Serum citrulline as a marker of acute cellular rejection for intestinal transplantation

Non-immunological biomarkers for assessment of villous abnormalities in patients with celiac disease

BACKGROUND AND AIM

Demonstration of villous abnormalities is an essential component of diagnosis of celiac disease (CeD) that requires duodenal biopsies. There is a need for non-invasive biomarker(s) that can predict the presence of villous abnormalities.

METHODS

Levels of plasma citrulline, plasma intestinal fatty acid binding protein (I-FABP), and serum regenerating gene 1α (Reg1α) were estimated in treatment naïve patients with CeD and controls. The levels of these biomarkers and their cyclical pattern were validated in a predicted model of enteropathy. Optimum diagnostic cut-off values were derived, and the results were further validated in a prospective validation cohort.

RESULTS

While level of plasma citrulline was significantly lower, the levels of plasma I-FABP and serum Reg1α were significantly higher in patients with CeD (n = 131) in comparison with healthy (n = 216) and disease controls (n = 133), and their levels reversed after a gluten-free diet (GFD). In the model of predicted enteropathy (n = 70), a sequential decrease and then increase in the level of plasma citrulline was observed; such a sequential change was not observed with I-FABP and Reg1α. The diagnostic accuracy for prediction of presence of villous abnormality was 89% and 78% if citrulline level was  ≤ 30 μM/L and I-FABP levels were ≥ 1100 pg/mL, respectively. The results were validated in a prospective validation cohort (n = 104) with a sensitivity and specificity of 79.5% and 83.1%, respectively, for predicting villous abnormalities of modified Marsh grade > 2 at calculated cut-off values of citrulline and I-FABP.

CONCLUSIONS

Plasma citrulline  ≤ 30 μM/L is the most consistent, highly reproducible non-invasive biomarker that can predict the presence of villous abnormality and has the potential for avoiding duodenal biopsies in 78% patients suspected to have CeD.

Intestinal Fatty Acid Binding Protein and Citrulline as Markers of Gut Injury and Prognosis in Patients With Acute Pancreatitis

OBJECTIVES

Severe acute pancreatitis (AP) is associated with high mortality due to systemic inflammatory response syndrome in the early phase and secondary infection in the later phase. Concomitant intestinal ischemia often results in gut injury. We studied intestinal fatty acid binding protein (IFABP) and citrulline levels as markers of gut injury to predict prognosis in AP.

METHODS

Acute pancreatitis patients at admission and controls were studied. Serum IFABP was measured by enzyme-linked immunosorbent assay and plasma citrulline by high-performance liquid chromatography technique. Ultrastructural changes in duodenal biopsy were also compared between the 2 groups.

RESULTS

The IFABP concentration was significantly higher in AP cases (n = 94) compared with controls (n = 100) (mean [standard deviation], 592.5 [753.6] vs 87.8 [67.6] pg/mL; P < 0.001) and in patients with severe AP versus mild AP (738.3 [955.3] vs 404.0 [263.3] pg/ mL, P = 0.03). Citrulline concentration was lower in AP versus controls (29.9 [33.8] vs 83.9 [60.1] μg/L, P < 0.001). We propose a model by which these biomarkers (IFABP >350 pg/mL and citrulline <18 μg/L) are able to predict poor prognosis in 33.9% of patients with AP. The gut injury was also validated via ultrastructural changes.

CONCLUSIONS

Intestinal fatty acid binding protein is a promising prognostic marker in acute pancreatitis.

Citrulline as a marker of intestinal function and absorption in clinical settings: A systematic review and meta-analysis

Background

Citrulline has been described as a marker of intestinal function or absorption but evidence varies according to clinical settings.

Objective

The objective of this article is to examine the evidence of plasma citrulline as a marker of intestinal function and absorption in various clinical settings.

Methods

Studies were examined for p values, means and standard deviations, correlation coefficients or other metrics depicting the association of citrulline with intestinal function. A random effects model was used to produce a pooled estimate. A hierarchical summary receiver operating curve model was fitted for diagnostic accuracy measures.

Results

Citrulline levels are correlated strongly with small bowel length in short bowel syndrome patients (r = 0.67). Citrulline is strongly negatively correlated (r = -0.56) with intestinal disease severity with regards to enteropathies (coeliac disease, tropical enteropathy, Crohn's disease, mucositis, acute rejection in intestinal transplantation). Citrulline cut-off levels have an overall sensitivity and specificity of 80% and 84% respectively. Citrulline levels in untreated coeliac patients compared to controls were reduced by 10 µmol/l. Citrulline levels increase with gluten-free diet and with improvement of enteropathy. Citrulline is decreased in critical illness and sepsis.

Conclusion

These findings allow us to advocate quite reasonably that citrulline is a marker of acute and chronic intestinal insufficiency.

Intestinal transplantation

Intestinal transplantation has now emerged as a lifesaving therapeutic option and standard of care for patients with irreversible intestinal failure. Improvement in survival over the years has justified expansion of the indications for intestinal transplantation beyond the original indications approved by Center for Medicare and Medicaid services. Management of patients with intestinal failure is complex and requires a multidisciplinary approach to accurately select candidates who would benefit from rehabilitation versus transplantation. Significant strides have been made in patient and graft survival with several advancements in the perioperative management through timely referral, improved patient selection, refinement in the surgical techniques and better understanding of the immunopathology of intestinal transplantation. The therapeutic efficacy of the procedure is well evident from continuous improvements in functional status, quality of life and cost-effectiveness of the procedure. This current review summarizes various aspects including current practices and evidence based recommendations of intestinal transplantation.

Serum citrulline as a diagnostic marker of sepsis-induced intestinal dysfunction

OBJECTIVE

To investigate the use of citrulline as an indicator for diagnosing septic acute intestinal dysfunction (SAID) in a rat model.

