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(Circulation. 2002;105:e46.)
© 2002 American Heart Association, Inc.

Correspondence

Eptifibatide and 7E3, but Not Tirofiban, Inhibit _vß₃ Integrin–Mediated Binding of Smooth Muscle Cells to Thrombospondin and Prothrombin

Robert M. Scarborough, PhD

COR Therapeutics, Inc, South San Francisco, Calif, E-mail rscarborough@corr.com

To the Editor:

The recent article in Circulation by Lele et al¹ reports _vß₃ inhibitory activity of eptifibatide. Surprisingly, the authors contend that this is a novel observation. Lele et al¹ demonstrate inhibition of _vß₃-mediated adhesion of human aortic smooth muscle cells (HASMCs) to thrombospondin (TSP) or prothrombin, human umbilical vein endothelial cell attachment to TSP or vitronectin, as well as the inhibition of TSP-induced proliferation of HASMCs mediated by _vß₃, with IC₅₀ values of approximately 30 µmol/L in each assay. All of these data are consistent with the original publication on the _IIbß₃ antagonist, eptifibatide, by Scarborough et al.² The authors reported that this agent is a highly selective integrin antagonist. They also showed that at higher concentrations, eptifibatide can inhibit vitronectin binding to purified _vß₃, can inhibit von Willebrand factor binding to purified _vß₃, and can inhibit M21 melanoma cell attachment to vitronectin mediated by _vß₃ with IC₅₀ values of 5, 40, and 7.5 µmol/L, respectively.² Why, then, would Lele et al make their conclusions of novelty with this nearly identical and supportive data and furthermore suggest that Scarborough et al² showed that eptifibatide does not interact with _vß₃? Mistakenly, in the last sentence of their report Lele et al state that eptifibatide is "a synthetic, cyclic peptide with a Lys-Gly-Asp (KGD) sequence." Thus, the authors appear to have mistaken the structure of eptifibatide from the original publication by Scarborough et al² and have drawn correspondingly incorrect conclusions about eptifibatide’s biological activity based on this mistaken identity. In fact, eptifibatide does not contain a KGD sequence but contains the homoArg-Gly-Asp (modified KGD) sequence (compound 34 in reference 2).^2,3 The original publications on eptifibatide^2,4 correctly reported the approximately 300- to 400-fold higher affinity of eptifibatide for _IIbß₃ versus _vß₃, supporting the view that eptifibatide is a highly selective integrin antagonist. Nevertheless, it is true that at therapeutic concentrations of eptifibatide,⁵ some inhibition of _vß₃ may occur based on data reported earlier² and pointed out by Lele et al.¹

References

1. Lele M, Sajid M, Wajih N, et al. Eptifibatide and 7E3, but not tirofiban, inhibit _vß₃ integrin–mediated binding of smooth muscle cells to thrombospondin and prothrombin. Circulation. 2001; 104: 582–587.[Abstract/Free Full Text]
   
2. Scarborough RM, Naughton MA, Teng W, et al. Design of potent and specific integrin antagonists. J Biol Chem. 1993; 268: 1066–1073.[Abstract/Free Full Text]
   
3. Scarborough RM, Gretler DD. Platelet glycoprotein IIb-IIIa antagonists as prototypical integrin blockers. novel parenteral and potential oral antithrombotic agents. J Med Chem. 2000; 43: 3453–3473.[Medline] [Order article via Infotrieve]
   
4. Phillips DR, Teng W, Arfsten A, et al. Effect of Ca²⁺ on GP IIb-IIIa interactions with Integrilin. Circulation. 1997; 96: 1488–1494.[Abstract/Free Full Text]
   
5. Gilchrist IC, O’Shea JC, Kosoglou T, et al. Pharmacodynamics and pharmacokinetics of higher-dose, double-bolus eptifibatide in percutaneous coronary intervention. Circulation. 2001; 104: 406–411.[Abstract/Free Full Text]
   

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Response

Manjiri Lele, MD; Mansoor Sajid, MD, PhD; Nadeem Wajih, PhD; George A. Stouffer, MD

Carolina Cardiovascular Biology Center, University of North Carolina, Chapel Hill, NC

We readily acknowledge the contributions of Dr Scarborough but disagree with his assertion that his work answered all relevant questions in regards to eptifibatide interactions with _vß₃ integrins (the vitronectin receptor) on vascular cells. Despite his prior studies, the paradigm until recently was that abciximab was the only glycoprotein IIb/IIIa inhibitor in clinical use that cross-reacted with _vß₃ at clinically relevant concentrations. This was illustrated by a recent review of GP IIb/IIIa inhibitors that stated: "Furthermore, abciximab binds to other receptors, such as the vitronectin and Mac-1 receptors, while tirofiban and eptifibatide bind specifically to the GP IIb/IIIa receptor."¹

A major emphasis of our work² was to demonstrate that eptifibatide directly interacts with _vß₃ on vascular cells in vitro and has functional effects at concentrations that are achieved clinically. Our studies used smooth muscle and endothelial cells, whereas Dr Scarborough used purified _vß₃ and M21 melanoma cells. The complexity of _vß₃ regulation, and the importance of directly studying vascular cells, is highlighted by recent work showing that _vß₃ can be activated, that the basal affinity state of _vß₃ differs among different cell types and that there are activation-dependent and activation-independent ligands.^3,4

The major conclusion of our studies, that functional inhibition of _vß₃ in vascular cells may occur at therapeutic concentrations of eptifibatide, is supported by our data as Dr Scarborough acknowledges. Further studies are now needed to delineate the clinical importance of antagonism of _vß₃ integrins by abciximab and eptifibatide.

Finally, we should have stated in our article that eptifibatide is a synthetic, cyclic peptide with a modified Lys-Gly-Asp (KGD) sequence. This error does not detract from our data as we used eptifibatide, provided by COR Therapeutics, for all of our experiments.

References

1. Bhatt DL, Topol EJ. Current role of platelet glycoprotein IIb/IIIa inhibitors in acute coronary syndromes. JAMA. 2000; 284: 1549–1558.[Abstract/Free Full Text]
   
2. Lele M, Sajid M, Wajih N, et al. Eptifibatide and 7E3, but not tirofiban, inhibit _vß₃ integrin–mediated binding of smooth muscle cells to thrombospondin and prothrombin. Circulation. 2001; 104: 582–587.
   
3. Suehiro K, Smith JW, Plow EF. The ligand recognition specificity of ß₃ integrins. J Biol Chem. 1996; 271: 10365–10371.[Abstract/Free Full Text]
   
4. Pampori N, Hato T, Stupack DG, et al. Mechanisms and consequences of affinity modulation of integrin _vß₃ detected with a novel patch-engineered monovalent ligand. J Biol Chem. 1999; 274: 21609–21616.[Abstract/Free Full Text]
   

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