The Effect of Phosphodiesterase-type 5 Inhibitors on Erectile Function: An Overview of Systematic Reviews

Efficacy and safety of PDE5 inhibitors in middle-aged and old patients with and without hypogonadism

PURPOSE

Although several reviews have evaluated the use of PDE5 inhibitors (PDE5i) for treating erectile dysfunction (ED), their specific use in middle-aged and old patients has not been fully evaluated. Given that elderly patients with ED often have a complex combination of systemic and sexual health risk factors, the safety and efficacy of PDE5i in such a context are hereby reviewed.

MATERIALS AND METHODS

A thorough examination of existing literature has been conducted on PubMed.

RESULTS

PDE5i has good safety and efficacy, but the situation is more complex for patients with hypogonadism than those with normal testosterone levels, with reduced responsiveness to PDE5i. In this case, combination therapy with testosterone is recommended, safe and effective.

CONCLUSIONS

Eliminating or reducing reversible risk factors and controlling or slowing the development of irreversible factors is an important foundation for using PDE5i to treat ED in all patients, especially middle-aged and elderly ones.

A novel standard-free detection of adulteration method for sildenafil derivatives in dietary supplements

The rapid and accurate detection of illegal adulteration of chemical drugs into dietary supplements is a big challenge in the food chemistry field. Detection of compounds without a standard reference is even more difficult; however, this is a common situation. Here in this study, a novel "standard-free detection of adulteration" (SFDA) method was proposed and phosphodiesterase-5 inhibitor derivatives were used as an example to figure out the possibility and reliability of this SFDA method. After analysis by quadrupole coupled time of flight-tandem mass spectrometry detection and multivariable statistics, six common fragment ions were chosen to indicate whether adulteration was present or not, while 20 characteristic fragment ions indicated whether adulteration was by nitrogen-containing heterocycles or by anilines. Furthermore, the quantitative methods were conducted by high-performance liquid chromatography-tandem mass spectrometry. In a word, this strategy allows for a quick determination of dietary supplement adulteration without any need for standard materials, improving the efficacy of food safety testing.

The effect of combination treatment with low-intensity shockwave therapy and daily tadalafil on severe erectile dysfunction: a double-blind, randomized, sham-controlled clinical trial

BACKGROUND

Patients with severe erectile dysfunction (ED) remain the most challenging group in terms of available noninvasive treatment modalities.

AIM

The study sought to assess the role of combination therapy with low-intensity shockwave therapy (LiST) and daily tadalafil 5 mg in a highly select group of patients with severe vasculogenic ED through a double-blind, randomized trial.

METHODS

Forty-eight sexually active men were randomly assigned to 12 sessions of LiST 3 times weekly and tadalafil 5 mg once daily (n = 34) or sham therapy and tadalafil (n = 17) for 4 weeks. Patients were assessed at 1 and 3 months after completion of treatment.

OUTCOMES

Improvement of erectile function was evaluated through the International Index of Erectile Function-Erectile Function domain (IIEF-EF) or 6-item IIEF and the Sexual Encounter Profile (SEP) diary. The primary outcome was the difference between the groups in the IIEF-EF at 3 months after completion of treatment. Secondary outcomes comprised (1) the difference between the groups in the IIEF-EF at 1 month after completion of treatment, (2) the difference between the groups in the "yes" responses to question 3 of the SEP diary at 1 and 3 months, and (3) the treatment-related adverse events. The number of patients attaining a minimal clinically important difference in the IIEF-EF (improvement of at least 7 points) was also assessed.

RESULTS

After treatment, the absolute scores in the IIEF-EF were higher in patients receiving LiST and tadalafil vs sham therapy and tadalafil both at the 1-month (12.1 ± 2.4 vs 10.2 ± 1.7; P = .002) and at the 3-month (12.9 ± 2.1 vs 10.8 ± 1.8; P < .001) evaluation. Between the 2 groups, the proportion of "yes" responses to question 3 of the SEP diary was not statistically significant, whereas the number of patients attaining a minimal clinically important difference in the IIEF-EF was statistically significant only at the 3-month evaluation. No adverse events occurred.

CLINICAL IMPLICATIONS

Application of LiST in patients with severe vasculogenic ED receiving daily dose tadalafil may further improve erectile function compared with tadalafil as a stand-alone treatment on the short term.

STRENGTHS AND LIMITATIONS

Although we provided the first study in the field, severe vasculogenic ED was defined based on medical history and clinical examination and not based on penile ultrasound measures.

