Phenylalanine kinetics in healthy volunteers and liver cirrhotics: implications for the phenylalanine breath test

Seconds-Resolved, In Situ Measurements of Plasma Phenylalanine Disposition Kinetics in Living Rats

Current knowledge of the disposition kinetics of endogenous metabolites is founded almost entirely on poorly time-resolved experiments in which samples are removed from the body for later, benchtop analysis. Here, in contrast, we describe real-time, seconds-resolved measurements of plasma phenylalanine collected in situ in the body via electrochemical aptamer-based (EAB) sensors, a platform technology that is independent of the reactivity of its targets and thus is generalizable to many. Specifically, using indwelling EAB sensors, we have monitored plasma phenylalanine in live rats with a few micromolar precision and a 12 s temporal resolution, identifying a large-amplitude, few-seconds phase in the animals' metabolic response that had not previously been reported. Using the hundreds of individual measurements that the approach provides from each animal, we also identify inter-subject variability, including statistically significant differences associated with the feeding status. These results highlight the power of in vivo EAB measurements, an advancement that could dramatically impact our understanding of physiology and provide a valuable new tool for the monitoring and treatment of metabolic disorders.

Malnutrition in cirrhosis: contribution and consequences of sarcopenia on metabolic and clinical responses

Malnutrition is the most common, reversible complication of cirrhosis that adversely affects survival, response to other complications, and quality of life. Sarcopenia, or loss of skeletal muscle mass, and loss of adipose tissue and altered substrate use as a source of energy are the 2 major components of malnutrition in cirrhosis. Current therapies include high protein supplementation especially as a late evening snack. Exercise protocols have the potential of aggravating hyperammonemia and portal hypertension. Recent advances in understanding the molecular regulation of muscle mass has helped identify potential novel therapeutic targets including myostatin antagonists, and mTOR resistance.

Clinical research on liver reserve function by 13C-phenylalanine breath test in aged patients with chronic liver diseases

Background

The objective of this study was to investigate whether the ¹³C-phenylalanine breath test could be useful for the evaluation of hepatic function in elderly volunteers and patients with chronic hepatitis B and liver cirrhosis.

Methods

L-[1-¹³C] phenylalanine was administered orally at a dose of 100 mg to 55 elderly patients with liver cirrhosis, 30 patients with chronic hepatitis B and 38 elderly healthy subjects. The breath test was performed at 8 different time points (0, 10, 20, 30, 45, 60, 90, 120 min) to obtain the values of Delta over baseline, percentage ¹³CO₂ exhalation rate and cumulative excretion (Cum). The relationships of the cumulative excretion with the ¹³C-%dose/h and blood biochemical parameters were investigated.

Results

The ¹³C-%dose/h at 20 min and 30 min combined with the cumulative excretion at 60 min and 120 min correlated with hepatic function tests, serum albumin, hemoglobin, platelet and Child-Pugh score. Prothrombin time, total and direct bilirubin were significantly increased, while serum albumin, hemoglobin and platelet, the cumulative excretion at 60 min and 120 min values decreased by degrees of intensity of the disease in Child-Pugh A, B, and C patients (P < 0.01).

Conclusions

The ¹³C-phenylalanine breath test can be used as a non-invasive assay to evaluate hepatic function in elderly patients with liver cirrhosis. The ¹³C-%dose/h at 20 min, at 30 min and cumulative excretion at 60 min may be the key value for determination at a single time-point. ¹³C-phenylalanine breath test is safe and helpful in distinguishing different stages of hepatic dysfunction for elderly cirrhosis patients.

Effect of alcohol consumption on the liver detoxication capacity as measured by [13C2]aminopyrine and L-[1-13C]phenylalanine breath tests

The aim of this study was to investigate the hepatic microsomal and cytosolic functions by using the 13CO2 breath test in healthy subjects either before or after consumption of red wine. Twelve adults received [13C2]aminopyrine and L-[1-13C]phenylalanine together with a standardised dinner. Expired air samples were taken over 6 h. After a wash-out period, the subjects consumed 0.4 ml ethanol per kg per day together with dinner over a 7.5-day period on average. Thereafter, 13C-tracer administration was repeated under identical conditions. The 13CO2 enrichments were measured by isotope ratio mass spectrometry. The mean cumulative percentage 13C-dose recovery after administration of [13C2]aminopyrine and L-[1-13C]phenylalanine either without or with red wine consumption amounted to 17.0+/-4.4 vs. 14.7+/-3.1% (p=0.170) and 14.0+/-2.8 vs. 11.5+/-3.9% (p=0.084), respectively. Moderate alcohol consumption does not induce significant short-term changes of the microsomal and the cytosolic function of the human liver in healthy subjects.

