A workflow to practically apply true dose considerations to in vitro testing for next generation risk assessment.

With the move away from safety testing assessment based on data generated in experimental animals the concept of Next Generation Risk Assessment (NGRA) has arisen which instead uses data from in silico and in vitro models. A key uncertainty in risk assessment is the actual dose of test chemical at the target site, and therefore surrogate dose metrics, such as nominal concentration in test media are used to describe in vitro effect (or no-effect) doses. The reliability and accuracy of the risk assessment therefore depends largely on our ability to understand and characterise the relationship between the dose metrics used and the actual biologically effective dose at the target site. The objective of this publication is to use 40 case study chemicals to illustrate how in vitro dose considerations can be applied to characterise the "true dose" and build confidence in the understanding of the biologically effective dose in in vitro test systems for the determination e.g. points of departure (PoDs) for NGRA. We propose a workflow that can be applied to assess whether the nominal test concentration can be considered a conservative dose metric for use in NGRA. The workflow examines the implications of volatility, stability, hydrophobicity, binding to plastic and serum, solubility, and the potential use of in silico models for some of these parameters. For the majority of the case study chemicals we found that the use of nominal concentrations in risk assessment would result in conservative decision making. However, for serval chemicals a potential for underestimation of the risk in humans in vivo based on in vitro nominal effect concentrations was identified, and approaches for refinement by characterisation of the actual effect concentration are proposed.

Structure-Based Exploration of Selectivity for ATM Inhibitors in Huntington's Disease.

Our group has recently shown that brain-penetrant ataxia telangiectasia-mutated (ATM) kinase inhibitors may have potential as novel therapeutics for the treatment of Huntington's disease (HD). However, the previously described pyranone-thioxanthenes (e.g., 4) failed to afford selectivity over a vacuolar protein sorting 34 (Vps34) kinase, an important kinase involved with autophagy. Given that impaired autophagy has been proposed as a pathogenic mechanism of neurodegenerative diseases such as HD, achieving selectivity over Vps34 became an important objective for our program. Here, we report the successful selectivity optimization of ATM over Vps34 by using X-ray crystal structures of a Vps34-ATM protein chimera where the Vps34 ATP-binding site was mutated to approximate that of an ATM kinase. The morpholino-pyridone and morpholino-pyrimidinone series that resulted as a consequence of this selectivity optimization process have high ATM potency and good oral bioavailability and have lower molecular weight, reduced lipophilicity, higher aqueous solubility, and greater synthetic tractability compared to the pyranone-thioxanthenes.

Evaluation of 5-(Trifluoromethyl)-1,2,4-oxadiazole-Based Class IIa HDAC Inhibitors for Huntington's Disease.

Using an iterative structure-activity relationship driven approach, we identified a CNS-penetrant 5-(trifluoromethyl)-1,2,4-oxadiazole (TFMO, 12) with a pharmacokinetic profile suitable for probing class IIa histone deacetylase (HDAC) inhibition in vivo. Given the lack of understanding of endogenous class IIa HDAC substrates, we developed a surrogate readout to measure compound effects in vivo, by exploiting the >100-fold selectivity compound 12 exhibits over class I/IIb HDACs. We achieved adequate brain exposure with compound 12 in mice to estimate a class I/IIb deacetylation EC50, using class I substrate H4K12 acetylation and global acetylation levels as a pharmacodynamic readout. We observed excellent correlation between the compound 12 in vivo pharmacodynamic response and in vitro class I/IIb cellular activity. Applying the same relationship to class IIa HDAC inhibition, we estimated the compound 12 dose required to inhibit class IIa HDAC activity, for use in preclinical models of Huntington's disease.

Optimization of Potent and Selective Ataxia Telangiectasia-Mutated Inhibitors Suitable for a Proof-of-Concept Study in Huntington's Disease Models.

Genetic and pharmacological evidence indicates that the reduction of ataxia telangiectasia-mutated (ATM) kinase activity can ameliorate mutant huntingtin (mHTT) toxicity in cellular and animal models of Huntington's disease (HD), suggesting that selective inhibition of ATM could provide a novel clinical intervention to treat HD. Here, we describe the development and characterization of ATM inhibitor molecules to enable in vivo proof-of-concept studies in HD animal models. Starting from previously reported ATM inhibitors, we aimed with few modifications to increase brain exposure by decreasing P-glycoprotein liability while maintaining potency and selectivity. Here, we report brain-penetrant ATM inhibitors that have robust pharmacodynamic (PD) effects consistent with ATM kinase inhibition in the mouse brain and an understandable pharmacokinetic/PD (PK/PD) relationship. Compound 17 engages ATM kinase and shows robust dose-dependent inhibition of X-ray irradiation-induced KAP1 phosphorylation in the mouse brain. Furthermore, compound 17 protects against mHTT (Q73)-induced cytotoxicity in a cortical-striatal cell model of HD.

