Clearance and renal extraction of technetium-99m-L,L-ethylenedicysteine, I-131-ortho-iodohippurate and I-125-iothalamate in pigs

99mTc-L,L-ethylenedicysteine (Tc-99m-EC) has been proposed as a Tc-99m-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. Tc-99m-EC, I-131-OIH and I-125-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 Tc-99m-EC and I-131-OIH (175 +/- 9 versus 178 +/- 8 ml min(-1), P = 0.43), whereas the difference between the total plasma clearance of Tc-99m-EC and I-131-OIH was highly significant (268 +/- 16 versus 185 +/- 9 ml min(-1), P=0.0001). Tc-99m-EC had a significant hepatic clearance of 83 +/- 10 ml min(-1) whereas the hepatic clearance of I-131-OIH was negligible. Renal plasma extraction of both Tc-99m-EC and I-131-OIH decreased significantly between 2 and 240 min post-injection from 0.85 to 0.45% for Tc-99m-EC and from 0.93 to 0.57% for I-131-OIH. Red blood cell binding of 99mTc-EC and I-131-OIH was 6.1% and 20%, respectively. The protein binding of Tc-99m-EC and I-131-OIH was 32% for both tracers. We conclude that Tc-99m-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.