Citrulline levels in a paediatric age group: does measurement on dried blood spots have additional value?

L-Citrulline in Neonates: From Bench to Bed Side

L-citrulline (L-CIT), a precursor to L-arginine (L-ARG), is a key contributor to the nitric oxide (NO) signaling pathway. Endothelial dysfunction, characterized by deficient nitric oxide synthesis, is implicated in the pathogenesis of various neonatal conditions such as necrotizing enterocolitis (NEC) and bronchopulmonary dysplasia (BPD) associated pulmonary hypertension (PH). This review summarizes the current evidence around the possible role of L-CIT supplementation in the treatment of these conditions. Detoxification of endogenously produced superoxide radicals is inadequate in preterm infants due to immature antioxidants that leads to the production of peroxynitrite, a reactive oxygen-free radical that is cytotoxic and causes damage to organelles and cellular membranes, further disrupting the coupling of endothelial NO synthase enzyme and the generation of high levels of reactive nitrogen and oxygen species. Animal studies in lipopolysaccharide-induced models of chorioamnionitis and hyperoxia- and inflammation-induced BPD-PH in rodent lung models revealed that L-CIT supplementation significantly mitigated structural changes in the pulmonary vasculature, preserved alveolar growth, and increased vascular endothelial growth factor gene expression, highlighting the anti-inflammatory and antioxidant effects of L-CIT supplementation. Similar benefits were noted in newborn piglet models of chronic hypoxia-induced PH and NEC. Pharmacokinetic studies in neonates have shown doses of 100-300 mg/kg/day to be safe and well tolerated. A few studies have shown the beneficial effects of L-CIT supplementation in pulmonary hypertension secondary to congenital heart disease, but evidence of efficacy in the neonatal population is lacking. While L-CIT shows promise in the treatment of various neonatal conditions, adequately powered studies to evaluate the safety and efficacy of L-CIT supplementation post-surgical NEC and BPD ± PH in the extremely preterm population are needed to translate this novel therapy to clinical practice.

Multi-dose enteral L-citrulline administration in premature infants at risk of developing pulmonary hypertension associated with bronchopulmonary dysplasia

OBJECTIVE

Information is needed to guide the design of randomized controlled trials (RCTs) evaluating L-citrulline therapy for premature infants with pulmonary hypertension associated with bronchopulmonary dysplasia (BPD-PH). Based on our single-dose pharmacokinetic study, we evaluated the ability of a multi-dose enteral L-citrulline strategy to achieve a target trough steady-state L-citrulline plasma concentration and its tolerability in premature infants.

STUDY DESIGN

Plasma L-citrulline concentrations were measured in six premature infants receiving 60 mg/kg L-citrulline every 6 h for 72 h before the first and last L-citrulline doses. L-citrulline concentrations were compared to concentration-time profiles from our previous study.

RESULTS

Target trough plasma L-citrulline concentrations were achieved in 2/6 subjects. No serious adverse events occurred.

CONCLUSIONS

Multi-dose L-citrulline was well tolerated. These results will assist in the design of phase II RCTs evaluating L-citrulline dosage strategies to achieve target plasma L-citrulline concentrations in infants at risk for BPD-PH.

CLINICAL TRIALS

gov ID: NCT03542812.

Multi-dose enteral L-citrulline administration in premature infants at risk of developing pulmonary hypertension associated with bronchopulmonary dysplasia

Objective

Information is needed to guide the design of randomized controlled trials (RCTs) evaluating L-citrulline as a therapy for premature infants with pulmonary hypertension associated with bronchopulmonary dysplasia (BPD-PH). Our goal was to evaluate the tolerability and ability to achieve a target steady-state L-citrulline plasma concentration in prematures treated enterally with a multi-dose L-citrulline strategy based on our single-dose pharmacokinetic study.

Study Design

Six prematures received 60 mg/kg of L-citrulline every 6 hours for 72 hours. Plasma L-citrulline concentrations were measured before the first and last L-citrulline doses. L-citrulline concentrations were compared to concentration-time profiles from our previous study.