CONCLUSION

The combination of 12 sessions LiST 3 times weekly and daily tadalafil for 4 weeks led to a 2-point difference in the IIEF-EF compared with sham therapy and daily tadalafil among patients with severe vasculogenic ED after 1 and 3 months from completion of treatment.

Differences in ocular adverse events associated with phosphodiesterase-5 inhibitors: a real-world pharmacovigilance study

OBJECTIVE

Our study aims to characterize the ocular safety profiles of phosphodiesterase type 5 (PDE5) inhibitors and explore the differences among different PDE5 inhibitors.

METHODS

We analyzed reports on ocular adverse events associated with sildenafil, vardenafil and tadalafil submitted to the FDA Adverse Event Reporting System (FAERS) database from the first quarter of 2004 to the first quarter of 2023. Disproportionality analysis was conducted to evaluate reporting risk profiles.

RESULTS

Among 61,211 reports qualifying for analysis, 5,127 involved sildenafil, 832 vardenafil, and 3,733 tadalafil. All PDE5 inhibitors showed increased reporting odds ratios (ROR) for ocular adverse events, with vardenafil highest (ROR 4.47) followed by sildenafil and tadalafil. Key ocular adverse events included cyanopsia, optic ischemic neuropathy, visual field defects, unilateral blindness and blindness. Sildenafil showed the highest disproportionality for cyanopsia (ROR 1148.11) while vardenafil and tadalafil showed the highest disproportionality for optic ischemic neuropathy. Time-to-onset analysis also revealed significant differences, with sildenafil having a later median time-to-onset compared to vardenafil and tadalafil.

CONCLUSIONS

This comprehensive pharmacovigilance study reveals distinct patterns of ocular adverse events associated with PDE5 inhibitors. These findings contribute to a better understanding of the safety profiles of PDE5 inhibitors and may guide healthcare professionals in clinical decision-making.

Application of terahertz Time-Domain spectroscopy and chemometrics-based whale optimization algorithm in PDE5 inhibitor detection

Combating the illicit use of PDE5 inhibitor drugs is a focal point in forensic science research. In order to achieve rapid identification of such drugs, this study applies terahertz time-domain spectroscopy combined with chemometrics to establish a fast and accurate detection method for PDE5 inhibitors. The optimal detection method is determined by comparing the spectral performance of three optical parameters, namely absorption coefficient, refractive index, and dielectric constant. Linear discriminant models based on different spectral parameters, whale optimization algorithm optimized extreme learning machine models, and whale optimization algorithm optimized random forest models are established. The effectiveness and performance of principal component analysis and competitive adaptive reweighted sampling algorithm for spectral feature data selection are also investigated. The PDE5 inhibitor identification model based on the competitive adaptive reweighted sampling - whale optimization algorithm - random forest (CARS-WOA-RF) model achieves an accuracy of 98.61%, and the identification model for two concentrations of Sildenafil achieves 100% accuracy. The results demonstrate that terahertz time-domain spectroscopy combined with chemometrics can effectively detect various common types of PDE5 inhibitor drugs and different concentrations.

Analysis of vacuum negative pressure therapy and traditional Chinese medicine lavage in combination with tadalafil for vascular erectile dysfunction

This study investigates the clinical effects of the novel Traditional Chinese Medicine (TCM) topical wash used in combination with negative pressure irrigation and tadalafil for the treatment of vascular erectile dysfunction. Eighty-seven patients with vascular erectile dysfunction were divided into an observation group and a control group. The observation group was administered negative pressure irrigation (TCM) in combination with oral tadalafil for four weeks, and the control group was administered oral tadalafil for four weeks. The observation group included 21 patients with arterial erectile dysfunction and 22 with intravenous erectile dysfunction. After treatment, IIEF-5, EHS, GAD scores, PSV, EDV and RI in observation group were improved compared with those before treatment (P = 0.000, 0.000, 0.000, L0.000/R0.000, L0.000/R0.000, L0.003/R0.000). Erectile function (IIEF-5, EHS) was significantly improved compared with the control group (P = 0.008, 0.002). In the observation group, there were 21 cases of arterial erectile dysfunction and 22 cases of intravenous erectile dysfunction. After treatment, PSV of arterial ED improved significantly (P = L0.000/R0.000), but EDV did not decrease significantly (P = L0.084/R 0.098). In patients with venous ED, PSV increased (P = L0.026/R0.032) and EDV decreased significantly (P = L0.000/R0.000). These findings suggest that TCM negative pressure lavage combined with tadalafil improves the blood supply of the penile artery, relaxes smooth muscle, and improves the closing mechanism of venous vessels in patients with vascular erectile dysfunction, ultimately improving the erectile function.

