Adenosine deaminase enzyme therapy prevents and reverses the heightened cavernosal relaxation in priapism

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.

Effect of Antioxidants Supplementation on Erectile Dysfunction: A Systematic Review and Meta-Analysis of Randomized Controlled Trials

INTRODUCTION

In Erectile dysfunction (ED) patients, phosphodiesterase type 5 (PDE5) inhibitors are considered as the first-line therapy. However, 30-50% of ED patients fail to follow this therapeutic option because of adverse events, lack of efficacy, or drug costs. Antioxidant supplementation is widely applied in clinical practice and viewed as a potential therapeutic option for ED. Therefore, it is attractive to assess the effect of antioxidants supplementation on ED patients.

OBJECTIVES

To evaluate the effects of antioxidants supplementation on ED.

METHODS

Published randomized controlled trials of antioxidants in ED were searched in the PubMed, Embase, and Cochrane Library databases from inception to October 3, 2021. Meta-analyses were carried out using a random-effects model. The results were presented as standard mean differences (SMDs) with their 95% confidence intervals (CIs).

RESULTS

Eighteen studies with 1,331 ED patients were included in the study. Compared with placebo, antioxidants alone treatment showed a statistical increase in International Index of Erectile Function (IIEF) score (SMD = 1.93; 95% CI: 0.15, 3.72; P = .034). Compared with placebo, antioxidants compound treatment elicited a significant increase in IIEF score (SMD = 2.74; 95% CI: 1.67, 3.81; P < .001) as well as sexual satisfaction score (SMD = 1.61; 95% CI: 0.63, 2.59; P = .001). Compared with the PDE5 inhibitors alone, combination of PDE5 inhibitors and antioxidants showed a significant increase in IIEF score (SMD = 1.1; 95% CI: 0.51, 1.68; P < .001) and sexual satisfaction score (SMD = 1.28; 95% CI: 0.06, 2.51; P = .04).

CONCLUSION

This study found that the effect of antioxidant alone treatment on ED may be limited. However, antioxidant compound treatment, as well as combination of PDE5 inhibitors and antioxidants, were associated with improved ED, and can be considered as an accessary therapeutic option for ED. Su L, Yang Z, Qu H, et al. Effect of Antioxidants Supplementation on Erectile Dysfunction: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. Sex Med Rev 2021;XX:XXX-XXX.

Relationship between Dietary Patterns with Benign Prostatic Hyperplasia and Erectile Dysfunction: A Collaborative Review

Interest in the role of dietary patterns has been consistently emerging in recent years due to much research that has documented the impact of metabolism on erectile dysfunction (ED) and/or benign prostatic hyperplasia (BPH). We conducted a non-systematic review of English articles published from 1964 to September 2021. The search terms were: (“dietary patterns” OR “diet”) AND/OR (“erectile dysfunction”) AND/OR (“benign prostatic hyperplasia”). In the present review, we have highlighted how the association between dietary patterns and two of the most frequent pathologies in urology, namely erectile dysfunction and benign prostatic hyperplasia, is present in the literature. The data suggested that a diet that is more adherent to the Mediterranean diet or that emphasizes the presence of vegetables, fruits, nuts, legumes, and fish or other sources of long-chain (n-3) fats, in addition to reduced content of red meat, may have a beneficial role on erectile function. At the same time, the same beneficial effects can be transferred to BPH as a result of the indirect regulatory effects on prostatic growth and smooth muscle tone, thus determining an improvement in symptoms. Certainly, in-depth studies and translational medicine are needed to confirm these encouraging data.

Quercetin enhances sexual behavior and improves ectonucleotidases activity in the hypothalamus of rats treated with cyclosporine

In order to confirm the erectile potential of Quercetin (Q) in cyclosporine-induced hypertensive rats, this research assesses the influence of Q on the ectonucleotidases and adenosine deaminase (ADA) in the brains of rats. Male Wistar rats (200 g-250 g) were divided into five groups at random: Normal control (CTRL), cyclosporine-induced hypertensive rats (25 mg kg day-1 ) (HT) group, positive control (Sildenafil [SIL], 5 mg kg day-1 ), Quercetin 25 mg kg day-1 (25 Q), and Quercetin 50 mg kg day-1 (50 Q). Aside from standard diet-fed male rats; cyclosporine was given i.p for the period of 30 days as well as Q orally while the female rats were only given a standard diet. The animals were subjected to sexual activity (copulation) after which the male rat hypothalamus was dissected for biochemical examination (E-NTPDase activities, ecto-5'-nucleotidase as well as ADA and also levels of nitric oxide [NO]). We observed that Q enhanced copulatory behavior as evident in mounting, intromission, ejaculation numbers, and latencies. A substantial (p < .05) increase in the activity of E-NTPDase (ATP and ADP as substrate) without any notable difference in the action of ecto-5' nucleotidase was facilitated by cyclosporine-induction when compared to the CTRL. The 50 mg/kg, however, had the highest reversal effect in accordance with dose manner. Also, cyclosporine increased ADA activity with a concomitant reduction of NO level while both doses of Q down-regulated ADA activity and, increased NO levels. Enhanced sexual behavior, modulation of ectonucleotidases as well as ADA activity and increased NO levels suggest that Q-rich plant foods may be promising sources of dietary phytonutrients for erectile dysfunction (ED) management. PRACTICAL APPLICATIONS: Behavioral and biochemical assays evaluated showed that Q significantly enhanced sexual behavior as well as improved ATP bioavailability in cyclosporine-induced erectile dysfunctional rats. The modulatory effects of Q on ectonucleotidases, along with its ability to minimize adenosine deaminase activity and increase nitric oxide levels, indicate that Q-rich plants and/or plant foods may be promising sources of dietary phytonutrients for erectile dysfunction management.