Results

Plasma L-citrulline concentrations agreed with the simulated concentration-time profiles. No serious adverse events occurred.

Conclusions

Simulations based on single-doses can be used to predict target multi-dose plasma L-citrulline concentrations. These results assist the design of RCTs evaluating the safety and effectiveness of L-citrulline therapy for BPD-PH. Clinical trials.gov ID: NCT03542812.

Dried blood spot reference intervals for 18 amino acids in a 1 to 6 years old pediatric cohort

OBJECTIVES

Reference intervals are indispensable for the accurate clinical interpretation of clinical laboratory tests. The reference intervals of amino acids in dried blood spots (DBS) from nonnewborn children are limited. In this study, we aim to establish the pediatric reference intervals for amino acids in DBS from healthy Chinese children aged from 1 to 6 years and to investigate the effect of sex and age.

DESIGN AND METHODS

In 301 healthy subjects aged from 1 to 6 years old, eighteen DBS amino acids were determined using ultra-performance chromatography-tandem mass spectrometry method. Amino acid concentrations were examined in relation to sex and age. Reference intervals were established according to the CLSI C28-A3 guidelines.

RESULTS

Reference intervals of 18 amino acids bounded by the 2.5 and 97.5 percentiles were calculated in DBS specimen. No significant influence of age was observed for all the target amino acid concentrations in 1- to 6-year-olds. Sex differences were found in leucine and aspartic acid.

CONCLUSIONS

The RIs established in the present study added value for diagnosing and managing the amino acid-related diseases in the pediatric population.

Impact of l-citrulline on nitric oxide signaling and arginase activity in hypoxic human pulmonary artery endothelial cells

Impaired nitric oxide (NO) signaling contributes to the development of pulmonary hypertension (PH). The l-arginine precursor, l-citrulline, improves NO signaling and has therapeutic potential in PH. However, there is evidence that l-citrulline might increase arginase activity, which in turn, has been shown to contribute to PH. Our major purpose was to determine if l-citrulline increases arginase activity in hypoxic human pulmonary artery endothelial cells (PAECs). In addition, to avoid potential adverse effects from high dose l-citrulline monotherapy, we evaluated whether the effect on NO signaling is greater using co-treatment with l-citrulline and another agent that improves NO signaling, folic acid, than either alone. Arginase activity was measured in human PAECs cultured under hypoxic conditions in the presence of l-citrulline (0-1 mM). NO production and endothelial nitric oxide synthase (eNOS) coupling, as assessed by eNOS dimer-to-monomer ratios, were measured in PAECs treated with l-citrulline and/or folic acid (0.2 μM). Arginase activity increased in hypoxic PAECs treated with 1 mM but not with either 0.05 or 0.1 mM l-citrulline. Co-treatment with folic acid and 0.1 mM l-citrulline increased NO production and eNOS dimer-to-monomer ratios more than treatment with either alone. The potential to increase arginase activity suggests that there might be plasma l-citrulline concentrations that should not be exceeded when using l-citrulline to treat PH. Rather than progressively increasing the dose of l-citrulline as a monotherapy, co-therapy with l-citrulline and folic acid merits consideration, due to the possibility of achieving efficacy at lower doses and minimizing side effects.

Pharmacokinetics of L-Citrulline in Neonates at Risk of Developing Bronchopulmonary Dysplasia-Associated Pulmonary Hypertension

BACKGROUND

Options to treat pulmonary hypertension (PH) in neonates with bronchopulmonary dysplasia (BPD) are few and largely ineffective. Improving the bioavailability of nitric oxide (NO) might be an efficacious treatment for BPD-PH. When administered orally, the NO-L-arginine precursor, L-citrulline, increases NO production in children and adults, however, pharmacokinetic (PK) studies of oral L-citrulline have not been performed in infants and children.

OBJECTIVES

This study characterized the PK of enterally administered L-citrulline in neonates at risk of developing BPD-PH to devise a model-informed dosing strategy.

