99mTc(CO)3(NTA): A superior 99mTc renal tubular tracer based on initial results in normal volunteers

Objectives 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 (ERPF). Our objectives were to compare the pharmacokinetics of these two tracers in normal volunteers.

Methods ^99mTc(CO)₃(NTA) was prepared with commercially available NTA and an IsoLink kit (Covidien) and isolated by RP HPLC. Two mCi of ^99mTc(CO)₃(NTA) were co-injected with 300 uCi of ¹³¹I-OIH and simultaneous imaging of each tracer was performed for 24 minutes. Plasma clearances were determined from 8 blood samples using the single-injection, two-compartment model. Plasma protein binding, red cell uptake and percent injected dose in the urine at 180 minutes were determined.

Results The clearance of ^99mTc(CO)₃(NTA) averaged 480 ± 82 mL/min compared to 482 ± 79 mL/min for ¹³¹I-OIH. The plasma protein binding of ^99mTc(CO)₃(NTA) averaged 45 ± 6% and red cell uptake was 12 ± 7% compared to 77 ± 3% and 20 ± 5% for ¹³¹I-OIH, respectively. At 3 h, 87 ± 4% of ^99mTc(CO)₃(NTA) was recovered in the urine compared to 90 ± 8% for ¹³¹I-OIH. Image quality with ^99mTc(CO)₃(NTA) was excellent and the renogram parameters were comparable to those of ¹³¹I-OIH.

Conclusions Preliminary results in normal volunteers suggest that the pharmacokinetic characteristics of ^99mTc(CO)₃(NTA) are comparable to those of ¹³¹I-OIH.

Research Support This work was supported by the National Institutes of Health, RO1 DK 38842.