phenoxyacetic acid for peptide-amide. A range of known peptide hormone sequences and other peptides with native C-termini were assembled by sequential incorporation of N alpha-Fmoc protected amino acids. Peptides were sidechain deprotected and cleaved from crowns with TFA/scavengers within 2 mL centrifuge tubes, and isolated by a series of ether/petrol wash and centrifugation steps. In this way it was possible to avoid a cleavage and isolation botteneck, allowing rapid processing of large numbers of peptides.

Tetrafibricin, a novel non-peptide fibrinogen receptor antagonist, induces conformational changes in glycoprotein IIb/IIIa

Arg-Gly-Asp (RGD) is an amino acid sequence in fibrinogen recognized by platelet glycoprotein (GP) IIb/IIIa. Recently, it was found that RGD peptide binding to GPIIb/IIIa leads to conformational changes in the complex that are associated with the acquisition of high-affinity fibrinogen-binding function. In this study, we found that tetrafibricin, a novel non-peptidic GPIIb/IIIa antagonist, induced similar conformational changes in GPIIb/IIIa as did RGD peptides. Tetrafibricin increased the binding of purified inactive GPIIb/IIIa to immobilized pl-80, a monoclonal antibody that preferentially recognizes ligand-occupied GPIIb/IIIa. Exposure of the pl-80 epitope by tetrafibricin was also observed on resting human platelets by flow cytometry. On intact platelets, the conformational changes transformed GPIIb/IIIa into a high-affinity receptor for fibrinogen and triggered subsequent platelet aggregation. Tetrafibricin is the first non-peptidic GPIIb/IIIa antagonist reported that has the capacity to induce conformational changes in GPIIb/IIIa.

Antiplatelet and antithrombotic efficacy of DMP 728, a novel platelet GPIIb/IIIa receptor antagonist

BACKGROUND

Currently used antiplatelet drugs, including aspirin, ticlopidine, and others, are effective against certain but not all of the many endogenous platelet activators. Because of their limited efficacy, a significant number of serious thromboembolic complications still occur, highlighting the need for a more effective therapy. Thus, we have identified a systemically active peptide analogue (DMP 728) of the arginine-glycine-aspartic acid (RGD) recognition sequence that mediates the binding of ligands such as fibrinogen to the platelet glycoprotein (GP) IIb/IIIa receptors. The goals of the present study were to determine the antiplatelet and antithrombotic efficacies of DMP 728 in various arterial thrombosis models.

METHODS AND RESULTS

DMP 728 demonstrated antiplatelet efficacy in vitro in inhibiting ADP-induced human platelet aggregation (IC50, 46 +/- 2 nmol/L) and fibrinogen binding to human platelets (IC50, 2.3 +/- 0.8 nmol/L) or purified human GPIIb/IIIa receptors (IC50, 0.6 +/- 0.1 nmol/L). DMP 728 demonstrated high affinity and specificity for human platelet GPIIb/IIIa over other adhesion molecules. In anesthetized mongrel dogs, DMP 728 at 0.001 to 1.0 mg/kg IV produced dose-dependent antiplatelet effects in inhibiting ex vivo platelet aggregation induced by ADP and in prolonging template bleeding time. DMP 728 effects on bleeding time prolongation were more rapidly reversible than those on platelet aggregation inhibition. A maximal antiplatelet effect for DMP 728 was demonstrated at 0.01 mg/kg IV bolus. The antithrombotic efficacy of DMP 728 was examined in vitro and in vivo after IV administration at different doses in various models of arterial thrombosis. In the coronary artery Folts model in dogs, DMP 728 demonstrated maximal antithrombotic efficacy at 0.01 mg/kg IV bolus with an ED50 of 0.005 mg/kg IV bolus in inhibiting cyclic flow reductions. Additionally, DMP 728 demonstrated 100% prevention of primary thrombosis and rethrombosis (P < .01) after treatment with different thrombolytics, including tissue plasminogen activator and streptokinase, in an electrolytically induced femoral artery thrombosis model in dogs.

CONCLUSIONS

Acute intravenous DMP 728 administration (0.001 to 1.0 mg/kg) has dose-dependent antiplatelet and antithrombotic effects in different arterial thrombosis models. These data suggest that DMP 728, a low-molecular-weight GPIIb/IIIa receptor antagonist, may have therapeutic potential as an effective antithrombotic agent in coronary and peripheral artery thromboembolic disorders.

