Pharmacokinetics of technetium-99m-MAG3 in humans.

Technetium-99m-mercaptoacetylglycylglycylglycine (99mTc-MAG3) is introduced to replace o-iodohippurate (OIH) for renal function studies. For interpretation of clinical findings, extensive pharmacokinetic studies were performed on patients. These showed that 99mTc-MAG3, compared with OIH, has a higher plasma-protein binding, an essentially higher intravascular concentration, a smaller volume of distribution and, with practically identical biologic half-lives, a correspondingly lower clearance. Simultaneous steady-state measurements resulted in a 1.5-fold higher clearance of OIH than of 99mTc-MAG3 (n = 124). Competitive inhibition of the tubular transport system by p-aminohippurate (PAH) (20 patients) revealed a distinctly higher suppression of the 99mTc-MAG3 clearance than of OIH which indicates a lower affinity of the 99mTc complex to the tubular cell. The plasma extraction efficiencies of both agents, measured during surgery (n = 5), did not indicate an extrarenal elimination of 99mTc-MAG3. This new radiopharmaceutical is a pragmatic alternative to OIH and offers advantages not only for scintigraphic imaging but is also suited for quantitative renal function studies.