Modeling cellular responses to serum and vitamin D in microgravity using a human kidney microphysiological system.

The microgravity environment aboard the International Space Station (ISS) provides a unique stressor that can help understand underlying cellular and molecular drivers of pathological changes observed in astronauts with the ultimate goals of developing strategies to enable long- term spaceflight and better treatment of diseases on Earth. We used this unique environment to evaluate the effects of microgravity on kidney proximal tubule epithelial cell (PTEC) response to serum exposure and vitamin D biotransformation capacity. To test if microgravity alters the pathologic response of the proximal tubule to serum exposure, we treated PTECs cultured in a microphysiological system (PT-MPS) with human serum and measured biomarkers of toxicity and inflammation (KIM-1 and IL-6) and conducted global transcriptomics via RNAseq on cells undergoing flight (microgravity) and respective controls (ground). Given the profound bone loss observed in microgravity and PTECs produce the active form of vitamin D, we treated 3D cultured PTECs with 25(OH)D3 (vitamin D) and monitored vitamin D metabolite formation, conducted global transcriptomics via RNAseq, and evaluated transcript expression of CYP27B1, CYP24A1, or CYP3A5 in PTECs undergoing flight (microgravity) and respective ground controls. We demonstrated that microgravity neither altered PTEC metabolism of vitamin D nor did it induce a unique response of PTECs to human serum, suggesting that these fundamental biochemical pathways in the kidney proximal tubule are not significantly altered by short-term exposure to microgravity. Given the prospect of extended spaceflight, more study is needed to determine if these responses are consistent with extended (>6 months) exposure to microgravity.

Development of a kidney microphysiological system hardware platform for microgravity studies.

Determining the physiological effects of microgravity on the human kidney is limited to relatively insensitive tests of biofluids (blood and urine) that do not return abnormal results until more than 50% of kidney function is lost. We have developed an "organ on chip" microphysiological model of the human kidney proximal tubule (PT-MPS) that can recapitulate many kidney functions and disease states and could play a critical role in determining mechanisms of early kidney dysfunction in microgravity. However, the ground-based PT-MPS system is incompatible with spaceflight as it requires a large pneumatic system coupled to a cell incubator for perfusion and intensive hand-on manipulation. Herein, we report the hardware engineering and performance of the Kidney Chip Perfusion Platform (KCPP), a small, advanced, semi-autonomous hardware platform to support kidney microphysiological model experiments in microgravity. The KCPP is composed of five components, the kidney MPS, the MPS housing and valve block, media cassettes, fixative cassettes, and the programable precision syringe pump. The system has been deployed twice to the ISSNL (aboard CRS-17 and CRS-22). From each set of ISSNL experiments and ground-based controls, we were able to recover PT-MPS effluent for biomarker analysis and RNA suitable for transcriptomics analysis demonstrating the usability and functionality of the KCPP.

Predicting complex kidney drug handling using a physiologically-based pharmacokinetic model informed by biomarker-estimated secretory clearance and blood flow.

Kidney function-adjusted drug dosing is currently based solely on the estimated glomerular filtration rate (GFR), however, kidney drug handling is accomplished by a combination of filtration, tubular secretion, and re-absorption. Mechanistic physiologically-based pharmacokinetic (PBPK) models recapitulate anatomic compartments to predict elimination from estimated perfusion, filtration, secretion, and re-absorption, but clinical applications are limited by a lack of empiric individual-level measurements of these functions. We adapted and validated a PBPK model to predict drug clearance from individual biomarker-based estimates of kidney perfusion and secretory clearance. We estimated organic anion transporter-mediated secretion via kynurenic acid clearance and kidney blood flow (KBF) via isovalerylglycine clearance in human participants, incorporating these measurements with GFR into the model to predict kidney drug clearance. We compared measured and model-predicted clearances of administered tenofovir and oseltamivir, which are cleared by both filtration and secretion. There were 27 outpatients (age 55 ± 15 years, mean iohexol-GFR [iGFR] 76 ± 31 mL/min/1.73 m2 ) in this drug clearance study. The mean observed and mechanistic model-predicted tenofovir clearances were 169 ± 102 mL/min and 163 ± 80 mL/min, respectively; estimated mean error of the mechanistic model was 37.1 mL/min (95% confidence interval [CI]: 24-52.9), compared to a mean error of 41.8 mL/min (95% CI: 25-61.6) from regression model. The mean observed and model-predicted oseltamivir carboxylate clearances were 183 ± 104 mL/min and 179 ± 89 mL/min, respectively; estimated mean error of the mechanistic model was 42.9 mL/min (95% CI: 29.7-56.4), versus error of 48.1 mL/min (95% CI: 31.2-67.3) from the regression model. Individualized estimates of tubular secretion and KBF improved the accuracy of PBPK model-predicted tenofovir and oseltamivir kidney clearances, suggesting the potential for biomarker-informed measures of kidney function to refine personalized drug dosing.