Mechanisms underlying priapism in sickle cell disease: targeting and key innovations on the preclinical landscape

Introduction: Priapism is prolonged penile erection in the absence of sexual arousal or desire and is a devastating condition affecting millions of patients with sickle cell disease (SCD) globally. Available drug treatments for SCD-related priapism remain limited and have been primarily reactive rather than preventive. Hence, there is an unmet need for new drug targets and pharmacologic therapies.Areas covered: We examine the molecular mechanisms underlying SCD-associated priapism evaluated mostly in animal models. In mouse models of SCD, molecular defects of priapism operating at the cavernous tissue level include reduced tonic NO/cGMP signaling, elevated oxidative/nitrosative stress, vascular adhesion molecule derangements, excessive adenosine and opiorphin signaling, dysregulated vasoconstrictive RhoA/ROCK signaling, and testosterone deficiency. We discuss the consequences of downregulated cGMP-dependent phosphodiesterase type 5 (PDE5) activity in response to these molecular signaling derangements, as the main effector mechanism causing unrestrained cavernous tissue relaxation that results in priapism.Expert opinion: Basic science studies are crucial for understanding the underlying pathophysiology of SCD-associated priapism. Understanding the molecular mechanisms could unearth new therapeutic targets for this condition based on these mechanisms. Treatment options should aim to improve deranged erection physiology regulatory signaling to prevent priapism and potentially restore or preserve erectile function.

Contemporary best practice in the evaluation and management of stuttering priapism

Stuttering priapism is rare and under-investigated clinical entity. Although it shares similarities with ischaemic priapism, by definition, stuttering priapism has distinct characteristics that advocate for a different management in the clinical setting. Therefore, the management of stuttering priapism aims primarily to prevent recurrence rather than the resolution of spontaneous attacks. A multimodal approach and the individualization of each case are essential because of the diversity of the condition and the plethora of proposed therapeutic strategies. Understanding the underlying pathophysiology and familiarity with contemporary, past and emerging future agents and therapeutic options are required in order to provide an optimal solution for each patient. In addition, patient counselling and the option to combine therapeutic strategies and challenge second-line therapies are essential weapons in the armament of the urologist. Although further clinical trials and studies are mandatory in order to obtain solid data and provide recommendations, all therapeutic options are analysed, with specific interest in the potential advantages and disadvantages. A structured evaluation procedure is also described.

Adenosine contribution to normal renal physiology and chronic kidney disease

Adenosine is a nucleoside that is particularly interesting to many scientific and clinical communities as it has important physiological and pathophysiological roles in the kidney. The distribution of adenosine receptors has only recently been elucidated; therefore it is likely that more biological roles of this nucleoside will be unveiled in the near future. Since the discovery of the involvement of adenosine in renal vasoconstriction and regulation of local renin production, further evidence has shown that adenosine signaling is also involved in the tubuloglomerular feedback mechanism, sodium reabsorption and the adaptive response to acute insults, such as ischemia. However, the most interesting finding was the increased adenosine levels in chronic kidney diseases such as diabetic nephropathy and also in non-diabetic animal models of renal fibrosis. When adenosine is chronically increased its signaling via the adenosine receptors may change, switching to a state that induces renal damage and produces phenotypic changes in resident cells. This review discusses the physiological and pathophysiological roles of adenosine and pays special attention to the mechanisms associated with switching homeostatic nucleoside levels to increased adenosine production in kidneys affected by CKD.

A2B Adenosine Receptor Agonist Improves Erectile Function in Diabetic Rats

Diabetes is an important risk factor for erectile dysfunction (ED). Recent studies have indicated that A2B adenosine receptor (ADORA2B) signaling is essential for penile erection. Thus, we hypothesize that diabetic ED may be attributed to impaired A2B adenosine signaling. To test this hypothesis, we generated diabetic rats by injecting streptozocin as animal model. After 12 weeks, immunohistochemistry staining was used to localize the expression of ADORA2B. Western Blot and quantitative PCR were employed to determine ADORA2B expression level. Intracavernosal pressure (ICP) measurement was used to evaluate erectile function. Diabetic rats received a single intravenous injection of BAY 60-6583, an ADORA2B agonist, or vehicle solution, at 60 min before the ICP measurement. The results showed that ADORA2B expressed in the nerve bundle, smooth muscle, and endothelium in penile tissue of control mice. Western Blot and quantitative PCR results indicated that the expression levels of ADORA2B protein and mRNA were significantly reduced in penile tissues of diabetic rats. Functional studies showed that the erectile response induced by electrical stimulation was remarkably decreased in diabetic rats, compared with age-matched control rats. However, at 60 min after BAY 60-6583 treatment, the erectile function was improved in diabetic rats, suggesting that enhancement of ADORA2B signaling may improve erectile function in diabetic ED. This preclinical study has revealed a previously unrecognized therapeutic possibility of BAY 60-6583 as an effective and mechanism-based drug to treat diabetic ED. In conclusion, we propose that impaired A2B adenosine signaling is one of the pathological mechanisms of diabetic ED.