Safety profile and signal detection of phosphodiesterase type 5 inhibitors for erectile dysfunction: a Food and Drug Administration Adverse Event Reporting System analysis

Background

Phosphodiesterase type 5 inhibitors (PDE5Is) are generally well tolerated but have been associated with uncommon and significant adverse events (AEs).

Aim

This study aims to investigate and compare the characteristics of AEs associated with PDE5Is used for erectile dysfunction and identify any safety signals in a postmarketing surveillance database between 2010 and 2021.

Methods

A descriptive analysis was conducted for all AEs reported to the Food and Drug Administration Adverse Event Reporting System for 4 PDE5Is-avanafil, sildenafil, tadalafil, and vardenafil-indicated for erectile dysfunction between January 2010 and December 2021. The frequency of the most reported AEs and outcomes were identified. A disproportionality analysis based on proportional reporting ratio (PRR) and reporting odds ratio (ROR) was conducted for the most common and clinically important AEs to identify signals to gain insights into potential differences in safety profiles.

Outcomes

The outcome measures of the study are frequency of reported AEs and outcomes following AE.

Results

A total of 29 236 AEs were reported for PDE5Is during the study period. The most reported AE was "drug ineffective" with 7115 reports (24.3%). Eight safety signals were detected across the 4 drugs. Key signals were sexual disorders (PRR, 3.13 [95% CI, 2.69-3.65]; ROR, 3.24 [95% CI, 2.77-3.79]) and death (PRR, 3.17 [2.5-4.01]; ROR, 3.211 [2.52-4.06]) for sildenafil, priapism (PRR, 3.63 [2.11-6.24]; ROR, 3.64 [2.12-6.26]) for tadalafil, and drug administration error (PRR, 2.54 [1.84-3.52]; ROR, 2.6 [1.86-3.63]) for vardenafil. The most reported outcomes were other serious events with 6685 events (67.2%) and hospitalization with 1939 events (19.5%).

Clinical Implications

The commonly reported AEs and detected signals may guide clinicians in treatment decision making for men with erectile dysfunction.

Strengths and Limitations

This is the first comprehensive report and disproportionality analysis on all types of AEs associated with PDE5Is used for erectile dysfunction in the United States. The findings should be interpreted cautiously due to limitations in the Adverse Event Reporting System, which includes self-reports, duplicate and incomplete reports, and biases in reporting and selection. Therefore, establishing a causal relationship between the reported AEs and the use of PDE5Is is uncertain, and the data may be confounded by other medications and indications.

Conclusion

PDE5Is demonstrate significantly increased risks of reporting certain clinically important AEs. While these events are not common, it is imperative to continually monitor PDE5I use at the levels of primary care to national surveillance to ensure safe utilization.

The Efficacy and Safety of Avanafil During a Treatment of Male Erectile Dysfunction: A Systematic Review and Meta-Analysis of Randomized Controlled Trials

Purpose

Erectile dysfunction (ED) contributes to a large burden and impairs the quality of life among males. Avanafil appears to be a promising treatment for ED; however, its efficacy and safety profile remain unclear. This study aimed to evaluate the efficacy and safety of avanafil for the treatment of ED.

Patients and Methods

An extensive search of PubMed, ScienceDirect, Web of Science, and Embase databases with 11 publications was performed, with outcomes evaluated are International Index of Erectile Function - Erectile Function (IIEF-EF), Sexual Encounter Profile (SEP), and Treatment-Emergent Adverse Events (TEAE). Statistical parameter Mean Difference (MD) and Risk Ratio (RR) with 95% Confidence Interval (CI) were used to measure effect size.

Results

The pooled estimates demonstrated that changes in IIEF-EF function (MD=4.39, 95% CI [3.41, 5.37], p<0.001), SEP-2 (RR=3.43, 95% CI [2.79, 4.22], p<0.001), SEP-3 (RR=2.30, 95% CI [2.01, 2.62], p<0.001), and TEAE (RR=1.49, 95% CI [1.12, 1.96], p=0.005) were significantly higher in the avanafil group than in the placebo group. Moreover, 200 mg avanafil was superior to that mg 100 mg-avanafil, indicated by the IIEF-EF score (MD=-1.15, 95% CI [-1.40, -0.89], p<0.001). In contrary, there were no significant differences in SEP-2 (RR=0.90, 95% CI [0.75, 1.08], p=0.26), SEP-3 (RR=0.92, 95% CI [0.81, 1.05], p=0.21) and TEAE (RR=1.00, 95% CI [0.87, 1.15], p=0.99) for both 100 mg and 200 mg doses.

