Metabolomic Profiling and Drug Interaction Characterization Reveal Riboflavin As a Breast Cancer Resistance Protein-Specific Endogenous Biomarker That Demonstrates Prediction of Transporter Activity In Vivo.

Advancement of endogenous biomarkers for drug transporters as a tool for assessing drug-drug interactions (DDIs) depends on initial identification of biomarker candidates and relies heavily on biomarker validation and its response to reference inhibitors in vivo. To identify endogenous biomarkers of breast cancer resistance protein (BCRP), we applied metabolomic approaches to profile plasma from Bcrp-/-, multidrug resistance protein (Mdr)1a/1b-/-, and Bcrp/Mdr1a/1b-/- mice. Approximately 130 metabolites were significantly altered in Bcrp and P-glycoprotein (P-gp) knockout mice, indicating numerous metabolite-transporter interactions. We focused on BCRP-specific substrates and identified riboflavin, which was significantly elevated in the plasma of Bcrp single- and Bcrp/P-gp double- but not P-gp single-knockout mice. Dual BCRP/P-gp inhibitor elacridar caused a dose-dependent increase of the area under the plasma concentration-time curve (AUC) of riboflavin in mice (1.51- and 1.93-fold increases by 30 and 150 mg/kg elacridar, respectively). In three cynomolgus monkeys, we observed approximately 1.7-fold increases in the riboflavin concentrations caused by ML753286 (10 mg/kg), which correlated well with the increase of sulfasalazine, a known BCRP probe in monkeys. However, the BCRP inhibitor had no effect on isobutyryl carnitine, arginine, or 2-arachidonoyl glycerol levels. Additionally, clinical studies on healthy volunteers indicated low intrasubject and intermeal variability of plasma riboflavin concentrations. In vitro experiments using membrane vesicles demonstrated riboflavin as a select substrate of monkey and human BCRP over P-gp. Collectively, this proof-of-principle study indicates that riboflavin is a suitable endogenous probe for BCRP activity in mice and monkeys and that future investigation of riboflavin as a blood-based biomarker of human BCRP is warranted. SIGNIFICANCE STATEMENT: Our results identified riboflavin as an endogenous biomarker candidate of BCRP. Its selectivity, sensitivity, and predictivity regarding BCRP inhibition have been explored. The findings of this study highlight riboflavin as an informative BCRP plasma biomarker in animal models. The utility of this biomarker requires further validation by evaluating the effects of BCRP inhibitors of different potencies on riboflavin plasma concentrations in humans. Ultimately, riboflavin may shed light on the risk assessment of BCRP DDIs in early clinical trials.

Discovery and Validation of Pyridoxic Acid and Homovanillic Acid as Novel Endogenous Plasma Biomarkers of Organic Anion Transporter (OAT) 1 and OAT3 in Cynomolgus Monkeys.

Perturbation of organic anion transporter (OAT) 1- and OAT3-mediated transport can alter the exposure, efficacy, and safety of drugs. Although there have been reports of the endogenous biomarkers for OAT1/3, none of these have all of the characteristics required for a clinical useful biomarker. Cynomolgus monkeys were treated with intravenous probenecid (PROB) at a dose of 40 mg/kg in this study. As expected, PROB increased the area under the plasma concentration-time curve (AUC) of coadministered furosemide, a known substrate of OAT1 and OAT3, by 4.1-fold, consistent with the values reported in humans (3.1- to 3.7-fold). Of the 233 plasma metabolites analyzed using a liquid chromatography-tandem mass spectrometry (LC-MS/MS)-based metabolomics method, 29 metabolites, including pyridoxic acid (PDA) and homovanillic acid (HVA), were significantly increased after either 1 or 3 hours in plasma from the monkeys pretreated with PROB compared with the treated animals. The plasma of animals was then subjected to targeted LC-MS/MS analysis, which confirmed that the PDA and HVA AUCs increased by approximately 2- to 3-fold by PROB pretreatments. PROB also increased the plasma concentrations of hexadecanedioic acid (HDA) and tetradecanedioic acid (TDA), although the increases were not statistically significant. Moreover, transporter profiling assessed using stable cell lines constitutively expressing transporters demonstrated that PDA and HVA are substrates for human OAT1, OAT3, OAT2 (HVA), and OAT4 (PDA), but not OCT2, MATE1, MATE2K, OATP1B1, OATP1B3, and sodium taurocholate cotransporting polypeptide. Collectively, these findings suggest that PDA and HVA might serve as blood-based endogenous probes of cynomolgus monkey OAT1 and OAT3, and investigation of PDA and HVA as circulating endogenous biomarkers of human OAT1 and OAT3 function is warranted.

