Effects of sildenafil (ViagraTM) administration on seminal parameters and post-ejaculatory refractory time in normal males*

Antonio Aversa1,3, Fernando Mazzilli1, Tiziana Rossi1, Michele Delfino1, Andrea M. Isidori1 and Andrea Fabbri1,2

1 Cattedra di Andrologia, Dipartimento di Fisiopatologia Medica, University of Rome La Sapienza, Italy and 2 Department of Endocrinology, St. Bartholomew's Hospital, London, UK


    Abstract
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 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
Sildenafil is a specific inhibitor of phosphodiesterase (PDE) type 5 and represents a powerful therapy for male erectile dysfunction (ED) of different aetiology. Recently, sildenafil has been shown to restore erections in temporary ED related to the need of semen collection for assisted reproductive techniques. In this study, we investigated whether sildenafil administration modifies seminal parameters and/or erectile function in normal healthy volunteers. In a double-blind, randomized, placebo-controlled, cross-over two period investigation we enrolled 20 healthy male volunteers (mean ± SE age 32 ± 0.5 years). Subjects were not using any medication for the 3 month period prior to the study and were engaged in a stable relationship with proven fertility. The effects of sildenafil (100 mg) on seminal parameters and erectile function after audiovisual sexual stimulation were evaluated by semen analysis and by colour-Duplex ultrasound (the Resistive Index) respectively. In all subjects, sildenafil caused no changes in seminal and erection parameters when compared to placebo. Interestingly, sildenafil administration led to a marked reduction of the post-ejaculatory refractory time (10.8 ± 0.9 min versus 2.6 ± 0.7 min for placebo and sildenafil respectively; P < 0.0001). These results indicate that in normal subjects acute sildenafil treatment does not modify semen characteristics and has a positive influence over the resumption of erections following ejaculation in the presence of a continuous erotic stimulus.

Key words: ejaculation/erectile function/phosphodiesterase inhibitor/reproductive behaviour/semen


    Introduction
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 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
The cyclic nucleotides cAMP and cGMP serve as second messengers for a wide variety of extracellular signals such as neurotransmitters, hormones, light, and odorants. Cyclic nucleotide phosphodiesterases (PDE) regulate intracellular levels of cAMP and cGMP by hydrolysing them to the corresponding 5' monophosphates (Conti et al., 1995Go). Nine different PDE isoenzymes (PDE1 to PDE9) have been described and found to be present at various concentrations in human tissues (Soderling et al., 1998aGo; Fabbri et al., 1999Go). Previous studies have shown that mRNA coding for cAMP-specific PDE (PDE4A) isoforms are present in mature rat and mouse germ cells (Naro et al., 1996Go) and that the expression of these isoforms is maximal in round spermatids and is maintained in mature spermatozoa (Soderling et al., 1998bGo). Nitric oxide synthase (Lewis et al., 1996Go) and two distinct PDE isoforms (PDE1 and PDE4) are present in human sperm cells (Fisch et al., 1998Go). Sildenafil is a specific and potent inhibitor of cGMP-specific phosphodiesterase (PDE) type-5, which is the predominant PDE isoenzyme responsible for the degradation of cGMP in the corpus cavernosum, and has also minor inhibitory effects on PDE6 and PDE1 activities (IC50 = 0.034 and 0.28 µmol/l respectively) (Morales et al., 1998Go). Sexual stimulation is mandatory for sildenafil to increase nitric oxide production and stimulate cGMP production which in turn causes trabecular smooth muscle relaxation, cavernosal arteries dilatation, increased intracavernosal pressure and penile erection (Fabbri et al., 1997Go; Goldstein et al., 1998). It has recently been shown that some degree of erectile dysfunction may be present in the male infertile partner especially when assisted reproductive techniques are necessary and these men have difficulties in producing spermatozoa on demand at the time of egg fertilization (Tur-Kaspa et al., 1999Go); this temporary erectile dysfunction can be successfully treated with sildenafil.

It has been demonstrated that human sperm cells contain as yet uncharacterized PDE isoforms which are different from PDE1 and PDE4, and that the in-vitro inhibition of sperm PDE1 and PDE4 isoenzymes by specific inhibitors stimulates acrosome reaction and sperm motility (Fisch et al., 1998Go). Also, it is known that after acute administration of 100 mg sildenafil, the drug reaches a concentration of 0.1–0.3 µmol/l in the ejaculate (Pfizer, ViagraTM data sheet). This concentration is consistent with a possible inhibitory interaction of sildenafil with sperm PDE isoforms (Fabbri et al., 1999Go). In the present study we evaluated the effects of sildenafil administration (100 mg) on seminal parameters in young healthy male volunteers. The effects of sildenafil on erectile response to audiovisual sexual stimulation were also investigated.


