A Model-Informed Drug Development Approach Supporting the Approval of an Unstudied Valbenazine Dose for Patients With Tardive Dyskinesia.

Valbenazine is a highly potent and selective inhibitor of synaptic vesicular monoamine transporter 2. The current therapeutic doses of valbenazine for tardive dyskinesia (TD) are 40, 60, or 80 mg capsules, given orally, once daily (QD). While 40 and 80 mg were investigated in phase 3 KINECT® 3 trial and initially approved, the approval of valbenazine 60 mg was based on the analysis utilizing the Model-informed drug development (MIDD) approach, facilitated through the US Food and Drug Administration's MIDD Pilot Program. This study aimed to demonstrate the efficacy of 60 mg QD dose through model simulations using an established exposure-response (E-R) relationship between valbenazine active metabolite [+]-α-dihydrotetrabenazine exposure and the change from baseline in Abnormal Involuntary Movement Scale total score (AIMS-CFB). A longitudinal E-R model was constructed based on the 40 and 80 mg data from the KINECT 3 trial. The final Emax model adequately predicted dose-dependent improvement in the primary endpoint and was used to interpolate AIMS-CFB for 60 mg at week 6. The efficacy of the unstudied 60 mg dose regimen is expected to be within the range of doses studied clinically with predicted mean AIMS-CFB (95% confidence interval) of -2.69 (-3.30, -2.13) between observed mean AIMS-CFB for 40 mg of -1.92 and 80 mg of -3.39. Results from this analysis provided the key evidence to establish efficacy of 60 mg QD without the need for an additional clinical trial. The availability of valbenazine 60 mg dose fills an existing medical need for patients with TD who could benefit from this third effective dose.

Profiling deutetrabenazine extended-release tablets for tardive dyskinesia and chorea associated with Huntington's disease.

INTRODUCTION: Tardive dyskinesia (TD) and Huntington's disease (HD)-associated chorea are persistent and disabling hyperkinetic disorders that can be treated with vesicular monoamine transporter type 2 (VMAT2) inhibitors, including the recently approved once-daily (QD) formulation of deutetrabenazine (DTBZ ER). While its efficacy and safety profile have not been directly investigated, currently available data confirms bioequivalence and similar bioavailability to the twice-daily formulation (DTBZ BID). AREAS COVERED: The authors briefly review the pivotal trials establishing efficacy of DTBZ for TD and HD-associated chorea, the pharmacokinetic data for bioequivalence between QD and BID dosing of DTBZ, as well as dose proportionality evidence, titration recommendations, and safety profile for DTBZ ER. EXPERT OPINION: Long-term data show that DTBZ is efficacious and well tolerated for the treatment of TD and HD-associated chorea. DTBZ ER likely demonstrates therapeutic equivalence with no new safety signals. Due to the lack of comparative clinical trial data, no evidence-based recommendation about choice of VMAT2 inhibitor or switching between VMAT2 inhibitors can be made about best practice. Ultimately, QD dosing may offer the chance of improved medication adherence, an important consideration in patients with complex treatment regimens and/or patients with cognitive decline.

Exploring the nigrostriatal and digestive interplays in Parkinson's disease using dynamic total-body [11C]CFT PET/CT.

