Exploring preferred binding domains of IgG1 mAbs to multimodal adsorbents using a combined biophysics and simulation approach

In this work, we employ a recently developed biophysical technique that uses diethylpyrocarbonate (DEPC) covalent labeling and mass spectrometry for the identification of mAb binding patches to two multimodal cation exchange resins at different pH. This approach compares the labeling results obtained in the bound and unbound states to identify residues that are sterically shielded and thus located in the mAb binding domains. The results at pH 6 for one mAb (mAb B) indicated that while the complementarity determining region (CDR) had minimal interactions with both resins, the FC domain was actively involved in binding. In contrast, DEPC/MS data with another mAb (mAb C) indicated that both the CDR and FC domains were actively involved in binding. These results corroborated chromatographic retention data with these two mAbs and their fragments and helped to explain the significantly stronger retention of both the intact mAb C and its Fab fragment. In contrast, labeling results with mAb C at pH 7, indicated that only the CDR played a significant role in resin binding, again corroborating chromatographic data. The binding domains identified from the DEPC/MS experiments were also examined using protein surface hydrophobicity maps obtained using a recently developed sparse sampling molecular dynamics (MD) approach in concert with electrostatic potential maps. These results demonstrate that the DEPC covalent labeling/mass spectrometry technique can provide important information about the domain contributions of multidomain proteins such as monoclonal antibodies when interacting with multimodal resins over a range of pH conditions.

Standardisation of ACPA tests: evaluation of a new candidate reference preparation

INTRODUCTION

Commercial assays measuring antibodies to citrullinated protein/peptide (ACPA) show poor quantitative agreement. The diagnostic industry has never adopted the International Union of Immunological Societies-Centers for Disease Control and Prevention (IUIS-CDC) ACPA reference standard. Recently, the National Institute for Biological Standards and Control (NIBSC) prepared a new candidate ACPA standard (18/204). We evaluated both reference materials using different commercially available ACPA assays.

MATERIALS AND METHODS

This is an international study in which the NIBSC candidate ACPA standard and the IUIS-CDC ACPA reference material were analysed together with 398 diagnostic samples from individuals with rheumatoid arthritis (RA) and in 1073 individuals who did not have RA using nine commercial ACPA assays.

RESULTS

For both reference materials and samples from individuals with RA and individuals who did not have RA, there were large differences in quantitative ACPA results between assays. For most assays, values for the IUIS-CDC standard were lower than values for NIBSC 18/204 and the IUIS-CDC/NIBSC ratio was comparable for several, but not all assays. When NIBSC 18/204 was used as a calibrator, an improvement in alignment of ACPA results across several of the evaluated assays was obtained. Moreover, NIBSC 18/204 could align clinical interpretation for some but not all assays.

CONCLUSION

Adoption of an international standard for ACPA determination is highly desirable. The candidate NIBSC 18/204 standard improved the standardisation and alignment of most ACPA assays and might therefore be recommended to be used as reference in commercial assays.

Performance characteristics of the first Food and Drug Administration (FDA)-cleared digital droplet PCR (ddPCR) assay for BCR::ABL1 monitoring in chronic myelogenous leukemia

Chronic myelogenous leukemia (CML) is a hematopoietic stem cell malignancy that accounts for 15-20% of all cases of leukemia. CML is caused by a translocation between chromosomes 9 and 22 which creates an abnormal fusion gene, BCR::ABL1. The amount of BCR::ABL1 transcript RNA is a marker of disease progression and the effectiveness of tyrosine kinase inhibitor (TKI) treatment. This study determined the analytical and clinical performance of a droplet digital PCR based assay (QXDx BCR-ABL %IS Kit; Bio-Rad) for BCR::ABL1 quantification. The test has a limit of detection of MR4.7 (0.002%) and a linear range of MR0.3-4.7 (50-0.002%IS). Reproducibility of results across multiple sites, days, instruments, and users was evaluated using panels made from BCR::ABL1 positive patient samples. Clinical performance of the assay was evaluated on patient samples and compared to an existing FDA-cleared test. The reproducibility study noted negligible contributions to variance from site, instrument, day, and user for samples spanning from MR 0.7-4.2. The assay demonstrated excellent clinical correlation with the comparator test using a Deming regression with a Pearson R of 0.99, slope of 1.037 and intercept of 0.1084. This data establishes that the QXDx™ BCR-ABL %IS Kit is an accurate, precise, and sensitive system for the diagnosis and monitoring of CML.