Incorporating Uremic Solute-mediated Inhibition of OAT1/3 Improves PBPK Prediction of Tenofovir Renal and Systemic Disposition in Patients with Severe Kidney Disease.

BACKGROUND: Dose modification of renally secreted drugs in patients with chronic kidney disease (CKD) has relied on serum creatinine concentration as a biomarker to estimate glomerular filtration (GFR) under the assumption that filtration and secretion decline in parallel. A discrepancy between actual renal clearance and predicted renal clearance based on GFR alone is observed in severe CKD patients with tenofovir, a compound secreted by renal OAT1/3. Uremic solutes that inhibit OAT1/3 may play a role in this divergence. METHODS: To examine the impact of transporter inhibition by uremic solutes on tenofovir renal clearance, we determined the inhibitory potential of uremic solutes hippuric acid, indoxyl sulfate, and p-cresol sulfate. The inhibition parameters (IC50) were incorporated into a previously validated mechanistic kidney model; simulated renal clearance and plasma PK profile were compared to data from clinical studies. RESULTS: Without the incorporation of uremic solute inhibition, the PBPK model failed to capture the observed data with an absolute average fold error (AAFE) > 2. However, when the inhibition of renal uptake transporters and uptake transporters in the slow distribution tissues were included, the AAFE value was within the pre-defined twofold model acceptance criterion, demonstrating successful model extrapolation to CKD patients. CONCLUSION: A PBPK model that incorporates inhibition by uremic solutes has potential to better predict renal clearance and systemic disposition of secreted drugs in patients with CKD. Ongoing research is warranted to determine if the model can be expanded to include other OAT1/3 substrate drugs and to evaluate how these findings can be translated to clinical guidance for drug selection and dose optimization in patients with CKD.

Metabolomic Profiling Identifies New Endogenous Markers of Tubular Secretory Clearance.

BACKGROUND: The proximal tubules eliminate protein-bound toxins and drugs through secretion. Measurements or estimates of GFR do not necessarily reflect the physiologically distinct process of secretion. Clinical assessment of this important intrinsic kidney function requires endogenous markers that are highly specific for secretory transport. METHODS: We used metabolomics profiling to identify candidate markers of tubular secretory clearance in 50 participants from a kidney pharmacokinetics study. We measured metabolites in three sequential plasma samples and a concurrent 10-hour timed urine sample using hydrophilic interaction liquid chromatography/high-resolution mass spectrometry. We quantified the association between estimated kidney clearance and normalized plasma peak height of each candidate solute to the clearance of administered furosemide, a protein-bound, avidly secreted medication. RESULTS: We identified 528 metabolites present in plasma and urine, excluding pharmaceuticals. We found seven highly (>50%) protein-bound and 49 poorly bound solutes with clearances significantly associated with furosemide clearance and 18 solute clearances favoring an association with furosemide clearance by the 90th percentile compared with GFR. We also found four highly bound and 42 poorly bound plasma levels that were significantly associated with furosemide clearance. CONCLUSIONS: We found several candidate metabolites whose kidney clearances or relative plasma levels are highly associated with furosemide clearance, an avidly secreted tracer medication of the organic anion transporters, highlighting their potential as endogenous markers of proximal tubular secretory clearance.

