Analysis of 2-methylcitric acid, methylmalonic acid, and total homocysteine in dried blood spots by LC-MS/MS for application in the newborn screening laboratory: A dual derivatization approach

Inborn errors of propionate, cobalamin and methionine metabolism are targets for Newborn Screening (NBS) in most programs world-wide, and are primarily screened by analyzing for propionyl carnitine (C3) and methionine in dried blood spot (DBS) cards using tandem mass spectrometry (MS/MS). Single-tier NBS approaches using C3 and methionine alone lack specificity, which can lead to an increased false-positive rate if conservative cut-offs are applied to minimize the risk of missing cases. Implementation of liquid chromatography tandem mass spectrometry (LC-MS/MS) second-tier testing for 2-methylcitric acid (MCA), methylmalonic acid (MMA), and homocysteine (HCY) from the same DBS card can improve disease screening performance by reducing the false-positive rate and eliminating the need for repeat specimen collection. However, DBS analysis of MCA, MMA, and HCY by LC-MS/MS is challenging due to limited specimen size and analyte characteristics leading to a combination of low MS/MS sensitivity and poor reverse-phase chromatographic retention. Sufficient MS response and analytical performance can be achieved for MCA by amidation using DAABD-AE and by butylation for MMA and HCY. Herein we describe the validation of a second-tier dual derivatization LC-MS/MS approach to detect elevated MCA, MMA, and HCY in DBS cards for NBS. Clinical utility was demonstrated by retrospective analysis of specimens, an interlaboratory method comparison, and assessment of external proficiency samples. Imprecision was <10.8% CV, with analyte recoveries between 90.2 and 109.4%. Workflows and analytical performance characteristics of this second-tier LC-MS/MS approach are amenable to implementation in the NBS laboratory.

Update on newborn dried bloodspot testing for cerebrotendinous xanthomatosis: An available high-throughput liquid-chromatography tandem mass spectrometry method

BACKGROUND

Cerebrotendinous xanthomatosis (CTX) is a rare genetic disorder of bile acid synthesis that can cause progressive neurological damage and premature death. Detection of CTX in the newborn period would be beneficial since an effective treatment is available. We previously described a liquid chromatography-electrospray ionization-tandem mass spectrometry (LC-ESI-MS/MS) test with potential to screen newborn dried bloodspots (DBS) for CTX. We report here modifications to the methodology and application of the modified test to analysis of DBS from a CTX-affected and unaffected newborns.

METHODS

The testing methodology utilizes keto derivatization to enable sensitive LC-ESI-MS/MS measurement of elevated 7α,12α-dihydroxy-4-cholesten-3-one (7α12αC4) in CTX newborn DBS. We report here method modifications, including use of a DBS extraction procedure used in newborn screening laboratories and a reduced analysis time of 2 min per sample.

RESULTS

Rapid isotope-dilution LC-ESI/MS/MS quantification of the ketosterol bile acid precursor 7α12αC4 provides a test that could readily discriminate a CTX positive newborn DBS sample (with a concentration of 104.4 ng/ml) from unaffected newborn samples (with a mean concentration of 4.1 ± 3.4 ng/ml; range 0.2-15.6 ng/ml, n = 39) analyzed in a blinded manner.

CONCLUSIONS

We provide additional evidence suggesting 7α12αC4 may be a promising test marker to screen newborn DBS for CTX. Early detection and intervention through newborn screening would greatly benefit those affected with CTX, preventing morbidity and mortality.