Men with Sickle Cell Anemia and Priapism Exhibit Increased Hemolytic Rate, Decreased Red Blood Cell Deformability and Increased Red Blood Cell Aggregate Strength

OBJECTIVES

To investigate the association between priapism in men with sickle cell anemia (SCA) and hemorheological and hemolytical parameters.

MATERIALS AND METHODS

Fifty-eight men with SCA (median age: 38 years) were included; 28 who had experienced priapism at least once during their life (priapism group) and 30 who never experienced this complication (control group). Twenty-two patients were treated with hydroxycarbamide, 11 in each group. All patients were at steady state at the time of inclusion. Hematological and biochemical parameters were obtained through routine procedures. The Laser-assisted Optical Rotational Cell Analyzer was used to measure red blood cell (RBC) deformability at 30 Pa (ektacytometry) and RBC aggregation properties (laser backscatter versus time). Blood viscosity was measured at a shear rate of 225 s-1 using a cone/plate viscometer. A principal component analysis was performed on 4 hemolytic markers (i.e., lactate dehydrogenase (LDH), aspartate aminotransferase (ASAT), total bilirubin (BIL) levels and reticulocyte (RET) percentage) to calculate a hemolytic index.

RESULTS

Compared to the control group, patients with priapism exhibited higher ASAT (p = 0.01), LDH (p = 0.03), RET (p = 0.03) levels and hemolytic indices (p = 0.02). Higher RBC aggregates strength (p = 0.01) and lower RBC deformability (p = 0.005) were observed in patients with priapism compared to controls. After removing the hydroxycarbamide-treated patients, RBC deformability (p = 0.01) and RBC aggregate strength (p = 0.03) were still different between the two groups, and patients with priapism exhibited significantly higher hemolytic indices (p = 0.01) than controls.

CONCLUSION

Our results confirm that priapism in SCA is associated with higher hemolytic rates and show for the first time that this complication is also associated with higher RBC aggregate strength and lower RBC deformability.

Targeted endothelial nanomedicine for common acute pathological conditions

Endothelium, a thin monolayer of specialized cells lining the lumen of blood vessels is the key regulatory interface between blood and tissues. Endothelial abnormalities are implicated in many diseases, including common acute conditions with high morbidity and mortality lacking therapy, in part because drugs and drug carriers have no natural endothelial affinity. Precise endothelial drug delivery may improve management of these conditions. Using ligands of molecules exposed to the bloodstream on the endothelial surface enables design of diverse targeted endothelial nanomedicine agents. Target molecules and binding epitopes must be accessible to drug carriers, carriers must be free of harmful effects, and targeting should provide desirable sub-cellular addressing of the drug cargo. The roster of current candidate target molecules for endothelial nanomedicine includes peptidases and other enzymes, cell adhesion molecules and integrins, localized in different domains of the endothelial plasmalemma and differentially distributed throughout the vasculature. Endowing carriers with an affinity to specific endothelial epitopes enables an unprecedented level of precision of control of drug delivery: binding to selected endothelial cell phenotypes, cellular addressing and duration of therapeutic effects. Features of nanocarrier design such as choice of epitope and ligand control delivery and effect of targeted endothelial nanomedicine agents. Pathological factors modulate endothelial targeting and uptake of nanocarriers. Selection of optimal binding sites and design features of nanocarriers are key controllable factors that can be iteratively engineered based on their performance from in vitro to pre-clinical in vivo experimental models. Targeted endothelial nanomedicine agents provide antioxidant, anti-inflammatory and other therapeutic effects unattainable by non-targeted counterparts in animal models of common acute severe human disease conditions. The results of animal studies provide the basis for the challenging translation endothelial nanomedicine into the clinical domain.

Beneficial and detrimental role of adenosine signaling in diseases and therapy

Adenosine is a major signaling nucleoside that orchestrates cellular and tissue adaptation under energy depletion and ischemic/hypoxic conditions by activation of four G protein-coupled receptors (GPCR). The regulation and generation of extracellular adenosine in response to stress are critical in tissue protection. Both mouse and human studies reported that extracellular adenosine signaling plays a beneficial role during acute states. However, prolonged excess extracellular adenosine is detrimental and contributes to the development and progression of various chronic diseases. In recent years, substantial progress has been made to understand the role of adenosine signaling in different conditions and to clarify its significance during the course of disease progression in various organs. These efforts have and will identify potential therapeutic possibilities for protection of tissue injury at acute stage by upregulation of adenosine signaling or attenuation of chronic disease progression by downregulation of adenosine signaling. This review is to summarize current progress and the importance of adenosine signaling in different disease stages and its potential therapeutic effects.

