Gender Differences in Binding Kinetics of Nicotinic Acetylcholine Receptors Using microPET/CT with 2-[¹⁸F]FA-85380

Objectives: Although men generally have higher smoking rates than women, nicotine addiction is a public health issue that concerns both genders. Despite its widespread epidemiological relevance, the precise mechanisms underlying nicotine dependence—as well as these gender differences that exist—remains poorly understood. Addiction to nicotine, the substance responsible for tobacco dependence, is predicated on the upregulation of the nicotinic acetylcholine receptor (nAChR) in the brain. Currently, serval positron emission tomography (PET) radiotracers, all of which are weak bases, are available for studies of the binding kinetics of nAChRs. They may be classified according to their differential kinetics based on their ligand pKas and receptor affinity. This study used one of high affinity radiotracer, 2-[¹⁸F]FA-85380, to examine the gender differences in binding kinetics of nAChRs.

Methods: The 2-[¹⁸F]FA-85380 radiotracer was synthesized from the commercially available precursor, 2-TMA-A85380, using an IBA Synthera V2 synthesis module equipped with Synthera preparative HPLC. The tracer (average injected activity = 143.6 uCi, specific activity >3000 mCi/μmole, 99% radiochemical purity) was injected into male and female C57BL/6J mice (3-6 months old) via intraperitoneal catheter within 30 seconds following the start of PET imaging. Dynamic PET acquisition was done for a continuous 180-minute scan and analyzed in 18x10-minute frames. The nAChR binding by 2-[¹⁸F]-A85380 in brain regions was assessed using the VivoQuant imaging analysis software with the 3D brain atlas and reported in standardized uptake value (SUV).

Results: The dynamic nAChR binding was first analyzed for the whole brain over 180-minute imaging period. The maximum binding capacity of the female is greater than that of males by 11.2% (0.632 vs. 0.564, SUV_max for females and males, respectively) while the binding peaks at 70 minutes in females compared to 50 minutes in males. The elevated binding capacity of the females remains throughout the rest of scanning time after the peak binding, demonstrating a slower dissociation rate of 2-[¹⁸F]-A85380 to nAChR. The brain was then segmented into 13 different functional regions. Assessment of regional nAChR binding capacities revealed highest binding activity in the thalamus (0.864 vs. 0.715, SUV_max for females and males, respectively) and lowest within the cerebellum (0.571 and 0.557, SUV_max for females and males, respectively) for both males and females. Consistently, females have higher nAChR binding than the males within all 13 brain regions. Mice with β2 subunit-knockout nAChR were additionally imaged with 2-[¹⁸F]-A85380. This gender difference was also observed in the knockout animals with female knockout demonstrating higher nAChR binding than the male knockout. However, regional differences were not observed in the β2-knockout mice.

Conclusions: The brain regional distribution of nAChR binding by 2-[¹⁸F]-A85380, specifically highest uptake within thalamus in contrast to the weakest binding within the cerebellum, is consistent with findings from human studies. The elevated maximum binding of 2-[¹⁸F]FA-85380 in females may suggest a higher density of nAChR in the female brain. In addition, one of the hypotheses of the nicotine binding mechanisms is that the ligand/receptor complex is trapped in the intracellular acidic vesicles. The slower dissociation rate observed in females may be indicative of stronger trapping. Together, these observed gender differences in mice may reflect gender differences in the response to nicotine exposure as well as to nicotine cessation treatments in humans. Therefore, more investigation into the binding kinetics of 2-[¹⁸F]FA-85380 and other nAChR ligands is necessary. The differences in kinetics and displaceable binding should provide insight into the cellular mechanism of nicotine induced nAChR upregulation and how nicotine agonists alter this process.