Empagliflozin in patients with type 2 diabetes mellitus and chronic obstructive pulmonary disease.

AIMS: Type 2 diabetes mellitus (T2DM) and chronic obstructive pulmonary disease (COPD) often co-exist, yielding increased risk of cardiovascular (CV) complications including heart failure (HF). We assessed risk of cardiorenal outcomes, mortality and safety in patients with versus without COPD in the EMPA-REG OUTCOME trial. METHODS: Patients (n = 7,020) with T2DM and CV disease (CVD) were treated with empagliflozin (10 mg or 25 mg) or placebo. Cox regression was used to assess COPD subgroup (placebo only) associations with, and treatment effects of empagliflozin versus placebo on first hospitalization for HF (HHF), CV death, all-cause mortality, incident/worsening nephropathy, and all-cause hospitalization. RESULTS: At baseline, patients with COPD (n = 707) had more HF and used insulin more frequently than those without COPD. During follow-up in the placebo group, those with baseline COPD had increased risk of HHF (HR 2.15 [95% CI 1.32, 3.49]), HHF/CV death (1.60 [1.10, 2.33]), incident/worsening nephropathy (1.68 [1.26, 2.24]), all-cause hospitalization (1.44 [1.19, 1.74]) and all-cause death (1.60 [1.09, 2.35]) compared to those without COPD. Empagliflozin consistently reduced all clinical outcomes, irrespective of COPD status (interaction p-values 0.14 to 0.96), with a confirmed safety profile. CONCLUSIONS: In patients with T2DM and CVD, COPD increased the risk of mortality and cardiorenal outcomes, including HF. Empagliflozin consistently reduced these outcomes versus placebo regardless of COPD, suggesting that empagliflozin's benefits in patients with T2DM and CVD are not mitigated by the presence of COPD.

Cardio/Kidney Composite End Points: A Post Hoc Analysis of the EMPA-REG OUTCOME Trial.

Background Cardio/kidney composite end points are clinically relevant but rarely analyzed in cardiovascular trials. This post hoc analysis of the EMPA-REG OUTCOME (Empagliflozin Cardiovascular Outcome Event Trial in Type 2 Diabetes Mellitus Patients) trial evaluated cardio/kidney composite end points by 2 statistical approaches. Methods and Results A total of 7020 patients with type 2 diabetes mellitus and established cardiovascular disease were treated with empagliflozin 10 or 25 mg (n=4687) or placebo (n=2333) on top of standard care. Cardio/kidney composite end points studied were: (1) cardiac or kidney death, kidney failure, hospitalization for heart failure, sustained decline in estimated glomerular filtration rate ≥40% from baseline, or sustained progression to macroalbuminuria; (2) cardiac or kidney death, kidney failure, hospitalization for heart failure, or sustained estimated glomerular filtration rate decline ≥40% from baseline; and (3) cardiac or kidney death, kidney failure, hospitalization for heart failure, or sustained doubling in serum creatinine from baseline. Cox regression using time-to-first-event analysis and win ratio (WR) using hierarchical order of events were applied. Empagliflozin reduced the risk of all cardio/kidney composites. The results varied only slightly between Cox and WR (eg, composite 1: hazard ratio, 0.56 [95% CI, 0.49-0.64]; WR, 1.76 [95% CI, 1.53-2.02]. WR prioritizes events by clinical importance; in particular, all fatal events are evaluated, whereas Cox regression ignores deaths when preceded by nonfatal events. Of the 285 cardio/kidney deaths in the analysis, 44 to 56 (15%-20%), depending on the composite, occurred after a nonfatal event and were not evaluated in Cox regression but evaluated by the WR. Conclusions By considering the clinical relevance of different event types, the WR represents an appropriate method to complement the traditional time-to-first-event analysis in cardio/kidney outcomes. Registration URL: https://www.clinicaltrials.gov; Unique identifier: NCT01131676.

Metabolic syndrome in patients with type 2 diabetes and atherosclerotic cardiovascular disease: a post hoc analyses of the EMPA-REG OUTCOME trial.

