Role of Nanotechnology in Erectile Dysfunction Treatment

Abstract

Introduction

The biological importance of nanotechnology based delivery vehicles for in vivo tissue regeneration is gaining acceptance by the medical community, however its relevance and incorporation into the treatment of sexual dysfunction is evolving and has not been well evaluated.

Aim

This article aims to provide scientific evidence examining the use of state of the art nanotechnology based delivery methodology in the treatment of erectile dysfunction (ED) in animal models and in patients.

Methods

In this review, we assessed current basic science literature examining the role of nanotechnology based delivery vehicles in the development of potential erectile dysfunction therapies.

Results

There are four primary areas where nanotechnology has been applied for ED treatment; 1) Topical delivery of drugs for on demand erectile function, 2) Injectable gels into the penis to prevent morphology changes post prostatectomy, 3) Hydrogels to promote CN regeneration/neuroprotection, and 4) Encapsulation of drugs to increase erectile function (primarily of PDE5 inhibitors).

Conclusions

Basic science studies provide evidence for a significant and evolving role of nanotechnology in the development of therapies for erectile dysfunction and suggest that properly administered nano based therapies may be potentially advantageous for treating male sexual dysfunction.

Introduction

Nanoparticle based drug delivery is a hot topic for translational therapy development for many diseases, including erectile dysfunction (ED). ED is a common debilitating condition in aging men, affecting 52% of men aged 40–69 (Massachusetts Male Aging Study) [1], and 22% of men under 40 [2]. Risk factors for developing ED are age, coronary artery disease, peripheral vascular disease, smoking, dyslipidemia, diabetes mellitus, and treatment for prostate cancer (including prostatectomy and radiation treatment) [1, 3]. A significant underlying cause of ED development is damage to the cavernous nerve (CN), a parasympathetic, peripheral nerve, which provides innervation to the penis. Cavernous nerve injury, which occurs with prostatectomy, diabetes and aging, results in down stream morphological remodeling of the penile corpora cavernosa, including smooth muscle apoptosis and ensuing fibrosis, which make the erectile tissue less able to respond to normal signaling mechanisms and standard of care treatments such as oral therapy with phosphodiesterase type 5 inhibitors (PDE5i). State of the art therapies, so far limited primarily to animal models, are designed to target both CN regeneration and prevention of penile morphological changes. This includes targeted and extended delivery of growth factors (involved in maintaining nerve architecture and penile smooth muscle) to the penis and CN, as well as transdermal delivery of PDE5i for “on demand” erectile function, both through increasing nitric oxide (NO) and temporary elevation of PDE5i.

The requirements of penile and CN delivery have some similarities as well as distinct challenges and requirements. For both, extended release of growth factors, with a biodegradable delivery vehicle that does not cause an immune response, are needed. For the CN, vehicles that provide a surface for regenerating axons to grow against, acting as guidance factors, are desirable to promote regeneration of axons. In animal models, flexible linear hydrogels are ideal for this purpose. While in patients the anatomy of the CN differs somewhat from that of rodent models, forming more of a neural net than one simple linear nerve structure (rat), however the same principle applies for regenerating neurons. For corpora cavernosal delivery, an additional shear stress is encountered with blood flow through the sinusoidal spaces and a delivery vehicle cannot impede blood flow or it will impair function. Self-assembling hydrogels, which can form in vivo, as a coating lining the sinusoidal spaces, is optimal. These potential therapies target long term ED development by suppressing architectural changes in the CN and penis. Another option is transdermal delivery of either nitric oxide (NO) or PDE5i, which can be useful for “on demand” erectile function. This type of topical delivery to the penis would avoid systemic side effects, which result from oral PDE5i use, and could avoid pain and anxiety of injection of other vasoactive drugs. Barriers to this type of delivery are the permeability of skin and tunica, prevention of skin changes in response to delivery vehicle and drug delivered, and potential toxicity to partners. In addition, similar target delivery vehicle requirements are needed as in CN and corpora cavernosal delivery (biodegradable, somewhat extended release, no immune response).

