Species-specific Bioactivation of Morpholines as a Causative of Drug Induced Liver Injury Observed in Monkeys

BACKGROUND: Everolimus, an allosteric mechanistic target of rapamycin (mTOR) inhibitor, recently demonstrated the therapeutic value of mTOR inhibitors for Central Nervous System (CNS) indications driven by hyperactivation of mTOR. A newer, potent brain-penetrant analog of everolimus, referred to as (1) in this manuscript [(S)-3-methyl-4-(7-((R)-3-methylmorpholino)-2-(thiazol-4-yl)-3H-imidazo[4,5-b]pyridin-5-yl)morpholine,(1)] catalytically inhibits mTOR function in the brain and increases the lifespan of mice with neuronal mTOR hyperactivation. INTRODUCTION: Early evaluation of the safety of 1 was conducted in cynomolgus monkeys in which oral doses were administered to three animals in a rising-dose fashion (from 2 to 30 mg/kg/day). 1 produced severe toxicity including the evidence of hepatic toxicity, along with non-dose proportional increases in drug exposure. Investigations of cross-species hepatic bioactivation of 1 were conducted to assess whether the formation of reactive drug metabolites was associated with the mechanism of liver toxicity. METHOD: 1 contained two morpholine rings known as structural alerts and can potentially form reactive intermediates through oxidative metabolism. Bioactivation of 1 was investigated in rat, human and monkey liver microsomes fortified with trapping agents such as methoxylamine or potassium cyanide. RESULTS: Our results suggest that bioactivation of the morpholine moieties to reactive intermediates may have been involved in the mechanism of liver toxicity observed with 1. Aldehyde intermediates trappable by methoxylamine were identified in rat and monkey liver microsomal studies. In addition, a total of four cyano conjugates arising from the formation of iminium ion intermediates were observed and identified. These findings may potentially explain the observed monkey toxicity. Interestingly, methoxylamine or cyano adducts of 1 were not observed in human liver microsomes. CONCLUSION: The bioactivation of 1 appears to be species-specific. Circumstantial evidence for the toxicity derived from 1 point to the formation of iminium ion intermediates trappable by cyanide in monkey liver microsomes. The cyano conjugates were only observed in monkey liver microsomes, potentially pointing to cause at least the hepatotoxicity observed in monkeys. In contrast, methoxylamine conjugates were detected in both rat and monkey liver microsomes, with only a trace amount in human liver microsomes. Cyano conjugates were not observed in human liver microsomes, challenging the team on the drugability and progressivity of 1 through drug development. The mechanisms for drug-induced liver toxicity are multifactorial. These results are highly suggestive that the iminium ion may be an important component in the mechanism of liver toxicity 1 observed in the monkey.

New Perspectives on Acyl Glucuronide Risk Assessment in Drug Discovery: Investigation of In vitro Stability, In situ Reactivity, and Bioactivation.

BACKGROUND: Acyl glucuronides of xenobiotics have been a subject of wide interest from the pharmaceutical industry with respect to biochemical reactivity, hepatic disposition, and enterohepatic circulation. The reactivity and lack of stability of an acyl glucuronide for a clinical candidate could pose major developability concerns. To date, multiple in vitro assays have been published to assess the risk associated with acyl glucuronides. Despite this fact, the translation of these findings to predicting clinical safety remains poor. METHODS: In the present investigation, we aimed to provide simplified in vitro strategy to understand the bioactivation potential of acyl glucuronides of 10 commercial, carboxylic acid containing drugs that have been categorized as "safe," "warning," or "withdrawn" with respect to their marketed use. Acyl migration was measured as a function of the number of peaks observed in LC-MSn analysis. In addition, we carried out reactive intermediate trapping studies with glutathione and methoxylamine to identify the key intermediates in the transacylation bioactivation and glycation pathways, respectively. We also conducted reaction phenotyping with recombinant UDP-glucuronosyltransferase (UGT) Supersomes® to investigate if the formation of acyl glucuronides could be linked to specific UGT isoform(s). RESULTS: Our results were in line with reported values in the literature. Our assay could be used in discovery research where half-life calculation completely eliminated the need to chemically synthesize the acyl glucuronide standard for risk assessment. We captured our results for risk assessment in a flow chart to simplify the various complex in vitro techniques historically presented. CONCLUSION: While the compounds tested from "withdrawn" and "warning category" all formed the glutathione adduct in buffer, none from "safe" category formed the glutathione adduct. In contrast, none of the compounds tested from any category formed methoxylamine conjugate, a reaction with putative aldehyde moiety formed via acyl migration. These results, highly favor the nucleophilic displacement as a cause of the reactivity rather than the acyl migration via aldehyde formation. The workflow presented could also be applied in the discovery setting to triage new chemical entities of interest.