Kidney microphysiological models for nephrotoxicity assessment.

Nephrotoxicity testing is an important step in preclinical development of new molecular entities (NMEs) and has traditionally been performed in 2-D cell culture systems and animal models. However, 2-D culture systems fail to replicate complex in vivo microenvironment and animal models face interspecies differences including the overexpression of drug transporters. In the last decade, 3-D microphysiological systems (MPS) have been developed to address these concerns. Here, we review recent advancements in kidney MPS and their application in drug-induced toxicity testing and kidney disease research. We find that current research is making significant progress addressing MPS limitations such as throughput, incorporating various regions of the nephron such as the glomerulus, and successfully modeling and predicting clinically relevant nephrotoxicity of current and new drugs.

Serum Protein Exposure Activates a Core Regulatory Program Driving Human Proximal Tubule Injury.

BACKGROUND: The kidneys efficiently filter waste products while retaining serum proteins in the circulation. However, numerous diseases compromise this barrier function, resulting in spillage of serum proteins into the urine (proteinuria). Some studies of glomerular filtration suggest that tubules may be physiologically exposed to nephrotic-range protein levels. Therefore, whether serum components can directly injure the downstream tubular portions of the kidney, which in turn can lead to inflammation and fibrosis, remains controversial. METHODS: We tested the effects of serum protein exposure in human kidney tubule microphysiologic systems and with orthogonal epigenomic approaches since animal models cannot directly assess the effect of serum components on tubules. RESULTS: Serum, but not its major protein component albumin, induced tubular injury and secretion of proinflammatory cytokines. Epigenomic comparison of serum-injured tubules and intact kidney tissue revealed canonical stress-inducible regulation of injury-induced genes. Concordant transcriptional changes in microdissected tubulointerstitium were also observed in an independent cohort of patients with proteinuric kidney disease. CONCLUSIONS: Our results demonstrate a causal role for serum proteins in tubular injury and identify regulatory mechanisms and novel pathways for intervention.

Microphysiological systems in absorption, distribution, metabolism, and elimination sciences.

The use of microphysiological systems (MPS) to support absorption, distribution, metabolism, and elimination (ADME) sciences has grown substantially in the last decade, in part driven by regulatory demands to move away from traditional animal-based safety assessment studies and industry desires to develop methodologies to efficiently screen and characterize drugs in the development pipeline. The past decade of MPS development has yielded great user-driven technological advances with the collective fine-tuning of cell culture techniques, fluid delivery systems, materials engineering, and performance enhancing modifications. The rapid advances in MPS technology have now made it feasible to evaluate critical ADME parameters within a stand-alone organ system or through interconnected organ systems. This review surveys current MPS developed for liver, kidney, and intestinal systems as stand-alone or interconnected organ systems, and evaluates each system for specific performance criteria recommended by regulatory authorities and MPS leaders that would render each system suitable for evaluating drug ADME. Whereas some systems are more suitable for ADME type research than others, not all system designs were intended to meet the recently published desired performance criteria and are reported as a summary of initial proof-of-concept studies.

Bridging the gap between in silico and in vivo by modeling opioid disposition in a kidney proximal tubule microphysiological system.

Opioid overdose, dependence, and addiction are a major public health crisis. Patients with chronic kidney disease (CKD) are at high risk of opioid overdose, therefore novel methods that provide accurate prediction of renal clearance (CLr) and systemic disposition of opioids in CKD patients can facilitate the optimization of therapeutic regimens. The present study aimed to predict renal clearance and systemic disposition of morphine and its active metabolite morphine-6-glucuronide (M6G) in CKD patients using a vascularized human proximal tubule microphysiological system (VPT-MPS) coupled with a parent-metabolite full body physiologically-based pharmacokinetic (PBPK) model. The VPT-MPS, populated with a human umbilical vein endothelial cell (HUVEC) channel and an adjacent human primary proximal tubular epithelial cells (PTEC) channel, successfully demonstrated secretory transport of morphine and M6G from the HUVEC channel into the PTEC channel. The in vitro data generated by VPT-MPS were incorporated into a mechanistic kidney model and parent-metabolite full body PBPK model to predict CLr and systemic disposition of morphine and M6G, resulting in successful prediction of CLr and the plasma concentration-time profiles in both healthy subjects and CKD patients. A microphysiological system together with mathematical modeling successfully predicted renal clearance and systemic disposition of opioids in CKD patients and healthy subjects.