METHODS

SD rats were divided into three groups: a normal group (A), a model group (B), and a glutamine group (C). Group B was divided into a 36-h group (B1) and a 72-h feeding group (B2). The concentrations of serum citrulline, intestinal fatty acid-binding protein (I-FABP) and intestinal glutamine and histopathological changes were measured.

RESULTS

The lengths of the villus and thicknesses of the mucosal layer in groups B1, B2 and C were significantly different from those in group A. Citrulline concentrations in groups B1, B2 and C were lower than in group A; the serum concentrations in group C were significantly greater than in groups B1 and B2. The I-FABP levels of groups B1, B2 and C were higher than group A; I-FABP levels of groups B1 and B2 were higher than group C. Intestinal glutamine levels of groups B1 and B2 were lower than groups A and C. The serum citrulline of group C was negatively correlated with I-FABP and Chiu's score.

CONCLUSIONS

Serum citrulline could be used as the diagnostic indicator of SAID.

The relationship between fasting plasma citrulline concentration and small intestinal function in the critically ill

Introduction

In this study, we aimed to evaluate whether fasting plasma citrulline concentration predicts subsequent glucose absorption in critically ill patients.

Methods

In a prospective observational study involving 15 healthy and 20 critically ill subjects, fasting plasma citrulline concentrations were assayed in blood samples immediately prior to the administration of a liquid test meal (1 kcal/ml; containing 3 g of 3-O-methylglucose (3-OMG)) that was infused directly into the small intestine. Serum 3-OMG concentrations were measured over the following 4 hours, with the area under the 3-OMG concentration curve (AUC) calculated as an index of glucose absorption.

Results

The groups were well matched in terms of age, sex and body mass index (BMI) (healthy subjects versus patients, mean (range) values: age, 47 (18 to 88) versus 49 (21 to 77) years; sex ratio, 60% versus 80% male; BMI, 25.2 (18.8 to 30.0) versus 25.5 (19.4 to 32.2) kg/m²). Compared to the healthy subjects, patients who were critically ill had reduced fasting citrulline concentration (26.5 (13.9 to 43.0) versus 15.2 (5.7 to 28.6) μmol/L; P < 0.01) and glucose absorption (3-OMG AUC, 79.7 (28.6 to 117.8) versus 61.0 (4.5 to 97.1) mmol/L/240 min; P = 0.05). There was no relationship between fasting citrulline concentration and subsequent glucose absorption (r = 0.28; P = 0.12).

Conclusions

Whereas both plasma citrulline concentrations and glucose absorption were reduced in critical illness, fasting plasma citrulline concentrations were not predictive of subsequent glucose absorption. These data suggest that fasting citrulline concentration does not appear to be a marker of small intestinal absorptive function in patients who are critically ill.

[Plasma citrulline concentration as a biomarker of intestinal function in short bowel syndrome and in intestinal transplant]

INTRODUCTION

Citrulline is a non-essential amino acid produced solely in the enterocyte. The aim of this study was to analyse the role of serum citrulline as a biomarker of enterocyte load in children with intestinal failure due to short bowel syndrome (SBS) and its relationship to enteral adaptation.

MATERIAL AND METHODS

Plasma citrulline concentration was determined by chromatography (normal value>15 μmol/L) in 57 patients (age 0.5-18 years) admitted to our Intestinal Rehabilitation Unit with intestinal failure. Those who were dehydrated, with renal insufficiency, or other conditions able to modify the results were excluded. Patients were divided into 4 groups: group i: SBS totally dependent on parenteral nutrition (PN); group ii: SBS under mixed enteral-parenteral nutrition; group iii: IF weaned from PN after a rehabilitation period; group iv: small bowel transplanted patients weaned from PN and taking a normal diet.

RESULTS

The mean ± SD plasma citrulline values were: group i (n=15): 7.1 ± 4.1; group ii (n=11): 15.8 ± 8.9; group iii (n=13): 20.6 ± 7.5; group iv (n=25): 28.8 ± 10.1. Values were significantly lower in group i in comparison with groups ii-iii-iv (P<.001), and in group ii in comparison with groups iii-iv (P<.001). A low citrulline was associated with remnant small bowel length (P<.001, r=0.85). In group iv citrulline levels decreased >50% in 3 patients who developed moderate-severe rejection, and in one patient who developed viral enteritis.

CONCLUSIONS

1. Plasma citrulline could be a sensitive and specific biomarker of the residual functional enterocyte load. 2. It is related to enteral feeding tolerance. 3. Its prognostic value in the process of intestinal adaptation and as a rejection marker in small bowel transplanted patients needs to be confirmed.

Serum citrulline levels as a marker of enterocyte function in patients after allogeneic hematopoietic stem cells transplantation - a pilot study

Background

Citrulline is an amino acid produced by enterocytes. Serum citrulline concentration has been proposed as a marker of enterocyte mass and function. Our study focused on evaluation of citrulline levels in patients with diarrhea related to toxic intestinal damage (mucositis), intestinal graft versus host disease (GVHD), and other etiology of diarrhea (e.g., dysmicrobia) after allogeneic stem cells transplantation (SCT).

Material/Methods

This was a prospective study in 11 adults (18 blood samples) with diarrhea developed after allogeneic SCT in 4/2011–1/2012 compared to twenty healthy control samples.

