Monoanionic 99m Tc-tricarbonyl-aminopolycarboxylate complexes with uncharged pendant groups: Radiosynthesis and evaluation as potential renal tubular tracers.

Introduction: ^99m Tc(CO) ₃ -nitrilotriacetic acid, ^99m Tc(CO) ₃ (NTA), is a new renal tubular agent with pharmacokinetic properties comparable to those of ¹³¹ I-OIH but the clearance of ^99m Tc(CO) ₃ (NTA) and ¹³¹ I-OIH is still less than the clearance of PAH, the gold standard for the measurement of effective renal plasma flow. At physiological pH, dianionic ^99m Tc(CO) ₃ (NTA) has a mononegative inner metal-coordination sphere and a mononegative uncoordinated carboxyl group. To evaluate alternate synthetic approaches, we assessed the importance of an uncoordinated carboxyl group, long considered essential for tubular transport, by evaluating the pharmacokinetics of three analogs with the ^99m Tc(CO) ₃ (NTA) metal-coordination sphere but with uncharged pendant groups.
Methods: ^99m Tc(CO) ₃ complexes with N-(2-acetamido)iminodiacetic acid (ADA), N-(2-hydroxyethyl)iminodiacetic acid (HDA) and N-(fluoroethyl)iminodiacetic acid (FEDA) were prepared using a tricarbonyl kit and isolated by HPLC. The pharmacokinetics were evaluated in Sprague-Dawley rats, with ¹³¹ I-OIH as an internal control; urine was analyzed for metabolites. Plasma protein binding and erythrocyte uptake were determined from the 10min blood samples. Re(CO) ₃ (FEDA), the analog of ^99m Tc(CO) ₃ (FEDA), was prepared and characterized.
Results: ^99m Tc(CO) ₃ (ADA), ^99m Tc(CO) ₃ (HDA) and ^99m Tc(CO) ₃ (FEDA) were efficiently prepared as a single species with high radiochemical purities (>99%). These new monoanionic ^99m Tc(CO) ₃ tracers with uncharged dangling groups all showed rapid blood clearance and high specificity for renal excretion. Activity in the urine, as a percent of ¹³¹ I-OIH at 10 and 60min, was 96% and 99% for ADA, 96% and 100% for HDA, and 100% and 99% for FEDA, respectively. Each new tracer was excreted unchanged in the urine. The Re(CO) ₃ (FEDA) structure adds compelling evidence that such ^99m Tc(CO) ₃ (NTA) analogs have metal-coordination spheres identical to that of ^99m Tc(CO) ₃ (NTA).
Conclusions: New tracers lacking the negatively charged pendant carboxyl group previously thought to be essential for rapid renal extraction, ^99m Tc(CO) ₃ (ADA), ^99m Tc(CO) ₃ (HDA) and ^99m Tc(CO) ₃ (FEDA), exhibit pharmacokinetics in rats comparable to those of ^99m Tc(CO) ₃ (NTA) and ¹³¹ I-OIH. Furthermore, these encouraging results in rats warrant evaluation of this new tracer type in humans.
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