Controlled labelling of tracer antibodies for time-resolved fluorescence-based immunoassays

Tracer antibodies, which are labelled with fluorescent or other type of reporter molecules, are widely employed in diagnostic immunoassays. Time-resolved fluorescence immunoassay (TRFIA), recognized as one of the most sensitive immunoassay techniques, utilizes tracers labelled with lanthanide ion (Ln) chelates. The conventional approach for conjugating isothiocyanate (ITC) Ln-chelates to antibodies involves random chemical targeting of the primary amino group of Lys residues, requiring typically overnight exposure to an elevated pH of 9-9.3 and leading to heterogeneity. Moreover, efforts to enhance the sensitivity of the assays by introducing a higher number of Ln-chelates per tracer antibody are associated with an elevated risk of targeting critical amino acid residues in the binding site, compromising the binding properties of the antibody. Herein, we report a method to precisely label recombinant antibodies with a defined number of Ln-chelates in a well-controlled manner by employing the SpyTag/SpyCatcher protein ligation technology. We demonstrate the functionality of the method with a full-length recombinant antibody (IgG) as well as an antibody fragment by producing site-specifically labelled antibodies for TRFIA for cardiac troponin I (cTnI) detection with a significant improvement in assay sensitivity compared to that with conventionally labelled tracer antibodies. Overall, our data clearly illustrates the benefits of the site-specific labelling strategy for generating high-performing tracer antibodies for TRF immunoassays.

Analytical Comparison of Pregnancy-Associated Plasma Protein-A (PAPP-A) Immunoassays for Biochemical Determination of Gestational Age

BACKGROUND

Accurate pregnancy dating is critical for maternal and child health and for counseling on safe and effective abortion methods. While last menstrual period and first trimester ultrasound are often used together to determine gestational age (GA), they have limited accuracy and availability, respectively. Prior studies have shown that pregnancy-associated plasma protein-A (PAPP-A) increases exponentially during pregnancy and has the potential to serve as a biochemical marker of GA. We aimed to analyze the relationship between sonographically determined GA and serum PAPP-A concentration measured by different immunoassays and to derive cutoff levels informative for the 70 days GA commonly recommended limit for medical abortion in outpatient settings.

METHODS

We compared technical characteristics of 4 commercially available PAPP-A immunoassays and tested 120 maternal serum samples (GA range: 34-231 days) along with contrived pool samples and traceable quality controls. These characteristics included area under the receiver operator characteristic (AUROC) plot, sensitivity and specificity based on cutoffs defined by the Youden Index, and likelihood ratios.

RESULTS

All 4 immunoassays had sensitivities and specificities ≥80%, and AUROC values ranging from 0.948 to 0.968. Marked differences among absolute PAPP-A values were noted depending on immunoassay. PAPP-A cutoff values at 70 days GA for each individual immunoassay were established along with procedural recommendations that increase equivalence among immunoassays.

CONCLUSIONS

Maternal serum PAPP-A levels correlated strongly with GA despite differences in immunoassay formats and absolute data output. Serum PAPP-A has biomarker potential for future development of a point-of-care test aimed at increasing access to medical abortion.

Pregnancy-Associated Plasma Protein a during Hemodialysis with Polyamide and Diacetate Cellulosic Membranes

Pregnancy-associated plasma protein A (PAPP-A) is a new prognostic factor of acute coronary syndrome in the general population. It is elevated in hemodialysis (HD) patients and at baseline, it was shown to be related to inflammation and oxidative stress. The aim of the study was to examine the relationship of PAPP-A and oxidative stress and inflammatory markers to HD treatment. Studied parameters were determined in 10 chronic HD patients treated with low flux polyamide (1st session) and diacetate cellulosic membranes (2nd session) at the beginning, after 15 minutes and at the end of the dialysis session. TRACE method (Time Resolved Amplified Cryptate Emission) was used for PAPP-A assessment. Results were evaluated with ANOVA.

PAPP-A levels did not depend on the type of HD membrane but changed significantly with the time of the HD session. They increased significantly from the beginning of HD to 15 min and then decreased to the end of the HD session – p<0.05 15 min of HD vs start, p<0.01 end vs start, p<0.0001 end vs 15 min of HD for polyamide membrane and p=0.05 15 min of HD vs start, p<0.01 end vs start, p<0.0001 end vs 15 min of HD for diacetate cellulosic membrane. Changes in other parameters and differences between membranes were only minimal.