PURPOSE: Dynamic total-body imaging enables new perspectives to investigate the potential relationship between the central and peripheral regions. Employing uEXPLORER dynamic [11C]CFT PET/CT imaging with voxel-wise simplified reference tissue model (SRTM) kinetic modeling and semi-quantitative measures, we explored how the correlation pattern between nigrostriatal and digestive regions differed between the healthy participants as controls (HC) and patients with Parkinson's disease (PD). METHODS: Eleven participants (six HCs and five PDs) underwent 75-min dynamic [11C]CFT scans on a total-body PET/CT scanner (uEXPLORER, United Imaging Healthcare) were retrospectively enrolled. Time activity curves for four nigrostriatal nuclei (caudate, putamen, pallidum, and substantia nigra) and three digestive organs (pancreas, stomach, and duodenum) were obtained. Total-body parametric images of relative transporter rate constant (R1) and distribution volume ratio (DVR) were generated using the SRTM with occipital lobe as the reference tissue and a linear regression with spatial-constraint algorithm. Standardized uptake value ratio (SUVR) at early (1-3 min, SUVREP) and late (60-75 min, SUVRLP) phases were calculated as the semi-quantitative substitutes for R1 and DVR, respectively. RESULTS: Significant differences in estimates between the HC and PD groups were identified in DVR and SUVRLP of putamen (DVR: 4.82 ± 1.58 vs. 2.58 ± 0.53; SUVRLP: 4.65 ± 1.36 vs. 2.84 ± 0.67; for HC and PD, respectively, both p < 0.05) and SUVREP of stomach (1.12 ± 0.27 vs. 2.27 ± 0.65 for HC and PD, respectively; p < 0.01). In the HC group, negative correlations were observed between stomach and substantia nigra in both the R1 and SUVREP values (r=-0.83, p < 0.05 for R1; r=-0.94, p < 0.01 for SUVREP). Positive correlations were identified between pancreas and putamen in both DVR and SUVRLP values (r = 0.94, p < 0.01 for DVR; r = 1.00, p < 0.001 for SUVRLP). By contrast, in the PD group, no correlations were found between the aforementioned target nigrostriatal and digestive areas. CONCLUSIONS: The parametric images of R1 and DVR generated from the SRTM model, along with SUVREP and SUVRLP, were proposed to quantify dynamic total-body [11C]CFT PET/CT in HC and PD groups. The distinction in correlation patterns of nigrostriatal and digestive regions between HC and PD groups identified by R1 and DVR, or SUVRs, may provide new insights into the disease mechanism.

The effect of potent CYP2D6 inhibition on the pharmacokinetics and safety of deutetrabenazine in healthy volunteers.

PURPOSE: Deutetrabenazine is a deuterated form of tetrabenazine with a confirmed lower rate of CYP2D6 metabolism of the active metabolites, α- and ß-HTBZ. In this study, we assessed the effect of paroxetine, a potent CYP2D6 inhibitor, on the pharmacokinetics and safety of deutetrabenazine and its metabolites. METHODS: In this open-label sequential drug-drug-interaction study, 24 healthy adults who were CYP2D6 extensive or intermediate metabolizers received a single deutetrabenazine 22.5-mg oral dose on days 1 and 11 and a single paroxetine 20-mg oral daily dose on days 4-12. Pharmacokinetics of deutetrabenazine and its metabolites were assessed on days 1-4 and 11-14. Paroxetine trough concentrations were obtained pre-dose on days 9-13. Safety examinations occurred throughout the study. RESULTS: Paroxetine administered under steady-state conditions, increased exposure of the deuterated active metabolites, α-HTBZ (1.2-fold Cmax and 1.8-fold AUC0-∞) and ß-HTBZ (2.1-fold Cmax and 5.6-fold AUC0-∞), and correspondingly, 1.6-fold Cmax and threefold AUC0-∞ for total (α + ß)-HTBZ. Sixteen subjects reported 45 adverse events and most were mild. Headache was the most common AE reported 8 times by 7 subjects (5 following paroxetine alone; 2 following deutetrabenazine + paroxetine). CONCLUSIONS: Paroxetine-induced increases in exposure to the active deutetrabenazine metabolites were less than those previously reported for tetrabenazine, a finding expected to reduce the burden of drug interaction. In addition, single doses of 22.5 mg deutetrabenazine, when given alone or in the presence of steady-state paroxetine (20 mg daily), were safe.

Pharmacokinetics of Deutetrabenazine and Tetrabenazine: Dose Proportionality and Food Effect.