Results

The median (interquartile range) of citrulline levels was significantly lower in the transplanted patients group compared to healthy controls: 9.3 (3.62–15.38) vs. 33.3 (26.82–36.23) μmol/L, p<0.0001. The median values of citrulline in patients with post-transplant toxic intestinal mucositis (n=8, days 1–22 post-transplant) vs. intestinal GVHD (n=7, day 43–142) vs. other etiology of diarrhea (e.g., dysmicrobia) (n=3, day 120–127) were: 9.55 (2.95–12.03) vs. 5 (3.85–9.05) vs. 15.6 (15.45–18.3) μmol/L resp. Serum citrulline levels were significantly higher in other (eg, dysmicrobic) etiology of diarrhea in comparison with mucositis (p=0.0336) and GVHD (p=0.0152).

Conclusions

Citrulline levels are very low shortly after the myeloablative FLU/MEL or BuCY2 conditioning allogeneic SCT due to the toxic intestinal damage. Significantly low levels of citrulline were also in patients with intestinal GVHD later on. Other observations in larger groups of patients are necessary before any specific recommendation for citrulline levels monitoring in intestinal GVHD can be made.

Plasma citrulline concentration as a marker for disease activity in patients with Crohn's disease

BACKGROUND

Citrulline is a nitrogen end product produced from the intermediary metabolism of glutamine through the enzymatically mediated urea cycle, almost exclusively in the enterocytes of small intestinal epithelium, with some synthesis in colonocytes. Intestinal dysfunction resulting from intestinal diseases or injuries affects intermediary metabolism, which includes citrulline synthesis. We sought to determine whether plasma citrulline was a biomarker for disease activity in patients with Crohn's disease with the hypothesis that citrulline concentration would be reduced during active disease.

METHODS

A total of 81 outpatients aged 18 to 65 years (mean, 40.6±15.4 y) with a known history of Crohn's disease were studied prospectively. Patients with prior small intestinal resection, or renal or hepatic insufficiency were excluded. Crohn's disease activity was measured by Harvey-Bradshaw Index (HBI) and was correlated to the plasma citrulline concentration measured simultaneously (ion chromatography). Spearman correlation coefficients were used to assess for an association between the 2 variables. Subgroup analyses of patients with isolated small intestinal disease and endoscopically or radiologic verified disease activity were also performed.

RESULTS

Twenty-two patients had isolated colonic disease and 59 had small intestinal involvement. Twenty-six of these patients had concurrent endoscopy and/or computed tomography or magnetic resonance imaging. On the basis of HBI scores, 32 patients had active disease (HBI ≥5) and 49 patients had inactive disease. The mean HBI scores were 4.8±5.5. The mean plasma citrulline concentration was normal, although was below normal in some patients. However, it failed to distinguish between active and inactive patients based on the HBI (active 27.8±8.8 μmol/L, inactive 27.8±11.1 μmol/L, P=0.991). There was no significant linear association between the ranks of citrulline and ranks of HBI (rs=0.012, P=0.915). Of the 59 patients with isolated small intestinal disease, there was no association between plasma citrulline concentration and the HBI (Spearman correlation coefficient, 0.073; P=0.583). There was no difference in plasma citrulline concentrations among those with confirmed inflammation by imaging or endoscopy (confirmed, 26.2±11.8; negative, 28.0±10.0; independent t test P=0.583).

CONCLUSIONS

Plasma citrulline concentration was not a marker of disease activity in patients with Crohn's disease. However, all patients studied were outpatients and it is possible that plasma citrulline concentration may be depressed only in patients with more severe disease or extensive small bowel involvement. In addition, plasma citrulline may be increased in the postabsorptional state, and for the most part, our patients were nonfasting. More studies are needed to further elucidate the role of citrulline as a marker of disease activity in patients with Crohn's disease. The possibility also exists that citrulline may be a better marker in patients with previous resection, and this group will require specific evaluation.

An in silico model of enterocytic glutamine to citrulline conversion pathway

Enterocyte is one of the main sites of amino acids metabolism and particularly of the citrulline biosynthesis. Working at the cellular scale and applying ordinary differential equations (ODEs) formalism, we have built a mathematical model of the enterocytic glutamine to citrulline conversion in the fasting state. This model enables us to test different physiopathological scenarios of enzyme activity loss. Results from two different approaches were compared: a standard approach (KA) based on the Michaelis-Menten assumptions and an association-dissociation approach (VH) based on the kinetic mass action law. For both approaches, ODEs system was numerically solved using Mathematica™. In both cases, the model correctly predicts the physiological plasma citrulline steady-state, but the two approaches present clear differences for metabolites of enzymes having a complex mechanism, challenging the validity of the KA approach in such cases. When physiopathological scenarios of enzyme activity loss are simulated, both approaches predict a very sharp transition from the physiological citrulline plasma level to the lack of its production: the concentration profiles of these simulations show a clear threshold of which characteristics vary with the involved enzyme. Moreover, amongst all enzymes included in the model, the ornithine aminotransferase (OAT) shows the highest sensitivity in the system whatever the approach used. This model points out the limits of the Michaelis-Menten approach to model complex enzyme mechanisms. It highlights the key role of OAT in the studied citrulline synthesis pathway and also suggests an order of magnitude about the optimal ratio of enzyme concentrations in this pathway.

Low serum citrulline concentration correlates with catheter-related bloodstream infections in children with intestinal failure

BACKGROUND

Serum citrulline concentration is used as a biomarker of enterocyte mass and enteral tolerance, and low serum concentrations are correlated with bacteremia in immunosuppressed adults undergoing hematopoietic stem cell transplant. The authors sought to determine if citrulline was associated with the development of catheter-related bloodstream infections (CRBSIs) in children with intestinal failure.

METHODS

Data were reviewed from 66 children treated in a multidisciplinary intestinal rehabilitation program, who had serum concentration citrulline measured between January 2007 and August 2009. All patients had a diagnosis of intestinal failure requiring parenteral nutrition (PN) support. Exclusion criteria included central venous catheter in situ <30 days, creatinine clearance <20 mL/minute, or a history of organ transplant/immunosuppression.

