Pharmacokinetic characteristics of 99mTc-Ethylene-l-dicysteine (99mTc-EC)

Ethylenedicysteine-99mTc (99mTc-EC) has been more and more commonly applied in dynamic studies as well as for clearance determinations. However, it was necessary to investigate in detail the pharmacokinetic characteristics of the radiopharmaceutical which may be important for its applicability in assessment of renal function.

Kidney images obtained from renoscintigraphy are characterised by excellent quality without visualisation of the organs adjacent to kidneys (liver, spleen). Renoscintigraphic curves demonstrate typical shapes with TMAX and T1/2 values not differing from the corresponding values obtained for other radiopharmaceuticals (99mTc-MAG3, 131I-OIH). In plasma, 99mTc-EC binds with proteins to a considerably lesser degree (c. 1/3) than 131I-OIH (c. 2/3), or 99mTc-MAG3 (> 9/10). No binding of 99mTc-EC with erythrocytes has been demonstrated, whereas 131I-OIH attaches to or penetrates the red blood cells (10-12%). 99mTc-EC is quickly excreted from the organism: 40 min after i.v. injection up to 70% of the administered radiopharmaceutical is found in urine, and at 1 and 1.5 h after the administration 80% and 95%, respectively. The distribution of 99mTc-EC in the organism can be described in a fully satisfactory way by means of an open two-compartment model, which allows this model to be used for clearance determinations. Comparison of the values of renal plasma clearance without collection of urine with the values determined by means of measurement of activity excreted with urine and mean blood concentration over a finite time interval leads to the conclusion that extrarenal plasma clearance of this compound (via the liver?) is negligible and amounts to c. 17 ml/min (5-6% of the total). The obtained correlation between clearance values for 99mTc-EC and 131I-OIH supports the contention that extrarenal excretion rate of 99mTc-EC (through the liver and bile ducts) is lower than the corresponding rates of either 131I-OIH or 99mTc-MAG3. A very close correlation between clearance values for 99mTc-EC and ERPF (131I-OIH clearance) and between their extraction constants (r = 0.91 and 0.92, respectively), allows for the introduction of 99mTc-EC to the assessment of renal function instead of 131I-OIH. Effective dose to the patient from unit activity of 99mTc-EC is comparable with that resulting from administration of other radiopharmaceuticals labelled with 99mTc.