P-selectin mediates the adhesion of sickle erythrocytes to the endothelium

Current and Future Therapeutics for Treating Patients with Sickle Cell Disease

Sickle cell disease (SCD) is the most common genetic blood disorder in the United States, with over 100,000 people suffering from this debilitating disease. SCD is caused by abnormal hemoglobin (Hb) variants that interfere with normal red blood cell (RBC) function. Research on SCD has led to the development and approval of several new SCD therapies in recent years. The recent FDA-approved novel gene therapies are potentially curative, giving patients an additional option besides a hematopoietic bone marrow transplant. Despite the promise of existing therapies, questions remain regarding their long-term pharmacological effects on adults and children. These questions, along with the exorbitant cost of the new gene therapies, justify additional research into more effective therapeutic options. Continual research in this field focuses on not only developing cheaper, more effective cures/treatments but also investigating the physiological effects of the current therapies on SCD patients, particularly on the brain and kidneys. In this article, we undertake a comprehensive review of ongoing clinical trials with completion dates in 2024 or later. Our exploration provides insights into the landscape of current therapeutics and emerging novel therapies designed to combat and potentially eradicate SCD, including the latest FDA-approved gene therapies.

Revolutionising healing: Gene Editing's breakthrough against sickle cell disease

Recent advancements in gene editing illuminate new potential therapeutic approaches for Sickle Cell Disease (SCD), a debilitating monogenic disorder caused by a point mutation in the β-globin gene. Despite the availability of several FDA-approved medications for symptomatic relief, allogeneic hematopoietic stem cell transplantation (HSCT) remains the sole curative option, underscoring a persistent need for novel treatments. This review delves into the growing field of gene editing, particularly the extensive research focused on curing haemoglobinopathies like SCD. We examine the use of techniques such as CRISPR-Cas9 and homology-directed repair, base editing, and prime editing to either correct the pathogenic variant into a non-pathogenic or wild-type one or augment fetal haemoglobin (HbF) production. The article elucidates ways to optimize these tools for efficacious gene editing with minimal off-target effects and offers insights into their effective delivery into cells. Furthermore, we explore clinical trials involving alternative SCD treatment strategies, such as LentiGlobin therapy and autologous HSCT, distilling the current findings. This review consolidates vital information for the clinical translation of gene editing for SCD, providing strategic insights for investigators eager to further the development of gene editing for SCD.

Platelets mediate the clearance of senescent red blood cells by forming prophagocytic platelet-cell complexes

ABSTRACT

In humans, ∼0.1% to 0.3% of circulating red blood cells (RBCs) are present as platelet-RBC (P-RBC) complexes, and it is 1% to 2% in mice. Excessive P-RBC complexes are found in diseases that compromise RBC health (eg, sickle cell disease and malaria) and contribute to pathogenesis. However, the physiological role of P-RBC complexes in healthy blood is unknown. As a result of damage accumulated over their lifetime, RBCs nearing senescence exhibit physiological and molecular changes akin to those in platelet-binding RBCs in sickle cell disease and malaria. Therefore, we hypothesized that RBCs nearing senescence are targets for platelet binding and P-RBC formation. Confirming this hypothesis, pulse-chase labeling studies in mice revealed an approximately tenfold increase in P-RBC complexes in the most chronologically aged RBC population compared with younger cells. When reintroduced into mice, these complexes were selectively cleared from the bloodstream (in preference to platelet-free RBC) through the reticuloendothelial system and erythrophagocytes in the spleen. As a corollary, patients without a spleen had higher levels of complexes in their bloodstream. When the platelet supply was artificially reduced in mice, fewer RBC complexes were formed, fewer erythrophagocytes were generated, and more senescent RBCs remained in circulation. Similar imbalances in complex levels and senescent RBC burden were observed in humans with immune thrombocytopenia (ITP). These findings indicate that platelets are important for binding and clearing senescent RBCs, and disruptions in platelet count or complex formation and clearance may negatively affect RBC homeostasis and may contribute to the known risk of thrombosis in ITP and after splenectomy.

Real-world experience of patients with sickle cell disease treated with crizanlizumab

In 2019, crizanlizumab was approved by the Food and Drug Administration (FDA) to reduce the rate of vaso-occlusive crisis in patients with sickle cell disease (SCD). We aimed to study the real-world effectiveness of crizanlizumab in our comprehensive sickle cell center. This was a retrospective cohort analysis of patients with SCD who received at least two consecutive doses of crizanlizumab. Clinically significant improvement was captured using the patient global impression of change scale (PGI-C). As of December 2022, there were 18 patients eligible for analysis with a median age of 30.5 years. Eight patients had the HbSS genotype, 7 HbSC, and 3 HbSB null genotype. Median duration of exposure to crizanlizumab was 53.6 weeks, and 16 (88.9%) patients received crizanlizumab for ≥26 weeks. Crizanlizumab was very well tolerated with no serious adverse events (grade ≥3) related to treatment. There was no significant difference in laboratory parameters. There was a remarkable improvement in patients' subjective response to crizanlizumab infusion. The median PGI-C score of our patients was 5, signifying moderately better with slight but noticeable changes. The morphine equivalent daily doses (MEDD) were lower after crizanlizumab infusion. MEDD prior to crizanlizumab was 90; after ≥2 consecutive crizanlizumab doses, it was 60. There was also a reduction in the hospital admissions, emergency, and urgent care visit for acute pain crisis in 6 (28%) patients. This study shows that crizanlizumab was associated with improvement in patients' response, both directly and indirectly related to the reduction of opioids used, which is consistent with results from the SUSTAIN trial.

Oral famotidine reduces the plasma level of soluble P-selectin in children with sickle cell disease

Plasma histamine levels are increased in patients with sickle cell disease (SCD), potentially promoting endothelial P-selectin expression and vaso-occlusion via histamine type 2 (H2) receptors. We conducted a prospective, non-comparative, single-centre study to determine whether famotidine, a H2 receptor antagonist, reduces P-selectin expression in SCD children. The median plasma P-selectin level was significantly reduced after 29 days of oral famotidine (53.2 ng/mL [IQR: 46.7-63.4] vs. 69.9 ng/mL [IQR: 53.6-84.2], median difference -10.2 ng/mL [IQR: -21.8 to -2.7], p = 0.005) in 28 patients. No effect was observed on other adhesion molecules, inflammation or haemolysis markers, except decreased reticulocyte count. No adverse events deemed related to famotidine were observed. Randomized controlled trials are now needed to assess the efficacy of famotidine in preventing vaso-occlusion in SCD.

Crizanlizumab in sickle cell disease

Vaso-occlusion in sickle cell disease (SCD) leads to a myriad of manifestations driving morbidity and mortality in patients with SCD. Increased leucocyte adhesion and P-selectin expression on platelets and endothelial cells is an inciting event that leads to obstruction of microcirculation by adhesion with rigid sickled red blood cells. Crizanlizumab is a first-in-class monoclonal antibody that inhibits P-selectin and has been shown to decrease the frequency of vaso-occlusive pain crises in patients with SCD in clinical trials. The role of crizanlizumab in other manifestations of SCD still needs further investigation.

