Matrix effects in clinical immunoassays and the effect of preheating and cooling analytical samples

Levels of Synaptic Proteins in Brain and Neurofilament Light Chain in Cerebrospinal Fluid and Plasma of OVT73 Huntington's Disease Sheep Support a Prodromal Disease State

BACKGROUND

Synaptic changes occur early in patients with Huntington's disease (HD) and in mouse models of HD. An analysis of synaptic changes in HD transgenic sheep (OVT73) is fitting since they have been shown to have some phenotypes. They also have larger brains, longer lifespan, and greater motor and cognitive capacities more aligned with humans, and can provide abundant biofluids for in vivo monitoring of therapeutic interventions.

OBJECTIVE

The objective of this study was to determine if there were differences between 5- and 10-year-old OVT73 and wild-type (WT) sheep in levels of synaptic proteins in brain and in neurofilament light chain (NfL) in cerebrospinal fluid (CSF) and plasma.

METHODS

Mutant huntingtin (mHTT) and other proteins were measured by western blot assay in synaptosomes prepared from caudate, motor, and piriform cortex in 5-year-old and caudate, putamen, motor; and piriform cortex in 10-year-old WT and OVT73 sheep. Levels of NfL, a biomarker for neuronal damage increased in many neurological disorders including HD, were examined in CSF and plasma samples from 10-year-old WT and OVT73 sheep using the Simoa NfL Advantage kit.

RESULTS

Western blot analysis showed mHTT protein expression in synaptosomes from OVT73 sheep was  23% of endogenous sheep HTT levels at both ages. Significant changes were detected in brain levels of PDE10A, SCN4B, DARPP32, calmodulin, SNAP25, PSD95, VGLUT 1, VAMP1, and Na+/K+-ATPase, which depended on age and brain region. There was no difference in NfL levels in CSF and plasma in OVT73 sheep compared to age-matched WT sheep.

CONCLUSIONS

These results show that synaptic changes occur in brain of 5- and 10-year-old OVT73 sheep, but levels of NfL in biofluids are unaffected. Altogether, the data support a prodromal disease state in OVT73 sheep that involves the caudate, putamen and cortex.

Differentiating iron-loading anemias using a newly developed and analytically validated ELISA for human serum erythroferrone

Erythroferrone (ERFE), the erythroid regulator of iron metabolism, inhibits hepcidin to increase iron availability for erythropoiesis. ERFE plays a pathological role during ineffective erythropoiesis as occurs in X-linked sideroblastic anemia (XLSA) and β-thalassemia. Its measurement might serve as an indicator of severity for these diseases. However, for reliable quantification of ERFE analytical characterization is indispensable to determine the assay’s limitations and define proper methodology. We developed a sandwich ELISA for human serum ERFE using polyclonal antibodies and report its extensive analytical validation. This new assay showed, for the first time, the differentiation of XLSA and β-thalassemia major patients from healthy controls (p = 0.03) and from each other (p<0.01), showing the assay provides biological plausible results. Despite poor dilution linearity, parallelism and recovery in patient serum matrix, which indicated presence of a matrix effect and/or different immunoreactivity of the antibodies to the recombinant standard and the endogenous analyte, our assay correlated well with two other existing ERFE ELISAs (both R² = 0.83). Nevertheless, employment of one optimal dilution of all serum samples is warranted to obtain reliable results. When adequately performed, the assay can be used to further unravel the human erythropoiesis-hepcidin-iron axis in various disorders and assess the added diagnostic value of ERFE.

Nonspecific Binding-Fundamental Concepts and Consequences for Biosensing Applications

Nature achieves differentiation of specific and nonspecific binding in molecular interactions through precise control of biomolecules in space and time. Artificial systems such as biosensors that rely on distinguishing specific molecular binding events in a sea of nonspecific interactions have struggled to overcome this issue. Despite the numerous technological advancements in biosensor technologies, nonspecific binding has remained a critical bottleneck due to the lack of a fundamental understanding of the phenomenon. To date, the identity, cause, and influence of nonspecific binding remain topics of debate within the scientific community. In this review, we discuss the evolution of the concept of nonspecific binding over the past five decades based upon the thermodynamic, intermolecular, and structural perspectives to provide classification frameworks for biomolecular interactions. Further, we introduce various theoretical models that predict the expected behavior of biosensors in physiologically relevant environments to calculate the theoretical detection limit and to optimize sensor performance. We conclude by discussing existing practical approaches to tackle the nonspecific binding challenge in vitro for biosensing platforms and how we can both address and harness nonspecific interactions for in vivo systems.

