Thrombosis and Hemostasis

Arginine-glycine-aspartic acid (RGD)– mimetic platelet inhibitors act by occupying the RGD recognition site of IIb/ 3 integrin (GPIIb/IIIa), thereby preventing the activated integrin from reacting with fibrinogen. Thrombocytopenia is a wellknown side effect of treatment with this class of drugs and is caused by Abs, often naturally occurring, that recognize IIb/ 3 in a complex with the drug being administered. RGD peptide and RGDmimetic drugs are known to induce epitopes (ligand-induced binding sites [LIBS]) in IIb/ 3 that are recognized by certain mAbs. It has been speculated, but not shown experimentally, that Abs from patients who develop thrombocytopenia when treated with an RGD-mimetic inhibitor similarly recognize LIBS determinants. We addressed this question by comparing the reactions of patient Abs and LIBS-specific mAbs against IIb/ 3 in a complex with RGD and RGD-mimetic drugs, and by examining the ability of selected non-LIBS mAbs to block binding of patient Abs to the liganded integrin. Findings made provide evidence that the patient Abs recognize subtle, druginduced structural changes in the integrin head region that are clustered about the RGD recognition site. The target epitopes differ from classic LIBS determinants, however, both in their location and by virtue of being largely drug-specific. (Blood. 2012;119(26):6317-6325)