Real-world trends in atrial fibrillation ablation indicate increasing durability of pulmonary vein isolation at repeat ablation.

BACKGROUND: Durable electrical isolation of pulmonary veins (PVs) is associated with better outcomes after atrial fibrillation (AF) ablation, but previous studies of AF recurrence have reported high rates of reconnection despite successful acute isolation. This study aims to quantify historical trends in the durability of PV isolation (PVI) as radiofrequency (RF) ablation catheters, additional ablation technologies, and associated workflows have evolved. METHODS: The study population included adult patients receiving a first repeat ablation for AF between September 2013 and July 2019 at the study site. All index ablations were performed at the same site with an RF catheter and included PVI. Three generations of irrigated RF catheters based on the same technology platform were used by the site during the timeframe of this study. RESULTS: A total of 224 patients were included in the analysis. At repeat ablation, the mean number of patients with at least one reconnected PV dropped significantly with subsequent catheter generation, from 78.3% to 56.7% to 27.0% (p < .0001). Moreover, the mean number of reconnected PVs were significantly reduced from 1.48 to 0.92 to 0.47 (p < .0001), representing a 68.3% reduction across the 3 generations of devices. CONCLUSION: Significant improvement in durable PVI was seen with successive generations of RF catheter over a 6-year period. In addition to catheter technology, ancillary advances in ablation technologies, workflows, and operator experience likely contributed to these improvements.

Immunogenicity of botulinum toxins.

Botulinum neurotoxins are formulated biologic pharmaceuticals used therapeutically to treat a wide variety of chronic conditions, with varying governmental approvals by country. Some of these disorders include cervical dystonia, post-stroke spasticity, blepharospasm, migraine, and hyperhidrosis. Botulinum neurotoxins also have varying governmental approvals for cosmetic applications. As botulinum neurotoxin therapy is often continued over many years, some patients may develop detectable antibodies that may or may not affect their biological activity. Although botulinum neurotoxins are considered "lower risk" biologics since antibodies that may develop are not likely to cross react with endogenous proteins, it is possible that patients may lose their therapeutic response. Various factors impact the immunogenicity of botulinum neurotoxins, including product-related factors such as the manufacturing process, the antigenic protein load, and the presence of accessory proteins, as well as treatment-related factors such as the overall toxin dose, booster injections, and prior vaccination or exposure. Detection of antibodies by laboratory tests does not necessarily predict the clinical success or failure of treatment. Overall, botulinum neurotoxin type A products exhibit low clinically detectable levels of antibodies when compared with other approved biologic products. This review provides an overview of all current botulinum neurotoxin products available commercially, with respect to the development of neutralizing antibodies and clinical response.

Steerable sheath visualizable under 3D electroanatomical mapping facilitates paroxysmal atrial fibrillation ablation with minimal fluoroscopy.

BACKGROUND: Advances in technology and workflows have facilitated substantial reductions in fluoroscopy utilization and procedure times for atrial fibrillation (AF) ablations. A recently available steerable sheath, visualizable on a 3D electroanatomical map (EAM), may further simplify low/zero fluoroscopy ablation workflows by facilitating understanding of the relative positions of the catheter and sheath. The objective of this study was to demonstrate feasibility, safety, procedural efficiency, and clinical effectiveness of incorporating the new visualizable sheath into a low-fluoroscopy workflow. METHODS: Consecutive de novo paroxysmal AF procedures were performed with a porous tip contact force catheter at a high-volume site between January 2018 and May 2019. Procedures performed with and without the VIZIGO™ EAM-visualizable sheath (Vizigo) were compared. All ablations employed the same standardized low-fluoroscopy workflow. Statistical analyses employed stabilized inverse probability of treatment weights (IPTW) to balance cohorts by operator and key patient characteristics. RESULTS: Cohorts of 142 Vizigo and 173 non-Vizigo patients were similar at baseline. Use of the Vizigo sheath was associated with approximately 10% improvement in catheter stability (p = 0.0005), 16% reduction in radiofrequency time (p < 0.0001), and 7% fewer ablations that used fluoroscopy (p = 0.0030). There was one cardiac tamponade in each cohort and no deaths, atrioesophageal fistulas, or strokes. Single-procedure freedom from atrial arrhythmia recurrence through 12 months was similar between cohorts (p = 0.9556). CONCLUSIONS: Use of a 3D EAM-visualizable sheath resulted in improved catheter stability, reduced radiofrequency time, and more procedures performed without fluoroscopy, without compromise to safety or effectiveness.