BACKGROUND: Patients with type 2 diabetes (T2D) and metabolic syndrome (MetS) are at greater cardiovascular risk than those with T2D without MetS. In the current report we aim to study the characteristics, cardio-renal outcomes and the effect of empagliflozin in patients with MetS enrolled in the EMPA-REG OUTCOME trial. METHODS: A total of 7020 patients with T2D and atherosclerotic cardiovascular disease were treated with empagliflozin (10 mg or 25 mg) or placebo for a median of 3.1 years. The World Health Organization MetS criteria could be determined for 6985 (99.5%) patients. We assessed the association between baseline MetS and multiple cardio-renal endpoints using Cox regression models, and we studied the change in the individual component over time of the MetS using mixed effect models. RESULTS: MetS at baseline was present in 5740 (82%) patients; these were more often white and had more often albuminuria and heart failure, had lower eGFR and HDL-cholesterol, and higher blood pressure, body mass index, waist circumference, and triglycerides. In the placebo group, patients with MetS had a higher risk of all outcomes including cardiovascular death: HR = 1.73 (95% CI 1.01-2.98), heart failure hospitalization: HR = 2.64 (95% CI 1.22, 5.72), and new or worsening nephropathy: HR = 3.11 (95% CI 2.17-4.46). The beneficial effect of empagliflozin was consistent on all cardio-renal outcomes regardless of presence of MetS. CONCLUSIONS: A large proportion of the EMPA-REG OUTCOME population fulfills the criteria for MetS. Those with MetS had increased risk of adverse cardio-renal outcomes. Compared with placebo, empagliflozin improved cardio-renal outcomes in patients with and without MetS. Trial registration Clinical Trial Registration: URL: https://www.clinicaltrials.gov . Unique identifier: NCT01131676.

Empagliflozin for Patients With Presumed Resistant Hypertension: A Post Hoc Analysis of the EMPA-REG OUTCOME Trial.

BACKGROUND: Type 2 diabetes (T2D) and resistant hypertension often coexist, greatly increasing risk of target-organ damage and death. We explored the effects of empagliflozin in patients with and without presumed resistant hypertension (prHT) in a post hoc analysis of EMPA-REG OUTCOME (NCT01131676). METHODS: Overall, 7,020 patients received empagliflozin 10, 25 mg, or placebo with median follow-up of 3.1 years. We defined baseline prHT as ≥3 classes of antihypertensive drugs including a diuretic and uncontrolled blood pressure (BP; systolic blood pressure (SBP) ≥140 and/or diastolic blood pressure ≥90 mm Hg) or ≥4 classes of antihypertensive, including a diuretic, and controlled BP. We explored the effect of empagliflozin on cardiovascular (CV) death, heart failure (HF) hospitalization, 3-point major adverse cardiac events, all-cause death, and incident/worsening nephropathy by Cox regression and BP over time by a mixed-repeated-measures-model analysis. RESULTS: 1,579 (22.5%) patients had prHT. The mean difference in change in SBP from baseline to week 12 vs. placebo was -4.5 (95% confidence interval, -5.9 to -3.1) mm Hg (P < 0.001) in prHT and -3.7 (-4.5, -2.9) mm Hg (P < 0.001) in patients without prHT. SBP was more frequently controlled (<130/80 mm Hg) with empagliflozin than with placebo. Patients with prHT had 1.5- to 2-fold greater risk of HF hospitalization, incident/worsening nephropathy, and CV death compared with those without prHT. Empagliflozin improved all outcomes in patients with and without prHT (interaction P > 0.1 for all outcomes). CONCLUSIONS: Empagliflozin induced a clinically relevant reduction in SBP and consistently improved all outcomes regardless of prHT status. Due to these dual effects, empagliflozin should be considered for patients with hypertension and T2D.

Adjuvant Subcutaneous Trastuzumab for HER2-Positive Early Breast Cancer: Subgroup Analyses of Safety and Active Medical Conditions by Body Weight in the SafeHer Phase III Study.