We can conclude that PAPP-A as a marker of cardiovascular damage shows significant changes during the HD session. Its initial increase might be ascribed to its release from complexes or storage. During dialysis, it might be destroyed or cleaved and removed as free fragments. Its levels both before and after the HD session are higher than in healthy subjects.

A sensitive time-resolved fluoroimmunoassay for determination of median levels of pregnancy-associated plasma protein a in pregnant women in China

Pregnancy-associated plasma protein A (PAPP-A) is an important serum marker for first trimester screening. Its weekly median value varies with ethnicity. A one-step time-resolved fluoroimmunoassay (TRFIA) using two monoclonal antibodies against PAPP-A with Eu(3+) chelates as labels has been developed. Using the assay described here, we evaluated 5,301 normal serum samples from Chinese women at 7-13 weeks of gestation. The detection limit using this assay was 1.2 mIU/L, and the maximum detection range was up to 10,000 mIU/L. The intra-assay and inter-assay coefficients of variation were <3.0% and <5.0%, respectively, and the mean recovery rate was 98.0%. PAPP-A concentrations measured in 516 maternal serum samples correlated well with those obtained by Dissociation-Enhanced Lanthanide Fluorescent Immunoassay (DELFIA) PAPP-A assay (r=0.988, P<0.001). The medians for 7-13 weeks of maternal serum PAPP-A were higher in the women from China compared to reports from other countries. The present assay possesses accuracy and high sensitivity and exhibits great potential for the clinical analysis of PAPP-A. Our investigation on the median concentrations of PAPP-A will help establish reference values that are specific for China and study the importance of ethnic factors in biochemical screening.

Detection of melanoma-derived cancer-testis antigen CT16 in patient sera by a novel immunoassay

Cancer-testis antigens (CTAs) are expressed mainly in various cancer tissues and in testis or placenta. Because of their restricted expression pattern, the CTAs can be potentially used for vaccine development and diagnostic applications. CTA CT16 has been found to be expressed in lung and renal cancers as well as in melanomas. Detection of CT16 protein directly from patient serum could facilitate monitoring of tumor growth and response to therapy in CT16-positive patients. A highly sensitive time-resolved fluorescence-based immunoassay measuring CTA CT16 in serum was developed. Generally, CTAs have not been measured directly from body fluids. CT16 level was detectable in 14 of 23 (61%) patients with metastatic melanoma, whereas none of the nine healthy volunteers collected by us had measurable CT16 level. For an unknown reason, 1 of 20 commercial control serum samples gave a positive result. The Wilcoxon-Mann-Whitney exact test showed statistically significant difference when patients with metastatic melanoma were compared to our control group (p = 0.006) or to the commercial set (p < 0.001). Four melanoma patients had exceptionally high serum CT16 level. CT16 did not correlate either with S100B, a recognized marker of progressing melanoma, or with unspecific serum marker lactate dehydrogenase. Elevation of CT16 titers preceded or followed the clinical diagnosis of disease progression in four patients with metastatic melanoma. As a conclusion, our results show that CT16 protein can be measured directly from patient serum, and the developed assay has a potential for clinical use.

Dry-Reagent Double-Monoclonal Assay for Cystatin C

BACKGROUND

Cystatin C is a low molecular weight cysteine proteinase inhibitor whose plasma or serum concentrations have been shown to be better correlated with glomerular filtration rate than serum creatinine concentrations. Routine assays for cystatin C are based on use of polyclonal antibodies and immunoturbidimetric and nephelometric designs. This study aimed to develop a double-monoclonal immunoassay for cystatin C.

METHODS

We tested functionality of 42 2-site antibody combinations involving 7 monoclonal antibodies with recombinant and plasma cystatin C. We developed a heterogeneous assay using 2 antibodies selected to give the best analytical performance. The assay used a dilution step and was based on a dry-reagent, all-in-one immunoassay concept with time-resolved fluorometry. The assay was performed on an automated immunoanalyzer in single wells that contained all the required assay components. We used heparin-derived plasma samples for methodological evaluation of the assay.

