Piet Swart

Phase II Metabolism in Human Skin: Skin Explants Show Full Coverage for Glucuronidation, Sulfation, N-Acetylation, Catechol Methylation, and Glutathione Conjugation

Although skin is the largest organ of the human body, cutaneous drug metabolism is often overlooked, and existing experimental models are insufficiently validated. This proof-of-concept study investigated phase II biotransformation of 11 test substrates in fresh full-thickness human skin explants, a model containing all skin cell types. Results show that skin explants have significant capacity for glucuronidation, sulfation, N-acetylation, catechol methylation, and glutathione conjugation. Novel skin metabolites were identified, including acyl glucuronides of indomethacin and diclofenac, glucuronides of 17β-estradiol, N-acetylprocainamide, and methoxy derivatives of 4-nitrocatechol and 2,3-dihydroxynaphthalene. Measured activities for 10 μM substrate incubations spanned a 1000-fold: from the highest 4.758 pmol·mg skin–1·h–1 for p-toluidine N-acetylation to the lowest 0.006 pmol·mg skin–1·h–1 for 17β-estradiol 17-glucuronidation. Interindividual variability was 1.4- to 13.0-fold, the highest being 4-methylumbelliferone and diclofenac glucuronidation. Reaction rates were generally linear up to 4 hours, although 24-hour incubations enabled detection of metabolites in trace amounts. All reactions were unaffected by the inclusion of cosubstrates, and freezing of the fresh skin led to loss of glucuronidation activity. The predicted whole-skin intrinsic metabolic clearances were significantly lower compared with corresponding whole-liver intrinsic clearances, suggesting a relatively limited contribution of the skin to the body’s total systemic phase II enzyme-mediated metabolic clearance. Nevertheless, the fresh full-thickness skin explants represent a suitable model to study cutaneous phase II metabolism not only in drug elimination but also in toxicity, as formation of acyl glucuronides and sulfate conjugates could play a role in skin adverse reactions.

Metabolism Studies of Unformulated Internally [3H]-Labeled Short Interfering RNAs in Mice

Absorption, distribution, metabolism, and excretion properties of two unformulated model short interfering RNA (siRNAs) were determined using a single internal [3H]-radiolabeling procedure, in which the full-length oligonucleotides were radiolabeled by Br/3H -exchange. Tissue distribution, excretion, and mass balance of radioactivity were investigated in male CD-1 mice after a single intravenous administration of the [3H]siRNAs, at a target dose level of 5 mg/kg. Quantitative whole-body autoradiography and liquid scintillation counting techniques were used to determine tissue distribution. Radiochromatogram profiles were determined in plasma, tissue extracts, and urine. Metabolites were separated by liquid chromatography and identified by radiodetection and high-resolution accurate mass spectrometry. In general, there was little difference in the distribution of total radiolabeled components after administration of the two unformulated [3H]siRNAs. The radioactivity was rapidly and widely distributed throughout the body and remained detectable in all tissues investigated at later time points (24 and 48 hours for [3H]MRP4 (multidrug resistance protein isoform 4) and [3H]SSB (Sjögren Syndrome antigen B) siRNA, respectively). After an initial rapid decrease, concentrations of total radiolabeled components in dried blood decreased at a much slower rate. A nearly complete mass balance was obtained for the [3H]SSB siRNA, and renal excretion was the main route of elimination (38%). The metabolism of the two model siRNAs was rapid and extensive. Five minutes after administration, no parent compound could be detected in plasma. Instead, radiolabeled nucleosides resulting from nuclease hydrolysis were observed. In the metabolism profiles obtained from various tissues, only radiolabeled nucleosides were found, suggesting that siRNAs are rapidly metabolized and that the distribution pattern of total radiolabeled components can be ascribed to small molecular weight metabolites.

