Tc-99m p-aminohippuric acid renal scintigraphy: results of preliminary animal and human studies

Purpose: Tc-99m p-aminohippuric acid (PAH) is a new radiopharmaceutical labeled with Tc-99m in presence of calcium three sodium salt of DTPA by use of Sn(II) - reduction method. Tc-99m PAH is rapidly secreted by the kidneys in a manner consistent with tubular secretion, as confirmed by rat probenecid studies. The present study aimed at evaluating Tc-99m PAH as renal imaging agent. Methods: After bolus injection of radiopharmaceutical in a Wister rat (17 MBq) and in 10 healthy subjects - 2 normal volunteers and 8 potential kidney. donors (111 MBq) dynamic images were acquired for 20 min in three frame groups. In all patients Tc-99m MAG3 and Tc-99m DTPA studies were also performed several days later, in l day intervals to acquire renal studies for comparison. The scintigraphic images and time/activity curves were analyzed. Results: Rat study gave typical three-phase renogram and satisfactory renal functional images with no significant extrarenal background. During Tc-99m PAH scintigraphy in humans rapid disappearance of radiopharmaceutical from the vascular system and high uptake in kidneys followed by its very fast elimination and low residual activity at the end of the study were registered. Anterior whole body images obtained at 40 min and 3 h post injection to evaluate the distribution of 99mTc-PAH within the body showed neither gallbladder nor liver activity. Mean values of the following semi-quantitative parameters were calculated; time to peak (Tmax): 3.65±0.91, time from peak to 50% of peak activity (T1/2): 6.91±2.71, percentage of residual activity in each kidney at the end of the study: 26±2.71 and split renal function: 51.26±3.92. Mean values of Tmax and T1/2 obtained with Tc-99m PAH were significantly lower in comparison with Tc-99m DTPA. Mean values of relative function did not show any significant difference between three tracers. Conclusion: Our preliminary results confirmed good biological characteristics of 99mTc-PAH, indicating its potential usefulness for routine renal scintigraphy. Due to its fast kinetics, short half-life and high-quality images, it could be suitable substitute for Tc-99m DTPA. (authors)