[11C]tropanyl benzilate-binding to muscarinic cholinergic receptors: methodology and kinetic modeling alternatives

In vivo studies of acetylcholinesterase activity using a labeled substrate, N-[11C]methylpiperdin-4-yl propionate ([11C]PMP)

Two esters, N-[11C]methylpiperidyl acetate ([11C]AMP) and N-[11C]methylpiperidyl propionate ([11C]PMP), were synthesized in no-carrier-added forms and evaluated as in vivo substrates for brain acetylcholinesterase (AChE). After peripheral injection in mice, each ester showed rapid penetration into the brain and a regional retention of radioactivity (striatum > cortex, hippocampus > cerebellum) reflecting known levels of AChE activity in the brain. Regional brain distributions after [11C]PMP administration showed better discrimination between regions of high, intermediate, and low AChE activities. Chromatographic analysis of blood and brain tissue extracts showed rapid and nearly complete hydrolysis of [11C]PMP within 10 min after injection. For both [11C]AMP and [11C]PMP, retention of radioactivity in all regions was reduced by pretreatment with diisopropylfluorophosphate (DFP), a specific irreversible AChE inhibitor. DFP treatment also significantly increased the proportions of unhydrolyzed ester in both blood and brain. Radioactivity localization in brain after peripheral injection was thus dependent on AChE-catalyzed hydrolysis to the hydrophilic product N-[11C]methylpiperidinol. PET imaging of [11C]AMP or [11C]PMP distributions in monkey brain showed clear accumulation of radioactivity in areas of highest AChE activity (striatum, cortex). These esters are thus in vivo substrates for brain AChE, with potential applications as in vivo imaging agents of enzyme action in the human brain. [11C]PMP, the ester with a slower rate of hydrolysis, appears to be the better candidate radiotracer for further development.

In vivo binding, pharmacokinetics and metabolism of the selective M2 muscarinic antagonists [3H]AF-DX 116 and [3H]AF-DX 384 in the anesthetized rat

The pharmacokinetics, in vivo binding and metabolism of two M2 muscarinic receptor antagonists, [3H]AF-DX 116 and [3H]AF-DX 384, were studied in anesthetized rats, which received either the tracer alone or following a saturating injection of atropine. Both radioligands were cleared from the circulation with distribution half-lives of 17 and 14 sec and elimination half-lives of 17 and 40 min for [3H]AF-DX 116 and [3H]AF-DX 384, respectively. A radioactive distribution, predominant in peripheral organs when compared to brain, was found at each time studied after tracer injection. Atropine-displaceable tracer uptake was evidenced at 20-40 min in brain (31%), submandibular glands (26%), spleen (37%) and notably heart (55%) for [3H]AF-DX 116 but only in heart (50%) for [3H]AF-DX 384 at 10-20 min. Regional brain sampling revealed a relatively uniform distribution of [3H]AF-DX 384 and a -45% atropine saturation effect (i.e., specific binding) in the thalamus 20 min after injection. Sequential thin-layer chromatographic studies performed on tissue extracts demonstrated the rapid appearance of labeled metabolites of both radiotracers in brain (but less so in liver) and especially in cardiac tissues, where almost 70% of total radioactivity still corresponded to authentic tracer 40 min after injection. Thus, based on their low blood-brain barrier permeability and the high presence of labeled metabolites in the central nervous system, AF-DX 116 and AF-DX 384 might be more helpful in the study of M2 muscarinic receptors present in heart rather than brain. Labeled with positron emittors, these M2 antagonists might be applicable to the pathophysiological study of disease states, such as cardiomyopathies.

Synthesis, in vivo biodistribution and dosimetry of [11C]N-methylpiperidyl benzilate ([11C]NMPB), a muscarinic acetylcholine receptor antagonist

4-N-Methylpiperidyl benzilate (NMPB), a high affinity antagonist for the muscarinic cholinergic receptor, has been synthesized in carbon-11-labeled form through the N-[11C]methylation of 4-piperidylbenzilate. The product was isolated by HPLC, and obtained in yields (> 100 mCi) and specific activities (500-3000 Ci/mmol) sufficient for in vivo evaluation in small animals. Time-dependent regional brain distributions in rats and mice showed high radiotracer uptake and retention in striatum and cortex, and low in cerebellum, consistent with muscarinic cholinergic receptor distributions. Radiotracer retention in tissues could be significantly reduced by pretreatment of animals with a large dose of a competing antagonist, quiniclidinyl benzilate. Whole body biodistribution in rats was used to calculate the expected human internal radiation dosimetry for this new radiopharmaceutical. These animal experiments formed the basis for subsequent introduction of [11C]NMPB into human use with positron emission tomography.