L-[1-13C]phenylalanine breath test in patients with chronic liver disease of different etiologies

The aim of this study was to compare the oxidation of L-[1-(13)C]phenylalanine ((13)C-PheOx) in patients with chronic liver failure due to different etiologies using L-[1-(13)C]phenylalanine breath test. Breath samples were collected before the administration of 100 mg L-[1-(13)C]phenylalanine, and every 10 min thereafter until completion of 1 h. Control subjects (n=9) presented a larger cumulative percentage of (13)C dose recovery (CPDR) than patients (n=124) with chronic liver disease, regardless of the etiology (7.5+/-0.7 vs. 4.2+/-0.2, p=0.001). No differences in CPDR were found considering the Child-Pugh (CP) class or etiology: alcoholic (CP A=7.7+/-0.7, CP B=4.1+/-0.5, CP C=2.0+/-0.3), hepatitis C virus (CP A=5.4+/-0.5, CP B=4.0+/-0.2, CP C=2.2+/-0.3), hepatocellular carcinoma (CP A=5.5+/-1.6, CP B=3.6+/-1.8, CP C=2.2+/-1.0); or cryptogenic cirrhotic patients (CP A=7.4+/-1.5, CP B=4.4+/-0.4, CP C=2.1+/-0.7). Results confirm that (13)C-PheOx decreases in patients with cirrhosis with respect to controls, notwithstanding the etiology.

Adaptation of the NDIR technology to13CO2breath tests under increased inspiratory O2concentrations

Nondispersive infrared spectroscopy (NDIR) allows the continuous analysis of respiratory gases. Due to its high selectivity, simple and robust setup, and small footprint, it is also used to support13CO2breath tests to assess bacterial growth in the stomach, gut, or liver function. CO2NDIR signals, however, are biased by oxygen in the gas matrix. This complicates NDIR-based breath tests, if the inspired oxygen concentration has to be adjusted to the subject's requirements, or hyperoxia-induced effects were studied. To avoid the oxygen-induced bias, a “dilution” approach was developed: expired gas is mixed with N2to lower the oxygen content down to the usual range of 15–20%. Accuracy and precision were tested using synthetic gas mixtures with increasing13CO2-to-12CO2ratios (13CO2/12CO2), either based on synthetic air with ∼20% volume O2or on pure O2. For samples with δ13C values smaller than 300 (or13CO2/12CO2smaller than 0.003), the dilution does not significantly increase the bias in the13CO2/12CO2determination, and the within-run imprecision is smaller than 1 δ13C. The practical use of this approach was validated in a pig study using a sepsis model reflecting a clinical situation that requires an increased oxygen concentration for respiration. The N2dilution eliminated the high bias in NDIR measurement, thus allowing the determination of the impact of oxygenation on glucose oxidation in patients ventilated with increased oxygen.

13C-breath tests: current state of the art and future directions

13C-breath tests provide a non-invasive diagnostic method with high patient acceptance. In vivo, human and also bacterial enzyme activities, organ functions and transport processes can be assessed semiquantitatively using breath tests. As the samples can directly be analysed using non-dispersive isotope selective infrared spectrometers or sent to analytical centres by normal mail breath tests can be easily performed also in primary care settings. The 13C-urea breath test which detects a Helicobacter pylori infection of the stomach is the most prominent application of stable isotopes. Determination of gastric emptying using test meals labelled with 13C-octanoic or 13C-acetic acid provide reliable results compared to scintigraphy. The clinical use of 13C-breath tests for the diagnosis of exocrine pancreatic insufficiency is still limited due to expensive substrates and long test periods with many samples. However, the quantification of liver function using hepatically metabolised 13C-substrates is clinically helpful in special indications. The stable isotope technique presents an elegant, non-invasive diagnostic tool promising further options of clinical applications. This review is aimed at providing an overview on the relevant clinical applications of 13C-breath tests.

Evaluation of postinjury hepatocyte function by central amino acid clearance

It has been demonstrated by other investigators that central plasma clearance of amino acids accurately predicts hepatocyte function in patients with liver disease and correlates with clinical outcome. This methodology has not heretofore been studied in the trauma patient. It is our hypothesis that central amino acid clearance in trauma patients is more reflective of hepatocyte function than traditional liver function tests. We examined the plasma amino acid clearance rates using L-[1-13C]phenylalanine. Clearance rates were compared to standard liver function tests (LFTs) and the sensitivity and predictability of the technique were determined. The study was conducted on uninjured control subjects and in seriously injured patients, both with and without significant liver injuries. Compared to baseline values in the control group, initial phenylalanine breath scores were reduced in the injured, but exceeded control levels at 7 days postinjury. These changes were statistically significant. There was no difference between those with and without liver trauma. LFTs showed inconsistent and conflicting results. Thus, central amino acid clearance measured by L-[1-13C]phenylalanine oxidation is depressed immediately following injury but reaches supranormal levels at 7 days postinjury. Compared to LFTs, amino acid clearance suggests initial hepatocyte suppression followed by hyperactivity and is a more accurate determinant of hepatocyte function.

Phenylalanine breath test as a method to evaluate hepatic dysfunction in obstructive jaundice

BACKGROUND

The purpose of this study was to investigate whether the hepatic dysfunction associated with obstructive jaundice can be measured from changes in expiratory 13CO2 levels after intravenous administration of l-[1-(13)C]phenylalanine, using a rat model of obstructive jaundice.

