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99mTc-L,L-ethylenedicysteine (99mTc-EC) has been proposed as a 99mTc-labelled alternative to radio-iodinated ortho-iodohippurate (OIH) for renal imaging and evaluation of renal function. The kinetics of this new renal function agent were studied by a single-injection plasma clearance technique in pigs. 99mTc-EC, 131I-OIH and 125I-iothalamate were injected and the plasma concentration of the three tracers was followed for 240 min. Renal, hepatic and total plasma clearance were calculated. There was no difference between the renal plasma clearance of 99mTc-EC and 131I-OIH (175 +/- 9 versus 178 +/- 8 ml min-1, P = 0.43), whereas the difference between the total plasma clearance of 99mTc-EC and 131I-OIH was highly significant (268 +/- 16 versus 185 +/- 9 ml min-1, P = 0.0001). 99mTc-EC had a significant hepatic clearance of 83 +/- 10 ml min-1 whereas the hepatic clearance of 131I-OIH was negligible. Renal plasma extraction of both 99mTc-EC and 131I-OIH decreased significantly between 2 and 240 min post-injection from 0.85 to 0.45% for 99mTc-EC and from 0.93 to 0.57% for 131I-OIH. Red blood cell binding of 99mTc-EC and 131I-OIH was 6.1% and 20%, respectively. The protein binding of 99mTc-EC and 131I-OIH was 32% for both tracers. We conclude that 99mTc-EC is not a suitable tracer for measuring renal function by the single-injection plasma clearance technique in pigs. This is due to a decreasing renal extraction and a significant hepatic clearance.

. This investigation was undertaken in order to determine whether the clearance of technetium-99m mercaptoacetyltriglycine (99mTc-MAG3) is more closely correlated to the clearance of iodine-125 orthoiodohippurate (125I-OIH) than to the clearance of chromium-51 ethylene diamine tetra-acetate (51Cr-EDTA) and whether there is a clinically significant extrarenal clearance of 99mTc-MAG3 . Fifty-one patients with a glomerular filtration rate (GFR) of 4-132ml/min were studied. After a simultaneous single injection of the three tracers, plasma clearance was measured from blood samples 0-5h post injection (p.i.) (0-24h in patients with GFR<15ml/min). Renal plasma clearance was measured 0-5h p.i. The ratio between the renal plasma clearance of 99mTc-MAG3 and 125I-OIH was 0.57. The ratio between the renal plasma clearance of 99mTc-MAG3 and 51Cr-EDTA was 2.57. The coefficient of variation (CV) of the 99mTc-MAG3/125I-OIH ratio was significantly smaller than the CV of the 99mTc-MAG3/51Cr-EDTA ratio (13.4% versus 31.2%). The corresponding plasma clearance ratios were 0.59 (CV=14.8%) and 2.48 (CV=27.0%). Plasma clearance overestimated renal plasma clearance by 7.0ml/min (5.4%) for 99mTc-MAG3 and by 4.1ml/min (8.4%) for 51Cr-EDTA. The difference in plasma and renal plasma clearance of 125I-OIH of 5.5ml/min did not reach statistical significance. Red blood cell binding of 99mTc-MAG3, 125I-OIH and 51Cr-EDTA was 2.0%, 14.6% and 0.2%, respectively. Protein binding of 99mTc-MAG3, 125I-OIH and 51Cr-EDTA was 86.3%, 61.1% and 5.9%, respectively. The volume of distribution of 99mTc-MAG3, 125I-OIH and 51Cr-EDTA was 16.3%, 27.0% and 19.4% of body weight. In conclusion, the clearances of 99mTc-MAG3 and 125I-OIH are more closely correlated than is the clearance of 99mTc-MAG3 with GFR. Extrarenal clearance of 99mTc-MAG3 is relatively smaller than extrarenal clearance of 51Cr-EDTA. Thus, plasma clearance of 99mTc-MAG3 can be used as a measure of renal tubular function.