Long-term safety and efficacy of tezepelumab in people with severe, uncontrolled asthma (DESTINATION): a randomised, placebo-controlled extension study

BACKGROUND

Tezepelumab is a human monoclonal antibody that blocks thymic stromal lymphopoietin. The drug has been tested previously in the phase 3 NAVIGATOR (NCT03347279) and SOURCE (NCT03406078) studies, and was subsequently approved as a treatment for severe asthma. This extension study recruited from NAVIGATOR and SOURCE and aimed to evaluate the long-term safety and efficacy of tezepelumab in individuals with severe, uncontrolled asthma.

METHODS

DESTINATION was a phase 3, multicentre, randomised, double-blind, placebo-controlled, long-term extension study. The study was done across 182 sites (including hospitals, clinics, medical centres, clinical trial centres, and private practices) in 18 countries. Participants (aged 12-80 years) were required to have good treatment compliance in the parent study. Randomisation was stratified by the parent study and all participants were re-randomised. Those who were previously randomised to receive tezepelumab in either parent study continued treatment of subcutaneous tezepelumab (210 mg every 4 weeks); those who were previously randomised to receive placebo in either parent study were re-randomised 1:1 to receive either subcutaneous tezepelumab (210 mg every 4 weeks) or placebo (every 4 weeks) using a randomisation list prepared by a computerised system. Total treatment duration (including the parent studies) was 104 weeks for all groups. Participants, investigators, and site staff were masked to treatment assignment. The primary endpoints were exposure-adjusted incidence of adverse events and serious adverse events and the secondary endpoint was the annualised asthma exacerbation rate; these were assessed from week 0 of the parent studies to week 104 of DESTINATION in all participants who were randomised and who received at least one dose of tezepelumab or placebo in either of the parent studies. The trial is registered with ClinicalTrials.gov, NCT03706079, and is closed to new participants.

FINDINGS

Participants were recruited between Jan 7, 2019, and Oct 15, 2020. For individuals who initially received tezepelumab (n=528) in NAVIGATOR, incidence of adverse events over 104 weeks was 49·62 (95% CI 45·16 to 54·39) per 100 patient-years, compared with 62·66 (56·93 to 68·81) for those receiving placebo (n=531; difference -13·04, 95% CI -17·83 to -8·18). For serious adverse events, incidence was 7·85 (6·14 to 9·89) per 100 patient-years for individuals who initially received tezepelumab and 12·45 (9·97 to 15·35) for those who received placebo (difference -4·59, -7·69 to -1·65). In SOURCE, incidence of adverse events was 47·15 (36·06 to 60·56) per 100 patient-years for those who initially received tezepelumab (n=74) and 69·97 (54·54 to 88·40) for those who received placebo (n=76; difference -22·82, -34·77 to -10·01). For serious adverse events, incidence was 13·14 (7·65 to 21·04) per 100 patient-years for those who initially received tezepelumab and 17·99 (10·66 to 28·44) for those who received placebo (difference -4·85, -14·88 to 4·53). Tezepelumab reduced the annualised asthma exacerbation rate over 104 weeks compared with placebo. In participants initially from NAVIGATOR, the annualised asthma exacerbation rate ratio over 104 weeks was 0·42 (95% CI 0·35 to 0·51); in those initially from SOURCE, the ratio over 104 weeks was 0·61 (0·38 to 0·96).

INTERPRETATION

Tezepelumab treatment was well tolerated for up to 2 years and resulted in sustained, clinically meaningful reductions in asthma exacerbations in individuals with severe, uncontrolled asthma. These findings are consistent with previous randomised, placebo-controlled studies and show the long-term safety and sustained efficacy of tezepelumab in individuals with severe, uncontrolled asthma.

FUNDING

AstraZeneca and Amgen.

DAMPs/PAMPs induce monocytic TLR activation and tolerance in COVID-19 patients; nucleic acid binding scavengers can counteract such TLR agonists

Millions of COVID-19 patients have succumbed to respiratory and systemic inflammation. Hyperstimulation of toll-like receptor (TLR) signaling is a key driver of immunopathology following infection by viruses. We found that severely ill COVID-19 patients in the Intensive Care Unit (ICU) display hallmarks of such hyper-stimulation with abundant agonists of nucleic acid-sensing TLRs present in their blood and lungs. These nucleic acid-containing Damage and Pathogen Associated Molecular Patterns (DAMPs/PAMPs) can be depleted using nucleic acid-binding microfibers to limit the patient samples' ability to hyperactivate such innate immune receptors. Single-cell RNA-sequencing revealed that CD16+ monocytes from deceased but not recovered ICU patients exhibit a TLR-tolerant phenotype and a deficient anti-viral response after ex vivo TLR stimulation. Plasma proteomics confirmed such myeloid hyperactivation and revealed DAMP/PAMP carrier consumption in deceased patients. Treatment of these COVID-19 patient samples with MnO nanoparticles effectively neutralizes TLR activation by the abundant nucleic acid-containing DAMPs/PAMPs present in their lungs and blood. Finally, MnO nanoscavenger treatment limits the ability of DAMPs/PAMPs to induce TLR tolerance in monocytes. Thus, treatment with microfiber- or nanoparticle-based DAMP/PAMP scavengers may prove useful for limiting SARS-CoV-2 induced hyperinflammation, preventing monocytic TLR tolerance, and improving outcomes in severely ill COVID-19 patients.