Biodistribution and Metabolism Studies of Lipid Nanoparticle–Formulated Internally [3H]-Labeled siRNA in Mice

Absorption, distribution, metabolism, and excretion properties of a small interfering RNA (siRNA) formulated in a lipid nanoparticle (LNP) vehicle were determined in male CD-1 mice following a single intravenous administration of LNP-formulated [3H]-SSB siRNA, at a target dose of 2.5 mg/kg. Tissue distribution of the [3H]-SSB siRNA was determined using quantitative whole-body autoradiography, and the biostability was determined by both liquid chromatography mass spectrometry (LC-MS) with radiodetection and reverse-transcriptase polymerase chain reaction techniques. Furthermore, the pharmacokinetics and distribution of the cationic lipid (one of the main excipients of the LNP vehicle) were investigated by LC-MS and matrix-assisted laser desorption ionization mass spectrometry imaging techniques, respectively. Following i.v. administration of [3H]-SSB siRNA in the LNP vehicle, the concentration of parent guide strand could be determined up to 168 hours p.d. (post dose), which was ascribed to the use of the vehicle. This was significantly longer than what was observed after i.v. administration of the unformulated [3H]-SSB siRNA, where no intact parent guide strand could be observed 5 minutes post dosing. The disposition of the siRNA was determined by the pharmacokinetics of the formulated LNP vehicle itself. In this study, the radioactivity was widely distributed throughout the body, and the total radioactivity concentration was determined in selected tissues. The highest concentrations of radioactivity were found in the spleen, liver, esophagus, stomach, adrenal, and seminal vesicle wall. In conclusion, the LNP vehicle was found to drive the kinetics and biodistribution of the SSB siRNA. The renal clearance was significantly reduced and its exposure in plasma significantly increased compared with the unformulated [3H]-SSB siRNA.

Metabolism and disposition of the metabotropic glutamate receptor 5 antagonist (mGluR5) mavoglurant (AFQ056) in healthy subjects.

The disposition and biotransformation of 14C-radiolabeled mavoglurant were investigated in four healthy male subjects after a single oral dose of 200 mg. Blood, plasma, urine, and feces collected over 7 days were analyzed for total radioactivity, mavoglurant was quantified in plasma by liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS), and metabolite profiles were generated in plasma and excreta by high-performance liquid chromatography (HPLC) and radioactivity detection. The chemical structures of mavoglurant metabolites were characterized by LC-MS/MS, wet-chemical and enzymatic methods, NMR spectroscopy, and comparison with reference compounds. Mavoglurant was safe and well tolerated in this study population. Mavoglurant absorption was ≥50% of dose reaching mean plasma Cmax values of 140 ng/ml (mavoglurant) and 855 ng-eq/ml (total radioactivity) at 2.5 and 3.6 hours, respectively. Thereafter, mavoglurant and total radioactivity concentrations declined with mean apparent half-lives of 12 and 18 hours, respectively. The elimination of mavoglurant occurred predominantly by oxidative metabolism involving primarily 1) oxidation of the tolyl-methyl group to a benzyl-alcohol metabolite (M7) and subsequently to a benzoic acid metabolite (M6), and 2) oxidation of the phenyl-ring leading to a hydroxylated metabolite (M3). The subjects were mainly exposed to mavoglurant and seven main metabolites, which combined accounted for 60% of 14C-AUC0–72 h (area under the concentration-time curve from time 0 to infinity). The primary steps of mavoglurant metabolism observed in vivo could partially be reproduced in vitro in incubations with human liver microsomes and recombinant cytochrome P450 enzymes. After 7 days, the mean balance of total radioactivity excretion was almost complete (95.3% of dose) with 36.7% recovered in urine and 58.6% in feces.

