Nc100668, a new tracer for imaging of venous thromboembolism: disposition and metabolism in rats.

The 99mTc-complex of NC100668 [Acetyl-Asn-Gln-Glu-Gln-Val-Ser-Pro-Tyr(3-iodo)-Thr-Leu-Leu-Lys-Gly-NC100194] is being evaluated for nuclear medical imaging of venous thromboembolism. NC100668 is a 13-amino acid peptide with a Tc-binding chelator [NC100194; -NH-CH2-CH2-N(CH2-CH2-NH-C(CH3)2-C(CH3)=N-OH)2] linked to the C-terminal end. Following injection in rats of [Asn-U-14C]NC100668 (labeling of the N-terminal amino acid), approximately 70% of the radioactivity was recovered in urine within 3 days. Following injection of [Lys-U-14C]NC100668 (labeling close to the C-terminal amino acid), radioactivity was cleared more slowly, with only 8% recovered in urine and approximately 80% of the radioactivity present in the body after 3 days. The highest concentration of radioactivity in the body following injection of [Lys-U-14C]NC100668 was observed in the kidney inner cortex; this probably represents 14C-labeled Lys, which is reabsorbed in the kidney tubules and incorporated into protein metabolism. Metabolite profiling by high-performance liquid chromatography with radiochemical detection revealed that following injection of [Asn-U-14C]NC100668, there is a rapid appearance in blood of one peak containing radioactive metabolite(s) originating from the N-terminal part of the molecule. In urine samples, only this radioactive peak was observed with no intact NC100668 remaining; this very hydrophilic N-terminal metabolite was probably either the N-terminal amino acid or a very short peptide. Liquid chromatography-mass spectrometry analyses of rat urine samples obtained after injection of nonlabeled NC100668 confirmed the identity of two metabolites generated from the C-terminal end of the molecule; Gly-NC100194 was identified as the major of these metabolites and NC100194 as a minor metabolite present at approximately one-tenth the amount of Gly-NC100194. No other metabolites were identified.