    Materials and methods
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 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
Study design
The study design consisted of a prospective double-blind, placebo-controlled, cross-over, two period investigation. The study blind was broken after completion of all studies. All subjects enrolled spontaneously in the study after giving an informed consent to the study protocol, which was approved by the Ethical Committee of our Institution. The inclusion criteria included normal erectile function, proven fertility, a stable relationship, normal electrocardiogram, no prior or concomitant serious illness or consumption of medications during the 3 month period prior to the study. In a double-blind fashion, 20 subjects (mean ± SE age 32 ± 0.5 years, range 28–37) were randomly assigned to one of the two study groups, that is 100 mg sildenafil (ViagraTM, Pfizer, Sandwich, UK) or placebo, by a designated hospital pharmacist. All subjects were asked to abstain from sexual activity, alcohol or cigarette smoking for at least 3 days before each session of the study and were given one tablet of the medication. All subjects were asked to ingest the test drug 1 h before admission to the office. The washout period consisted of a 7 day period in which no medication was given. After this time, all subjects were crossed over to receive the alternative treatment. Differences between treatments were analysed by Student's t-test for paired data and analysis of variance.

Semen processing
All semen specimens were collected 1 h after sildenafil consumption and evaluated according to World Health Organization guidelines (WHO, 1992). A combined Bryan and Leishman strain (WHO) was used to assess sperm morphology. Sperm kinematics [motility percentages, straight-line velocity (VSL), curvilinear velocity (VCL) and linearity (LIN)] were assessed by a superimposed image analysis system (SIAS) (Mazzilli et al., 1995Go; 1999Go). This system is based upon superimposition of images and allows for a very accurate motility analysis; in fact, the operator can verify visually if each automatic track corresponds to the real superimposed sperm track and, when necessary, correct it.

Erectile and ejaculatory function studies
Erectile function at baseline was assessed by the administration of the Sexual Health Inventory for Men – IIEF 5 to each subject (Rosen et al., 1997Go). The erection quality after either treatment was assessed during audiovisual sexual stimulation (AVSS) by each subject and evaluated by an examiner (A.A.) by utilizing colour-power Doppler sonography parameters [(the peak systolic velocities (PSV), the end diastolic velocity (EDV) and the resistive index (RI)] and the number and morphology of helicine arterioles (Aversa et al., 1999Go). The time to re-obtain erection after ejaculation [post-ejaculatory refractory time (PERT)] was measured with the stop-watch technique by asking the subject to keep self-stimulating immediately after ejaculation and concomitantly by keeping on watching a different AVSS.


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 Materials and methods
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Table IGo shows the seminal parameters after sildenafil or placebo administration. Mean values of sperm number, motility and sperm abnormality percentages did not show significant variations in the two groups considered.


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Table I. Semen analysis in 20 fertile healthy subjects after double-blind placebo-controlled administration of 100 mg sildenafil tablet
 
Figure 1Go shows the erectile and ejaculatory parameters in the two groups of treatment. In all subjects after sildenafil administration while the penile haemodynamic parameters of erection were not different from placebo (Figure 1Go, upper panel), the post-ejaculatory refractory times were markedly reduced by 4- to 5-fold (Figure 1Go, lower panel).



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Figure 1. Effects of double-blind placebo or 100 mg sildenafil administration on cavernous arteries inflow (upper panel) and post-ejaculatory refractory time (lower panel). Peak systolic velocity (PSV) represents the average value obtained from two cavernous artery measurements. *P < 0.0001 sildenafil versus placebo.

 
No modifications in mean blood pressure and heart beat values were found after the two treatments (data not shown). Reported adverse events were facial flushing and headache which were more frequent with sildenafil than placebo (80% versus 5% and 50% versus 5% respectively, P < 0.0001). Side-effects were always of mild intensity and transient, all resolving after 1 h from drug administration.


    Discussion
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 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
The recent appearance of sildenafil (ViagraTM) in the European markets represented a milestone for the future management of men with erectile dysfunction and a breakthrough compared with previously available treatments, e.g. intracavernosal and intraurethral prostaglandin therapies, vacuum devices or penile implants. In fact, oral therapy always represents a more suitable option for all men suffering from erectile dysfunction and for the couple in the long-term follow-up (Fabbri et al., 1999Go). Studies on the efficacy and safety of sildenafil in men with erectile dysfunction are well known (Goldstein et al., 1998; Morales et al., 1998Go). There are also anecdotal reports that sildenafil may improve sexual performance in potent subjects. To our knowledge, this is the first report on the effects of sildenafil administration on reproductive parameters and sexual behaviour in normal healthy volunteers.