Analysis of Drug-Drug Interaction Labeling Language and Clinical Recommendations for Newly Approved Drugs Evaluated With Digoxin, Midazolam, and S-Warfarin.

PURPOSE: To best promote drug tolerability and efficacy in the clinic, data from drug-drug interaction (DDI) evaluations and subsequent translation of the results to DDI prevention and/or management strategies must be incorporated into the US Food and Drug Administration (FDA) product labeling in a consistent manner because differences in language might result in varied interpretations. This analysis aimed to assess the consistency in DDI labeling language in New Drug Applications (NDAs). METHODS: NDAs of recently approved drugs (2012-2020) that increase the exposure of digoxin, midazolam, and S-warfarin, index substrates of P-glycoprotein, cytochrome P450 (CYP) 3A, and CYP2C9 activity, respectively, were fully reviewed. Noninhibitors were also evaluated to appreciate the extent of mechanistic extrapolation in case of negative index studies. FINDINGS: After a systematic review of the DDI studies available in NDAs, FDA-approved labeling, and commonly used clinical tertiary resources, differences in DDI results presentation and resulting clinical recommendations were found, even for inhibitors that affect similarly the exposure of the same index substrate. Studies with negative results were often reported in the labels without providing mechanistic interpretation, thus limiting the possible extrapolation of this information to other known substrates. IMPLICATIONS: The variability in language affects how the information was presented to clinicians in tertiary resources. Strategies that aim to improve the translation of mechanistic DDI index studies into consistent labeling recommendations are briefly discussed in this review.

Kidney Organoid and Microphysiological Kidney Chip Models to Accelerate Drug Development and Reduce Animal Testing.

Kidneys are critical for the elimination of many drugs and metabolites via the urine, filtering waste and maintaining proper fluid and electrolyte balance. Emerging technologies incorporating engineered three-dimensional (3D) in vitro cell culture models, such as organoids and microphysiological systems (MPS) culture platforms, have been developed to replicate nephron function, leading to enhanced efficacy, safety, and toxicity evaluation of new drugs and environmental exposures. Organoids are tiny, self-organized three-dimensional tissue cultures derived from stem cells that can include dozens of cell types to replicate the complexity of an organ. In contrast, MPS are highly controlled fluidic culture systems consisting of isolated cell type(s) that can be used to deconvolute mechanism and pathophysiology. Both systems, having their own unique benefits and disadvantages, have exciting applications in the field of kidney disease modeling and therapeutic discovery and toxicology. In this review, we discuss current uses of both hPSC-derived organoids and MPS as pre-clinical models for studying kidney diseases and drug induced nephrotoxicity. Examples such as the use of organoids to model autosomal dominant polycystic kidney disease, and the use of MPS to predict renal clearance and nephrotoxic concentrations of novel drugs are briefly discussed. Taken together, these novel platforms allow investigators to elaborate critical scientific questions. While much work needs to be done, utility of these 3D cell culture technologies has an optimistic outlook and the potential to accelerate drug development while reducing the use of animal testing.

Prediction of Kidney Drug Clearance: A Comparison of Tubular Secretory Clearance and Glomerular Filtration Rate.