Nitrergic Mechanisms for Management of Recurrent Priapism

INTRODUCTION

Priapism is a condition involving prolonged penile erection unrelated to sexual interest or desire. The ischemic type, including its recurrent variant, is often associated with both physical and psychological complications. As such, management is of critical importance. Ideal therapies for recurrent priapism should address its underlying pathophysiology.

AIM

To review the available literature on priapism management approaches particularly related to nitrergic mechanisms.

METHODS

A literature review of the pathophysiology and management of priapism was performed using PubMed.

MAIN OUTCOME MEASURE

Publications pertaining to mechanisms of the molecular pathophysiology of priapism.

RESULTS

Nitrergic mechanisms are characterized as major players in the molecular pathophysiology of priapism. PDE5 inhibitors represent an available therapeutic option with demonstrated ability in attenuating these underlying nitrergic derangements. Several additional signaling pathways have been found to play a role in the molecular pathophysiology of priapism and have also been associated with these nitrergic mechanisms.

CONCLUSION

An increasing understanding of the molecular pathophysiology of priapism has led to the discovery of new potential targets. Several mechanism-based therapeutic approaches may become available in the future.

How I treat priapism

Priapism is a disorder of persistent penile erection unrelated to sexual interest or desire. This pathologic condition, specifically the ischemic variant, is often associated with devastating complications, notably erectile dysfunction. Because priapism demonstrates high prevalence in patients with hematologic disorders, most commonly sickle cell disease (SCD), there is significant concern for its sequelae in this affected population. Thus, timely diagnosis and management are critical for the prevention or at least reduction of cavernosal tissue ischemia and potential damage consequent to each episode. Current guidelines and management strategies focus primarily on reactive treatments. However, an increasing understanding of the molecular pathophysiology of SCD-associated priapism has led to the identification of new potential therapeutic targets. Future agents are being developed and explored for use in the prevention of priapism.

Elevated adenosine signaling via adenosine A2B receptor induces normal and sickle erythrocyte sphingosine kinase 1 activity

Erythrocyte possesses high sphingosine kinase 1 (SphK1) activity and is the major cell type supplying plasma sphingosine-1-phosphate, a signaling lipid regulating multiple physiological and pathological functions. Recent studies revealed that erythrocyte SphK1 activity is upregulated in sickle cell disease (SCD) and contributes to sickling and disease progression. However, how erythrocyte SphK1 activity is regulated remains unknown. Here we report that adenosine induces SphK1 activity in human and mouse sickle and normal erythrocytes in vitro. Next, using 4 adenosine receptor-deficient mice and pharmacological approaches, we determined that the A2B adenosine receptor (ADORA2B) is essential for adenosine-induced SphK1 activity in human and mouse normal and sickle erythrocytes in vitro. Subsequently, we provide in vivo genetic evidence that adenosine deaminase (ADA) deficiency leads to excess plasma adenosine and elevated erythrocyte SphK1 activity. Lowering adenosine by ADA enzyme therapy or genetic deletion of ADORA2B significantly reduced excess adenosine-induced erythrocyte SphK1 activity in ADA-deficient mice. Finally, we revealed that protein kinase A-mediated extracellular signal-regulated kinase 1/2 activation functioning downstream of ADORA2B underlies adenosine-induced erythrocyte SphK1 activity. Overall, our findings reveal a novel signaling network regulating erythrocyte SphK1 and highlight innovative mechanisms regulating SphK1 activity in normal and SCD.

Metformin in vitro and in vivo increases adenosine signaling in rabbit corpora cavernosa

INTRODUCTION

In subjects with erectile dysfunction responding poorly to sildenafil, metformin was reported to improve erections.

AIMS

The aim of this study is to investigate metformin's mechanism of action on erectile function, particularly focusing on adenosine (ADO) and nitric oxide (NO) signaling in an animal model of high-fat diet (HFD)-induced metabolic syndrome.

METHODS

In vitro contractility studies of penile strips. Penile expression of genes related to ADO or NO signaling was also evaluated.

MAIN OUTCOME MEASURE

In vitro contractility studies were used to investigate the effect of in vivo and ex vivo metformin administration on ADO- or acetylcholine (Ach)-induced relaxation of penile strips from HFD as compared with animals fed a regular diet (RD).

RESULTS

Expression of ADO receptor type 3 (A3 R), ADO deaminase (ADA), AMP deaminase type 1 (AMPD1), and 2 (AMPD2) was decreased in HFD as compared with RD. Accordingly, in HFD the ADO relaxant effect was potentiated as compared with RD (P < 0.02). In vivo metformin treatment in both RD and HFD significantly increased the ADO relaxing effect (P < 0.0001 and P < 0.01, respectively, vs. relative untreated groups) although to a different extent. In fact, the half-maximal inhibitory concentration (IC50 )/IC50 ratio in RD increased fourfold vs. HFD (RD IC50 ratio = 13.75 ± 2.96; HFD IC50 ratio = 2.85 ± 0.52). In corpora cavernosa (CC) from HFD, in vivo metformin (i) normalized A3 R, ADA, and AMPD1; (ii) further decreased AMPD2; (iii) increased dimethylarginine dimethylamino-hydrolase; and (iv) partially restored impaired Ach-induced relaxation. Ex vivo metformin time and dose dependently increased the relaxant effect of ADO in RD. The potentiating effect of metformin on ADO-induced relaxation was significantly reduced by preincubation with NO synthase inhibitor N(ω) -Nitro-L-arginine methyl ester hydrochloride (L-NAME). Interestingly, in vivo testosterone supplementation in HFD rabbits (i) increased penile expression of endothelial NO synthase and AMPD2 and (ii) restored metformin's potentiating effect on ADO-induced relaxation up to RD level.

