Inhibition of Dihydroorotate Dehydrogenase Overcomes Differentiation Blockade in Acute Myeloid Leukemia.

While acute myeloid leukemia (AML) comprises many disparate genetic subtypes, one shared hallmark is the arrest of leukemic myeloblasts at an immature and self-renewing stage of development. Therapies that overcome differentiation arrest represent a powerful treatment strategy. We leveraged the observation that the majority of AML, despite their genetically heterogeneity, share in the expression of HoxA9, a gene normally downregulated during myeloid differentiation. Using a conditional HoxA9 model system, we performed a high-throughput phenotypic screen and defined compounds that overcame differentiation blockade. Target identification led to the unanticipated discovery that inhibition of the enzyme dihydroorotate dehydrogenase (DHODH) enables myeloid differentiation in human and mouse AML models. In vivo, DHODH inhibitors reduced leukemic cell burden, decreased levels of leukemia-initiating cells, and improved survival. These data demonstrate the role of DHODH as a metabolic regulator of differentiation and point to its inhibition as a strategy for overcoming differentiation blockade in AML.

Health Consequences of Thymus Removal in Adults.

BACKGROUND: The function of the thymus in human adults is unclear, and routine removal of the thymus is performed in a variety of surgical procedures. We hypothesized that the adult thymus is needed to sustain immune competence and overall health. METHODS: We evaluated the risk of death, cancer, and autoimmune disease among adult patients who had undergone thymectomy as compared with demographically matched controls who had undergone similar cardiothoracic surgery without thymectomy. T-cell production and plasma cytokine levels were also compared in a subgroup of patients. RESULTS: After exclusions, 1420 patients who had undergone thymectomy and 6021 controls were included in the study; 1146 of the patients who had undergone thymectomy had a matched control and were included in the primary cohort. At 5 years after surgery, all-cause mortality was higher in the thymectomy group than in the control group (8.1% vs. 2.8%; relative risk, 2.9; 95% confidence interval [CI], 1.7 to 4.8), as was the risk of cancer (7.4% vs. 3.7%; relative risk, 2.0; 95% CI, 1.3 to 3.2). Although the risk of autoimmune disease did not differ substantially between the groups in the overall primary cohort (relative risk, 1.1; 95% CI, 0.8 to 1.4), a difference was found when patients with preoperative infection, cancer, or autoimmune disease were excluded from the analysis (12.3% vs. 7.9%; relative risk, 1.5; 95% CI, 1.02 to 2.2). In an analysis involving all patients with more than 5 years of follow-up (with or without a matched control), all-cause mortality was higher in the thymectomy group than in the general U.S. population (9.0% vs. 5.2%), as was mortality due to cancer (2.3% vs. 1.5%). In the subgroup of patients in whom T-cell production and plasma cytokine levels were measured (22 in the thymectomy group and 19 in the control group; mean follow-up, 14.2 postoperative years), those who had undergone thymectomy had less new production of CD4+ and CD8+ lymphocytes than controls (mean CD4+ signal joint T-cell receptor excision circle [sjTREC] count, 1451 vs. 526 per microgram of DNA [P = 0.009]; mean CD8+ sjTREC count, 1466 vs. 447 per microgram of DNA [P<0.001]) and higher levels of proinflammatory cytokines in the blood. CONCLUSIONS: In this study, all-cause mortality and the risk of cancer were higher among patients who had undergone thymectomy than among controls. Thymectomy also appeared be associated with an increased risk of autoimmune disease when patients with preoperative infection, cancer, or autoimmune disease were excluded from the analysis. (Funded by the Tracey and Craig A. Huff Harvard Stem Cell Institute Research Support Fund and others.).

Image-seq: spatially resolved single-cell sequencing guided by in situ and in vivo imaging.

