An in vitro approach to investigate ocular metabolism of a topical, selective ß1-adrenergic blocking agent, betaxolol.

1. Topical glaucoma treatments have often been limited by poor absorption and bioavailability. Betaxolol, a selective ß1-blocker, has been well studied for its pharmacokinetics and disposition. Limited ocular, betaxolol metabolism data is available despite a growing number of novel ocular treatments. 2. In vitro ocular fractions indicated the formation of an active metabolite, across rat, rabbit and human, which was only observed historically in the liver. 3. Ocular metabolic profiles of preclinical toxicology species, rat and rabbit, were not predictive of human in vitro ocular data. M1 was specific to human and only captured by the liver data. 4. Liver S9 over predicted the extent of ocular metabolism compared to ocular fractions. Rabbit liver S9 fractions demonstrated extensive glucuronidation and higher parent turn-over in 1 h as compared to other matrices. 5. This research assesses in vitro species and organ differences across preclinical species and human. The complex data set highlights the need for an in vitro ocular system to explore poorly documented ocular metabolism.

Identification of saturated and unsaturated fatty acids released during microsomal incubations.

1. In vitro clearance in liver microsomes is routinely measured in drug discovery and development for new chemical entities. Literature reports indicate that long chain fatty acids such as arachidonic, linoleic and oleic acids may be released over a period of time during microsomal incubations. Some fatty acids have been shown to interfere with oxidative and conjugative reactions in microsomes, thus potentially inhibiting microsomal clearance of compounds. 2. The present study was aimed at deciphering the fatty acids present or released from microsomes. Analytical methods were developed to characterize and quantitatively assess the fatty acids without chemical derivatization in rat, monkey and human liver microsomes. Additionally, incubations with uridine-5'-diphosphoglucuronic acid (UDPGA) were utilized to trap the released fatty acids as their glucuronate esters, which were characterized and confirmed by high-resolution LC-MS/MS. 3. Our results indicate for the first time that timnodonic, trans-eicosenoic, gondoic, behenic, and nervonic acid were released during microsomal incubations. Additionally, α- and γ-linolenic, timnodonic, palmitoleic, linoleic, arachidonic, palmitic, oleic, and stearic acid were identified as their corresponding acyl-glucuronides in rat, monkey and human liver microsomes, providing the first direct evidence that the released fatty acids are capable of forming glucuronides under incubation conditions.

Metabolism of bromopride in mouse, rat, rabbit, dog, monkey, and human hepatocytes.

Bromopride (BRP) has been utilized clinically for treatment of nausea, vomiting and gastro-intestinal motility disorders. The pharmacokinetics of BRP have been characterized in dogs and humans; however, the metabolic profile of BRP has not been well studied. The present study was aimed at better understanding BRP metabolism across species. We investigated biotransformation of BRP in mouse, rat, rabbit, dog, monkey, and human hepatocytes with the help of LC-MS(n) and accurate mass measurement. Mice, rats, dogs, and monkeys are relevant in drug discovery and development as pre-clinical species to be compared with humans, whereas rabbits were efficacy models for BRP. Overall, twenty metabolites of BRP were identified across hepatocytes from the six species. Monkeys offered the most coverage for humans, in terms of number of metabolites identified. Interestingly, M14, an N-sulfate metabolite of BRP, was identified as a human-specific metabolite. BRP metabolism had only been reported in dog plasma and urine, historically. Our investigation is the first documentation of in vitro metabolism of BRP in the six species reported here. Metabolites M1, M2, M4-M10, M12, M13, and M15-M20 have not been previously reported. In summary, this report documents seventeen metabolites of BRP for the first time, thus providing a deeper insight into the biotransformation of BRP.

Oxidative ipso substitution of 2,4-difluoro-benzylphthalazines: identification of a rare stable quinone methide and subsequent GSH conjugate.

