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^99mTc-MAEC Complexes: New Renal Radiopharmaceuticals Combining Characteristics of ^99mTc-MAG3 and ^99mTc-EC

¹ Department of Radiology, Emory University School of Medicine, Atlanta, Georgia
² Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana

^99mTc-Mercaptoacetyltriglycine (^99mTc-MAG3) and ^99mTc-L,L-ethylenedicysteine (^99mTc-LL-EC) are useful renal radiopharmaceuticals; however, both agents have renal clearances less than that of ¹³¹I-orthoiodohippurate (¹³¹I-OIH), and ^99mTc-LL-EC exists in dianionic and monoanionic forms at physiologic pH. In an effort to develop a superior ^99mTc agent with a rapid clearance comparable with that of ¹³¹I-OIH, we have designed a new ligand system, mercaptoacetamide-ethylene-cysteine (MAEC), which combines important structural features of both MAG3 and EC. Methods: Biodistribution and clearance studies were performed on Sprague–Dawley rats using syn- and anti-^99mTc-L- and -D-MAEC coinjected with ¹³¹I-OIH. Studies were also performed by coinjecting each isomer (74 MBq [2 mCi]) and 7.4–11.1 MBq (200–300 µCi) of ¹³¹I-OIH in 3 volunteers with dual-isotope imaging performed using a camera system fitted with a high-energy collimator. Blood samples were obtained from 3 to 90 min after injection and urine samples were obtained at 30, 90, and 180 min. Results: In the rats, <10% of the injected dose remained in the blood at 10 min after injection for all isomers, and the urine dose at 60 min ranged from 84% to 99% that of ¹³¹I-OIH. The clearances of syn- and anti-^99mTc-L-MAEC in the rats were higher than the clearances for the D-isomers (P 0.02) and were 102% and 105% that of ¹³¹I-OIH, respectively. In humans, the plasma protein binding of the ^99mTc-MAEC complexes ranged from 82% to 89%. All 4 complexes provided excellent renal images. The ^99mTc-MAEC complex/¹³¹I-OIH plasma clearance ratio in humans ranged from 45% (anti-^99mTc-L-MAEC) to 74% (syn-^99mTc-D-MAEC) with the 180-min urine excretion equivalent to that of ¹³¹I-OIH for all 4 complexes. Conclusion: Initial data in humans suggest that syn-^99mTc-D-MAEC has a higher clearance than that of ^99mTc-MAG3; however, none of the ^99mTc-MAEC tracers have a clearance equivalent to that of ¹³¹I-OIH and further ligand design is needed.

Key Words: ^99mTc-mercaptoacetamide-ethylene-cysteine • ^99mTc-ethylenedicysteine • ^99mTc-mercaptoacetyltriglycine • renal radiopharmaceuticals

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