The APC-EPCR-PAR1 axis in sickle cell disease

Sickle Cell Disease (SCD) is a group of inherited hemoglobinopathies. Sickle cell anemia (SCA) is caused by a homozygous mutation in the β-globin generating sickle hemoglobin (HbS). Deoxygenation leads to pathologic polymerization of HbS and sickling of erythrocytes. The two predominant pathologies of SCD are hemolytic anemia and vaso-occlusive episodes (VOE), along with sequelae of complications including acute chest syndrome, hepatopathy, nephropathy, pulmonary hypertension, venous thromboembolism, and stroke. SCD is associated with endothelial activation due to the release of danger-associated molecular patterns (DAMPs) such as heme, recurrent ischemia-reperfusion injury, and chronic thrombin generation and inflammation. Endothelial cell activation is mediated, in part, by thrombin-dependent activation of protease-activated receptor 1 (PAR1), a G protein coupled receptor that plays a role in platelet activation, endothelial permeability, inflammation, and cytotoxicity. PAR1 can also be activated by activated protein C (APC), which promotes endothelial barrier protection and cytoprotective signaling. Notably, the APC system is dysregulated in SCD. This mini-review will discuss activation of PAR1 by APC and thrombin, the APC-EPCR-PAR1 axis, and their potential roles in SCD.

Sickle red blood cell-derived extracellular vesicles activate endothelial cells and enhance sickle red cell adhesion mediated by von Willebrand factor

Endothelial activation and sickle red blood cell (RBC) adhesion are central to the pathogenesis of sickle cell disease (SCD). Quantitatively, RBC-derived extracellular vesicles (REVs) are more abundant from SS RBCs compared with healthy RBCs (AA RBCs). Sickle RBC-derived REVs (SS REVs) are known to promote endothelial cell (EC) activation through cell signalling and transcriptional regulation at longer terms. However, the SS REV-mediated short-term non-transcriptional response of EC is unclear. Here, we examined the impact of SS REVs on acute microvascular EC activation and RBC adhesion at 2 h. Compared with AA REVs, SS REVs promoted human pulmonary microvascular ECs (HPMEC) activation indicated by increased von Willebrand factor (VWF) expression. Under microfluidic conditions, we found abnormal SS RBC adhesion to HPMECs exposed to SS REVs. This enhanced SS RBC adhesion was reduced by haeme binding protein haemopexin or VWF cleaving protease ADAMTS13 to a level similar to HPMECs treated with AA REVs. Consistent with these observations, haemin- or SS REV-induced microvascular stasis in SS mice with implanted dorsal skin-fold chambers that was inhibited by ADAMTS13. The adhesion induced by SS REVs was variable and was higher with SS RBCs from patients with increased markers of haemolysis (lactate dehydrogenase and reticulocyte count) or a concomitant clinical diagnosis of deep vein thrombosis. Our results emphasise the critical contribution made by REVs to the pathophysiology of SCD by triggering acute microvascular EC activation and abnormal RBC adhesion. These findings may help to better understand acute pathophysiological mechanism of SCD and thereby the development of new treatment strategies using VWF as a potential target.

Advances in protein glycosylation and its role in tissue repair and regeneration

After tissue damage, a series of molecular and cellular events are initiated to promote tissue repair and regeneration to restore its original structure and function. These events include inter-cell communication, cell proliferation, cell migration, extracellular matrix differentiation, and other critical biological processes. Glycosylation is the crucial conservative and universal post-translational modification in all eukaryotic cells [1], with influential roles in intercellular recognition, regulation, signaling, immune response, cellular transformation, and disease development. Studies have shown that abnormally glycosylation of proteins is a well-recognized feature of cancer cells, and specific glycan structures are considered markers of tumor development. There are many studies on gene expression and regulation during tissue repair and regeneration. Still, there needs to be more knowledge of complex carbohydrates' effects on tissue repair and regeneration, such as glycosylation. Here, we present a review of studies investigating protein glycosylation in the tissue repair and regeneration process.

Contemporary Management and Prevention of Vaso-Occlusive Crises (VOCs) in Adults With Sickle Cell Disease

Sickle cell disease (SCD) is a hematological disorder that primarily affects individuals of African descent from sub-Saharan Africa and along the mediterranean. The main complications leading to hospitalizations include vaso-occlusive crises (VOCs) and acute chest syndrome (ACS). Therefore, the main objective of this paper was to identify and evaluate evidence-based management and prevention of VOCs in patients with SCD. A literature search of PubMed, Medline Cochrane and Google Scholar database (January 1985 to April 2020) was performed using the following search terms "vaso-occlusive crises", "sickle cell disease", "hydroxyurea", "L-glutamine", "voxelotor", "crizanlizumab", "treatment" and "prevention" as well as a combination of these terms. All English-language interventional studies assessing the efficacy and safety of VOC outcomes were evaluated. Literature was excluded if published in a language other than English or if it was a review article. A total of 69 articles were identified and there were 7 articles that met the search criteria. Majority of the studies focused on mean and median annual rates of VOCs as primary outcomes while median time to first sickle cell crises, median rates of hospitalizations etc were evaluated as secondary outcomes. After reviewing the literature, many patients with VOCs will still benefit from hydroxyurea therapy since long term efficacy data and cost is still a concern for the newer agents including L-glutamine, voxelotor and crizanlizumab. Other factors such as cost or compliance may also be taken into consideration when making recommendations for therapy.

Population Pharmacokinetics and Pharmacodynamics of Crizanlizumab in Healthy Subjects and Patients with Sickle Cell Disease

BACKGROUND AND OBJECTIVES

Crizanlizumab is a humanized monoclonal antibody against P-selectin for the prevention of vaso-occlusive crises in sickle cell disease (SCD). The objective of this study was to investigate crizanlizumab population pharmacokinetics (PK) and pharmacodynamics (PD), as well as influential covariates.

METHODS

A population PK model for crizanlizumab was developed from healthy volunteer and SCD patient data, using a two-compartment intravenous infusion model utilizing a target-mediated drug disposition (TMDD) approach. The relationship between crizanlizumab concentration and ex vivo P-selectin inhibition was fitted to a non-linear sigmoidal Emax model. Covariate selection was performed in a stepwise manner.

RESULTS

Crizanlizumab exhibits nonlinear pharmacokinetics in the wide dose range of 0.2-8 mg/kg body weight. The population pharmacokinetic base model incorporated body weight as covariate in the form of allometric scaling wherein the exponents were fixed to 0.8. SCD patients had higher baseline soluble P-selectin concentration, resulting in a higher estimated initial target concentration. The typical individual in the model is a 70 kg SCD patient with normal renal function and a baseline albumin of 43 g/L; CL was 0.012 L/h while V_ss was 5.2 L. For the population PD model, none of the identified additional factors beyond PD assay and covariates, such as body weight at baseline nor patient type differences, led to relevant differences in P-selectin % inhibition.

CONCLUSIONS

Renal and hepatic impairments, concomitant hydroxyurea usage, and presence of anti-drug antibody are not expected to impact the exposure of crizanlizumab. The model allows for extrapolating the PK of crizanlizumab to pediatric population and evaluation of alternative regimens and route of administration. TRIAL REGISTRATION NUMBER [DATE OF REGISTRATION]: SUSTAIN (CSEG101A2201 Phase 2), ClinicalTrials.gov identifier: NCT01895361 [10 July 2013]; CSEG101A2202 (Phase 2), ClinicalTrials.gov identifier: NCT03264989 [29 August 2017].