Tissue function depends on cellular organization. While the properties of individual cells are increasingly being deciphered using powerful single-cell sequencing technologies, understanding their spatial organization and temporal evolution remains a major challenge. Here, we present Image-seq, a technology that provides single-cell transcriptional data on cells that are isolated from specific spatial locations under image guidance, thus preserving the spatial information of the target cells. It is compatible with in situ and in vivo imaging and can document the temporal and dynamic history of the cells being analyzed. Cell samples are isolated from intact tissue and processed with state-of-the-art library preparation protocols. The technique therefore combines spatial information with highly sensitive RNA sequencing readouts from individual, intact cells. We have used both high-throughput, droplet-based sequencing as well as SMARTseq-v4 library preparation to demonstrate its application to bone marrow and leukemia biology. We discovered that DPP4 is a highly upregulated gene during early progression of acute myeloid leukemia and that it marks a more proliferative subpopulation that is confined to specific bone marrow microenvironments. Furthermore, the ability of Image-seq to isolate viable, intact cells should make it compatible with a range of downstream single-cell analysis tools including multi-omics protocols.

Limited plasticity of monocyte fate and function associated with epigenetic scripting at the level of progenitors.

Myeloid cell heterogeneity is known, but whether it is cell-intrinsic or environmentally-directed remains unclear. Here, an inducible/reversible system pausing myeloid differentiation allowed the definition of clone-specific functions that clustered monocytes into subsets with distinctive molecular features. These subsets were orthogonal to the classical/nonclassical categorization and had inherent, restricted characteristics that did not shift under homeostasis, after irradiation, or with infectious stress. Rather, their functional fate was constrained by chromatin accessibility established at or before the granulocyte-monocyte or monocyte-dendritic progenitor level. Subsets of primary monocytes had differential ability to control distinct infectious agents in vivo. Therefore, monocytes are a heterogeneous population of functionally restricted subtypes defined by the epigenome of their progenitors that are differentially selected by physiologic challenges with limited plasticity to transition from one subset to another.

ThermoBRET: A Ligand-Engagement Nanoscale Thermostability Assay Applied to GPCRs.

Measurements of membrane protein thermostability reflect ligand binding. Current thermostability assays often require protein purification or rely on pre-existing radiolabelled or fluorescent ligands, limiting their application to established targets. Alternative methods, such as fluorescence-detection size exclusion chromatography thermal shift, detect protein aggregation but are not amenable to high-throughput screening. Here, we present a ThermoBRET method to quantify the relative thermostability of G protein coupled receptors (GPCRs), using cannabinoid receptors (CB1 and CB2 ) and the ß2 -adrenoceptor (ß2 AR) as model systems. ThermoBRET reports receptor unfolding, does not need labelled ligands and can be used with non-purified proteins. It uses Bioluminescence Resonance Energy Transfer (BRET) between Nanoluciferase (Nluc) and a thiol-reactive fluorescent dye that binds cysteines exposed by unfolding. We demonstrate that the melting point (Tm ) of Nluc-fused GPCRs can be determined in non-purified detergent solubilised membrane preparations or solubilised whole cells, revealing differences in thermostability for different solubilising conditions and in the presence of stabilising ligands. We extended the range of the assay by developing the thermostable tsNLuc by incorporating mutations from the fragments of split-Nluc (Tm of 87 °C versus 59 °C). ThermoBRET allows the determination of GPCR thermostability, which is useful for protein purification optimisation and drug discovery screening.

Disruption of dNTP homeostasis by ribonucleotide reductase hyperactivation overcomes AML differentiation blockade.

Differentiation blockade is a hallmark of acute myeloid leukemia (AML). A strategy to overcome such a blockade is a promising approach against the disease. The lack of understanding of the underlying mechanisms hampers development of such strategies. Dysregulated ribonucleotide reductase (RNR) is considered a druggable target in proliferative cancers susceptible to deoxynucleoside triphosphate (dNTP) depletion. Herein, we report an unanticipated discovery that hyperactivating RNR enables differentiation and decreases leukemia cell growth. We integrate pharmacogenomics and metabolomics analyses to identify that pharmacologically (eg, nelarabine) or genetically upregulating RNR subunit M2 (RRM2) creates a dNTP pool imbalance and overcomes differentiation arrest. Moreover, R-loop-mediated DNA replication stress signaling is responsible for RRM2 activation by nelarabine treatment. Further aggravating dNTP imbalance by depleting the dNTP hydrolase SAM domain and HD domain-containing protein 1 (SAMHD1) enhances ablation of leukemia stem cells by RRM2 hyperactivation. Mechanistically, excessive activation of extracellular signal-regulated kinase (ERK) signaling downstream of the imbalance contributes to cellular outcomes of RNR hyperactivation. A CRISPR screen identifies a synthetic lethal interaction between loss of DUSP6, an ERK-negative regulator, and nelarabine treatment. These data demonstrate that dNTP homeostasis governs leukemia maintenance, and a combination of DUSP inhibition and nelarabine represents a therapeutic strategy.

