Correlation between plasma levels of 7alpha-hydroxy-4-cholesten-3-one and cholesterol 7alpha-hydroxylation rates in vivo in hyperlipidemic patients

A simple surrogate approach for the quantitation of C4 (7α-hydroxy-4-cholesten-3-one) in human serum via LC-MS/MS and its application in a clinical study

We present a validated LC-MS/MS assay for the quantitation of 7α-hydroxy-4-cholesten-3-one (C4), a key intermediate in the bile acid synthesis pathway from cholesterol, in human serum. A surrogate matrix approach was employed to overcome the challenges posed by the endogenous C4 levels in the biological matrix. Human serum samples were spiked with stable isotope labeled internal standard (SIL-IS), processed using supported liquid extraction (SLE), and analyzed by LC-MS/MS. Parallelism was successfully demonstrated between human serum (authentic matrix) and 5 % bovine serum albumin in phosphate buffered saline containing 0.1 % tween 20 (5 % BSA in PBST) (surrogate matrix). The assay's linear analytical range was established from 0.200 to 200 ng/mL. This validated LC-MS/MS method exhibited excellent accuracy and precision. The overall accuracy was between 97.9 % and 101 % with %CV less than 4.0 % for C4 in human serum. C4 was found to be stable in human serum for up to 24.7 h at room temperature, up to 34 days when stored at -25 °C or - 80 °C, and after five freeze/thaw cycles. The assay has been successfully applied to human serum samples to support a clinical study.

Comparative Study of the Dose-Dependence of OATP1B Inhibition by Rifampicin Using Probe Drugs and Endogenous Substrates in Healthy Volunteers

PURPOSE

To evaluate association of the dose-dependent effect of rifampicin, an OATP1B inhibitor, on the plasma concentration-time profiles among OATP1B substrates drugs and endogenous substrates.

METHODS

Eight healthy volunteers received atorvastatin (1 mg), pitavastatin (0.2 mg), rosuvastatin (0.5 mg), and fluvastatin (2 mg) alone or with rifampicin (300 or 600 mg) in a crossover fashion. The plasma concentrations of these OATP1B probe drugs, total and direct bilirubin, glycochenodeoxycholate-3-sulfate (GCDCA-S), and coproporphyrin I, were determined.

RESULTS

The most striking effect of 600 mg rifampicin was on atorvastatin (6.0-times increase) and GCDCA-S (10-times increase). The AUC_0-24h of atorvastatin was reasonably correlated with that of pitavastatin (r² = 0.73) and with the AUC_0-4h of fluvastatin (r² = 0.62) and sufficiently with the AUC_0-24h of rosuvastatin (r² = 0.32). The AUC_0-24h of GCDCA-S was reasonably correlated with those of direct bilirubin (r² = 0.74) and coproporphyrin I (r² = 0.78), and sufficiently with that of total bilirubin (r² = 0.30). The AUC_0-24h of GCDCA-S, direct bilirubin, and coproporphyrin I were reasonably correlated with that of atorvastatin (r² = 0.48-0.70) [corrected].

CONCLUSION

These results suggest that direct bilirubin, GCDCA-S, and coproporphyrin I are promising surrogate probes for the quantitative assessment of potential OATP1B-mediated DDI.

The bile acid-sequestering resin sevelamer eliminates the acute GLP-1 stimulatory effect of endogenously released bile acids in patients with type 2 diabetes

AIMS

Discovery of the specific bile acid receptors farnesoid X receptor (FXR) and Takeda G protein-coupled receptor 5 (TGR5) in enteroendocrine L cells has prompted research focusing on the impact of bile acids on glucagon-like peptide-1 (GLP-1) secretion and glucose metabolism. The aim of the present study was to assess the GLP-1 secretory and gluco-metabolic effects of endogenously released bile, with and without concomitant administration of the bile acid-sequestering resin, sevelamer, in patients with type 2 diabetes.