RESULTS

From a relative epitope map involving 7 cystatin C–specific antibodies, we selected a pair of antibodies for a 2-site sandwich-type dry-reagent assay. Total assay time was 15 min, and 10 μL of a 100-fold diluted sample was used. The analytical detection limit (background + 3SD) and functional detection limit (CV 20%) were 0.01 mg/L and 0.02 mg/L, respectively. Within-run and total assay imprecision were &lt;4.7% and &lt;5.6% (at 0.84–3.2 mg/L), respectively, and plasma recoveries of added cystatin C were 94%–110%. Regression analysis with the Roche particle-enhanced immunoturbidimetric method yielded the following (SD): slope, 1.391 (0.029); y-intercept, −0.152 (0.045) mg/L; Sy⊻x=0.294 mg/L (n=131).

CONCLUSIONS

The developed assay enables rapid and reliable measurement of cystatin C.

PAPP-A and free ss-hCG stability in first trimester serum using PerkinElmer AutoDELFIA and DELFIA Xpress systems

BACKGROUND

In this study we aim to investigate the stability of free-beta-hCG and PAPP-A over time in serum and whole blood in typical routine temperatures.

METHODS

Serum pools were stored under the following temperatures: 30 degrees C, room temperature, refrigerator temperature and -20 degrees C, for up to 240 days. Stability of the markers in whole blood was examined in a shorter study and compared to serum. Samples were analysed using the AutoDELFIA and DELFIA Xpress analysers.

RESULTS

On the AutoDELFIA, considering a 10% change acceptable, PAPP-A levels are stable in serum for 142 days at refrigerator temperature, 37 days at room temperature and 20 days at 30 degrees C. Free-beta hCG is stable in serum for 94 days at refrigerator temperature, 3 days at room temperature and 12 h at 30 degrees C. There was no significant change with either analyte after -20 degrees C storage for up to 240 days or after six repeated freeze-thaw cycles. In whole blood, free-beta hCG levels increased more rapidly compared to serum, especially at 30 degrees C.

CONCLUSION

Normal handling of samples is only likely to minimally effect the risk assessment of chromosomal anomalies. However, careful attention should be paid to minimise the increase of free-beta hCG levels in samples shipped as whole blood.

Temporal course of pregnancy-associated plasma protein-A in angioplasty-treated ST-elevation myocardial infarction patients and potential significance of concomitant heparin administration

Pregnancy-associated plasma protein-A (PAPP-A) is a putative plaque instability marker. In acute coronary syndromes, the disrupted culprit plaque contains abundant PAPP-A, and circulating PAPP-A levels predict clinical outcomes. Determinants of circulating PAPP-A levels, however, are not fully understood, and the potential role of concomitant heparin administration has not previously been evaluated. The purposes of the present study were to evaluate in-hospital levels of PAPP-A compared with other circulating biomarkers in patients with acute myocardial infarctions and to explore the potential impact of concomitant heparin administration on PAPP-A levels in an animal experiment. Group A comprised 84 patients with ST elevation myocardial infarctions (STEMIs) who were treated with heparin and transferred for primary percutaneous coronary intervention. Plasma samples were obtained at the time of primary percutaneous coronary intervention, twice within the next 24 hours, and subsequently daily during hospitalization. Levels of PAPP-A, troponin T, soluble cluster of differentiation 163, N-terminal-pro-brain natriuretic peptide, and high-sensitivity C-reactive protein were determined. PAPP-A levels were also determined in 2 historical cohorts not given heparin: 14 patients with STEMIs (group B) and 56 patients with non-ST elevation myocardial infarctions (group C). The impact of concomitant heparin administration on the clearance of PAPP-A from the circulation was also explored in mice. In group A, PAPP-A levels were increased in the initial plasma sample in 95% of patients presenting within 3 hours of symptom onset, whereas increased levels of troponin T, soluble cluster of differentiation 163, N-terminal-pro-brain natriuretic peptide, and high-sensitivity C-reactive protein were detectable in 45%, 15%, 50%, and 35% of patients, respectively. Compared with group A, lower levels of PAPP-A were observed in the initial plasma samples drawn in groups B (p = 0.07) and C (p <0.001) not given heparin. In the animal experiment, concomitant heparin administration resulted in increased levels of PAPP-A and delayed clearance of PAPP-A from the circulation. In conclusion, PAPP-A is markedly elevated in the earliest hours after the onset of symptoms in patients with STEMIs treated with heparin and primary percutaneous coronary intervention, and in animal studies, heparin administration is associated with a significant increase in PAPP-A levels, presumably because of the detachment of PAPP-A from the vessel wall. If future studies confirm that concomitant heparin administration also increases PAPP-A levels in humans, the prognostic role of PAPP-A in patients with STEMIs needs to be reevaluated.