Regulation of ligand binding to glycoprotein IIb-IIIa (integrin alpha IIb beta 3) in isolated platelet membranes

The major platelet integrin, glycoprotein IIb-IIIa, binds soluble fibrinogen only after platelet activation. To investigate the mechanism by which platelets convert glycoprotein IIb-IIIa into a functional fibrinogen receptor, we characterized the opening and closing of fibrinogen-binding sites in isolated platelet membranes and compared the regulatory properties of membrane-bound glycoprotein IIb-IIIa with those of the detergent-solubilized receptor. Basal fibrinogen binding to the membranes possessed many of the properties of fibrinogen binding to activated platelets; however, less than 10% of glycoprotein IIb-IIIa in the membranes was capable of binding fibrinogen. Preincubating the membranes with either an activating glycoprotein IIb-IIIa antibody or alpha-chymotrypsin increased fibrinogen binding. In contrast, agents that require intracellular mediators, such as platelet agonists, guanine-nucleotide-binding-protein activators and purified protein kinase C, did not stimulate fibrinogen binding to the membranes, suggesting that cytosolic factor(s) may be required for activation of the receptor in platelets. Occupancy of glycoprotein IIb-IIIa in the membranes with RGD (Arg-Gly-Asp)-containing peptides reversibly exposed neoantigenic epitopes and fibrinogen-binding sites in the receptor. These conformational changes required membrane fixation to be maintained following peptide removal. Similar results were obtained with purified glycoprotein IIb-IIIa incorporated into phospholipid vesicles, indicating that the resting state of the receptor is favoured in these environments. In contrast, when the conformation of detergent-solubilized glycoprotein IIb-IIIa was altered by exposure to RGD-containing peptides, the receptor remained active even after incorporation into phospholipid vesicles. These results demonstrate that platelet membranes are a useful model in which to study the regulation of glycoprotein IIb-IIIa and suggest that the environment surrounding the receptor may have a profound influence on this process.

Epitope mapping of the cat (Felis domesticus) major allergen Fel d I by overlapping synthetic peptides and monoclonal antibodies against native and denatured Fel d I

The major cat allergen Fel d I is a homodimer of which each monomer consists of two disulfide-linked polypeptide chains: chain 1 (70 amino acid residues) and chain 2 (92 amino acid residues). Twenty-one synthetic peptides of 14 amino acid residues length, overlapping by seven residues and spanning the entire sequence of both chains, were synthesized. These peptides were coupled to CNBr-activated Sepharose-4B and used as solid-phase antigens in epitope-mapping studies with monoclonal antibodies against native and reduced/alkylated Fel d I. Two monoclonal antibodies directed against reduced/alkylated chain 1 bound to the overlapping peptides 53-66 and 60-70 of chain 1. The monoclonal antibody directed against reduced/alkylated chain 2 bound to the overlapping peptides 36-49 and 43-56 of chain 2. Binding specificity was demonstrated by inhibition by reduced/alkylated Fel d I for all three monoclonal antibodies. Another monoclonal antibody against reduced/alkylated Fel d I had been found to bind predominantly to reduced/alkylated chain 2 on immunoblot in previous studies. It bound to peptides 1-16 and 60-70 of chain 1 and peptides 1-14 and 50-63 of chain 2; it is therefore probably directed against a conformational epitope formed by these four regions. Possibly because of low affinity of this monoclonal antibody, specificity of its binding could not be verified by inhibition studies. A panel of monoclonal antibodies directed against native Fel d I bound to peptides 1-16 and 60-70 of chain 1 and peptides 1-14 and 43-56 of chain 2.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding interactions of kistrin with platelet glycoprotein IIb-IIIa: analysis by site-directed mutagenesis

The binding interactions between platelet fibrinogen receptor, glycoprotein (GP) IIb-IIIa, and kistrin, a snake venom disintegrin protein that contains the adhesion site recognition sequence Arg-Gly-Asp (RGD) and potently inhibits platelet aggregation, have been investigated by site-directed mutagenesis of a synthetic kistrin gene. Kistrin was expressed as a fusion protein in Escherichia coli under control of the alkaline phosphatase promoter. This construction included the stII signal sequence to direct secretion to the periplasmic space and one synthetic (Z) domain of Staphylococcal protein A to allow affinity purification using IgG Sepharose. Kistrin was cleaved from the Z-domain by site-specific proteolysis using a mutant subtilisin BPN' and purified by reverse-phase HPLC. This approach facilitated the rapid purification of a set of 43 alanine replacement mutants whose relative affinity for GP IIb-IIIa was measured by competition with immobilized kistrin and by inhibition of platelet aggregation in human platelet-rich plasma. Alanine replacements at R49, G50, and D51 led to weaker inhibitors of platelet aggregation by 90-fold, 2-fold, and > 200-fold, respectively. The conservative D51E mutant was still > 100-fold less potent whereas R49K had a minor effect (1.8-fold), implying the critical nature of the aspartate for high affinity binding. However, mutations outside of the RGD region led to proteins indistinguishable from kistrin, suggesting no substantial secondary binding interactions. Furthermore, reduced kistrin is not active. We therefore propose that a favorable conformation of the RGD region alone is responsible for the high affinity binding of kistrin to GP IIb-IIIa.