BACKGROUND: Although proximal tubular secretion is the primary mechanism of kidney drug elimination, current kidney drug dosing strategies are on the basis of eGFR. METHODS: In a dedicated pharmacokinetic study to compare GFR with tubular secretory clearance for predicting kidney drug elimination, we evaluated stable outpatients with eGFRs ranging from 21 to 140 ml/min per 1.73 m2. After administering single doses of furosemide and famciclovir (metabolized to penciclovir), we calculated their kidney clearances on the basis of sequential plasma and timed urine measurements. Concomitantly, we quantified eight endogenous secretory solutes in plasma and urine using liquid chromatography-tandem mass spectrometry and measured GFR by iohexol clearance (iGFR). We computed a summary secretion score as the scaled average of the secretory solute clearances. RESULTS: Median iGFR of the 54 participants was 73 ml/min per 1.73 m2. The kidney furosemide clearance correlated with iGFR (r=0.84) and the summary secretion score (r=0.86). The mean proportionate error (MPE) between iGFR-predicted and measured furosemide clearance was 30.0%. The lowest MPE was observed for the summary secretion score (24.1%); MPEs for individual secretory solutes ranged from 27.3% to 48.0%. These predictive errors were statistically indistinguishable. Penciclovir kidney clearance was correlated with iGFR (r=0.83) and with the summary secretion score (r=0.91), with similar predictive accuracy of iGFR and secretory clearances. Combining iGFR with the summary secretion score yielded only modest improvements in the prediction of the kidney clearance of furosemide and penciclovir. CONCLUSIONS: Secretory solute clearance measurements can predict kidney drug clearances. However, tight linkage between GFR and proximal tubular secretory clearance in stable outpatients provides some reassurance that GFR, even when estimated, is a useful surrogate for predicting secretory drug clearances in such patients.

Microphysiological system modeling of ochratoxin A-associated nephrotoxicity.

Ochratoxin A (OTA) is one of the most abundant mycotoxin contaminants in food stuffs and possesses carcinogenic, nephrotoxic, teratogenic, and immunotoxic properties. Specifically, a major concern is severe nephrotoxicity, which is characterized by degeneration of epithelial cells of the proximal tubules and interstitial fibrosis. However, the mechanism of OTA toxicity, as well as the genetic risk factors contributing to its toxicity in humans has been elusive due to the lack of adequate models that fully recapitulate human kidney function in vitro. The present study attempts to evaluate dose-response relationships, identify the contribution of active transport proteins that govern the renal disposition of OTA, and determine the role of metabolism in the bioactivation and detoxification of OTA using a 3D human kidney proximal tubule microphysiological system (kidney MPS). We demonstrated that LC50 values of OTA in kidney MPS culture (0.375-1.21 µM) were in agreement with clinically relevant toxic concentrations of OTA in urine. Surprisingly, no enhancement of kidney injury biomarkers was evident in the effluent of the kidney MPS after OTA exposure despite significant toxicity observed by LIVE/DEAD staining. Instead, these biomarkers decreased in an OTA concentration-dependent manner. Furthermore, the effect of 1-aminobenzotriazole (ABT) and 6-(7-Nitro-2,1,3-benzoxadiazol-4-ylthio) hexanol (NBDHEX), pan-inhibitors of P450 and glutathione S-transferase (GST) enzymes, respectively, on OTA-induced toxicity in kidney MPS was examined. These studies revealed significant enhancement of OTA-induced toxicity by NBDHEX (3 µM) treatment, whereas ABT (1 mM) treatment decreased OTA-induced toxicity, suggesting roles for GSTs and P450 enzymes in the detoxification and bioactivation of OTA, respectively. Analysis of transcriptional changes using RNA-sequencing of kidney MPS treated with different concentrations of OTA revealed downregulation of several nuclear factor (erythroid derived-2)-like 2 (NRF2)-regulated genes by OTA treatment, including GSTs. The transcriptional repression of GSTs is likely playing a key role in OTA toxicity via attenuation of glutathione conjugation/detoxification. The sequential molecular events may explain the mechanism of toxicity associated with OTA. Additionally, OTA transport studies using kidney MPS in the presence and absence of probenecid (1 mM) suggested a role for organic anionic membrane transporter(s) in the kidney specific disposition of OTA. Our findings provide a clearer understanding of the mechanism of OTA-induced kidney injury, which may support changes in risk assessment, regulatory agency policies on allowable exposure levels, and determination of the role of genetic factors in populations at risk for OTA nephrotoxicity.