RESULTS

A total of 15 patients were excluded because of the above criteria. In this cohort of 51 patients, 26 (51%) developed CRBSIs. Both groups were similar in terms of gestational age, diagnosis, nutrition status, and biochemical liver function tests. The mean (± standard deviation [SD]) minimum serum citrulline concentration was significantly lower in patients who developed CRBSIs (6.7 ± 4.6 µmol/L) than in those who did not (11.3 ± 6.4 µmol/L, P = .004). Multivariate logistic regression analysis identified lower minimum serum citrulline concentration and longer central venous catheter duration as independently associated with CRBSI (P = .003 and P = .038, respectively).

CONCLUSIONS

Low serum citrulline concentration and longer central venous catheter time are independently associated with CRBSI in children with intestinal failure. Serum citrulline concentration may be a useful biomarker to identify patients with intestinal failure who are at high risk of developing a CRBSI.

Assessment of Small Bowel Function in Critical Illness: Potential Role of Citrulline Metabolism

Small intestinal function in critically ill patients should ideally be assessed in order to determine the preferred feeding route, timing, and composition of enteral nutrition. Additionally, evaluation of small bowel function may lead to new insights aimed to maintain enterocyte integrity. Critically ill patients are likely to have impaired enterocyte function mainly as a consequence of diminished splanchnic blood flow associated with mucosal hyperpermeability and bacterial translocation, a pathological state believed to be pivotal in the development of sepsis and multiple organ dysfunction syndrome (MODS). However, feasible and validated clinical tools to reliably assess enterocyte function are lacking. This explorative review discusses the promising role of citrulline, a nonprotein amino acid almost exclusively generated by the enterocyte, as a biomarker reflecting enterocyte function in critically ill patients. Citrulline metabolism, its potential as enterocyte biomarker, and literature on citrulline in critically illness will be discussed. Finally, a novel test for enterocyte function, the citrulline generation test (enterocytic citrulline production upon stimulation with enteral or intravenous glutamine) will be considered briefly.

Citrulline evaluation in bowel transplantation

INTRODUCTION

The purpose of this study was to evaluate the correlation of plasma citrulline and rejection episodes in intestinal transplantation.

METHODS

From January 2007 until present, we performed citrulline assays on our small bowel patients. We investigated the correlation of these assays with the rejection status of the patients. The rejection status of the graft was defined based on graft biopsies.

RESULTS

Of 5195 citrulline samples, average serum citrulline levels decreased significantly when the patients presented a rejection episode. We found the following: no rejection, 17.38 microm/L; mild rejection, 13.05 microm/L; moderate rejection, 7.98 microm/L; and severe rejection, 6.05 microm/L. Our current emphasis is to determine the predictive power of citrulline with other biomarkers versus as a separate and isolated measurement.

CONCLUSIONS

In our study, citrulline levels correlated significantly with the rejection status of the graft. Serial follow-up of the patients using this assay may alert us to the possibility of increased alloreactivity and rejection episodes.

Diagnosing malabsorption in the intensive care unit

Malabsorption as a result of decreased intestinal function is a frequently occurring problem in intensive care units. Small bowel dysfunction may lead to malnutrition and may predispose patients to infectious complications (sepsis) and may be linked to increased hospitalization duration, morbidity and mortality. There are several small bowel function tests, such as faecal fat excretion and sugar absorption tests, but data specifically applicable to the intensive care setting are limited. Moreover, there are multiple confounders such as altered intestinal permeability, changed transit time and renal dysfunction. A more specific test, fasting plasma citrulline concentration, has poor test characteristics for detection of decreased intestinal function in patients with dysfunctional enterocytes. Therefore, a citrulline generation test was recently developed in order to measure the conversion of glutamine (administered either enterally or intravenously) into citrulline. This conversion takes place exclusively in a properly functioning enterocyte; therefore, citrulline generation upon glutamine stimulation may be a promising biomarker for assessment of the small bowel function.

Plasma citrulline concentration reflects enterocyte mass in children with short bowel syndrome

Plasma citrulline was recently shown to reflect the residual functional enterocyte mass in various situations characterized by intestinal failure. However, few data are available in children with short bowel syndrome. The objective of this study was to assess the value of citrulline assays in this situation. Prospective plasma citrulline assays were performed in 31 children with short bowel syndrome. Median age was 16 mo (range, 1 mo to 15 y), and median follow-up was 14 mo (6-40 mo). The energy supplied by parenteral nutrition (PN), served to assess intestinal failure severity. Plasma citrulline at inclusion showed a positive correlation with residual short bowel length. Subsequent values correlated negatively with intestinal failure severity. Plasma citrulline increased over time during or after weaning from PN (from 15.8 +/- 11.5 microM to 19.3 +/- 3.8 microM) but remained stable and low in patients who continued to need PN (6.5 +/- 3.0 microM at inclusion and 7.7 +/- 6.0 microM at last follow-up). No weaned patients had a residual short bowel length less than 40 cm and plasma citrulline less than 11 microM. Our findings constitute the first evidence that serial plasma citrulline assays help to monitor residual small bowel adaptation in children.