METHODS AND RESULTS

Ten premature neonates (≤ 28 weeks gestation) were administered a single dose of 150 mg/kg (powder form solubilized in sterile water) oral L-citrulline at 32 ± 1 weeks postmenstrual age. Due to the need to limit blood draws, time windows were used to maximize the sampling over the dosing interval by assigning neonates to one of two groups (ii) samples collected pre-dose and at 1- and 2.5-h post-dose, and (ii) pre-dose and 0.25- and 3-h post-dose. The L-arginine concentrations (µmol/L) and the L-citrulline (µmol/L) plasma concentration-time data were evaluated using non-compartmental analysis (Phoenix WinNonlin version 8.1). Optimal dosage strategies were derived using a simulation-based methodology. Simulated doses of 51.5 mg or 37.5 mg/kg given four times a day produced steady-state concentrations close to a target of 50 µmol/L. The volume of distribution (V/F) and clearance (CL/F) were 302.89 ml and 774.96 ml/h, respectively, with the drug exhibiting a half-life of 16 minutes. The AUC from the time of dosing to the time of last concentration was 1473.3 h*μmol/L, with Cmax and Tmax of 799 μmol/L and 1.55 h, respectively.

CONCLUSION

This is the first PK study in neonates presenting data that can be used to inform dosing strategies in future randomized controlled trials evaluating enteral L-citrulline as a potential treatment to reduce PH associated with BPD in premature neonates.

REGISTRATION

Clinical trials.gov Identifier: NCT03542812.

Association of zinc deficiency with infectious complications in pediatric hematopoietic stem cell transplantation patients

BACKGROUND

Zinc plays essential roles in immune function and epithelial integrity. Patients undergoing hematopoietic stem cell transplantation (HSCT) often have low plasma zinc levels because of poor intake and diarrhea. We hypothesized that patients with zinc deficiency before HSCT had worse infectious complications after HSCT compared with patients with normal zinc levels. Citrulline, a marker of intestinal integrity, was also hypothesized to be lower in patients with zinc deficiency.

PATIENTS AND METHODS

Thirty patients undergoing HSCT at Ramathibodi Hospital during March 2020-September 2021 were enrolled. Blood samples for plasma zinc and citrulline were collected during the HSCT period. The 14- and 90-day outcomes after HSCT were prospectively recorded.

RESULTS

Twelve of 30 (40%) patients had zinc deficiency before HSCT. Zinc-deficient patients were younger (median (interquartile range): 6 (8.8) vs 13 (5.8) years old; p = 0.017). Zinc levels tended to increase after admission in both groups. Patients with zinc deficiency had lower citrulline levels than those with normal zinc levels. Citrulline levels decreased in both groups after stem cell infusion, and the level was not significantly different between the two groups. Zinc-deficient patients had a higher rate of bacterial infection within 90 days after HSCT than those with normal zinc levels (6 in 12 patients (50.0%) vs 1 in 18 patients (5.6%); odds ratio [OR]: 17.0; 95% confidence interval [CI]: 1.68-171.70; p = 0.016). This remained significant after adjustments for age (adjusted OR: 12.31; 95% CI: 1.084-139.92; p = 0.043).

CONCLUSION

The prevalence of zinc deficiency in pediatric patients undergoing HSCT was high. Zinc-deficient patients had lower citrulline levels and higher incidence of bacterial infection after HSCT. However, citrulline level was not different between patients with and without bacterial infections. It is worth to investigate whether zinc supplementation before HSCT can reduce bacterial infection after HSCT.

Plasma Citrulline as a Biomarker for Early Diagnosis of Necrotizing Enterocolitis in Preterm Infants

OBJECTIVE

Citrulline synthesized by healthy enterocytes and decreases with injury. This work aimed to study plasma citrulline concentrations (CITs) as a biomarker to differentiate among infants presenting with early nonspecific signs and symptoms of necrotizing enterocolitis (NEC) with those who will develop NEC. Further to study the correlation between posttreatment CIT with time to full feeds (TTFF) and length of stay (LOS).