Targeting SELPLG/P-selectin glycoprotein ligand 1 in preclinical ARDS: Genetic and epigenetic regulation of the SELPLG promoter

We previously identified a missense single nucleotide polymorphism rs2228315 (G>A, Met62Ile) in the selectin-P-ligand gene (SELPLG), encoding P-selectin glycoprotein ligand 1 (PSGL-1), to be associated with increased susceptibility to acute respiratory distress syndrome (ARDS). These earlier studies demonstrated that SELPLG lung tissue expression was increased in mice exposed to lipopolysaccharide (LPS)- and ventilator-induced lung injury (VILI) suggesting that inflammatory and epigenetic factors regulate SELPLG promoter activity and transcription. In this report, we used a novel recombinant tandem PSGL1 immunoglobulin fusion molecule (TSGL-Ig), a competitive inhibitor of PSGL1/P-selectin interactions, to demonstrate significant TSGL-Ig-mediated decreases in SELPLG lung tissue expression as well as highly significant protection from LPS- and VILI-induced lung injury. In vitro studies examined the effects of key ARDS stimuli (LPS, 18% cyclic stretch to simulate VILI) on SELPLG promoter activity and showed LPS-mediated increases in SELPLG promoter activity and identified putative promoter regions associated with increased SELPLG expression. SELPLG promoter activity was strongly regulated by the key hypoxia-inducible transcription factors, HIF-1α, and HIF-2α as well as NRF2. Finally, the transcriptional regulation of SELPLG promoter by ARDS stimuli and the effect of DNA methylation on SELPLG expression in endothelial cell was confirmed. These findings indicate SELPLG transcriptional regulation by clinically-relevant inflammatory factors with the significant TSGL-Ig-mediated attenuation of LPS and VILI highly consistent with PSGL1/P-selectin as therapeutic targets in ARDS.

Adhesion molecules and cerebral microvascular hemodynamic abnormalities in sickle cell disease

Cerebrovascular abnormalities are a common feature of sickle cell disease that may be associated with risk of vaso-occlusive pain crises, microinfarcts, and cognitive impairment. An activated endothelium and adhesion factors, VCAM-1 and P-selectin, are implicated in sickle cell vasculopathy, including abnormal hemodynamics and leukocyte adherence. This study examined the association between cerebral expression of these adhesion factors and cortical microvascular blood flow dynamics by using in-vivo two-photon microscopy. We also examined the impact of blood transfusion treatment on these markers of vasculopathy. Results showed that sickle cell mice had significantly higher maximum red blood cell (RBC) velocity (6.80 ± 0.25 mm/sec, p ≤ 0.01 vs. 5.35 ± 0.35 mm/sec) and more frequent blood flow reversals (18.04 ± 0.95 /min, p ≤ 0.01 vs. 13.59 ± 1.40 /min) in the cortical microvasculature compared to controls. In addition, sickle cell mice had a 2.6-fold (RFU/mm2) increase in expression of VCAM-1 and 17-fold (RFU/mm2) increase in expression of P-selectin compared to controls. This was accompanied by an increased frequency in leukocyte adherence (4.83 ± 0.57 /100 μm/min vs. 2.26 ± 0.37 /100 μm/min, p ≤ 0.001). We also found that microinfarcts identified in sickle cell mice were 50% larger than in controls. After blood transfusion, many of these parameters improved, as results demonstrated that sickle cell mice had a lower post-transfusion maximum RBC velocity (8.30 ± 0.98 mm/sec vs. 11.29 ± 0.95 mm/sec), lower frequency of blood flow reversals (12.80 ± 2.76 /min vs. 27.75 ± 2.09 /min), and fewer instances of leukocyte adherence compared to their pre-transfusion imaging time point (1.35 ± 0.32 /100 μm/min vs. 3.46 ± 0.58 /100 μm/min). Additionally, we found that blood transfusion was associated with lower expression of adhesion factors. Our results suggest that blood transfusion and adhesion factors, VCAM-1 and P-selectin, are potential therapeutic targets for addressing cerebrovascular pathology, such as vaso-occlusion, in sickle cell disease.

Stroke in sickle cell disease and the promise of recent disease modifying agents

Sickle cell disease (SCD) is an inherited hemoglobinopathy affecting approximately 100,000 individuals in the United States. Cerebrovascular disease is among the most common and debilitating complications of SCA, with 53% experiencing silent cerebral infarct by age 30 and 3.8% experiencing overt stroke by age 40 years. This review highlights the burden of cerebrovascular disease in SCD, including both stroke and silent cerebral infarct (SCI). We then discuss the pathophysiology of stroke and cerebral fat embolism in the absence of a patent foramen ovale. This review also reveals that options for primary and secondary stroke prevention in SCD are still limited to hydroxyurea and blood transfusion, and that the role of aspirin and anticoagulation in SCD stroke has not been adequately studied. Limited data suggest that the novel disease-modifying agents for SCD management may improve renal dysfunction, leg ulcers, and lower the abnormally high TCD flow velocity. Further research is urgently needed to investigate their role in stroke prevention in SCD, as these novel agents target the main stroke contributors in SCD - hemolysis and vaso-occlusion. This literature review also explores the role of healthcare disparities in slowing progress in SCD management and research in the United States, highlighting the need for more investment in patient and clinician education, SCD management, and research.

Emerging functional microfluidic assays for the study of thromboinflammation in sickle cell disease

PURPOSE OF REVIEW

This review briefly summarizes the significant impact of thromboinflammation in sickle cell disease in relation to recent advances in biomarkers that are used in functional microfluidic assays.

RECENT FINDINGS

Sickle cell disease (SCD) is an inherited hemoglobinopathy that affects 100 000 Americans and millions worldwide. Patients with SCD exhibit chronic haemolysis, chronic inflammation and thrombosis, and vaso-occlusion, triggering various clinical complications, including organ damage and increased mortality and morbidity. Recent advances in functional microfluidic assays provide direct biomarkers of disease, including abnormal white blood cell and red blood cell adhesion, cell aggregation, endothelial degradation and contraction, and thrombus formation.

SUMMARY

Novel and emerging functional microfluidic assays are a promising and feasible strategy to comprehensively characterize thromboinflammatory reactions in SCD, which can be used for personalized risk assessment and tailored therapeutic decisions.

Update on Treatment Options for Stuttering Priapism

Purpose of Review

There is a paucity of peer-reviewed evidence to guide medical management of stuttering priapism. The purpose of this review is to summarize the current understanding regarding the pathophysiology of priapism and management options for stuttering priapism.

Recent Findings

Conducting large-scale, randomized, placebo-controlled trials that elucidate the optimal treatment of stuttering priapism is challenging. Therefore, recent treatment guidelines are based upon small case series, retrospective studies, and expert opinions. Nonetheless, multiple compounds from various drug classes have shown promise in treating stuttering priapism, and a few pharmacotherapies such as Crizanlizumab are currently under active investigation.