CONCLUSION

Metformin in vivo and ex vivo increases ADO signaling in CC, most probably interfering with NO formation and ADO breakdown.

Opiorphin is a master regulator of the hypoxic response in corporal smooth muscle cells

Men with sickle cell disease (SCD) risk developing priapism. Recognizing that SCD is a disease of hypoxia, we investigated the effect of hypoxia on gene expression in corporal smooth muscle (CSM) cells. Rat CSM cells in vitro were treated with CoCl2 or low oxygen tension to mimic hypoxia. Hypoxic conditions increased expression of genes previously associated with priapism in animal models. Variable coding sequence a1 (Vcsa1; the rat opiorphin homologue, sialorphin), hypoxia-inducible factor 1a (Hif-1a), and A2B adenosine receptor (a2br) were increased by 10-, 4-, and 6-fold, respectively, by treatment with CoCl2, whereas low oxygen tension caused increases in expression of 3-, 4-, and 1.5-fold, respectively. Sialorphin-treated CSM cells increased expression of Hif-1a and a2br by 4-fold, and vcsa1-siRNA treatment reduced expression by ∼50%. Using a Hif-1a inhibitor, we demonstrated up-regulation of a2br by sialorphin is dependent on Hif-1a, and knockdown of vcsa1 expression with vcsa1-siRNA demonstrated that hypoxic-up-regulation of Hif-1a is dependent on vcsa1. In CSM from a SCD mouse, there was 15-fold up-regulation of opiorphin at a life stage prior to priapism. We conclude that in CSM, opiorphins are master regulators of the hypoxic response. Opiorphin up-regulation in response to SCD-associated hypoxia activates CSM "relaxant" pathways; excessive activation of these pathways results in priapism.

The role of adenosine signaling in sickle cell therapeutics

Data suggest a role for adenosine signaling in the pathogenesis of sickle cell disease (SCD). Signaling through the adenosine A2A receptor (A2AR) has demonstrated beneficial effects. Activation of A2ARs decreases inflammation with SCD by blocking activation of invariant natural killer T cells. Decreased inflammation may reduce the severity of vasoocclusive crises. Adenosine signaling through the adenosine A2B receptor (A2BR) may be detrimental in SCD. Whether adenosine signaling predominantly occurs through A2ARs or A2BRs may depend on differing levels of adenosine and disease state (steady state versus crisis). There may be opportunities to develop novel therapeutic approaches targeting A2ARs and/or A2BRs for patients with SCD.

Targeted Drug Delivery to Endothelial Adhesion Molecules

Endothelial cells represent important targets for therapeutic and diagnostic interventions in many cardiovascular, pulmonary, neurological, inflammatory, and metabolic diseases. Targeted delivery of drugs (especially potent and labile biotherapeutics that require specific subcellular addressing) and imaging probes to endothelium holds promise to improve management of these maladies. In order to achieve this goal, drug cargoes or their carriers including liposomes and polymeric nanoparticles are chemically conjugated or fused using recombinant techniques with affinity ligands of endothelial surface molecules. Cell adhesion molecules, constitutively expressed on the endothelial surface and exposed on the surface of pathologically altered endothelium—selectins, VCAM-1, PECAM-1, and ICAM-1—represent good determinants for such a delivery. In particular, PECAM-1 and ICAM-1 meet criteria of accessibility, safety, and relevance to the (patho)physiological context of treatment of inflammation, ischemia, and thrombosis and offer a unique combination of targeting options including surface anchoring as well as intra- and transcellular targeting, modulated by parameters of the design of drug delivery system and local biological factors including flow and endothelial phenotype. This review includes analysis of these factors and examples of targeting selected classes of therapeutics showing promising results in animal studies, supporting translational potential of these interventions.

New insights into sickle cell disease: a disease of hypoxia

PURPOSE OF REVIEW

Sickle cell disease (SCD) is a devastating genetic disorder caused by a single amino acid substitution in β-globin. Although the condition was first described more than a 100 years ago, treatment options remain scarce and unsatisfactory. This review summarizes recent findings that may provide novel insight into therapeutic approaches to SCD treatment.

RECENT FINDINGS

Because of insufficient numbers of erythrocytes for oxygen delivery, SCD patients constantly face hypoxia. Adenosine is well known as a key signaling nucleoside that orchestrates a multifaceted physiological response to hypoxia. Recent studies have revealed that adenosine concentrations are significantly elevated in SCD and contribute to disease pathology by activating adenosine receptors on red blood cells. Apart from adenosine, hypoxia also causes hemoglobin release via hemolysis. Studies on free hemoglobin in circulation have uncovered another two important molecules: nitric oxide and heme oxygenase-1.