Bioenergetic Differences in the Airway Epithelium of Lean Versus Obese Asthmatics Are Driven by Nitric Oxide and Reflected in Circulating Platelets

Aims: Asthma, characterized by airway obstruction and hyper-responsiveness, is more severe and less responsive to treatment in obese subjects. While alterations in mitochondrial function and redox signaling have been implicated in asthma pathogenesis, it is unclear whether these mechanisms differ in lean versus obese asthmatics. In addition, we previously demonstrated that circulating platelets from asthmatic individuals have altered bioenergetics; however, it is unknown whether platelet mitochondrial changes reflect those observed in airway epithelial cells. Herein we hypothesized that lean and obese asthmatics show differential bioenergetics and redox signaling in airway cells and that these alterations could be measured in platelets from the same individual. Results: Using freshly isolated bronchial airway epithelial cells and platelets from lean and obese asthmatics and healthy individuals, we show that both cell types from obese asthmatics have significantly increased glycolysis, basal and maximal respiration, and oxidative stress compared with lean asthmatics and healthy controls. This increased respiration was associated with enhanced arginine metabolism by arginase, which has previously been shown to drive respiration. Inducible nitric oxide synthase (iNOS) was also upregulated in cells from all asthmatics. However, due to nitric oxide synthase uncoupling in obese asthmatics, overall nitric oxide (NO) bioavailability was decreased, preventing NO-dependent inhibition in obese asthmatic cells that was observed in lean asthmatics. Innovation and Conclusion: These data demonstrate bioenergetic differences between lean and obese asthmatics that are, in part, due to differences in NO signaling. They also suggest that the platelet may serve as a useful surrogate to understand redox, oxidative stress and bioenergetic changes in the asthmatic airway.

Step-Up Therapy in Black Children and Adults with Poorly Controlled Asthma

BACKGROUND

Morbidity from asthma is disproportionately higher among black patients than among white patients, and black patients constitute the minority of participants in trials informing treatment. Data indicate that patients with inadequately controlled asthma benefit more from addition of a long-acting beta-agonist (LABA) than from increased glucocorticoids; however, these data may not be informative for treatment in black patients.

METHODS

We conducted two prospective, randomized, double-blind trials: one involving children and the other involving adolescents and adults. In both trials, the patients had at least one grandparent who identified as black and had asthma that was inadequately controlled with low-dose inhaled glucocorticoids. We compared combinations of therapy, which included the addition of a LABA (salmeterol) to an inhaled glucocorticoid (fluticasone propionate), a step-up to double to quintuple the dose of fluticasone, or both. The treatments were compared with the use of a composite measure that evaluated asthma exacerbations, asthma-control days, and lung function; data were stratified according to genotypic African ancestry.

RESULTS

When quintupling the dose of fluticasone (to 250 μg twice a day) was compared with adding salmeterol (50 μg twice a day) and doubling the fluticasone (to 100 μg twice a day), a superior response occurred in 46% of the children with quintupling the fluticasone and in 46% of the children with doubling the fluticasone and adding salmeterol (P = 0.99). In contrast, more adolescents and adults had a superior response to added salmeterol than to an increase in fluticasone (salmeterol-low-dose fluticasone vs. medium-dose fluticasone, 49% vs. 28% [P = 0.003]; salmeterol-medium-dose fluticasone vs. high-dose fluticasone, 49% vs. 31% [P = 0.02]). Neither the degree of African ancestry nor baseline biomarkers predicted a superior response to specific treatments. The increased dose of inhaled glucocorticoids was associated with a decrease in the ratio of urinary cortisol to creatinine in children younger than 8 years of age.

CONCLUSIONS

In contrast to black adolescents and adults, almost half the black children with poorly controlled asthma had a superior response to an increase in the dose of an inhaled glucocorticoid and almost half had a superior response to the addition of a LABA. (Funded by the National Heart, Lung, and Blood Institute; BARD ClinicalTrials.gov number, NCT01967173.).

Mometasone or Tiotropium in Mild Asthma with a Low Sputum Eosinophil Level

BACKGROUND

In many patients with mild, persistent asthma, the percentage of eosinophils in sputum is less than 2% (low eosinophil level). The appropriate treatment for these patients is unknown.

METHODS

In this 42-week, double-blind, crossover trial, we assigned 295 patients who were at least 12 years of age and who had mild, persistent asthma to receive mometasone (an inhaled glucocorticoid), tiotropium (a long-acting muscarinic antagonist), or placebo. The patients were categorized according to the sputum eosinophil level (<2% or ≥2%). The primary outcome was the response to mometasone as compared with placebo and to tiotropium as compared with placebo among patients with a low sputum eosinophil level who had a prespecified differential response to one of the trial agents. The response was determined according to a hierarchical composite outcome that incorporated treatment failure, asthma control days, and the forced expiratory volume in 1 second; a two-sided P value of less than 0.025 denoted statistical significance. A secondary outcome was a comparison of results in patients with a high sputum eosinophil level and those with a low level.

RESULTS

A total of 73% of the patients had a low eosinophil level; of these patients, 59% had a differential response to a trial agent. However, there was no significant difference in the response to mometasone or tiotropium, as compared with placebo. Among the patients with a low eosinophil level who had a differential treatment response, 57% (95% confidence interval [CI], 48 to 66) had a better response to mometasone, and 43% (95% CI, 34 to 52) had a better response to placebo (P = 0.14). In contrast 60% (95% CI, 51 to 68) had a better response to tiotropium, whereas 40% (95% CI, 32 to 49) had a better response to placebo (P = 0.029). Among patients with a high eosinophil level, the response to mometasone was significantly better than the response to placebo (74% vs. 26%) but the response to tiotropium was not (57% vs. 43%).

CONCLUSIONS

The majority of patients with mild, persistent asthma had a low sputum eosinophil level and had no significant difference in their response to either mometasone or tiotropium as compared with placebo. These data provide equipoise for a clinically directive trial to compare an inhaled glucocorticoid with other treatments in patients with a low eosinophil level. (Funded by the National Heart, Lung, and Blood Institute; SIENA ClinicalTrials.gov number, NCT02066298.).