Deutetrabenazine (Austedo, Teva), an approved treatment of chorea in Huntington's disease and tardive dyskinesia in adult patients, is a rationally designed deuterated form of tetrabenazine. Two studies assessed the pharmacokinetics and safety of deutetrabenazine compared with tetrabenazine, and the effects of food on absorption of the deuterated active metabolites, α-dihydrotetrabenazine (α-HTBZ) and ß-dihydrotetrabenazine (ß-HTBZ). One study was an open-label 2-part study in healthy volunteers; the first part included a crossover single dose of two 15 mg candidate deutetrabenazine formulations in fed and fasted states compared with tetrabenazine 25 mg in the fasted state, and the second part included single and repeated dosing of the commercial formulation of deutetrabenazine (7.5, 15, and 22.5 mg) compared with tetrabenazine 25 mg. The second study was an open-label 5-way crossover study in healthy volunteers (n = 32) to evaluate relative bioavailability of 4 dose levels of the commercial formulation of deutetrabenazine (6, 12, 18, and 24 mg) with a standard meal and 18 mg with a high-fat meal. Both studies confirmed longer half-lives for active metabolites and lower peak-to-trough fluctuations for the sum of the metabolites (total [α+ß]-HTBZ) following deutetrabenazine compared with tetrabenazine (3- to 4-fold and 11-fold, respectively) in steady-state conditions. Deutetrabenazine doses estimated to provide total (α+ß)-HTBZ exposure comparable to tetrabenazine 25 mg were 11.4-13.2 mg. Food had no effect on exposure to total (α+ß)-HTBZ, as measured by AUC. Although the total (α+ß)-HTBZ Cmax of deutetrabenazine was increased by ≈50% in the presence of food, it remained lower than that of tetrabenazine.

Beta Cell Imaging-From Pre-Clinical Validation to First in Man Testing.

There are presently no reliable ways to quantify human pancreatic beta cell mass (BCM) in vivo, which prevents an accurate understanding of the progressive beta cell loss in diabetes or following islet transplantation. Furthermore, the lack of beta cell imaging hampers the evaluation of the impact of new drugs aiming to prevent beta cell loss or to restore BCM in diabetes. We presently discuss the potential value of BCM determination as a cornerstone for individualized therapies in diabetes, describe the presently available probes for human BCM evaluation, and discuss our approach for the discovery of novel beta cell biomarkers, based on the determination of specific splice variants present in human beta cells. This has already led to the identification of DPP6 and FXYD2ga as two promising targets for human BCM imaging, and is followed by a discussion of potential safety issues, the role for radiochemistry in the improvement of BCM imaging, and concludes with an overview of the different steps from pre-clinical validation to a first-in-man trial for novel tracers.

Intranasal delivery of tetrabenazine nanoemulsion via olfactory region for better treatment of hyperkinetic movement associated with Huntington's disease: Pharmacokinetic and brain delivery study.

Tetrabenazine reduces chorea symptoms associated with Huntington's disease by depleting monoamines in pre-synaptic vesicles. It exhibits low aqueous solubility and undergoes first pass metabolism due to which it has low oral bioavailability. The aim of present work was to formulate intranasal tetrabenazine loaded nanoemulsion for better management and treatment of hyperkinesia related with Huntington's disease. A quality by design (QbD) technique was employed as statistical multivariate approach for formulation and optimization of nanoemulsion. Optimized formulation showed droplet size of 106.80 ±â€¯1.96 nm with polydispersity index (PDI) value of 0.198 ±â€¯0.005 and -9.63 ±â€¯0.63 mV zeta potential. Ex-vivo drug permeation studies were carried out and found that the formulation has an augmented permeation by 1.68 times as compared to tetrabenazine suspension. MTT assay on neuro-2a cell lines showed that tetrabenazine loaded nanoemulsion displayed better cell viability than placebo and aqueous drug solution at ½ × Cmax, Cmax and 2 × Cmax. Pharmacokinetic parameters in brain after intranasal administration of tetrabenazine nanoemulsion were found to be Cmax = 3.497 ±â€¯0.275 µg/mL, AUC0-12 = 29.196 ±â€¯0.870 µg h/mL and elimination rate constant (ke) = 0.097 ±â€¯0.012 h-1 where as in plasma the pharmacokinetic parameters were Cmax = 1.400 ±â€¯0.084 µg/mL, AUC0-12 = 12.925 ±â€¯0.340 µg h/mL and ke = 0.061 ±â€¯0.010 h-1. Histopathological studies of porcine nasal mucosa showed that nasal mucosa remains intact when treated with tetrabenazine loaded nanoemulsion. Thus it can be concluded from study that optimized nanoemulsion formulation of a tetrabenazine was robust and its delivery through nasal route is a viable alternative to other routes of administration for treatment of hyperkinesia associated with Huntington's disease.

Pharmacokinetic and Metabolic Profile of Deutetrabenazine (TEV-50717) Compared With Tetrabenazine in Healthy Volunteers.

