99mTc-MAG3: Dynamic studies in patients with renal disease

Animal studies have suggested that ^99mTc-mercapto-acetylglycyl-glycyl-glycine (^99mTc-MAG3) might be suitable for the determination of the renal plasma flow (RPF) because of its high renal clearance. In this study ¹³¹I-orthoiodohippurate (¹³¹I-OIH) and ^99mTc-MAG3 (labeling always >95%) were administered simultaneously in 11 patients (creatinine clearance ranging from 14 to 130 ml/min per 1.73 m²) to measure effective RPF(ERPF) using the standard technique (UV/P). Glomerular filtration rate (GFR; clearance of ¹²⁵I-thalamate, ¹²⁵I-OT) was also measured. The mean ratio of ^99mTc-MAG3 clearance to ¹³¹I-IOH clearance was 0.55±0.02 (SEM), P<0.01, n=16, and was independant of GFR and ERPF. To study this difference in renal handling ofthe radiopharmaceuticals, renal extractions by the right kidney were determined in another six patients after a single shot of the agents. Renal extraction of ^99mTc-MAG3 was 0.60±0.03 after 5 min, and 0.41±0.08 after 30 min. Renal extraction of ¹³¹I-OIH amounted to 0.86±0.04 and 0.77±0.03, respectively. Using renal extractions of 0.41 and 0.77, respectively, it appeared that calculated renal plasma flows measured simultaneously with ^99mTc-MAG3 and ¹³¹I-OIH were similar. Protein binding 30 min after the priming dose was 66% for ^99mTc-MAG3 and 47% for ¹²³I-OIH. We conclude that in spite of a high renal clearance (ratio to ¹²⁵I-OT clearance 2.69±0.27), ^99mTc-MAG3 seems unsuitable for an accurate determination of the RPF. This conclusion is strongly supported bythe observation that the ratio of ^99mTc-MAG3 to ¹³¹I-IOH clearance markedly decreased from 0.66 to 0.32 in a patient with an increase in urinary protein loss.