In this review, state of the art nanotechnology based delivery vehicles designed for future translation to ED patients, and which are also useful for ED research in animal models, are discussed. Nanotechnology is defined as studies conducted at the nanoscale (between 1 to 100 nanometers) and involves manipulation of matter on the atomic, molecular and supramolecular scale. These potential therapies are divided into four methods of delivery, which are: 1.) Topical delivery of drugs for on demand erectile function, 2.) Injectable self-assembling hydrogels for extended growth factor release to the corpora cavernosa of the penis, 3.) Application of hydrogels for extended release of proteins to regenerate the CN, and 4.) Encapsulation of drugs for oral delivery, to increase erectile function. Recent publications in each area and their potential benefits and pitfalls will be discussed.

Results

1. Topical delivery of drugs for on demand erectile function

The aim of this study by Han et al., [4] was to determine if nanoparticles that encapsulate known erectogenic agents, such as tadalafil, sialorphin and NO, improve erectile function with transdermal delivery, thus avoiding systemic side effects. Many currently used erectogenic agents such as PDE5i have significant adverse side effects including headache, facial flushing, nasal congestion and dyspepsia. The transdermal delivery system employed a nanoparticle based hybrid hydrogel/glass composed of PEG, TMOS, chitosan, sodium nitrite, and glucose, which were delivered as a suspension in carboxymethylcellulose [4]. When hydrated, the suspension releases the encapsulated material and was applied as a gel to the glans and penile shaft of a rodent Sprague Dawley aging rat model of ED. Results showed that spontaneous erections were visible 4.5 minutes after gel application of NO and after 9 minutes with sialorphin. NO generating erection lasted 1.42 minutes and sialorphin generating erection lasted 8 minutes. Tadalafil releasing nanoparticles resulted in erection 1 hour after application [4]. The study concluded that nanoparticles encapsulating erectogenic agents resulted in spontaneous erection when applied to the penis and could be useful for drug delivery to promote erection. This type of “on demand” erection may have potential application for aging ED patients. Several interesting questions arose from this study including the duration in which the drug delivery was effective in eliciting an erection, the in vivo penetration and release kinetics of the nanomaterials, and if the nanomaterial would have an adverse effect on sexual partner physiology and function.

In this second topical drug delivery study, the authors answer some of the questions that arose in their previous transdermal delivery of erectogenic agents in an aging rat ED model [5]. In this work they examined if topically applied NO-releasing nanoparticles induce erection in a cavernous nerve (CN) resection model, which mimics post prostatectomy erectile function. They hypothesize that the NO-releasing nanoparticles are effective for “on demand” erection by increasing penile blood flow. The nanoparticle utilized for this study includes both PEG and chitosan as additives to a basic tetramethylorthosilicate (TMOS) recipe for sol-gel preparation which is referred to as a hydrogel/glass composite [6]. NO is spontaneously generated through the reduction of nitrite to NO, which is facilitated by the hydrogen bonding network provided by the nanoparticle platform [5]. It was effective with respect to NO formation, retention and slow-sustained release and was applied with coconut oil or hyaluronic acid as a carrier one week after bilateral cavernous nerve resection. Control rats received empty vehicle. In response to NO treatment, 6 of 10 rats exhibited spontaneous erections of ~one minute duration. The onset of spontaneous erections was from 5–37 minutes after application and occurred for at least 45 minutes [5]. Similar results were obtained for application of NO nanoparticles delivered in a coconut oil base rather than DMSO gel. No spontaneous erections were observed after application with empty nanoparticle vehicle. Release kinetics show that the nanoparticles release NO within a few minutes and release occurs in a continuous manner for ~5 hours, and was attenuated by 7 hours. Microcirculatory blood flow, measured in a hamster model, significantly increased with NO treatment, and was sustained over 90 minutes [5]. This study concluded that NO delivered by the nanoparticle platform could be potentially useful for penile rehabilitation to facilitate oxygenation post prostatectomy. Whether nerve injury induced corpora cavernosal changes are suppressed and penile architecture maintained/rescued in the post prostatectomy model, remains to be evaluated.