Effect of hemodiafiltration on pregnancy-associated plasma protein A (PAPP-A) and related parameters

BACKGROUND

Dialysis patients are at high risk of vascular/cardiovascular complications with multifactorial pathogenesis, and pregnancy-associated plasma protein A (PAPP-A) is one of the new markers related to cardiovascular risk. Because hemodiafiltration (HDF) is supposed to be better for cardiovascular status, the aim of this study was to describe whether it has any advantage concerning changes of PAPP-A and related molecules during the session in comparison with hemodialysis (HD).

METHODS

The studied group consisted of 20 chronic hemodialysis patients. In each patient, PAPP-A and related parameters-IGFBP-4 (insulin like growth factor binding protein), IGF-I (insulin like growth factor), and two MMPs (matrix metalloproteinases)-2 and 9-were determined both during a single online HDF session (high-flux polysulfone membrane HF80, postdilution) and during a single HD session (low-flux polysulfone membrane F6, F7) at time 0 (start), 15 min, 120 min, and 240 min (end) of the session.

RESULTS

PAPP-A, elevated at baseline in dialysis patients, changes significantly both during HDF and HD without significant differences between these two procedures (mean levels during HDF were 24.3, 53.9, 24.3, and 27.3 mIU/L). It increases more than two-fold from 0 to 15 min of the session (p < 0.001) and then decreases until the end of the session (p < 0.001). MMP-2 decreased slightly during both sessions (p < 0.001), and changes of other molecules were only minimal.

CONCLUSION

A single HDF session compared to HD has no advantage in the decrease of PAPP-A and other tested molecules, all of them related to cardiovascular risk. Studies aimed at a long-term effect of both procedures on these parameters would be needed to further evaluate these therapeutical strategies.

Measurement and clinical significance of circulating PAPP-A in ACS patients

BACKGROUND

The rupture of coronary atherosclerotic plaque and subsequent thrombus formation are major events underlying acute coronary syndromes (ACS). Pregnancy associated plasma protein A (PAPP-A), a biomarker of plaque rupture, has been studied in patients with ACS. This review aimed to provide an overview of clinical utility of PAPP-A in ACS patients and analytical issues adhering to immunological PAPP-A measurement.

METHODS

The literature relating to PAPP-A in ACS, the molecular structure and immunodetection of PAPP-A was reviewed. PubMed was used to search the relevant articles published from 1974 to 2006.

RESULTS

Higher PAPP-A concentrations have been found in patients with ACS than in patients with stable angina and subjects without coronary artery disease. Elevated PAPP-A concentrations have also been shown to associate with adverse cardiac events in ACS patients. The prognostic value of PAPP-A appears to be independent of cardiac troponins. Noteworthy, the PAPP-A form that accounts for increase in ACS is uncomplexed with the proform of eosinophil major basic protein (proMBP). However, PAPP-A assays applied in clinical studies published thus far detect total PAPP-A. Consequently, the clinical value may be non-optimal when total PAPP-A is measured in ACS patients. In addition, the clinical value can also be affected by the analytical factors that exert an effect on the performance of PAPP-A assays.

CONCLUSIONS

PAPP-A appears to be a very promising biomarker useful in the clinical management of ACS patients. However, more prospective and interventional studies with carefully established immunoassays are required to validate its clinical utility.

Immunofluorometric Point-of-Care Assays for the Detection of Acute Coronary Syndrome-Related Noncomplexed Pregnancy-Associated Plasma Protein A

Background: We recently reported that the pregnancy-associated plasma protein A (PAPP-A) form specifically related to acute coronary syndromes (ACS) is not complexed with the proform of eosinophil major basic protein (proMBP). The aim of this study was to develop rapid point-of-care immunoassays for the measurement of the noncomplexed PAPP-A.

Methods: We developed immunofluorometric noncompetitive dry-reagent assays for total PAPP-A with 2 PAPP-A subunit-specific monoclonal antibodies and for PAPP-A/proMBP complex with 1 PAPP-A subunit-specific antibody and 1 proMBP subunit-specific antibody. The concentration of noncomplexed PAPP-A was determined as the difference of the results obtained with the 2 assays.