MATERIALS AND METHODS

We performed a randomized, placebo-controlled, double-blinded cross-over study including 15 metformin-treated patients with type 2 diabetes. During 4 experimental study days, either sevelamer 3200 mg or placebo in combination with intravenous infusion of cholecystokinin (CCK) (0.4 pmol sulfated CCK-8/kg/min) or saline was administered in randomized order. The primary endpoint was plasma GLP-1 excursions as measured by incremental area under the curve. Secondary endpoints included plasma responses of glucose, triglycerides, insulin, CCK, fibroblast growth factor-19 and 7α-hydroxy-4-cholesten-3-one (C4). In addition, gallbladder dynamics, gastric emptying, resting energy expenditure, appetite and ad libitum food intake were assessed.

RESULTS

CCK-mediated gallbladder emptying was demonstrated to elicit a significant induction of GLP-1 secretion compared to saline, whereas concomitant single-dose administration of the bile acid sequestrant sevelamer was shown to eliminate the acute bile acid-induced increase in plasma GLP-1 excursions.

CONCLUSIONS

Single-dose administration of sevelamer eliminated bile acid-mediated GLP-1 secretion in patients with type 2 diabetes, which could be explained by reduced bile acid stimulation of the basolaterally localized TGR5 on enteroendocrine L cells.

LC-MS/MS quantification of 7α-hydroxy-4-cholesten-3-one (C4) in rat and monkey plasma

7α-hydroxy-4-cholesten-3-one (C4) is an oxidative enzymatic product of cholesterol metabolism via cholesterol 7α-hydroxylase, an enzyme also known as cholesterol 7-alpha-monooxygenase or cytochrome P450 7A1 (CYP7A1). C4 is a stable intermediate in the rate limiting pathway of bile acid biosynthesis. Previous studies showed that plasma C4 levels correlated with CYP7A1 enzymatic activity and could serve as a biomarker for bile acid synthesis. Here we developed and qualified a simple and robust high-throughput method using liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) to quantify C4 in rat and monkey plasma. As C4 being an endogenous compound, this method used calibration standards in 50/50: acetonitrile/water (v/v). In order to mimic the incurred samples, quality control samples were prepared in the authentic plasma. Stable isotope labeled C4 (C4-d₇) was used as the internal standard. The sample volume for analysis was 20μL and the sample preparation method was protein precipitation with acetonitrile. The average endogenous C4 concentrations, from 10 different lots of rat and monkey plasma, were 53.0±16.5ng/mL and 6.8±5.6ng/mL, respectively. Based on these observed endogenous C4 levels, the calibration curve ranges were established at 1-200ng/mL and 0.5-100ng/mL for rat assay and monkey assay, respectively. The method was qualified with acceptable accuracy, precision, linearity, and specificity. Matrix effect, recovery, and plasma stability of bench-top, freeze-thaw, and long-term frozen storage were also evaluated. The method has been successfully applied to pre-clinical studies.

A simple, fast, sensitive and robust LC-MS/MS bioanalytical assay for evaluating 7α-hydroxy-4-cholesten-3-one biomarker in a clinical program

AIM

Serum 7α-hydroxy-cholesten-3-one (C4) has been reported as a biomarker to assess CYP7A1 enzyme activity and bile acid synthesis. To support a clinical program, a sensitive and reliable assay without derivatization was required for the analysis of C4 in human serum. Methodology & results: A systematic approach was used to optimize mass spectrometry, LC and sample extraction conditions, therefore, significantly improved assay sensitivity, and achieved the required quantification limit without derivatization. A surrogate matrix approach was used to overcome the interference from endogenous C4. A stable isotope-labeled C4 was used as internal standard. The samples were extracted using a simple protein precipitation method with 2% formic acid in acetonitrile.

CONCLUSION

A simple, fast, sensitive and robust UHPLC-MS/MS method for the quantification of 0.50 ng/ml C4 in 100 µl human serum was developed and fit for purpose validated. The method was successfully applied to the bioanalysis of C4 in a clinical study.

