Quantitative analysis of [¹⁸F]CHL2310, a novel PET ligand for cholesterol 24-Hydroxylase, in nonhuman primate brain

Cholesterol 24-hydroxylase (CYP46A1) is a brain-specific enzyme catalyzes the conversion of cholesterol into 24S-hydroxycholesterol, thereby playing a pivotal role in maintaining cerebral cholesterol homeostasis. Dysregulation of this pathway has been implicated in various neurological disorders, including Alzheimer's disease, Huntington's disease, epilepsy, and depression. To enable quantitative assessment of CYP46A1 activity in vivo, a CYP46A1-targeted PET radiotracer with high specificity and favorable pharmacokinetics is envisioned. This study aimed to evaluate the metabolic stability, target specificity, and pharmacokinetic properties of [¹⁸F]CHL2310, a novel CYP46A1 PET radioligand, in non-human primates.

[¹⁸F]CHL2310 was synthesized using a tosylate precursor. Baseline and TAK-935 pre-blocked PET imaging with arterial blood sampling were performed in non-human primates. Whole-blood and plasma radioactivity, plasma free fraction, and metabolite analysis were conducted to generate metabolite-corrected plasma input functions. Regional brain time-activity curves were fitted using one-tissue compartment model, two-tissue compartment model and Logan graphical analysis to estimate total distribution volume. CYP46A1 occupancy by TAK-935 was quantified using Lassen plots, enabling the assessment of the dose-occupancy relationship. Non-displaceable binding potential was calculated using either non-displaceable distribution volume or the cerebellum as a reference region. Test-retest variability was evaluated in baseline scans from the same subject. Additionally, whole-body PET imaging was performed in male and female monkeys to estimate human radiation dosimetry.

[¹⁸F]CHL2310 was synthesized with a non-decay-corrected radiochemical yield of 6.7 ± 1.5% and radiochemical purity >99%. The tracer demonstrated high specific binding in NHP brain with reasonable metabolic stability and a free fraction of 13.1 ± 0.8% in plasma. TACs were well described by two-tissue compartment model and Logan graphical analysis. TAK-935 exhibited dose-dependent CYP46A1 occupancy with a half-maximal inhibitory dose of 0.0095 mg/kg, derived from Lassen plots. The averaged test-retest variability of tissue distribution volumes was -3.0 ± 4.8%, and the averaged absolute TRV was 4.4 ± 3.5%. The effective radiation dose for humans was estimated as 0.013 mSv/MBq.

[¹⁸F]CHL2310 shows high in vivo specificity, favorable pharmacokinetic properties, and robust quantitative performance in non-human primates. These characteristics support its potential as a PET radiotracer for imaging CYP46A1 in human studies.