Summary

Stuttering priapism is an under-investigated disorder with a complex pathophysiology. Currently, there is no wildly adopted universal therapeutic strategy. Further research is warranted to identify the appropriate treatment of stuttering priapism and to determine the long-term side effects of current pharmacotherapies.

Endothelial VWF is critical for the pathogenesis of vaso-occlusive episode in a mouse model of sickle cell disease

Vaso-occlusive episode (VOE) is a common and critical complication of sickle cell disease (SCD). Its pathogenesis is incompletely understood. von Willebrand factor (VWF), a multimeric plasma hemostatic protein synthesized and secreted by endothelial cells and platelets, is increased during a VOE. However, whether and how VWF contributes to the pathogenesis of VOE is not fully understood. In this study, we found increased VWF levels during tumor necrosis factor (TNF)-induced VOE in a humanized mouse model of SCD. Deletion of endothelial VWF decreased hemolysis, vascular occlusion, and organ damage caused by TNF-induced VOE in SCD mice. Moreover, administering ADAMTS13, the VWF-cleaving plasma protease, reduced plasma VWF levels, decreased inflammation and vaso-occlusion, and alleviated organ damage during VOE. These data suggest that promoting VWF cleavage via ADAMTS13 may be an effective treatment for reducing hemolysis, inflammation, and vaso-occlusion during VOE.

The oral ferroportin inhibitor vamifeport improves hemodynamics in a mouse model of sickle cell disease

Sickle cell disease (SCD) is an inherited hemolytic anemia caused by a single point mutation in the β-globin gene of hemoglobin that leads to synthesis of sickle hemoglobin (HbS) in red blood cells (RBCs). HbS polymerizes in hypoxic conditions, leading to intravascular hemolysis, release of free hemoglobin and heme, and increased adhesion of blood cells to the endothelial vasculature, which causes painful vaso-occlusion and organ damage. HbS polymerization kinetics are strongly dependent on the intracellular HbS concentration; a relatively small reduction in cellular HbS concentration may prevent HbS polymerization and its sequelae. We hypothesized that iron restriction via blocking ferroportin, the unique iron transporter in mammals, might reduce HbS concentration in RBCs, thereby decreasing hemolysis, improving blood flow, and preventing vaso-occlusive events. Indeed, vamifeport (also known as VIT-2763), a clinical-stage oral ferroportin inhibitor, reduced hemolysis markers in the Townes model of SCD. The RBC indices of vamifeport-treated male and female Townes mice exhibited changes attributable to iron-restricted erythropoiesis: decreased corpuscular hemoglobin concentration mean and mean corpuscular volume, as well as increased hypochromic and microcytic RBC fractions. Furthermore, vamifeport reduced plasma soluble VCAM-1 concentrations, which suggests lowered vascular inflammation. Accordingly, intravital video microscopy of fluorescently labeled blood cells in the microvasculature of Townes mice treated with vamifeport revealed diminished adhesion to the endothelium and improved hemodynamics. These preclinical data provide a strong proof-of-concept for vamifeport in the Townes model of SCD and support further development of this compound as a potential novel therapy in SCD.

Role of P-Selectin in the Development of Hemostasis Disorders in COVID-19

This is a review of data on the impact of COVID-19 on blood clotting. An important feature of the pathogenesis of severe acute respiratory syndrome caused by the SARS-Co-2 coronavirus is the risk of thrombotic complications including microvascular thrombosis, venous thromboembolism, and stroke. These thrombotic complications, like thrombocytopenia, are markers of the severe form of COVID-19 and are associated with multiple organ failure and increased mortality. One of the central mechanisms of this pathology is dysregulation of the adhesive protein P-selectin. The study of the mechanisms of changes in hemostasis and vascular pathology, and the role in these processes of biomarkers of thrombogenesis, and primarily of P-selectin of various origins (platelets, endothelial cells, and plasma), can bring some clarity to the understanding of the pathogenesis and therapy of COVID-19.

Recent advances in "sickle and niche" research - Tribute to Dr. Paul S Frenette

In this retrospective, we review the two research topics that formed the basis of the outstanding career of Dr. Paul S. Frenette. In the first part, we focus on sickle cell disease (SCD). The defining feature of SCD is polymerization of the deoxygenated mutant hemoglobin, which leads to a vicious cycle of hemolysis and vaso-occlusion. We survey important discoveries in SCD pathophysiology that have led to recent advances in treatment of SCD. The second part focuses on the hematopoietic stem cell (HSC) niche, the complex microenvironment within the bone marrow that controls HSC function and homeostasis. We detail the cells that constitute this niche, and the factors that these cells use to exert control over hematopoiesis. Here, we trace the scientific paths of Dr. Frenette, highlight key aspects of his research, and identify his most important scientific contributions in both fields.

Assessment of Reticulocyte and Erythrocyte Parameters From Automated Blood Counts in Vaso-Occlusive Crisis on Sickle Cell Disease

Sickle cell disease is a complex genetic disease involving cell adhesion between red blood cells, white blood cells, platelets and endothelial cells, inducing painful vaso-occlusive crisis (VOC). We assessed reticulocyte and erythrocyte parameters in a cohort of confirmed SCD patients, and investigated whether a combination of these routine laboratory biomarkers of haemolysis could be used to predict VOC development. Reticulocyte and erythrocyte parameters were evaluated using the Sysmex XN-9000 analyser. A total of 98 patients with SCD were included, 72 in steady state and 26 in VOC. Among the 72 patients in steady state, 22 developed a VOC in the following year (median: 3 months [2-6]). The following parameters were increased in SCD patients with VOC development compared to SCD patients without VOC development in the following year: reticulocyte count (94.6 10⁹/L [67.8-128] vs. 48.4 10⁹/L [24.9-87.5]), immature reticulocyte count (259 10⁹/L [181-334] vs. 152 10⁹/L [129-208]) reticulocyte/immature reticulocyte fraction (IRF) ratio (6.63 10⁹/(L^*%) [4.67-9.56] vs. 4.94 10⁹/(L^*%) [3.96-6.61]), and medium fluorescence reticulocytes (MFR) (19.9% [17.4-20.7] vs. 17.1% [15.95-19.75]). The association of a reticulocyte count of >189.4 10⁹/L and an MFR of >19.75% showed a sensitivity of 81.8% and a specificity of 88% to predict VOC development in the following year. Based on our findings, a combination of routine laboratory biomarkers, as reticulocyte count, immature reticulocyte count and fluorescent reticulocyte fraction at steady state, could be used to predict VOC development in SCD.