Aldehyde Oxidase Activity in Fresh Human Skin

Human aldehyde oxidase (AO) is a molybdoflavoenzyme that commonly oxidizes azaheterocycles in therapeutic drugs. Although high metabolic clearance by AO resulted in several drug failures, existing in vitro–in vivo correlations are often poor and the extrahepatic role of AO practically unknown. This study investigated enzymatic activity of AO in fresh human skin, the largest organ of the body, frequently exposed to therapeutic drugs and xenobiotics. Fresh, full-thickness human skin was obtained from 13 individual donors and assayed with two specific AO substrates: carbazeran and zoniporide. Human skin explants from all donors metabolized carbazeran to 4-hydroxycarbazeran and zoniporide to 2-oxo-zoniporide. Average rates of carbazeran and zoniporide hydroxylations were 1.301 and 0.164 pmol⋅mg skin–1⋅h–1, resulting in 13 and 2% substrate turnover, respectively, after 24 hours of incubation with 10 μM substrate. Hydroxylation activities for the two substrates were significantly correlated (r2 = 0.769), with interindividual variability ranging from 3-fold (zoniporide) to 6-fold (carbazeran). Inclusion of hydralazine, an irreversible inhibitor of AO, resulted in concentration-dependent decrease of hydroxylation activities, exceeding 90% inhibition of carbazeran 4-hydroxylation at 100 μM inhibitor. Reaction rates were linear up to 4 hours and well described by Michaelis-Menten enzyme kinetics. Comparison of carbazeran and zoniporide hydroxylation with rates of triclosan glucuronidation and sulfation and p-toluidine N-acetylation showed that cutaneous AO activity is comparable to tested phase II metabolic reactions, indicating a significant role of AO in cutaneous drug metabolism. To our best knowledge, this is the first report of AO enzymatic activity in human skin.

The impact of early human data on clinical development: there is time to win.

Modern accelerator mass spectrometry (AMS) methods enable the routine application of this technology in drug development. By the administration of a (14)C-labelled microdose or microtrace, pharmacokinetic (PK) data, such as mass balance, metabolite profiling, and absolute bioavailability (AB) data, can be generated easier, faster, and at lower costs. Here, we emphasize the advances and impact of this technology for pharmaceutical companies. The availability of accurate intravenous (iv) PK and human absorption, distribution, metabolism, and excretion (ADME) information, even before or during Phase I trials, can improve the clinical development plan. Moreover, applying the microtrace approach during early clinical development might impact the number of clinical pharmacology and preclinical safety pharmacology studies required, and shorten the overall drug discovery program.

A Unique Automation Platform for Measuring Low Level Radioactivity in Metabolite Identification Studies

Generation and interpretation of biotransformation data on drugs, i.e. identification of physiologically relevant metabolites, defining metabolic pathways and elucidation of metabolite structures, have become increasingly important to the drug development process. Profiling using 14C or 3H radiolabel is defined as the chromatographic separation and quantification of drug-related material in a given biological sample derived from an in vitro, preclinical in vivo or clinical study. Metabolite profiling is a very time intensive activity, particularly for preclinical in vivo or clinical studies which have defined limitations on radiation burden and exposure levels. A clear gap exists for certain studies which do not require specialized high volume automation technologies, yet these studies would still clearly benefit from automation. Use of radiolabeled compounds in preclinical and clinical ADME studies, specifically for metabolite profiling and identification are a very good example. The current lack of automation for measuring low level radioactivity in metabolite profiling requires substantial capacity, personal attention and resources from laboratory scientists. To help address these challenges and improve efficiency, we have innovated, developed and implemented a novel and flexible automation platform that integrates a robotic plate handling platform, HPLC or UPLC system, mass spectrometer and an automated fraction collector.

Phenotypic and metabolic investigation of a CSF-1R kinase receptor inhibitor (BLZ945) and its pharmacologically active metabolite

