Inhibition of rifampicin- and quinine-dependent antibody binding by anti-GPIX mAbs.

MAIPA studies were performed in the presence and absence of rifampicin (A) or quinine (B) with patient serum and a mAb as indicated in the figure. (A) The results of the rifampicin-induced serum binding to platelets in the presence or absence of rifampicin and a cross-blocking anti-GPIX mAb (SZ1) or a non–cross-blocking anti-GPIX mAb (FMC25). Binding was not observed in the absence of rifampicin. The rifampicin-dependent antibody did not inhibit the binding of FMC25, giving a positive result. Conversely, the drug-dependent antibody almost completely inhibited SZ1 binding, giving a negative result. (B) The results of 1 representative quinine-induced thrombocytopenia patient's serum binding to platelets in the presence or absence of quinine and an anti-GPIX mAb (FMC25 or SZ1). No binding was observed in the absence of quinine. The quinine-dependent antibody did not inhibit the binding of FMC 25, again giving a positive result. Conversely, the quinine-dependent antibody inhibited SZ1 binding, giving a negative result. The striped box indicates binding in the presence of the drug; the filled box indicates binding in the absence of the drug; Qn, quinine; Rif, rifampicin; Pt, patient.