Deutetrabenazine (Austedo, Teva Pharmaceuticals) is a deuterated form of tetrabenazine. It is the first deuterated drug to receive US regulatory approval and is approved for treatment of chorea in Huntington's disease and tardive dyskinesia. Two oral single dose studies comparing deutetrabenazine (25 mg) with tetrabenazine (25 mg) in healthy volunteers evaluated the impact of deuteration on pharmacokinetics of the active metabolites, alpha-dihydrotetrabenazine (α-HTBZ) and beta-dihydrotetrabenazine (ß-HTBZ), metabolite profile, safety, and tolerability. In the two-way, cross-over study, the mean elimination half-life of deuterated total (α + ß)-HTBZ was doubled compared with nondeuterated total (α + ß)-HTBZ, with a twofold increase in overall mean exposure (area under the concentration-time curve from zero to infinity (AUC0-inf )) and a marginal increase in mean peak plasma concentration (Cmax ). In the mass balance and metabolite profiling study, there were no novel plasma or urinary metabolites of [14 C]-deutetrabenazine relative to [14 C]-tetrabenazine. Specific deuteration in deutetrabenazine resulted in a superior pharmacokinetic profile and an increased ratio of active-to-inactive metabolites, attributes considered to provide significant benefits to patients.

Impact of Deuterium Substitution on the Pharmacokinetics of Pharmaceuticals.

OBJECTIVE: Stable heavy isotopes of hydrogen, carbon, and other elements have been incorporated into drug molecules, largely as tracers for quantitation during the drug development process. Studies involving the human use of drugs labeled with deuterium suggest that these compounds may offer some advantages when compared with their nondeuterated counterparts. Deuteration has gained attention because of its potential to affect the pharmacokinetic and metabolic profiles of drugs. Deutetrabenazine (Austedo, Teva Pharmaceutical Industries, Ltd) is the first deuterated drug to receive Food and Drug Administration approval. This deuterated form of the drug tetrabenazine is indicated for the treatment of chorea associated with Huntington's disease as well as tardive dyskinesia. Ongoing clinical trials suggest that a number of other deuterated compounds are being evaluated for the treatment of human diseases and not merely as research tools. DATA SOURCES: A search of the MEDLINE (1946 to present) database was undertaken using the Ovid interface. The search was conducted using the heading deuterium and then limited to Administration & Dosage, Adverse Effects, Pharmacokinetics, Pharmacology, Poisoning, Therapeutic Use, and Toxicity. STUDY SELECTION AND DATA EXTRACTION: All articles were reviewed and those with human information were included. Review articles were likewise interrogated for additional published human data. CONCLUSIONS: Deuterated compounds may, in some cases, offer advantages over nondeuterated forms, often through alterations in clearance. Deuteration may also redirect metabolic pathways in directions that reduce toxicities. The approval of additional deuterated compounds may soon follow. Clinicians will need to be familiar with the dosing, efficacy, potential side effects, and unique metabolic profiles of these new entities.

Comparing pharmacologic mechanism of action for the vesicular monoamine transporter 2 (VMAT2) inhibitors valbenazine and deutetrabenazine in treating tardive dyskinesia: does one have advantages over the other?

The two approved treatments for tardive dyskinesia both inhibit the vesicular monoamine transporter type 2 (VMAT2) yet have pharmacologic properties that distinguish one from the other. Knowing these differences may help optimize which treatment to select for individual patients.

Deutetrabenazine for the treatment of Huntington's chorea.

INTRODUCTION: Huntington's disease (HD) is an inherited neurodegenerative disorder for which no disease-modifying treatment is currently available. Only symptomatic treatment can be offered. Chorea is the most common motor manifestation of HD and may interfere with daily activities, reduce quality of life, and cause injury. Areas covered: Deutetrabenazine is the first deuterated drug and second drug after tetrabenazine, the classic vesicular monoamine transporter type 2 (VMAT2) inhibitor, to receive approval for the treatment of chorea associated with HD. This review, based largely on a detailed PubMed search, will summarize the pharmacological properties, clinical evidence of efficacy and tolerability of deutetrabenazine in the treatment of HD chorea. Expert commentary: Due to differences in pharmacology and pharmacokinetics, deutetrabenazine has shown promise that it is at least as effective as tetrabenazine in the treatment of HD chorea but has a lower risk of adverse effects. The role of VMAT2 inhibitors in the treatment of hyperkinetic movement disorders is expanding due to their efficacy and favorable tolerability profiles.