Genotoxicity of 4-(piperazin-1-yl)-8-(trifluoromethyl)pyrido[2,3-e][1,2,4] triazolo[4,3-a]pyrazine, a Potent H4 Receptor Antagonist for the Treatment of Allergy: Evidence of Glyoxal Intermediate Involvement.

BACKGROUND: 4-(piperazin-1-yl)-8-(trifluoromethyl)pyrido[2,3-e][1,2,4]triazolo[4,3-a]pyrazine (1) is a small-molecule which demonstrated a sub-nM inhibitory potency toward the histamine H4 receptor (H4R). However, it was found to be mutagenic in an in vitro Ames assay. Metabolic bioactivation of 1 could potentially arise from the piperazine moiety by forming reactive intermediates such as glyoxal, aldehyde-imine and/or iminium ion, which could all lead to genotoxicity. The aim of this study was to investigate bioactivation of 1 to determine the potential causes of the genotoxicity and mitigate liabilities in this scaffold. METHODS: 1 was investigated for its genotoxicity in phenobarbital and ß-naphthoflavone induced Sprague Dawley rat liver S9 fractions. Trapping agents such as o-phenylenediamine was used postincubation. RESULTS: Following metabolic profiling of 1, two oxidative metabolites were observed and identified in phenobarbital- and ß -naphthoflavone induced Sprague Dawley rat liver S9 fractions. Metabolic pathway of 1 was primarily mediated by the metabolism of the piperazine moiety. The trapped glyoxal was identified by using high resolution LC-MS instrument. Structural characterization of the trapped glyoxal was determined by comparison of retention time, accurate mass measurement and Collision Induced Dissociation (CID) spectra to authentic standard. CONCLUSION: In the present investigation, a novel method was developed to trap glyoxal, which may potentially be liberated from piperazine moiety. These findings led to modifications on the piperazine ring to mitigate the bioactivation pathways leading to mutagenicity. Subsequently, the next generation compounds with modified piperazine moiety, retained H4R inhibitory potency in vitro and were not genotoxic in the Ames mutagenicity assay.

Investigation of Ocular Bioactivation Potential and the Role of Cytochrome P450 2D Enzymes in Rat.

BACKGROUND: Timolol is clinically administered topically (ocular) to reduce intraocular pressure and treat open-angle glaucoma. Ocular administration of timolol in low doses (0.5% w/v in the form of eye drops) has led to challenges for in vivo metabolite identification. An understanding of drug metabolism in the eye is important for clinical ocular therapeutics and potential drug candidates. METHODS: We aimed to investigate the metabolism of timolol in rat ocular and liver S9 fractions, as well as rat ocular tissue and plasma following a 0.5% topical (ocular) dose of timolol. We explored the potential in vitro metabolic bioactivation in the eye/liver by conducting trapping studies for putative aldehyde and iminium ion intermediates that may arise from the morpholine functionality. RESULTS: Oxidative metabolism of timolol to its major metabolite (M4) in ocular S9 and recombinant rat cytochrome P450 (CYP) isoforms supports the possible role of rat ocular CYP2D2, 2D4, and/or 2D18. Observation of N-acetyl-timolol (M5) is suggestive that the ocular N-acetyltransferases may also play a larger role in ocular disposition of timolol, a previously unreported finding. This research is the first comprehensive report of in vitro ocular metabolism of timolol in rat. CONCLUSION: This study also indicates that in vitro hepatic metabolism is over-predictive of ocular metabolism following topically ocular dosed timolol. The research, herein, highlights the eye as an organ capable of first pass metabolism for topical drugs. Thus, new ophthalmologic considerations for studying and designing long term topical therapies in preclinical species are needed in drug discovery.

Do We Need to Study Metabolism and Distribution in the Eye: Why, When, and Are We There Yet?