Results: The assays were linear from 0.5 to 300 mIU/L. The analytical detection limit and functional detection limit (CV &lt;20%) were 0.18 mIU/L and 0.27 mIU/L for total PAPP-A assay and 0.23 mIU/L and 0.70 mIU/L for PAPP-A/proMBP assay, respectively. The total assay imprecisions were &lt;10%, and recoveries were 88%–107% for both assays. The mean difference (95% limits of agreement) between the new total PAPP-A assay and a previously reported total PAPP-A assay was −3.2% (−45.7% to 39.3%; n = 546; P = 0.0019). In serum samples from 159 non-ACS individuals, median concentrations (interquartile range) were 2.42 (1.14) mIU/L for total PAPP-A, 2.20 (1.18) mIU/L for PAPP-A/proMBP, and 0.18 (0.63) mIU/L for noncomplexed PAPP-A. Total PAPP-A and PAPP-A/proMBP, but not noncomplexed PAPP-A, correlated with age (r = 0.290, P = 0.0002; r = 0.230, P = 0.0035; r = 0.075, P = 0.3483, respectively).

Conclusions: The new assays described revealed that noncomplexed PAPP-A is found only in negligible amounts in non-ACS samples.

Protein detection using biobarcodes

Over the past 50 years the development of assays for the detection of protein analytes has been driven by continuing demands for higher levels of sensitivity and multiplexing. The result has been a progression of sandwich-type immunoassays, starting with simple radioisotopic, colorimetric, or fluorescent labeling systems to include various enzymatic or nanostructure-based signal amplification schemes, with a concomitant sensitivity increase of over 1 million fold. Multiplexing of samples and tests has been enabled by microplate and microarray platforms, respectively, or lately by various molecular barcoding systems. Two different platforms have emerged as the current front-runners by combining a nucleic acid amplification step with the standard two-sided immunoassay. In both, the captured protein analyte is replaced by a multiplicity of oligonucleotides that serve as surrogate targets. One of these platforms employs DNA or RNA polymerases for the amplification step, while detection is by fluorescence. The other is based on gold nanoparticles for both amplification as well as detection. The latter technology, now termed Biobarcode, is completely enzyme-free and offers potentially much higher multiplexing power.

Immunoassays Developed for Pregnancy-Associated Plasma Protein-A (PAPP-A) in Pregnancy May Not Recognize PAPP-A in Acute Coronary Syndromes

Background: Pregnancy-associated plasma protein-A (PAPP-A) concentrations are increased in the circulation of patients with acute coronary syndromes (ACS) and are associated with future adverse cardiac events. PAPP-A in ACS differs from PAPP-A in pregnancy in that PAPP-A in ACS is not complexed with the proform of eosinophil major basic protein (proMBP). We investigated the effect of antibody selection on the utility of PAPP-A assays for measurement of PAPP-A in pregnancy and/or ACS, and whether immunoassays for PAPP-A in pregnancy are suitable for PAPP-A in ACS.

Methods: We constructed 2-site sandwich time-resolved immunofluorometric assays using 22 monoclonal antibodies raised against pregnancy serum PAPP-A. All antibodies were studied in pairs, with each antibody used as either capture or tracer. We compared the reactivity of each antibody combination with PAPP-A/proMBP complex derived from pregnancy sera or with uncomplexed PAPP-A extracted from atherosclerotic plaques. Recombinant human PAPP-A and proMBP were also used to determine the specificity of the antibodies. We confirmed all major findings with serum samples collected from patients with myocardial infarction.

Results: Six monoclonal antibodies reacted with the proMBP subunit of the PAPP-A/proMBP complex. Epitopes of 3 proMBP-reactive antibodies largely overlapped, but were well separated from those of another group of 3 proMBP-reactive antibodies. Assays using any of the 6 proMBP-reactive antibodies failed to detect PAPP-A in ACS. In addition, some 2-site assays capable of detecting PAPP-A in pregnancy were almost incapable of detecting PAPP-A in ACS, although the individual epitopes remained detectable in PAPP-A in ACS.

Conclusions: Immunoassays developed for PAPP-A in pregnancy may not be suitable for PAPP-A in ACS. Assays for PAPP-A in ACS should be based on careful antibody selection and subjected to extensive testing with clinical ACS samples.