Porous tip radiofrequency ablation catheter reduced heart failure-related complications and healthcare resource utilization in paroxysmal atrial fibrillation patients.

Introduction: A 56-hole porous tip radiofrequency catheter was developed to provide more uniform cooling with less fluid delivery than a prior 6-hole irrigated design. This study aimed to evaluate the impact of contact force (CF) ablation with the porous tip on complications (congestive heart failure [CHF] and non-CHF related), healthcare resource utilization, and procedural efficiency in patients undergoing de novo paroxysmal atrial fibrillation (PAF) ablations in a real-world setting. Methods: Consecutive de novo PAF ablations were performed between February 2014 and March 2019 by six operators at a single US academic center. The 6-hole design was used through December 2016 with the 56-hole porous tip adopted in October 2016. The outcomes of interest included symptomatic CHF presentation and CHF-related complications. Results: Of 174 patients who were included, mean age was 61.1 ± 10.8 years, 67.8% were male, and 25.3% had a history of CHF. Ablation with the porous tip catheter significantly decreased fluid delivery (1177 vs. 1912 mL with the 6-hole design; p < .0001). CHF-related complications within 7 days, particularly fluid overload, were substantially reduced with the porous tip (15.2% vs. 5.3% of patients; p = .0281) and the proportion of patients with symptomatic CHF presentation within 30 days postablation was significantly lower (14.7% vs. 32.5%; p = .0058). Conclusion: The 56-hole porous tip led to significantly reduced CHF-related complications and healthcare utilization in PAF patients undergoing CF catheter ablation when compared to the prior 6-hole design. This reduction likely results from the significant decrease in fluid delivery during the procedure.

Safety of the open-irrigated ablation catheter for radiofrequency ablation: safety analysis from six clinical studies.

BACKGROUND: The open-irrigated catheter is used most frequently for atrial and ventricular radiofrequency ablation (RFA), and is often considered as the standard by which new ablation systems are compared. But few data have been published concerning its safety. This report provides a comprehensive safety analysis of the use of an open-irrigated catheter for RFA of atrial flutter, ventricular tachycardia, and atrial fibrillation in 1,275 patients in six rigorously monitored, prospective, multicenter studies. METHODS: This analysis is of data from six studies conducted as part of both Food and Drug Administration-mandated investigational device exemption studies and postapproval studies. The six studies span a period of more than 10 years. All serious RFA complications and vascular access complications that occurred within seven days postprocedure were included. RESULTS: The number of patients who experienced any acute serious RFA complication in these studies combined was 4.9% (63/1,275). The two earliest studies were conducted when the open-irrigated catheter was first introduced, and accounted for 55.6% of the complications. In the first atrial flutter ablation study, RFA complications decreased by 60% (15.4%-6.2%) after a proctoring program was initiated during the study. For all studies, vascular access complications ranged between from 0.5%-4.7%, and no stroke or transient ischemic attack was reported within 7 days postprocedure. No significant pulmonary vein stenosis was reported from the atrial fibrillation studies. CONCLUSION: A proctoring program, careful fluid management, and absence of char and coagulum contributed to the safe use of the open-irrigated RFA catheter.

Predictive value of atrial fibrillation during the postradiofrequency ablation blanking period.