An integrated Taguchi and response surface methodological approach for the optimization of an HPLC method to determine glimepiride in a supersaturatable self-nanoemulsifying formulation

We studied the application of Taguchi orthogonal array (TOA) design during the development of an isocratic stability indicating HPLC method for glimepiride as per TOA design; twenty-seven experiments were conducted by varying six chromatographic factors. Percentage of organic phase was the most significant (p < 0.001) on retention time, while buffer pH had the most significant (p < 0.001) effect on tailing factor and theoretical plates. TOA design has shortcoming, which identifies the only linear effect, while ignoring the quadratic and interaction effects. Hence, a response surface model for each response was created including the linear, quadratic and interaction terms. The developed models for each response found to be well predictive bearing an acceptable adjusted correlation coefficient (0.9152 for retention time, 0.8985 for tailing factor and 0.8679 for theoretical plates). The models were found to be significant (p < 0.001) having a high F value for each response (15.76 for retention time, 13.12 for tailing factor and 9.99 for theoretical plates). The optimal chromatographic condition uses acetonitrile - potassium dihydrogen phosphate (pH 4.0; 30 mM) (50:50, v/v) as the mobile phase. The temperature, flow rate and injection volume were selected as 35 ± 2 °C, 1.0 mL min(-1) and 20 μL respectively. The method was validated as per ICH guidelines and was found to be specific for analyzing glimepiride from a novel supersaturatable self-nanoemulsifying formulation.

Critical role of bioanalytical strategies in investigation of clinical PK observations, a Phase I case study

RG7652 is a human immunoglobulin 1 (IgG1) monoclonal antibody (mAb) targeting proprotein convertase subtilisin/kexin type 9 (PCSK9) and is designed for the treatment of hypercholesterolemia. A target-binding enzyme-linked immunosorbent assay (ELISA) was developed to measure RG7652 levels in human serum in a Phase I study. Although target-binding assay formats are generally used to quantify free therapeutic, the actual therapeutic species being measured are affected by assay conditions, such as sample dilution and incubation time, and levels of soluble target in the samples. Therefore, in the presence of high concentrations of circulating target, the choice of reagents and assay conditions can have a significant effect on the observed pharmacokinetic (PK) profiles. Phase I RG7652 PK analysis using the ELISA data resulted in a nonlinear dose normalized exposure. An investigation was conducted to characterize the ELISA to determine whether the assay format and reagents may have contributed to the PK observation. In addition, to confirm the ELISA results, a second orthogonal method, liquid chromatography tandem mass spectrometry (LC-MS/MS) using a signature peptide as surrogate, was developed and implemented. A subset of PK samples, randomly selected from half of the subjects in the 6 single ascending dose (SAD) cohorts in the Phase I clinical study, was analyzed with the LC-MS/MS assay, and the data were found to be comparable to the ELISA data. This paper illustrates the importance of reagent characterization, as well as the benefits of using an orthogonal approach to eliminate bioanalytical contributions when encountering unexpected observations.

Acute exacerbation of chronic hepatitis B: the dilemma of differentiation from acute viral hepatitis B

Exacerbations of chronic hepatitis B are common in endemic countries. Acute exacerbation of chronic hepatitis B virus (CHB-AE) causing derangement of liver functions may be seen in a flare of HBV in immune clearance phase or as a reactivation of HBV in patients with inactive or resolved HBV infection. While reactivation of HBV is usually seen in HBsAg positive patients, it is being increasingly recognized in patients with apparently resolved HBV infection who do not have HBsAg in serum but have IgG antibody to core antigen (anti-HBc) in the serum, especially so in patients on chemotherapy, immunosuppressive therapy or undergoing hematopoietic stem cell transplantation. In an icteric patient who is HBsAg positive, it may be difficult to differentiate CHB-AE from acute viral hepatitis B (AVH-B). Both may have similar clinical presentation and even IgM anti-HBc, the traditional diagnostic marker of AVH-B, may also appear at the time of exacerbation of CHB. The differentiation between CHB-AE and AVH-B is important not only for prognostication but also because management strategies are different. Most cases of AVH-B will resolve on their own, HBsAg clearance is achieved spontaneously in 90-95% of adults and treatment is rarely indicated except in the few with severe/fulminant disease. In contrast, in CHB-AE, the onset of jaundice may lead to decompensation of liver disease and treatment is warranted. The mechanisms of acute exacerbation and the differentiating features between AVH-B and CHB-AE are reviewed.