BACKGROUND: This SafeHer subgroup analysis assessed the safety of fixed-dose subcutaneous trastuzumab (H SC) as an adjuvant therapy in HER2-positive early breast cancer (EBC) by body weight. PATIENTS AND METHODS: Patients with HER2-positive EBC not previously treated with anti-HER2 therapy received H SC 600 mg (every 3 weeks for 18 cycles), with neoadjuvant or adjuvant chemotherapy or without adjuvant chemotherapy. Adverse events (AEs) were assessed throughout treatment and at final follow-up (28 ±5 days after last treatment). Subgroups were categorized by body weight, Asian origin, and chemotherapy administration. All analyses were descriptive. RESULTS: Of 2,577 patients enrolled, 2,573 received ≥1 dose of study medication and were included in this safety analysis. Median body weight at baseline was 67.0 kg (range 33.6-150.0 kg). Any-grade AEs occurred in 88.7% (2,282/2,573) of the overall population, versus 87.1% (590/677) of the lowest bodyweight quartile (≤59 kg), 90.0% (561/623) of the highest quartile (>77 kg), and 86.5% (327/378) of the Asian population. Grade ≥3 AEs occurred in 23.2% (596/2,573) of the overall population, 17.9% (121/677) of the lowest bodyweight quartile, 26.8% (167/623) of the highest quartile, and 15.3% (58/378) of the Asian population. The highest bodyweight quartile had the highest incidence of medical conditions at baseline (highest quartile, 75.6%; lowest quartile, 56.1%). CONCLUSION: These data support the use of fixed-dose H SC as an adjuvant therapy in HER2-positive EBC and confirm the comparable safety profile of H SC in patients with low body weight or of Asian origin versus the overall population in SafeHer. ClinicalTrials.gov: NCT01566721. IMPLICATIONS FOR PRACTICE: The safety profile of fixed-dose subcutaneous trastuzumab (H SC) was comparable between patients in the lowest bodyweight subgroup and the overall patient population, and also between patients of Asian origin (of whom a higher proportion often fall within the lower bodyweight quartiles) and the overall population. The safety data from this SafeHer subgroup analysis therefore support the use of fixed-dose H SC 600 mg administered every 3 weeks as an adjuvant therapy for patients with HER2-positive early breast cancer across different bodyweight subgroups and in the Asian patient population.

Switching between intravenous and subcutaneous trastuzumab: Safety results from the PrefHer trial.

AIM: To assess the safety and tolerability of switching between subcutaneous (SC) and intravenous (IV) trastuzumab in the PrefHer study (NCT01401166). PATIENTS AND METHODS: Patients with HER2-positive early breast cancer completed (neo)adjuvant chemotherapy and were randomised to receive four cycles of SC trastuzumab, via single-use injection device (SID; Cohort 1) or hand-held syringe (Cohort 2), followed by four cycles of IV, or vice versa (the crossover period presented here) as part of their 18 standard cycles of adjuvant trastuzumab treatment. Adverse events (AEs) were reported using standard criteria. RESULTS: Overall, fewer AEs were reported during the IV treatment periods, regardless of administration sequence (IV→SC or SC→IV). Differences in AEs between the SC and IV periods were partly due to variances in grade 1 and 2 local injection site reactions (ISRs) and systemic administration-related reactions (ARRs) and these occurred mainly during SC treatment, as expected. When ISRs and ARRs were excluded, rates of AEs were higher during the first treatment period, compared with the second, in both treatment sequences; otherwise there was no clear pattern in the type of AEs reported. Rates of clinically important events, including grade ≥3 AEs, serious AEs, AEs leading to study drug discontinuation and cardiac AEs, were low and similar between treatment arms (<5%). There were no grade 4 or 5 AEs. No new safety signals for trastuzumab were observed. CONCLUSIONS: PrefHer revealed that switching from IV to SC trastuzumab (hand-held syringe or SID) or vice versa did not impact the known safety profile of trastuzumab.

11 years' follow-up of trastuzumab after adjuvant chemotherapy in HER2-positive early breast cancer: final analysis of the HERceptin Adjuvant (HERA) trial.