TLR Signaling Rescues Fungicidal Activity in Syk-Deficient Neutrophils.

An impaired neutrophil response to pathogenic fungi puts patients at risk for fungal infections with a high risk of morbidity and mortality. Acquired neutrophil dysfunction in the setting of iatrogenic immune modulators can include the inhibition of critical kinases such as spleen tyrosine kinase (Syk). In this study, we used an established system of conditionally immortalized mouse neutrophil progenitors to investigate the ability to augment Syk-deficient neutrophil function against Candida albicans with TLR agonist signaling. LPS, a known immunomodulatory molecule derived from Gram-negative bacteria, was capable of rescuing effector functions of Syk-deficient neutrophils, which are known to have poor fungicidal activity against Candida species. LPS priming of Syk-deficient mouse neutrophils demonstrates partial rescue of fungicidal activity, including phagocytosis, degranulation, and neutrophil swarming, but not reactive oxygen species production against C. albicans, in part due to c-Fos activation. Similarly, LPS priming of human neutrophils rescues fungicidal activity in the presence of pharmacologic inhibition of Syk and Bruton's tyrosine kinase (Btk), both critical kinases in the innate immune response to fungi. In vivo, neutropenic mice were reconstituted with wild-type or Syk-deficient neutrophils and challenged i.p. with C. albicans. In this model, LPS improved wild-type neutrophil homing to the fungal challenge, although Syk-deficient neutrophils did not persist in vivo, speaking to its crucial role on in vivo persistence. Taken together, we identify TLR signaling as an alternate activation pathway capable of partially restoring neutrophil effector function against Candida in a Syk-independent manner.

Accuracy of routine external ventricular drain placement following a mixed reality-guided twist-drill craniostomy.

OBJECTIVE: The traditional freehand placement of an external ventricular drain (EVD) relies on empirical craniometric landmarks to guide the craniostomy and subsequent passage of the EVD catheter. The diameter and trajectory of the craniostomy physically limit the possible trajectories that can be achieved during the passage of the catheter. In this study, the authors implemented a mixed reality-guided craniostomy procedure to evaluate the benefit of an optimally drilled craniostomy to the accurate placement of the catheter. METHODS: Optical marker-based tracking using an OptiTrack system was used to register the brain ventricular hologram and drilling guidance for craniostomy using a HoloLens 2 mixed reality headset. A patient-specific 3D-printed skull phantom embedded with intracranial camera sensors was developed to automatically calculate the EVD accuracy for evaluation. User trials consisted of one blind and one mixed reality-assisted craniostomy followed by a routine, unguided EVD catheter placement for each of two different drill bit sizes. RESULTS: A total of 49 participants were included in the study (mean age 23.4 years, 59.2% female). The mean distance from the catheter target improved from 18.6 ± 12.5 mm to 12.7 ± 11.3 mm (p = 0.0008) using mixed reality guidance for trials with a large drill bit and from 19.3 ± 12.7 mm to 10.1 ± 8.4 mm with a small drill bit (p < 0.0001). Accuracy using mixed reality was improved using a smaller diameter drill bit compared with a larger bit (p = 0.039). Overall, the majority of the participants were positive about the helpfulness of mixed reality guidance and the overall mixed reality experience. CONCLUSIONS: Appropriate indications and use cases for the application of mixed reality guidance to neurosurgical procedures remain an area of active inquiry. While prior studies have demonstrated the benefit of mixed reality-guided catheter placement using predrilled craniostomies, the authors demonstrate that real-time quantitative and visual feedback of a mixed reality-guided craniostomy procedure can independently improve procedural accuracy and represents an important tool for trainee education and eventual clinical implementation.

