Sex- and age-specific sensitivities of the endocannabinoid system in Alzheimer's disease revealed by PET imaging with [¹⁸F]FMPEP-<i>d</i>₂ and [¹⁸F]MAGL-2102

The endocannabinoid system is a critical brain signaling pathway that is dysregulated in various brain disorders, including Alzheimer's disease (AD). Cannabinoid-targeted therapies and imaging approaches have gained increasing interest; however, the biological impact of the endocannabinoid system in disease needs further validation. We aimed to study changes in cannabinoid receptor 1 (CB1) and monoacylglycerol lipase (MAGL), components of endocannabinoid signaling and degradation, in a mouse model of AD by PET imaging. <b>Methods:</b> [¹⁸F]FMPEP-<i>d</i> ₂ and [¹⁸F]MAGL-2102 were produced on a commercial radiosynthesis module. PET-CT images with both tracers were acquired in a knock-in mouse model of AD bearing mutated human amyloid precursor protein (<i>App^NL-G-F</i> ) at 3 ages, and compared to wild-type mice. Excised brains were used for <i>in vitro</i> autoradiography with [¹⁸F]FMPEP-<i>d</i> ₂ and [¹⁸F]MAGL-2102, immunofluorescence, and western blotting. Male wild-type and 5xFAD mice were chronically treated with MAGL inhibitor JZL184 and imaged with [¹⁸F]MAGL-2102 two days after ending treatment. <b>Results:</b> PET imaging showed sex-, age- and genotype-dependent changes in CB1 and MAGL availability. At 4-months (early-stage β-amyloid pathology), female <i>App^NL-G-F</i> mice had lower CB1 availability, and MAGL availability was increased in male <i>App^NL-G-F</i> _, compared to wild-types_. At 8-months, no genotype differences in CB1 were observed, yet MAGL availability was reduced in <i>App^NL-G-F</i> frontal cortex, and male <i>App^NL-G-F</i> mice exhibited higher MAGL than transgenic females brain-wide. At 12-months (late-stage β-amyloid pathology), significantly lower uptake of [¹⁸F]FMPEP-<i>d</i> ₂ was observed in <i>App^NL-G-F</i> compared to wild-type, with no changes in [¹⁸F]MAGL-2102 binding. <i>App^NL-G-F</i> plaque staging was confirmed by Thioflavin-S staining. Imaging findings were supplemented by autoradiography, immunofluorescence, and western blots. [¹⁸F]MAGL-2102 availability was responsive to target engagement of the MAGL inhibitor JZL184 in wild-type and 5xFAD mice. <b>Conclusions:</b> The present study showed dynamic age-, sex- and pathology-related changes in CB1 and MAGL availability from early-stage β-amyloid pathology, suggesting that the endocannabinoid system is a useful target for diagnostics and treatment of AD. Finally, these results highlight that endocannabinoid sex differences should be considered in diagnostics and drug development.