Separate or Combined Treatments with Daily Sildenafil, Molsidomine, or Muscle‐Derived Stem Cells Prevent Erectile Dysfunction in a Rat Model of Cavernosal Nerve Damage

ABSTRACT

Introduction

Long‐term daily administration of phosphodiesterase type 5 (PDE5) inhibitors in the rat prevents or reverses corporal veno‐occlusive dysfunction (CVOD) and smooth muscle cell (CSMC) loss and fibrosis, in both aging and bilateral cavernosal nerve resection (BCNR) models for erectile dysfunction. In the aging rat model, corporal implantation of skeletal muscle‐derived stem cells (MDSC) reverses CVOD. Nitric oxide (NO) and cyclic guanosine monophosphate can modulate stem cell lineage.

Aim

To investigate in the BCNR model the effects of sildenafil at lower doses, alone or in combination with MDSC or the NO donor molsidomine, on CVOD and the underlying corporal histopathology.

Main Outcomes Measures

CVOD, histological, and biochemical markers in rat corporal tissue.

Methods

Rats subjected to BCNR were maintained for 45 days either untreated, or received sildenafil in the water or retrolingually at 10, 2.5, and 1.25 mg/kg/day (medium, low, and very low doses), or intraperitoneal molsidomine, or MDSC implantation into the corpora cavernosa separately or in combination. Cavernosometry evaluated CVOD. Histopathology was assessed on penile sections by Masson trichrome, immunohistochemistry for α‐smooth muscle actin (ASMA), or immunofluorescence for neuronal nitric oxide synthase (nNOS)/neurofilament 70, and in fresh tissue by Western blot for various markers and picrosirius red for collagen.

Results

All treatments normalized erectile function (drop rate), and most increased the CSMC/collagen ratio and ASMA expression in corporal tissue sections, and reduced collagen content in the penile shaft. MDSC also increased nNOS and brain‐derived neurotrophic factor. The combination treatment was not superior to MDSC or sildenafil given alone, and upregulated PDE5.

Conclusions.  Lowering the dose of a continuous long‐term sildenafil administration still maintained the prevention of CVOD in the BCNR rat previously observed, but it was less effective on the underlying histopathology. As in the aging rat model, MDSC also counteracted CVOD, but supplementation with very low‐dose sildenafil did not improve the outcome. Kovanecz I, Rivera S, Nolazco G, Vernet D, Segura D, Gharib S, Rajfer J, and Gonzalez‐Cadavid NF. Separate or combined treatments with daily sildenafil, molsidomine, or muscle‐derived stem cells, prevent erectile dysfunction in a rat model of cavernosal nerve damage. J Sex Med 2012;9:2814–2826.

Introduction

Erectile dysfunction (ED) is the most prevalent complication of prostate cancer treatment. Post‐radical prostatectomy ED can occur in up to 70% of these patients even in centers of excellence that apply nerve‐sparing techniques 1, 2, 3, severely affecting the quality of life of patients, and posing a heavy burden on healthcare costs 4, 5, 6. The fear of this complication is often a deterrent for the patient to opt for radical prostatectomy even when it is the treatment of choice.

In post‐radical prostatectomy/radiotherapy, and possibly cryotherapy 7, 8, cavernosal nerve damage causes first an acute neurogenic ED by interfering with the nitrergic neurotransmission originated from the brain, mediated by nitric oxide (NO) [9], and then a neuropraxia of the corpora cavernosa leading to fibrosis, apoptosis, and loss of corporal smooth muscle cells (CSMC) [10]. This impairs the compliance of the tissue and its ability to retain blood during erection causing corporal veno‐occlusive dysfunction (CVOD) 10, 11, 12. This is a form of vasculogenic ED that is the prevalent type of ED [13]. The frequent resistance of post‐radical prostatectomy CVOD to oral phosphodiesterase type 5 (PDE5) inhibitors, or intracorporeal injections applied “on demand” to induce erections [14], is because vasodilation by the oral PDE5 inhibitors requires the integrity of both the cavernosal nerve to transmit the sexual stimulus and the CSMC to respond, and local vasodilators are ineffective when the corporal tissue is injured [8].

There is therefore a considerable clinical interest in finding new strategies for penile regeneration and this has spurred experimental studies in animal models of post‐RP, essentially in rats and mice where the cavernosal nerve damage is induced by bilateral cavernosal nerve resection (BCNR) 11, 12, 15, 16, transection (BCNT) [17], or controlled bilateral cavernosal nerve crush (BCNC) [18]. Using BCNR procedures in the rat, it has been demonstrated that long‐term continuous administration of the three PDE5 inhibitors, sildenafil, tadalafil, and vardenafil, partially prevents CVOD by counteracting the underlying corporal histopathology by an antifibrotic mechanism 11, 15, 16, 19. This is different from the vasodilator mechanism that operates in the conventional “on demand” palliative interventions to facilitate penile erection upon sexual stimulation. However, since the daily doses of PDE5 inhibitors in these BCNR studies were, when translated from rats to humans, about 2–3‐fold higher than the usual ones used on demand, and so far only a few clinical studies with nonconclusive results have been performed on this modality with standard doses 14, 20, further experimental studies with lower doses are needed to improve this approach.

In addition to this pharmacological therapy, alternative strategies, such as the use of stem cells, either by themselves or in combination with long‐term continuous administration of PDE5 inhibitors need to be examined. Despite the promising results in several studies with different stem cell types in animal models of ED associated with aging and diabetes 17, 21, 22, 23, 24, 25, 26, 27, only a few reports are available for ED post‐radical prostatectomy models, and they share some limitations [28]. This field was opened in the BCNC model by applying embryonic stem cells [29], which suffer from the fact that their clinical use for non‐life‐threatening conditions is unlikely. These cells failed to be detected at late stages, the correction of ED by electrical field stimulation (EFS) was partial, and no studies on corporal damage or CVOD were performed.

Some of these experimental concerns on stem cell therapy for ED apply to a study using skeletal muscle‐derived stem cells (MDSC) in a BCNT model [17], and, more recently, in a short report on the amelioration of CVOD in the BCNR model, compounded by questions on the “MDSC” population employed [30]. MDSC were effective in correcting an impaired EFS response in aged rats and to convert to smooth muscle cells (SMC) and other differentiated cells in the corpora cavernosa and in the injured vagina 27, 31. To our knowledge, no combination of PDE5 inhibitors and stem cell treatments or with NO donors has been tested for ED in animal models, even if this modality was studied for myocardial infarction with some inconclusive results [32].

In the present work, we have investigated whether sildenafil given for 45 days to BCNR rats at 1/2 through 1/16 the doses previously tested, and the NO donor molsidomine, in combination or not with sildenafil, could prevent CVOD and improve the underlying corporal histopathology, and whether similar or better effects could be obtained with MDSC alone or in the presence of a chronic very low dose of sildenafil.