High-Throughput Assay to Screen Small Molecules for Their Ability to Prevent Sickling of Red Blood Cells

Sickle cell disease (SCD) is an inherited disorder of hemoglobin (Hb); approximately 300,000 babies are born worldwide with SCD each year. In SCD, fibers of polymerized sickle Hb (HbS) form in red blood cells (RBCs), which cause RBCs to develop their characteristic "sickled" shape, resulting in hemolytic anemia and numerous vascular complications including vaso-occlusive crises. The development of novel antisickling compounds will provide new therapeutic options for patients with SCD. We developed a high-throughput "sickling assay" that is based on an automated high-content imaging system to quantify the effects of hypoxia on the shape and size of RBCs from HbSS SCD patients (SS RBCs). We used this assay to screen thousands of compounds for their ability to inhibit sickling. In the assay, voxelotor (an FDA-approved medication used to treat SCD) prevented sickling with a z'-factor > 0.4, suggesting that the assay is capable of identifying compounds that inhibit sickling. We screened the Broad Repurposing Library of 5393 compounds for their ability to prevent sickling in 4% oxygen/96% nitrogen. We identified two compounds, SNS-314 mesylate and voxelotor itself, that successfully prevented sickling. SNS-314 mesylate prevented sickling in the absence of oxygen, while voxelotor did not, suggesting that SNS-314 mesylate acts by a mechanism that is different from that of voxelotor. The sickling assay described in this study will permit the identification of additional, novel antisickling compounds, which will potentially expand the therapeutic options for SCD.

Selectin-Mediated Signaling-Shedding Light on the Regulation of Integrin Activity in Neutrophils

As a consequence of tissue injury or infection, neutrophils are recruited in a stepwise recruitment process from the bloodstream into the surrounding tissue. Selectins are a family of adhesion molecules comprised of L-, E-, and P-selectin. Differences in expression patterns, protein structure, and ligand binding characteristics mediate distinct functions of each selectin. Interactions of selectins and their counter-receptors mediate the first contact of neutrophils with the endothelium, as well as subsequent neutrophil rolling along the endothelial surface. For efficient neutrophil recruitment, activation of β₂-integrins on the cell surface is essential. Integrin activation can be elicited via selectin- as well as chemokine-mediated inside-out signaling resulting in integrin conformational changes and clustering. Dysregulation of selectin-induced integrin activation on neutrophils is involved in the development of severe pathological disease conditions including leukocyte adhesion deficiency (LAD) syndromes in humans. Here, we review molecular mechanisms involved in selectin-mediated signaling pathways in neutrophils and their impact on integrin activation, neutrophil recruitment, and inflammatory diseases.

Endothelial Cell Adhesion Molecules- (un)Attainable Targets for Nanomedicines

Endothelial cell adhesion molecules have long been proposed as promising targets in many pathologies. Despite promising preclinical data, several efforts to develop small molecule inhibitors or monoclonal antibodies (mAbs) against cell adhesion molecules (CAMs) ended in clinical-stage failure. In parallel, many well-validated approaches for targeting CAMs with nanomedicine (NM) were reported over the years. A wide range of potential applications has been demonstrated in various preclinical studies, from drug delivery to the tumor vasculature, imaging of the inflamed endothelium, or blocking immune cells infiltration. However, no NM drug candidate emerged further into clinical development. In this review, we will summarize the most advanced examples of CAM-targeted NMs and juxtapose them with known traditional drugs against CAMs, in an attempt to identify important translational hurdles. Most importantly, we will summarize the proposed strategies to enhance endothelial CAM targeting by NMs, in an attempt to offer a catalog of tools for further development.

l-glutamine, crizanlizumab, voxelotor, and cell-based therapy for adult sickle cell disease: Hype or hope?

For more than two decades, hydroxyurea was the only therapeutic agent approved by the Food and Drug Administration (FDA) for sickle cell disease (SCD). Although curative allogeneic hematopoietic stem cell transplants (allo-HSCT) were also available, only very few patients underwent the procedure due to lack of matched-related donors. However, therapeutic options for SCD patients increased dramatically in the last few years. Three new agents, l-glutamine, crizanlizumab, and voxelotor, were approved by the FDA for use in SCD patients. The number of SCD patients who underwent allo-HSCT also increased as a result of advances in the prevention of graft failure and graft-versus-host disease from using mismatched donor HSC. More recently gene therapy was made available on clinical trials. The increased treatment options for SCD have led to a sense of optimism and excitement among many physicians that these new approaches would alter the clinical course and disease burden. Although these newer agents do provide hope to SCD patients, the hyped-up responses need to be evaluated in the context of reality. In this review, we will discuss and compare these new agents and cell-based therapy, evaluate their clinical and economic impacts, and examine their roles in reducing the disease burden.

A critical evaluation of crizanlizumab for the treatment of sickle cell disease

INTRODUCTION

P-selectin is a key adhesion molecule in the pathogenesis of sickle cell disease, including acute painful event(s). Many of the mediators activated in prototypical pain crisis are also involved in other complications seen in this population. Crizanlizumab is a monoclonal antibody approved in the US in 2019 for patients of all genotypes of sickle cell disease. By blocking P-selectin, it effectively prevents acute painful event(s) and has a manageable safety profile.

AREAS COVERED

In this review, we provide an overview of the (i) biology of P-selectin in sickle cell disease, (ii) various agents inhibiting P-selectin, (iii) pharmacology of crizanlizumab, (iv) preclinical and clinical data on crizanlizumab, and (v) its potential for other indications, ongoing studies, regulatory status, and cost issues. Further, we describe its position among other approved agents in sickle cell disease and project future directions as well.

EXPERT OPINION

Crizanlizumab holds great promise in modulating the natural history of sickle cell disease and may have pleotropic effects. Studies are ongoing to define its role in preventing other sickle cell-related complications, non-sickle cell inflammatory states, and thrombotic disorders.

Long-Term Effects of Allogeneic Hematopoietic Stem Cell Transplantation on Systemic Inflammation in Sickle Cell Disease Patients

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is the only currently available curative treatment for sickle cell disease (SCD). However, the effects of HSCT on SCD pathophysiology are poorly elucidated. Here, we assessed red blood cell (RBC) adhesiveness, intensity of hemolysis, vascular tone markers and systemic inflammation, in SCD patients treated with allogeneic HSCT. Thirty-two SCD patients were evaluated before and on long-term follow-up after HSCT. Overall survival was 94% with no severe (grade III-IV) graft-vs-host disease and a 22% rejection rate (graft failure). Hematological parameters, reticulocyte counts, and levels of lactate dehydrogenase (LDH), endothelin-1 and VCAM-1 normalized in SCD patients post-HSCT. Expression of adhesion molecules on reticulocytes and RBC was lower in patients with sustained engraftment. Levels of IL-18, IL-15 and LDH were higher in patients that developed graft failure. Increased levels of plasma pro-inflammatory cytokines, mainly TNF-α, were found in SCD patients long-term after transplantation. SCD patients with sustained engraftment after allo-HSCT showed decreased reticulocyte counts and adhesiveness, diminished hemolysis, and lower levels of vascular tonus markers. Nevertheless, systemic inflammation persists for at least five years after transplantation, indicating that allo-HSCT does not equally affect all aspects of SCD pathophysiology.