11ß-hydroxysteroid dehydrogenase type 1 gene knockout attenuates atherosclerosis and in vivo foam cell formation in hyperlipidemic apoE⁻/⁻ mice.

BACKGROUND: Chronic glucocorticoid excess has been linked to increased atherosclerosis and general cardiovascular risk in humans. The enzyme 11ß-hydroxysteroid dehydrogenase type 1 (11ßHSD1) increases active glucocorticoid levels within tissues by catalyzing the conversion of cortisone to cortisol. Pharmacological inhibition of 11ßHSD1 has been shown to reduce atherosclerosis in murine models. However, the cellular and molecular details for this effect have not been elucidated. METHODOLOGY/PRINCIPAL FINDINGS: To examine the role of 11ßHSD1 in atherogenesis, 11ßHSD1 knockout mice were created on the pro-atherogenic apoE⁻/⁻ background. Following 14 weeks of Western diet, aortic cholesterol levels were reduced 50% in 11ßHSD1⁻/⁻/apoE⁻/⁻ mice vs. 11ßHSD1⁺/⁺/apoE⁻/⁻ mice without changes in plasma cholesterol. Aortic 7-ketocholesterol content was reduced 40% in 11ßHSD1⁻/⁻/apoE⁻/⁻ mice vs. control. In the aortic root, plaque size, necrotic core area and macrophage content were reduced ∼30% in 11ßHSD1⁻/⁻/apoE⁻/⁻mice. Bone marrow transplantation from 11ßHSD1⁻/⁻/apoE⁻/⁻ mice into apoE⁻/⁻ recipients reduced plaque area 39-46% in the thoracic aorta. In vivo foam cell formation was evaluated in thioglycollate-elicited peritoneal macrophages from 11ßHSD1⁺/⁺/apoE⁻/⁻ and 11ßHSD1⁻/⁻/apoE⁻/⁻ mice fed a Western diet for ∼5 weeks. Foam cell cholesterol levels were reduced 48% in 11ßHSD1⁻/⁻/apoE⁻/⁻ mice vs. control. Microarray profiling of peritoneal macrophages revealed differential expression of genes involved in inflammation, stress response and energy metabolism. Several toll-like receptors (TLRs) were downregulated in 11ßHSD1⁻/⁻/apoE⁻/⁻ mice including TLR 1, 3 and 4. Cytokine release from 11ßHSD1⁻/⁻/apoE⁻/⁻-derived peritoneal foam cells was attenuated following challenge with oxidized LDL. CONCLUSIONS: These findings suggest that 11ßHSD1 inhibition may have the potential to limit plaque development at the vessel wall and regulate foam cell formation independent of changes in plasma lipids. The diminished cytokine response to oxidized LDL stimulation is consistent with the reduction in TLR expression and suggests involvement of 11ßHSD1 in modulating binding of pro-atherogenic TLR ligands.

A general liquid chromatography/mass spectroscopy-based assay for detection and quantitation of methyltransferase activity.

Methyltransferases form a large class of enzymes, most of which use S-adenosylmethionine as the methyl donor. In fact, S-adenosylmethionine is second only to ATP in the variety of reactions for which it serves as a cofactor. Several methods to measure methyltransferase activity have been described, most of which are applicable only to specific enzymes and/or substrates. In this work we describe a sensitive liquid chromatography/mass spectroscopy-based methyltransferase assay. The assay monitors the conversion of S-adenosylmethionine to S-adenosylhomocysteine and can be applied to any methyltransferase and substrate of interest. We used the well-characterized enzyme catechol O-methyltransferase to demonstrate that the assay can monitor activity with a variety of substrates, can identify new substrates, and can be used even with crude preparation of enzyme. Furthermore, we demonstrate the utility of the assay for kinetic characterization of enzymatic activity.