Rare SOX2+ Airway Progenitor Cells Generate KRT5+ Cells that Repopulate Damaged Alveolar Parenchyma following Influenza Virus Infection

Recent studies have implicated keratin 5 (KRT5)⁺ cells in repopulation of damaged lung tissue following severe H1N1 influenza virus infection. However, the origins of the cells repopulating the injured alveolar region remain controversial. We sought to determine the cellular dynamics of lung repair following influenza infection and define whether nascent KRT5⁺ cells repopulating alveolar epithelium were derived from pre-existing alveolar or airway progenitor cells. We found that the wound-healing response begins with proliferation of SOX2⁺ SCGB1A1^- KRT5^- progenitor cells in airways. These cells generate nascent KRT5⁺ cells as an early response to airway injury and yield progeny that colonize damaged alveolar parenchyma. Moreover, we show that local alveolar progenitors do not contribute to nascent KRT5⁺ cells after injury. Repopulation of injured airway and alveolar regions leads to proximalization of distal airways by pseudostratified epithelium and of alveoli by airway-derived epithelial cells that lack the normal characteristics of mature airway or alveolar epithelium.

Role of Matrix Metalloproteinases-1 and -2 in Interleukin-13-Suppressed Elastin in Airway Fibroblasts in Asthma

Elastin synthesis and degradation in the airway and lung parenchyma contribute to airway mechanics, including airway patency and elastic recoil. IL-13 mediates many features of asthma pathobiology, including airway remodeling, but the effects of IL-13 on elastin architecture in the airway wall are not known. We hypothesized that IL-13 modulates elastin expression in airway fibroblasts from subjects with allergic asthma. Twenty-five subjects with mild asthma (FEV1, 89 ± 3% predicted) and 30 normal control subjects (FEV1, 102 ± 2% predicted) underwent bronchoscopy with endobronchial biopsy. Elastic fibers were visualized in airway biopsy specimens using Weigert's resorcin-fuchsin elastic stain. Airway fibroblasts were exposed to IL-13; a pan-matrix metalloproteinase (MMP) inhibitor (GM6001); specific inhibitors to MMP-1, -2, -3, and -8; and combinations of IL-13 with MMP inhibitors in separate conditions in serum-free media for 48 hours. Elastin (ELN) expression as well as MMP secretion and activity were quantified. Results of this study show that elastic fiber staining of airway biopsy tissue was significantly associated with methacholine PC20 (i.e., the provocative concentration of methacholine resulting in a 20% fall in FEV1 levels) in patients with asthma. IL-13 significantly suppressed ELN expression in asthmatic airway fibroblasts as compared with normal control fibroblasts. The effect of IL-13 on ELN expression was significantly correlated with postbronchodilator FEV1/FVC in patients with asthma. MMP inhibition significantly stimulated ELN expression in patients with asthma as compared with normal control subjects. Specific inhibition of MMP-1 and MMP-2, but not MMP-3 or MMP-8, reversed the IL-13-induced suppression of ELN expression. In asthma, MMP-1 and MMP-2 mediate IL-13-induced suppression of ELN expression in airway fibroblasts.

A Protocol for the Comprehensive Flow Cytometric Analysis of Immune Cells in Normal and Inflamed Murine Non-Lymphoid Tissues

Flow cytometry is used extensively to examine immune cells in non-lymphoid tissues. However, a method of flow cytometric analysis that is both comprehensive and widely applicable has not been described. We developed a protocol for the flow cytometric analysis of non-lymphoid tissues, including methods of tissue preparation, a 10-fluorochrome panel for cell staining, and a standardized gating strategy, that allows the simultaneous identification and quantification of all major immune cell types in a variety of normal and inflamed non-lymphoid tissues. We demonstrate that our basic protocol minimizes cell loss, reliably distinguishes macrophages from dendritic cells (DC), and identifies all major granulocytic and mononuclear phagocytic cell types. This protocol is able to accurately quantify 11 distinct immune cell types, including T cells, B cells, NK cells, neutrophils, eosinophils, inflammatory monocytes, resident monocytes, alveolar macrophages, resident/interstitial macrophages, CD11b^- DC, and CD11b⁺ DC, in normal lung, heart, liver, kidney, intestine, skin, eyes, and mammary gland. We also characterized the expression patterns of several commonly used myeloid and macrophage markers. This basic protocol can be expanded to identify additional cell types such as mast cells, basophils, and plasmacytoid DC, or perform detailed phenotyping of specific cell types. In examining models of primary and metastatic mammary tumors, this protocol allowed the identification of several distinct tumor associated macrophage phenotypes, the appearance of which was highly specific to individual tumor cell lines. This protocol provides a valuable tool to examine immune cell repertoires and follow immune responses in a wide variety of tissues and experimental conditions.

Novel inactivating mutation of the calcium-sensing receptor in a young woman with mild hypercalcaemia

A young woman with mild hypercalcaemia and an inappropriately normal serum parathyroid hormone had parathyroid scintigraphy suggestive of an active ectopic parathyroid tissue in the superior mediastinum. Urinary calcium to creatinine clearance ratio was low, and a subsequent genetic analysis confirmed a novel mutation (Q164K) in the calcium sensing receptor gene, consistent with familial hypocalciuric hypercalcaemia. We propose that this mutation accounts for her clinical and investigational findings, although a double pathology of Q164K and an ectopic parathyroid adenoma is also conceivable.

Nitric oxide mediates relative airway hyporesponsiveness to lipopolysaccharide in surfactant protein A-deficient mice

Surfactant protein A (SP-A) mediates innate immune cell responses to LPS, a cell wall component of gram-negative bacteria that is found ubiquitously in the environment and is associated with adverse health effects. Inhaled LPS induces lung inflammation and increases airway responsiveness (AR). However, the role of SP-A in mediating LPS-induced AR is not well-defined. Nitric oxide (NO) is described as a potent bronchodilator, and previous studies showed that SP-A modulates the LPS-induced production of NO. Hence, we tested the hypothesis that increased AR, observed in response to aerosolized LPS exposure, would be significantly reduced in an SP-A-deficient condition. Wild-type (WT) and SP-A null (SP-A(-/-)) mice were challenged with aerosolized LPS. Results indicate that despite similar inflammatory indices, LPS-treated SP-A(-/-) mice had attenuated AR after methacholine challenge, compared with WT mice. The attenuated AR could not be attributed to inherent differences in SP-D concentrations or airway smooth muscle contractile and relaxation properties, because these measures were similar between WT and SP-A(-/-) mice. LPS-treated SP-A(-/-) mice, however, had elevated nitrite concentrations, inducible nitric oxide synthase (iNOS) expression, and NOS activity in their lungs. Moreover, the administration of the iNOS-specific inhibitor 1400W completely abrogated the attenuated AR. Thus, when exposed to aerosolized LPS, SP-A(-/-) mice demonstrate a relative airway hyporesponsiveness that appears to be mediated at least partly via an iNOS-dependent mechanism. These findings may have clinical significance, because recent studies reported associations between surfactant protein polymorphisms and a variety of lung diseases.

