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

Abstract

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.

Keywords: 18F-PF-06445974; PET; phosphodiesterase-4B (PDE4B).

Graphical abstract

FIGURE 1.

Distribution of radioactivity in the brain of a healthy volunteer after injection of ¹⁸F-PF-06445974 and the participant’s coregistered MRI scan (top). The PET image displays the mean concentration (conc.) of radioactivity from 0 to 120 min and is expressed as SUV (bottom). The highest uptake was in the thalamus, which is marked with cross hairs.

FIGURE 2.

Concentration of plasma parent radioligand ¹⁸F-PF-0644974 separated from radiometabolite (A) and total radioactivity (B) in whole brain of 3 healthy human participants. The plasma time–activity curve after peak was fit to a triexponential curve. The brain time–activity curve was fit to a 2-tissue-compartment model. Concentrations in plasma and brain are expressed as SUV. Plasma parent is plotted on a log scale, and brain activity is plotted on a linear scale. Conc. = concentration.

FIGURE 3.

The apparent value of total distribution volume (V_T) from 5 human participants (A) and 3 monkeys (B). V_T values never achieved stability during the scans. V_T values increased linearly by 10% during the last 40 min in humans and also by 10% during the last 60 min in monkeys.

FIGURE 4.

Radiochromatograms showing the composition of radioactivity extracted at 180 min from plasma (A) and brain (B) of a rat injected with ¹⁸F-PF-06445974. In plasma (A), parent radioligand (blue peak) comprised 32% of total radioactivity, and the adjacent radiometabolite (peak marked with an asterisk) was 62%. At the same time, in the brain (B), parent radioligand comprised 99% of total radioactivity, and the adjacent radiometabolite was only 1%.

FIGURE 5.

Concentrations of parent radioligand in brain and plasma of wild-type and efflux transporter knockout mice at 120 min after ¹⁸F-PF-06445974 injection. (A) For the parent radioligand, the ratio of brain-to-plasma concentration was 93 in wild-type and 188 in mice with a knockout of both BCRP and P-gp. Thus, the brain-to-plasma ratio of parent radioligand was 2 times higher in knockout mice than in wild-type mice. (B) For the radiometabolite, the ratio of brain-to-plasma concentration was 0.2 in wild-type and 4.0 in BCRP and P-gp knockout mice. Thus, the brain-to-plasma ratio of parent radioligand was 20 times higher in knockout mice than in wild-type mice. Concentrations are expressed as percent SUV (%SUV).

FIGURE 6.

(A) PET and coregistered MRI of brain in a rhesus monkey. The animal was scanned at baseline and after blockade by PF-06445974 (0.1 mg/kg intravenously injected 10 min before radioligand). Because of prolonged tachycardia (heart rate up to 190 bpm), the blocked scan was terminated after 75 min. The PET images show mean concentration of radioactivity SUV from 0 to 70 min. (B) Radioactivity in whole brain after injection of ¹⁸F-PF-06445974. (C) Concentration in arterial plasma of plasma parent radioligand ¹⁸F-PF-06445974 separated from radiometabolite. Because there were so few plasma samples, compartmental modeling could not generate a reliable measure of enzyme density (total distribution volume, V_T). Measured as average concentration of radioactivity from 10 to 60 min (SUV_10–60), nonradioactive PF-06445974 blocked 92% of radioligand uptake into brain. Conc. = concentration.

FIGURE 7.

Schema of the cAMP cascade (black arrows) and negative feedback via protein kinase A (PKA) (red arrows). Several neurotransmitters act via receptors coupled to G proteins (Gs) to stimulate adenylate cyclase (AC), which produces cAMP, activates PKA, and then phosphorylates cAMP response element-binding protein (CREB), which moves to the nucleus and increases expression of brain-derived neurotrophic factor (BDNF) and other genes. Negative feedback is provided by PKA, which phosphorylates and activates PDE4, which metabolizes cAMP, thereby terminating the cAMP signal.