Conclusion

This review highlights the potential use of this drug in ED treatment. Further large-scale Randomized Controlled Trials investigations involving various racial groups are required to confirm these findings.

Canthin-6-Ones: Potential Drugs for Chronic Inflammatory Diseases by Targeting Multiple Inflammatory Mediators

Chronic inflammatory disease (CID) is a category of medical conditions that causes recurrent inflammatory attacks in multiple tissues. The occurrence of CID is related to inappropriate immune responses to normal tissue substances and invading microbes due to many factors, such as defects in the immune system and imbalanced regulation of commensal microbes. Thus, effectively keeping the immune-associated cells and their products in check and inhibiting aberrant activation of the immune system is a key strategy for the management of CID. Canthin-6-ones are a subclass of β-carboline alkaloids isolated from a wide range of species. Several emerging studies based on in vitro and in vivo experiments reveal that canthin-6-ones may have potential therapeutic effects on many inflammatory diseases. However, no study has yet summarized the anti-inflammatory functions and the underlying mechanisms of this class of compounds. This review provides an overview of these studies, focusing on the disease entities and the inflammatory mediators that have been shown to be affected by canthin-6-ones. In particular, the major signaling pathways affected by canthin-6-ones, such as the NLR family pyrin domain containing 3 (NLRP3) inflammasome and the NF-κB signaling pathway, and their roles in several CIDs are discussed. Moreover, we discuss the limitations in studies of canthin-6-ones and provide possible solutions. In addition, a perspective that may suggest possible future research directions is provided. This work may be helpful for further mechanistic studies and possible therapeutic applications of canthin-6-ones in the treatment of CID.

Nitroprusside─Expanding the Potential Use of an Old Drug Using Nanoparticles

For more than 70 years, sodium nitroprusside (SNP) has been used to treat severe hypertension in hospital emergency settings. During this time, a few other clinical uses have also emerged such as in the treatment of acute heart failure as well as improving mitral incompetence and in the intra- and perioperative management during heart surgery. This drug functions by releasing nitric oxide (NO), which modulates several biological processes with many potential therapeutic applications. However, this small molecule has a short lifetime, and it has been administered through the use of NO donor molecules such as SNP. On the other hand, SNP also has some setbacks such as the release of cyanide ions, high water solubility, and very fast NO release kinetics. Currently, there are many drug delivery strategies that can be applied to overcome many of these limitations, providing novel opportunities for the use of old drugs, including SNP. This Perspective describes some nitroprusside properties and highlights new potential therapeutic uses arising from the use of drug delivery systems, mainly silica-based nanoparticles. There is a series of great opportunities to further explore SNP in many medical issues as reviewed, which deserves a closer look by the scientific community.

Erectile Dysfunction: Pharmacological Pathways with Understudied Potentials

Erectile dysfunction (ED) is a public health concern worldwide. In the past, it was perceived as a phenomenon attributed to age advancement. However, more individuals are affected every year that do not fall under that age criterion. Epidemiological research revealed that this abnormality has an association with endothelial dysfunction connected to several cardiovascular (CV) risk factors. Currently, ED is interpreted as a clinical marker for future adverse events and not only as a present health issue that negatively affects the quality of life. The management of ED involves lifestyle modifications, therapeutic optimization for comorbid conditions, and pharmacological and psychosexual therapy. Phosphodiesterase type 5 (PDE5) inhibitors are the first-line pharmacological agents to be prescribed for such a condition. Nonetheless, other pharmacological pathways and agents remain underinvestigated or were investigated at some stage. This review aimed to present to future researchers interested in this field with some pharmacological agents that showed favorable effects on a limited number of studies on human subjects or experimental models.

Treatment of Sexual Dysfunction in Women with Systemic Autoimmune Rheumatic Disorders: A Systematic Review

Introduction

Female sexual dysfunction (SD) is an under-recognized and undertreated problem in patients with systemic autoimmune rheumatic disorders (SARDs).

