To date, studies suggest that biological signaling by nitric oxide (NO) is primarily mediated by cGMP, which is synthesized by NO-activated guanylyl cyclases and broken down by cyclic nucleotide phosphodiesterases (PDEs). Effects of cGMP occur through three main groups of cellular targets: cGMP-dependent protein kinases (PKGs), cGMP-gated cation channels, and PDEs. cGMP binding activates PKG, which phosphorylates serines and threonines on many cellular proteins, frequently resulting in changes in activity or function, subcellular localization, or regulatory features. The proteins that are so modified by PKG commonly regulate calcium homeostasis, calcium sensitivity of cellular proteins, platelet activation and adhesion, smooth muscle contraction, cardiac function, gene expression, feedback of the NO-signaling pathway, and other processes. Current therapies that have successfully targeted the NO-signaling pathway include nitrovasodilators (nitroglycerin), PDE5 inhibitors [sildenafil (Viagra and Revatio), vardenafil (Levitra), and tadalafil (Cialis and Adcirca)] for treatment of a number of vascular diseases including angina pectoris, erectile dysfunction, and pulmonary hypertension; the PDE3 inhibitors [cilostazol (Pletal) and milrinone (Primacor)] are used for treatment of intermittent claudication and acute heart failure, respectively. Potential for use of these medications in the treatment of other maladies continues to emerge.

cGMP-Dependent Protein Kinases and cGMP Phosphodiesterases in Nitric Oxide and cGMP Action

Erectile Dysfunction: AUA Guideline.

This review assesses the efficacy and harms of oral PDE-5 inhibitors and hormonal treatments for ED and the effect of measuring serum hormone levels on treatment outcomes for ED. Data, primarily fr...

Oral phosphodiesterase-5 inhibitors and hormonal treatments for erectile dysfunction: a systematic review and meta-analysis.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1743-6109.2009.01627.x

Pharmacotherapy for erectile dysfunction.

Objectives To determine the incidence and characteristics of randomised controlled trials that report using the modified intention to treat approach, and how the approach is described.

Design Systematic review.

Data sources PubMed, Embase, Cochrane central register of controlled trials, ISI Web of Knowledge, Ovid, HighWire Press, Science-Direct, Ingenta, Medscape, BioMed Central, Springer, and Wiley, from inception to December 2006.

Main outcome measures Incidence of trials in which use of modified intention to treat was reported, and how the approach was described (classified according to the type and number of deviations from the intention to treat approach).

Results 475 randomised controlled trials reported use of a modified intention to treat analysis. Of these, 76 (16%) were published in five highly cited general medical journals. The incidence of all trials that reported use of modified intention to treat published in journals indexed in Medline increased from 0.006% in 1982-6 to 0.5% in 2002-6 (P<0.001 for linear trend). When the description of the modified intention to treat was examined in each trial, 192 (40%) reported one type of deviation from the intention to treat approach, 261 (55%) reported two or more types, and 22 (5%) did not describe any type. In 266 (56%) of the trials the deviation was related to the treatment received, in 196 (41%) to a post baseline assessment, in 118 (25%) to a baseline assessment, in 108 (23%) to a target condition, and in 23 (5%) to follow-up. Post-randomisation exclusions occurred in 380 (80%) trials. The results reported by 270 of the 352 (77%) superiority trials favoured the drug under investigation. All of the 123 trials using equivalence or non-inferiority methods to investigate interventions reported results that favoured their assumptions.

Conclusions Randomised controlled trials that report using a modified intention to treat are increasingly being published in the medical literature. The descriptions of such an approach were ambiguous, and may cover any type of descriptions for exclusion, such as missing data and deviation from protocol. Explicit statements about post-randomisation exclusions should replace the ambiguous terminology of modified intention to treat.

https://www.bmj.com/content/bmj/340/bmj.c2697.full.pdf

Modified intention to treat reporting in randomised controlled trials: systematic review.

In a previous study assessing tadalafil for the treatment of erectile dysfunction (ED), tadalafil 20 mg was shown to improve erectile function for up to 36 hours vs placebo. This study sought to demonstrate the effectiveness of both 10- and 20-mg tadalafil vs placebo at 2 prespecified assigned times of 24 and 36 hours postdosing. This double-blind, placebo-controlled, parallel-group study randomized 483 men with ED into 6 groups according to a combination of treatment (placebo, tadalafil 10 or 20 mg) and assigned time (24 or 36 hours) for intercourse attempts. Patients were stratified by baseline ED severity based on Erectile Function Domain scores. The study had 4 phases: a 4-week run-in (no ED medication taken); a 2- to 4-week equilibration (dosing as needed); a 4- to 6-week assessment; and a 6-month open-label extension. During the assessment phase, men took a total of 4 doses of study medication, each dose separated by more than or equal to 7 days. Efficacy was measured as the mean per-patient percentage of successful intercourse attempts (Sexual Encounter Profile Diary Question 3: SEP3) during the assessment phase. Men taking either 10- or 20-mg tadalafil had a significant increase in SEP3 from baseline scores vs placebo at both 24 hours (P = .038 and <.001 for 10 and 20 mg, respectively) and 36 hours (P < .001 for both doses) postdose. The mean per-patient percentages of successful intercourse attempts for the 24-hour time point were 41.8%, 55.8%, and 67.3% for placebo and tadalafil 10 and 20 mg, respectively; for the 36-hour time point, the mean per-patient percentages were 32.8%, 56.2%, and 61.9% for placebo and tadalafil 10 and 20 mg, respectively. The most common treatment-emergent adverse events were headache, back pain, dyspepsia, and nasopharyngitis. Both 10- and 20-mg tadalafil improved erectile function for up to 36 hours postdosing in men with ED of varied severity.

https://onlinelibrary.wiley.com/doi/pdf/10.2164/jandrol.04126

Tadalafil improved erectile function at twenty-four and thirty-six hours after dosing in men with erectile dysfunction: US trial.

Because sildenafil citrate is a treatment, not a cure, for erectile dysfunction (ED), many men may choose to use it for an extended period. Men with ED who had previously completed 1 of 4 double-blind trials with short-term open-label extension (combined duration, 0.9-1.2 years) were eligible for this 4-year, open-label, extension study, which assessed the safety and effectiveness of flexible doses (25, 50, and 100 mg sildenafil) used as needed. Adverse events that were serious or led to dosing changes or discontinuation (temporary or permanent) were recorded. Many of the 979 participants (mean age, 58 [range, 27-82] years; mean ED duration, 4.5 years) had concomitant hypertension (28%), diabetes (22%), or hyperlipidemia (14%). Overall, 37 (3.8%) had treatment-related adverse events (none serious) requiring dosage change or discontinuation and 62 (6.3%) discontinued because of insufficient response. At each yearly assessment, more than 94% of participants responded affirmatively to the questions: "Are you satisfied with the effect of treatment on your erections?" and "If yes, has treatment improved your ability to engage in sexual activity?" These results argue against the loss of tolerability or the development of tachyphylaxis over a prolonged period of as needed, flexible-dose sildenafil treatment of men with ED.

Long-term safety and effectiveness of sildenafil citrate in men with erectile dysfunction

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Tadalafil improved erectile function at twenty-four and thirty-six hours after dosing in men with erectile dysfunction: US trial.

Long-term safety and effectiveness of sildenafil citrate in men with erectile dysfunction