Metabolomics studies hold promise for the discovery of pathways linked to disease processes. Cardiovascular disease (CVD) represents the leading cause of death and morbidity worldwide. Here we used a metabolomics approach to generate unbiased small-molecule metabolic profiles in plasma that predict risk for CVD. Three metabolites of the dietary lipid phosphatidylcholine—choline, trimethylamine N-oxide (TMAO) and betaine—were identified and then shown to predict risk for CVD in an independent large clinical cohort. Dietary supplementation of mice with choline, TMAO or betaine promoted upregulation of multiple macrophage scavenger receptors linked to atherosclerosis, and supplementation with choline or TMAO promoted atherosclerosis. Studies using germ-free mice confirmed a critical role for dietary choline and gut flora in TMAO production, augmented macrophage cholesterol accumulation and foam cell formation. Suppression of intestinal microflora in atherosclerosis-prone mice inhibited dietary-choline-enhanced atherosclerosis. Genetic variations controlling expression of flavin monooxygenases, an enzymatic source of TMAO, segregated with atherosclerosis in hyperlipidaemic mice. Discovery of a relationship between gut-flora-dependent metabolism of dietary phosphatidylcholine and CVD pathogenesis provides opportunities for the development of new diagnostic tests and therapeutic approaches for atherosclerotic heart disease.

/pdf/gut-flora-metabolism-of-phosphatidylcholine-promotes-437q5mia6l.pdf

Gut flora metabolism of phosphatidylcholine promotes cardiovascular disease

Background Recent studies in animals have shown a mechanistic link between intestinal microbial metabolism of the choline moiety in dietary phosphatidylcholine (lecithin) and coronary artery disease through the production of a proatherosclerotic metabolite, trimethylamine-N-oxide (TMAO). We investigated the relationship among intestinal microbiota-dependent metabolism of dietary phosphatidylcholine, TMAO levels, and adverse cardiovascular events in humans. Methods We quantified plasma and urinary levels of TMAO and plasma choline and betaine levels by means of liquid chromatography and online tandem mass spectrometry after a phosphatidylcholine challenge (ingestion of two hard-boiled eggs and deuterium [d9]-labeled phosphatidylcholine) in healthy participants before and after the suppression of intestinal microbiota with oral broad-spectrum antibiotics. We further examined the relationship between fasting plasma levels of TMAO and incident major adverse cardiovascular events (death, myocardial infarction,...

/pdf/intestinal-microbial-metabolism-of-phosphatidylcholine-and-4sbnwjtcl8.pdf

Intestinal Microbial Metabolism of Phosphatidylcholine and Cardiovascular Risk

Non-lethal Inhibition of Gut Microbial Trimethylamine Production for the Treatment of Atherosclerosis

Rationale: Trimethylamine- N -oxide (TMAO), a gut microbial-dependent metabolite of dietary choline, phosphatidylcholine (lecithin), and l-carnitine, is elevated in chronic kidney diseases (CKD) and associated with coronary artery disease pathogenesis. Objective: To both investigate the clinical prognostic value of TMAO in subjects with versus without CKD, and test the hypothesis that TMAO plays a direct contributory role in the development and progression of renal dysfunction. Methods and Results: We first examined the relationship between fasting plasma TMAO and all-cause mortality over 5-year follow-up in 521 stable subjects with CKD (estimated glomerular filtration rate, 2 ). Median TMAO level among CKD subjects was 7.9 μmol/L (interquartile range, 5.2–12.4 μmol/L), which was markedly higher ( P P P P =0.036). Among non-CKD subjects, elevated TMAO levels portend poorer prognosis within cohorts of high and low cystatin C. In animal models, elevated dietary choline or TMAO directly led to progressive renal tubulointerstitial fibrosis and dysfunction. Conclusions: Plasma TMAO levels are both elevated in patients with CKD and portend poorer long-term survival. Chronic dietary exposures that increase TMAO directly contributes to progressive renal fibrosis and dysfunction in animal models.

