The in vivo and in vitro metabolic profile of 99mTc-NC100668, a new tracer for imaging venous thromboembolism: Identification and biodistribution of the principal radiolabeled metabolite

99m Tc-NC100668 [Acetyl-Asn-Gln-Glu-Gln-Val-Ser-Pro-Tyr(3-iodo)-Thr-Leu-Leu-Lys-Gly-NC100194] is a radiopharmaceutical imaging agent being developed to aid the diagnosis of thromboembolism. The stability profile of 99m Tc-NC100668 was investigated by high-performance liquid chromatography (HPLC) after in vitro exposure to blood and plasma obtained from rat and human, as well as to urine and bile obtained from rat. The metabolic profile of 99m Tc-NC100668 exposed to human and rat hepatic S9 (a liver homogenate-rich cytochrome P450) was also studied. The profile of 99m Tc-labeled species in plasma, urine, and bile was investigated following i.v. administration of 99m Tc-NC100668 to rat. The major species observed in vitro and in vivo consisted of the 99m Tc-chelator (NC100194) [ N,N -Bis( N -(1,1-dimethyl-2-(hydroxylimino-)propyl)aminoethyl)aminoethylamine] attached to the C-terminal amino acid residue and referred to as 99m Tc-complex of Gly-NC100194. The identity of the major metabolite was confirmed by cochromatography with an authentic standard and the genuine metabolite using a second HPLC method. The minor metabolites were sodium pertechnetate ( 99m Tc) and 99m Tc-NC100194. In addition, a small number of other species were transiently observed in vitro; they were not investigated further. The biodistribution of the major metabolite was studied in male Wistar rats. The affinity of the major metabolite toward plasma clot was established using a plasma clot-forming assay. A minor uptake of 99m Tc-complex of Gly-NC100194 in the plasma clot and a rapid removal from the body were noted. In conclusion, the metabolites of 99m Tc-NC100668 are not anticipated to have a negative impact on the ability of the test substance to image blood clots.