An Improved Vascularized, Dual-Channel Microphysiological System Facilitates Modeling of Proximal Tubular Solute Secretion.

A vascularized human proximal tubule model in a dual-channel microphysiological system (VPT-MPS) was developed, representing an advance over previous, single-cell-type kidney microphysiological systems. Human proximal tubule epithelial cells (PTECs) and human umbilical vein endothelial cells (HUVECs) were cocultured in side-by-side channels. Over 24 h of coculturing, PTECs maintained polarized expression of Na+/K+ ATPase, tight junctions (ZO-1), and OAT1. HUVECs showed the absence of ZO-1 but expressed endothelial cell marker (CD-31). In time-lapse imaging studies, fluorescein isothiocyanate (FITC)-dextran passed freely from the HUVEC vessel into the supporting extracellular matrix, confirming the leakiness of the endothelium (at 80 min, matrix/intravessel fluorescence ratio = 0.2). Dextran-associated fluorescence accumulated in the matrix adjacent to the basolateral aspect of the PTEC tubule with minimal passage of the compound into the tubule lumen observed (at 80 min, tubule lumen/matrix fluorescence ratio = 0.01). This demonstrates that the proximal tubule compartment is the rate-limiting step in the secretion of compounds in VPT-MPS. In kinetic studies with radiolabeled markers, p-aminohippuric acid (PAH) exhibited greater output into the tubule lumen than did paracellular markers mannitol and FITC-dextran (tubule outflow/vessel outflow concentration ratio of 7.7% vs 0.5 and 0.4%, respectively). A trend toward reduced PAH secretion by 45% was observed upon coadministration of probenecid. This signifies functional expression of renal transporters in PTECs that normally mediate the renal secretion of PAH. The VPT-MPS holds the promise of providing an in vitro platform for evaluating the renal secretion of new drug candidates and investigating the dysregulation of tubular drug secretion in chronic kidney disease.

Heterologous Expression and Functional Characterization of Novel CYP2C9 Variants Identified in the Alaska Native People.

CYP2C9 is a major form of human liver cytochrome P450 that is responsible for the oxidative metabolism of several widely used low-therapeutic index drugs, including (S)-warfarin and phenytoin. In a cohort of Alaska Native people, ultrarare or novel CYP2C9 protein variants, M1L (rs114071557), N218I (rs780801862), and P279T (rs182132442, CYP2C9*29), are expressed with higher frequencies than the well characterized CYP2C9*2 and CYP2C9*3 alleles. We report here on their relative expression in lentivirus-infected HepG2 cells and the functional characterization of purified reconstituted enzyme variants expressed in Escherichia coli toward (S)-warfarin, phenytoin, flurbiprofen, and (S)-naproxen. In the infected HepG2 cells, robust mRNA and protein expression were obtained for wild-type, N218I, and P279T variants, but as expected, the M1L variant protein was not translated in this liver-derived cell line. His-tagged wild-type protein and the N218I and P279T variants, but not M1L, expressed well in E. coli and were highly purified after affinity chromatography. Upon reconstitution with cytochrome P450 oxidoreductase and cytochrome b5, the N218I and P279T protein variants metabolized (S)-warfarin, phenytoin, flurbiprofen, and (S)-naproxen to the expected monohydroxylated or O-demethylated metabolites. Steady-state kinetic analyses revealed that the relative catalytic efficiency ratios of (S)-warfarin metabolism by the P279T and N218I variants were 87% and 24%, respectively, of wild-type CYP2C9 protein. A similar rank ordering was observed for metabolism of phenytoin, flurbiprofen, and (S)-naproxen. We conclude that carriers of the variant N218I and, especially, the M1L alleles would be at risk of exacerbated therapeutic effects from drugs that rely on CYP2C9 for their metabolic clearance. SIGNIFICANCE STATEMENT: Novel gene variants of CYP2C9-M1L, and N218I, along with P279T (CYP2C9*29)-are expressed in Alaska Native people at relatively high frequencies. In vitro characterization of their functional effects revealed that each variant confers reduced catalytic efficiency toward several substrates, including the low-therapeutic index drugs (S)-warfarin and phenytoin. These data provide the first functional information for new, common CYP2C9 variants in this understudied population. The data may help guide dose adjustments in allele carriers, thus mitigating potential healthcare disparities.