BACKGROUND: Recurrent arrhythmia following catheter ablation of atrial fibrillation (AF) may present early, during a standard 3-month blanking period. Early recurrence has been correlated to late recurrence, but the degree to which its absence predicts longer-term success has not been quantified. OBJECTIVE: The purpose of this study was to explore and quantify the relationship between early and late arrhythmia recurrence, specifically the negative predictive value, that is, the degree to which absence of blanking period recurrence predicts absence of late recurrence. METHODS: A systematic literature review and meta-analysis were conducted using statistical methods of a diagnostic test accuracy review. Studies of AF ablation using point-by-point radiofrequency, with repeated monitoring of arrhythmia recurrence including asymptomatic recurrence, and with separate data by AF type, were eligible. RESULTS: Nine studies met the prespecified eligibility criteria. For paroxysmal AF, 89% (confidence interval [CI] 82%-94%) of patients free from early recurrence remained free from late recurrence. The estimate for persistent AF was similar (91%; CI 75%-97%). This finding was robust in sensitivity analyses. Patients with early recurrence had a wider range of likely outcomes with longer-term follow-up. CONCLUSION: Freedom from AF recurrence during the blanking period is highly predictive of longer-term success in catheter ablation. Clinical trials in this area may be able to leverage these findings to more quickly assess the potential utility of new ablation technologies and methods, for example, by using early surrogate measures of success.

Real-world characteristics and readmissions among patients undergoing ablation for ventricular tachycardia: a retrospective database analysis of commercially insured patients in the USA.

BACKGROUND: Radiofrequency catheter ablation is an effective treatment to alleviate symptoms and reduce recurrent implantable cardioverter-defibrillator (ICD/CRT-D) shocks in patients with ventricular tachycardia (VT). OBJECTIVE: To assess the characteristics and outcomes (complications, inpatient readmissions) of commercially insured patients in the USA undergoing ablation for ischaemic or non-ischaemic VT. METHODS: Patients aged 18-64 years with a primary diagnosis of VT who underwent ablation between 2006 and 2015 were identified using the IBM MarketScan Commercial Database. The rate of complications including vascular complications, pericarditis, pulmonary embolism and pericardial tamponade over a 30-day post-ablation period (including index admission) was examined. Inpatient readmissions (VT-related, heart failure (HF)-related and non-VT arrhythmia-related) over the 12-month post-ablation period were examined. A Cox regression model was used to determine factors associated with inpatient readmissions. RESULTS: 5242 patients (488 with ischaemic and 4754 with non-ischaemic VT) met the study criteria. The majority of VT ablations occurred in an outpatient setting (57% for ischaemic and 66% for non-ischaemic VT). Among complications, vascular complications were most frequent (2.05% among ischaemic and 1.6% among non-ischaemic VT patients) over the 30-day post-ablation period. Among ischaemic VT patients, 17%, 7.6% and 4.7% had VT-related, HF-related and non-VT arrhythmia-related inpatient readmissions, respectively in the 12-month post-ablation period. For non-ischaemic VT patients, these numbers were 7.5%, 1.7% and 3.1%, respectively. Inpatient setting (vs outpatient), baseline ICD/CRT-D implantation, HF comorbidity and ≥2 prior hospitalisations were associated with a higher risk of post-ablation VT-related inpatient readmissions among ischaemic VT patients. Similar factors also were associated with a higher risk of post-ablation VT-related inpatient readmission among non-ischaemic VT patients. CONCLUSION: Setting of ablation and comorbidity status were found to influence readmission rates. Complication and readmission rates following VT ablation were low indicating towards the favourable safety profile of VT ablation.

The industrialization of ablation: a highly standardized and reproducible workflow for radiofrequency ablation of atrial fibrillation.