Insights into Selectin Inhibitor Design from Endogenous Isomeric Ligands of SLea and SLex

Selectins interact with cell-surface glycans to promote the initial tethering and rolling of leukocytes, and these interactions are targets for designs of inhibitors to neutralize diseases related to excessive inflammatory responses in many cardiovascular and immune dysfunctions, as well as tumor markers in different cancers. The isomeric endogenous tetrasaccharides, sialyl Lewis X (sLe^x) and sialyl Lewis A (sLe^a), are minimal sugar structures required for selectin binding. Understanding their subtle structural variances and significant advanced binding strengths of sLe^a over sLe^x could benefit the rational designs for selectin inhibitors. Modeling based on the E-selectin-sLe^x crystal structure in the present study demonstrated that the N-acetyl group of GlcNAc in sLe^x could form steric hindrances in the E-selectin-sLe^x complex, but the hydroxy methylene group of GlcNAc in sLe^a at the same position allows for stronger binding interactions. The subsequent designed inhibitor with a synthetic accessible linker molecule that has no exo-cyclic moieties replacing GlcNAc displayed comparable dynamic and energetic binding features to sLe^a. The present study deciphered the clues from endogenous isomeric sLe^a and sLe^x and provided insights into designing selectin inhibitors with simplified synthesis.

Longitudinal assessment of adhesion to vascular cell adhesion molecule-1 at steady state and during vaso-occlusive crises in sickle cell disease

Sickle cell disease (SCD) is characterized by frequent and unpredictable vaso‐occlusive crises (VOCs). Sickle erythrocytes (SSRBCs) contribute to VOCs by participating in a series of adhesive events with blood cells and the vascular endothelium. Adhesion assays have been used to evaluate the relationship between SSRBC adhesion and SCD severity. We developed a standardized, clinical flow adhesion assay of whole blood to vascular cell adhesion molecule (FA‐WB‐VCAM). The objective of this study was to assess the variability and clinical predictive value of FA‐WB‐VCAM in a six‐month longitudinal, observational study (ELIPSIS) in SCD subjects during at‐home, steady‐state and self‐reported VOCs, and following VOC resolution. We observed a strong relationship between FA‐WB‐VCAM and SCD severity. Adhesion indices were significantly lower in SCD subjects on hydroxycarbamide and increased during VOCs; at‐home VOCs had significantly higher FA‐WB‐VCAM than steady‐state and contact VOCs. SCD subjects with a high frequency of self‐reported VOCs had a pro‐adhesive phenotype at steady state and were stratified into a high‐adhesive phenotype cohort; two years prospectively we observed a higher frequency of VOCs in the high‐adhesion cohort. This study supports stratifying SCD subjects based on steady‐state FA‐WB‐VCAM and suggests that FA‐WB‐VCAM may be a plausible surrogate end‐point for SCD severity.

How I approach disease-modifying therapy in children with sickle cell disease in an era of novel therapies

Finally,after decades of stagnation, the therapeutic landscape for sickle cell disease (SCD) is changing with an increasing number of novel therapeutics. Hydroxyurea remains the primary disease-modifying therapy and, when started early in life with maintenance of an optimal dose, can reduce many SCD-related complications. To complement hydroxyurea, there are a growing number of pharmacologic options with additional efforts focused on the development and optimization of curative therapies. Here, we review current treatment options and provide recommendations as to how to approach the treatment of children and adolescents within this evolving therapeutic landscape to allow for full and healthy lives.

The Role of RBC Oxidative Stress in Sickle Cell Disease: From the Molecular Basis to Pathologic Implications

Sickle cell disease (SCD) is an inherited monogenic disorder and the most common severe hemoglobinopathy in the world. SCD is characterized by a point mutation in the β-globin gene, which results in hemoglobin (Hb) S production, leading to a variety of mechanistic and phenotypic changes within the sickle red blood cell (RBC). In SCD, the sickle RBCs are the root cause of the disease and they are a primary source of oxidative stress since sickle RBC redox state is compromised due to an imbalance between prooxidants and antioxidants. This imbalance in redox state is a result of a continuous production of reactive oxygen species (ROS) within the sickle RBC caused by the constant endogenous Hb autoxidation and NADPH oxidase activation, as well as by a deficiency in the antioxidant defense system. Accumulation of non-neutralized ROS within the sickle RBCs affects RBC membrane structure and function, leading to membrane integrity deficiency, low deformability, phosphatidylserine exposure, and release of micro-vesicles. These oxidative stress-associated RBC phenotypic modifications consequently evoke a myriad of physiological changes involved in multi-system manifestations. Thus, RBC oxidative stress in SCD can ultimately instigate major processes involved in organ damage. The critical role of the sickle RBC ROS production and its regulation in SCD pathophysiology are discussed here.

Flow adhesion of whole blood to P-selectin: a prognostic biomarker for vaso-occlusive crisis in sickle cell disease

Blood cell adhesion to P-selectin and vascular cell adhesion molecule-1 (VCAM-1) contributes to the pathophysiology of vaso-occlusion crisis (VOC) events in individuals with sickle cell disease (SCD). We evaluated the use of standardized flow adhesion biomarkers in a six-month, 35-subjects longitudinal study (ELIPSIS). Flow adhesion of whole blood on P-selectin (FA-WB-Psel) and VCAM1 (FA-WB-VCAM), and of isolated white blood cells on P-selectin (FA-WBC-Psel) and VCAM-1 (FA-WBC-VCAM) were elevated on VOC days compared with non-VOC days, but only FA-WB-Psel reached statistical significance (P = 0·015). Optimal cut-off values were established with Cox regression models for FA-WB-Psel [46 cells/mm²; hazard ratio (HR): 2·3; 95% confidence interval (CI):1·4-4·0; P = 0·01] and FA-WB-VCAM (408 cells/mm², HR:1·8; 95% CI: 0·9-3·45; P = 0·01). A combined (FA-WB-Psel and FA-WB-VCAM) multimarker risk score was also significantly (P = 0·0006) correlated with VOC risk that was two-fold higher for intermediate and 5·64-fold higher for high score. The concordance (C)-index for the multimarker score was 0·63 in the six-month period (95% CI: 0·56-0·70), indicating a better ability to distinguish patient risk of VOC, compared to individual biomarkers FA-WB-VCAM (C-index: 0·57; 95% CI: 0·49-0·65) or FA-WB-Psel (C-index: 0·58; 95% CI: 0·53-0·62). The presented multimarker score can be used to risk-stratify individuals with SCD during their steady state into low, intermediate, and high-risk strata for self-reported VOCs. Such risk stratification could help focus healthcare resources more efficiently to maintiain health, personalize treatment selection to each patient's individual needs, and potentially reduce healthcare costs.

Pain in sickle cell disease: current and potential translational therapies

Pain is a major comorbidity of sickle cell disease (SCD). Patients with SCD may suffer from both acute and chronic pain. Acute pain is caused by recurrent and unpredictable episodes of vaso-occlusive crises (VOC), whereas the exact etiology of chronic pain is still unknown. Opioids are the mainstay for pain treatment, but the opioid epidemic has significantly altered access to prescription opioids and has brought concerns over their long-term use into the forefront, which have negatively impacted the treatment of sickle pain. Opioids remain potent analgesics but growing opioid-phobia has led to the realization of an unmet need to develop nonopioid therapies that can provide relief for severe sickle pain. This realization has contributed to the approval of 3 different drugs by the Food and Drug Administration (FDA) for the treatment of SCD, particularly to reduce VOC and/or have an impact on the pathobiology of SCD. In this review, we outline the challenges and need for validation of side-effects of opioids and provide an update on the development of mechanism-based translational therapies, specifically targeting pain in SCD.