The next study which examined topical delivery of drugs for on demand erectile function was by Park et al [7], which aimed to determine if alcoholic hydrogels containing prostaglandin E1 ethyl ester (prodrug for PGE1) improve erectile function in a cat model. The goal of the study was to avoid the inconvenience associated with injection. The type of nanoparticle employed was an alcoholic hydrogel, which was applied as a gel to the penis. The encapsulated material, prostaglandin E1 ethyl ester, was released with hydration of the hydrogel. The prodrug is stable in aqueous solution but is hydrolyzed in skin homogenates within 4 hours [7]. They examined skin penetration kinetics, which exhibited a release rate of 7.6 and 1.8 μg/cm²/hr (PGE1-EE and PGE). Several penetration enhancers were examined, with limonene and cineole being the most useful, enhancing the flux four fold over unenhanced hydrogel. Pharmacodynamic studies in a cat model showed increased intracavernosal pressure (ICP) with prostaglandin E1 ethyl ester treatment [7]. It was concluded that this alcoholic hydrogel carrier might be a beneficial alternative to injection. Questions remain regarding the duration of effect and potential adverse effects on patient partners.

Several studies examined transdermal delivery of PDE5i to the penis to avoid potential systemic side effects when given orally. There are two types of liposomes commonly used for transdermal drug delivery. These are ethosomes and transferosomes. In this study by Fahmy et al., [8], ethosomes, which are lipid vesicles which incorporate high levels of ethanol to deliver drugs across the skin layers, were examined in vitro for their usefulness in delivering the PDE5i vardenafil, to rat skin, for application to improve erectile function through penile application. Vardenafil is a more potent and selective PDE5i than sildenafil, however it has low oral bioavailability because of low solubility and extensive metabolism in vivo. Ethosomes are hypothesized to be more useful for transdermal delivery than conventional lipid vesicles since they penetrate deeper into the skin. The diffusion of vardenafil from prepared nanoethosomes was examined in vitro using an automated Franz diffusion cell apparatus. The optimized formula was investigated in vitro for delivery to rat skin in vivo using confocal laser scanning microscopy images, which confirmed enhanced diffusion release of vardenafil in rat skin. Results showed that the optimized formula produced nanoethosomes with an average size of 128nm and an entrapment efficiency of 76.23%. Vardenafil nanoethosomes showed a significant improvement in permeation [8]. Transdermal bioavailability of vardenafil from the nanoethosome film was approximately twofold higher than the oral bioavailability from an aqueous suspension [8]. It was concluded that ethosome delivery of vardenafil is a promising transdermal drug delivery system, which could result in management of ED for a longer period with reduced dosage. The next step would be to examine the proposed nanoparticles in vivo, to determine how well they enhance erectile function under real life conditions in an ED model.

Transferosomes are flexible and ultradeformable vesicular systems composed of phospholipids and a single-chain surfactant that acts to destabilize the lipid bilayer, thus improving penetration through the skin. In a study by Badr-Eldin et al [9], optimized sildenafil citrate nano-transfersomal transdermal films with enhanced and controlled permeation were formulated, with the objective to alleviate skin penetration challenges experienced by ethosomes, which can be restricted to outer skin layers. The permeability of sildenafil citrate containing transferosomes was examined in vitro using Wistar rat skin in a Franz diffusion cell apparatus [9]. The transferosomes were unilamellar and spherical in shape with vesicular size of 130 nm and exhibited enhanced skin permeation, plasma bioavailability and extended absorption of sildenafil citrate [9]. While in vivo animal studies have not yet been performed, it was concluded that optimized sildenafil citrate nano-transfersomal films maybe beneficial in reducing dose and administration frequency.

More conventional lipid carriers can also be used to enhance drug solubility and bioavailability of PDE5i such as the solid lipid nanoparticles used for Avanafil delivery in a rat skin in vitro model, which is similar to that used for ethosome and transferosome delivery of PDE5i. The goal of the study by Kurakula et al., [10] was to formulate and optimize nanoparticles with subsequent loading into hydrogel films for Avanafil transdermal delivery. The Avanafil nanoparticles were tested in vitro in rat skin using Franz diffusion cells [10]. The nanoparticles were spherical in shape and the optimized nanoparticles showed particle size and entrapment efficiency of 86nm and 85.01%. Nanoparticles with Avanafil were able to penetrate into deeper skin layers [10]. The study concluded that transdermal films loaded with Avanafil nanoparticles might be a useful alternative to oral PDE5i administration. In vivo Avanafil nanoparticle delivery was not tested in this study.