SUMMARY

The core of SCD pathology is erythrocyte sickling under hypoxic conditions, leading to vaso-occlusion and hemolysis. Deeper and more comprehensive understanding of SCD as a disease of hypoxia will provide us new therapeutic targets for SCD treatment.

Sildenafil citrate-restored eNOS and PDE5 regulation in sickle cell mouse penis prevents priapism via control of oxidative/nitrosative stress

Sildenafil citrate revolutionized the practice of sexual medicine upon its federal regulatory agency approval approximately 15 years ago as the prototypical phosphodiesterase type 5 inhibitor indicated for the treatment of male erectile dysfunction. We now provide scientific support for its alternative use in the management of priapism, a clinical disorder of prolonged and uncontrolled penile erection. Sildenafil administered continuously to sickle cell mice, which show a priapism phenotype, reverses oxidative/nitrosative stress effects in the penis, mainly via reversion of uncoupled endothelial nitric oxide synthase to the functional coupled state of the enzyme, which in turn corrects aberrant signaling and function of the nitric oxide/cyclic GMP/protein kinase G/phosphodiesterase type 5 cascade. Priapism tendencies in these mice are reverted partially toward normal neurostimulated erection frequencies and durations after sildenafil treatment in association with normalized cyclic GMP concentration, protein kinase G activity and phosphodiesterase type 5 activity in the penis. Thus, sildenafil exerts pleiotropic effects in the penis that extend to diverse erection disorders.

Sickle cell vaso-occlusion causes activation of iNKT cells that is decreased by the adenosine A2A receptor agonist regadenoson

Adenosine A2A receptor (A2AR) agonists reduce invariant natural killer T (iNKT) cell activation and decrease inflammation in sickle cell disease (SCD) mice. We conducted a phase 1 trial of the A2AR agonist regadenoson in adults with SCD. The target dose was 1.44 μg/kg/h. iNKT cell activation was evaluated using antibodies targeting the p65 subunit of nuclear factor-κB (phospho-NF-κB p65), interferon-γ (IFN-γ), and A2AR. Regadenoson was administered to 27 adults with SCD. We examined 21 patients at steady state and 6 during painful vaso-occlusive crises (pVOC). iNKT cell activation was also measured in 14 African-American controls. During pVOC, the fraction of iNKT cells demonstrating increased phospho-NF-κB p65 and A2AR expression was significantly higher compared with controls (P < .01) and steady-state patients (P < .05). IFN-γ expression was also significantly higher compared with controls (P = .02). After a 24-hour infusion of regadenoson during pVOC, phospho-NF-κB p65 activation in iNKT cells decreased compared to baseline by a median of 48% (P = .03) to levels similar to controls and steady-state SCD. No toxicities were identified. Infusional regadenoson administered to adults with SCD at 1.44 μg/kg/h during pVOC decreases activation of iNKT cells without toxicity.

Stuttering priapism: insights into pathogenesis and management

Priapism is defined as a persistent, painful erection that continues beyond, or is unrelated to, sexual stimulation. It may be categorized as either ischemic (low/absent flow) or nonischemic (high flow). Stuttering priapism is a variant of the ischemic type that is characterized by repetitive, transient, painful, self-limiting episodes of priapism. It is associated with various hematological disorders, including sickle cell disease and pharmacological treatments. The consequences of ineffective treatment of priapism are erectile dysfunction and impaired quality of life due to chronic pain and physical disfigurement. Many of the existing medical therapeutic options for treatment of stuttering priapism are nonmechanistic and associated with significant adverse effects. However, the scientific knowledge of stuttering priapism has transitioned in the past few years, from a condition that is poorly understood to one that has borne a burst of evolving molecular science. In this review, the pathophysiology of priapism is discussed, with particular emphasis on new molecular effectors and mechanisms. Novel treatment methods, as well as potential future agents, based on the emerging molecular evidence are discussed.

Sickling cells, cyclic nucleotides, and protein kinases: the pathophysiology of urogenital disorders in sickle cell anemia

Sickle cell anemia is one of the best studied inherited diseases, and despite being caused by a single point mutation in the HBB gene, multiple pleiotropic effects of the abnormal hemoglobin S production range from vaso-occlusive crisis, stroke, and pulmonary hypertension to osteonecrosis and leg ulcers. Urogenital function is not spared, and although priapism is most frequently remembered, other related clinical manifestations have been described, such as nocturia, enuresis, increased frequence of lower urinary tract infections, urinary incontinence, hypogonadism, and testicular infarction. Studies on sickle cell vaso-occlusion and priapism using both in vitro and in vivo models have shed light on the pathogenesis of some of these events. The authors review what is known about the deleterious effects of sickling on the genitourinary tract and how the role of cyclic nucleotides signaling and protein kinases may help understand the pathophysiology underlying these manifestations and develop novel therapies in the setting of urogenital disorders in sickle cell disease.