The European Medicines Agency Review of Crizanlizumab for the Prevention of Recurrent Vaso-Occlusive Crises in Patients With Sickle Cell Disease

Crizanlizumab is a monoclonal antibody that binds to P-selectin. On October 28, 2020, a conditional marketing authorization valid through the European Union (EU) was issued for crizanlizumab for the prevention of recurrent vaso-occlusive crises (VOCs) in patients with sickle cell disease aged 16 years or older. Crizanlizumab was evaluated in a phase 2, double-blind, placebo-controlled randomized multicenter trial comparing high-dose (5 mg/kg) crizanlizumab, low-dose (2.5 mg/kg) crizanlizumab and placebo in patients with a history of 2-10 VOCs in the previous year. Patients who were receiving concomitant hydroxycarbamide (HC) as well as those not receiving HC were included in the study. The primary endpoint of the trial was the annual rate of sickle cell-related pain crises as adjudicated by a central review committee. High-dose crizanlizumab led to a 45.3% lower median annual rate of sickle cell-related pain crises compared to placebo (P = 0.010), with no statistically significant difference for the low dose. Treatment with high-dose crizanlizumab led to similar incidences of adverse events (AEs), grade 3 AEs, and serious AEs compared to placebo. Most frequently observed AEs that occurred more often in the crizanlizumab arm compared to placebo were infusion related reactions (34.8% versus 21%), arthralgia (18.2% versus 8.1%), diarrhea (10.6% versus 3.2%), and nausea (18.2% versus 11.3%). The aim of this article is to summarize the scientific review of the application leading to regulatory approval in the EU.

Gene therapy as the new frontier for Sickle Cell Disease

Sickle Cell Disease (SCD) is one of the most common monogenic disorders caused by a point mutation in the β-globin gene. This mutation results in polymerization of hemoglobin (Hb) under reduced oxygenation conditions, causing rigid sickle-shaped RBCs and hemolytic anemia. This clearly defined fundamental molecular mechanism makes SCD a prototypical target for precision therapy. Both the mutant β-globin protein and its downstream pathophysiology are pharmacological targets of intensive research. SCD also is a disease well-suited for biological interventions like gene therapy. Recent advances in hematopoietic stem cell (HSC) transplantation and gene therapy platforms, like Lentiviral vectors and gene editing strategies, expand the potentially curative options for patients with SCD. This review discusses the recent advances in precision therapy for SCD and the preclinical and clinical advances in autologous HSC gene therapy for SCD.

Integrin VLA-4 as a PET imaging biomarker of hyper-adhesion in transgenic sickle mice

In sickle cell disease (SCD), very late antigen-4 (VLA-4 or integrin α4β1) mediates the adhesion of reticulocytes to inflamed, proinflammatory endothelium, a key process in promoting vaso-occlusive episodes (VOEs). We hypothesized that a radionuclide tracer targeting VLA-4 could be harnessed as a positron emission tomography (PET) imaging biomarker of VOEs. We tested the VLA-4 peptidomimetic PET tracer 64Cu-CB-TE1A1P-PEG4-LLP2A (64Cu-LLP2A) for imaging hyper-adhesion-associated VOEs in the SCD Townes mouse model. With lipopolysaccharide (LPS)-induced VOEs, 64Cu-LLP2A uptake was increased in the bone marrow of the humeri and femurs, common sites of VOEs in SCD mice compared with non-SCD mice. Treatment with a proven inhibitor of VOEs (the anti-mouse anti-P-selectin monoclonal antibody [mAb] RB40.34) during LPS stimulation led to a reduction in the uptake of 64Cu-LLP2A in the humeri and femurs to baseline levels, implying blockade of VOE hyper-adhesion. Flow cytometry with Cy3-LLP2A demonstrated an increased percentage of VLA-4-positive reticulocytes in SCD vs non-SCD mice in the bone and peripheral blood after treatment with LPS, which was abrogated by anti-P-selectin mAb treatment. These data, for the first time, show in vivo imaging of VLA-4-mediated hyper-adhesion, primarily of SCD reticulocytes, during VOEs. PET imaging with 64Cu-LLP2A may serve as a valuable, noninvasive method for identifying sites of vaso-occlusion and may provide an objective biomarker of disease severity and anti-P-selectin treatment efficacy in patients with SCD.

Red blood cell adhesion to ICAM-1 is mediated by fibrinogen and is associated with right-to-left shunts in sickle cell disease

Sickle cell disease (SCD), which afflicts 100 000 Americans, as well as millions worldwide, is associated with anemia, lifelong morbidity, and early mortality. Abnormal adhesion of sickle red blood cells (RBCs) to activated vascular endothelium may contribute acutely to the initiation of painful vaso-occlusive crises and chronically to endothelial damage in SCD. Sickle RBCs adhere to activated endothelium through several adhesion mechanisms. In this study, using whole blood from 17 people with heterozygous SCD (HbS variant) and 55 people with homozygous SCD (HbSS) analyzed in an in vitro microfluidic assay, we present evidence for the adhesion of sickle RBCs to immobilized recombinant intercellular adhesion molecule 1 (ICAM-1). We show that sickle RBC adhesion to ICAM-1 in vitro is associated with evidence of hemolysis in vivo, marked by elevated lactate dehydrogenase levels, reticulocytosis, and lower fetal hemoglobin levels. Further, RBC adhesion to ICAM-1 correlates with a history of intracardiac or intrapulmonary right-to-left shunts. Studies of potential ICAM-1 ligands on RBC membranes revealed that RBC-ICAM-1 interactions were mediated by fibrinogen bound to the RBC membrane. We describe, for the first time, RBC rolling behavior on ICAM-1 under high shear rates. Our results suggest that firm adhesion of sickle RBCs to ICAM-1 most likely occurs in postcapillary venules at low physiological shear rates, which is facilitated by initial rolling in high shear regions (eg, capillaries). Inhibition of RBC and ICAM-1 interactions may constitute a novel therapeutic target in SCD.

P-selectin deficiency promotes liver senescence in sickle cell disease mice

Sickle cell disease (SCD) is caused by a homozygous mutation in the β-globin gene, which leads to erythrocyte sickling, vasoocclusion, and intense hemolysis. P-selectin inhibition has been shown to prevent vasoocclusive events in patients with SCD; however, the chronic effect of P-selectin inhibition in SCD remains to be determined. Here, we used quantitative liver intravital microscopy in our recently generated P-selectin-deficient SCD mice to show that chronic P-selectin deficiency attenuates liver ischemia but fails to prevent hepatobiliary injury. Remarkably, we find that this failure in resolution of hepatobiliary injury in P-selectin-deficient SCD mice is associated with the increase in cellular senescence and reduced epithelial cell proliferation in the liver. These findings highlight the importance of investigating the long-term effects of chronic P-selectin inhibition therapy on liver pathophysiology in patients with SCD.

