YM-264 is a selective, potent and orally active platelet-activating factor (PAF) antagonist with a pKi value of 8.85 for rabbit platelet membranes. IC50 & Target: pKi: 8.85 (PAF, rabbit platelet membranes)^[1]. In Vitro: The anti-platelet-activating factor effect of YM-264 is examined in vitro. YM-264 inhibits [³H] platelet-activating factor binding to rabbit platelet membranes with a pK_ivalue of 8.85. YM-264 inhibits the platelet-activating factor-induced human, rabbit and guinea-pig platelet aggregation with pA2 values of 8.68, 8.33 and 8.14, respectively^[1]. In Vivo: There are no significant differences in baseline airway responsiveness between control and YM-264 treated groups. Airway hyperresponsiveness induced by antigen exposure is significantly inhibited by the administration of YM-264. The baseline Rrs is 0.40 (0.02) cm H₂O/mL/s in the control group (n=6). In the YM-264 treated groups, the baseline Rrs is 0.39 (0.01) and 0.36 (0.01) cm H₂O/mL/s at a doses of 1 mg/kg (n=5) and 3 mg/kg (n=6), respectively. The Rrs during the IAR significantly increase from baseline to 0.92 (0.10) cm H₂O/mL/s in control (p=0.0002), 0.81 (0.12) in YM-264 1 mg/kg (p=0.01), and 1.06 (0.29) in YM-264 3 mg/kg (p=0.048). Reelevation of Rrs in the late phase is observed in the control group after antigen challenge. At this phase, Rrs significantly increase to 0.72 (0.10) cm H₂O/mL/s (p=0.0101) from the baseline (0.40) at 6 h after the exposure of antigen. In contrast, YM-264 at the doses of 1 and 3 mg/kg show significant inhibition of reelevation of Rrs as compared with control. YM-264 inhibit the eosinophil infiltration dose dependently^[2].