Steven R. Thomas

The synthesis and preclinical evaluation in rhesus monkey of [18F]MK-6577 and [11C]CMPyPB glycine transporter 1 positron emission tomography radiotracers

Two positron emission tomography radiotracers for the glycine transporter 1 (GlyT1) are reported here. Each radiotracer is a propylsulfonamide‐containing benzamide and was labeled with either carbon‐11 or fluorine‐18. [11C]CMPyPB was synthesized by the alkylation of a 3‐hydroxypyridine precursor using [11C]MeI, and [18F]MK‐6577 was synthesized by a nucleophilic aromatic substitution reaction using a 2‐chloropyridine precursor. Each tracer shows good uptake into rhesus monkey brain with the expected distribution of highest uptake in the pons, thalamus, and cerebellum and lower uptake in the striatum and gray matter of the frontal cortex. In vivo blockade and chase studies of [18F]MK‐6577 showed a large specific signal and reversible binding. In vitro autoradiographic studies with [18F]MK‐6577 showed a large specific signal in both rhesus monkey and human brain slices and a distribution consistent with the in vivo results and those reported in the literature. In vivo metabolism studies in rhesus monkeys demonstrated that only more‐polar metabolites are formed for each tracer. Of these two tracers, [18F]MK‐6577 was more extensively characterized and is a promising clinical positron emission tomography tracer for imaging GlyT1 and for measuring GlyT1 occupancy of therapeutic compounds. Synapse, 2011.

4-N-linked-heterocyclic piperidine derivatives with high affinity and selectivity for human dopamine D4 receptors

The syntheses of a number of different N-linked heterocyclic pyrazole replacements based on the structure 1 are described (compounds 3-12) as hD4 ligands. After further optimisation the best compound identified was 13 which has high affinity for hD4 (5.2 nM) and >300-fold selectivity for hD4 receptors over hD2 and hD3 receptors.

Accelerated metabolite identification by “Extraction-NMR”

Examples of the use of extraction-NMR, an efficient and rapid method to obtain structural information on metabolites without prior separation, are described. Crude ethyl acetate extracts of in vitro microsomal incubations were analysed by NMR spectroscopy. The region downfield of 5.5 ppm in the proton spectra of these crude extracts was found to be essentially clear of endogenous resonances. As a consequence, sites of aromatic hydroxylation can often be determined without prior separation of the crude extracts. Sites of metabolism close to the aromatic region may also be accessible via two-dimensional (2D) homonuclear experiments (e.g. COSY, NOESY, TOCSY). One-dimensional (1D) and 2D fluorine experiments also can provide additional information on the structure of metabolites. Depending on the complexity of the aromatic region of the parent compound, signal overlap and the relative abundance of the individual components, extraction-NMR has the potential to provide information for unambiguous structure elucidation of two or three major metabolites. Should extraction-NMR produce inconclusive results, i.e. too many metabolites are present or metabolism occurred exclusively on aliphatic regions, it is possible to re-use the extraction-NMR sample and proceed with traditional methods of analysis.

Asymmetric synthesis of the northern segment of ephedradine C

An asymmetric synthesis of the dihydrobenzofuran segment of ephedradine C has been achieved. Key steps include a chiral oxazolidinone-mediated aldol reaction to form a β-hydroxy ester, followed by a novel debenzylation and concomitant intramolecular cyclisation with iodotrimethylsilane. An asymmetric Michael reaction with a homochiral lithium amide was used to form the third and final chiral centre. The absolute stereochemistry of these three centres was confirmed by X-ray crystal-structure determinations.