Current status of small bowel and multivisceral transplantation

Intestinal transplantation has shown exceptional growth over the past 20 years with remarkable progress. As with other solid organ transplants, intestinal transplantation has moved out of the experimental realm to become the stan dard of care for many patients with intestinal failure. Intestinal transplantation may soon be extended routinely to patients who, although not strictly meeting the criteria for intestinal failure, may benefit from intestinal transplantation, such as patients who have nonresectable indolent tumors or diffuse thrombosis of the portomesenteric system. As clinical experience has increased with intestinal transplantation, outcomes have improved. The currently reported 1-year graft and patient survival rate is 80%, which approaches that for other solid abdominal organs. Unfortunately, most of the gains in survival are seen in the first postoperative year, with long-term survival remaining basically unchanged since the early 1990s. With improved outcomes, more centers have entered into the intestinal transplant arena. In the United States alone, 20 centers performed at least one intestinal transplant in 2007. Increase in access to intestinal transplantation and more widespread awareness of this option likely will result in a consistent increase in the number of yearly transplants for the foreseeable future. Immunosuppressive regimens continue to evolve, with induction therapy being the major change in the past 5 years. Although rejection rates in the first year after transplant have been reduced by induction therapy, long-term side effects of heavy immunosuppression continue to weigh negatively on transplant outcomes. The future for immunosuppression lies in two areas: (1) individual monitoring of the immunosuppression level for each individual patient and (2) development of serum and tissue markers for the early identification of rejection. It is likely that a combination of technologies will allow immunosuppression to be tailored to each recipient. Development of these approaches to immunosuppression is necessary to predict graft dysfunction ahead of irreversible graft injury and allows adjustments in immunosuppression before the onset of rejection. Intestinal transplantation continues to be performed only in situations in which all other therapeutic modalities have failed. No randomized trials compare intestinal transplantation to long-term PN to establish guidelines for a timely referral for this treatment option. Late referral remains a crippling problem in the field of intestinal transplantation, with a great number of patients in need of simultaneous liver transplantation at the time of listing for intestinal transplantation. Early referral for isolated intestinal transplant will reduce the need for simultaneous multiorgan transplants and increase the residual organs available for patients in need of (primarily) liver transplantation.

Is serum citrulline measurement clinically useful in coeliac disease?

Citrulline (CIT), a non-protein amino acid in circulating blood, is almost exclusively contained in the enterocytes of small bowel mucosa and may represent a reliable marker of functioning enterocyte mass. The aim of this study was to evaluate the clinical utility of measuring serum citrulline levels in a group of patients affected by coeliac disease (CD). Fifty healthy volunteers, 21 patients with untreated coeliac disease and 6 patients with refractory coeliac disease took part in the study. Serum citrulline levels and duodenal lesions were evaluated at the time of diagnosis, and after at least 24 months of gluten-free diet. Serum citrulline concentrations were determined by ion exchange chromatography. In comparison to healthy volunteers, serum citrulline concentrations were significantly lower in untreated and refractory coeliac disease patients. No significant difference was found between untreated and refractory coeliac disease patients and between patients with different patterns of clinical presentation or various degrees of duodenal lesions. After a gluten-free diet, the mean of serum citrulline concentration was increased in all but one patient. Although, as expected, serum citrulline levels turned out to be low in coeliac disease, the clinical utility of their measurement is, at least, questionable in this condition.

Therapeutic efficacy of intestinal and multivisceral transplantation: survival and nutrition outcome

The clinical introduction of intestinal transplantation has added a new dimension and offered a valid therapeutic option for patients with irreversible intestinal failure. In the year 2000, the Center for Medicare & Medicaid Services (CMS) recognized intestinal, combined liver-intestinal, and multivisceral transplantation as the standard of care for patients with irreversible intestinal and parenteral nutrition (PN) failure. Accordingly, the indications for the procedure are currently limited to those who develop life-threatening PN complications. However, a recent improvement in survival similar to other solid organ transplant recipients should justify lifting the current restricted criteria, and the procedure should be considered before the development of PN failure. Equally important is the awareness of the recent evolution in nutrition management and outcome after transplantation. Early and progressive enteral feeding using a complex polymeric formula is safe and effective after successful transplantation. Full nutrition autonomy is universally achievable among most intestinal and multivisceral recipients, with enjoyment of unrestricted oral diet. Such a therapeutic benefit is commonly maintained among long-term survivors, with full rehabilitation and restoration of quality of life.

Manipulation of citrulline availability in humans

To determine whether circulating citrulline can be manipulated in vivo in humans, and, if so, whether citrulline availability affects the levels of related amino acids, nitric oxide, urinary citrulline, and urea nitrogen, 10 healthy volunteers were studied on 3 separate days: 1) under baseline conditions; 2) after a 24-h treatment with phenylbutyrate (0.36 g.kg(-1).day(-1)), a glutamine "trapping" agent; and 3) during oral L-citrulline supplementation (0.18 g.kg(-1).day(-1)), in randomized order. Plasma, erythrocyte (RBC), and urinary citrulline concentrations were determined by gas chromatography-mass spectrometry at 3-h intervals between 1100 and 2000 on each study day. Regardless of treatment, RBC citrulline was lower than plasma citrulline, with an RBC-to-plasma ratio of 0.60 +/- 0.04, and urinary citrulline excretion accounted for <1% of the citrulline load filtered by kidney. Phenylbutyrate induced an approximately 7% drop in plasma glutamine (P = 0.013), and 18 +/- 14% (P < 0.0001) and 19 +/- 17% (P < 0.01) declines in plasma and urine citrulline, respectively, with no alteration in RBC citrulline. Oral L-citrulline administration was associated with 1) a rise in plasma, urine, and RBC citrulline (39 +/- 4 vs. 225 +/- 44 micromol/l, 0.9 +/- 0.3 vs. 6.2 +/- 3.8 micromol/mmol creatinine, and 23 +/- 1 vs. 52 +/- 9 micromol/l, respectively); and 2) a doubling in plasma arginine level, without altering blood urea or urinary urea nitrogen excretion, and thus enhanced nitrogen balance. We conclude that 1) depletion of glutamine, the main precursor of citrulline, depletes plasma citrulline; 2) oral citrulline can be used to enhance systemic citrulline and arginine availability, because citrulline is bioavailable and very little citrulline is lost in urine; and 3) further studies are warranted to determine the mechanisms by which citrulline may enhance nitrogen balance in vivo in humans.