The liver is known to be the principal site of drug metabolism. Depending on the route of administration, especially in cases of topical and local delivery, evaluation of local drug metabolism in extrahepatic tissues is vital to assess fraction of the drug metabolized. This parameter becomes important from the point of view of drug availability or the contribution to overall clearance. Examples include fraction metabolized in the gut for oral drugs and contribution of pulmonary or renal clearance to total clearance of a drug. Diseases of the eye represent a rising unmet medical need and a number of therapeutics are currently being developed in the form of small molecules and biologics. Treatment of ocular diseases has expanded to explore various topical formulations and local short- and long-term therapies by ocular routes of administration. Until recently, metabolism in the eye for any species, including human, was not well documented, but this topic is gaining wide interest. Many in vitro-ex vivo models, each with separate pros and cons, are being used for studying ocular metabolism. This review is aimed at providing a perspective on the relevance and application of ocular metabolism, melanin binding, and the use of tissue- and cell-derived ocular models in discovery and preclinical development.

In vitro ocular metabolism and bioactivation of ketoconazole in rat, rabbit and human.

Oral ketoconazole is clinically administered for treatment of severe cases for fungal keratitis. Pharmacodynamics and efficacy of oral and topical (ocular) ketoconazole have been explored in rabbit. However, metabolism of ketoconazole in the eye in any species is not well explored in any preclinical species or human. An understanding of ocular drug metabolism in the eye is crucial for ocular therapeutics to facilitate the risk assessment and development of potential drug candidates for the clinic. We aimed to investigate the metabolism of ketoconazole in rat, rabbit and human ocular S9 fractions. Metabolism in liver S9 fractions was also studied for a direct comparison. Eleven putative metabolites were identified in the in vitro incubations. Of these metabolites, six were present in rat ocular S9 whereas eight were present in rabbit and human ocular matrices. Metabolic pathways in rabbit and human ocular fractions suggested the formation of reactive intermediates in rabbit and human liver and ocular S9 incubations, which was confirmed with trapping studies. Herein, we report eight human ocular metabolites of ketoconazole for the first time. To the best of our knowledge, this is the first report of ocular metabolic pathways and ocular bioactivation of ketoconazole in preclinical species and human.

Ocular Metabolism of Levobunolol: Historic and Emerging Metabolic Pathways.

Although ocular transport and delivery have been well studied, metabolism in the eye is not well documented, even for clinically available medications such as levobunolol, a potent and nonselective ß-adrenergic receptor antagonist. Recently, we reported an in vitro methodology that could be used to evaluate ocular metabolism across preclinical species and humans. The current investigation provides detailed in vitro ocular and liver metabolism of levobunolol in rat, rabbit, and human S9 fractions, including the formation of equipotent active metabolite, dihydrolevobunolol, with the help of high-resolution mass spectrometry. 11 of the 16 metabolites of levobunolol identified herein, including a direct acetyl conjugate of levobunolol observed in all ocular and liver fractions, have not been reported in the literature. The study documents the identification of six human ocular metabolites that have never been reported. The current investigation presents evidence for ocular and hepatic metabolism of levobunolol via non-cytochrome P450 pathways, which have not been comprehensively investigated to date. Our results indicated that rat liver S9 and human ocular S9 fractions formed the most metabolites. Furthermore, liver was a poor in vitro surrogate for eye, and rat and rabbit were poor surrogates for human in terms of the rate and extent of levobunolol metabolism.

Antioxidant and α-glucosidase inhibitory phenolics isolated from highbush blueberry flowers.

Blueberries have been extensively researched, but there are limited studies on other parts of the plant. Here we report the first phytochemical examination of highbush blueberry (Vaccinium corymbosum) flowers, which yielded 21 phenolics. The compounds were identified from extensive NMR and mass spectral analyses and included five caffeic acid (1-5), three coumaric acid (6-8), and two cinnamyl alcohol (9-10) derivatives, eight flavonol glycosides (11-18), and three phenylpropanoid-substituted catechins (19-21). The isolates were evaluated for antioxidant and α-glucosidase inhibitory activities. Overall, the flavonol glycosides and phenylpropanoid-substituted catechins showed superior antioxidant activities compared to the positive controls, vitamin C (IC(50)=63µM) and butylated hydroxytoluene (IC(50)=1548µM). Similarly, these phenolic sub-classes were more potent α-glucosidase inhibitors than the clinical drug, acarbose (IC(50)=200µM). Thus, non-consumed parts of food plants may be exploited as sources of bioactive compounds beyond their edible parts alone for nutraceutical and functional food applications.