In this study by Elnaggar et al., [11], the authors aimed to determine if sildenafil citrate loaded nanostructured lipid carriers (NLCs) and solid lipid nanoparticles (SLNs) can deliver sildenafil under conditions of in vitro transdermal application to human skin in a Franz diffusion cell assembly. Nanoparticle optimization including size, entrapment efficiency and in vitro release were examined [11]. Results showed that SLNs and NLCs were optimized in the 180 and 100 nm range with good entrapment efficiency. Both SLNs and NLCs had enhanced in vitro release and transdermal permeation of sildenafil citrate. Release kinetics showed higher initial release for faster onset followed by slower controlled release for longer duration [11]. It was concluded that the lipid nanoparticles might be beneficial for local sildenafil citrate release through the penile skin, and thus avoid the side effects, which frequently occur with systemic administration. The next question to address is if the lipid carriers can function as well in vivo.

The last transdermal delivery study to discuss is by Ali et al [12] in which the goal was to formulate topical nanocarriers of the low-cost vasodilator, papaverine hydrochloride, as an alternative to painful penile injections. Intracavernosal injections and intraurethral therapy are considered second-line therapies because of high dropout rates, pain on the injection site, prolonged erections and priapism [13]. Up to 50% of men who begin intracavernous therapy discontinue treatment because of discomfort and lack of spontaneity. Penile skin and the tunica act as a physical barrier to transdermal delivery of drugs, resulting in reduced efficacy. Transdermal nano-transferosome was used as a nanocarrier to enhance the penetration of papaverine to the penis. For the methods, several formulations were prepared and characterized for their encapsulation efficiency, particle size, and cumulative drug release [12]. The best formula was incorporated into 2% (w/v) hydroxypropyl methylellulose hydrogel base. The gel containing transferosomal papaverine hydrochloride and that containing free papaverine, were clinically compared using color flow Doppler measurements in 9 men with ED. Results show an entrapment efficiency of 72%, and low particle size of 220nm. Release kinetics show 73% drug release within 2 hours. Clinical evaluation showed increase in cavernous artery diameter from .53mm to .78 mm and the increase in the peak systolic flow velocity from 5.95 cm/second to 12.2cm/second [12]. 3 of 9 patients had grade 3 erections while 1 patient had grade 4 erections. Five patients had no response [12]. It was concluded that transferosomes maybe useful in some ED patients as a carrier of papaverine hydrochloride.

2. Injectable self-assembling hydrogels for extended release, intracavernosal delivery to the penis

The second method in which nanoparticles are being developed as potential therapies for ED, and as useful delivery vehicles for ED research, are injectable hydrogels that form in vivo within the corpora cavernosa and can deliver proteins for extended periods. Many self-assembly strategies utilize micellar or vesicular objects in aqueous suspension, which can then be injected into the penis. In this study by Bond et al [14], a unique family of materials called peptide amphiphiles (PA) were utilized, which self-assemble into high-aspect-ratio nanofibers. At appropriate concentrations, these nanofibers entangle to form macroscopic hydrogels. The benefit of this methodology for protein delivery in vivo is that the unassembled PAs can be injected with the protein of interest and a trigger (change in pH or ion concentration) to cause in vivo gelation, as a thin gel layer lining the sinusoidal spaces of the corpora cavernosa. The hydrogel assembled is stable, has complementary physical properties to the surrounding soft tissue [15–18], is non-invasive, biodegradable and does not cause a measurable immune response. Proteins are protected within the gel matrix and modification is not needed for delivery [15, 19–20].

Optimization of PA volume, injection methodology into the penis, release kinetics of the protein from the PA, duration of release and potential immune response were evaluated. An in vitro release assay showed that ~80% of the SHH protein was released from the PA by 6 days [14]. Efficacy of Sonic hedgehog (SHH) protein delivery to the corpora cavernosa (CN) of the penis by PA was examined in a Sprague Dawley rat ED model in which the cavernous nerve was resected. The apoptotic index after CN resection was examined at several points after CN injury. SHH delivered by PA decreased the apoptotic index and increased SHH protein in the corpora cavernosa in a time dependent manner as the PA degraded. SHH protein was labeled with a fluorescent dye and when delivered by PA showed incorporation into corpora cavernosal smooth muscle [14]. These studies concluded that PA technology was effective in delivering SHH protein in vivo to the penis and in suppressing the apoptotic index after CN resection. In a follow up study utilizing the same PA methodology, SHH was shown to also suppress collagen induction in response to CN injury [21]. Thus SHH delivered by PA has translational potential to suppress penile remodeling in response to CN injury (prostatectomy model).