Review of deutetrabenazine: a novel treatment for chorea associated with Huntington's disease.

Deutetrabenazine was recently approved for the treatment of chorea in Huntington's disease (HD) and is the first deuterated medication that has been US Food and Drug Administration (FDA)-approved for therapeutic use. In this article, we review deutetrabenazine's drug design, pharmacokinetics, drug interactions, efficacy, adverse events, comparison with tetrabenazine, dosage, and administration. Deutetrabenazine is a deuterated form of tetrabenazine and is a vesicular monoamine transporter 2 (VMAT2) inhibitor. The substitution of deuterium for hydrogen at key positions in the tetrabenazine molecule allows a longer drug half-life and less frequent daily dosing. Deutetrabenazine is administered twice daily up to a maximum daily dose of 48 mg, which corresponds to a similar daily dose of 100 mg of tetrabenazine. In a Phase III clinical trial (First-HD), there was a statistically significant improvement of chorea in HD subjects, as well as improvements in global impression of change as assessed by both patients and clinicians. This improvement was seen without significant adverse effects as the overall tolerability profile of deutetrabenazine was similar to placebo. Somnolence was the most commonly reported symptom in the deutetrabenazine group. In a study where subjects converted from tetrabenazine to deutetrabenazine in an open-label fashion (ARC-HD) and indirect comparison studies between tetrabenazine and deutetrabenazine, there is a suggestion that while efficacy for chorea is similar, the data may slightly favor tetrabenazine, but adverse effects and tolerability strongly favor deutetrabenazine. These data have not been replicated in true head-to-head studies. Current evidence supports that deutetrabenazine is an effective therapeutic treatment option for chorea in HD and may provide a more favorable adverse effect profile than tetrabenazine. However, more data are needed, particularly in the form of head-to-head studies between deutetrabenazine and other treatment options as well as longer term clinical experience with deutetrabenazine.

Developing a cassette microdosing approach to enhance the throughput of PET imaging agent screening.

Cassette dosing is also known as N-in-One dosing: several compounds are simultaneously administrated to a single animal and then the samples are rapidly detected by LC-MS/MS. This approach is a successful strategy to enhance the efficiency of drug discovery and reduce animal usage. However, no report on the utility of the cassette approach in radiotracer discovery has appeared in the literature. This study designed a cassette microdose with LC-MS/MS method to enhance the throughput for screening radiopharmaceutical biodistribution in the rat brain directly. Three unradiolabeled compounds (FPBM FPBM2 and AV-133) were chosen as model drugs administrated intravenously to the rats as a cassette as opposed to discrete study. The rat brain biodistribution data, target localization, the differential uptake ratio (%ID/g) and the brain tissue-specific binding ratio were obtained by the LC-MS/MS analysis. These data matched very well with the values obtained by the standard radioactivity measurements. Moreover, no significant differences between discrete dosing and cassette dosing were observed. By circumventing the need for radiolabeled molecules, this method may be high-throughput and safe for the research and development of new PET imaging agents. The combination of cassette microdosing and LC-MS/MS would be a medium throughput screening tool at an early stage in the discovery/development process of PET imaging agents.

Nigrostriatal proteasome inhibition impairs dopamine neurotransmission and motor function in minipigs.

Parkinson's disease (PD) is characterized by degeneration of dopaminergic neurons in the substantia nigra leading to slowness and stiffness of limb movement with rest tremor. Using ubiquitin proteasome system inhibitors, rodent models have shown nigrostriatal degeneration and motor impairment. We translated this model to the Göttingen minipig by administering lactacystin into the medial forebrain bundle (MFB). Minipigs underwent positron emission tomography (PET) imaging with (+)-α-[11C]dihydrotetrabenazine ([11C]DTBZ), a marker of vesicular monoamine transporter 2 availability, at baseline and three weeks after the unilateral administration of 100 µg lactacystin into the MFB. Compared to their baseline values, minipigs injected with lactacystin showed on average a 36% decrease in ipsilateral striatal binding potential corresponding to impaired presynaptic dopamine terminals. Behaviourally, minipigs displayed asymmetrical motor disability with spontaneous rotations in one of the animals. Immunoreactivity for tyrosine hydroxylase (TH) and HLA-DR-positive microglia confirmed asymmetrical reduction in nigral TH-positive neurons with an inflammatory response in the lactacystin-injected minipigs. In conclusion, direct injection of lactacystin into the MFB of minipigs provides a model of PD with reduced dopamine neurotransmission, TH-positive neuron reduction, microglial activation and behavioural deficits. This large animal model could be useful in studies of symptomatic and neuroprotective therapies with translatability to human PD.