In vitro metabolite identification and GSH trapping studies in human liver microsomes were conducted to understand the bioactivation potential of compound 1 [2-(6-(4-(4-(2,4-difluorobenzyl)phthalazin-1-yl)piperazin-1-yl)pyridin-3-yl)propan-2-ol], an inhibitor of the Hedgehog pathway. The results revealed the formation of a unique, stable quinone methide metabolite (M1) via ipso substitution of a fluorine atom and subsequent formation of a GSH adduct (M2). The stability of this metabolite arises from extensive resonance-stabilized conjugation of the substituted benzylphthalazine moiety. Cytochrome P450 (P450) phenotyping studies revealed that the formation of M1 and M2 were NADPH-dependent and primarily catalyzed by CYP3A4 among the studied P450 isoforms. In summary, an unusual and stable quinone methide metabolite of compound 1 was identified, and a mechanism was proposed for its formation via an oxidative ipso substitution.

An experimental approach to enhance precursor ion fragmentation for metabolite identification studies: application of dual collision cells in an orbital trap.

Recent advancements in mass spectrometry including data-dependent scanning and high-resolution mass spectrometry have aided metabolite profiling for non-radiolabeled xenobiotics. However, narrowing down a site of metabolism is often limited by the quality of the collision-induced dissociation (CID)-based precursor ion fragmentation. An alternative dissociation technique, higher energy collisional dissociation (HCD), enriches compound fragmentation and yields 'triple-quadrupole-like fragmentation'. Applying HCD along with CID and data-dependent scanning could enhance structural elucidation for small molecules. Liquid chromatography/multi-stage mass spectrometry (LC/MS(n) ) experiments with CID and HCD fragmentation were carried out for commercially available compounds on a hybrid linear ion trap orbital trap mass spectrometer equipped with accurate mass measurement capability. The developed method included stepped normalized collision energy (SNCE) parameters to enhance MS fragmentation without tuning for individual compounds. All the evaluated compounds demonstrated improved fragmentation under HCD as compared with CID. The results suggest that an LC/MS(n) method that incorporated both SNCE HCD- and CID-enabled precursor ion fragmentation afforded comprehensive structural information for the compounds under investigation. A dual collision cell approach was remarkably better than one with only CID MS(n) in an orbital trap. It is evident that such an acquisition method can augment the identification of unknown metabolites in drug discovery by improving fragmentation efficiency of both the parent compound and its putative metabolite(s).

Evaluation of pharmaceutical excipients as cosolvents in 4-methyl umbelliferone glucuronidation in human liver microsomes: applications for compounds with low solubility.

Standard incubation procedures for carrying out microsomal assays involve the use of less than 1% w/v organic solvents to minimize the potential inhibitory effects of organic solvents on metabolic activity. This presents a practical limitation for poorly soluble xenobiotics, which cannot be incubated at concentrations high enough to obtain a V(max), and therefore subsequent values for K(m) and Cl(int) cannot be calculated. Our goal was to study the application of a variety of pharmaceutical excipients to aid the solubilization of compounds in vitro in glucuronidation incubations, without affecting the reaction kinetics. In vitro glucuronidation incubations were carried out in human liver microsomes with 4-methylumbelliferone (4-MU) and the kinetics of 4-MU glucuronidation in the presence of excipients were compared to that in control incubations without any excipients. In addition, IC(75) values were calculated for each excipient. We observed that HPBCD (Hydroxypropyl-ß-cyclodextrin) may be employed in in vitro glucuronidation incubations up to 0.5% w/v without affecting the Cl(int) of 4-MU. Although NMP (N-methyl-2-pyrrolidone) and DMA (N,N-dimethylacetamide); showed low IC(75) values approximately 0.1% w/v each, neither excipients altered the Cl(int) of 4-MUG (4-methylumbelliferyl-ß-D-glucuronide) formation. Our studies point toward possible applications of pharmaceutical excipients to carry out in vitro glucuronidation of substrates with poor aqueous solubility, in order to estimate Cl(int) and subsequently scaled organ clearance values.