Platform Reagents Enable Synthesis of Ligand-Directed Covalent Probes: Study of Cannabinoid Receptor 2 in Live Cells.

Pharmacological modulation of cannabinoid receptor type 2 (CB2R) holds promise for the treatment of neuroinflammatory disorders, such as Alzheimer's disease. Despite the importance of CB2R, its expression and downstream signaling are insufficiently understood in disease- and tissue-specific contexts. Herein, we report the first ligand-directed covalent (LDC) labeling of CB2R enabled by a novel synthetic strategy and application of platform reagents. The LDC modification allows visualization and study of CB2R while maintaining its ability to bind other ligands at the orthosteric site. We employed in silico docking and molecular dynamics simulations to guide probe design and assess the feasibility of LDC labeling of CB2R. We demonstrate selective, covalent labeling of a peripheral lysine residue of CB2R by exploiting fluorogenic O-nitrobenzoxadiazole (O-NBD)-functionalized probes in a TR-FRET assay. The rapid proof-of-concept validation with O-NBD probes inspired incorporation of advanced electrophiles suitable for experiments in live cells. To this end, novel synthetic strategies toward N-sulfonyl pyridone (N-SP) and N-acyl-N-alkyl sulfonamide (NASA) LDC probes were developed, which allowed covalent delivery of fluorophores suitable for cellular studies. The LDC probes were characterized by a radioligand binding assay and TR-FRET experiments. Additionally, the probes were applied to specifically visualize CB2R in conventional and imaging flow cytometry as well as in confocal fluorescence microscopy using overexpressing and endogenously expressing microglial live cells.

How dimensions can guide surgical planning and training: a systematic review of Kambin's triangle.

OBJECTIVE: The authors sought to analyze the current literature to determine dimensional trends across the lumbar levels of Kambin's triangle, clarify the role of imaging techniques for preoperative planning, and understand the effect of inclusion of the superior articular process (SAP). This compiled knowledge of the triangle is needed to perform successful procedures, reduce nerve root injuries, and help guide surgeons in training. METHODS: The authors performed a search of multiple databases using combinations of keywords: Kambin's triangle, size, measurement, safe triangle, and bony triangle. Articles were included if their main findings included measurement of Kambin's triangle. The PubMed, Scopus, Ovid, Cochrane, Embase, and Medline databases were systematically searched for English-language articles with no time frame restrictions through July 2022. RESULTS: Eight studies comprising 132 patients or cadavers were included in the study. The mean ± SD age was 66.69 ± 9.6 years, and 53% of patients were male. Overall, the size of Kambin's triangle increased in area moving down vertebral levels, with L5-S1 being the largest (133.59 ± 4.36 mm2). This trend followed a linear regression model when SAP was kept (p = 0.008) and removed (p = 0.003). There was also a considerable increase in the size of Kambin's triangle if the SAP was removed. CONCLUSIONS: Here, the authors have provided the first reported systematic review of the literature of Kambin's triangle, its measurements at each lumbar level, and key areas of debate related to the definition of the working safe zone. These findings indicate that CT is heavily utilized for imaging of the safe zone, the area of Kambin's triangle tends to increase caudally, and variation exists between patients. Future studies should focus on using advanced imaging techniques for preoperative planning and establishing guidelines for surgeons.

The TEMPI syndrome.

The TEMPI syndrome is a rare and acquired disorder characterized by 5 salient features, which compose its name: (1) telangiectasias; (2) elevated erythropoietin and erythrocytosis; (3) monoclonal gammopathy; (4) perinephric fluid collections; and (5) intrapulmonary shunting. Complete resolution of symptoms following treatment with plasma cell-directed therapy supports the hypothesis that the monoclonal antibody is causal and pathogenic. Understanding the basis of the TEMPI syndrome will depend on the identification of additional patients and a coordinated international effort.

Aldehyde dehydrogenase 3a2 protects AML cells from oxidative death and the synthetic lethality of ferroptosis inducers.