MATERIALS AND METHODS

Under pentobarbital anesthesia, bile duct ligation (BDL) was performed (n=12). In the control group, simple laparotomy was performed (n=12). On postoperative day 7, 20 mg/kg l-[1-(13)C]phenylalanine was administered via the femoral vein. Phenylalanine breath test (PBT) was performed for 30 min. We compared single point of 13CO2 level (SPT: T: min) and sum of 13CO2 output (ST) values between BDL and control rats. We examined the correlation of SPT or ST with phenylalanine hydroxylase activity (PHA) and blood chemical parameters.

RESULTS

Both SPT and ST values decreased in BDL compared to control 3 min after the start of PBT (SP10; 103+/-12 (per thousand) versus 84+/-16 (per thousand) P=0.025). PHA/g liver in BDL was significantly decreased compared to control (40.81+/-4.80 (U) versus 28.93+/-9.60 (U) P=0.008). PHA/g liver was correlated with SPT with correlation coefficient of more than 0.715, 10 min after the start of PBT, and the maximum correlation was at SP20 (r=0.801). Blood chemical parameters were correlated with S15 (total bilirubin, r=-0.717; alkaline phosphatase, r=-0.841; gamma-glutamyl transpeptidase, r=-0.759; alanine aminotransferase, r=-0.776; albumin, r=0.819).

CONCLUSIONS

In the breath test with intravenously l-[1-(13)C]phenylalanine, hepatic dysfunction associated with obstructive jaundice could be measured in a short period.

13C-methacetin breath test as liver function test in patients with chronic hepatitis C virus infection

BACKGROUND

The 13C-methacetin breath test enables the quantitative evaluation of the cytochrome P450-dependent liver function.

AIM

To find out whether this breath test is sensitive in noncirrhotic patients also with chronic hepatitis C in early stages of fibrosis.

METHODS

Sixty-one healthy controls and 81 patients with chronic hepatitis C underwent a 13C-methacetin breath test. In all patients, a liver biopsy was performed. The liver histology was classified according to the histology activity index-Knodell score.

RESULTS

Delta over baseline values of the patients at 15 min significantly differed from controls (19.2 +/- 9.2 per thousand vs. 24.1 +/- 5.7 per thousand; P < 0.003). The cumulative recovery after 30 min in patients was 11.4 +/- 4.8% and in healthy controls 13.8 +/- 2.8% (P < 0.002). However, patients with early fibrosis (histology activity index IVB) did not differ in delta over baseline values of the patients at 15 min (23.2 +/- 7.9 per thousand vs. 22.6 +/- 7.2 per thousand; P = 0.61) or cumulative recovery (13.6 +/- 3.7% vs. 13.2 +/- 3.8%; P = 0.45) from patients with more advanced fibrosis (histology activity index IVC). Patients with clinically nonsymptomatic cirrhosis (histology activity index IVD; Child A) metabolized 13C-methacetin to a significantly lesser extent (delta over baseline values of the patients at 15 min: 8.3 +/- 4.9 per thousand; P < 0.005 and cumulative recovery after 30 min: 5.6 +/- 3.2%; P < 0.003). The 13C-methacetin breath test identified cirrhotic patients with 95.0% sensitivity and 96.7% specificity.

CONCLUSION

The non-invasive 13C-methacetin breath test reliably distinguishes between early cirrhotic (Child A) and noncirrhotic patients, but fails to detect early stages of fibrosis in patients with chronic hepatitis C.

Evaluation of hepatic function with 13C-methacetin breath test in patients with liver cirrhosis

AIM: To investigate the characteristics of the ¹³C-methacetin breath test (¹³C-MBT) as a tool to monitor hepatic function of patients with liver cirrhosis.

METHODS: Forty-two patients with liver cirrhosis and thirty-one age- and sex-matched healthy controls underwent ¹³C-MBT.¹³C-methacetin (75 mg) was given orally, and the levels of ¹³CO₂ delta over baseline (DOB), metabolisation velocity (MV) and cumulative percentage doses (CD) before and 10, 20, 30, 40, 50, 60, 80, 100 and 120 min after substrate administration breath samples was determined and compared, by non-dispersive infrared spectrometry.

RESULTS: The ¹³CO₂ peak percent doses of healthy controls, Child class A and Child class B patients were 20.3 ±3.5%, 7.5±1.8% or 6.5±3.3% and 4.5±1.3%, and occurred at 20 min, 20 or 80 min and 40 min, respectively. The differences of¹³CO₂ peak percent doses among healthy controls, Child class A and Child class B patients were obvious (P <0.05). No ¹³CO₂peak percent doses occurred in Child class C patients. The differences of ¹³CO₂ peak metabolic velocity were obvious (P <0.05) between healthy controls (28.8±5.3%) and Child class A patients (9.4±2.4%). No ¹³CO₂ peak metabolic velocity occurred in Child class B and Child class C patients. The ¹³CO₂ cumulative percentage doses of healthy controls, Child class A , Child class B and Child class C patients at 120 min after substrate administration were 31.2±4.5%, 13.8±3.7%, 8.2±2.2% and 2.4±0.8% (P <0.05), respectively.

CONCLUSION: The severity of liver damage can be directly and effectively evaluated by ¹³C-MBT in patients with cirrhosis.