Development and Technical Validation of an Immunoassay for the Detection of APP669-711 (Aß-3-40) in Biological Samples.

The ratio of amyloid precursor protein (APP)669-711 (Aß-3-40)/Aß1-42 in blood plasma was reported to represent a novel Alzheimer's disease biomarker. Here, we describe the characterization of two antibodies against the N-terminus of Aß-3-x and the development and "fit-for-purpose" technical validation of a sandwich immunoassay for the measurement of Aß-3-40. Antibody selectivity was assessed by capillary isoelectric focusing immunoassay, Western blot analysis, and immunohistochemistry. The analytical validation addressed assay range, repeatability, specificity, between-run variability, impact of pre-analytical sample handling procedures, assay interference, and analytical spike recoveries. Blood plasma was analyzed after Aß immunoprecipitation by a two-step immunoassay procedure. Both monoclonal antibodies detected Aß-3-40 with no appreciable cross reactivity with Aß1-40 or N-terminally truncated Aß variants. However, the amyloid precursor protein was also recognized. The immunoassay showed high selectivity for Aß-3-40 with a quantitative assay range of 22 pg/mL-7.5 ng/mL. Acceptable intermediate imprecision of the complete two-step immunoassay was reached after normalization. In a small clinical sample, the measured Aß42/Aß-3-40 and Aß42/Aß40 ratios were lower in patients with dementia of the Alzheimer's type than in other dementias. In summary, the methodological groundwork for further optimization and future studies addressing the Aß42/Aß-3-40 ratio as a novel biomarker candidate for Alzheimer's disease has been set.

Commutability of the certified reference materials for the standardization of ß-amyloid 1-42 assay in human cerebrospinal fluid: lessons for tau and ß-amyloid 1-40 measurements.

BACKGROUND: The core Alzheimer's disease cerebrospinal fluid (CSF) biomarkers total tau (T-tau), phosphorylated tau (P-tau), ß-amyloid 1-42 (Aß42) and ß-amyloid 1-40 (Aß40) are increasing in importance and are now part of the research criteria for the diagnosis of the disease. The main aim of this study is to evaluate whether a set of certified reference materials (CRMs) are commutable for Aß42 and to serve as a feasibility study for the other markers. This property is a prerequisite for the establishment of CRMs which will then be used by manufacturers to calibrate their assays against. Once the preanalytical factors have been standardized and proper selection criteria are available for subject cohorts this harmonization between methods will allow for universal cut-offs to be determined. METHODS: Thirty-four individual CSF samples and three different CRMs where analyzed for T-tau, P-tau, Aß42 and Aß40, using up to seven different commercially available methods. For Aß40 and Aß42 a mass spectrometry-based procedure was also employed. RESULTS: There were strong pairwise correlations between the different methods (Spearman's ρ>0.92) for all investigated analytes and the CRMs were not distinguishable from the individual samples. CONCLUSIONS: This study shows that the CRMs are commutable for the different assays for Aß42. For the other analytes the results show that it would be feasible to also produce CRMs for these. However, additional studies are needed as the concentration interval for the CRMs were selected based on Aß42 concentrations only and did in general not cover satisfactory large concentration intervals for the other analytes.

Assessing the commutability of reference material formats for the harmonization of amyloid-ß measurements.

BACKGROUND: The cerebrospinal fluid (CSF) amyloid-ß (Aß42) peptide is an important biomarker for Alzheimer's disease (AD). Variability in measured Aß42 concentrations at different laboratories may be overcome by standardization and establishing traceability to a reference system. Candidate certified reference materials (CRMs) are validated herein for this purpose. METHODS: Commutability of 16 candidate CRM formats was assessed across five CSF Aß42 immunoassays and one mass spectrometry (MS) method in a set of 48 individual clinical CSF samples. Promising candidate CRM formats (neat CSF and CSF spiked with Aß42) were identified and subjected to validation across eight (Elecsys, EUROIMMUN, IBL, INNO-BIA AlzBio3, INNOTEST, MSD, Simoa, and Saladax) immunoassays and the MS method in 32 individual CSF samples. Commutability was evaluated by Passing-Bablok regression and the candidate CRM termed commutable when found within the prediction interval (PI). The relative distance to the regression line was assessed. RESULTS: The neat CSF candidate CRM format was commutable for almost all method comparisons, except for the Simoa/MSD, Simoa/MS and MS/IBL where it was found just outside the 95% PI. However, the neat CSF was found within 5% relative distance to the regression line for MS/IBL, between 5% and 10% for Simoa/MS and between 10% and 15% for Simoa/MSD comparisons. CONCLUSIONS: The neat CSF candidate CRM format was commutable for 33 of 36 method comparisons, only one comparison more than expected given the 95% PI acceptance limit. We conclude that the neat CSF candidate CRM can be used for value assignment of the kit calibrators for the different Aß42 methods.

Cerebrospinal Fluid Aß42/40 Corresponds Better than Aß42 to Amyloid PET in Alzheimer's Disease.

BACKGROUND: Decreased concentrations of amyloid-ß 1-42 (Aß42) in cerebrospinal fluid (CSF) and increased retention of Aß tracers in the brain on positron emission tomography (PET) are considered the earliest biomarkers of Alzheimer's disease (AD). However, a proportion of cases show discrepancies between the results of the two biomarker modalities which may reflect inter-individual differences in Aß metabolism. The CSF Aß42/40 ratio seems to be a more accurate biomarker of clinical AD than CSF Aß42 alone. OBJECTIVE: We tested whether CSF Aß42 alone or the Aß42/40 ratio corresponds better with amyloid PET status and analyzed the distribution of cases with discordant CSF-PET results. METHODS: CSF obtained from a mixed cohort (n = 200) of cognitively normal and abnormal research participants who had undergone amyloid PET within 12 months (n = 150 PET-negative, n = 50 PET-positive according to a previously published cut-off) was assayed for Aß42 and Aß40 using two recently developed immunoassays. Optimal CSF cut-offs for amyloid positivity were calculated, and concordance was tested by comparison of the areas under receiver operating characteristic (ROC) curves (AUC) and McNemar's test for paired proportions. RESULTS: CSF Aß42/40 corresponded better than Aß42 with PET results, with a larger proportion of concordant cases (89.4% versus 74.9%, respectively, p < 0.0001) and a larger AUC (0.936 versus 0.814, respectively, p < 0.0001) associated with the ratio. For both CSF biomarkers, the percentage of CSF-abnormal/PET-normal cases was larger than that of CSF-normal/PET-abnormal cases. CONCLUSION: The CSF Aß42/40 ratio is superior to Aß42 alone as a marker of amyloid-positivity by PET. We hypothesize that this increase in performance reflects the ratio compensating for general between-individual variations in CSF total Aß.