Inhibition of thrombin by iopromide in vitro

Iopromide is a nonionic, iodinated, monomeric, radiographic contrast agent used in various indications, including coronary angiography and visceral an d peripheral arteriography. Nonionic contrast media have been postulated to increase thrombogenicity when compared with ionic contrast media. The goal of this study was to characterize the interaction of iopromide with thromb in, specifically to determine the rate, extent, specificity, and reversibil ity of the thrombin inhibition by iopromide, the integrity of the thrombin- iopromide complex, and the inhibitory potency of iopromide using a validate d assay methodology. Iopromide was mixed with purified thrombin or pooled s erum from healthy male and female donors. The final concentrations of iopro mide in the presence of estimated physiologic concentrations of thrombin (1 nmol/L) were 0-184 mmol/L. After incubation for defined time intervals, th e activity of thrombin was determined by adding substrate and measuring the absorbance of the generated chromophores at 405 nm. The possible inhibitio n of the protease trypsin by iopromide was investigated to evaluate the spe cificity of thrombin inhibition by iopromide. Iopromide was compared with T hromstop, a known thrombin inhibitor, to assess the relative potency of iop romide. The inhibition of thrombin by iopromide was immediate, rapidly reve rsible, and proportionate to the iopromide concentrations. The minimum inhi bitory concentration of iopromide was 50 mmol/L. At the highest iopromide c oncentration tested, 184 mmol/L, the mean inhibition of thrombin activity w as 44.5%. The mean concentration of iopromide associated with a 50% inhibit ion was 206 mmol/L. The inhibitory potency of iopromide was 4 x 10(6) times smaller than that of Thromstop. The inhibition of thrombin by iopromide is specific, because trypsin was not inhibited by iopromide. The results indi cate that in vitro iopromide at clinically relevant concentrations partiall y inhibits thrombin activity. However, the in vitro model used does not con sider other factors that may be relevant for the overall coagulation respon se in vivo.