STUDY DESIGN

This is a prospective study which included infants < 32 weeks gestational age (GA) with 9 infants each in Group 1 (stage 2/3 NEC), Group 2 (with stage 1 NEC-like presentation), and Group 3 (healthy GA-matched infants). CIT was measured in Groups 1 and 2 within 24 hours of presentation and again in Group 1 after treatment.

RESULTS

The three groups were similar in clinical characteristics. Median CIT (µmol/L) in Group 1 (15.4 [interquartile range, IQR: 7.3-18.0]) was lower than Group 2 (22.2 [IQR: 18.3-27.3], p = 0.02) and Group 3 (24.9 [IQR: 19.8-31.9], p = 0.009). Posttreatment CIT in Group 1 did not correlate with TTFF (r = 0.15; p = 0.69) and LOS (r =  - 0.33; p = 0.38).

CONCLUSION

CIT was lower in infants with NEC as compared with healthy controls and those infants with nonspecific signs of NEC. CIT after treatment does not correlate with TTFF and LOS.

KEY POINTS

· Citrulline is produced by enterocytes.. · It is decreased in infants with necrotizing enterocolitis early in disease.. · It can be used as a biomarker for early diagnosis of necrotizing enterocolitis..

Use of serum citrulline concentrations from routine newborn screen as a biomarker for necrotizing enterocolitis

PURPOSE

Necrotizing enterocolitis (NEC), a leading cause of mortality and morbidity in preterm neonates, lacks a reliable biomarker. Citrulline is primarily produced by enterocytes and correlates with intestinal function. Serum citrulline concentration (CIT) is routinely measured in routine newborn screening (NBS). The purpose of the study is to test if CIT from NBS may predict the occurrence of NEC and whether it correlates with the time to full feeds (TTFF) and length of stay (LOS), serving as a biomarker of NEC and intestinal health.

METHODS

In a retrospective case control study conducted on neonates with gestational age of 26-32 weeks, we compared CIT levels between cases (neonates with NEC) and controls (next-born neonate). NBS was collected within first 24 h, at day 5 and when the neonates achieved full feeds and were compared using non-parametric tests.

RESULTS

There was no difference in CIT between the controls and cases on day 1 [11.42 (7.42-14.84 vs. 11.93 (6.85-18.8) µmol/L, p = 0.55], on day 5 [11.99 (7.99-16.55) vs. 13.70 (7.42-26.83) µmol/L, p = 0.05], or at full feeds [14.86 (6.85-25.69) vs. 15.7 (7.42-26.26) µmol/L, p = 0.87]. CIT on day 1 did not correlate with TTFF (r = 0.08, p = 0.53) or LOS (r = 0.23, p = 0.06), respectively).

CONCLUSIONS

CIT from routine NBS does not serve as a biomarker to predict NEC in preterm neonates.

Reduced early dried blood spot citrulline levels in preterm infants with meconium obstruction of prematurity

BACKGROUND

Citrulline is a non-protein amino acid synthesized in the enterocytes of the small bowel. Recent studies have reported that plasma citrulline levels correlate with functional enterocyte mass.

AIMS

This study aimed to determine the normal dried blood spot (DBS) citrulline levels and to determine the existence of a correlation between citrulline levels and meconium obstruction of prematurity (MOP).

STUDY DESIGN AND SUBJECTS

A retrospective cohort study was performed involving 285 infants born at less than 32weeks gestation who were admitted to the neonatal intensive care unit between Oct 2009 and Aug 2014.

OUTCOME MEASURES

We analyzed the DBS citrulline levels, which are routinely measured via newborn screening at 7days following birth, using liquid chromatography-MS/MS. We investigated the relationship between DBS citrulline levels and clinical parameters such as gestational age (GA), body measurements at birth, gender, or the presence or absence of either necrotizing enterocolitis or MOP.

RESULTS

A total of 229 infants with a median GA of 29.6weeks and a median birth weight of 1160g were included. DBS citrulline levels were not associated with GA, body measurements at birth or gender. DBS citrulline levels were significantly decreased when patients presented with MOP (p=0.037).