Abstract 1. 4-[2((1R,2R)-2-Hydroxycyclohexylamino)-benzothiazol-6-yloxyl]-pyridine-2-carboxylic acid methylamide (BLZ945) is a small molecule inhibitor of CSF-1R kinase activity within osteoclasts designed to prevent skeletal related events in metastatic disease. Key metabolites were enzymatically and structurally characterized to understand the metabolic fate of BLZ945 and pharmacological implications. The relative intrinsic clearances for metabolites were derived from in vitro studies using human hepatocytes, microsomes and phenotyped with recombinant P450 enzymes. 2. Formation of a pharmacologically active metabolite (M9) was observed in human hepatocytes. The M9 metabolite is a structural isomer (diastereomer) of BLZ945 and is about 4-fold less potent. This isomer was enzymatically formed via P450 oxidation of the BLZ945 hydroxyl group, followed by aldo–keto reduction to the alcohol (M9). 3. Two reaction phenotyping approaches based on fractional clearances were applied to BLZ945 using hepatocytes and liver microsomes. The fraction metabolized (fm) or contribution ratio was determined for each metabolic reaction type (oxidation, glucuronidation or isomerization) as well as for each metabolite. The results quantitatively illustrate contribution ratios of the involved enzymes and pathways, e.g. the isomerization to metabolite M9 accounted for 24% intrinsic clearance in human hepatocytes. In summary, contribution ratios for the Phase I and Phase II pathways can be determined in hepatocytes.

ADME studies of [5-3H]-2′-O-methyluridine nucleoside in mice: a building block in siRNA therapeutics

The chemical modification 2′‐O‐methyl of nucleosides is often used to increase siRNA stability towards nuclease activities. However, the metabolic fate of modified nucleosides remains unclear. Therefore, the aim of this study was to determine the mass balance, pharmacokinetic, and absorption, distribution, metabolism, and excretion (ADME)‐properties of tritium‐labeled 2′‐O‐methyluridine, following a single intravenous dose to male CD‐1 mice. The single intravenous administration of [5‐3H]‐2′‐O‐methyluridine was well tolerated in mice. Radioactivity was rapidly and widely distributed throughout the body and remained detectable in all tissues investigated throughout the observation period of 48 h. After an initial rapid decline, blood concentrations of total radiolabeled components declined at a much slower rate. [3H]‐2′‐O‐Methyluridine represented a minor component of the radioactivity in plasma (5.89% of [3H]‐AUC0‐48 h). Three [3H]‐2′‐O‐methyluridine metabolites namely uridine (M1), cytidine (M2), and uracil (M3) were the major circulating components representing 32.8%, 8.11%, and 23.6% of radioactivity area under the curve, respectively. The highest concentrations of total radiolabeled components and exposures were observed in kidney, spleen, pineal body, and lymph nodes. The mass balance, which is the sum of external recovery of radioactivity in excreta and remaining radioactivity in carcass and cage wash, was complete. Renal excretion accounted for about 52.7% of the dose with direct renal excretion of the parent in combination with metabolism to the endogenous compounds cytidine, uracil, cytosine, and cytidine.

ViewLux™ Microplate Imager for Metabolite Profiling: Validation and Applications in Drug Development

Generation of early information on metabolic pathways, metabolite structures and their systemic exposure is a highly time consuming activity during the drug development process. Since these data have become of higher interest for the health authorities, efforts have been made to provide results as early as possible. ViewLux UltraHTS Microplate Imager is an instrument originally designed for high throughput in biological assays requiring luminescence, absorbance or fluorescence detections. In this work, we evaluate the capability of the new generation of the instrument for both 14C and 3H detection. We discuss data processing of the Viewlux, especially the background subtraction in comparison to conventional TopCount® instruments, as this has an impact on the limit of detection for samples containing low amounts of radioactivity like samples originating from ADME studies. We demonstrate that the limit of detection can be lowered by prolonging the exposure time for 3H labeled compounds up to 2 h. We validated the ViewLux for our metabolite profiling applications (in vitro and in vivo ADME samples) in early drug development using UPLC followed by fraction collection in 384 well plates and demonstrated for our applications that limits of detection of 2.2 and 24 dpm/well for 14C and 3H, respectively could be reached and that the throughput could be increased by at least two fold compared to conventional Topcount detection. We also demonstrate in this work how endogenous interferences resulting in false positive peaks in samples containing low amounts of radioactivity can be overcome by using a customized light filter.