A Hormonal Contraceptive for Men: How Close are We?

L. Martini (Eds.) Progress in Brain Research, Vol. 181 ISSN: 0079-6123 Copyright
2010 Elsevier B.V. All rights reserved.

CHAPTER 15

A hormonal contraceptive for men: how close are we? Ilpo Huhtaniemi
Department of Surgery and Cancer, Imperial College London, Hammersmith Campus, London, United Kingdom

Abstract: Novel contraceptive methods for men are still not available, and the opinions about their need among experts and lay public are polarized between enthusiasm and scepticism. Of the different strategies, hormonal methods aimed at suppression of spermatogenesis have been most extensively studies, are most promising, and are the only approach with the potential of breakthrough in the near future. Their principle is to block pituitary gonadotropin secretion, which will eliminate the endocrine stimulus for testicular androgen production, thereby eliminating its support for spermatogenesis. Testosterone alone or in combination with progestin is the most promising lead. However, many obstacles still have to be overcome before a practical and acceptable method is available. The reasons for the slow progress are partly biological and partly practical and economical. It is difficult to design a method that would be effective in most men, have no side effects and be reversible, economical, and acceptable by all cultures. Unfortunately, the pharmaceutical industry is currently not participating in the development work, and the research in the field is suffering from lack of political and financial support. Ironically, with relative modest additional effort a hormonal contraceptive method for men would be available. We review in this chapter the main principles of hormonal male contraception, the results of the latest clinical trials and shed light on some future perspectives in the field. Keywords: male contraception; spermatogenesis; gonadotropins; sex steroids; testosterone; progestins

vasectomy, still no modern reversible contraceptive methods are available for men. Although widely used they are not optimal due to the condom’s limited user efficacy and vasectomy’s lack of reversibility. The current situation is a real missed opportunity in the quest for controlling the world population explosion, because now half of potential contraception users are left out. Although population overgrowth is not a concern of the developed world, the opportunity for men to participate more actively in family planning is an important gender equality issue. A novel male method would also provide contraception for couples who cannot use any of the currently available female methods, for example, during post-partum contraception.

Introduction World overpopulation remains one of the main challenges of mankind, and it even contributes to global warming by directly impinging on the level of the human carbon print. Besides sociopolitical measures, such as limiting family size by law and increasing the literacy rate of women, attempts to improve the quality and prevalence of usage of contraceptive methods are of major importance. Conspicuously, apart from condoms and


Corresponding author. Tel.: þ44-(0)20-75942104; Fax: þ44-(0)20-75942184; E-mail: [email protected]

DOI: 10.1016/S0079-6123(08)81015-1

273

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than 60 years ago. Today, it is much more difficult to launch a new drug, and this in particular applies to contraceptives to be used by healthy individuals for many years.

There has always existed the popular cynicism and scepticism that if the ‘male pill’ were available, men would not use it and women would not trust the men. However, investigations have shown that if novel male methods were available they would be welcomed by the majority of men and women in all nationalities and religious groups (Anderson and Baird, 2002; Heinemann et al., 2005; Zhang et al., 2006). Moreover, only 2% of women in a stable relationship would not trust their partner to use the contraceptive (Glasier et al., 2000). The task to produce a new contraceptive for either sex is formidable, because it has to be highly effective, safe, reversible, easily accessible, inexpensive and culturally acceptable; it should not affect potency and libido; and it should be free from side effects. Biologically, the task with the male contraceptive is more challenging than it was for women. It was relatively easy to develop a hormone preparation that inhibited ovulation of a single oocyte once a month, whereas to stop the production or to inactivate sperm produced at a rate of 1000 per every heartbeat is much more difficult. Development of a male method has lagged behind for multiple reasons, which include the availability of safe and effective female methods and the purported reluctance of men to use contraceptive methods. Furthermore, the standards for safety and efficacy for new medications are now much stricter

