Coligand effects on the solution stability, biodistribution and metabolism of the 99mTc-labeled cyclic RGDfK tetramer

In this study, we present the evaluation of two new ternary ligand 99mTc complexes [99mTc(HYNIC tetramer)(tricine)(L)] [L=isonicotinic acid (ISONIC) and 2,5-pyridinedicarboxylic acid (PDA)] as potential radiotracers for tumor imaging. Athymic nude mice bearing MDA-MB-435 human breast cancer xenografts were used to evaluate their biodistribution and metabolic properties. Solution stability data showed that [99mTc(HYNIC tetramer)(tricine)(L)] (L=ISONIC and PDA) had significant decomposition (14% and 35%, respectively) at 6 h in the absence of excess ISONIC or PDA coligand. Biodistribution data clearly showed that [99mTc(HYNIC tetramer)(tricine)(PDA)] had a much lower uptake in most organs of interest than [99mTc(HYNIC tetramer)(tricine)(ISONIC)] during the 2-h study period. Results from metabolism studies revealed that ∼50% of [99mTc(HYNIC tetramer)(tricine)(ISONIC)] remained intact in fecal samples at 120 min postinjection, whereas only 10% of [99mTc(HYNIC tetramer)(tricine)(PDA)] remained intact in fecal samples. The extent of metabolism correlated well with radiotracer solution stability. The results from this and our previous studies clearly demonstrated that coligands [trisodium triphenylphosphine-3,3′,3″-trisulfonate (TPPTS), ISONIC and PDA] have a significant impact on the tumor uptake, excretion kinetics and metabolism of the 99mTc-labeled cyclic RGDfK tetramer. Among the three radiotracers evaluated in this tumor-bearing animal model, [99mTc(HYNIC tetramer)(tricine)(TPPTS)] remained the best with respect to blood clearance, tumor uptake and target/background ratios.