Localization of the cross-linking sites of RGD and KQAGDV peptides to the isolated fibrinogen receptor, the human platelet integrin glycoprotein IIb/IIIa. Influence of peptide length

The non-covalent and Ca(2+)-dependent heterodimer GPIIb/IIIa, formed by platelet glycoproteins IIb (GPIIb) and IIIa (GPIIIa), also known as the integrin alpha IIb beta 3, is the inducible receptor for fibrinogen and other adhesive proteins on the surface of activated platelets. A fraction of the isolated GPIIb/IIIa in solution binds RGD or KQAGDV inhibitory peptides and, upon peptide removal, apparently acquires the capacity to bind fibrinogen ('activated' GPIIb/IIIa) [Du, X., Plow, E. F., Frelinger, A. L., III, O'Toole, T. E., Loftus, J. C. & Ginsberg, M. H. (1991) Cell 65, 409-416]. Photoaffinity labelling was used here to study the ligand binding site(s) of GPIIb/IIIa in solution, for which the peptides CKRKRKRKRRGDV (alpha 1), CGRGDF (alpha 2), CYHHLGGAKQAGDV (gamma 1) and CGAKQAGDV (gamma 2) were synthesized with a photoactivable cross-linker group and a fluorescent reporter group attached to the N-terminal cysteine residue. Contrary to the situation in activated platelets, both GPIIb and GPIIIa were equally labelled by the four peptides and the cross-linking sites were localized by protein chemical analyses of the fluorescently labelled tryptic peptides of both subunits. Thus, the localization of the cross-linking sites in GPIIb varies considerably with the peptide length and is very different from that localization observed in activated platelets: alpha 2 and gamma 2 were found cross-linked to the N-terminal of both the heavy (GPIIbH 42-73) and the light (GPIIbL2 30-75) chains of GPIIb; while the longer peptides alpha 1 and gamma 1 were cross-linked to the C-terminal of GPIIbH within the 696-724 and 752-768 peptide stretches, respectively. On the other hand, the cross-linking sites of the four inhibitory peptides in GPIIIa were found mainly within the proteolysis susceptible region, between the N-terminal (GPIIIa 1-52) and the core (GPIIb 423-622) highly disulphide-bonded domains, observing that the longer the peptide the closer the cross-linking site is to the N-terminal of GPIIIa: alpha 1 at GPIIIa 63-87 and 303-350; gamma 1 at GPIIIa 9-37; alpha 2 at GPIIIa 151-191; and gamma 2 at GPIIIa 303-350. These results led us to the following conclusions. (a) The GPIIIa 100-400 region contributes to the ligand-binding domain in GPIIb/IIIa both in solution and in activated platelets.(ABSTRACT TRUNCATED AT 400 WORDS)

Aminopeptidase resistant Arg-Gly-Asp analogs are stable in plasma and inhibit platelet aggregation

Tetrapeptides containing the sequence Arg-Gly-Asp (RGD) antagonize fibrinogen binding to its platelet receptor (gp IIb/IIIa, integrin alpha IIb beta 3) and inhibit platelet aggregation in vitro. The peptides RGDS and RGDY(Me)-NH2 were rapidly degraded when incubated in human, rat, and dog plasma. HPLC analysis indicated that amino acids were sequentially removed from the peptide N-terminus, and this degradation was prevented by the aminopeptidase inhibitor bestatin. Analogs of RGDY(Me)-NH2 with an acetylated or deleted alpha-amino group were prepared. Both analogs were stable when incubated in plasma, blocked 125I-fibrinogen binding to activated platelets (IC50 = 10-30 microM) and inhibited ADP induced platelet aggregation (IC50 = 10-30 microM). This study concludes that aminopeptidase rapidly degrades RGD peptides in plasma, an important issue for in vivo testing of RGD peptides and analogs. RGD analogs intrinsically stabilized against aminopeptidase are stable in plasma and are important tools for antithrombotic studies involving antagonism of gp IIb/IIIa.

Kistrin, a polypeptide platelet GPIIb/IIIa receptor antagonist, enhances and sustains coronary arterial thrombolysis with recombinant tissue-type plasminogen activator in a canine preparation

BACKGROUND

Kistrin is a 68-amino acid polypeptide from the venom of the Malayan pit viper Agkistrodon rhodostoma, which inhibits the platelet GPIIb/IIIa receptor. Its effect on thrombolysis, reocclusion, and bleeding associated with administration of recombinant tissue-type plasminogen activator (rt-PA) was studied in a canine model of coronary artery thrombosis.