Objectives

To summarize and evaluate the existing treatment modalities for SD in females with SARDs.

Methods

A systematic review was conducted following the PRISMA guidelines. Electronic databases were searched up to April 2022 for studies that assessed the use of pharmacological and non-pharmacological treatment modalities for the management of SD in females with SARDs. Randomized and observational studies were included. (PROSPERO: CRD42022296381).

Results

Seven studies with 426 females with SD were included. Seven different treatment modalities belonging to 5 different classes (androgen therapy, phosphodiesterase-5 inhibitors, exercise, education and local creams) were evaluated in patients with systemic lupus erythematosus, rheumatoid arthritis and systemic sclerosis. The majority of the studies were of low methodological quality. Standardized patient education and 8-week aerobic walking programs were successful in improving female SD. Local creams improved dyspareunia in females with systemic sclerosis. Testosterone did not significantly improve SD in patients with systemic lupus erythematosus. Accordingly, tadalafil did not result in a significant improvement of SD in females with systemic sclerosis, based on the Female Sexual Function Index.

Conclusion

There is a lack of sufficient evidence to recommend a certain management strategy for SD in females with SARDs. Nonpharmacological therapy and lubricant creams may be beneficial in females with SARDs. No benefit was demonstrated after androgen therapy or tadalafil. Still, no definite conclusions can be drawn due to the important limitations of the available literature. Overall, our results may be considered preliminary and further research in the field is mandatory.

Synchronous Surgical Management of Erectile Dysfunction and Stress Urinary Incontinence: A Systematic Review and Meta-Analysis of Reoperation Rates

INTRODUCTION

The efficacy, safety, and postoperative outcomes of synchronous surgery for concomitant erectile dysfunction (ED) and stress urinary incontinence (SUI) remain unclear.

OBJECTIVES

We performed a systematic review and meta-analysis to evaluate the available synchronous surgical approaches for concomitant ED and SUI and to assess the reoperation rates compared to asynchronous surgery and surgery only for ED or SUI.

METHODS

We searched PubMed, Cochrane Library, and Embase databases until June 2022 for relevant studies. Based on data availability, we performed a meta-analysis of odds ratios (ORs) comparing reoperation rates after synchronous surgery in patients with concomitant ED and SUI versus asynchronous surgery, as well as surgery solely for ED or SUI (PROSPERO: CRD42022326941).

RESULTS

We included 18 studies in the systematic review (16,517 patients) and 5 in the meta-analysis. Comparing synchronous implantation of penile prosthesis and artificial urinary sphincter (AUS) versus asynchronous surgery, no statistically significant differences were observed in the reoperation rates [OR:0.98, 95% confidence interval (CI): 0.52-1.84, I²:0%). Comparing synchronous implantation of both penile prosthesis and AUS versus implantation of only a penile prosthesis or an AUS, combined surgery was associated with higher reoperation rates (OR:2.02, 95%CI: 1.29-3.16, I²:36% and OR:1.7, 95%CI: 1.25-2.32, I²:0%, respectively). Synchronous surgery led to high satisfaction rates and significant improvement in ED and SUI. Evidence for the combination of penile prosthesis with a male sling or the ProACT device is low, but data suggests it may be safe and effective. The synchronous placement of a Mini-Jupette sling and penile prosthesis represents a promising treatment modality for the correction of ED and mild SUI and/or climacturia.

CONCLUSIONS

Synchronous penile prosthesis and AUS implantation appears safe and effective in patients with severe ED and SUI. Further high-quality studies are mandatory to strengthen the current scarce evidence for synchronous surgery in patients with ED and SUI. Pyrgidis N, Barham DW, Hammad M, et al. Synchronous Surgical Management of Erectile Dysfunction and Stress Urinary Incontinence: A Systematic Review and Meta-Analysis of Reoperation Rates. Sex Med Rev 2022;XX:XXX-XXX.