Acid-stress-induced changes in enterohaemorrhagic Escherichia coli O157 : H7 virulence

Enterohaemorrhagic Escherichia coli (EHEC) O157 : H7 is naturally exposed to a wide variety of stresses including gastric acid shock, and yet little is known about how this stress influences virulence. This study investigated the impact of acid stress on several critical virulence properties including survival, host adhesion, Shiga toxin production, motility and induction of host-cell apoptosis. Several acid-stress protocols with relevance for gastric passage as well as external environmental exposure were included. Acute acid stress at pH 3 preceded by acid adaptation at pH 5 significantly enhanced the adhesion of surviving organisms to epithelial cells and bacterial induction of host-cell apoptosis. Motility was also significantly increased after acute acid stress. Interestingly, neither secreted nor periplasmic levels of Shiga toxin were affected by acid shock. Pretreatment of bacteria with erythromycin eliminated the acid-induced adhesion enhancement, suggesting that de novo protein synthesis was required for the enhanced adhesion of acid-shocked organisms. DNA microarray was used to analyse the transcriptome of an EHEC O157 : H7 strain exposed to three different acid-stress treatments. Expression profiles of acid-stressed EHEC revealed significant changes in virulence factors associated with adhesion, motility and type III secretion. These results document profound changes in the virulence properties of EHEC O157 : H7 after acid stress, provide a comprehensive genetic analysis to substantiate these changes and suggest strategies that this pathogen may use during gastric passage and colonization in the human gastrointestinal tract.

Debriefing in the intensive care unit: a feedback tool to facilitate bedside teaching

OBJECTIVE

To develop an assessment tool for bedside teaching in the intensive care unit (ICU) that provides feedback to residents about their performance compared with clinical best practices.

METHOD

We reviewed the literature on the assessment of resident clinical performance in critical care medicine and summarized the strengths and weaknesses of these assessments. Using debriefing after simulation as a model, we created five checklists for different situations encountered in the ICU--areas that encompass different Accreditation Council for Graduate Medical Education core competencies. Checklists were designed to incorporate clinical best practices as defined by the literature and institutional practices as defined by the critical care professionals working in our ICUs. Checklists were used at the beginning of the rotation to explicitly define our expectations to residents and were used during the rotation after a clinical encounter by the resident and supervising physician to review a resident's performance and to provide feedback to the resident on the accuracy of the resident's self-assessment of his or her performance.

RESULTS

Five "best practice" checklists were developed: central catheter placement, consultation, family discussions, resuscitation of hemorrhagic shock, and resuscitation of septic shock. On average, residents completed 2.6 checklists per rotation. Use of the cards was fairly evenly distributed, with the exception of resuscitation of hemorrhagic shock, which occurs less frequently than the other encounters in the medical ICU. Those who used more debriefing cards had higher fellow and faculty evaluations. Residents felt that debriefing cards were a useful learning tool in the ICU.

CONCLUSIONS

Debriefing sessions using checklists can be successfully implemented in ICU rotations. Checklists can be used to assess both resident performance and consistency of practice with respect to published standards of care in critical care medicine.

Bisphosphonate space measurement and dosage requirement for intravenous clodronate treatment of Paget's disease

It has been claimed that the bisphosphonate space, a scintigraphic technique which simultaneously estimates bone uptake and renal clearance of bisphosphonate, can be used to predict the dose of bisphosphonate required to induce biochemical remission in Paget's disease. In a prospective study of 15 newly diagnosed patients with Paget's disease, bisphosphonate space measurements were made prior to treatment with intravenous clodronate. Treatment with clodronate 0.6 g was given four times weekly until the alkaline phosphatase (ALP) suppressed below the upper level of the reference range (120 u/l) or reached a plateau, and the cumulative dose required (1.8-7.8 g) was calculated. Overall, the bisphosphonate space correlated poorly with the total dose requirement (r = 0.441, P = 0.100), but the relationship was weakened by an outlier, who had the poorest renal function. Excluding this subject improved the correlation coefficient (r = 0.852, P <0.0001). The relationship between dose requirement and log10 initial ALP was not as strong (r = 0.672, P <0.01). However, for both ALP and the bisphosphonate space the 95% prediction intervals were wide. We conclude that the bisphosphonate space does relate to dose requirements for intravenous bisphosphonates, but that it is unreliable when renal function is poor, and may not offer much gain over the pretreatment ALP levels. Both ALP and the bisphosphonate space have wide prediction intervals, and are therefore poor guides to dose requirement.

