Evaluation of effective renal plasma flow with I-127 ortho-iodohippurate and I-123 ortho-iodohippurate in rabbits

Rationale and Objectives. Radiolabeled ortho-iodohippurate is commonly employed for evaluating effective renal plasma flow (ERPF) by means of either in vivo scintigraphy and/or plasma clearance curves. A new method has recently been developed for measuring levels of stable iodine (iodine-127) in biologic samples, based on the detection of x-ray fluorescence photons. In this study, the authors assessed the potential of the new system to evaluate ERPF by using an iodinated contrast medium with adequate glomerular filtration and tubular secretion properties. Materials and Methods. A commercial system was used to evaluate ERPF after intravenous injection of stable I-127 ortho-iodohippurate. The results were compared with the clearance values of I-123 ortho-iodohippurate, considered the reference standard. Seven rabbits under general anesthesia were given intravenous injections of I-123 ortho-iodohippurate and I-127 ortho-iodohippurate. The corresponding plasma curves were evaluated from 4 to 60 minutes to calculate ERPF as the dose/integral of plasma curve. Results. The initial distribution volumes of I-123 ortho-iodohippurate (149.4 mL/kg ± 12.1) and I-127 ortho-iodohippurate (148.8 mL/kg ± 11.8) were virtually superimposable, thus confirming the chemical identity of the two compounds. The plasma clearance values for I-127 ortho-iodohippurate (11.15 mL/min kg^-1 ± 1.44) were slightly (not significantly) higher than those for I-123 ortho-iodohippurate (10.49 mL/min kg^-1 ± 1.41), perhaps because of a relative 'mass' load effect of the iodinated medium. Conclusion. The results obtained in this study demonstrate the feasibility of the new system for evaluating ERPF, provided that a compound with adequate glomerular filtration and tubular secretion properties is employed.