Metabolic alterations in cancer represent convergent effects of oncogenic mutations. We hypothesized that a metabolism-restricted genetic screen, comparing normal primary mouse hematopoietic cells and their malignant counterparts in an ex vivo system mimicking the bone marrow microenvironment, would define distinctive vulnerabilities in acute myeloid leukemia (AML). Leukemic cells, but not their normal myeloid counterparts, depended on the aldehyde dehydrogenase 3a2 (Aldh3a2) enzyme that oxidizes long-chain aliphatic aldehydes to prevent cellular oxidative damage. Aldehydes are by-products of increased oxidative phosphorylation and nucleotide synthesis in cancer and are generated from lipid peroxides underlying the non-caspase-dependent form of cell death, ferroptosis. Leukemic cell dependence on Aldh3a2 was seen across multiple mouse and human myeloid leukemias. Aldh3a2 inhibition was synthetically lethal with glutathione peroxidase-4 (GPX4) inhibition; GPX4 inhibition is a known trigger of ferroptosis that by itself minimally affects AML cells. Inhibiting Aldh3a2 provides a therapeutic opportunity and a unique synthetic lethality to exploit the distinctive metabolic state of malignant cells.

Biased M1-muscarinic-receptor-mutant mice inform the design of next-generation drugs.

Cholinesterase inhibitors, the current frontline symptomatic treatment for Alzheimer's disease (AD), are associated with low efficacy and adverse effects. M1 muscarinic acetylcholine receptors (M1 mAChRs) represent a potential alternate therapeutic target; however, drug discovery programs focused on this G protein-coupled receptor (GPCR) have failed, largely due to cholinergic adverse responses. Employing novel chemogenetic and phosphorylation-deficient, G protein-biased, mouse models, paired with a toolbox of probe molecules, we establish previously unappreciated pharmacologically targetable M1 mAChR neurological processes, including anxiety-like behaviors and hyper-locomotion. By mapping the upstream signaling pathways regulating these responses, we determine the importance of receptor phosphorylation-dependent signaling in driving clinically relevant outcomes and in controlling adverse effects including 'epileptic-like' seizures. We conclude that M1 mAChR ligands that promote receptor phosphorylation-dependent signaling would protect against cholinergic adverse effects in addition to driving beneficial responses such as learning and memory and anxiolytic behavior relevant for the treatment of AD.

Severe autoimmune hemolytic anemia following receipt of SARS-CoV-2 mRNA vaccine.

BACKGROUND: Large clinical trials have demonstrated the overall safety of vaccines for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). However, reports have emerged of autoimmune phenomena, including vaccine-associated myocarditis, immune thrombocytopenia, and immune thrombotic thrombocytopenia. CASE PRESENTATION: Here we present a novel case of a young woman who developed life-threatening autoimmune hemolytic anemia (AIHA) after her first dose of a SARS-CoV-2 mRNA vaccine. Notably, initial direct antiglobulin testing was negative using standard anti-IgG reagents, which are "blind" to certain immunoglobulin (IgG) isotypes. Further testing using an antiglobulin reagent that detects all IgG isotypes was strongly positive and confirmed the diagnosis of AIHA. The patient required transfusion with 13 units of red blood cells, as well as treatment with corticosteroids, rituximab, mycophenolate mofetil, and immune globulin. CONCLUSION: As efforts to administer SARS-CoV-2 vaccines continue globally, clinicians must be aware of potential autoimmune sequelae of these therapies.

Spine surgery under awake spinal anesthesia: an Egyptian experience during the COVID-19 pandemic.

