ANTITHROMBOTIC MAP OF THE N-TERMINAL DOMAIN OF THE GPIIIA SUBUNIT OF THE HUMAN PLATELET FIBRINOGEN RECEPTOR (GPIIB-IIIA) DETERMINED IN-VIVOBY MONOCLONAL, IMMUNOCHEMICAL INHIBITION OF ACUTE ARTERIAL THROMBOSIS

The inhibition of the platelet fibrinogen receptor, the glycoprotein I lb-IIIa (GPIIb-IIIa) or integrin alpha IIb beta 3, has recently became an accepted practice in clinical cardiology, The interest lies now in the improvement of the antithrombotic activity and the minimization o f the secondary effects of the receptor inhibitors, by their evaluatio n in vivo in the different dynamic conditions and pathological states under which these inhibitors have to perform, In this paper, we functi onally map in vivo the N-terminal domain of the GPIIIa subunit, using the antithrombotic activity of five murine monoclonal antibodies (mabs ) (P37, P40, 95-1, P95-2 and P97), all of them inhibitors of platelet aggregation in vitro and directed to this ligand binding domain of the human fibrinogen receptor, Competition experiments have shown that th ese mabs bind with high affinity (5-7 nM) and compete very strongly am ong themselves for binding to human resting platelets, except P40, whi ch neither binds nor competes, These antibodies were assayed in a dog model of acute thrombosis in the carotid artery, which were induced 15 min after their intravenous administration (0.8 mg/kg). The antithrom botic activity was quantified by the measurement of the [In-111]oxine- labelled platelet deposition at the site of the arterial lesion and wa s expressed as the percentage of the total circulating platelets, Anti body P37, directed to the GPIIIa 101-109 sequence, decreased the plate let deposition 630-fold with respect to control animals, P95-2, P97 an d P95-1 decreased the platelet deposition 160-, 32- and 25-fold, respe ctively, while P40, directed to the GPIIIa 260-302 sequence, did not s how any antithrombotic activity, We conclude that all the mabs directe d to the N-terminal domain of GPIIla, which inhibit platelet aggregati on ill vitro and whose epitopes are very close to each other and expos ed in resting platelets, have high antithrombotic activity in vivo, wh ich varies depending on the actual location of the epitopes in the rec eptor topography, Among these antibodies, P37, the strongest receptor inhibitor in vivo and whose epitope is most probably the closest to th e fibrinogen binding site(s), seems the best candidate for comparative studies in animal models with today's best GPIIb-IIIa inhibitors and for clinical trials in humans in order to arrest or prevent thrombosis , reocclusion and late restenosis.