Critical sample pretreatment in monitoring dried blood spot citrulline

BACKGROUND

Plasma Citrulline concentration has been correlated to functional enterocyte mass. Dried blood spot (DBS) analysis using LC-MSMS reduces sample amount needed. We optimized DBS elution to increase precision and accuracy of DBS LC-MSMS analysis.

METHOD

DBS control samples were eluted in varying pH (2.2-7.0) and for varying times (15-75 min) and Cit, Arg and Orn were analyzed using LC-MSMS, with and without derivatization. In 20 volunteers, the DBS LC-MSMS assay was correlated with a plasma ion exchange HPLC method.

RESULTS

For Citrulline an extraction optimum was obtained at pH 2.6, whereas lower Arginine concentrations were found using low extraction pH. Increasing elution times lead to increased concentrations. Within-run CV was higher with, compared to without derivatization. No close association could be found between plasma HPLC and DBS LC-MSMS concentrations.

CONCLUSION

Analysis of amino acids on DBS using LC-MSMS should be optimized regarding the purpose of the assay. In our study, most optimal results were obtained without derivatization and elution in pH 2.6 for 45 min. Cellular amino acids in DBS might influence the correlation of Cit with severity of enteral disorders. Therefore, further evaluation of DBS Cit as a marker for enteral disorders is warranted.

Plasma citrulline concentration: a reliable marker of small bowel absorptive capacity independent of intestinal inflammation

OBJECTIVES

Postabsorptive plasma citrulline concentration has been proposed as a reliable marker of small bowel absorptive capacity in short bowel patients. The aim of this study was to address the potentially confounding impact of intestinal inflammation.

METHODS

Fifty-five patients were selected according to diagnosis, small bowel length, and degree of bowel inflammation. (a) Crohn's disease (CD) with massive small bowel resection leaving </=50 cm (N = 6), (b) CD with 50-150 cm remaining (N = 9), (c) CD with no resection but active inflammation (high C-reactive protein [CRP] and Crohn's Disease Activity Index [CDAI] >220) (N = 7), (d) CD without resection or active inflammation (normal CRP and CDAI <150) (N = 9), (e) mesenteric infarction (MI) with resection leaving </=50 cm (N = 6), (f) MI leaving 50-150 cm (N = 6), (g) active celiac disease (N = 6), (h) healthy volunteers (N = 6). Postabsorptive fasting plasma citrulline was measured using reverse-phase, high performance liquid chromatography. Absorptive capacity and permeability were also measured after oral sugar-mix ingestion (5 g lactulose, 1 g L-rhamnose, 0.5 g D-xylose).

RESULTS

The plasma citrulline strongly correlated with small bowel length (P < 0.0001) and xylose absorption (P < 0.001). No correlation was found with CDAI, permeability, CRP, albumin, sedimentation rate, white cell count, or platelet count. Citrulline was significantly higher (P < 0.0004) in CD and MI patients with a remnant small bowel length of 50-150 cm (mean 21.0 micromol/L) than in those with length </=50 cm (mean 9.2 micromol/L).

CONCLUSIONS

Plasma citrulline concentration is a simple and reliable surrogate for small bowel absorptive capacity and is not influenced by intestinal inflammation.

Biomarkers for radiation-induced small bowel epithelial damage: An emerging role for plasma Citrulline

Reduction of cancer treatment-induced mucosal injury has been recognized as an important target for improving the therapeutic ratio as well as reducing the economic burden associated with these treatment related sequellae. Clinical studies addressing this issue are hampered by the fact that specific objective parameters, which enable monitoring of damage in routine clinical practice, are lacking. This review summarizes pros and cons of currently available endpoints for intestinal injury. The metabolic background and characteristics of plasma citrulline, a recently investigated biomarker specifically for small intestinal injury, are discussed in more detail.

Role and significance of plasma citrulline in the early phase after small bowel transplantation in pigs

A reliable serological marker of acute cellular rejection (ACR) after small bowel transplantation (SBTx) is still missing. Plasma citrulline level (PCL) reflects the functional integrity of intestinal mucosa which is partially lost during ACR. The aim of our study was to investigate the role of PCL as marker of ACR after SBTx. Eighteen German landrace pigs were used and divided into three groups. Group 1 (G1), autologous SBTx (n = 4) as control; group 2 (G2), allogeneic SBTx without immunosuppression (IS) (n = 7) and group 3 (G3), allogeneic SBTx with IS (n = 7). IS consisted of tacrolimus and steroids without induction treatment. Observation period was 14 days. Mucosal biopsies were obtained intraoperatively and daily using a Thiry-Vella loop. ACR was differentiated into indeterminate, mild, moderate and severe using a standardized grading schema. PCL was measured daily. An ACR onset occurred generally from postoperative day 4 both in G2 and G3 as mild form and developed differently in the two groups: moderate to severe in G2 and indeterminate to mild in G3. A significant decline of PCL occurred only in cases of moderate and severe ACR, but not in cases of indeterminate and mild ACR. The PCL failed as a marker in the early diagnosis of ACR and became reliable only when advanced mucosal damage was present.