Pregnancy-associated plasma protein A: a biomarker in acute ST-elevation myocardial infarction (STEMI)

BACKGROUND

Elevated circulating levels of pregnancy-associated plasma protein A (PAPP-A), a novel marker of atherosclerotic plaque instability, are associated with increased risk of future cardiac events in patients with acute coronary syndromes (ACS). However, little is known of the kinetics or clinical significance of circulating PAPP-A after plaque rupture in acute ST-elevation myocardial infarction (STEMI).

AIM

To evaluate the 48-hour release of pregnancy-associated plasma protein A (PAPP-A) and its association with 12-month outcome in patients with acute ST-elevation myocardial infarction (STEMI).

METHODS

Sixty-two consecutive STEMI patients were included (40 men and 22 women, median age 67.5 years (range 34-84)), of whom 54 (87.1%) received reperfusion therapy. PAPP-A was measured at admission and 6-12, 24 and 48 hours thereafter. In 14 patients, samples were obtained also at 1, 2 and 4 hours.

RESULTS

There was an early peak of circulating PAPP-A during the first 12 hours from symptom onset, followed by rapid normalization. A second, late PAPP-A elevation was noticed in 20/62 patients (32.3%). Admission PAPP-A >10.0 mIU/L (highest tertile) was associated (P = 0.049) with increased 12-month risk of cardiovascular death or non-fatal myocardial infarction. Moreover, the combination of failed early reperfusion together with late PAPP-A elevation was strongly (7/13 versus 10/49 patients, P = 0.016) associated with adverse outcome. Admission PAPP-A did not correlate with admission C-reactive protein or cardiac troponin I.

CONCLUSIONS

PAPP-A is elevated early in STEMI and then declines rapidly, a pattern consistent with release from the ruptured plaque. The variability of PAPP-A kinetics at 48 hours reflects the success of reperfusion. This study also shows that PAPP-A may have prognostic value in STEMI.

PAPP-A: a marker of plaque instability. Is it ready for prime time?

Pregnancy-associated plasma protein-A (PAPP-A) is a zinc-binding metalloproteinase that was first identified circulating in the serum of pregnant women, but has recently been suggested to be a predictor of risk for cardiovascular events. Publications on PAPP-A, primarily from the last 4-5 years, were reviewed extensively. A special emphasis was placed on those that address the development of methods for the detection of PAPP-A in human serum/plasma and those that study the diagnostic/prognostic accuracy of PAPP-A in patients with acute coronary syndrome. This review shall focus on the possible mechanism of action of PAPP-A, clinical studies involving PAPP-A, PAPP-A assays and its molecular structure. All clinical studies to date related to the diagnostic/prognostic accuracy of PAPP-A (a total of eight studies) were included. They suggest that elevations in PAPP-A correlate with higher risk for the development of cardiovascular adverse events. However, there are conflicting data as to whether PAPP-A levels correlate with cardiac troponins or C-reactive protein. Thus, there is room for speculation as to whether PAPP-A is involved in inflammatory processes, and as to what its role is in plaque rupture and subsequent cardiac ischemic events. Furthermore, new insights into the molecular structure of PAPP-A could aid in the development of more specific assays that may help unravel the clinical utility of PAPP-A.

Molecular Distinction of Circulating Pregnancy-Associated Plasma Protein A in Myocardial Infarction and Pregnancy

Background: In the blood of pregnant women, pregnancy-associated plasma protein A (PAPP-A) is present as a covalent complex with the proform of eosinophil major basic protein (proMBP). Recently, increased serum concentrations of PAPP-A have been found in acute coronary syndromes (ACS). The aim of this study was to investigate whether the circulating PAPP-A in ACS is the same as that in pregnancy.

Methods: We developed two time-resolved immunofluorometric assays based on a relative epitope map constructed by the use of 17 monoclonal antibodies. One assay, which measured total PAPP-A, used two PAPP-A subunit-specific antibodies. The other assay, which measured PAPP-A/proMBP complex, used one proMBP subunit-specific antibody and one PAPP-A subunit-specific antibody. Serum samples from four patients with myocardial infarction (MI), three pregnant women in their first trimester, and one in her third trimester were fractionated by gel filtration on a Superose™ 6 precision column. The two assays were used to analyze fractions obtained by gel filtration as well as serum samples serially collected from four other MI patients.

