Robust Quantification of Phosphodiesterase-4D in Monkey Brain with PET and 11C-Labeled Radioligands That Avoid Radiometabolite Contamination

Abstract

Phosphodiesterase-4D (PDE4D) has emerged as a significant target for treating neuropsychiatric disorders, but no PET radioligand currently exists for robustly quantifying human brain PDE4D to assist biomedical research and drug discovery. A prior candidate PDE4D PET radioligand, namely [¹¹C]T1650, failed in humans because of poor time stability of brain PDE4D-specific signal (indexed by total volume of distribution), likely due to radiometabolites accumulating in brain. Its nitro group was considered to be a source of the brain radiometabolites. Methods: We selected 5 high-affinity and selective PDE4D inhibitors, absent of a nitro group, from our prior structure-activity relationship study for evaluation as PET radioligands. Results: All 5 radioligands were labeled with ¹¹C (half-time, 20.4 min) in useful yields and with high molar activity. All displayed sizable PDE4D-specific signals in rhesus monkey brain. Notably, [¹¹C]JMJ-81 and [¹¹C]JMJ-129 exhibited excellent time stability of signal (total volume of distribution). Furthermore, as an example, [¹¹C]JMJ-81 was found to be free of radiometabolites in ex vivo monkey brain, affirming that this radioligand can provide robust quantification of brain PDE4D with PET. Conclusion: Given their high similarity in structures and metabolic profiles, both [¹¹C]JMJ-81 and [¹¹C]JMJ-129 warrant further evaluation in human subjects. [¹¹C]JMJ-129 shows a higher PDE4D specific-to-nonspecific binding ratio and will be the first to be evaluated.

Keywords: 11C; PET; inhibitors; phosphodiesterase-4D (PDE4D).

Graphical abstract

FIGURE 1.

Chemical structures of T1650 and 5 new PDE4D radioligand candidates. Asterisks denote sites for ¹¹C labeling.

FIGURE 2.

Illustration of radioligand production and evaluation. DMF = dimethylformamide.

FIGURE 3.

(A and B) Time–activity curves after injection of [¹¹C]JMJ-81 for monkey brain (A) and unchanged radioligand in plasma (B) at baseline and after block by rolipram (0.5 mg/kg, intravenously) in same monkey. (C) V_T values at 90 min after injection of [¹¹C]JMJ-81 across monkey brain regions at baseline and after block by rolipram (0.5 mg/kg intravenously). SUV is calculated as radioactivity in brain (Bq) per g of brain divided by injected activity (Bq) per g of body weight (36).

FIGURE 4.

(A and B) Lassen plots for whole brain in 2 monkeys for [¹¹C]JMJ-81 (A) and [¹¹C]JMJ-129 (B), based on V_T measures from 2-tissue-compartment modeling. (C and D) V_T time-stability curves for whole brain, frontal cortex, and cerebellum for [¹¹C]JMJ-81 (C) and [¹¹C]JMJ-129 (D) from 2 monkeys. V_T values are normalized to end of scan values. Rolipram dose was 0.5 mg/kg for monkey 1 in [¹¹C]JMJ-81 experiment only. Other experiments used rolipram at 0.2 mg/kg. WB = whole brain.

FIGURE 5.

(A) V_T estimated by Logan analysis of regional monkey brain data for [¹¹C]JMJ-81 and [¹¹C]JMJ-129 at baseline and after PDE4D block with BPN14770 (3 mg/kg, intravenously). (B) PET images of radioactivity concentration (SUV) for [¹¹C]JMJ-81 and [¹¹C]JMJ-129 in monkey brain at baseline and after PDE4D block. Radioactivity concentrations were averaged from 20 to 60 min.

FIGURE 6.

(A) Relative in vitro stabilities of radioligands ([¹¹C]T1650, [¹¹C]JMJ-81, and [¹¹C]JMJ-129) after incubation with fresh rat whole blood, plasma, and homogenized fresh brain tissues for at least 30 min at 37°C (n = 1). (B) Relative ex vivo stability of radioligands in rat brain and plasma at 30 min after injection (n = 3). (C) Representative radiochromatograms for HPLC of plasma and brain harvested from monkey at 30 min after injection of [¹¹C]JMJ-81.