Electronic structure of high-spin iron(III)-alkylperoxo complexes and its relation to low-spin analogues: reaction coordinate of O-O bond homolysis

The spectroscopic properties of the high-spin Fe(III)-alkylperoxo model complex [Fe(6-Me(3)TPA)(OH(x))(OO(t)Bu)](x)(+) (1; TPA = tris(2-pyridylmethyl)amine, (t)Bu = tert-butyl, x = 1 or 2) are defined and related to density functional calculations of corresponding models in order to determine the electronic structure and reactivity of this system. The Raman spectra of 1 show four peaks at 876, 842, 637, and 469 cm(-1) that are assigned with the help of normal coordinate analysis, and corresponding force constants have been determined to be 3.55 mdyn/A for the O-O and 2.87 mdyn/A for the Fe-O bond. Complex 1 has a broad absorption feature around 560 nm that is assigned to a charge-transfer (CT) transition from the alkylperoxo to a t(2g) d orbital of Fe(III) with the help of resonance Raman profiles and MCD spectroscopy. An additional contribution to the Fe-O bond arises from a sigma interaction between and an e(g) d orbital of iron. The electronic structure of 1 is compared to the related low-spin model complex [Fe(TPA)(OH(x))(OO(t)Bu)](x)(+) and the reaction coordinate for O-O homolysis is explored for both the low-spin and the high-spin Fe(III)-alkylperoxo systems. Importantly, there is a barrier for homolytic cleavage of the O-O bond on the high-spin potential energy surface that is not present for the low-spin complex, which is therefore nicely set up for O-O homolysis. This is reflected by the electronic structure of the low-spin complex having a strong Fe-O and a weak O-O bond due to a strong Fe-O sigma interaction. In addition, the reaction coordinate of the Fe-O homolysis has been investigated, which is a possible decay pathway for the high-spin system, but which is thermodynamically unfavorable for the low-spin complex.

Electronic structure of high-spin iron(III)-alkylperoxo complexes and its relation to low-spin analogues: reaction coordinate of O-O bond homolysis

The spectroscopic properties of the high-spin Fe(III)-alkylperoxo model complex [Fe(6-Me(3)TPA)(OH(x))(OO(t)Bu)](x)(+) (1; TPA = tris(2-pyridylmethyl)amine, (t)Bu = tert-butyl, x = 1 or 2) are defined and related to density functional calculations of corresponding models in order to determine the electronic structure and reactivity of this system. The Raman spectra of 1 show four peaks at 876, 842, 637, and 469 cm(-1) that are assigned with the help of normal coordinate analysis, and corresponding force constants have been determined to be 3.55 mdyn/A for the O-O and 2.87 mdyn/A for the Fe-O bond. Complex 1 has a broad absorption feature around 560 nm that is assigned to a charge-transfer (CT) transition from the alkylperoxo to a t(2g) d orbital of Fe(III) with the help of resonance Raman profiles and MCD spectroscopy. An additional contribution to the Fe-O bond arises from a sigma interaction between and an e(g) d orbital of iron. The electronic structure of 1 is compared to the related low-spin model complex [Fe(TPA)(OH(x))(OO(t)Bu)](x)(+) and the reaction coordinate for O-O homolysis is explored for both the low-spin and the high-spin Fe(III)-alkylperoxo systems. Importantly, there is a barrier for homolytic cleavage of the O-O bond on the high-spin potential energy surface that is not present for the low-spin complex, which is therefore nicely set up for O-O homolysis. This is reflected by the electronic structure of the low-spin complex having a strong Fe-O and a weak O-O bond due to a strong Fe-O sigma interaction. In addition, the reaction coordinate of the Fe-O homolysis has been investigated, which is a possible decay pathway for the high-spin system, but which is thermodynamically unfavorable for the low-spin complex.

Spectroscopic properties and electronic structure of low-spin Fe(III)-alkylperoxo complexes: homolytic cleavage of the O-O bond

The spectroscopic properties, electronic structure, and reactivity of the low-spin Fe(III)-alkylperoxo model complex [Fe(TPA)(OH(x))(OO(t)Bu)](x+) (1; TPA = tris(2-pyridylmethyl)amine, (t)Bu = tert-butyl, x = 1 or 2) are explored. The vibrational spectra of 1 show three peaks that are assigned to the O-O stretch (796 cm(-1)), the Fe-O stretch (696 cm(-)(1)), and a combined O-C-C/C-C-C bending mode (490 cm(-1)) that is mixed with upsilon(FeO). The corresponding force constants have been determined to be 2.92 mdyn/A for the O-O bond which is small and 3.53 mdyn/A for the Fe-O bond which is large. Complex 1 is characterized by a broad absorption band around 600 nm that is assigned to a charge-transfer (CT) transition from the alkylperoxo pi*(upsilon) to a t(2g) d orbital of Fe(III). This metal-ligand pi bond is probed by MCD and resonance Raman spectroscopies which show that the CT state is mixed with a ligand field state (t(2g) --> e(g)) by configuration interaction. This gives rise to two intense transitions under the broad 600 nm envelope with CT character which are manifested by a pseudo-A term in the MCD spectrum and by the shapes of the resonance Raman profiles of the 796, 696, and 490 cm(-1) vibrations. Additional contributions to the Fe-O bond arise from sigma interactions between mainly O-O bonding donor orbitals of the alkylperoxo ligand and an e(g) d orbital of Fe(III), which explains the observed O-O and Fe-O force constants. The observed homolytic cleavage of the O-O bond of 1 is explored with experimentally calibrated density functional (DFT) calculations. The O-O bond homolysis is found to be endothermic by only 15 to 20 kcal/mol due to the fact that the Fe(IV)=O species formed is highly stabilized (for spin states S = 1 and 2) by two strong pi and a strong sigma bond between Fe(IV) and the oxo ligand. This low endothermicity is compensated by the entropy gain upon splitting the O-O bond. In comparison, Cu(II)-alkylperoxo complexes studied before [Chen, P.; Fujisawa, K.; Solomon, E. I. J. Am. Chem. Soc. 2000, 122, 10177] are much less suited for O-O bond homolysis, because the resulting Cu(III)=O species is less stable. This difference in metal-oxo intermediate stability enables the O-O homolysis in the case of iron but directs the copper complex toward alternative reaction channels.

