PET measurement of the in vivo affinity of 11C-(R)-rolipram and the density of its target, phosphodiesterase-4, in the brains of conscious and anesthetized rats

Abstract

A variety of phosphodiesterases hydrolyze and terminate the effects of the intracellular second messenger 3′,5′-cyclic adenosine monophosphate (cAMP). Phosphodiesterase subtype 4 (PDE4) is particularly abundant in the brain and has been imaged with 11C-(R)-rolipram, a selective inhibitor of PDE4. We sought to measure in vivo both the binding site density (B max) and the radioligand affinity (1/KD) of 11C-(R)-rolipram in the rat brain. We also studied 2 critical factors in small-animal PET scans: the influence of anesthesia and the difference in binding under in vivo and in vitro conditions. Methods: In vivo, Bmax and KD were measured in PET saturation experiments by the administration of 11C-(R)-rolipram and various doses of carrier (R)-rolipram in conscious and isoflurane-anesthetized rats. The metabolite-corrected arterial input function was measured in each scan. To image conscious rats, the head of the rat was fixed in a holder and the animals were trained to comply with this apparatus. Bound and free (R)-rolipram levels were calculated under transient equilibrium conditions (i.e., at the time of peak specific binding). Results: The Bmax and KD of conscious rats were significantly greater than those of anesthetized rats, by 29% and 59%, respectively. In addition, the in vitro KD was 3-7 times greater than was the in vivo KD, although the Bmax was similar in both conditions. Conclusion: The in vivo Bmax and KD of (R)-rolipram were successfully measured in both conscious and anesthetized rats. KD was affected to a greater extent than was Bmax by the 2 conditions. That is, KD was increased in the conscious rat, compared with in the anesthetized rat, and KD was increased in vitro, compared with in vivo. The current study shows that the rat, a readily available species for research, can be used to measure in vivo both affinity and density of radioligand targets, which can later be directly assessed with standard in vitro techniques. Copyright © 2009 by the Society of Nuclear Medicine, Inc.