"Natural" is not synonymous with "Safe": Toxicity of natural products alone and in combination with pharmaceutical agents.

During the 25 years since the US Congress passed the Dietary Supplement Health and Education Act (DSHEA), the law that transformed the US Food and Drug Administration's (FDA's) authority to regulate dietary supplements, the dietary supplement market has grown exponentially. Retail sales of herbal products, a subcategory of dietary supplements, have increased 83% from 2008 to 2018 ($4.8 to $8.8 billion USD). Although consumers often equate "natural" with "safe", it is well recognized by scientists that constituents in these natural products (NPs) can result in toxicity. Additionally, when NPs are co-consumed with pharmaceutical agents, the precipitant NP can alter drug disposition and drug delivery, thereby enhancing or reducing the therapeutic effect of the object drug(s). With the widespread use of NPs, these effects can be underappreciated. We present a summary of a symposium presented at the Annual Meeting of the Society of Toxicology 2019 (12 March 2019) that discussed potential toxicities of NPs alone and in combination with drugs.

A new LC-MS assay for the quantitative analysis of vitamin K metabolites in human urine.

Vitamin K (VK), in both its phylloquinone and menaquinone forms, has been hypothesized to undergo ω- and ß-oxidation on its hydrophobic side chain in order to generate the observed urinary metabolites, K acid I and K acid II, which are excreted primarily as glucuronide conjugates. Synthetic standards of K acid I, K acid II, and a putative intermediate metabolite, menaquinone (MK)1 ω-COOH, were used to develop and optimize a new atmospheric pressure negative chemical ionization LC-MS/MS assay for the quantitation of these compounds in urine from untreated individuals and subjects treated with a high dose VK supplement. VK catabolites were extracted from urine, deconjugated, and converted to their methyl ester derivatives using previously reported methodology. The assay showed a high degree of sensitivity, with limits of detection below 10-50 fmol of metabolite per milliliter of urine, as well as an inter-assay precision of 8-12%. Metabolite standards provided unambiguous evidence for MK1 ω-COOH as a new human urinary metabolite of VK. This assay provides a minimally invasive, highly sensitive, and specific alternative for monitoring VK status in humans.

Human liver-kidney model elucidates the mechanisms of aristolochic acid nephrotoxicity.

Environmental exposures pose a significant threat to human health. However, it is often difficult to study toxicological mechanisms in human subjects due to ethical concerns. Plant-derived aristolochic acids are among the most potent nephrotoxins and carcinogens discovered to date, yet the mechanism of bioactivation in humans remains poorly understood. Microphysiological systems (organs-on-chips) provide an approach to examining the complex, species-specific toxicological effects of pharmaceutical and environmental chemicals using human cells. We microfluidically linked a kidney-on-a-chip with a liver-on-a-chip to determine the mechanisms of bioactivation and transport of aristolochic acid I (AA-I), an established nephrotoxin and human carcinogen. We demonstrate that human hepatocyte-specific metabolism of AA-I substantially increases its cytotoxicity toward human kidney proximal tubular epithelial cells, including formation of aristolactam adducts and release of kidney injury biomarkers. Hepatic biotransformation of AA-I to a nephrotoxic metabolite involves nitroreduction, followed by sulfate conjugation. Here, we identify, in a human tissue-based system, that the sulfate conjugate of the hepatic NQO1-generated aristolactam product of AA-I (AL-I-NOSO3) is the nephrotoxic form of AA-I. This conjugate can be transported out of liver via MRP membrane transporters and then actively transported into kidney tissue via one or more organic anionic membrane transporters. This integrated microphysiological system provides an ex vivo approach for investigating organ-organ interactions, whereby the metabolism of a drug or other xenobiotic by one tissue may influence its toxicity toward another, and represents an experimental approach for studying chemical toxicity related to environmental and other toxic exposures.