BACKGROUND OR PURPOSE: The purpose of this analysis was to report on efficacy of a standardized workflow for atrial fibrillation (AF) ablation using technology advances such as 3D imaging and contact force sensing in a real-world setting. METHODS: Consecutive AF ablations from 2014 to 2015 at a high-volume site in Belgium were included. The workflow consisted of a pre-specified procedure sequence including 3D modeling followed by radiofrequency encircling of the pulmonary veins (25 W posterior wall, 35 W anterior wall) with a THERMOCOOL SMARTTOUCH® Catheter guided by CARTO VISITAG™ Module (2.5 mm/5 s stability, 50% > 7 g) and ablation index (targets: 550 anterior wall, 400 posterior wall). Efficiency endpoints were procedure time, fluoroscopy time, and radiation dose. The primary effectiveness endpoint was freedom from atrial arrhythmia recurrence. RESULTS: A total of 605 paroxysmal AF (PAF) and 182 persistent AF (PsAF) patients were followed for 436 ± 199 days. Mean procedure times were short (PAF: 96.1 ± 26.2 min; PsAF: 109.2 ± 35.6 min) with most procedures (90.6% PAF; 81.3% PsAF) completed in ≤ 120 min. Minimal fluoroscopy was utilized (PAF: 6.1 ± 3.8 min, 5.9 ± 3.4 Gy*cm2; PsAF: 6.9 ± 4.7 min, 7.4 ± 4.9 Gy*cm2). Freedom from atrial arrhythmia recurrence was higher for PAF than PsAF patients (OR: 2.0, 95% CI: 1.4-2.9, p = 0.0003), but adjusted mean rates were high in both groups (81.0% vs. 67.9%). Rates were adjusted for prior ablation and age (at 65 years). CONCLUSION: AF ablation using a standardized workflow resulted in low procedure times and variability, with minimal fluoroscopy exposure. Long-term freedom from atrial arrhythmia recurrence was high in both PAF and PsAF populations.

Atrial fibrillation ablation with a spring sensor-irrigated contact force-sensing catheter compared with other ablation catheters: systematic literature review and meta-analysis.

OBJECTIVES: The objective of our review was to systematically assess available evidence on the effectiveness, safety and efficiency of a spring sensor-irrigated contact force (CF) catheter (THERMOCOOL SMARTTOUCH Catheter (ST)) for percutaneous ablation of paroxysmal or persistent atrial fibrillation (AF), compared with other ablation catheters, or with the ST with the operator blinded to CF data. DESIGN: Systematic literature review and meta-analysis. BACKGROUND: Emerging evidence suggests improved clinical outcomes of AF ablation using CF-sensing catheters; however, reviews to date have included data from multiple, distinct CF technologies. METHODS: We conducted a systematic review and meta-analysis of published studies comparing the use of ST versus other ablation catheters for the treatment of AF. A comprehensive search of electronic and manual sources was conducted. The primary endpoint was freedom from recurrent atrial tachyarrhythmia (AT) at 12 months. Procedural and safety data were also analysed. RESULTS: Thirty-four studies enrolling 5004 patients were eligible. The use of ST was associated with increased odds of freedom from AT at 12 months (71.0%vs60.8%; OR 1.454, 95% CI 1.12 to 1.88, p=0.004) over the comparator group, and the effect size was most evident in paroxysmal AF patients (75.6%vs64.7%; OR 1.560, 95% CI 1.09 to 2.24, p=0.015). Procedure and fluoroscopy times were shorter with ST (p=0.05 and p<0.01, respectively, vs comparator groups). The reduction in procedure time is estimated at 15.5 min (9.0%), and fluoroscopy time 4.8 min (18.7%). Complication rates, including cardiac tamponade, did not differ between groups. CONCLUSIONS: Compared with the use of other catheters, AF ablation using the CF-sensing ST catheter for AF is associated with improved success rates, shorter procedure and fluoroscopy times and similar safety profile.

Safety and Long-Term Success of Persistent Atrial Fibrillation Ablation Using THERMOCOOL SMARTTOUCH® Catheter: Real-World Experience.