Adenosine signaling in normal and sickle erythrocytes and beyond

Sickle cell disease (SCD) is a debilitating hemolytic genetic disorder with high morbidity and mortality affecting millions of individuals worldwide. Although SCD was discovered more than a century ago, no effective mechanism-based prevention and treatment are available due to poorly understood molecular basis of sickling, the fundamental pathogenic process of the disease. SCD patients constantly face hypoxia. One of the best-known signaling molecules to be induced under hypoxic conditions is adenosine. Recent studies demonstrate that hypoxia-mediated elevated adenosine signaling plays an important role in normal erythrocyte physiology. In contrast, elevated adenosine signaling contributes to sickling and multiple life threatening complications including tissue damage, pulmonary dysfunction and priapism. Here, we summarize recent research on the role of adenosine signaling in normal and sickle erythrocytes, progression of the disease and therapeutic implications. In normal erythrocytes, both genetic and pharmacological studies demonstrate that adenosine can enhance 2,3-bisphosphoglycerate (2,3-BPG) production via A(2B) receptor (ADORA2B) activation, suggesting that elevated adenosine has an unrecognized role in normal erythrocytes to promote O(2) release and prevent acute ischemic tissue injury. However, in sickle erythrocytes, the beneficial role of excessive adenosine-mediated 2,3-BPG induction becomes detrimental by promoting deoxygenation, polymerization of sickle hemoglobin and subsequent sickling. Additionally, adenosine signaling via the A(2A) receptor (ADORA2A) on invariant natural killer T (iNKT) cells inhibits iNKT cell activation and attenuates pulmonary dysfunction in SCD mice. Finally, elevated adenosine coupled with ADORA2BR activation is responsible for priapism, a dangerous complication seen in SCD. Overall, the research reviewed here reveals a differential role of elevated adenosine in normal erythrocytes, sickle erythrocytes, iNK cells and progression of disease. Thus, adenosine signaling represents a potentially important therapeutic target for the treatment and prevention of disease.

Adenosine signaling: good or bad in erectile function?

The erectile status of penile tissue is governed largely by the tone of cavernosal smooth muscle cells, which is determined by the balance of vascular relaxants and constrictors. Vascular relaxants play a key role in regulating the tone of cavernosal smooth muscle and thus the initiation and maintenance of penile erection. Early studies drew attention to the potential role of adenosine signaling in this process. However, the serendipitous discovery of the effect of sildenafil on erectile physiology drew more attention toward nitric oxide (NO) as a vasodilator in the process of penile erection, and a recently discovered, unexpected erectile phenotype of adenosine deaminase-deficient mice reemphasizes the importance of adenosine as a key regulatory of erectile status. Adenosine, like NO, is a potent and short-lived vasorelaxant that functions via cyclic nucleotide second messenger signaling to promote smooth muscle relaxation. Recent studies reviewed here show that adenosine functions to relax the corpus cavernosum and promote penile erection. Excess adenosine in penile tissue contributes to the disorder called priapism, and impaired adenosine signaling is associated with erectile dysfunction. More recent research summarized in this review reveals that adenosine functions as a key endogenous vasodilator in the initiation and maintenance of normal penile erection. This new insight highlights adenosine signaling pathways operating in penile tissue as significant therapeutic targets for the treatment of erectile disorders.

Targeted interception of signaling reactive oxygen species in the vascular endothelium

Reactive oxygen species (ROS) are implicated as injurious and as signaling agents in human maladies including inflammation, hyperoxia, ischemia-reperfusion and acute lung injury. ROS produced by the endothelium play an important role in vascular pathology. They quench, for example, nitric oxide, and mediate pro-inflammatory signaling. Antioxidant interventions targeted for the vascular endothelium may help to control these mechanisms. Animal studies have demonstrated superiority of targeting ROS-quenching enzymes catalase and superoxide dismutase to endothelial cells over nontargeted formulations. A diverse arsenal of targeted antioxidant formulations devised in the last decade shows promising results for specific quenching of endothelial ROS. In addition to alleviation of toxic effects of excessive ROS, these targeted interventions suppress pro-inflammatory mechanisms, including endothelial cytokine activation and barrier disruption. These interventions may prove useful in experimental biomedicine and, perhaps, in translational medicine.

Priapism in sickle-cell disease: a hematologist's perspective

INTRODUCTION

Priapism is a familiar problem to hematologists, well known for its association with sickle-cell disease (SCD). It also occurs in a variety of other hematological illnesses, nearly all forms of congenital hemolytic anemia, including other hemoglobinopathies and red blood cell membranopathies and enzymopathies.

AIM

Provide urologists with a comprehensive review of priapism in SCD, with an emphasis on the perspective of a practicing hematologist.

METHODS

Medline searches through July 2010 were conducted using the terms priapism, erectile dysfunction, and sickle cell.

MAIN OUTCOME MEASURES

Expert opinion was based on review of the medical literature related to this subject matter.

RESULTS

In men with SCD, large epidemiological studies have linked the risk of priapism to clinical markers of the severity of intravascular hemolysis. Extracellular hemoglobin and arginase released during hemolysis has been implicated in reducing nitric oxide bioavailability, although the relevance of hemolysis to vascular dysfunction has been challenged by some scientists. Consistent with the role of impairment of the nitric oxide axis, mice genetically deficient in nitric oxide production have also been shown to develop priapic activity. Provocative new data indicate that hemolysis-linked dysregulation of adenosine signaling in the penis contributes to priapism in sickle cell mice. Serious questions have arisen regarding the efficacy of mainstays of textbook dogma for treatment of acute severe priapism, including intravenous fluids, alkalinization, and exchange transfusion, and there is increasing acceptance for early aspiration and irrigation of the corpus cavernosum.