There are several other types of hydrogels that have been utilized in the study of ED in animal models. In this study by Bae et al., [22] basic fibroblast growth factor (bFGF) was delivered via a nanoparticle-based hydrogel injected subcutaneously in a rat ED model, at the same time as adipose derived stem cells into the corproa cavernosa of the penis. The aim of the study was to determine if bFGF and stem cell treatment improved erectile function in a CN compression/crush (clamp for 2 minutes duration) model. The nanoparticle used for this study was PEG-tyramine hydrogel (gelatin-poly (ethylene glycol)- tryamine hydrogel [22]. Release kinetics performed under non-physiological conditions, showed release of bFGF for ~7 days, however most of the protein was released within 2.5 days. Results show significant improvement in intracavernosal pressure (ICP) with bFGF or stem cell therapy alone when evaluated at 4 weeks after CN injury and further improvement with combined therapy [22]. It was concluded that bFGF hydrogel improved erectile function almost as much as stem cell treatment. Some questions arose with this study. The rats utilized for experimentation were 70 days old at the beginning of the study. At this age the penis continues to grow and add collagen until adulthood is reached (P115–120) [21]. It would be interesting to examine if findings are similar in an adult animal, which is not actively undergoing growth of the penile tissue. It is also of interest that bFGF hydrogel was injected subcutaneously and yet had an effect on corpora cavernosal function. It is unclear if bFGF was able to enter the corpora cavernosa or if it was taken up by nerve endings to undergo retrograde transport to the CN to promote regeneration.

An additional component to some hydrogel delivery vehicles is the incorporation of magnetic nanoparticles, which allow for limited movement of nanoparticles in vivo with application of external magnets. This is potentially advantageous for localization of compounds or stem cells to a specific region. The goal of this study by Lin et al [23] was to investigate if NanoShuttle magnetic nanoparticle delivery of stem cells injected into the corpora cavernosa, could maintain stem cells longer in the corpora cavernosa, so as to further improve erectile dysfunction in a rat CN crush model, over conventional stem cell delivery. The significance of localizing stem cells is that most stem cells, when injected, travel to the bone marrow and other places in the body (cavernous nerve), thus exerting paracrine effects rather than direct effects on the corporal tissue. For detailed discussion of stem cell in rat models and potential paracrine effects, see Albersen et al 2016 [23]. NanoShuttle is a biocompatible magnetic nano-particle assembly (approximately 50nm) consisting of gold nanoparticles, iron oxide and poly-L-lysine. NanoShuttle magnetizes cells by electrostatically and nonspecifically attaching to cell membranes via poly-L-lysine (approximately 50pg per cell) with no effect on cell proliferation or viability [24]. For the methods, adipose derived stem cells were magnetized with NanoShuttle magnetic nanoparticles to create Nano-adipose derived stem cells and were injected into the corpora cavernosa with and without external magnet placement on the rat body for 6 hours, after bilateral CN crush (ultrafine hemostat for 30 seconds) in a Sprague Dawley rat ED model. After 4 weeks the ICP was evaluated [24]. Results show that NanoShuttle magnetic nanoparticles were successfully bound to adipose derived stem cells and they migrated toward the magnet. In vivo Nano-adipose derived stem cells were retained in the corpora cavernosa using the magnet for up to 3 days but remained only 1 day without the magnet. ICP, α-actin and PECAM-1 were higher in the Nano-adipose derived stem cell with magnet treatment group [24]. It was concluded that the use of magnetic nanoparticles retained more stem cells in the corpora cavernosal tissue, where it exerts a positive effect on erectile function after CN injury. Of significance in this study is that the rats in which experiments were performed were only 56 days old. At this time the rat is still undergoing puberty, is not yet capable of erectile function (63 days old), and is undergoing significant growth with active developmental pathways. It is unclear if the methodology would be as effective, and yield similar results, if the study was performed in an adult rat (P115–120), where the corpora cavernosal tissue was not actively growing.