Evaluation of Pancreatic VMAT2 Binding with Active and Inactive Enantiomers of [18F]FP-DTBZ in Healthy Subjects and Patients with Type 1 Diabetes.

PURPOSE: Previous studies demonstrated the utility of [18F]fluoropropyl-(+)-dihydrotetrabenazine ([18F]FP-(+)-DTBZ) as a positron emission tomography (PET) radiotracer for the vesicular monoamine transporter type 2 (VMAT2) to quantify beta cell mass in healthy control (HC) and type 1 diabetes mellitus (T1DM) groups. Quantification of specific binding requires measurement of non-displaceable uptake. Our goal was to identify a reference tissue (renal cortex or spleen) to quantify pancreatic non-specific binding of [18F]FP-(+)-DTBZ with the inactive enantiomer, [18F]FP-(-)-DTBZ. This was the first human study of [18F]FP-(-)-DTBZ. PROCEDURES: Six HCs and four T1DM patients were scanned on separate days after injection of [18F]FP-(+)-DTBZ or [18F]FP-(-)-DTBZ. Distribution volumes (VT) and standardized uptake values (SUVs) were compared between groups. Three methods for calculation of non-displaceable uptake (VND) or reference SUV were applied: (1) use of [18F]FP-(+)-DTBZ reference VT as VND, assuming VND is uniform across organs; (2) use of [18F]FP-(-)-DTBZ pancreatic VT as VND, assuming that VND is uniform between enantiomers in the pancreas; and (3) use of a scaled [18F]FP-(+)-DTBZ reference VT as VND, assuming that a ratio of non-displaceable uptake between organs is uniform between enantiomers. Group differences in VT (or SUV), binding potential (BPND), or SUV ratio (SUVR) were estimated using these three methods. RESULTS: [18F]FP-(-)-DTBZ VT values were different among organs, and VT(+) and VT(-) were also different in the renal cortex and spleen. Method 3 with the spleen to estimate VND (or reference SUV) gave the highest non-displaceable uptake and the largest HC vs. T1DM group differences. Significant group differences were also observed in VT (or SUV) with method 1 using spleen. SUV was affected by differences in the input function between groups and between enantiomers. CONCLUSIONS: Non-displaceable uptake was different among organs and between enantiomers. Use of scaled spleen VT values for VND is a suitable method for quantification of VMAT2 in the pancreas.

Reference tissue models in the assessment of 11 C-DTBZ binding to the VMAT2 in rat striatum: A test-retest reproducibility study.

Dopaminergic PET imaging is a useful tool to assess the dopaminergic integrity and to follow-up longitudinal studies. The aim of this study was to evaluate the reliability and reproducibility of different reference tissue-based methods to determine the non-displaceable binding potential (BPND ) as a quantitative measure of 11 C-DTBZ binding to the VMAT2 in rat striatum using cerebellum as reference region. Eight healthy Wistar rats underwent two microPET scans at the age of 12 (test) and 20 weeks (retest). BPND was determined using the simplified reference tissue model, Logan reference tissue model, and multilinear reference tissue models (MRTMo and MRTM2). Additionally, a striatal-to-cerebellar-ratio (SCR) analysis was performed. The reproducibility between the two scans was assessed using the interclass correlation coefficients (ICC) and the variability index. Repeatability indices showed acceptable ICC = 0.66 (SCR) to excellent ICC = 0.98 (MRTM2) reliability for this study and a variability ranging from 12.26% (SCR) to 3.28% (MRTM2). To the best of our knowledge, this is the first report on longitudinal studies for 11 C-DTBZ in rats using reference tissue methods. Excellent intersubject and intrasubject reproducibility was obtained with the multilinear reference MRTM2, suggesting this as the best method to compare longitudinal studies, whereas the SCR method had poor reliability. Logan method, however, is a method simple to compute that shows accurate reproducibility with a reasonable level of inter- and intra-subject variability allowing crossover studies to follow-up the uptake of 11 C-DTBZ in rat striatum.