CONCLUSIONS

Early DBS citrulline levels were not associated with either GA or body measurements at birth but were reduced among preterm infants with MOP compared with the control infants. These results may be indicative of abnormal fetal intestinal development and reduced functional enterocyte mass among preterm infants with MOP.

High Protein Diet and Huntington's Disease

Huntington's disease (HD) is a neurodegenerative disorder caused by the huntingtin (HTT) gene with expanded CAG repeats. In addition to the apparent brain abnormalities, impairments also occur in peripheral tissues. We previously reported that mutant Huntingtin (mHTT) exists in the liver and causes urea cycle deficiency. A low protein diet (17%) restores urea cycle activity and ameliorates symptoms in HD model mice. It remains unknown whether the dietary protein content should be monitored closely in HD patients because the normal protein consumption is lower in humans (~15% of total calories) than in mice (~22%). We assessed whether dietary protein content affects the urea cycle in HD patients. Thirty HD patients were hospitalized and received a standard protein diet (13.7% protein) for 5 days, followed by a high protein diet (HPD, 26.3% protein) for another 5 days. Urea cycle deficiency was monitored by the blood levels of citrulline and ammonia. HD progression was determined by the Unified Huntington's Disease Rating Scale (UHDRS). The HPD increased blood citrulline concentration from 15.19 μmol/l to 16.30 μmol/l (p = 0.0378) in HD patients but did not change blood ammonia concentration. A 2-year pilot study of 14 HD patients found no significant correlation between blood citrulline concentration and HD progression. Our results indicated a short period of the HPD did not markedly compromise urea cycle function. Blood citrulline concentration is not a reliable biomarker of HD progression.

Citrulline concentration in routinely collected neonatal dried blood spots cannot be used to predict necrotising enterocolitis

AIM

Low citrulline concentration is a marker of low functional enterocyte mass, which may predispose neonates to necrotising enterocolitis (NEC). We hypothesised that citrulline could be used to assess the NEC risk that could not be accounted for by gestational age and birthweight. This study investigated whether citrulline concentrations routinely measured in neonatal dried blood spots (DBS) could predict NEC.

METHODS

We used national Danish registries to retrospectively identify all 361 babies born between 2003 and 2009 who were diagnosed with NEC and had a valid citrulline concentration measured from a DBS sample. The control group comprised 1083 healthy newborns, with three controls for every newborn with NEC, matched for birthweight and gestational age.

RESULTS

Neonatal dried blood spots were collected between 2 and 21 days of life, with a median of 8 days. The results showed that NEC was not associated with low citrulline concentration, either in a direct comparison between the NEC and control groups or in a multivariate logistic regression (p = 0.73).

CONCLUSION

The findings of this study show that the citrulline concentrations found in routine DBS samples between 2003 and 2009 did not predict NEC in newborn babies.

Reduced plasma citrulline levels in low birth weight infants with necrotizing enterocolitis

BACKGROUND

Citrulline, a nonprotein amino acid, is an intermediate of the urea cycle and synthesized in small intestine. Lower plasma citrulline levels were associated with reduced function of enterocytes. Necrotizing enterocolitis (NEC) causes high morbidity and mortality, and leads impaired intestinal functions.

METHODS

Plasma citrulline levels of neonates with a gestational age <32 weeks and ≤1,500 gm who developed NEC stage II/III were measured by high-performance liquid chromatography.

RESULTS

We enrolled 36 preterm infants including 20 with NEC and 16 controls. Median citrulline levels of NEC and control groups were 8.6 and 20.18 μmol/l (P < 0.05), and cut off level of citrulline was 13.15 μmol/l with a sensitivity of 80% and a specificity of 82%. Median arginine levels of NEC and control groups were 22.02 and 39.89 μmol/l (P < 0.05), and cut off level of arginine was 28.52 μmol/l with a sensitivity of 70% and a specificity of 75%. Blood sampling day, gender, parenteral, and enteral nutrition did not affect the amino acid levels.

CONCLUSION

We found lower plasma citrulline and arginine levels in preterm infants with NEC. Further studies are needed to determine most appropriate levels to predict recovery and prognosis of NEC, and treatment options with these amino acids in preterm infants.