Results: Pregnancy-related PAPP-A was eluted as a single peak with a molecular mass of ∼700 kDa, whereas ACS-related PAPP-A was also eluted as a single peak but with a molecular mass of ∼530 kDa. Pregnancy-related PAPP-A was detected equally by the two assays, whereas increased ACS-related PAPP-A was detected only by the assay for total PAPP-A.

Conclusions: Our results provide the first evidence that circulating ACS-related PAPP-A is different from circulating pregnancy-related PAPP-A in that it is not complexed with proMBP. These findings provide a solid foundation for the design of immunoassays to accurately measure atherosclerosis-associated plasma protein A in the circulation.

Evaluation of pregnancy-associated plasma protein A as a prognostic indicator in acute coronary syndrome patients

BACKGROUND

Higher circulating concentrations of pregnancy-associated plasma protein A (PAPP-A), a potential proatherosclerotic metalloproteinase, have been associated with increased risk for acute coronary syndrome (ACS). Our goal was to determine the ability of circulating concentrations of PAPP-A to predict adverse events in patients presenting to the Emergency Department (ED) with symptoms of ACS.

METHODS

A total of 346 patients with symptoms of ACS were included in the study. Serum samples obtained immediately after enrollment were analyzed for PAPP-A and cardiac troponin T (cTnT). The occurrence of adverse events during a 30-day follow-up period was recorded, and receiver-operating characteristic (ROC) curve analysis was performed to evaluate the prognostic characteristics of PAPP-A and cTnT.

RESULTS

A total of 33 (9.5 %) patients developed adverse events during the follow up period. At a cut-off concentration of 0.22 mIU/l, PAPP-A was a predictor of adverse events with a sensitivity and specificity (95% C.I.) of 66.7% (48.2-82.0) and 51.1% (45.4-56.8), respectively. The sensitivity and specificity of cTnT were 51.5% (33.6-69.2) and 82.1% (77.4-86.2), respectively, using a 0.01-ng/ml cut-off value, which was obtained using ROC analysis.

CONCLUSIONS

PAPP-A appears to be a modest predictor of adverse events in patients presenting to the ED with ACS symptoms, being inferior to cTnT in predicting adverse events in an ED setting. PAPP-A appears to be as sensitive as cTnT, but it is less specific.

Circulating pregnancy-associated plasma protein a predicts outcome in patients with acute coronary syndrome but no troponin I elevation

BACKGROUND

Risk stratification in troponin (cTn)-negative acute coronary syndrome (ACS) remains a clinical challenge. We investigated the predictive value of circulating pregnancy-associated plasma protein A (PAPP-A), a novel marker of atherosclerotic plaque activity, in these patients.

METHODS AND RESULTS

Two hundred consecutive hospitalized ACS patients were included, of whom 136 (69 men and 67 women; mean+/-SD age, 66+/-16 years) remained cTnI-negative for up to 24 hours. PAPP-A was measured at admission, 6 to 12 hours, and 24 hours. During 6-month follow-up, 26 (19.1%) of the cTnI-negative patients reached a primary end point (cardiovascular death, myocardial infarction, or revascularization). At a cutoff level of 2.9 mIU/L, elevated PAPP-A was an independent predictor of adverse outcome (adjusted risk ratio [RR], 4.6; 95% confidence interval, 1.8 to 11.8; P=0.002). Another independent predictor was admission CRP >2.0 mg/L (RR, 2.6; P=0.03).

CONCLUSIONS

Measurement of plasma PAPP-A, a zinc-binding matrix metalloproteinase, is a strong independent predictor of ischemic cardiac events and need of revascularization in patients who present with suspected myocardial infarction but remain troponin negative.

Time-resolved fluorescence resonance energy transfer assay for point-of-care testing of urinary albumin

BACKGROUND

Microalbuminuria is an established early marker of diabetic nephropathy and an important cardiovascular risk factor in diabetes and hypertension. We aimed to develop a rapid point-of-care assay for the measurement of urine albumin.

METHODS

The competitive homogeneous assay used an albumin-specific monoclonal antibody labeled with a stable fluorescent europium chelate as donor and an albumin labeled with cyanine 5 (Cy5) as acceptor. The assay was performed at room temperature in single microtitration wells that contained all the required dry-form reagents. The close proximity between the two labels in the immune complex allowed fluorescence resonance energy to be transferred from the pulse-excited europium chelate to the acceptor Cy5. The emission of long-lived energy transfer signal from the sensitized Cy5 was measured at 665 nm with time-resolved fluorometry that eliminated short-lived background.