Stereospecific alkane hydroxylation by non-heme iron catalysts: mechanistic evidence for an Fe(V)=O active species

High-valent iron-oxo species have frequently been invoked in the oxidation of hydrocarbons by both heme and non-heme enzymes. Although a formally Fe(V)=O species, that is, [(Por(*))Fe(IV)=O](+), has been widely accepted as the key oxidant in stereospecific alkane hydroxylation by heme systems, it is not established that such a high-valent state can be accessed by a non-heme ligand environment. Herein we report a systematic study on alkane oxidations with H(2)O(2) catalyzed by a group of non-heme iron complexes, that is, [Fe(II)(TPA)(CH(3)CN)(2)](2+) (1, TPA = tris(2-pyridylmethyl)amine) and its alpha- and beta-substituted analogues. The reactivity patterns of this family of Fe(II)(TPA) catalysts can be modulated by the electronic and steric properties of the ligand environment, which affects the spin states of a common Fe(III)-OOH intermediate. Such an Fe(III)-peroxo species is high-spin when the TPA ligand has two or three alpha-substituents and is proposed to be directly responsible for the selective C-H bond cleavage of the alkane substrate. The thus-generated alkyl radicals, however, have relatively long lifetimes and are susceptible to radical epimerization and trapping by O(2). On the other hand, 1 and the beta-substituted Fe(II)(TPA) complexes catalyze stereospecific alkane hydroxylation by a mechanism involving both a low-spin Fe(III)-OOH intermediate and an Fe(V)=O species derived from O-O bond heterolysis. We propose that the heterolysis pathway is promoted by two factors: (a) the low-spin iron(III) center which weakens the O-O bond and (b) the binding of an adjacent water ligand that can hydrogen bond to the terminal oxygen of the hydroperoxo group and facilitate the departure of the hydroxide. Evidence for the Fe(V)=O species comes from isotope-labeling studies showing incorporation of (18)O from H(2)(18)O into the alcohol products. (18)O-incorporation occurs by H(2)(18)O binding to the low-spin Fe(III)-OOH intermediate, its conversion to a cis-H(18)O-Fe(V)=O species, and then oxo-hydroxo tautomerization. The relative contributions of the two pathways of this dual-oxidant mechanism are affected by both the electron donating ability of the TPA ligand and the strength of the C-H bond to be broken. These studies thus serve as a synthetic precedent for an Fe(V)=O species in the oxygen activation mechanisms postulated for non-heme iron enzymes such as methane monooxygenase and Rieske dioxygenases.

"Intermolecular" trapping of a nonheme Fe(IV)=O intermediate

The reactions of Fe(III)(TPA) (TPA = tris(2-pyridylmethyl)amine) complexes with 2-methyl-1-phenyl-2-propyl-hydroperoxide (MPPH) delivered by syringe pump under anaerobic conditions afford nearly quantitative conversion of MPPH to products derived from the benzyl radical. These results unequivocally show that MPPH breaks down by O-O bond homolysis, leading to the formation of the benzyl radical and a high valent Fe(IV)=O species. Without added substrates, the benzyl radical reacts with the high valent species to form benzyl alcohol or benzyl halides. The Fe(IV)=O species can also effect the two-electron oxidation of added substrates such as thioanisole, cyclohexanol, and cyclooctene under appropriate conditions. The oxidation of thioanisole and cyclohexanol is likely facilitated by pre-equilibrium binding of the substrate to the metal center, allowing these substrates to intercept the high valent iron-oxo species as it forms. These results suggest the importance of close proximity to direct the high valent metal center down a desired pathway.

Models for extradiol cleaving catechol dioxygenases: syntheses, structures, and reactivities of iron(II)-monoanionic catecholate complexes

Crystallographic and spectroscopic studies of extradiol cleaving catechol dioxygenases indicate that the enzyme-substrate complexes have both an iron(II) center and a monoanionic catecholate. Herein we report a series of iron(II)-monoanionic catecholate complexes, [(L)Fe(II)(catH)](X) (1a, L = 6-Me(3)-TPA (tris(6-methyl-2-pyridylmethyl)amine), catH = CatH (1,2-catecholate monoanion); 1b, L = 6-Me(3)-TPA, catH = DBCH (3,5-di-tert-butyl-1,2-catecholate monoanion); 1c, L = 6-Me(2)-bpmcn (N,N'-dimethyl-N,N'-bis(6-methyl-2-pyridylmethyl)-trans-1,2-diaminocyclohexane), catH = CatH; 1d, L = 6-Me(2)-bpmcn, catH = DBCH), that model such enzyme complexes. The crystal structure of [(6-Me(2)-bpmcn)Fe(II)(DBCH)](+) (1d) shows that the DBCH ligand binds to the iron asymmetrically as previously reported for 1b, with two distinct Fe-O bonds of 1.943(1) and 2.344(1) A. Complexes 1 react with O(2) or NO to afford blue-purple iron(III)-catecholate dianion complexes, [(L)Fe(III)(cat)](+) (2). Interestingly, crystallographically characterized 2d, isolated from either reaction, has the N-methyl groups in a syn configuration, in contrast to the anti configuration of the precursor complex, so epimerization of the bound ligand must occur in the course of isolating 2d. This notion is supported by the fact that the UV-vis and EPR properties of in situ generated 2d(anti) differ from those of isolated 2d(syn). While the conversion of 1 to 2 in the presence of O(2) occurs without an obvious intermediate, that in the presence of NO proceeds via a metastable S = (3)/(2) [(L)Fe(catH)(NO)](+) adduct 3, which can only be observed spectroscopically but not isolated. Intermediates 3a and 3b subsequently disproportionate to afford two distinct complexes, [(6-Me(3)-TPA)Fe(III)(cat)](+) (2a and 2b) and [(6-Me(3)-TPA)Fe(NO)(2)](+) (4) in comparable yield, while 3d converts to 2d in 90% yield. Complexes 2b and anti-2d react further with O(2) over a 24 h period and afford a high yield of cleavage products. Product analysis shows that the products mainly derive from intradiol cleavage but with a small extent of extradiol cleavage (89:3% for 2b and 78:12% for anti-2d). The small amounts of the extradiol cleavage products observed may be due to the dissociation of an alpha-methyl substituted pyridyl arm, generating a complex with a tridentate ligand. Surprisingly, syn-2d does not react with O(2) over the course of 4 days. These results suggest that there are a number of factors that influence the mode and rate of cleavage of catechols coordinated to iron centers.