In a similar study by Zhu et al., [25], adipose tissue-derived stem cells labeled with superparamagnetic iron oxide nanoparticles (SPIONs) were utilized in a streptozotocin-induced diabetic rat ED model with application of an external magnet to localize nanoparticles injected into the penis. For the methods, 70 day old rats were injected with streptozotocin and rats with glucose greater than 300mg/dl were used for the study. Rats were injected with the SPIONS and with external placement of a magnet over the injection site for 30 minutes. Four weeks after SPION labeled stem cell treatment, ICP was measured [25]. Results showed that SPIONs incorporated into adipose derived stem cells did not appear to exert negative effects on their function. In rats treated with stem cells without SPIONS, erectile function was improved 30% while incorporation of SPIONS improved erectile function 42% (12% difference) [25]. It was concluded that magnetic targeting of the stem cells contributed to longer retention in the corpora cavernosa and had positive effects on erectile function. It is unclear if diabetes induced in a developing rat is comparable to morphological changes that occur when diabetes is induced in an adult rat.

3. Application of hydrogels for extended release of proteins to regenerate the CN

The third method in which nanoparticles have been incorporated into ED research and for the development of ED therapies, involves delivery of growth factors to the CN in an extended release manner to preserve CN function and promote regeneration, in CN injury models that mimic injury incurred during prostatectomy.

In this study by Angeloni et al [26], the authors examine whether SHH protein delivered by linear peptide amphiphile technology, can promote CN regeneration, improve erectile function and prevent penile remodeling in a Sprague Dawley rat CN crush (30 second crush with forceps) model. The nanoparticle utilized for these studies are monodomain gels containing aligned peptide amphiphile (PA) nanofibers [27]. The PA used for these studies is similar to those in Bond et al., 2010, however the PA is able to achieve and retain a three dimensional structure that is linear, with the protein intercalated between the nanofibers as the gel forms. This allows for placement of the monodomain gels on top of the CN after crush injury, to release SHH protein in an extended release manner. The advantage of this type of delivery is that it is biodegradable, provides directional guidance to regenerating axons and can deliver proteins over extended periods. The release kinetics of SHH protein from the monodomain gels shows a 90% release of protein from the gel by 75 hours (3–4 days) [26]. Results show a 60% improvement in ICP 6 weeks after CN crush injury with significantly improved CN architecture [26]. A later manuscript by this group shows reduced apoptotic index in the penis with SHH delivery by monodomain gel to the CN at the time of crush injury [28]. SHH protein delivered by monodomain gel is also neuroprotective, resulting in less CN injury and penile apoptosis [28]. It was concluded that SHH protein delivery by monodomain gel containing PA nanofibers promotes CN regeneration, improves erectile function, and decreases smooth muscle apoptosis.

In this study by Kim et al., [29] a hyaluronic acid based hydrogel was used to deliver nerve growth factor (NGF) to the CN at the time of crush injury (30 second), in conjunction with human adipose-derived stem cell dripped onto the CN crush site of a rat model. Four weeks after injury the erectile function was measured by ICP [28]. Hyaluronic acid-poly(ethylene oxide) hydrogel is biocompatible, biodegradable, and allows for extended release of bioactive molecules [30–32]. Approximately half of the NGF that was released from the hydrogel (10% of protein loaded) occurred in the first 3.5 days. By 35 days 20% of the loaded NGF was released [29]. Results showed a 76% decrease in erectile function with CN injury. This improved 53% with stem cell treatment, 40% with NGF-hydrogel treatment, and 65% with combined treatment [29]. It was concluded that combined NGF hydrogel treatment, along with stem cell therapy enhances post CN injury erectile function. CN architecture was not assessed with treatment to determine the mechanism of erectile function improvement.