RESULTS

The assay procedure required 12 min for a 10- micro L urine sample. The working range was from 10 to approximately 320 mg/L, and the lower limit of detection was 5.5 mg/L. The within- and between-run CVs were 6.9-10% and 7.5-13%, respectively. Recovery was 103-122%. The assay correlated well (r(2) = 0.98; n = 37) with a laboratory-based immunoassay, although mean (SD) results were 7 (29)% lower.

CONCLUSIONS

The speed and ease of performance of this assay recommend it for near-patient use. The assay is the first to combine a fluorescence resonance energy transfer-type rapid competitive assay with an all-in-one dry reagent.

Highly sensitive immunoassay of free prostate-specific antigen in serum using europium(III) nanoparticle label technology

BACKGROUND

Recent proceedings in utilization of europium(III) chelate-dyed polystyrene nanoparticles as labels have combined the advantages of an enhanced monovalent binding affinity and a high specific activity of nanoparticle-antibody bioconjugate. Our objective was to evaluate the performance of the nanoparticle label technology with biological samples in an immunoassay of free prostate-specific antigen (PSA-F) using a standard microtitration well platform.

METHODS

Long-lifetime luminescent europium(III)-chelate nanoparticles, 107 nm in diameter, were coated with a PSA-F specific monoclonal antibody. The two-step noncompetitive immunoassay was performed in a microtitration well coated with a second monoclonal antibody. The signal of the surface-bound nanoparticle-antibody bioconjugates was measured directly from the bottom of the well using a standard time-resolved plate fluorometer.

RESULTS

The detection limit (mean + 2SD) of the nanoparticle-based PSA-F assay was 0.21 ng/l using a 20-microl sample volume. The assay response was linear up to 5 microg/l, and the functional sensitivity was approximately 0.5 ng/l. The within-run imprecision for spiked serum samples at concentrations 0.0005-0.5 microg/l was 6.4-21.8%, and the within-run and between-run imprecisions for serum samples at concentrations 0.2-2.5 microg/l were 3.4-7.2% and 4.4-7.6%, respectively. The concentrations obtained from serum samples correlated well with the reference immunoassay; slope = 1.018 +/- 0.018; intercept = 0.012 +/- 0.021 microg/l; S(y/x) = 0.112 microg/l; r = 0.993; n = 51.

CONCLUSIONS

The developed method demonstrated acceptable performance characteristics allowing clinical studies utilizing patient samples with extremely low concentrations of PSA-F. The present assay detected PSA-F in most of samples from prostatectomized men and in few samples from healthy women that were nondetectable according to the reference immunoassay.

Release patterns of pregnancy associated plasma protein A (PAPP-A) in patients with acute coronary syndromes

OBJECTIVE

Pregnancy associated plasma protein A (PAPP-A) has recently been shown to be associated with acute coronary syndromes (ACS). The goal of this study was to investigate its release patterns in patients with ACS.

DESIGN

PAPP-A concentrations in plasma samples serially collected after admissions from 15 patients with ACS were measured. The levels of PAPP-A were compared with a reference range determined from 80 normal subjects. The associations between PAPP-A and myoglobin (Mb), C-reactive protein (CRP), fatty-acid-binding protein (FABP) and creatine kinase MB (CK-MB) were determined.

RESULTS

Various release patterns were observed with 2-10-fold changes of PAPP-A in the different patients. Increases in PAPP-A levels above the reference range could appear early at 2 h or late at 30 h after onset of chest pain. Only in 4 of the 15 cases were significantly elevated PAPP-A levels detected before 6 h after onset. Elevations early after admission showed rapid decline whereas later elevations were more persistent. No associations between PAPP-A and Mb, CRP, FABP and CK-MB were found. However, a weak but significant association to cardiac troponin I (cTn I) was found.

CONCLUSION

PAPP-A is an additional marker for ACS, but does not seem to be a useful early marker for acute myocardial infarction (AMI). The possible clinical utility of PAPP-A calls for extensive studies of chest pain patients using serial sampling combined with short- and long-term outcome studies.