CONCLUSION

For patients with sickle cell with recurrent priapism, there is very limited evidence for a medical prophylaxis role for hydroxyurea, etilefrine, pseudoephedrine, leuprolide, sildenafil, and other agents. Recent publications have highlighted nitric oxide and adenosine signal transduction pathways as worthy of additional research. Research and clinical management of sickle-cell priapism is strengthened by multidisciplinary collaboration between hematologists and urologists.

Potential risks of chronic sildenafil use for priapism in sickle cell disease

INTRODUCTION

Priapism is a common concern in sickle cell disease. With a high frequency of recurrences and serious long-term sequela, a preventative, rather than traditionally reactive approach, needs to be taken in these patients. Reports have shown successful use of sildenafil as a prophylactic treatment but have failed to address adverse outcomes, including vasoocclusive pain crises, of chronic sildenafil therapy in sickle cell patients.

AIMS

We wish to draw attention to the potential adverse outcomes of this therapy on the overall state of the patient's disease for consideration in future studies.

METHODS

We used sildenafil in a patient suffering from almost daily attacks of priapism.

RESULTS

Sildenafil was successful in decreasing the frequency of priapism; however, our patient experienced an increased frequency of vasoocclusive crises, something not previously addressed.

CONCLUSION

Future studies of sildenafil use in sickle cell disease need to assess the global state of the disease, not just the frequency of priapism.

Impaired erectile function in CD73-deficient mice with reduced endogenous penile adenosine production

INTRODUCTION

Adenosine has been implicated in normal and abnormal penile erection. However, a direct role of endogenous adenosine in erectile physiology and pathology has not been established.

AIM

To determine the functional role of endogenous adenosine production in erectile function.

METHODS

CD73-deficient mice (CD73(-/-)) and age-matched wild-type (WT) mice were used. Some WT mice were treated with alpha, beta-methylene adenosine diphosphate (ADP) (APCP), a CD73-specific inhibitor. High-performance liquid chromatography was used to measure adenosine levels in mouse penile tissues. In vivo assessment of intracorporal pressure (ICP) normalized to mean arterial pressure (MAP) in response to electrical stimulation (ES) of the cavernous nerve was used.

MAIN OUTCOME MEASUREMENT

The main outcome measures of this study were the in vivo assessment of initiation and maintenance of penile erection in WT mice and mice with deficiency in CD73 (ecto-5'-nucleotidase), a key cell-surface enzyme to produce extracellular adenosine.

RESULTS

Endogenous adenosine levels were elevated in the erected state induced by ES of cavernous nerve compared to the flaccid state in WT mice but not in CD73(-/-) mice. At cellular levels, we identified that CD73 was highly expressed in the neuronal, endothelial cells, and vascular smooth muscle cells in mouse penis. Functionally, we found that the ratio of ES-induced ICP to MAP in CD73(-/-) mice was reduced from 0.48 ± 0.03 to 0.33 ± 0.05 and ES-induced slope was reduced from 0.30 ± 0.13 mm Hg/s to 0.15 ± 0.05 mm Hg/s (both P < 0.05). The ratio of ES-induced ICP to MAP in APCP-treated WT mice was reduced from 0.49 ± 0.03 to 0.38 ± 0.06 and ES-induced slope was reduced from 0.29 ± 0.11 mm Hg/s to 0.19 ± 0.04 mm Hg/s (both P < 0.05).

CONCLUSION

Overall, our findings demonstrate that CD73-dependent production of endogenous adenosine plays a direct role in initiation and maintenance of penile erection.

In vitro and in vivo animal models in priapism research

INTRODUCTION

Priapism is an enigmatic yet devastating clinical phenomenon. In the last two decades, the use of various animal models to study this disorder has dramatically advanced our understanding of this mysterious disorder.

AIM

This report reviews various animal models used to study ischemic priapism and informs basic science researchers the broad view of priapism research.

METHODS

Retrospective review of pertinent literature from the last two decades via PubMed search using the keywords "ischemic priapism" and "priapism model."

MAIN OUTCOME MEASURES

  Findings on the animal models used in ischemic priapism research and its advantages and limitations.

RESULTS

In vitro and in vivo animal models varying from dogs, cats, rabbits, rats to mice were used in priapism research. In vitro models included: (i) corpora cavernosa smooth muscle (CCSM) strip in organ bath; (ii) corporal tissue binding assay; (iii) CCSM cell culture under hypoxia/anoxia. In vivo models could be categorized as: (i) pharmacologically induced by corpus cavernosum medicine injection; (ii) ventilation induced by tidal volume control; (iii) mechanical induced by a constrictor band placed around the base of the penis combined with induced erection; (iv) genetic engineered by intracorporal gene transfer, transgenic, or gene knock-out.

CONCLUSIONS

The ischemic priapism animal models are shifting from pharmaceutically or mechanically induced to genetically engineered. The knowledge generated by those models is enhancing our understanding and management of this clinical challenge.