Effect of acetylcholinesterase inhibitors on the binding of nicotinic alpha4beta2 receptor PET radiotracer, (18)F-nifene: A measure of acetylcholine competition.

Acetylcholinesterase inhibitors (AChEI's) are used to treat Alzheimer's disease (AD), and the putative mode of action is to increase acetylcholine (ACh) levels. Our goal is to evaluate competition of ACh with nicotinic alpha4beta2 receptor PET agonist radiotracer, 2-[(18)F]fluoro-3-[2-((S)-3-pyrrolinyl)methoxy]pyridine ((18)F-nifene). This ability to measure ACh-(18)F-nifene competition may have potential to assess efficacy of AChEI's in vivo. In vitro studies in rat brain slices used two AChEI's, physostigmine (PHY) and galanthamine (GAL). Brain slices were incubated with (18)F-nifene and various concentrations of PHY (0.2-20 microM) or GAL (0.4-4 microM) prior to (18)F-nifene treatment. For ACh competition, slices were also incubated with PHY + 100 nM ACh or GAL + 100 nM ACh or 100 nM ACh alone. Nonspecific binding of (18)F-nifene was determined using 300 microM nicotine. In the in vitro rat brain homogenate binding assay, ACh inhibited (3)H-cytisine binding to alpha4beta2 receptors with K(i) values of 19.2 nM (with PHY) and 34.7 microM (no PHY) indicating approximately 1.8 x 10(3) weaker binding of ACh in the absence of AChEI. Binding of (18)F-nifene was not affected by PHY (0.2-20 microM) or ACh 100 nM alone but decreased substantially by PHY + ACh 100 nM in all brain regions (down by >40% of control in thalamus). Similarly, for GAL (4 microM) no effect on (18)F-nifene binding occurred but GAL (0.4-4 microM) + ACh 100 nM showed a reduction of (18)F-nifene binding in all brain regions (down by approximately 15%). The reduction in both cases is a result of ACh competition with (18)F-nifene in the presence of AChEI. These preliminary in vitro results suggest that ACh is able to compete with (18)F-nifene at the alpha4beta2 receptors in the presence of PHY or GAL. The effect is AChEI-concentration dependent and is greater for PHY than GAL. Thus (18)F-nifene has promise for assessing ACh levels and AChEI effects in vivo.