11C-PABA as a PET Radiotracer for Functional Renal Imaging: Preclinical and First-in-Human Study

para-Aminobenzoic acid (PABA) has been previously used as an exogenous marker to verify completion of 24-h urine sampling. Therefore, we hypothesized that PABA radiolabeled with 11C might allow high-quality dynamic PET of the kidneys with less radiation exposure than other agents because of its shorter biologic and physical half-life. We evaluated if 11C-PABA can visualize renal anatomy and quantify function in healthy rats and rabbits and in a first-in-humans study on healthy volunteers. Methods: Healthy rats and rabbits were injected with 11C-PABA intravenously. Subsequently, dynamic PET was performed, followed by postmortem tissue-biodistribution studies. 11C-PABA PET was directly compared with the current standard, 99mTc-mercaptoacetyltriglycin, in rats. Three healthy human subjects also underwent dynamic PET after intravenous injection of 11C-PABA. Results: In healthy rats and rabbits, dynamic PET demonstrated a rapid accumulation of 11C-PABA in the renal cortex, followed by rapid excretion through the pelvicalyceal system. In humans, 11C-PABA PET was safe and well tolerated. There were no adverse or clinically detectable pharmacologic effects in any subject. The cortex was delineated on PET, and the activity gradually transited to the medulla and then pelvis with high spatiotemporal resolution. Conclusion:11C-PABA demonstrated fast renal excretion with a very low background signal in animals and humans. These results suggest that 11C-PABA might be used as a novel radiotracer for functional renal imaging, providing high-quality spatiotemporal images with low radiation exposure.