In vitro binding affinity vs. in vivo site occupancy: A PET study of four diastereomers of dihydrotetrabenazine (DTBZ) in monkey brain

Evaluation of trans- and cis-4-[18F]Fluorogabapentin for Brain PET Imaging

Gabapentin, a selective ligand for the α2δ subunit of voltage-dependent calcium channels, is an anticonvulsant medication used in the treatment of neuropathic pain, epilepsy, and other neurological conditions. We recently described two radiofluorinated derivatives of gabapentin (trans-4-[18F]fluorogabapentin, [18F]tGBP4F, and cis-4-[18F]fluorogabapentin, [18F]cGBP4F) and showed that these compounds accumulate in the injured nerves in a rodent model of neuropathic pain. Given the use of gabapentin in brain diseases, here we investigate whether these radiofluorinated derivatives of gabapentin can be used for imaging α2δ receptors in the brain. Specifically, we developed automated radiosynthesis methods for [18F]tGBP4F and [18F]cGBP4F and conducted dynamic PET imaging in adult rhesus macaques with and without preadministration of pharmacological doses of gabapentin. Both radiotracers showed very high metabolic stability, negligible plasma protein binding, and slow accumulation in the brain. [18F]tGBP4F, the isomer with higher binding affinity, showed low brain uptake and could not be displaced, whereas [18F]cGBP4F showed moderate brain uptake and could be partially displaced. Kinetic modeling of brain regional time-activity curves using a metabolite-corrected arterial input function shows that a one-tissue compartment model accurately fits the data. Graphical analysis using Logan or multilinear analysis 1 produced similar results as compartmental modeling, indicating robust quantification. This study advances our understanding of how gabapentinoids work and provides an important advancement toward imaging α2δ receptors in the brain.

9-Cyclopropylmethoxy-dihydrotetrabenazine and its stereoisomers as vesicular monoamine transporter-2 inhibitors

Aim: To separate and evaluate 9-cyclopropylmethoxy-dihydrotetrabenazine (13a) and its stereoisomers for their high affinity for vesicular monoamine transporter-2 (VMAT2). Method: Stereoisomers of 13a were separated and configurations were ascertained by chiral chromatography and crystal diffraction combined with ¹H-¹H NOESY assay. Possible binding modes of eight stereoisomers and VMAT2 were explored by molecular docking assays. The VMAT2 affinity of the stereoisomers, inhibition in vivo and pharmacokinetics in rats were evaluated. Results: Three stereoisomers were obtained: P1, P2 and P3, and all had similar VMAT2 binding modes. P2 [(2R, 3R, 11bR)-13a] showed the highest potential VMAT2 binding activity (K_i = 0.75 nM), decreased locomotor activity in rats and had an oral absolute bioavailability of 92.0%. Conclusion: P2 has good efficacy and pharmacokinetic properties and warrants further development to treat tardive dyskinesia.

PET Imaging of VMAT2 with the Novel Radioligand [18F]FE-DTBZ-d4 in Nonhuman Primates: Comparison with [11C]DTBZ and [18F]FE-DTBZ

The vesicular monoamine transporter type 2 (VMAT2) is believed to be responsible for the uptake of monoamines into the vesicles of the synaptic terminals. Two VMAT2 radioligands [¹¹C]DTBZ and [¹⁸F]FP-DTBZ have been used to assess the degree of nigrostriatal deficit in Parkinson’s disease (PD) using positron emission tomography (PET). [¹⁸F]FE-DTBZ-d4, the nondeuterated analogue of [¹⁸F]FE-DTBZ showed similar imaging properties with better stability against defluorination. Therefore, [¹⁸F]FE-DTBZ-d4 draws attention to be investigated as an imaging marker for VMAT2 in the brain. The aim of this study was to investigate the brain kinetics and quantification of [¹⁸F]FE-DTBZ-d4 in nonhuman primates (NHPs), with comparison to [¹¹C]DTBZ and [¹⁸F]FE-DTBZ. Radiolabeling was successfully achieved either by one-step ¹¹C-methylation or by a two-step fluorine-18 nucleophilic substitution reaction. The stability and radiochemical yield were analyzed with high-performance liquid chromatography (HPLC). Three female cynomolgus monkeys were included in the study and underwent a total of 12 positron emission tomography (PET) measurements. Each monkey was examined with each tracer. In addition, two pretreatment and one displacement PET measurements with tetrabenazine (2.0 mg/kg) were performed for [¹⁸F]FE-DTBZ-d4. All PET measurements were conducted using a high-resolution research tomograph (HRRT) system. Radiometabolites were measured in monkey plasma using gradient radio-HPLC. [¹⁸F]FE-DTBZ-d4 (SUV: 4.28 ± 1.01) displayed higher brain uptake compared to both [¹⁸F]FE-DTBZ (SUV: 3.43 ± 0.54) and [¹¹C]DTBZ (SUV: 3.06 ± 0.32) and faster washout. Binding potential (BP_ND) values of [¹⁸F]FE-DTBZ-d4 in different brain regions (putamen: 5.5 ± 1.4; caudate: 4.4 ± 1.1; midbrain: 1.4 ± 0.4) were higher than those of [¹¹C]DTBZ and [¹⁸F]FE-DTBZ. [¹⁸F]FE-DTBZ showed faster radiometabolism in plasma compared to [¹¹C]DTBZ and [¹⁸F]FE-DTBZ-d4. [¹⁸F]FE-DTBZ-d4 is a suitable radioligand for quantification of VMAT2 in the nonhuman primate brain, with better imaging properties than [¹¹C]DTBZ and [¹⁸F]FE-DTBZ. A preliminary comparison suggests that [¹⁸F]FE-DTBZ-d4 has increased stability against defluorination compared to the nondeuterated analogue.