OBJECTIVE: Despite tremendous advancements in biomedical science and surgical technique, spine surgeries are still associated with considerable rates of morbidity and mortality, particularly in the elderly. Multiple novel techniques have been employed in recent years to adequately treat spinal diseases while mitigating the perioperative morbidity associated with traditional spinal surgery. Some of these techniques include minimally invasive methods and novel anesthetic and analgesic methods. In recent years, awake spine surgery with spinal anesthesia has gained attention as an alternative to general anesthesia (GA). In this study, the authors retrospectively reviewed a single-institution Egyptian experience with awake spine surgery using spinal anesthesia during the COVID-19 pandemic. METHODS: Overall, 149 patients who were admitted to As-Salam International Hospital in Cairo for lumbar and lower thoracic spine surgeries, between 2019 and 2020, were retrospectively reviewed. Patient demographics and comorbidities were collected and analyzed. Visual analog scale (VAS) and Oswestry Disability Index (ODI) scores were assessed at different time intervals including preoperatively, immediately after surgery, and 1 year postoperatively. Patient satisfaction was queried through a questionnaire assessing patient preference for traditional anesthesia or spinal anesthesia. RESULTS: Of the 149 patients who successfully received spine surgery with spinal anesthesia, there were 49 males and 100 females. The cohort age ranged from 22 to 85 years with a mean of 47.5 years. The operative time ranged from 45 to 300 minutes with a mean estimated blood loss (EBL) of 385 ± 156 mL. No major cardiopulmonary or intraoperative complications occurred, and patients were able to eat immediately after surgery. Patients were able to ambulate without an assistive device 6 to 8 hours after surgery. Decompression and fusion patients were discharged on postoperative days 2 and 3, respectively. VAS and ODI scores demonstrated excellent pain relief, which was maintained at the 1-year postoperative follow-up. No 30- or 90-day readmissions were recorded. Of 149 patients, 124 were satisfied with spinal anesthesia and would recommend spinal anesthesia to other patients. The remaining patients were not satisfied with spinal anesthesia but reported being pleased with their postoperative clinical and functional outcomes. One patient was converted to GA due to the duration of the procedure. CONCLUSIONS: Patients who received spinal anesthesia for awake spine surgery experienced short stays in the hospital, no readmissions, patient satisfaction, and well-controlled pain. The results of this study have validated the growing body of literature that demonstrates that awake spine surgery with spinal anesthesia is safe and associated with superior outcomes compared with traditional GA. Additionally, the ability to address chronic debilitating conditions, such as spinal conditions, with minimal use of valuable resources, such as ventilators, proved useful during the COVID-19 pandemic and could be a model should other stressors on healthcare systems arise, especially in developing areas of the world.

Development of High-Specificity Fluorescent Probes to Enable Cannabinoid Type 2 Receptor Studies in Living Cells.

Pharmacological modulation of cannabinoid type 2 receptor (CB2R) holds promise for the treatment of numerous conditions, including inflammatory diseases, autoimmune disorders, pain, and cancer. Despite the significance of this receptor, researchers lack reliable tools to address questions concerning the expression and complex mechanism of CB2R signaling, especially in cell-type and tissue-dependent contexts. Herein, we report for the first time a versatile ligand platform for the modular design of a collection of highly specific CB2R fluorescent probes, used successfully across applications, species, and cell types. These include flow cytometry of endogenously expressing cells, real-time confocal microscopy of mouse splenocytes and human macrophages, as well as FRET-based kinetic and equilibrium binding assays. High CB2R specificity was demonstrated by competition experiments in living cells expressing CB2R at native levels. The probes were effectively applied to FACS analysis of microglial cells derived from a mouse model relevant to Alzheimer's disease.

An unappreciated role for neutrophil-DC hybrids in immunity to invasive fungal infections.

Neutrophils are classically defined as terminally differentiated, short-lived cells; however, neutrophils can be long-lived with phenotypic plasticity. During inflammation, a subset of neutrophils transdifferentiate into a population called neutrophil-DC hybrids (PMN-DCs) having properties of both neutrophils and dendritic cells. While these cells ubiquitously appear during inflammation, the role of PMN-DCs in disease remains poorly understood. We observed the differentiation of PMN-DCs in pre-clinical murine models of fungal infection: blastomycosis, aspergillosis and candidiasis. Using reporter strains of fungal viability, we found that PMN-DCs associate with fungal cells and kill them more efficiently than undifferentiated canonical neutrophils. During pulmonary blastomycosis, PMN-DCs comprised less than 1% of leukocytes yet contributed up to 15% of the fungal killing. PMN-DCs displayed higher expression of pattern recognition receptors, greater phagocytosis, and heightened production of reactive oxygen species compared to canonical neutrophils. PMN-DCs also displayed prominent NETosis. To further study PMN-DC function, we exploited a granulocyte/macrophage progenitor (GMP) cell line, generated PMN-DCs to over 90% purity, and used them for adoptive transfer and antigen presentation studies. Adoptively transferred PMN-DCs from the GMP line enhanced protection against systemic infection in vivo. PMN-DCs pulsed with antigen activated fungal calnexin-specific transgenic T cells in vitro and in vivo, promoting the production of interferon-γ and interleukin-17 in these CD4+ T cells. Through direct fungal killing and induction of adaptive immunity, PMN-DCs are potent effectors of antifungal immunity and thereby represent innovative cell therapeutic targets in treating life-threatening fungal infections.