/pdf/gut-microbiota-dependent-trimethylamine-n-oxide-tmao-pathway-3qx87fkj6t.pdf

Gut Microbiota-Dependent Trimethylamine N-oxide (TMAO) Pathway Contributes to Both Development of Renal Insufficiency and Mortality Risk in Chronic Kidney Disease

An updated review of the existing knowledge regarding uremic toxins facilitates the design of experimental studies. We performed a literature search and found 621 articles about uremic toxicity published after a 2003 review of this topic. Eighty-seven records provided serum or blood measurements of one or more solutes in patients with CKD. These records described 32 previously known uremic toxins and 56 newly reported solutes. The articles most frequently reported concentrations of β2-microglobulin, indoxyl sulfate, homocysteine, uric acid, and parathyroid hormone. We found most solutes (59%) in only one report. Compared with previous results, more recent articles reported higher uremic concentrations of many solutes, including carboxymethyllysine, cystatin C, and parathyroid hormone. However, five solutes had uremic concentrations less than 10% of the originally reported values. Furthermore, the uremic concentrations of four solutes did not exceed their respective normal concentrations, although they had been previously described as uremic retention solutes. In summary, this review extends the classification of uremic retention solutes and their normal and uremic concentrations, and it should aid the design of experiments to study the biologic effects of these solutes in CKD.

Normal and Pathologic Concentrations of Uremic Toxins

L-Carnitine is a naturally occurring compound that facilitates the transport of fatty acids into mitochondria for beta-oxidation. Exogenous L-carnitine is used clinically for the treatment of carnitine deficiency disorders and a range of other conditions. In humans, the endogenous carnitine pool, which comprises free L-carnitine and a range of short-, medium- and long-chain esters, is maintained by absorption of L-carnitine from dietary sources, biosynthesis within the body and extensive renal tubular reabsorption from glomerular filtrate. In addition, carrier-mediated transport ensures high tissue-to-plasma concentration ratios in tissues that depend critically on fatty acid oxidation. The absorption of L-carnitine after oral administration occurs partly via carrier-mediated transport and partly by passive diffusion. After oral doses of 1-6g, the absolute bioavailability is 5-18%. In contrast, the bioavailability of dietary L-carnitine may be as high as 75%. Therefore, pharmacological or supplemental doses of L-carnitine are absorbed less efficiently than the relatively smaller amounts present within a normal diet.L-Carnitine and its short-chain esters do not bind to plasma proteins and, although blood cells contain L-carnitine, the rate of distribution between erythrocytes and plasma is extremely slow in whole blood. After intravenous administration, the initial distribution volume of L-carnitine is typically about 0.2-0.3 L/kg, which corresponds to extracellular fluid volume. There are at least three distinct pharmacokinetic compartments for L-carnitine, with the slowest equilibrating pool comprising skeletal and cardiac muscle.L-Carnitine is eliminated from the body mainly via urinary excretion. Under baseline conditions, the renal clearance of L-carnitine (1-3 mL/min) is substantially less than glomerular filtration rate (GFR), indicating extensive (98-99%) tubular reabsorption. The threshold concentration for tubular reabsorption (above which the fractional reabsorption begins to decline) is about 40-60 micromol/L, which is similar to the endogenous plasma L-carnitine level. Therefore, the renal clearance of L-carnitine increases after exogenous administration, approaching GFR after high intravenous doses. Patients with primary carnitine deficiency display alterations in the renal handling of L-carnitine and/or the transport of the compound into muscle tissue. Similarly, many forms of secondary carnitine deficiency, including some drug-induced disorders, arise from impaired renal tubular reabsorption. Patients with end-stage renal disease undergoing dialysis can develop a secondary carnitine deficiency due to the unrestricted loss of L-carnitine through the dialyser, and L-carnitine has been used for treatment of some patients during long-term haemodialysis. Recent studies have started to shed light on the pharmacokinetics of L-carnitine when used in haemodialysis patients.

Pharmacokinetics of L-carnitine.

BACKGROUND Trimethylamine (TMA) is a short-chain tertiary aliphatic amine that is derived from the diet either directly from the consumption of foods high in TMA or by the intake of food high in precursors to TMA, such as trimethylamine-N-oxide (TMNO), choline and L-carnitine. The clinical significance of TMA may be related to its potential to contribute to neurological toxicity and 'uraemic breath' in patients with end-stage renal disease (ESRD). METHODS Concentrations of TMA and TMNO in plasma from 10 healthy adults (not on haemodialysis) and 10 adults with ESRD undergoing haemodialysis (pre- and post-dialysis) were determined by gas chromatography-mass spectrometry. RESULTS The concentrations of TMA and TMNO in pre-dialysis plasma (1.39+/-0.483 and 99.9+/-31.9 microM, respectively) were significantly (P<0.05) higher than the corresponding levels in healthy subjects (0.418+/-0.124 and 37.8+/-20.4 microM, respectively). However, there were no significant differences between post-dialysis and healthy subject plasma concentrations. In the ESRD patients, there was a significant (P<0.05) reduction in plasma TMA (from 1.39+/-0.483 to 0.484+/-0.164 microM) and TMNO (from 99.9+/-31.9 to 41.3+/-18.8 microM) during a single haemodialysis session. CONCLUSIONS TMA and TMNO accumulate between haemodialysis sessions in ESRD patients, but are efficiently removed during a single haemodialysis session.