BACKGROUND: Clinical trials have shown that trastuzumab, a recombinant monoclonal antibody against HER2 receptor, significantly improves overall survival and disease-free survival in women with HER2-positive early breast cancer, but long-term follow-up data are needed. We report the results of comparing observation with two durations of trastuzumab treatment at a median follow-up of 11 years, for patients enrolled in the HERA (HERceptin Adjuvant) trial. METHODS: HERA (BIG 1-01) is an international, multicentre, open-label, phase 3 randomised trial of 5102 women with HER2-positive early breast cancer, who were enrolled from hospitals in 39 countries between Dec 7, 2001, and June 20, 2005. After completion of all primary therapy (including, surgery, chemotherapy, and radiotherapy as indicated), patients were randomly assigned (1:1:1) to receive trastuzumab for 1 year (once at 8 mg/kg of bodyweight intravenously, then 6 mg/kg once every 3 weeks) or for 2 years (with the same dose schedule), or to the observation group. Primary endpoint is disease-free survival, and analyses are in the intention-to-treat population. Hazard ratios (HRs) were estimated from Cox models, and survival curves were estimated by the Kaplan-Meier method. Comparison of 2 years versus 1 year of trastuzumab is based on 366-day landmark analyses. This study is registered with ClinicalTrials.gov (NCT00045032). FINDINGS: Of the 5102 women randomly assigned in the HERA trial, three patients had no evidence of having provided written informed consent to participate. We followed up the intention-to-treat population of 5099 patients (1697 in observation, 1702 in 1-year trastuzumab, and 1700 in 2-years trastuzumab groups). After a median follow-up of 11 years (IQR 10·09-11·53), random assignment to 1 year of trastuzumab significantly reduced the risk of a disease-free survival event (HR 0·76, 95% CI 0·68-0·86) and death (0·74, 0·64-0·86) compared with observation. 2 years of adjuvant trastuzumab did not improve disease free-survival outcomes compared with 1 year of this drug (HR 1·02, 95% CI 0·89-1·17). Estimates of 10-year disease-free survival were 63% for observation, 69% for 1 year of trastuzumab, and 69% for 2 years of trastuzumab. 884 (52%) patients assigned to the observation group selectively crossed over to receive trastuzumab. Cardiac toxicity remained low in all groups and occurred mostly during the treatment phase. The incidence of secondary cardiac endpoints was 122 (7·3%) in the 2-years trastuzumab group, 74 (4·4%) in the 1-year trastuzumab group, and 15 (0·9%) in the observation group. INTERPRETATION: 1 year of adjuvant trastuzumab after chemotherapy for patients with HER2-positive early breast cancer significantly improves long-term disease-free survival, compared with observation. 2 years of trastuzumab had no additional benefit. FUNDING: F Hoffmann-La Roche (Roche).

Role of Troponins I and T and N-Terminal Prohormone of Brain Natriuretic Peptide in Monitoring Cardiac Safety of Patients With Early-Stage Human Epidermal Growth Factor Receptor 2-Positive Breast Cancer Receiving Trastuzumab: A Herceptin Adjuvant Study Cardiac Marker Substudy.

Purpose Women receiving trastuzumab with chemotherapy are at risk for trastuzumab-related cardiac dysfunction (TRCD). We explored the prognostic value of cardiac markers (troponins I and T, N-terminal prohormone of brain natriuretic peptide [NT-proBNP]) to predict baseline susceptibility to develop TRCD. We examined whether development of cardiac end points or significant left ventricular ejection fraction (LVEF) drop was associated with markers' increases. Patients and Methods Cardiac marker assessments were coupled with LVEF measurements at different time points for 533 patients from the Herceptin Adjuvant (HERA) study who agreed to participate in this study. Patients with missing marker assessments were excluded, resulting in 452 evaluable patients. A primary cardiac end point was defined as symptomatic congestive heart failure of New York Heart Association class III or IV, confirmed by a cardiologist, and a significant LVEF drop, or death of definite or probable cardiac causes. A secondary cardiac end point was defined as a confirmed significant asymptomatic or mildly symptomatic LVEF drop. Results Elevated baseline troponin I (> 40 ng/L) and T (> 14 ng/L), occurring in 56 of 412 (13.6%) and 101 of 407 (24.8%) patients, respectively, were associated with an increased significant LVEF drop risk (univariate analysis: hazard ratio, 4.52; P < .001 and hazard ratio, 3.57; P < .001, respectively). Few patients had their first elevated troponin value recorded during the study (six patients for troponin I and 25 patients for troponin T). Two patients developed a primary and 31 patients a secondary cardiac end point (recovery rate of 74%, 23 of 31). For NT-proBNP, higher increases from baseline were seen in patients with significant LVEF drop. Conclusion Elevated troponin I or T before trastuzumab is associated with increased risk for TRCD. A similar conclusion for NT-proBNP could not be drawn because of the lack of a well-established elevation threshold; however, higher increases from baseline were seen in patients with TRCD compared with patients without.