In the first phase of the study, we evaluated sperm parameters in the two groups studied. Nitric oxide synthase (Lewis et al., 1996Go) and at least two distinct PDE isoforms (PDE1 and PDE4) have been demonstrated to be present in human sperm cells. Specific inhibition of PDE1 and PDE4 by 8-methoxy-isobutyl-methylxanthine and rolipram selectively stimulated the acrosome reaction and sperm motility respectively (Fisch et al., 1998Go). After assimilation sildenafil circulates in plasma at micromolar concentrations which can cause a minor inhibition of PDE6 and PDE1 activities (ED50 = 1:10 and 1:100 of PDE5 inhibition respectively) and determine transient side-effects (Morales et al., 1998Go). In our study the maximal therapeutic dose of sildenafil was administered (100 mg) in order to achieve maximal drug concentration in seminal fluid and investigate eventual sildenafil–spermatozoa interactions. It has been reported that in healthy volunteers, after acute 100 mg oral administration, sildenafil is present in seminal fluid in a concentration range of 0.1–0.3 µmol/l (Pfizer, ViagraTM data sheet). At this concentration sildenafil has the potential to interact with PDE1 and perhaps with other as yet uncharacterized PDE isoforms (30–40% out of total PDE activity) present in sperm cells (Fisch et al., 1998Go; Fabbri et al., 1999Go). In the present study we showed that in subjects with proven fertility, sildenafil treatment did not cause any significant changes in the semen parameters studied, i.e. sperm number, and percentages of sperm abnormalities and motility. This finding excludes the presence of acute effects of sildenafil treatment on the male fertility profile and emphasizes the possibility of a potential use of the drug during assisted reproductive techniques, when a temporary erectile dysfunction may occur related to the need to produce spermatozoa on demand at the time of insemination (Tur-Kaspa et al., 1999Go). Nevertheless, since the improvements in seminal parameters from PDE inhibitors are small (Fisch et al., 1998Go), the effects of sildenafil in oligozoospermic males may be more pronounced and will be further evaluated in future studies.

In the second phase of the study we compared the erectile and ejaculatory functions after sildenafil and placebo. We clearly showed that, even if penile haemodynamic parameters were unchanged, a marked reduction in post-ejaculatory refractory time occurred after sildenafil compared with placebo. The lack of sildenafil-induced changes in penile rigidity in normal potent subjects is consistent with the assumption that the nitric oxide/cGMP pathway is maximally activated in these subjects. Ejaculatory disturbances are common clinical problems, reported in approximately 40% of adult males (Spector and Carey, 1990Go). Sildenafil did not influence the ejaculation time and/or ejaculatory attitudes in any of our subjects, but markedly reduced the PERT by 4- to 5-fold. It is known that the post-ejaculatory interval is very variable in different species, e.g. from less than 30 s in Syrian hamsters, some minutes in Norway rats, to hours or days in some mammals (Meisel and Sachs, 1994Go). It encompasses two phases: an absolute and a relative refractory period. During the former, the male is insensitive to sexual stimuli as well as being hyporesponsive to many other stimuli (Meisel and Sachs, 1994Go). The length of the PERT is biochemically regulated. It is increased by dopamine-receptor blockade and electrolytic lesions of dopaminergic neurons located in the substantia nigra (McIntosh and Barfield, 1984aGo), whereas it is reduced by specific treatment with serotonergic neurotoxins and electrolytic lesions of serotonergic neurons in the dorsal raphe nucleus (McIntosh and Barfield, 1984aGo,bGo). These observations are consistent with the presence of both a dopaminergic and a serotonergic control system, which normally exert a positive and inhibitory influence over the resumption of mating following ejaculation respectively. Since dopamine and serotonin do not utilize cGMP as second messenger (Schwartz et al., 1998Go; Duman, 1998Go), it is unlikely that sildenafil-induced reduction of the PERT is due to an interaction with central monoaminergic control pathways. On the other hand, sildenafil-induced reduction of the post-ejaculatory interval may be explained by its relatively long plasma half-life (~4 h) (Morales et al., 1998Go) and a consequent prolonged inhibition of intracavernosal PDE5. These observations indicate that sildenafil has a positive influence on the resumption of erection after ejaculation and has the potential to facilitate multiple instances of sexual intercourse in the presence of a continuous erotic stimulus. This is of clinical relevance and implies that use of caution is necessary, especially in impotent patients suitable for sildenafil treatment and in whom prolonged sexual activity is contra-indicated, e.g. subjects with a medical history of coronary heart disease.

In conclusion, even if further studies are needed to evaluate the effects of chronic sildenafil treatment on fertility capacity, our results indicate that sildenafil has no acute impact on sperm function. More important, the ability of sildenafil to reduce the post-ejaculatory refractory time in the presence of a continuous erotic stimulus adds a new aspect of interest in the research area concerning the regulatory mechanisms of male copulatory behaviour.


    Notes
 
3 To whom correspondence should be addressed at: Cattedra di Andrologia, Dipartimento di Fisiopatologia Medica, Policlinico Umberto I, Viale del Policlinico, 00161 Rome, Italy Back

* This paper was presented at the First International Consultation on Erectile Dysfunction, Paris, 1–3 July, 1999. Back


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 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
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Submitted on August 16, 1999; accepted on October 12, 1999.