Fig. 8: M-tirofiban is a pure αIIbβ3 antagonist.

a Histograms (mean + s.d., n = 3 independent experiments) showing binding of mAb AP5 to human platelets in presence of buffer (B), tirofiban (T; 150 nM), and M-tirofiban (M; 1.5 μM; white histograms), and before (gray histograms) or after (dark gray histograms) addition of ADP (5 μM). Numbers represent p values (Student’s t-test, two-tailed). No significant differences were found between buffer and M-tirofiban before (p = 0.273) or after (p = 0.81; both Student’s t-test, two-tailed) ADP addition. b Kinetics of clot retraction in absence or presence of tirofiban, or M-tirofiban (mean +/− s.e., five experiments with three donors). Kinetics of clot retraction was not different between buffer control (contr.) and M-tirofiban (p = 0.61, F-test, two-tailed). c, d Dose–response curves (mean +/− s.e.) comparing displacement of Alexa488-labeled Hr10 binding to inactive (c, n = 5) or mAb PT-25-activated αIIbβ3- K562 (d, n = 3) by increasing concentrations of tirofiban or M-tirofiban in presence of physiologic concentrations of Mg²⁺ and Ca²⁺ (1 mM each). The respective IC₅₀s were 51.3 +/− 19.2 nM and 257.2 +/− 88.0 nM for inactive αIIbβ3, and 16.9 +/− 2.4 nM and 247.1 +/− 29.3 nM for active αIIbβ3. The lower affinities of both compounds are explained by the requirement for more inhibitor to displace high affinity binding of Hr10 (compared to binding of FB in Fig. 7c) to αIIbβ3.