A generic standard additions based method to determine endogenous analyte concentrations by immunoassays to overcome complex biological matrix interference

We describe a novel generic method to derive the unknown endogenous concentrations of analyte within complex biological matrices (e.g. serum or plasma) based upon the relationship between the immunoassay signal response of a biological test sample spiked with known analyte concentrations and the log transformed estimated total concentration. If the estimated total analyte concentration is correct, a portion of the sigmoid on a log-log plot is very close to linear, allowing the unknown endogenous concentration to be estimated using a numerical method. This approach obviates conventional relative quantification using an internal standard curve and need for calibrant diluent, and takes into account the individual matrix interference on the immunoassay by spiking the test sample itself. This technique is based on standard additions for chemical analytes. Unknown endogenous analyte concentrations within even 2-fold diluted human plasma may be determined reliably using as few as four reaction wells.

A generic standard additions based method to determine endogenous analyte concentrations by immunoassays to overcome complex biological matrix interference

We describe a novel generic method to derive the unknown endogenous concentrations of analyte within complex biological matrices (e.g. serum or plasma) based upon the relationship between the immunoassay signal response of a biological test sample spiked with known analyte concentrations and the log transformed estimated total concentration. If the estimated total analyte concentration is correct, a portion of the sigmoid on a log-log plot is very close to linear, allowing the unknown endogenous concentration to be estimated using a numerical method. This approach obviates conventional relative quantification using an internal standard curve and need for calibrant diluent, and takes into account the individual matrix interference on the immunoassay by spiking the test sample itself. This technique is based on standard additions for chemical analytes. Unknown endogenous analyte concentrations within even 2-fold diluted human plasma may be determined reliably using as few as four reaction wells.

Free analyte QC concept: a novel approach to prove correct quantification of free therapeutic protein drug/biomarker concentrations

Quantification of free drug concentrations is highly challenging due to the dynamic drug–ligand equilibrium, which may result in incorrect results. Current QC concepts do not adequately cover all of the important influencing factors: the assay itself (format and procedure); the calibration concept; the sample preparation; and the sample storage. Here, we propose a ‘free analyte QC concept’ that enables quantitative testing of these four factors and, thus, provides best possible proof of correct free drug quantification. The principle of the free analyte QC concept and an example of its application for a free drug assay is described. A comparison of this novel approach with current approaches and how the new concept fits (or does not fit) with current regulatory guidelines is discussed.

Matrix effect and recovery terminology issues in regulated drug bioanalysis

Understanding the meaning of the terms used in the bioanalytical method validation guidance is essential for practitioners to implement best practice. However, terms that have several meanings or that have different interpretations exist within bioanalysis, and this may give rise to differing practices. In this perspective we discuss an important but often confusing term – ‘matrix effect (ME)’ – in regulated drug bioanalysis. The ME can be interpreted as either the ionization change or the measurement bias of the method caused by the nonanalyte matrix. The ME definition dilemma makes its evaluation challenging. The matrix factor is currently used as a standard method for evaluation of ionization changes caused by the matrix in MS-based methods. Standard additions to pre-extraction samples have been suggested to evaluate the overall effects of a matrix from different sources on the analytical system, because it covers ionization variation and extraction recovery variation. We also provide our personal views on the term ‘recovery’.