Accumulation of trimethylamine and trimethylamine- N -oxide in end-stage renal disease patients undergoing haemodialysis

Trimethylamine (TMA) is a volatile tertiary aliphatic amine that is derived from the diet either directly from the consumption of foods containing TMA, or by the intake of food containing precursors to TMA such as trimethylamine-N-oxide (TMNO), choline and L-carnitine. Following oral absorption in humans, TMA undergoes efficient N-oxidation to TMNO, a reaction catalyzed by the flavin-containing monooxygenase (FMO) isoform 3 enzyme. TMNO subsequently undergoes excretion in the urine, although, evidence also suggests that metabolic retro-reduction of TMNO can occur. Whilst the pharmacokinetics of TMA and TMNO has not been fully elucidated in humans, a number of studies provide information on the likely fate of dietary derived TMA. Trimethylaminuria is a condition that is characterized by a deficiency in FMO3 enzyme activity, resulting in the excretion of increased amounts of TMA in bodily fluids such as urine and sweat, and breath. A human FMO3 database has been established and currently twenty-eight variants of the FMO3 gene have been reported including twenty-four missense, three nonsense, and one gross deletion mutation. Whilst TMA and TMNO are generally regarded as non-toxic substances, they are of clinical interest because of their potential to form the carcinogen N-nitrosodimethylamine.

Trimethylamine: metabolic, pharmacokinetic and safety aspects.

Objective

L-Carnitine is an endogenous molecule involved in fatty acid metabolism. Secondary carnitine deficiency may develop in patients with end-stage renal disease undergoing long-term hemodialysis because of dialytic loss. In these patients L-carnitine can be administered to restore plasma and tissue levels. The objective of this study was to evaluate the pharmacokinetics of intravenous L-carnitine in patients undergoing long-term hemodialysis.



Methods

Twelve patients undergoing three dialysis sessions/week received L-carnitine intravenously (20 mg · kg−1) at the end of each dialysis session for 9 weeks. Plasma samples were analyzed for L-carnitine, acetyl-L-carnitine, and total carnitine by HPLC.



Results

Under baseline conditions, the mean ± SD predialysis plasma concentration of L-carnitine was 19.5 ± 5.6 μmol/L, decreasing to 5.6 ± 1.9 μmol/L at the end of the dialysis session. These concentrations were substantially lower than endogenous levels in healthy human beings. Under baseline conditions the extraction ratios of L-carnitine and acetyl-L-carnitine by the dialyser were 0.74 ± 0.07 and 0.71 ± 0.11, respectively. During repeated dosing, there was accumulation of L-carnitine in plasma, and after 9 weeks of dosing, the predialysis and postdialysis plasma levels were 191 ± 54.1 and 41.8 ± 13.0 μmol/L, respectively. The predialysis and postdialysis plasma levels of L-carnitine decreased once dosing was ceased but had not returned to pretreatment levels after 6 weeks.



Conclusion

The study demonstrated that removal of L-carnitine by hemodialysis is extremely efficient and that patients undergoing hemodialysis had plasma concentrations that were substantially lower than normal, particularly during dialysis. During repeated administration of L-carnitine, the predialysis and postdialysis concentrations of the compound increased steadily, reaching an apparent steady state after about 8 weeks. It is proposed that this accumulation arose from the distribution of l-carnitine into a deep tissue pool that includes skeletal muscle.



Clinical Pharmacology & Therapeutics (2000) 68, 238–249; doi: 10.1067/mcp.2000.108850

Pharmacokinetics of L-carnitine in patients with end-stage renal disease undergoing long-term hemodialysis

https://www.kidney-international.org/article/S0085-2538(15)50221-4/pdf

Gianfranco Fornasini

Papers

Pharmacokinetics of L-carnitine.

Accumulation of trimethylamine and trimethylamine- N -oxide in end-stage renal disease patients undergoing haemodialysis

Trimethylamine: metabolic, pharmacokinetic and safety aspects.

Pharmacokinetics of L-carnitine in patients with end-stage renal disease undergoing long-term hemodialysis

Impact of hemodialysis on endogenous plasma and muscle carnitine levels in patients with end-stage renal disease.