Development and characterization of a CNS-penetrant benzhydryl hydroxamic acid class IIa histone deacetylase inhibitor.

We have identified a potent, cell permeable and CNS penetrant class IIa histone deacetylase (HDAC) inhibitor 22, with >500-fold selectivity over class I HDACs (1,2,3) and ∼150-fold selectivity over HDAC8 and the class IIb HDAC6 isoform. Dose escalation pharmacokinetic analysis demonstrated that upon oral administration, compound 22 can reach exposure levels in mouse plasma, muscle and brain in excess of cellular class IIa HDAC IC50 levels for ∼8 h. Given the interest in aberrant class IIa HDAC function for a number of neurodegenerative, neuromuscular, cardiac and oncology indications, compound 22 (also known as CHDI-390576) provides a selective and potent compound to query the role of class IIa HDAC biology, and the impact of class IIa catalytic site occupancy in vitro and in vivo.

Potent, Selective, and CNS-Penetrant Tetrasubstituted Cyclopropane Class IIa Histone Deacetylase (HDAC) Inhibitors.

Potent and selective class IIa HDAC tetrasubstituted cyclopropane hydroxamic acid inhibitors were identified with high oral bioavailability that exhibited good brain and muscle exposure. Compound 14 displayed suitable properties for assessment of the impact of class IIa HDAC catalytic site inhibition in preclinical disease models.

Lead optimization toward proof-of-concept tools for Huntington's disease within a 4-(1H-pyrazol-4-yl)pyrimidine class of pan-JNK inhibitors.

Through medicinal chemistry lead optimization studies focused on calculated properties and guided by X-ray crystallography and computational modeling, potent pan-JNK inhibitors were identified that showed submicromolar activity in a cellular assay. Using in vitro ADME profiling data, 9t was identified as possessing favorable permeability and a low potential for efflux, but it was rapidly cleared in liver microsomal incubations. In a mouse pharmacokinetics study, compound 9t was brain-penetrant after oral dosing, but exposure was limited by high plasma clearance. Brain exposure at a level expected to support modulation of a pharmacodynamic marker in mouse was achieved when the compound was coadministered with the pan-cytochrome P450 inhibitor 1-aminobenzotriazole.

Development of a series of aryl pyrimidine kynurenine monooxygenase inhibitors as potential therapeutic agents for the treatment of Huntington's disease.

We report on the development of a series of pyrimidine carboxylic acids that are potent and selective inhibitors of kynurenine monooxygenase and competitive for kynurenine. We describe the SAR for this novel series and report on their inhibition of KMO activity in biochemical and cellular assays and their selectivity against other kynurenine pathway enzymes. We describe the optimization process that led to the identification of a program lead compound with a suitable ADME/PK profile for therapeutic development. We demonstrate that systemic inhibition of KMO in vivo with this lead compound provides pharmacodynamic evidence for modulation of kynurenine pathway metabolites both in the periphery and in the central nervous system.

Assessment of chloroquine treatment for modulating autophagy flux in brain of WT and HD mice.

BACKGROUND: Increasing mutant huntingtin (mHTT) clearance through the autophagy pathway may be a way to treat Huntington's disease (HD). Tools to manipulate and measure autophagy flux in brain in vivo are not well established. OBJECTIVE: To examine the in vivo pharmacokinetics and pharmacodynamics of the lysosomal inhibitor chloroquine (CQ) and the levels of selected autophagy markers to determine usefulness of CQ as a tool to study autophagy flux in brain. METHODS: Intraperitoneal injections of CQ were administered to WT and HD(Q175/Q175) mice. CQ levels were measured by LC-MS/MS in WT brain, muscle and blood at 4 to 24 hours after the last dose. Two methods of tissue preparation were used to detect by Western blot levels of the macroautophagy markers LC3 II and p62, the chaperone mediated autophagy receptor LAMP-2A and the late endosome/lysosomal marker RAB7. RESULTS: Following peripheral administration, CQ levels were highest in muscle and declined rapidly between 4 and 24 hours. In the brain, CQ levels were greater in the cortex than striatum, and levels persisted up to 24 hours post-injection. CQ treatment induced changes in LC3 II and p62 that were variable across regions and tissue preparations. HD(Q175/Q175) mice exposed to CQ had variable but diminished levels of LC3 II, p62 and LAMP-2A, and increased levels of RAB7. Higher levels of mHTT were found in the membrane compartment of CQ treated HD mice. CONCLUSION: Our findings suggest that the response of brain to CQ treatment, a blocker of autophagy flux, is variable and not as robust as it has been demonstrated in vitro, suggesting that CQ treatment has limitations for modulating autophagy flux in vivo. Alternative methods, compounds, and technologies need to be developed to further investigate autophagy flux in vivo, especially in the brain.