Concurrent Assessment of Deformability and Adhesiveness of Sickle Red Blood Cells by Measuring Perfusion of an Adhesive Artificial Microvascular Network

Biomarker development is a key clinical research need in sickle cell disease (SCD). Hemorheological parameters are excellent candidates as abnormal red blood cell (RBC) rheology plays a critical role in SCD pathophysiology. Here we describe a microfluidic device capable of evaluating RBC deformability and adhesiveness concurrently, by measuring their effect on perfusion of an artificial microvascular network (AMVN) that combines microchannels small enough to require RBC deformation, and laminin (LN) coating on channel walls to model intravascular adhesion. Each AMVN device consists of three identical capillary networks, which can be coated with LN (adhesive) or left uncoated (non-adhesive) independently. The perfusion rate for sickle RBCs in the LN-coated networks (0.18 ± 0.02 nL/s) was significantly slower than in non-adhesive networks (0.20 ± 0.02 nL/s), and both were significantly slower than the perfusion rate for normal RBCs in the LN-coated networks (0.22 ± 0.01 nL/s). Importantly, there was no overlap between the ranges of perfusion rates obtained for sickle and normal RBC samples in the LN-coated networks. Interestingly, treatment with poloxamer 188 decreased the perfusion rate for sickle RBCs in LN-coated networks in a dose-dependent manner, contrary to previous studies with conventional assays, but in agreement with the latest clinical trial which showed no clinical benefit. Overall, these findings suggest the potential utility of the adhesive AMVN device for evaluating the effect of novel curative and palliative therapies on the hemorheological status of SCD patients during clinical trials and in post-market clinical practice.

P-selectin-deficient mice to study pathophysiology of sickle cell disease

P-selectin–deficient SCD mice are protected from lung vaso-occlusion. P-selectin–deficient SCD mice will be useful in assessing the benefits of anti–P-selectin therapy in diverse complications of SCD.

Sickle cell vaso-occlusion: The dialectic between red cells and white cells

The pathophysiology of sickle cell anemia, a hereditary hemoglobinopathy, has fascinated clinicians and scientists alike since its description over 100 years ago. A single gene mutation in the HBB gene results in the production of abnormal hemoglobin (Hb) S, whose polymerization when deoxygenated alters the physiochemical properties of red blood cells, in turn triggering pan-cellular activation and pathological mechanisms that include hemolysis, vaso-occlusion, and ischemia-reperfusion to result in the varied and severe complications of the disease. Now widely regarded as an inflammatory disease, in recent years attention has included the role of leukocytes in vaso-occlusive processes in view of the part that these cells play in innate immune processes, their inherent ability to adhere to the endothelium when activated, and their sheer physical and potentially obstructive size. Here, we consider the role of sickle red blood cell populations in elucidating the importance of adhesion vis-a-vis polymerization in vaso-occlusion, review the direct adhesion of sickle red cells to the endothelium in vaso-occlusive processes, and discuss how red cell- and leukocyte-centered mechanisms are not mutually exclusive. Given the initial clinical success of crizanlizumab, a specific anti-P selectin therapy, we suggest that it is appropriate to take a holistic approach to understanding and exploring the complexity of vaso-occlusive mechanisms and the adhesive roles of the varied cell types, including endothelial cells, platelets, leukocytes, and red blood cells.

P-Selectin Blockade in the Treatment of Painful Vaso-Occlusive Crises in Sickle Cell Disease: A Spotlight on Crizanlizumab

Microvascular vaso-occlusion driven pain crisis is the hallmark of sickle cell disease with profound morbidity and increased mortality. Selectins, most notably P-selectins have an integral role in this phenomenon. P-selection was first identified in 1989. In 2019, after 3 decades of basic, translational, and clinical work with this pathway, the US Food and Drug Administration approved a P-selectin antibody, crizanlizumab to reduce frequency of pain crisis in patients more than 16 years with sickle cell disease. We review the fundamentals of P-selectin pathobiology, P-selectin blocking agents, clinical data with the use of crizanlizumab and prospects of this novel class of drugs in the context of other treatments for painful vaso-occlusive episodes.

Medical treatment of recurrent ischaemic priapism: a review of current molecular therapeutics and a new clinical management paradigm

OBJECTIVES

To examine the current molecular therapeutics in the medical treatment of recurrent ischemic priapism (RIP). To propose a stepwise clinical management paradigm for the treatment of RIP.

METHODS

We performed a literature search using the PubMed database for the terms 'recurrent ischemic priapism' and 'stuttering priapism' up until December 2020. We assessed pre-clinical and clinical studies regarding medical management of RIP and molecular pathophysiology. Case series and randomized trials were evaluated by study quality and patient outcomes to determine a potential clinical management scheme.

RESULTS

Recent research has fostered an improved understanding of the underlying molecular pathophysiology of RIP that has paved the way forward for developing new therapeutic agents. Medications targeting neurovascular, hormonal and haematological mechanisms associated with RIP show great promise towards remedying this condition. A host of therapeutic agents operating across different mechanistic directions may be implemented according to a clinical management scheme to potentially optimize RIP outcomes.

CONCLUSION

RIP remains a medically neglected condition with current management focused on treating the acute condition rather than modulating the course of disease. Continued research into the molecular mechanisms of RIP and standardized clinical pathways can improve the quality of care for patients suffering from this condition.

Pilot assessment of omega-3 fatty acids and potassium thiocyanate in sickle cell anemia patients with conditional peak systolic cerebral artery blood velocity

OBJECTIVE

The purpose of this pilot study was to explore the effect of omega-3 fatty acids and potassium thiocyanate on conditional peak systolic cerebral artery blood velocity in children with sickle cell anemia (SCA).

METHODS

Transcranial doppler ultrasonography (TCD) was done on 232 SCA children, and 21 found with conditional peak systolic blood velocity (PSV) of 200-249 cm/s in internal carotid, middle or anterior cerebral arteries. These were randomized to receive omega-3 fatty acids and potassium thiocyanate with standard treatment of SCA (test group, N = 14), or standard treatment only (control group, N = 7). After 3 months of treatment, PSV was measured again.

RESULTS

Right middle cerebral artery PSV was significantly reduced in the test relative to the control groups (p = 0.04). PSV returned to normal in 79% of the test versus 43% of the control group; and increased to abnormal in one member of the control group, but none of the test group.

CONCLUSIONS

The pilot data suggest that in SCA, omega-3 fatty acids and potassium thiocyanate might reduce conditional blood velocity to normal, or prevent progression to abnormal values. A larger, randomized, clinical trial is required to further address the current gap in management of conditional TCD blood velocity.

Management of Sickle Cell Disease Complications Beyond Acute Chest Syndrome

Sickle cell disease results in numerous complications that can lead to significant morbidity and mortality. Amongst them, acute chest syndrome is the leading cause of mortality. As a result, most providers are in tune with this complication and well versed with management. As sickle cell patients now live longer, they face a multitude of other complications that if left unattended, can lead to significant morbidity and mortality as well. It is critical to look beyond acute chest syndrome and adopt a more comprehensive approach to the management of the sickle cell patient.

Severe Acute Chest Syndrome in a Sickle Cell Patient That Required Repeated Exchange Transfusion

Acute chest syndrome (ACS) is one of the major causes of mortality and morbidity in patients with sickle cell disease (SCD). ACS usually presents in a more severe form in adults older than 20 years. High clinical suspicion should be maintained in SCD patients who presents with painful crises. This case report presents an interesting severe form of ACS that, quite unusually, required repeated exchanged blood transfusion to achieve clinical improvement.