Phosphodiesterase-5 Inhibitory Activity of Canthin-6-One Alkaloids and the Roots of Eurycoma longifolia and Eurycoma harmandiana

Eurycoma longifolia (EL) and Eurycoma harmandiana (EH) are natural medicinal plants belonging to the Simaroubaceae family, and are well-known for their ability to enhance male sexual performance. The present study investigated the phosphodiesterase-5 (PDE-5) inhibitory activity of intact roots of EL and EH. Additionally, canthin-6-one alkaloids, β-carboline alkaloids, and quassinoids were also screened for PDE-5 inhibitory activity. We developed in vitro root and callus cultures of EL and EH to determine their PDE-5 inhibitory activity. Our results indicated that canthin-6-one alkaloids, which include canthin-6-one-9-O-β-D-glucopyranoside, 9-methoxycanthin-6-one, canthin-6-one, and 9-hydroxycanthin-6-one, exhibited PDE-5 enzymatic inhibitory activity, with IC₅₀ values of 2.86±0.23, 3.30±1.03, 4.31±0.52, and 4.66±1.13 μM, respectively. The ethanolic extract of the intact roots of EL and EH, and the in vitro root culture of EH had large amounts of canthin-6-one alkaloids (1.50±0.04, 2.12±0.03, and 3.48±0.08 mg/g dry weight, respectively), and showed potent PDE-5 inhibition. Our findings indicate that in vitro root cultures of EH may be used to replace intact plants, and canthin-6-one-9-O-β-D-glucopyranoside should be further investigated for development as a health supplement.

The Effect of Combination Treatment with Low-Intensity Shockwave Therapy and Tadalafil on Mild and Mild-To-Moderate Erectile Dysfunction: A Double-Blind, Randomized, Placebo-Controlled Clinical Trial

BACKGROUND

Combination of different first-line treatments for erectile dysfunction (ED) has emerged as a promising therapeutic approach.

AIM

To conduct the first double-blind, randomized, placebo-controlled clinical trial to evaluate the efficacy and safety of combination therapy with low-intensity shockwave therapy (LiST) and tadalafil vs LiST and placebo in patients with mild or mild-to-moderate vasculogenic ED.

METHODS

Fifty sexually active patients fulfilling the eligibility criteria were randomly assigned to 6 sessions of LiST twice weekly for 3 weeks and tadalafil (n = 25) or placebo (n = 25) once daily for 4 weeks. Patients were evaluated at 1, 3, and 6 months after completion of the treatment protocol.

OUTCOMES

The primary outcome was the mean change from baseline in the International Index of Erectile Function-Erectile Function (IIEF-EF) domain between the 2 groups at 3 months after treatment. Erectile function was also assessed at 1 and 6 months. The number of patients attaining a minimal clinically important difference (MCID) in the IIEF-EF, as well as the safety of combination therapy were evaluated.

RESULTS

Adjusting for the baseline values, IIEF-EF improved by 0.8 points more (95% confidence interval [CI] = -0.2 to 1.9, P = .12) at 1 month, 1 point more (95% CI = 0.1-1.9, P = .02) at 3 months and 1.7 points more (95% CI = 0.8-2.7, P < .001) at 6 months in patients treated with combination therapy compared to monotherapy. The number of patients attaining a MCID in the IIEF-EF between the 2 groups improved significantly only at the 3-month evaluation. No adverse events were reported during the whole study period.

CLINICAL IMPLICATIONS

Combination of LiST twice weekly for 3 weeks and tadalafil 5 mg once daily for 4 weeks may further ameliorate mild or mild-to-moderate vasculogenic ED compared to LiST monotherapy.

STRENGTHS & LIMITATIONS

We conducted the first randomized trial exploring the role of LiST and tadalafil in the management of ED. Conversely, our study lacks external validity due to its single-center design.

CONCLUSION

The addition of daily low-dose tadalafil during application of LiST may further improve erectile function compared to application of LiST as a standalone treatment in patients with mild or mild-to-moderate vasculogenic ED. Still, further high-quality studies are warranted to corroborate our findings.

Cyclic Nucleotide (cNMP) Analogues: Past, Present and Future

Cyclic nucleotides are important second messengers involved in cellular events, and analogues of this type of molecules are promising drug candidates. Some cyclic nucleotide analogues have become standard tools for the investigation of biochemical and physiological signal transduction pathways, such as the Rp-diastereomers of adenosine and guanosine 3′,5′-cyclic monophosphorothioate, which are competitive inhibitors of cAMP- and cGMP-dependent protein kinases. Next generation analogues exhibit a higher membrane permeability, increased resistance against degradation, and improved target specificity, or are caged or photoactivatable for fast and/or targeted cellular imaging. Novel specific nucleotide analogues activating or inhibiting cyclic nucleotide-dependent ion channels, EPAC/GEF proteins, and bacterial target molecules have been developed, opening new avenues for basic and applied research. This review provides an overview of the current state of the field, what can be expected in the future and some practical considerations for the use of cyclic nucleotide analogues in biological systems.