Design, synthesis, and biological evaluation of potent and selective class IIa histone deacetylase (HDAC) inhibitors as a potential therapy for Huntington's disease.

Inhibition of class IIa histone deacetylase (HDAC) enzymes have been suggested as a therapeutic strategy for a number of diseases, including Huntington's disease. Catalytic-site small molecule inhibitors of the class IIa HDAC4, -5, -7, and -9 were developed. These trisubstituted diarylcyclopropanehydroxamic acids were designed to exploit a lower pocket that is characteristic for the class IIa HDACs, not present in other HDAC classes. Selected inhibitors were cocrystallized with the catalytic domain of human HDAC4. We describe the first HDAC4 catalytic domain crystal structure in a "closed-loop" form, which in our view represents the biologically relevant conformation. We have demonstrated that these molecules can differentiate class IIa HDACs from class I and class IIb subtypes. They exhibited pharmacokinetic properties that should enable the assessment of their therapeutic benefit in both peripheral and CNS disorders. These selective inhibitors provide a means for evaluating potential efficacy in preclinical models in vivo.

Metabolism and pharmacokinetics of JM6 in mice: JM6 is not a prodrug for Ro-61-8048.

Understanding whether regulation of tryptophan metabolites can ameliorate neurodegeneration is of high interest to investigators. A recent publication describes 3,4-dimethoxy-N-(4-(3-nitrophenyl)-5-(piperidin-1-ylmethyl)thiazol-2-yl)benzenesulfonamide (JM6) as a novel prodrug for the kynurenine 3-monooxygenase (KMO) inhibitor 3,4-dimethoxy-N-(4-(3-nitrophenyl)thiazol-2-yl)benzenesulfonamide (Ro-61-8048) that elicits therapeutic effects in mouse models of Huntington's and Alzheimer's diseases (Cell 145:863-874, 2011). Our evaluation of the metabolism and pharmacokinetics of JM6 and Ro-61-8048 indicate instead that Ro-61-8048 concentrations in mouse plasma after JM6 administration originate from a Ro-61-8048 impurity (<0.1%) in JM6. After a 0.05 mg/kg Ro-61-8048 oral dose alone or coadministered with 10 mg/kg JM6 to mice, the Ro-61-8048 areas under the concentration-time curves (AUCs) from 0 to infinity were similar (4300 and 4900 nM × h, respectively), indicating no detectable contributions of JM6 metabolism to the Ro-61-8048 AUCs. JM6 was stable in incubations under acidic conditions and Ro-61-8048 was not a product of JM6 metabolism in vitro (plasma, blood, or hepatic models). Species differences in the quantitative rate of oxidative metabolism indicate that major circulating JM6 metabolite(s) in mice are unlikely to be major in humans: JM6 is rapidly metabolized via the piperidyl moiety in mouse (forming an iminium ion reactive intermediate) but is slowly metabolized in human (in vitro), primarily via O-dealkylation at the phenyl ring. Our data indicate that JM6 is not a prodrug for Ro-61-8048 and is not a potent KMO inhibitor.

Oral administration of the pimelic diphenylamide HDAC inhibitor HDACi 4b is unsuitable for chronic inhibition of HDAC activity in the CNS in vivo.

Histone deacetylase (HDAC) inhibitors have received considerable attention as potential therapeutics for a variety of cancers and neurological disorders. Recent publications on a class of pimelic diphenylamide HDAC inhibitors have highlighted their promise in the treatment of the neurodegenerative diseases Friedreich's ataxia and Huntington's disease, based on efficacy in cell and mouse models. These studies' authors have proposed that the unique action of these compounds compared to hydroxamic acid-based HDAC inhibitors results from their unusual slow-on/slow-off kinetics of binding, preferentially to HDAC3, resulting in a distinctive pharmacological profile and reduced toxicity. Here, we evaluate the HDAC subtype selectivity, cellular activity, absorption, distribution, metabolism and excretion (ADME) properties, as well as the central pharmacodynamic profile of one such compound, HDACi 4b, previously described to show efficacy in vivo in the R6/2 mouse model of Huntington's disease. Based on our data reported here, we conclude that while the in vitro selectivity and binding mode are largely in agreement with previous reports, the physicochemical properties, metabolic and p-glycoprotein (Pgp) substrate liability of HDACi 4b render this compound suboptimal to investigate central Class I HDAC inhibition in vivo in mouse per oral administration. A drug administration regimen using HDACi 4b dissolved in drinking water was used in the previous proof of concept study, casting doubt on the validation of CNS HDAC3 inhibition as a target for the treatment of Huntington's disease. We highlight physicochemical stability and metabolic issues with 4b that are likely intrinsic liabilities of the benzamide chemotype in general.