99mTc(CO)3(NTA): A 99mTc Renal Tracer with Pharmacokinetic Properties Comparable to Those of 131I-OIH in Healthy Volunteers

Studies in rats showed that the pharmacokinetics of the tricarbonyl core radiopharmaceutical 99mTc(CO)3-nitrilotriacetic acid, 99mTc(CO)3(NTA), were essentially identical to those of 131I ortho-iodohippuran (131I-OIH), the clinical gold standard for the measurement of effective renal plasma flow. Our objective was to compare the pharmacokinetics of these 2 tracers in healthy volunteers. Methods: 99mTc(CO)3(NTA) was prepared with commercially available NTA and a commercially available kit and isolated by reversed-phase high-performance liquid chromatography. Approximately 74 MBq (2 mCi) of 99mTc(CO)3(NTA) were coinjected with 9.25 MBq (250 μCi) of 131I-OIH in 9 volunteers, and simultaneous imaging of each tracer was performed for 24 min. Plasma clearances were determined from 8 blood samples obtained 3–90 min after injection using the single-injection, 2-compartment model. Plasma protein binding, red cell uptake, and percentage injected dose in the urine at 30 and 180 min were determined. Results: There was no difference in the plasma clearances of 99mTc(CO)3(NTA) and 131I-OIH, 475 ± 105 mL/min versus 472 ± 108 mL/min, respectively. The plasma protein binding and red cell uptake of 99mTc(CO)3(NTA) were 43% ± 5% and 9% ± 6%, respectively; both values were significantly lower (P < 0.001) than the plasma protein binding (75% ± 3%) and red cell uptake (17% ± 5%) of 131I-OIH. There was no significant difference in the percentage injected dose recovered in the urine at 30 min and at 3 h; for comparison, the percentage dose in the urine at 3 h was 91% ± 4% for 99mTc(CO)3(NTA) and 91% ± 6% for 131I-OIH (P = 0.96). Image quality with 99mTc(CO)3(NTA) was excellent, and the renogram parameters were similar to those of 131I-OIH. Conclusion: Preliminary results in healthy volunteers suggest that the pharmacokinetic behavior of 99mTc(CO)3(NTA) is comparable to that of 131I-OIH.