Ss. Rebello et al., Role of extracellular ionized calcium in the in vitro assessment of GPIIb/IIIa receptor antagonists, J THROMB TH, 9(1), 2000, pp. 23-28
Several preclinical studies have found a poor correlation between the ex vi
vo platelet inhibitory potency and the in vivo antithrombotic efficacy of G
PIIb/IIIa receptor antagonists. The present study was designed to examine t
he differential in vitro potencies of c7E3, MK-383, DMP-728, and SM-20302 i
n inhibiting ex vivo platelet aggregation under normocalcemic and hypocalce
mic conditions. Human blood was collected in either trisodium citrate (0.37
%) or PPACK (20 mu g/mL). Platelet aggregation assays were performed in pla
telet-rich plasma from citrate-anticoagulated blood (cPRP) and PPACK-antico
agulated blood (pPRP) using ADP (20 mu M) and TRAP (10 mu M) as agonists in
the presence of c7E3, MK-383, DMP-728, or SM-20302. The concentration of i
onized calcium in cPRP was 16-19 times lower than that in pPRP. The IC(5)0
of c7E3 for inhibiting ADP-induced platelet aggregation in cPRP (2.76 +/- 0
.11 mu g/mL) was 1.6 times lower than that in pPRP (4.46 +/- 0.48 mu g/mL;
P < 0.05). Similarly, the IC(5)0 for c7E3 for inhibiting TRAP-induced plate
let aggregation in cPRP (4.52 +/- 0.34 mu g/mL) was 1.7 times lower than th
at in pPRP (7.69 +/- 0.43 mu g/mL; P < 0.05). MK-383, DMP-728, and SM-20302
also demonstrated 1.96-, 1.15-, and 1.43-fold lower IC(5)0 values, respect
ively, in cPRP as compared with pPRP. Chelation of ionized calcium in pPRP
led to a progressive increase in platelet inhibition by all the antagonists
. These results suggest that the observed in vitro inhibitory potency of a
GPIIb/IIIa receptor antagonist is markedly enhanced when trisodium citrate
is used as an anticoagulant to collect blood for ex vivo assay. These findi
ngs indicate that dosing regimens for GPIIb/IIIa receptor antagonists based
on the platelet inhibition profile in citrate may provide misleading infor
mation with respect to their true in vivo antithrombotic efficacy.