Nicotinic acetylcholine receptors (alpha4beta2) and their relationship to cognitive and mood symptoms in Parkinson disease: A 2-[18F]-F-A-85380 PET study

Acetylcholine receptors in dementia and mild cognitive impairment

PURPOSE

To clarify whether changes in the cholinergic transmission occur early in the course of Alzheimer's disease (AD), we carried out positron emission tomography (PET) with the radioligand 2-[(18)F]F-A-85380, which is supposed to be specific for alpha4beta2 nicotinic acetylcholine receptors (nAChRs).

METHOD

We included patients with moderate to severe AD and patients with amnestic mild cognitive impairment (MCI), presumed to present preclinical AD.

RESULTS

Both patients with AD and MCI showed significant reductions in alpha4beta2 nAChRs in brain regions typically affected by AD pathology. These findings indicate that a reduction in alpha4beta2 nAChRs occurs during early symptomatic stages of AD. The alpha4beta2 nAChR availability in these regions correlated with the severity of cognitive impairment, indicating a stage sensitivity of the alpha4beta2 nAChR status.

CONCLUSION

Together, our results provide evidence for the potential of 2-[(18)]F-A-85380 nAChR PET in the diagnosis of patients at risk for AD. Because of the extraordinary long acquisition time with 2-[(18)F]F-A-85380, we developed the new alpha4beta2 nAChR-specific radioligands (+)- and (-)-[(18)F]norchloro-fluoro-homoepibatidine (NCFHEB) and evaluated them preclinically. (-)-[(18)F]NCFHEB shows twofold higher brain uptake and significantly shorter acquisition times. Therefore, (-)-[(18)F]NCFHEB should be a suitable radioligand for larger clinical investigations.

A simplified method for the measurement of nonmetabolized 2-[18F]F-A-85380 in blood plasma using solid-phase extraction

Quantification of alpha(4)beta(2)* nicotinic acetylcholine receptors using 2-[(18)F]fluoro-3-(2(S)-azetidinylmethoxy)pyridine (2-[(18)F]FA) and positron emission tomography (PET) imaging requires measurement of nonmetabolized radioligand in blood plasma, which was previously accomplished using high-performance liquid chromatography (HPLC). Here, we introduce a one-step solid-phase extraction (SPE) method for measuring the concentration of nonmetabolized 2-[(18)F]FA. This method allows many samples to be processed in a short period of time. SPE effectively separated 2-[(18)F]FA from radioactive metabolites typically observed in blood plasma after administration of radioligand in humans. Measurements of the 2-[(18)F]FA parent fraction in healthy human volunteers obtained using the SPE method were nearly identical to those obtained using HPLC (1.3+/-5% average underestimation of SPE), and reproducibility was good within and between runs (2% and 6% coefficient of variation, respectively). SPE recovery of 2-[(18)F]FA from blood plasma was not appreciably diminished (3+/-0.6%) by a larger volume of blood plasma loaded onto the cartridge, suggesting the possibility of increasing the plasma sample volume at later times in a PET study to improve measurement sensitivity. 2-[(18)F]FA was stable in blood stored on ice over 8 h and in saline at low concentrations (<2 MBq/ml) but not at high concentrations (ca. 130 MBq/ml). Using SPE, the elimination half-life and full body distribution volume of 2-[(18)F]FA in healthy human volunteers were estimated as 4.2+/-0.8 h and 220+/-70 L, respectively. These results suggest that SPE is the method of choice for the determination of the plasma 2-[(18)F]FA concentration when measurement of individual metabolites is not required.