Deuterated 18F-9-O-hexadeutero-3-fluoropropoxyl-(+)-dihydrotetrabenazine (D6-FP-(+)-DTBZ): A vesicular monoamine transporter 2 (VMAT2) imaging agent.

INTRODUCTION: Vesicular monoamine transporters 2 (VMAT2) in the brain serve as transporter for packaging monoamine in vesicles for normal CNS neurotransmission. Several VMAT2 imaging agents, [11C]-(+)-DTBZ, dihydrotetrabenazine and [18F]FP-(+)-DTBZ (9-O-fluoropropyl-(+)-dihydro tetrabenazine, a.k.a. [18F]AV-133), are useful for studying the changes in brain function related to monoamine transmission by in vivo imaging. Deuterated analogs have been reported targeting VMAT2 binding sites. METHODS: A novel deuterated [18F]9-O-hexaduterofluoropropyl-(+)-dihydrotetrabenazine, [18F]D6-FP-(+)-DTBZ, [18F]1, was prepared as a VMAT2 imaging agent. This 18F agent which targeted VMAT2 was evaluated by in vitro binding, in vivo biodistribution and microPET imaging studies in rodents. RESULTS: The one step radiolabeling reaction led to the desired [18F]D6-FP-(+)-DTBZ, [18F]1, which showed excellent binding affinity to VMAT2 (Ki=0.32±0.07nM) comparable to that of FP-(+)-DTBZ (Ki=0.33±0.02nM) using [18F]FP-(+)-DTBZ and rat striatum membrane homogenates. In vivo biodistribution in normal rats showed that 1, exhibited excellent brain uptake and comparable high ratio of striatum to cerebellum (target/background) ratio at 1h after injection (ratio of 6.05±0.43 vs 5.66±0.72 for [18F]FP-(+)-DTBZ vs [18F]1, respectively). MicroPET imaging studies in rats further confirm that the striatum with high VMAT2 concentration was clearly delineated in normal rat brain after iv injection of [18F]1. We observed minor changes of metabolism in rat plasma between these two agents; however, the changes showed little effect on regional brain uptake and retention. CONCLUSIONS: The results reported here lend support for using [18F]D6-FP-(+)-DTBZ, [18F]1, as in vivo PET imaging agent for VMAT2 binding in the brain.

Differences in Dihydrotetrabenazine Isomer Concentrations Following Administration of Tetrabenazine and Valbenazine.

BACKGROUND: Tetrabenazine (TBZ) activity is thought to result from four isomeric dihydrotetrabenazine (HTBZ) metabolites ([+]-α-HTBZ, [-]-α-HTBZ, [+]-ß-HTBZ, [-]-ß-HTBZ). Each isomer has a unique profile of vesicular monoamine transporter 2 (VMAT2) inhibition and off-target binding. Previously published data only report total isomer (α) and (ß) concentrations. We developed a method to quantify the individual HTBZ isomers in samples from patients with Huntington's disease receiving TBZ. For comparison, concentrations of [+]-α-HTBZ, the single active metabolite shared by valbenazine (VBZ) and TBZ, were determined in samples from patients with tardive dyskinesia receiving VBZ. METHODS: A liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was developed and validated for quantitation of the four individual HTBZ isomers. Concentrations were determined in serum from patients with Huntington's disease administered TBZ and plasma from patients with tardive dyskinesia administered VBZ once daily. RESULTS: In patients administered TBZ, [-]-α-HTBZ and [+]-ß-HTBZ were the most abundant HTBZ isomers while [-]-ß-HTBZ and [+]-α-HTBZ were present as minor metabolites. Only [+]-α-HTBZ was observed in patients administered VBZ. CONCLUSIONS: Based on relative abundance and potency, [+]-ß-HTBZ appears to be the primary contributor to VMAT2 inhibition by TBZ, a finding in contrast with the generally held assertion that [+]-α-HTBZ is the major contributor. [-]-α-HTBZ, the other abundant TBZ metabolite, has much lower VMAT2 inhibitory potency than [+]-ß-HTBZ, but increased affinity for other CNS targets, which may contribute to off-target effects of TBZ. In contrast, pharmacological activity for VBZ is derived primarily from [+]-α-HTBZ. Individual HTBZ isomer concentrations provide a more clinically relevant endpoint for assessing on- and off-target effects of TBZ than total isomer concentrations.