BACKGROUND: To investigate the real-world clinical experience of persistent atrial fibrillation (persAF) ablation using the THERMOCOOL SMARTTOUCH® catheter with contact force (CF)-sensing ability in a prospective, multicenter registry. METHODS: Patients with persAF (excluding long-standing persAF) undergoing ablation were enrolled. Primary adverse events (AEs), 12-month success, quality of life (QoL), and correlation of success with CF were assessed. RESULTS: Overall, 150 patients with persAF (age 61.6 ± 9.4 years; 76.0% male; 90.7% Caucasian; left ventricular ejection fraction 56.9% ± 10.3%; left atrial diameter 41.5 ± 7.9 mm) underwent catheter insertion (safety cohort); 142 met eligibility criteria and were ablated (evaluable cohort). Confirmation of entrance block for all targeted pulmonary veins was achieved in 99.3% of patients. The primary AE rate was 4.0% (6/150), and 12-month success was 63.1% (95% confidence interval: 54.2%-71.4%). A non-significant trend towards higher success was observed in patients with isoproterenol/adenosine challenge vs. those without (73.1% vs. 60.2%, respectively; P=0.065). Investigators stayed within their pre-selected CF working range (catheter-tissue contact stability) 79.7% ± 12.7% of the time. When investigators stayed within the CF range ≥80% vs. <80% of the time, ablation success was 69.2% vs. 58.5%, respectively (P=0.285). QoL improved significantly at 6 months and was sustained through the 12-month follow-up (P<0.0001). CONCLUSIONS: Symptom control in a real-world setting of persAF ablation using the THERMOCOOL SMARTTOUCH® catheter was 63.1%, with significant improvements in QoL, and trended non-significantly towards increased success in patients receiving isoproterenol/adenosine challenge and when investigators stayed within their pre-selected CF range ≥80% of the time.

High mobility group A2 potentiates genotoxic stress in part through the modulation of basal and DNA damage-dependent phosphatidylinositol 3-kinase-related protein kinase activation.

The high mobility group A2 (HMGA2) protein belongs to the architectural transcription factor HMGA family, playing a role in chromosomal organization and transcriptional regulation. We and others have previously reported that ectopic HMGA2 expression is associated with neoplastic transformation and anchorage-independent cell proliferation. Here, we reported a correlation between increased HMGA2 expression and enhanced chemosensitivity towards topoisomerase II inhibitor, doxorubicin, in breast cancer cells. Using cells exhibiting differential HMGA2 expression and small interfering RNA technique, we showed that HMGA2 expression modulates cellular response to the genotoxicity of DNA double-strand breaks. Notably, HMGA2 enhances doxorubicin-elicited cell cycle delay in sub-G1 and G2-M and augments cell cycle dysregulation on cotreatment of doxorubicin and caffeine. We further reported that HMGA2 induces a persistent Ser139 phosphorylation of histone 2A variant X, analogous to the activation by doxorubicin-mediated genotoxic stress. Moreover, this HMGA2-dependent enhancement of cytotoxicity is further extended to other double-strand breaks elicited by cisplatin and X-ray irradiation and is not restricted to one cell type. Together, we postulated that the enhanced cytotoxicity by double-strand breaks in HMGA2-expressing cells is mediated, at least in part, through the signaling pathway of which the physiologic function is to maintain genome integrity. These findings should contribute to a greater understanding of the role of HMGA2 in promoting tumorigenesis and conveying (chemo)sensitivity towards doxorubicin and other related double-strand breaks.

Paroxysmal AF catheter ablation with a contact force sensing catheter: results of the prospective, multicenter SMART-AF trial.