Metabolism of bile acids in the post-prandial state

The modulation of energy expenditure by dietary administration of cholic acid in mice promoted interest in studying bile acid(s) (BA) as adjuvants in the treatment of metabolic diseases such as obesity and diabetes. Bile acids can modulate intermediary metabolism by acting directly on nuclear as well as G-protein-coupled receptors or indirectly through changes in gut microbiota. Despite the potential of BA to affect intermediary metabolism, plasma kinetics and changes in individual BA in blood in the post-prandial state have been neglected for a long time. Minutes after ingestion of a meal (or a glucose challenge), the plasma BA concentration increases as a result of the secretion of bile into the duodenum, followed by intestinal absorption and a systemic circulation spillover. A large inter-individual variability of post-prandial kinetics of plasma BA is documented. Factors such as gender, diet composition, circadian oscillations, and individual capacities for the synthesis and transport of BA play important roles in determining this variability and are discussed in the present short review in light of new findings.

Obesity is associated with changes in oxysterol metabolism and levels in mice liver, hypothalamus, adipose tissue and plasma

Oxysterols are bioactive lipids derived from cholesterol that are linked to inflammatory processes. Because obesity and metabolic syndrome are characterized by inflammation and altered cholesterol metabolism, we sought to investigate the variations of oxysterol levels and their metabolic pathways induced by obesity in the liver, hypothalamus, adipose tissue and plasma. To this end, we used diet-induced and genetic (ob/ob and db/db) models of obesity. Among the oxysterols measured, we found that 4β-oxysterol levels were consistently decreased in the high-fat diet study, at different time-points, and in the ob/ob model. Overall, we did not find any correlation between cytochromes mRNA expression and variations of oxysterol levels. We also measured the levels of hepatic primary bile acids, in these three models and found similar profiles between HFD and ob/ob mice. However, although they are downstream metabolites of oxysterols, the variations in bile acid levels did not reflect the variations of their precursors. Our data show that, when considering oxysterol metabolism, the high-fat diet and ob/ob models are more closely related when compared to the db/db model. However, we were able to discriminate between lean and obese phenotypes based on liver oxysterol (4β-hydroxycholesterol, 27- hydroxycholesterol, 7-hydroxycholestenone) levels and enzyme (CYP3A11, CYP27A1, CYP7A1) expression.

Evaluation of cholesterol metabolism in cerebrotendinous xanthomatosis

BACKGROUND

Cerebrotendinous xanthomatosis (CTX) is a treatable bile acid disorder caused by mutations of CYP27A1. The pathogenesis of neurological damage has not been completely explained. Oral chenodeoxycholic acid (CDCA) can lead to clinical stabilization, but in a subgroup of patients the disease progresses despite treatment. In the present study, we aimed at clarifying cholesterol metabolism abnormalities and their response to CDCA treatment, in order to identify reliable diagnostic and prognostic markers and understand if differences exist between stable patients and those with neurological progression.

METHODS

We enrolled 19 untreated CTX patients and assessed serum profile of bile acids intermediates, oxysterols, cholesterol, lathosterol, and plant sterols. Then we performed a long-term follow up during CDCA therapy, and compared biochemical data with neurological outcome.

RESULTS

We observed increase of cholestanol, 7α-hydroxy-4-cholesten-3-one (7αC4), lathosterol, and plant sterols, whereas 27-hydroxycholesterol (27-OHC) was extremely low or absent. CDCA treatment at a daily dose of 750 mg normalized all biochemical parameters except for 7αC4 which persisted slightly higher than normal in most patients, and 27-OHC which was not modified by therapy. Biochemical evaluation did not reveal significant differences between stable and worsening patients.

DISCUSSION

Cholestanol and 7αC4 represent important markers for CTX diagnosis and monitoring of therapy. Treatment with CDCA should aim at normalizing serum 7αC4 as well as cholestanol, since 7αC4 better mirrors 7α-hydroxylation rate and is thought to be correlated with cholestanol accumulation in the brain. Assessment of serum 27-OHC is a very good tool for biochemical diagnosis at any stage of disease. Lathosterol and plant sterols should be considered as additional markers for diagnosis and monitoring of therapy. Further studies including long-term assessment of bile acid intermediates in cerebrospinal fluid are needed in patients who show clinical progression despite treatment.

