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-labe lled alternative to radioiodinated ortho-iodohippurate (OIH) for renal imag ing and evaluation of renal function. The kinetics of this new renal functi on agent were studied by a single-injection plasma clearance technique in p igs. Tc-99m-EC, I-131-OIH and I-125-iothalamate were injected and the plasm a concentration of the three tracers was followed for 240 min. Renal, hepat ic and total plasma clearance were calculated. There was no difference betw een 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 hep atic clearance of 83 +/- 10 ml min(-1) whereas the hepatic clearance of I-1 31-OIH was negligible. Renal plasma extraction of both Tc-99m-EC and I-131- OIH decreased significantly between 2 and 240 min postinjection from 0.85 t o 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 prote in 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 th e single-injection plasma clearance technique in pigs. This is due to a dec reasing renal extraction and a significant hepatic clearance.