Resonance Raman studies of the iron(II)--alpha-keto acid chromophore in model and enzyme complexes

The bidentate coordination of an alpha-keto acid to an iron(II) center via the keto group and the carboxylate gives rise to metal-to-ligand charge-transfer transitions between 400 and 600 nm in model complexes and in alpha-ketoglutarate-dependent dioxygenases. Excitation into these absorption bands of the Fe(II)TauD(alpha-KG) complex (TauD = taurine/alpha-ketoglutarate dioxygenase, alpha-KG = alpha-ketoglutarate) elicits two resonance Raman features at 460 and 1686 cm(-1), both of which are sensitive to (18)O labeling. Corresponding studies of model complexes, the six-coordinate [Fe(II)(6-Me(3)-TPA)(alpha-keto acid)](+) and the five-coordinate [Fe(II)(Tp(Ph2))(alpha-keto acid)] (6-Me(3)-TPA = tris[(6-methyl-2-pyridyl)methyl]amine, Tp(Ph2) = hydrotris(3,5-diphenylpyrazol-1-yl)borate), lead to the assignment of these two features to the Fe(II)(alpha-keto acid) chelate mode and the nu(C==O) of the keto carbonyl group, respectively. Furthermore, the chelate mode is sensitive to the coordination number of the metal center; binding of a sixth ligand to the five-coordinate [Fe(II)(Tp(Ph2))(benzoylformate)] elicits a 9--20 cm(-1) downshift. Thus, the 10 cm(-1) upshift of the chelate mode observed for Fe(II)TauD(alpha-KG) upon the addition of the substrate, taurine, is associated with the conversion of the six-coordinate metal center to a five-coordinate center, as observed for the iron center of clavaminate synthase from X-ray crystallography (Zhang, Z.; et al. Nat. Struct. Biol. 2000, 7, 127-133) and MCD studies (Zhou, J.; et al. J. Am. Chem. Soc. 1998, 120, 13539--13540). These studies provide useful insights into the initial steps of the oxygen activation mechanism of alpha-ketoglutarate-dependent dioxygenases.

A mechanistic study of the reaction between a diiron(II) complex [FeII(2)(mu-OH)2(6-Me3-TPA)2](2+) and O2 to form a diiron(III) peroxo complex

A kinetic study of the reaction between a diiron(II) complex [Fe(II)(2)(mu-OH)(2)(6-Me(3)-TPA)(2)](2+) 1, where 6-Me(3)-TPA = tris(6-methyl-2-pyridylmethyl)amine, and dioxygen is presented. A diiron(III) peroxo complex [Fe(III)(2)(mu-O)(mu-O(2))(6-Me(3)-TPA)(2)](2+) 2 forms quantitatively in dichloromethane at temperatures from -80 to -40 degrees C. The reaction is first order in [Fe(II)(2)] and [O(2)], with the activation parameters DeltaH(double dagger) = 17 +/- 2 kJ mol(-1) and DeltaS(double dagger) = -175 +/- 20 J mol(-1) K(-1). The reaction rate is not significantly influenced by the addition of H(2)O or D(2)O. The reaction proceeds faster in more polar solvents (acetone and acetonitrile), but the yield of 2 is not quantitative in these solvents. Complex 1 reacts with NO at a rate about 10(3) faster than with O(2). The mechanistic analysis suggests an associative rate-limiting step for the oxygenation of 1, similar to that for stearoyl-ACP Delta(9)-desaturase, but distinct from the probable dissociative pathway of methane monoxygenase. An eta(1)-superoxo Fe(II)Fe(III) species is a likely steady-state intermediate during the oxygenation of complex 1.

Catalysis research of relevance to carbon management: progress, challenges, and opportunities

The goal of the "Opportunities for Catalysis Research in Carbon Management" workshop was to review within the context of greenhouse gas/carbon issues the current state of knowledge, barriers to further scientific and technological progress, and basic scientific research needs in the areas of H2 generation and utilization, light hydrocarbon activation and utilization, carbon dioxide activation, utilization, and sequestration, emerging techniques and research directions in relevant catalysis research, and in catalysis for more efficient transportation engines. Several overarching themes emerge from this review. First and foremost, there is a pressing need to better understand in detail the catalytic mechanisms involved in almost every process area mentioned above. This includes the structures, energetics, lifetimes, and reactivities of the species thought to be important in the key catalytic cycles. As much of this type of information as is possible to acquire would also greatly aid in better understanding perplexing, incomplete/inefficient catalytic cycles and in inventing new, efficient ones. The most productive way to attack such problems must include long-term, in-depth fundamental studies of both commercial and model processes, by conventional research techniques and, importantly, by applying various promising new physicochemical and computational approaches which would allow incisive, in situ elucidation of reaction pathways. There is also a consensus that more exploratory experiments, especially high-risk, unconventional catalytic and model studies, should be undertaken. Such an effort will likely require specialized equipment, instrumentation, and computational facilities. The most expeditious and cost-effective means to carry out this research would be by close coupling of academic, industrial, and national laboratory catalysis efforts worldwide. Completely new research approaches should be vigorously explored, ranging from novel compositions, fabrication techniques, reactors, and reaction conditions for heterogeneous catalysts, to novel ligands and ligation geometries (e.g., biomimetic), reaction media, and activation methods for homogeneous ones. The interplay between these two areas involving various hybrid and single-site supported catalyst systems should also be productive. Finally, new combinatorial and semicombinatorial means to rapidly create and screen catalyst systems are now available. As a complement to the approaches noted above, these techniques promise to greatly accelerate catalyst discovery, evaluation, and understanding. They should be incorporated in the vigorous international research effort needed in this field.