Proteomic analysis of detergent resistant membrane domains during early interaction of macrophages with rough and smooth Brucella melitensis.

The plasma membrane contains discrete nanometer-sized domains that are resistant to non-ionic detergents, and which are called detergent resistant membrane domains (DRMDs) or lipid rafts. Exposure of host cells to pathogenic bacteria has been shown to induce the re-distribution of specific host proteins between DRMDs and detergent soluble membranes, which leads to the initiation of cell signaling that enable pathogens to access host cells. DRMDs have been shown to play a role in the invasion of Brucella into host macrophages and the formation of replicative phagosomes called Brucella-containing vacuoles (BCVs). In this study we sought to characterize changes to the protein expression profiles in DRMDs and to respective cellular pathways and networks of Mono Mac 6 cells in response to the adherence of rough VTRM1 and smooth 16 M B. melitensis strains. DRMDs were extracted from Mono Mac 6 cells exposed for 2 minutes at 4°C to Brucella (no infection occurs) and from unexposed control cells. Protein expression was determined using the non-gel based quantitative iTRAQ (Isobaric Tags for Relative and Absolute Quantitation) mass spectrometry technique. Using the identified iTRAQ proteins we performed enrichment analyses and probed constructed human biochemical networks for interactions and metabolic reactions. We identified 149 proteins, which either became enriched, depleted or whose amounts did not change in DRMDs upon Brucella exposure. Several of these proteins were distinctly enriched or depleted in DRMDs upon exposure to rough and smooth B. melitensis strains which results in the differential engagement of cellular pathways and networks immediately upon Brucella encounter. For some of the proteins such as myosin 9, small G protein signaling modulator 3, lysine-specific demethylase 5D, erlin-2, and voltage-dependent anion-selective channel protein 2, we observed extreme differential depletion or enrichment in DRMDs. The identified proteins and pathways could provide the basis for novel ways of treating or diagnosing Brucellosis.

A plea for neutral comparison studies in computational sciences.

In computational science literature including, e.g., bioinformatics, computational statistics or machine learning, most published articles are devoted to the development of "new methods", while comparison studies are generally appreciated by readers but surprisingly given poor consideration by many journals. This paper stresses the importance of neutral comparison studies for the objective evaluation of existing methods and the establishment of standards by drawing parallels with clinical research. The goal of the paper is twofold. Firstly, we present a survey of recent computational papers on supervised classification published in seven high-ranking computational science journals. The aim is to provide an up-to-date picture of current scientific practice with respect to the comparison of methods in both articles presenting new methods and articles focusing on the comparison study itself. Secondly, based on the results of our survey we critically discuss the necessity, impact and limitations of neutral comparison studies in computational sciences. We define three reasonable criteria a comparison study has to fulfill in order to be considered as neutral, and explicate general considerations on the individual components of a "tidy neutral comparison study". R codes for completely replicating our statistical analyses and figures are available from the companion website http://www.ibe.med.uni-muenchen.de/organisation/mitarbeiter/020_professuren/boulesteix/plea2013.

Soluble MD2 increases TLR4 levels on the epithelial cell surface.

