Histamine H3 inverse agonist PET tracers labelled with carbon-11 or fluorine-18

The PET tracer [11C]MK-6884 quantifies M4 muscarinic receptor in rhesus monkeys and patients with Alzheimer's disease

[Figure: see text].

Preclinical Characterization of 18F-MK-6240, a Promising PET Tracer for In Vivo Quantification of Human Neurofibrillary Tangles

A PET tracer is desired to help guide the discovery and development of disease-modifying therapeutics for neurodegenerative diseases characterized by neurofibrillary tangles (NFTs), the predominant tau pathology in Alzheimer disease (AD). We describe the preclinical characterization of the NFT PET tracer ¹⁸F-MK-6240.

METHODS

In vitro binding studies were conducted with ³H-MK-6240 in tissue slices and homogenates from cognitively normal and AD human brain donors to evaluate tracer affinity and selectivity for NFTs. Immunohistochemistry for phosphorylated tau was performed on human brain slices for comparison with ³H-MK-6240 binding patterns on adjacent brain slices. PET studies were performed with ¹⁸F-MK-6240 in monkeys to evaluate tracer kinetics and distribution in the brain. ¹⁸F-MK-6240 monkey PET studies were conducted after dosing with unlabeled MK-6240 to evaluate tracer binding selectivity in vivo.

RESULTS

The ³H-MK-6240 binding pattern was consistent with the distribution of phosphorylated tau in human AD brain slices. ³H-MK-6240 bound with high affinity to human AD brain cortex homogenates containing abundant NFTs but bound poorly to amyloid plaque-rich, NFT-poor AD brain homogenates. ³H-MK-6240 showed no displaceable binding in the subcortical regions of human AD brain slices and in the hippocampus/entorhinal cortex of non-AD human brain homogenates. In monkey PET studies, ¹⁸F-MK-6240 displayed rapid and homogeneous distribution in the brain. The ¹⁸F-MK-6240 volume of distribution stabilized rapidly, indicating favorable tracer kinetics. No displaceable binding was observed in self-block studies in rhesus monkeys, which do not natively express NFTs. Moderate defluorination was observed as skull uptake.

CONCLUSION

¹⁸F-MK-6240 is a promising PET tracer for the in vivo quantification of NFTs in AD patients.

11C-GSK189254: a selective radioligand for in vivo central nervous system imaging of histamine H3 receptors by PET

The histamine H(3) receptor is a G-protein-coupled presynaptic auto- and heteroreceptor whose activation leads to a decrease in the release of several neurotransmitters including histamine, acetycholine, noradrenaline, and dopamine. H(3) receptor antagonists such as 6-[(3-cyclobutyl-2,3,4,5-tetrahydro-1H-3-benzazepin-7-yl)oxy]-N-methyl-3-pyridinecarboxamide hydrochloride (GSK189254) can increase the release of these neurotransmitters and thus may offer potential therapeutic benefits in diseases characterized by disturbances of neurotransmission. The aim of this study was to synthesize and evaluate (11)C-labeled GSK189254 ((11)C-GSK189254) for imaging the histamine H(3) receptor in vivo by PET.

METHODS

GSK189254 exhibits high affinity (0.26 nM) and selectivity for the human histamine H(3) receptor. Autoradiography experiments were performed using (3)H-GSK189254 to evaluate its in vitro binding in porcine brain tissues. GSK189254 was labeled by N-alkylation using (11)C-methyl iodide in good yields, radiochemical purity, and specific activity. A series of PET experiments was conducted to investigate (11)C-GSK189254 binding in the porcine brain.

RESULTS

In vitro autoradiography demonstrated specific (3)H-GSK189254 binding in the porcine brain; therefore, (11)C-GSK189254 was evaluated in vivo in pigs and showed good brain penetration and high uptake in regions such as the striatum and cortices, known to contain high densities of the histamine H(3) receptors. The radioligand kinetics were reversible, and quantitative analysis was achieved with a 2-tissue-compartmental model yielding the distribution volume as the outcome measure of interest. The distribution volume was reduced to a homogeneous level in all regions after blocking by the coadministration of either unlabeled GSK189254 or ciproxifan, a structurally distinct histamine H(3) antagonist. Further coadministration studies allowed for the estimation of the radioligand affinity (0.1 nM) and the density of histamine H(3) receptor sites in the cerebellum (0.74 nM), cortex (2.05 nM), and striatum (2.65 nM).

CONCLUSION

These findings suggest that (11)C-GSK189254 possesses appropriate characteristics for the in vivo imaging of the histamine H(3) receptor by PET.