Principles of hormonal male contraception One of the most widely tested principles of male contraception is the hormonal approach. It was discovered 70 years ago that treatment of men with testosterone (T) effectively suppresses their spermatogenesis (Heckel, 1939; McCullagh and McGurl, 1939). The explanation for this seemingly paradoxical response is in fact simple: when the circulating level of T increases by administration of exogenous hormone, enhanced negative feedback suppresses the secretion of the two pituitary gonadotropins, luteinizing hormone (LH) and follicle-stimulating hormone (FSH) (Figs. 1 and 2). LH is necessary for the stimulation of T production in testicular Leydig cells and for the maintenance of high intra-testicular level of this hormone, which is the most important hormone stimulating spermatogenesis (Fig. 3). FSH respectively enhances T action by maintaining the supporting function of Sertoli cells for spermatogenesis. The action of both gonadotropins is essential for qualitatively and quantitatively normal spermatogenesis. The anti-gonadotropic action of T can be boosted by

Hyp GnRH Analogs Testosterone Estradiol Progesterone

GnRH

Testosterone

GnRHR

Progestins

Pit

LH

FSH

LHR FSHR

Testes Testes

Inhibin

Fig. 1. The hypothalamic–pituitary–testicular axis, feedback loops of endogenous sex steroids (on the left) and sites of inhibitory actions of contraceptive hormones (on the right). Hyp = hypothalamus, Pit = pituitary gland.

275 4.0

4.0

LH (IU/L)

3.5

FSH (IU/L)

3.5

3.0

3.0

2.5

2.5

2.0

2.0

1.5

1.5

1.0

1.0

0.5

0.5

0.0

0.0 0

1

4

6

14

18

24

42 44 Week

FL 4 FL 8 FL 12 FL 16 FL 20 FL 24

0

1

4

6

14

18

24

42 44 Week

FL 4 FL 8 FL 12 FL 16 FL 20 FL 24

Fig. 2. The suppressive effect of treatment with T undecanoate/etonogestrel combination for 44 weeks on serum levels of LH and FSH. The two lines depict effects of two slightly different doses of the hormones. The suppression of gonadotropins is almost instantaneous, but their recovery following cessation of treatment occurs with some delay. The spermatogenic suppression in these subjects is depicted in Fig. 4. From Mommers et al. (2008) with permission.

Testosterone concentration

combining it with other anti-gonadotropic agents, such as gonadotropin-releasing hormone (GnRH) analogues and progestins (Fig. 1). Inclusion of androgen in the contraceptive regimen is necessary in order to maintain the normal extragonadal androgenic (potency and libido) and anabolic (muscle strength, bone density) effects of testicular

androgens. Suppression of spermatogenesis is reached with this type of contraceptive treatment in three to four months (Fig. 4), and it takes roughly the same time for sperm counts to return to the starting level after cessation of treatment. The slow onset of contraceptive efficacy is due to the biology of spermatogenesis with the 70-day

Steroid therapy Without androgenic effect

With androgenic effect

100 x Threshold testicular T level to maintain spermatogenesis

Add-back androgen

Testis Serum

Testis Serum

Testis Serum

Fig. 3. Schematic presentation of the 100-fold higher level of T in the testis tissue compared with peripheral serum (left) and the effect of anti-gonadotropic steroid treatment on these levels when the agent has no androgenic effect (middle) and when the agent is T (right). For the contraceptive efficacy it is important that the intra-testicular levels of T decrease below the threshold needed to maintain spermatogenesis. If the anti-gonadotropic treatment does not contain androgen, an addback dose of androgen (e.g. T) is needed to provide sufficient circulating androgen levels to maintain extragonadal androgen effects.

276 (a) 100

Percentage of men with a sperm concentration < cut off

90 80 70 60

Cut off:

50 40

< detection limit < =0.1 million per ml < =1 million per ml < =3 million per ml

30 20 10 0 0

6

12

16

20

24

30/34

42/44 Week

Fig. 4. An example of response of spermatogenesis to contraceptive treatment with androgen (T undecanoate i.m.) and progestin (etonogestrel implant) combination treatment. Spermatogenic suppression with sufficient contraceptive efficacy (sperm count <1 million/mL) is reached by about 90% in the men in 12 weeks. From Mommers et al. (2008) with permission.