BACKGROUND: Catheter ablation is important for treatment of paroxysmal atrial fibrillation (PAF). Limited animal and human studies suggest a correlation between electrode-tissue contact and radiofrequency lesion generation. OBJECTIVES: The study sought to assess the safety and effectiveness of an irrigated, contact force (CF)-sensing catheter in the treatment of drug refractory symptomatic PAF. METHODS: A prospective, multicenter, nonrandomized study was conducted. Enrollment criteria included: ≥3 symptomatic episodes of PAF within 6 months of enrollment and failure of ≥1 antiarrhythmic drug (Class I to IV). Ablation included pulmonary vein isolation with confirmed entrance block as procedural endpoint. RESULTS: A total of 172 patients were enrolled at 21 sites, where 161 patients had a study catheter inserted and 160 patients underwent radiofrequency application. Procedural-related serious adverse events occurring within 7 days of the procedure included tamponade (n = 4), pericarditis (n = 3), heart block (n = 1, prior to radiofrequency application), and vascular access complications (n = 4). By Kaplan-Meier analyses, 12-month freedom from atrial fibrillation/atrial flutter/atrial tachycardia recurrence was 72.5%. The average CF per procedure was 17.9 ± 9.4 g. When the CF employed was between investigator selected working ranges ≥80% of the time during therapy, outcomes were 4.25 times more likely to be successful (p = 0.0054; 95% confidence interval: 1.53 to 11.79). CONCLUSIONS: The SMART-AF trial demonstrated that this irrigated CF-sensing catheter is safe and effective for the treatment of drug refractory symptomatic PAF, with no unanticipated device-related adverse events. The increased percent of time within investigator-targeted CF ranges correlates with increased freedom from arrhythmia recurrence. Stable CF during radiofrequency application increases the likelihood of 12-month success. (THERMOCOOL® SMARTTOUCH® Catheter for Treatment of Symptomatic Paroxysmal Atrial Fibrillation; NCT01385202).

Efficacy, tolerability, and immunogenicity of onabotulinumtoxina in a randomized, double-blind, placebo-controlled trial for cervical dystonia.

OBJECTIVE: To evaluate the efficacy, tolerability, and neutralizing antibodies in the treatment of cervical dystonia with onabotulinumtoxinA (BOTOX). METHODS: Subjects received onabotulinumtoxinA (containing original bulk toxin) treatment in a 10-week open-label period (period 1). Eligible subjects who completed this period were randomized to onabotulinumtoxinA or placebo in a 10-week double-blind period (period 2). The primary outcome measures were the Cervical Dystonia Severity Scale and the physician Global Assessment Scale at week 6 in period 2. Serum samples for immunogenicity tests were taken at baseline and study exit. The potential impact of preexisting neutralizing antibodies (nAbs) was examined across subgroups for period 1 and by analysis of covariance for period 2. RESULTS: Of 214 subjects enrolled in period 1, 170 enrolled in period 2 and received placebo (n = 82) or onabotulinumtoxinA (n = 88). In period 1, subjects with preexisting nAbs responded similarly to those without preexisting nAbs. In period 2, onabotulinumtoxinA produced significantly greater improvements than placebo on the Cervical Dystonia Severity Scale (-1.81 vs -0.31 points; P = 0.012) and physician Global Assessment Scale (61.7% vs. 41.6% improved; P = 0.022) at the primary time point week 6, using baseline severity and neutralizing antibody (nAb) status at study entry as covariates. Two subjects seroconverted from nAb negative at baseline to nAb positive at study exit but remained responsive to onabotulinumtoxinA during both the open and blinded treatment periods. Rhinitis and treatment-related dysphagia were reported significantly more frequently with onabotulinumtoxinA than placebo. CONCLUSION: OnabotulinumtoxinA was well tolerated and more effective than placebo for the treatment of cervical dystonia. Subject nAb status at baseline was not a clear predictor of response to onabotulinumtoxinA.

Suppression of nonhomologous end joining repair by overexpression of HMGA2.