Measurement Uncertainty Impacts Diagnosis of Diabetes Mellitus: Reliable Minimal Difference of Plasma Glucose Results

INTRODUCTION

The diagnosis of diabetes mellitus is based on suitable cut-off values of specific biomarkers, such as the concentration of glucose in plasma. The German Diabetes Association has very recently published a clinical practice guideline on the definition, classification and diagnosis of diabetes mellitus that recommends measurements of plasma glucose concentration have an imprecision defined as a minimal difference (MD) of at a fasting plasma glucose concentration of 7.0 mmol/L. To obtain reliable values for the MD, we investigated long-term and short-term measurement uncertainty.

METHODS

The imprecision was determined by two approaches: (1) a long-term dataset with imprecision based on the Guideline of the German Medical Association on Quality Assurance in Medical Laboratory Examinations (Rili-BAEK), in a medical laboratory operating 24/7, using internal quality control (IQC) data for four concentrations during a 10-year period; and (2) a detailed short-term dataset with imprecision assessed by hourly measurements of control materials. These datasets were used to calculate the MD cut-off (MD_cut-off) as: [Formula: see text]  = 2  [Formula: see text], where SD is the standard deviation and k = 2 represents a confidence level of 95%.

RESULTS

The MD_cut-off of ≤ 0.7 mmol/L at a fasting plasma glucose concentration of 7.0 mmol/L (MD_cut-off 7.0) for the long-term and the short-term approaches were 0.44 and 0.40 mmol/L, respectively. The MD_cut-off 7.0 from both approaches was therefore below the recommended value of 0.7 mmol/L. It was noted that the variability in performance within and between instruments can be covered by reporting the long-term MD_cut-off 7.0 across all connected instruments. In this study, stable results for the MD_cut-off 7.0 were obtained after 1 year.

CONCLUSION

Imprecision as measured by IQC data is remarkably stable over many years of operation. Current imprecision assessment usually focuses on only single instruments, whereas clinicians perceive the measurement as the result of the combined analytical performance of all instruments used for a certain assay. In the clinical setting, the MD may be a more useful measure of imprecision, and we suggest deriving the MD_cut-off combined from all instruments and control cycles that are used in the patient care setting for a given analyte.

Identification of high thrombotic risk triple-positive antiphospholipid syndrome patients is dependent on anti-cardiolipin and anti-β2glycoprotein I antibody detection assays

Essentials Triple-positivity is associated with a high risk for a first thrombotic event and recurrence. Identification of triple-positives is dependent on the solid phase assay used. In triple-positivity, IgM only adds value in thrombotic risk stratification together with IgG. Thrombotic risk in triple-positive patients with IgM only, depends on the platform.

ABSTRACT

Background The antiphospholipid syndrome (APS) is characterized by thrombosis and/or pregnancy morbidity with the persistent presence of antiphospholipid antibodies (aPL). Triple-positivity (i.e. positivity for lupus anticoagulant [LAC], anti-cardiolipin [aCL] and anti-β2glycoprotein I [aβ2GPI] antibodies) is associated with a high thrombotic risk. Objectives We investigated the variability in triple-positivity detection by measuring the same samples with four commercially available solid phase assays. In addition, the added clinical value of aPL in LAC-positive patients was investigated, as well as the association of IgM triple-positivity and thrombosis. Patients/Methods We included 851 patients from seven European medical centers. Anti-CL and aβ2GPI IgG/IgM antibodies were determined by four platforms: BioPlex® 2200, ImmunoCap® EliA, ACL AcuStar® and QUANTA Lite ELISA® . Results Triple-positivity detection by solid phase assays varied, ranging from 89 up to 118 in thrombotic APS patients (n = 258), of which 86 were detected independent of the platform. Lupus anticoagulant positivity resulted in an odds ratio (OR) for thrombosis of 3.4; triple-positivity (irrespective of the isotype) increased the OR from 4.3 up to 5.2, dependent on the platform. Triple-positivity solely for the IgM isotype did not increase the OR for thrombosis compared with LAC positivity. The highest OR for thrombosis was reached for positivity for IgG and IgM aβ2GPI and aCL (8.6 up to 28.9). Conclusions Triple-positivity proved to be highly associated with thrombosis, but identification is assay dependent. Within triple-positivity, IgM antibodies only have an added clinical value in patients positive for IgG antibodies.