Investigating the Influence of Tracer Kinetics on Competition-Kinetic Association Binding Assays: Identifying the Optimal Conditions for Assessing the Kinetics of Low-Affinity Compounds.

An increased appreciation of the importance of optimizing drug-binding kinetics has lead to the development of various techniques for measuring the kinetics of unlabeled compounds. One approach is the competition-association kinetic binding method first described in the 1980s. The kinetic characteristics of the tracer employed greatly affects the reliability of estimated kinetic parameters, a barrier to successfully introducing these kinetic assays earlier in the drug discovery process. Using a modeling and Monte Carlo simulation approach, we identify the optimal tracer characteristics for determining the kinetics of the range of unlabeled ligands typically encountered during the different stages of a drug discovery program (i.e., rapidly dissociating, e.g., k off = 10 minute-1 low-affinity "hits" through to slowly dissociating e.g., k off = 0.01 minute-1 high-affinity "candidates"). For more rapidly dissociating ligands (e.g., k off = 10 minute-1), the key to obtaining accurate kinetic parameters was to employ a tracer with a relatively fast off-rate (e.g., k off = 1 minute-1) or, alternatively, to increase the tracer concentration. Reductions in assay start-time ≤1second and read frequency ≤5 seconds significantly improved the reliability of curve fitting. Timing constraints are largely dictated by the method of detection, its inherent sensitivity (e.g., TR-FRET versus radiometric detection), and the ability to inject samples online. Furthermore, we include data from TR-FRET experiments that validate this simulation approach, confirming its practical utility. These insights into the optimal experimental parameters for development of competition-association assays provide a framework for identifying and testing novel tracers necessary for profiling unlabeled competitors, particularly rapidly dissociating low-affinity competitors.

Pharmacological Characterization of a Novel 5-Hydroxybenzothiazolone-Derived ß 2-Adrenoceptor Agonist with Functional Selectivity for Anabolic Effects on Skeletal Muscle Resulting in a Wider Cardiovascular Safety Window in Preclinical Studies.

The anabolic effects of ß 2-adrenoceptor (ß 2-AR) agonists on skeletal muscle have been demonstrated in various species. However, the clinical use of ß 2-AR agonists for skeletal muscle wasting conditions has been limited by their undesired cardiovascular effects. Here, we describe the preclinical pharmacological profile of a novel 5-hydroxybenzothiazolone (5-HOB) derived ß 2-AR agonist in comparison with formoterol as a representative ß 2-AR agonist that have been well characterized. In vitro, 5-HOB has nanomolar affinity for the human ß 2-AR and selectivity over the ß 1-AR and ß 3-AR. 5-HOB also shows potent agonistic activity at the ß 2-AR in primary skeletal muscle myotubes and induces hypertrophy of skeletal muscle myotubes. Compared with formoterol, 5-HOB demonstrates comparable full-agonist activity on cAMP production in skeletal muscle cells and skeletal muscle tissue-derived membranes. In contrast, a greatly reduced intrinsic activity was determined in cardiomyocytes and cell membranes prepared from the rat heart. In addition, 5-HOB shows weak effects on chronotropy, inotropy, and vascular relaxation compared with formoterol. In vivo, 5-HOB significantly increases hind limb muscle weight in rats with attenuated effects on heart weight and ejection fraction, unlike formoterol. Furthermore, changes in cardiovascular parameters after bolus subcutaneous treatment in rats and rhesus monkeys are significantly lower with 5-HOB compared with formoterol. In conclusion, the pharmacological profile of 5-HOB indicates superior tissue selectivity compared with the conventional ß 2-AR agonist formoterol in preclinical studies and supports the notion that such tissue-selective agonists should be investigated for the safe treatment of muscle-wasting conditions without cardiovascular limiting effects.