Studies of HCQ interaction with proteins by ellipsometry. (A) Ellipsometry tracing showing β2GPI (5 μg/mL) binding to phospholipid bilayer and the effect of HCQ (1 mg/mL) on the phospholipid-bound protein. The phospholipid-bound β2GPI was rapidly dissociated from the bilayer by the addition of drug. (B) Ellipsometry tracing showing polyclonal aPL IgG (0.5 mg/mL) binding to phospholipid bilayer and the effects of HCQ at 1 mg/mL on the phospholipid-bound protein. The phospholipid-bound aPL IgG was also readily dissociated from the bilayer by the addition of drug. (C) Binding isotherm of β2GPI to the phospholipid bilayer. In the presence of HCQ (1 mg/mL), the binding of β2GPI was significantly reduced and the K_d was significantly increased compared with the buffer control (1.44 ± 0.15 μM vs 0.53 ± 0.07 μM; P < .001). (D) Binding isotherm of aPL mAb, IS4, in HBSA to phospholipid bilayer. In the presence of HCQ (1 mg/mL), the binding of aPL mAb was significantly reduced and the K_d was significantly increased compared with the buffer controls (0.36 ± 0.03 μM vs 0.12 ± 0.01 μM; P < .001).