The accessory protein MD2 has been implicated in LPS-mediated activation of the innate immune system by functioning as a co-receptor with TLR4 for LPS binding at the cell surface. Epithelial cells that play a role in primary immune response, such as in the lung or gut, often express TLR4, but are dependent on circulating soluble MD2 (sMD2) to bind TLR4 to assemble the functional receptor. In this study, we show that sMD2 incubation with HEK293 epithelial cells transfected with TLR4 increases the cell surface levels of TLR4 in the absence of LPS. Dose response studies reveal that a threshold sMD2 concentration (approximately 450 nM) stimulates maximal TLR4 levels on the cell surface, whereas higher concentrations of sMD2 (approximately 1800 nM) reduce these enhanced TLR4 levels. We show evidence that MD2 multimer formation is increased at these higher concentrations of sMD2 and that addition of LPS to sMD2-stimulated cells masks the enhanced TLR4 cell surface levels, most likely due to the LPS-induced downregulation of TLR4 by endocytosis following receptor stimulation. All together, these results support a model in which sMD2 binds to TLR4 and increases TLR4 levels at the cell surface by preventing TLR4 turnover through the endocytic pathway. Thus, sMD2 may prime epithelial cells for enhanced immunoresponsive function prior to LPS exposure.

Targeting toll-like receptor signaling pathways for design of novel immune therapeutics.

The Toll-like receptor (TLR) family plays a fundamental role in host innate immunity by mounting a rapid and potent inflammatory response to pathogen infection. TLRs recognize distinct microbial components and activate intracellular signaling pathways that induce expression of host inflammatory genes. Extensive research in the past decade to understand TLR-mediated mechanisms of innate immunity has enabled pharmaceutical companies to begin to develop novel therapeutics for the purpose of controlling inflammatory disease. Initially, extracellular TLR agonists were designed to compete with natural microbial ligands for binding to TLRs. More recently, basic research to identify new targets for drug development has begun to explore modulation of TLR intracellular signaling pathways, in addition to TLR ligand binding. In this review, we will discuss recent strategies, including the use of decoy peptides and mimetics, plant polyphenols, and chemically modified antisense oligonucleotides, that inhibit different molecular events in TLR signaling pathways to modulate the inflammatory response. The molecular mechanisms of these inhibitors range from interference with protein-protein interactions between signaling proteins, to inhibition of transcription factor activity, to perturbation of the plasma membrane, and are derived from host, pathogen, and plant sources and by rational design. Taken together, these studies represent promising avenues for the development of novel tailored immune therapeutics that can relieve the great toll inflicted by inflammatory disease on human health and quality of life.

Development and characterization of Ni-NTA-bearing microspheres.

