Abstract
1147
Objectives: Rolipram is a selective inhibitor of phosphodiesterase 4 (PDE4), the enzyme that catabolizes the second messenger cAMP. The enzyme activity of PDE4 is regulated by phosphorylation with greater activity in the phosphorylated than the non-phosphorylated form. Studies using recombinant DNAs showed the phosphorylated form is more sensitive to inhibition by rolipram. Thus, rolipram binding may provide indirect measurement of the activity of PDE4. Methods: In vivo Bmax and Kd were measured by estimating specifically bound (B) and free (F) (R)-[C-11]rolipram under transient equilibrium in PET experiments without blocking (n=5, mass dose: 0.75±0.12 μg/kg), partially (n=5, 0.015–0.0015 mg/kg) and fully blocking (n=2, 0.9–1.0 mg/kg) with non-radiolabeled (R)-rolipram. PET images were acquired with the ATLAS, and metabolite-corrected arterial input function was measured in all scans. B under transient equilibrium was estimated by performing two-compartmental fitting by constraining K1/k2 to the value obtained in full blocking scans. F was estimated from the activity in nondisplaceable compartment and plasma free fraction. Bmax and Kd were calculated by nonlinear fitting using one- and two-binding site models for the saturation curve created from B and F obtained above. At the end of the baseline scans, animals were sacrificed and samples were obtained to measure Bmax and Kd by homogenate binding. Results: Nonlinear fitting provided excellent fitting for the in vivo saturation curves with R2>0.90 for both one and two binding site models. F-test did not show a significant difference in goodness-of-fit between these models not supporting the presence of two binding sites. In vivo and in vitro measurements showed similar Bmax. Kd measured in vivo tended to be smaller than in vitro values. Conclusions: (R)-rolipram has greater affinity in vivo implying greater levels of PDE4 phosphorylation.
Research Support (if any): NIMH Z01-MH-002795-04
- Society of Nuclear Medicine, Inc.