The value of plasma citrulline to predict mucosal injury in intestinal allografts

Diagnosis of intestinal transplant rejection depends on clinical assessment, endoscopy and most importantly, histology of intestinal biopsies. Plasma citrulline levels (P-Cit) reflect functional enterocyte mass in nontransplant patients and have been evaluated in two small series after transplant. This study was designed to determine the sensitivity and specificity of P-Cit as diagnostic tool for allograft injury, especially to distinguish between viral enteritis and rejection. We prospectively collected 403 P-Cit samples within 24 h of intestinal biopsy in 49 patients. P-Cit levels were correlated with the mucosal damage and histopathological diagnoses. P-Cit levels in bowels with significant mucosal damage (i.e. moderate or severe rejection, viral enteritis, PTLD, ischemia reperfusion injury, allergic enteritis) were significantly lower than in intestines with no or mild injury (i.e. indeterminate or mild rejection, nonspecific enteritis): 22.9 +/- 15.4 versus 38 +/- 23.2 nmol/mL (p < 0.0001). Sensitivity and specificity of the test were 80% and 58.1% for rejection, and 56.5% and 66% for viral enteritis, thereby unable to distinguish between both entities. In conclusion, P-Cit reflects the extent of mucosal injury regardless of the etiology, but does not seem to be a predictive marker for rejection or viral enteritis, as its values may decline only when diffuse mucosal damage has occurred.

An association of lower serum citrulline levels within 30 days of acute rejection in patients following small intestine transplantation

INTRODUCTION

In a prospective protocol we studied whether serum citrulline level within 30 days of an acute rejection was predictive of the episode.

METHODS

An acute rejection episode was defined as the date of occurrence of any biopsy-proven rejection in which treatment was initiated until two successive biopsies showed no further rejection. We compared the mean citrulline level based on values determined within 30 days of the start of an acute rejection episode with the mean citrulline level measured on the same patient during a rejection-free period. Serum citrulline measurements were available immediately prior to the occurrence of rejection for 22 patients who experienced 37 episodes.

RESULTS

For the 12 episodes of mild rejection, the mean serum citrulline level +/- SE (standard error) was 15.0 + 2.3 micromol/L prior to rejection and 18.8 +/- 2.4 micromol/L during the rejection-free periods. A paired t test of the mean differences was not significant (P = 17). For the 25 episodes of moderate or severe rejection, the mean serum citrulline level was 12.4 +/- 1.1 micromol/L before rejection and 18.8 +/- 2.0 micromol/L during the rejection-free periods. A paired t test of the mean difference was statistically significant (P = .002).

CONCLUSIONS

Although further study of citrulline as a marker for the early detection of acute rejection episodes is needed, our hope is that its use will help to prevent some of these early episodes from evolving into full-blown moderate or severe grades of rejection.

Utilization of Dried Blood Spot Citrulline Level as a Noninvasive Method for Monitoring Graft Function Following Intestinal Transplantation

BACKGROUND

Citrulline concentrations have been proposed as a marker for intestinal allograft rejection. We instituted dried blood spot (DBS) specimen monitoring of citrulline to simplify sample collection posttransplant. This study demonstrates the correlation between plasma and dried blood spot specimen citrulline concentrations after intestinal transplantation.

METHODS

Plasma and DBS samples were analyzed by hydrophilic interaction chromatography tandem mass spectrometry. Comparison of the strength of linear correlation was made according to the type of surgery, sonication time, DBS citrulline levels, and the time interval between the blood sample collection and the assay date.

RESULTS

A very strong linear correlation exists between the plasma and DBS citrulline concentrations (r=0.87; P<0.001). The correlation between plasma and DBS citrulline concentrations was maintained when evaluating only the intestinal transplant recipients. There was no significant difference in the strength of linear correlation according to sonication time, cirtrulline concentrations, or length of time to assay date.

CONCLUSIONS

DBS citrulline monitoring will ease sample collection following intestinal transplantation and improve the ability to detect intestinal dysfunction and rejection by a noninvasive means.

Almost all about citrulline in mammals

Citrulline (Cit, C6H13N3O3), which is a ubiquitous amino acid in mammals, is strongly related to arginine. Citrulline metabolism in mammals is divided into two fields: free citrulline and citrullinated proteins. Free citrulline metabolism involves three key enzymes: NO synthase (NOS) and ornithine carbamoyltransferase (OCT) which produce citrulline, and argininosuccinate synthetase (ASS) that converts it into argininosuccinate. The tissue distribution of these enzymes distinguishes three "orthogonal" metabolic pathways for citrulline. Firstly, in the liver, citrulline is locally synthesized by OCT and metabolized by ASS for urea production. Secondly, in most of the tissues producing NO, citrulline is recycled into arginine via ASS to increase arginine availability for NO production. Thirdly, citrulline is synthesized in the gut from glutamine (with OCT), released into the blood and converted back into arginine in the kidneys (by ASS); in this pathway, circulating citrulline is in fact a masked form of arginine to avoid liver captation. Each of these pathways has related pathologies and, even more interestingly, citrulline could potentially be used to monitor or treat some of these pathologies. Citrulline has long been administered in the treatment of inherited urea cycle disorders, and recent studies suggest that citrulline may be used to control the production of NO. Recently, citrulline was demonstrated as a potentially useful marker of short bowel function in a wide range of pathologies. One of the most promising research directions deals with the administration of citrulline as a more efficient alternative to arginine, especially against underlying splanchnic sequestration of amino acids. Protein citrullination results from post-translational modification of arginine; that occurs mainly in keratinization-related proteins and myelins, and insufficiencies in this citrullination occur in some auto-immune diseases such as rheumatoid arthritis, psoriasis or multiple sclerosis.

Serum citrulline levels correlate with enteral tolerance and bowel length in infants with short bowel syndrome

OBJECTIVE

To determine if serum levels of CIT (a nonprotein amino acid synthesized by the intestine) correlate with total parenteral nutrition (PN)-independence in children with short bowel syndrome (SBS).

STUDY DESIGN

We prospectively obtained serum amino acid profiles over a 24-month interval from all infants with SBS 3 weeks to 4 years of age. Remaining small intestine length was recorded at surgery, and percent enteral calories tolerated (enteral calories divided by enteral plus parenteral calories x 100) was determined in 24 infants with SBS and 21 age-matched controls (blood drawn for non-gastrointestinal symptoms).