Age-associated alterations in cholesterol homeostasis: evidence from a cross-sectional study in a Northern Italy population

BACKGROUND

The modifications of cholesterol metabolism associated with aging are ill-defined. The objective of this study was to define age-associated alterations of the different metabolic pathways controlling cholesterol homeostasis by analyzing circulating sterols.

METHODS

We analyzed serum samples collected from 201 adult (75 male, 126 female) subjects within the epidemiological MICOL study (Multicentrica Italiana Colelitiasi). The age range was 38-79 years; 103 had evidence of gallstones. The concentrations of the different sterols, recognized as markers of the main pathways of cholesterol homeostasis, were analyzed by gas chromatography-mass spectrometry, including lathosterol (synthesis), campesterol and sitosterol (absorption), and 7α-hydroxy-4-cholesten-3-one (degradation to bile acids).

RESULTS

A significant direct correlation was detected between age and cholesterol levels (r =0.34, P<0.01). The lathosterol/cholesterol ratio was lower in older age quartiles (P<0.05 by analysis of variance), with an inverse correlation between the lathosterol/cholesterol ratio and age (r=-0.32, P<0.01). Such correlation was particularly evident in females. The campesterol/cholesterol and sitosterol/cholesterol ratios were inversely correlated with aging in control, but not in gallstone patients. The levels of 7α-hydroxy-4-cholesten-3-one were not correlated with age.

CONCLUSION

These data show a reduction of cholesterol synthesis with aging which is associated with increased circulating cholesterol levels. The finding might be related to a reduced metabolic need for cholesterol in advancing age, leading to a downregulation of the main mechanisms of cholesterol intake in the liver. A different age-related behavior was observed in gallstone-free versus gallstone patients regarding cholesterol absorption. The possible implications in terms of the pharmacological management of hypercholesterolemia in the elderly remain to be defined.

The use of stable and radioactive sterol tracers as a tool to investigate cholesterol degradation to bile acids in humans in vivo

Alterations of cholesterol homeostasis represent important risk factors for atherosclerosis and cardiovascular disease. Different clinical-experimental approaches have been devised to study the metabolism of cholesterol and particularly the synthesis of bile acids, its main catabolic products. Most evidence in humans has derived from studies utilizing the administration of labeled sterols; these have several advantages over in vitro assay of enzyme activity and expression, requiring an invasive procedure such as a liver biopsy, or the determination of fecal sterols, which is cumbersome and not commonly available. Pioneering evidence with administration of radioactive sterol derivatives has allowed to characterize the alterations of cholesterol metabolism and degradation in different situations, including spontaneous disease conditions, aging, and drug treatment. Along with the classical isotope dilution methodology, other approaches were proposed, among which isotope release following radioactive substrate administration. More recently, stable isotope studies have allowed to overcome radioactivity exposure. Isotope enrichment studies during tracer infusion has allowed to characterize changes in the degradation of cholesterol via the "classical" and the "alternative" pathways of bile acid synthesis. Evidence brought by tracer studies in vivo, summarized here, provides an exceptional tool for the investigation of sterol metabolism, and integrate the studies in vitro on human tissue.

Patterns of cholesterol metabolism: pathophysiological and therapeutic implications for dyslipidemias and the metabolic syndrome

Investigating cholesterol metabolism, which derives from balancing cholesterol synthesis and absorption, opens new perspectives in the pathogenesis of dyslipidemias and the metabolic syndrome (MS). Cholesterol metabolism is studied by measuring plasma levels of campesterol, sitosterol and cholestanol, that is, plant sterols which are recognised as surrogate cholesterol-absorption markers and lathosterol or squalene, that is, cholesterol precursors, which are considered surrogate cholesterol-synthesis markers. This article presents current knowledge on cholesterol synthesis and absorption, as evaluated by means of cholesterol precursors and plant sterols, and discusses patterns of cholesterol balance in the main forms of primary hyperlipidaemia and MS. Understanding the mechanism(s) underlying these patterns of cholesterol synthesis and absorption will help to predict the response to hypolipidemic treatment, which can then be tailored to ensure the maximum clinical benefit for patients.