Understanding the molecular details associated with aberrant high mobility group A2 (HMGA2) gene expression is key to establishing the mechanism(s) underlying its oncogenic potential and effect on the development of therapeutic strategies. Here, we report the involvement of HMGA2 in impairing DNA-dependent protein kinase (DNA-PK) during the nonhomologous end joining (NHEJ) process. We showed that HMGA2-expressing cells displayed deficiency in overall and precise DNA end-joining repair and accumulated more endogenous DNA damage. Proper and timely activation of DNA-PK, consisting of Ku70, Ku80, and DNA-PKcs subunits, is essential for the repair of DNA double strand breaks (DSB) generated endogenously or by exposure to genotoxins. In cells overexpressing HMGA2, accumulation of histone 2A variant X phosphorylation at Ser-139 (gamma-H2AX) was associated with hyperphosphorylation of DNA-PKcs at Thr-2609 and Ser-2056 before and after the induction of DSBs. Also, the steady-state complex of Ku and DNA ends was altered by HMGA2. Microirradiation and real-time imaging in living cells revealed that HMGA2 delayed the release of DNA-PKcs from DSB sites, similar to observations found in DNA-PKcs mutants. Moreover, HMGA2 alone was sufficient to induce chromosomal aberrations, a hallmark of deficiency in NHEJ-mediated DNA repair. In summary, a novel role for HMGA2 to interfere with NHEJ processes was uncovered, implicating HMGA2 in the promotion of genome instability and tumorigenesis.

Retrocyclin: a primate peptide that protects cells from infection by T- and M-tropic strains of HIV-1.

Human bone marrow expresses a pseudogene that encodes an antimicrobial peptide homologous to rhesus monkey circular minidefensins (delta-defensins). We prepared the putative ancestral human peptide by solid-phase synthesis and named it "retrocyclin." Retrocyclin did not cause direct inactivation of HIV-1, and its modest antibacterial properties resembled those of its rhesus homologs. Nevertheless, retrocyclin had a remarkable ability to inhibit proviral DNA formation and to protect immortalized and primary human CD4(+) lymphocytes from in vitro infection by both T-tropic and M-tropic strains of HIV-1. Confocal fluorescent microscopy studies performed with BODIPY-FL-labeled RC-101, a close analog of retrocyclin, showed that the peptide formed patch-like aggregates on the surface of CD4(+) cells. These findings suggest that retrocyclin interferes with an early stage of HIV-1 infection and that retrocyclin-like agents might be useful topical agents to prevent sexually acquired HIV-1 infections.

SMAP-29 has two LPS-binding sites and a central hinge.

The CD spectra of SMAP-29, an antimicrobial peptide from sheep, showed disordered structure in aqueous buffers, and significant helicity in membrane-like environments, including SDS micelles, lipopolysaccharide (LPS) dispersions, and trifluoroethanol buffer systems. A structure determined by NMR in 40% perdeuterated trifluoroethanol indicated that residues 8-17 were helical, residues 18-19 formed a hinge, and residues 20-28 formed an ordered, hydrophobic segment. SMAP-29 was flexible in 40% trifluoroethanol, forming two sets of conformers that differed in the relative orientation of the N-terminal domain. We used a chromogenic Limulus assay to determine the EC50 of the peptide (the concentration that bound 50% of the added LPS). Studies with full-length and truncated SMAP-29 molecules revealed that each end of the holopeptide contained an LPS-binding domain. The higher affinity LPS-binding domain was situated in the flexible N-terminal portion. LPS binding to full-length SMAP-29 showed positive cooperativity, so the EC50 of the peptide (2.6 microm) was considerably lower than that of the individual LPS-binding domains. LPS-binding studies with a mixture of truncated peptides revealed that this cooperativity was primarily intramolecular (i.e. involving the N- and C-terminal LPS-binding sites of the same peptide molecule). CAP-18[106 -142], an antimicrobial cathelicidin peptide of rabbits, resembled SMAP-29 in that it contained N- and C-terminal LPS-binding domains, had an EC50 of 2.5 microm, and bound LPS with positive cooperativity. We conclude that the presence of multiple binding sites that function cooperatively allow peptides such as SMAP-29 and CAP-18 to bind LPS with high affinity.