4. Encapsulation of drugs for oral delivery, to increase erectile function

The last method in which nanoparticles have been incorporated in ED research and development of treatment modalities, is by encapsulation of drugs that improve erectile function, for oral delivery. In this study by Hosny et al., [33], the authors prepare sildenafil citrate for delivery from solid lipid nanoparticles in an effort to avoid the poor solubility and extensive first-pass metabolism of sildenafil, which can result in low (40%) bioavailability and short elimination half-life (4 hr). Solid lipid nanoparticles were prepared, and in vitro drug release, stability and in vivo pharmacokinetics were studied in a rabbit model [33]. Results show the average particle size was 28.5 nm with 95.34% entrapment efficiency. Release kinetics showed a controlled drug release over 24hr. Bioavailability of sildenafil citrate was enhanced by >1.87 fold, and the mean residence time was longer than that for the commercially available tablet [33]. In was concluded that solid lipid nanoparticles may be a promising carrier for sustained/prolonged sildenafil citrate release with enhanced oral bioavailability.

Discussion/Conclusions

Are we ready for clinical trial?

In this study we reviewed state of the art nanotechnology based vehicles for delivery of proteins and stem cells to the penis and CN in order to improve erectile function, and “on demand” erectile function requirements by providing novel delivery of PDE5i and vasodilators. These vehicles have significant potential for development as translational therapies for ED patients, and will move ED research forward as the medications, growth factors, stem cells, and signaling molecules that were studied have been established as promising agents to improve erectile function. Nanotechnology based vehicles offer unique design and usefulness for drug delivery and can enhance growth factor effects by providing a scaffold with improved release, penetration, bioavailability, and localized, sustained delivery. So far in vivo and in vitro studies in animal models are promising and demonstrate significant and measurable improvements in erection with no measurable immune response. Future application and translation require toxicity and pharmacology studies to ensure no adverse reaction with degradation and break down of the nano-materials, in vivo in small and large animal models. To date little is known about the toxicity of the nano-materials under investigation for ED research. The studies presented in this review are state-of-the-art with regard to delivery of biological materials, however the status of the field has not progressed to pre-clinical toxicity evaluation. This is an important next step for many of the nano-materials presented. The necessity of FDA approval will be the most challenging barrier to clinical trial and application. FDA approval is a lengthy and costly process and most scientists in the field have little mentoring about how to make this transition.

Table 1.

Publications and nanomaterials used for each study

1.) Topical delivery of drugs for on demand erectile function	2.) Injectable self-assembling hydrogels for extended growth factor release to the corpora cavernosa of the penis	3.) Application of hydrogels for extended release of proteins to regenerate the CN	4.) Encapsulation of drugs for oral delivery, to increase erectile function
Han et al., [4]: hybrid hydrogel/glass composed of PEG, TMOS, chitosan, sodium nitrite, and glucose, which were delivered as a suspension in carboxymethylcellulose	Bond et al [14]: peptide amphiphiles (PA) were utilized, which self-assemble into high-aspect-ratio nanofibers. At appropriate concentrations, these nanofibers entangle to form macroscopic hydrogels	Angeloni et al [26]: monodomain gels containing aligned peptide amphiphile (PA) nanofibers [27]	Hosny et al., [33]: Solid lipid nanoparticles
Friedmen et al., [6]: PEG and chitosan as additives to a basic tetramethylorthosilicate (TMOS) recipe for sol-gel preparation which is referred to as a hydrogel/glass composite	Bae et al., [22]: nanoparticle-based hydrogel injected subcutaneously	Kim et al., [29]: hyaluronic acid based hydrogel
Park et al [7]: alcoholic hydrogels containing prostaglandin E1 ethyl ester (prodrug for PGE1	Lin et al [23]: NanoShuttle magnetic nanoparticle delivery of stem cells
Fahmy et al., [8]: ethosomes, which are lipid vesicles which incorporate high levels of ethanol to deliver drugs across the skin layers	Zhu et al., [25]: adipose tissue-derived stem cells labeled with superparamagnetic iron oxide nanoparticles (SPIONs)
Badr-Eldin et al [9]: transferosomes are flexible and ultradeformable vesicular systems composed of phospholipids and a single-chain surfactant that acts to destabilize the lipid bilayer, thus improving penetration through the skin
Ali et al [12]: transdermal nano-transferosome was used as a nanocarrier to enhance the penetration of papaverine

Summary Sentence.

Nanotechnology can be a valuable tool to promote cavernous nerve regeneration and improve erectile dysfunction in prostatectomy and diabetic animal models.