METHODS AND RESULTS

Coronary patency was monitored for 2 hours by ultrasonic flow probe and repeated coronary angiography. The rt-PA was given as 0.45-mg/kg bolus injections at 15-minute intervals until recanalization or to a maximum of four boluses. Four groups of four or five dogs were studied: a control group that received intravenous heparin (4,000-unit bolus and 1,000 units each hour) and three groups that received heparin and 0.48, 0.24, or 0.12 mg/kg kistrin, administered as a 10% bolus injection and an infusion during a 60-minute period. In the control group, reflow occurred in four of five dogs within 37 +/- 47 minutes but was followed by cyclic reflow and reocclusion. Kistrin at a dose of 0.48 and 0.24 mg/kg reduced the time to reflow to 6 +/- 5 and 10 +/- 3 minutes, respectively, and abolished reocclusion. With 0.12 mg/kg kistrin, reflow occurred in all four animals, within 27 +/- 23 minutes, and reocclusion occurred in two animals. Kistrin induced a dose-related prolongation of the template bleeding time: with 0.48 mg/kg kistrin, the bleeding time was prolonged from 3.8 +/- 1.3 minutes before infusion to 29 +/- 2 minutes during infusion, but it was shortened to 8.3 +/- 2.6 minutes at 90 minutes after the end of infusion. Kistrin also caused a dose-related inhibition of platelet aggregation with ADP and collagen: with 0.48 mg/kg kistrin, platelet aggregation was abolished during the infusion but had partially recovered toward the end of the observation period. Pathological examination of recanalized coronary arterial segments of dogs given 0.48 or 0.24 mg/kg kistrin revealed widely patent arteries with some platelets layered on the damaged intimal surface.

CONCLUSIONS

Kistrin increases the rate and extent of thrombolysis with a reduced dose of rt-PA, and it prevents reocclusion. At an effective dose, it is associated with a transient prolongation of the bleeding time and inhibition of platelet aggregation. Kistrin may offer promise as adjunctive treatment to thrombolytic agents in patients with acute myocardial infarction.

Adhesive interactions of platelets and their blockade

Formation of thrombi, which constitute the main mechanism of occlusive cardiovascular diseases, is mediated by blood platelets and fibrinogen. At least three stimulatory pathways can activate platelets, yet only one is sensitive to inhibition by aspirin (cyclooxygenase). Aspirin-insensitive pathways, mediated by protein kinase C and myosin light-chain kinase, lead to a change of platelet shape, with an attendant striking increase in their surface (pseudopods) followed by exposure of receptors for fibrinogen and vWf on GPIIb-IIIa. Another receptor for vWf (GPIb), independent of known pathways of platelet activation, seems to function primarily in vessels with a high shear rate. The multistep processes of platelet activation can be circumvented by the blockade of platelet receptors for adhesive molecules, present in subendothelium and in plasma. However, platelet receptors exposed on GPIIb-IIIa share common structural features with the endothelial receptor for vitronectin. Blockade of platelet GPIIb-IIIa with synthetic peptides containing the RGD sequence, or with certain monoclonal antibodies, may inadvertently cause detachment, or prevent attachment, of endothelial cells in a zone of vascular injury. The peptide analogs of human fibrinogen gamma chain sequence 400-411 possess high selectivity for platelet GPIIb-IIIa because they do not cause detachment of endothelial cells. Thus, endothelial regrowth in the zone of vascular injury following thrombolysis and/or angioplasty will go unperturbed. The significance of adhesive proteins interacting with their receptors transcends the issue of the fundamental mechanism of platelet aggregation of platelet thrombus formation. A molecular model of the adhesive interaction between fibrinogen domains and GPIIb-IIIa will probably be the most amenable to construction. Once such a model is established and its allosteric regulation is unraveled, its utility for further development of improved antiplatelet receptor blockers as antithrombotic drugs, that are both selective and potent will become a reality.

A beta 3 integrin mutation abolishes ligand binding and alters divalent cation-dependent conformation

The ligand-binding function of integrin adhesion receptors depends on divalent cations. A mutant alpha IIb beta 3 integrin (platelet gpIIb/IIIa) that lacks ligand recognition shows immunologic evidence of a perturbed interaction with divalent cations. This was found to be caused by a G----T mutation that resulted in an Asp119----Tyr119 substitution in the beta 3 subunit. This residue is proximal to bound ligand and is in a conserved region among integrins that are enriched in oxygenated residues. The spacing of these residues aligns with the calcium-binding residues in EF hand proteins, suggesting interaction with receptor-bound divalent cation as a mechanism of ligand binding common to all integrins.