First-in-Human Evaluation of ¹⁸F-PF-06445974, a PET Radioligand That Preferentially Labels Phosphodiesterase-4B

Phosphodiesterase-4 (PDE4), which metabolizes the second messenger cyclic adenosine monophosphate (cAMP), has 4 isozymes: PDE4A, PDE4B, PDE4C, and PDE4D. PDE4B and PDE4D have the highest expression in the brain and may play a role in the pathophysiology and treatment of depression and dementia. This study evaluated the properties of the newly developed PDE4B-selective radioligand ¹⁸F-PF-06445974 in the brains of rodents, monkeys, and humans. Methods: Three monkeys and 5 healthy human volunteers underwent PET scans after intravenous injection of ¹⁸F-PF-06445974. Brain uptake was quantified as total distribution volume (V _T) using the standard 2-tissue-compartment model and serial concentrations of parent radioligand in arterial plasma. Results: ¹⁸F-PF-06445974 readily distributed throughout monkey and human brain and had the highest binding in the thalamus. The value of V _T was well identified by a 2-tissue-compartment model but increased by 10% during the terminal portions (40 and 60 min) of the monkey and human scans, respectively, consistent with radiometabolite accumulation in the brain. The average human V _T values for the whole brain were 9.5 ± 2.4 mL ⋅ cm^-3 Radiochromatographic analyses in knockout mice showed that 2 efflux transporters-permeability glycoprotein (P-gp) and breast cancer resistance protein (BCRP)-completely cleared the problematic radiometabolite but also partially cleared the parent radioligand from the brain. In vitro studies with the human transporters suggest that the parent radioligand was a partial substrate for BCRP and, to a lesser extent, for P-gp. Conclusion: ¹⁸F-PF-06445974 quantified PDE4B in the human brain with reasonable, but not complete, success. The gold standard compartmental method of analyzing brain and plasma data successfully identified the regional densities of PDE4B, which were widespread and highest in the thalamus, as expected. Because the radiometabolite-induced error was only about 10%, the radioligand is, in the opinion of the authors, suitable to extend to clinical studies.