A metabolic enzyme for S-nitrosothiol conserved from bacteria to humans

Considerable evidence indicates that NO biology involves a family of NO-related molecules and that S-nitrosothiols (SNOs) are central to signal transduction and host defence. It is unknown, however, how cells switch off the signals or protect themselves from the SNOs produced for defence purposes. Here we have purified a single activity from Escherichia coli, Saccharomyces cerevisiae and mouse macrophages that metabolizes S-nitrosoglutathione (GSNO), and show that it is the glutathione-dependent formaldehyde dehydrogenase. Although the enzyme is highly specific for GSNO, it controls intracellular levels of both GSNO and S-nitrosylated proteins. Such 'GSNO reductase' activity is widely distributed in mammals. Deleting the reductase gene in yeast and mice abolishes the GSNO-consuming activity, and increases the cellular quantity of both GSNO and protein SNO. Furthermore, mutant yeast cells show increased susceptibility to a nitrosative challenge, whereas their resistance to oxidative stress is unimpaired. We conclude that GSNO reductase is evolutionarily conserved from bacteria to humans, is critical for SNO homeostasis, and protects against nitrosative stress.

Double-strand hydrolysis of plasmid DNA by dicerium complexes at 37 degrees C

Significant effort has been made to develop synthetic metal complexes that hydrolyze DNA. Here we report a new dicerium complex, Ce(2)(HXTA) (HXTA = 5-methyl-2-hydroxy-1,3-xylene-alpha,alpha-diamine-N,N,N',N'-tetraacetic acid), which can hydrolyze DNA at pH 8 and 37 degrees C. This complex hydrolyzes DNA restriction fragments to give products with high regioselectivity, affording >90% 5'-OPO(3) and 3'-OH ends, like the products of DNA hydrolyzing enzymes. Ce(2)(HXTA) also hydrolyzes Litmus 29 plasmid DNA to afford both nicked and linear DNA. Analysis of the relative amounts of supercoiled, nicked, and linear DNA present show that there is one double-strand cleavage per ten single-strand cleavages, indicating that the linear DNA formed cannot be the result of two random single-strand cleavage events. The kinetics of nicked and linear DNA formation are comparable, both being associated with apparent first-order rate constants of approximately 1 x 10(-)(4) s(-)(1) for complex concentrations of 10(-)(5)-10(-)(4) M. These observations suggest that similar factors affect the hydrolysis of the first and second DNA strands and that cleaving the phosphodiester bond is likely the rate determining step in both cases. This is the first detailed study of a metal complex shown to mimic DNA hydrolases in their capability to effect double-strand DNA hydrolysis regioselectively at the 3'-O-P bond.

A nonheme iron(II) complex that models the redox cycle of lipoxygenase

The air-stable complex [Fe(6-Me3-TPA) (O2CAr)]+ [1; 6-Me3-TPA = tris(6-methyl-2-pyridylmethyl)amine] has been synthesized as a model for the iron(II) site of lipoxygenase. This iron(II) complex reacts with 0.5 equiv ROOH to form a yellow species, which has been formulated as [FeIII(OH)(6-Me3-TPA) (O2CAr)]+ (2) by electrospray mass spectrometry. Addition of more ROOH converts 2 into a purple species, which is characterized by electrospray ionization mass spectrometry and resonance Raman spectroscopy as [FeIII(OOR)(6-Me3-TPA)(O2CAr)]+. The purple species is metastable and decomposes via Fe-O bond homolysis to regenerate the starting iron(II) complex. These metal-centered transformations parallel the changes observed for lipoxygenase in its reaction with its product hydroperoxide.

N-donor effects on carboxylate binding in mononuclear iron(II) complexes of a sterically hindered benzoate ligand

Using the sterically hindered 2,6-dimesitylbenzoate ligand Mes2ArCO2-, a series of mononuclear Fe(II) carboxylate complexes has been obtained with the general formula (Mes2ArCO2)2Fe(base)2 (base = 1-methylimidazole (MeIm), pyridine (Py), 2-picoline (2-Pic), 2,5-lutidine (2,5-Lut), 2,6-lutidine (2,6-Lut), (base)2 = N,N,N',N'-tetramethylethylenediamine (TMEDA)). For the monodentate base adducts, single-crystal X-ray diffraction studies revealed several different structural types ranging from distorted tetrahedral to distorted octahedral that correlate with the degree of alpha-substitution of the N-donors. Increasing alpha-substitution leads to the lengthening of the Fe-N bond, which in turn results in a change in carboxylate binding mode from eta 1 to eta 2. We surmise that this change is due to an electrostatic effect and is driven by increasing the Lewis acidity of the Fe center. Such a simple process for inducing carboxylate shifts could play a critical role in biological systems.

"FUR"--one size suits all

This work used amalgamated data from previous projects in order to test the concept that when organ function is expressed in terms of tracer kinetics, the results are independent of patient size or gender. Dynamic gamma camera studies were analysed by measuring the rate of movement of tracers from the blood into various organs. These rates were expressed as a "fractional uptake rate" (FUR), which is the fraction of tracer in the blood taken up by the organ per unit time. As these values were small, it was convenient to express the FUR per million seconds. The FUR was calculated using the expression FUR = SLOPE (of Rutland-Patlak plot), multiplied by B(0) (the blood curve value back-extrapolated to time zero), and divided by the TOTAL amount of tracer injected. Data were used from adult patients between the ages of 20 and 49 years who had normal organ function. Organ/tracer groups studied were the skeletal uptake of 99mTc-MDP, the renal uptake of 99mTc-MAG3, the renal uptake of 99mTc-MDP, the renal uptake of 99mTc-DTPA, the hepatic uptake of 99mTc-colloid, the splenic uptake of 99mTc-colloid, and the hepatic uptake of 99mTc-DISIDA. Each organ/tracer group was divided into three subgroups according to patient size (smallest, middle and largest), and also into subgroups according to gender. Comparison of these subgroups did not show any significant size- or gender-related differences in FUR values. It is concluded that for patients with normally functioning organs the FUR is independent of patient size or gender. Thus, the FUR is a valuable way of expressing organ function, particularly in patients with unusual or rapidly changing body size, such as children.