Abstract

The ^99mTc-complex of NC100668 [Acetyl-Asn-Gln-Glu-Gln-Val-Ser-Pro-Tyr(3-iodo)-Thr-Leu-Leu-Lys-Gly-NC100194] is a new tracer tested 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. The present study was performed following injection of ^99mTc-NC100668 in healthy human volunteers with five dose levels of NC100668 (20-2000 μg) and a constant radioactivity dose. The rate at which the radioactivity was cleared from blood was independent of gender and dose of NC100668; more than half of the 82% urinary clearance of radioactivity was obtained 2 h postinjection. The radioactivity in blood was reduced to 50% of initial values within 12 min; this was followed by a more gradual decrease with a half-life of 1.2 h and a terminal elimination half-life of 10.5 h. The plasma concentration of NC100668 decreased rapidly with an initial half-life of 5 to 10 min. The half-life after this initial phase could be estimated for only two of the subjects in the highest-dose group because the NC100668 concentration in the other samples at these time points was below the limit of detection of the liquid chromatography/mass spectrometry (LC/MS) method. LC/MS analyses of urine samples revealed the identity of two metabolites generated from the C-terminal end of the molecule; Gly-NC100194 was identified as the major metabolite and NC100194 as a minor metabolite. The estimated sum of these two metabolites is in the same magnitude as the recoveries of ^99mTc in these samples, indicating that most of the ^99mTc excreted in urine is bound to one of these metabolites.