Initial evaluation of new 99mTc(CO)3 renal imaging agents having carboxyl-rich thioether ligands and chemical characterization of ReCO3 analogues

INTRODUCTION: The first human studies of a characterized radiopharmaceutical containing a {(99m)Tc(CO)(3)}(+) core, Na[(99m)Tc(CO)(3)(LAN)], demonstrated that Na[(99m)Tc(CO)(3)(LAN)] was an excellent renal imaging agent; however, its clearance was less than that of (131)I-orthoiodohippurate ((131)I-OIH), and it did not provide a direct measure of effective renal plasma flow. In order to develop a (99m)Tc renal agent with pharmacokinetic properties equivalent to those of (131)I-OIH, we investigated the (99m)Tc(CO)(3)/Re(CO)(3) complexes formed from carboxymethylmercaptosuccinic acid (CMSAH(3)) and thiodisuccinic acid (TDSAH(4)). Once the ligand is bound to (99m)Tc(CO)(3) through a thioether and two carboxyl groups, the complexes have at least one unbound carboxyl group, essential for the interaction with the renal tubular transporter. METHODS: X-ray crystal structural analysis of [NMe(4)][Re(CO)(3)(CMSAH)] was performed to interpret the nature of (99m)Tc tracers. CMSAH(3) and TDSAH(4) were radiolabeled by incubating each ligand and the precursor [(99m)Tc(CO)(3)(H(2)O)(3)](+) at 70 degrees C (pH 7) for 30 min. The products were purified by reversed-phase high-performance liquid chromatography, and biodistribution studies were performed in Sprague-Dawley rats, with (131)I-OIH as an internal control at 10 and 60 min. RESULTS: Radiolabeling CMSAH(3) and TDSAH(4) with the [(99m)Tc(CO)(3)(H(2)O)(3)](+) precursor gave products quantitatively. Analysis of the Re(CO)(3) complexes with the CMSAH(3) and TDSAH(4) ligands demonstrates that ligands are bound in (99m)Tc/Re(CO)(3) complexes through a thioether and two deprotonated carboxyl groups (forming tridentate dianionic moieties, generally with two 5-membered chelate rings). Renal excretion at 60 min (activity in the urine as a percentage of (131)I-OIH) was 68+/-1% for Na(3)[(99m)Tc(CO)(3)(TDSA)] but was 98+/-1% for Na(2)[(99m)Tc(CO)(3)(CMSA)]. CONCLUSION: In rats, Na(2)[(99m)Tc(CO)(3)(CMSA)] is extracted by the kidneys and eliminated in the urine almost as rapidly as (131)I-OIH; consequently, Na(2)[(99m)Tc(CO)(3)(CMSA)] may provide a direct measure of effective renal plasma flow, and further evaluation in humans is warranted.