BACKGROUND: For ease of purification, proteins are often expressed with a short affinity sequence of five or six adjacent histidine residues (His-tag). This His-tag binds to the metal of metal chelator complexes such as Ni(2+)-nitrilotriacetic acid (Ni-NTA) or -iminodiacetic acid (Ni-IDA). Chromatography resins bearing covalently attached metal chelator complexes are used widely for the easy affinity purification of His-tagged proteins or peptides. Because Ni-NTA microspheres were not commercially available at the beginning of our studies, we prepared and characterized such microspheres to immobilize His-tagged proteins and study their interactions. Our microspheres are of three types: (a) metal chelator complexes bound covalently to polystyrene microspheres, (b) metal chelator complexes bound covalently to silica microspheres, and (c) lipid-linked metal chelator complexes adsorbed to silica microspheres forming self-assembled bilayer membranes where the metal chelators have lateral mobility. METHODS: The microspheres bearing covalently attached Ni-chelator were synthesized by reacting a primary amine-bearing Ni-NTA ligand with carboxy-functionalized microspheres and then loading with Ni(2+). Microspheres with laterally mobile metal chelator were made by incubating glass microspheres with liposomes containing phosphatidylcholine (PC) and the metal chelating lipid 1,2-dioleoyl-sn-glycero-3-[(N (5-amino-1-carboxypentyl)iminodiacetic acid)succinyl]. Binding of a His-tagged enhanced green fluorescent protein (EGFP) was used to characterize these microspheres by flow cytometry for their specificity, sensitivity, capacity and stability. RESULTS: While all micospheres specifically bind His-tagged proteins, the conditions to achieve this are different for the polystyrene- and silica-based spheres. All three types of microspheres bind His-EGFP with saturation occurring at 30-50 nM and an apparent avidity (concentration of half-maximal binding) of approximately 1 to 2 x 10(-8) M at pH 7.4. Binding of His-EGFP is inhibited by imidazole or ethylene-diaminetetraacetic acid (EDTA). Polystyrene Ni-NTA microspheres showed significant nonspecific binding as measured by binding in the presence of imidazole or EDTA or by binding of fluorescent proteins lacking a His-tag. This nonspecific binding of proteins to and aggregation of polystyrene spheres could only be prevented by the inclusion of low concentrations of Tween 20, but not by including bovine serum albumin (BSA), polyethylene glycols, or polyvinylpyrrolidones as blocking agents. In contrast, silica-based microspheres with covalently attached Ni-NTA or silica microspheres bearing adsorbed bilayers that contain Ni-NTA-lipid showed little nonspecific binding in the presence of BSA. Our results on the stability of immobilization indicate that washing destabilizes the binding of His-tagged proteins to Ni-NTA microspheres. This binding consists of two interactions of different affinities. We also demonstrate that limited multiplexed analysis with differently sized silica microspheres bearing the Ni-NTA-lipid is feasible. CONCLUSIONS: The microspheres described are well suited to selectively immobilize His-tagged proteins to analyze their interactions by flow cytometry. The affinity and kinetic stability of the interaction of His-tagged proteins with Ni-NTA are insufficient to use Ni-NTA microspheres in multiplexed analysis formats where different His-tagged proteins are bound to distinct microspheres. Improvements towards this end (improved chelators and/or improved affinity tags) are critical for extending the use of this method. We are currently working on novel chelators to strengthen the stability of immobilization of His-tagged proteins to surfaces. Such improvements would greatly enhance the analysis of interactions of immobilized His-tagged proteins and could make the development of microsphere-based arrays with His-tagged protein/antibody possible.

Analysis of cholera toxin-ganglioside interactions by flow cytometry.

Cholera toxin entry into mammalian cells is mediated by binding of the pentameric B subunit (CTB) to ganglioside GM(1) in the cell membrane. We used flow cytometry to quantitatively measure in real time the interactions of fluorescently labeled pentameric cholera toxin B-subunit (FITC-CTB) with its ganglioside receptor on microsphere-supported phospholipid membranes. A model that describes the multiple steps of this mode of recognition was developed to guide our flow cytometric experiments and extract relevant equilibrium and kinetic rate constants. In contrast to previous studies, our approach takes into account receptor cross-linking, an important feature for multivalent interactions. From equilibrium measurements, we determined an equilibrium binding constant for a single subunit of FITC-CTB binding monovalently to GM(1) presented in bilayers of approximately 8 x 10(7) M(-1) while that for binding to soluble GM(1)-pentasaccharide was found to be approximately 4 x 10(6) M(-1). From kinetic measurements, we determined the rate constant for dissociation of a single site of FITC-CTB from microsphere-supported bilayers to be (3.21 +/- 0.03) x 10(-3) s(-1), and the rate of association of a site on FITC-CTB in solution to a GM(1) in the bilayer to be (2.8 +/- 0.4) x 10(4) M(-1) s(-1). These values yield a lower estimate for the equilibrium binding constant of approximately 1 x 10(7) M(-1). We determined the equilibrium surface cross-linking constant [(1.1 +/- 0.1) x 10(-12) cm(2)] and from this value and the value for the rate constant for dissociation derived a value of approximately 3.5 x 10(-15) cm(2) s(-1) for the forward rate constant for cross-linking. We also compared the interaction of the receptor binding B-subunit with that of the whole toxin (A- and B-subunits). Our results show that the whole toxin binds with approximately 100-fold higher avidity than the pentameric B-subunit alone which is most likely due to the additional interaction of the A(2)-subunit with the membrane surface. Interaction of cholera toxin B-subunit and whole cholera toxin with gangliosides other than GM(1) revealed specific binding only to GD1(b) and asialo-GM(1). These interactions, however, are marked by low avidity and require high receptor concentrations to be observed.