Development of a bead immunoassay to measure Vi polysaccharide-specific serum IgG after vaccination with the Salmonella enterica serovar Typhi Vi polysaccharide

Vi polysaccharide from Salmonella enterica serotype Typhi is used as one of the available vaccines to prevent typhoid fever. Measurement of Vi-specific serum antibodies after vaccination with Vi polysaccharide by enzyme-linked immunosorbent assay (ELISA) may be complicated due to poor binding of the Vi polysaccharide to ELISA plates resulting in poor reproducibility of measured antibody responses. We chemically conjugated Vi polysaccharide to fluorescent beads and performed studies to determine if a bead-based immunoassay provided a reproducible method to measure vaccine-induced anti-Vi serum IgG antibodies. Compared to ELISA, the Vi bead immunoassay had a lower background and therefore a greater signal-to-noise ratio. The Vi bead immunoassay was used to evaluate serum anti-Vi IgG in 996 subjects from the city of Kolkata, India, before and after vaccination. Due to the location being one where Salmonella serotype Typhi is endemic, approximately 45% of the subjects had protective levels of anti-Vi serum IgG (i.e., 1 microg/ml anti-Vi IgG) before vaccination, and nearly 98% of the subjects had protective levels of anti-Vi serum IgG after vaccination. Our results demonstrate that a bead-based immunoassay provides an effective, reproducible method to measure serum anti-Vi IgG responses before and after vaccination with the Vi polysaccharide vaccine.

Antibodies to amino acid 200-239 (p200) of Ro52 as serological markers for the risk of developing congenital heart block

Maternal autoantibodies to the p200-epitope of Ro52 have been suggested to correlate with development of congenital heart block. The aim of the present study was to evaluate the clinical relevance and predictive value of p200-antibodies in high-risk pregnancies. Sera from 515 Finnish, Swedish and American women were included in the study. Sera originated from 202 mothers with an infant affected by second- or third-degree atrioventricular block (AVB), 177 mothers with rheumatic disease having infants with normal heart rate and female blood donors (n = 136). A novel serological assay for Ro52 p200-antibodies with intra- and inter-assay variability of 3% and 3.8% respectively was developed. Mothers of children affected by AVB II-III had significantly higher p200-antibody levels than mothers with rheumatic disease having children with normal heart rate (P < 0.001). In the Swedish cohort, a distinction between foetuses with normal conduction, AVB I, AVB II and III was possible. A significant difference in anti-p200 levels between AVB I and AVB II-III groups compared with foetuses with normal conduction (P < 0.05 and P < 0.01) was observed. Using p200-antibodies as a second step analysis in Ro52-positive pregnancies increased the positive predictive value for foetal cardiac involvement (AVB I, II or III) from 0.39 (0.27-0.51) to 0.53 (0.37-0.68). In conclusion, Ro52 p200-antibodies may occur in women with unaffected children, but levels are significantly higher in mothers of children with congenital heart block and are suggested as a relevant marker in evaluating the risk for foetal AV block.

A new efficient method for eliminating the interference effect of human serum and increasing the sensitivity and recovery rate of enzyme immunoassay

A new, very simple method for increasing the sensitivity and recovery rate of enzyme-linked immunosorbent assay (ELISA) for the precise quantification of antigen in human serum is described. The assay design uses CATNF6A4c IgG2a monoclonal antibody and biotinylated anti-human tumor necrosis factor-alpha (hTNF-alpha) polyclonal mouse IgG as the capture and tracer antibodies, respectively. The assay is completed within 4 hours at room temperature and is capable of detecting both recombinant and native human TNF-alpha. The assay incorporates the use of saturated ammonium sulfate (SAS) as a component of the dilution buffer to amplify the resultant signal from antigen containing human serum and eliminating the endogenous interference of native human serum. SAS worked optimally at the final concentrations, ranging from 1.2% to 11%. The addition of SAS to the dilution buffer resulted in a dramatic increase in both sensitivity and recovery rate of the ELISA. The results demonstrated that 50 microL of dilution buffer, containing SAS, enabled the precise quantification of human TNF-alpha in 100 microL of human serum samples and eliminated the interference of native serum, which seemed to be related to complement proteins. Therefore, dilution buffer containing SAS, at a defined concentration, seemed to be a potential candidate for resolving sensitivity and recovery problems usually encountered in immunoassays when measurement was performed with native serum samples. The proposed technique provides an easy, practical, and consistent method for ELISA when using human native serum.