RESULTS

Mean CIT for controls was 31 +/- 2 micromol/L. In patients with SBS (n = 24), serum CIT correlated linearly with percent enteral calories (R = 0.85; P <.001) and with bowel length (R = 0.47; P < or =.03). CIT level in patients with SBS weaned off PN was 30 +/- 2 micromol/L; in those subsequently weaned off PN, 20 +/- 2 micromol/L; and in those who would remain PN-dependent, 11 +/- 2 micromol/L ( P < or =.01). Serum CIT > or =19 micromol/L had 94% sensitivity and 67% specificity for being off or coming off total PN.

CONCLUSIONS

Serum CIT level >19 micromol/L in children with SBS is associated with development of enteral tolerance and may be a useful predictive test.

Hypocitrullinemia in patients with MELAS: an insight into the “MELAS paradox”

L-citrulline, classified as a nonessential amino acid, is synthesized predominantly via Delta-1-pyrroline carboxylate synthase in the endothelial cells of the small intestine. In mammals, small quantities of citrulline are also produced in nitric oxide synthase-expressing cells. Considering the fact that the enzymes involved in the endogenous synthesis of L-citrulline are all located in the mitochondria and the fact that citrulline is a component of the citrulline-nitric oxide (NO) cycle, we hypothesized that the distinct clinical, biochemical, and morphological characteristics of MELAS, a maternally inherited mitochondrial disorder, might be due to alterations in nitric oxide homeostasis. Analysis of serum from MELAS patients showed that levels of plasma arginine were similar in both patients and in controls. However, levels of citrulline in MELAS patients were significantly lower than in controls, and there was a clear inverse correlation between arginine and citrulline levels in these patients. We found no correlation between the level of heteroplasmy and the plasma levels of either arginine or citrulline. We discuss the depressed citrulline levels in MELAS patients, who have an unusual and paradoxical pattern of vascular respiratory chain expression, in the context of NO homeostasis.

Monitoring myeloablative therapy-induced small bowel toxicity by serum citrulline concentration: a comparison with sugar permeability tests

BACKGROUND

Intestinal mucositis is an important cause of cancer treatment-related morbidity and mortality, carrying a serious economic burden. Currently, objective parameters are lacking that would enable the monitoring of gut damage in routine clinical practice, thus hindering the development of clinical studies designed to investigate potential new strategies aimed at reducing or preventing this side effect. The authors investigated the characteristics of serum citrulline concentration compared with sugar permeability tests with respect to its use as a marker for cancer treatment-induced small bowel injury.

METHODS

In this prospective study, 10 patients with hematologic malignancies who were receiving myeloablative therapy had gut toxicity assessed with sugar permeability tests. Serum citrulline concentrations also were determined using archival serum samples. The association between both parameters and their respective characteristics were analyzed and compared with data from the literature.

RESULTS

Sensitivity and specificity were better for the citrulline assay compared with sugar permeability tests. Maximum gut damage assessed with the citrulline assay was observed 1-2 weeks earlier compared with the sugar permeability test. Similarly, citrulline indicated recovery of gut damage at 3 weeks after transplantation, whereas most sugar permeability tests remained abnormal.

CONCLUSIONS

The simplicity of the method, the low costs, and the lack of drawbacks to the method make the citrulline assay the first choice for measuring and monitoring treatment-related gut damage and provides an objective parameter for cancer treatment-related gut toxicity.

Plasma citrulline concentration: a surrogate end point for radiation-induced mucosal atrophy of the small bowel. A feasibility study in 23 patients

PURPOSE

Plasma citrulline, a nitrogen end product of glutamine metabolism in small-bowel enterocytes, was suggested as a marker of radiation-induced small-bowel epithelial cell loss in mice after single-dose whole-body irradiation. Our objective was to evaluate the feasibility of citrulline as a marker for radiation-induced small-intestinal mucosal atrophy in patients during and after abdominal fractionated radiotherapy.

METHODS AND MATERIALS

Twenty-three patients were studied weekly during treatment and at intervals of 2 weeks and 3 and 6 months after treatment by postabsorptive plasma citrulline concentration and clinical toxicity grading. The interrelationship between these variables and the correlation with small-bowel dose and volume parameters were investigated.

RESULTS

During fractionated radiotherapy, citrulline concentration significantly decreased as a function of the radiation dose (p < 0.001) and the volume of small bowel treated (p = 0.001). The plasma citrulline concentration correlated with clinical toxicity during the last 3 weeks of treatment. As a whole, citrulline concentration correlated better with radiation dose and volume parameters than clinical toxicity grading.

CONCLUSIONS

In patients treated with fractionated radiation therapy for abdominal or pelvic cancer sites, plasma citrulline concentration may be a simple objective marker for monitoring epithelial cell loss, a major event in acute radiation-induced small-bowel toxicity.

The metabolic basis of arginine nutrition and pharmacotherapy

As an essential precursor for the synthesis of proteins and other molecules with enormous biological importance (including nitric oxide, urea, ornithine, proline, polyamines, glutamate, creatine, agmatine, and dimethylarginines), arginine displays remarkable metabolic and regulatory versatility. Evidence available to date provides a sound reason to classify arginine as an essential amino acid for young mammals (including parenterally fed human infants) and as a conditionally essential amino acid for adults under such conditions as trauma, burn injury, massive small-bowel resection, and renal failure. Arginine administration reverses endothelial dysfunction, enhances wound healing, prevents the early stages of tumorigenesis, and improves cardiovascular, reproductive, pulmonary, renal, digestive, and immune functions. Arginine or its effective precursor citrulline may hold great promise as a nutritional or pharmacotherapeutic treatment for a wide array of human diseases.