Single Dose and Repeat Once-Daily Dose Safety, Tolerability and Pharmacokinetics of Valbenazine in Healthy Male Subjects.

Valbenazine (VBZ) is a vesicular monoamine transporter 2 (VMAT2) inhibitor approved for the treatment of tardive dyskinesia. The safety, tolerability and pharmacokinetics of VBZ following single and repeat once-daily (QD) dosing were evaluated in 2 randomized, single-center, double-blind studies in healthy male subjects. In the first study, 2 cohorts of 8 subjects were administered single doses (SD) of placebo (PBO; N = 2/period) or VBZ (N = 6/period; 1, 2, 5, or 12.5 mg for Cohort 1 and 12.5, 25, 50, or 75 mg for Cohort 2) using a sequential escalation scheme. The second study consisted of 2 phases. In the initial phase, subjects were administered SD PBO (N = 2/period) or VBZ (N = 6/period; 75, 100, 125 or 150 mg) with sequential escalation. In the second phase, subjects received PBO, or 50 or 100 mg VBZ (N = 4:8:8) QD for 8 days (Cohort 1) or PBO or 50 mg VBZ (N = 6:6) QD for 8 days (Cohort 2). For both studies, plasma concentrations of VBZ and its active metabolite, NBI-98782, were determined. Safety was assessed throughout the studies. PK parameters were determined using noncompartmental methods. In both studies, VBZ was rapidly absorbed with peak concentrations typically observed within 1.5 hours. Peak NBI-98782 concentrations were typically observed at 4.0 to 9.0 hours. Terminal elimination half-life for both VBZ and NBI-98782 was ~20 hours. Across the 1 to 150 mg SD range evaluated across the studies, VBZ and NBI-98782 Cmax and AUC increased dose-proportionally from 50 to 150 mg and more than dose-proportionally from 1 to 50 mg. QD VBZ and NBI-98782 Cmax and AUC parameters were also dose-proportional between the 50 and 100 mg doses. Steady-state for both analytes appeared to be achieved by Day 8. The accumulation index was ~1.5 for VBZ and ~2.5 for NBI-98782. Peak to trough fluctuation was approximately 250% for VBZ and 70% for NBI-98782. Across both studies, NBI-98782 exposure was approximately 20%-30% that of VBZ based on molar ratios. In the first study, the maximum-tolerated dose was not achieved; headache (2 events) was the only treatment-emergent adverse event (TEAE) reported by more than one subject. In the second study, fatigue (4 events) was the only TEAE reported by more than one subject following SD VBZ. Following QD VBZ, the TEAEs of fatigue, insomnia, disturbance in attention, and nervousness were dose-dependent; the latter three TEAEs were considered dose-limiting. Subject withdrawals due to TEAEs were 1 each for PBO and 50 mg VBZ QD, and 3 for 100 mg VBZ QD. Clinically relevant effects on laboratory parameters, vital signs or ECGs were limited to increased CPK (SD: 1 each for 5 mg VBZ and PBO), ALT (QD: 1 each for 50 and 100 mg VBZ and PBO), and triglycerides (QD: 1 each for 50 mg VBZ and PBO). VBZ has an acceptable safety profile and predictable pharmacokinetics that result in stable concentrations of active compounds with low peak-to-trough fluctuation following once-daily dosing.

Valbenazine: First Global Approval.

Valbenazine (Ingrezza™) is an orally bioavailable, selective, vesicular monoamine transporter 2 (VMAT2) inhibitor being developed by Neurocrine Biosciences for the treatment of various central nervous system disorders. Valbenazine has been approved in the USA for the treatment of adults with tardive dyskinesia (TD), is at various stages of development in other countries for TD and is in phase 2 development in the USA for Tourette syndrome. This article summarizes the milestones in the development of valbenazine leading to its first global approval in the USA for the treatment of adults with TD.