Rapid Feasibility Studies of Tracers for Positron Emission Tomography: High-Resolution PET in Small Animals with Kinetic Analysis

Abstract

The development of methods for production of a radiotracer for use in human studies with positron emission tomography (PET) is often a time-consuming process of optimizing radiolabelling yields and handling procedures. Sometimes the radiotracer is not the original drug, but rather a derivative with unknown in vivo pharmacological properties. We have developed a fast and simple method of testing putative new PET tracers in vivo in small animals. The procedure has been validated in rats with different PET tracers with known kinetic and pharmacological properties ([2-18F]2-fluoro-2-deoxy-d-glucose, [ N-methyl-11C]Ro 15-1788, and [15O]butanol). The tracer concentration in arterial blood was continuously measured to obtain the brain input function. Following image reconstruction of the scans, time–activity curves of selected regions of interest were generated. Estimations of CMRglc (1.0 ± 0.2 μmol g−1 min−1), CBF (1.4 ± 0.4 ml g−1 min−1) and transport rate constants for [ N-methyl-11C]Ro 15-1788 (K1 = 0.44 ± 0.01 ml g−1 min−1 and k2 = 0.099 ± 0.005 min−1) as well as calculated first pass extraction (0.32 ±0.1) are in reasonable agreement with literature values. Small animal studies require minimal amounts of radioactivity and can be performed without sterility and toxicology tests. They may serve as a preliminary basis for radiation safety calculations because whole body scans can be performed even with a head scanner. The major advantage of this procedure in comparison to ex vivo autoradiography is that very few experiments are necessary to reliably determine the properties of the blood–brain barrier transport of the radiotracer and the possible whole brain receptor binding characteristics.