spermatogenic cycle. The immature germ cells have to be washed off from the testis before the seminal fluid is free from residual sperm. Androgen treatment suppresses both gonadotropins, but apparently the suppression of LH, and LH-dependent T production, is crucial for the contraceptive effect. T can be considered the master switch of spermatogenesis, while FSH contributes to the quality and quantity of sperm. With the unravelling of structure and FSH-suppressing functions of inhibin (Fig. 1), there were high hopes that the principle of FSH suppression could be developed into a male contraceptive. However, neither men with inactivating FSH receptor mutation (Tapanainen et al., 1997) nor knockout mice for FSHb (Kumar et al., 1997) or FSH receptor (Abel et al., 2000) are azoospermic, and the inhibin/FSH approach has been largely abandoned. Although the hormonal approach to male contraception is promising, there are still multiple conceptual and practical obstacles before the final product is available for general use. A recent Cochrane Review on steroid hormones for contraception in men (Grimes et al., 2007) concluded: ‘No hormonal

birth control for men is ready for general use. Most trials were small pilot studies trying to differentiate hormone treatments. Larger trials with better methods are needed to test good leads in this area.’ Several comprehensive reviews on male hormonal contraception have recently summarized the older literature (Anderson and Baird, 2002; Grimes et al., 2007; Matthiesson and McLachlan, 2006; Page et al., 2008; Wenk and Nieschlag, 2006). We therefore concentrate below on the latest developments in the field and provide some predictions for future directions.

The various hormonal regimens tested for male contraception Testosterone alone Although the concept of T-based male contraception is old, the first seminal proof-of-concept trials on the use of T were carried out only about 20 years ago by the World Health Organization (Table 1) (WHO, 1990, 1996). It was established in these studies, using a treatment of 200 mg of T

Table 1. Male 'real-life' hormonal contraceptive efficacy trials without concomitant usage of a female method

Study

Number of couples

Failure to suppress þ dropouts

Became azoospermic N (%)

Completed efficacy phase N (%)

Pregnancies in efficacy phase N (%/person-year)

WHO (1990)

225

114

157 (57)

119 (53)

1 (0.8)

WHO (1996)

357

75

268 (75) 81 (23)b

280 (78)

4 (1.4)a

Gu et al. (2003) Turner et al. (2003)

305

9

296 (97)

280 (92)

1 (0.18)

55

4

49 (89) 2 (3.6)c

51 (93)

0 (0)

1045

43

1002 (96)d

733 (70)

9 (0.6)

Gu et al. (2009)

TE, testosterone enanthate; TU, testosterone undecanoate; DMPE, dimedroxyprogesterone acetate. a All pregnancies in oligozoospermic men. b Oligozoospermia 0.1–3 × 106 per mL in efficacy phase. c Oligozoospermia 0.1–1 × 106 per mL in efficacy phase. d Including severe oligozoospermia <1 × 106 per mL.

Treatment þ length of efficacy phase

Ethnicity

TE (200 mg/week) for 12 months TE (200 mg/week) for 12 months

Mixed

TU i.m. (500 mg/month) for six months DMPA (300 mg/3 month) þ 200 mg T implant for 12 months TU i.m. (500 mg/month) for 30 months

Chinese

Mixed

Caucasian

Chinese

278 Testosterone (T)

OH

O

OCO(CH2)5CH3

O

T enanthate

OH

OCO(CH2)9CH3

T undecanoate O

O

CH3

7α-methyl19-nor-T (MENT)

Fig. 5. The structure of testosterone (T), the two most widely used T esters (enanthate and undecanoate) used in the contraceptive trials, as well as a promising selective androgen receptor modulator (SARM), 7a-methyl-19-nor-T (MENT).

enanthate intra-muscular (i.m.) once weekly for 12 months (Fig. 5), that hormonal regimens to induce azoospermia can provide highly effective, sustained, and reversible male contraception with minimum side effects. The suppression of sperm counts to £3 × 106 per mL was originally considered sufficient for contraceptive efficacy, but the cutoff level was later reduced to £1 × 106 per mL (Nieschlag, 2009). Of the men completing the treatment, 89–100% became azoospermic in Chinese centres, but only 45–70% in those on Caucasian men. The better contraceptive efficacy of T treatment in Asian men has been documented in several subsequent studies, but its reasons remain unclear (see below). The insufficient contraceptive efficacy of T alone in Caucasian men prompted investigators to attempt combination of T with other antigonadotropic agents in Europe and North America (see below). The Chinese investigators have continued testing the T-alone regimen. A large phase III trial was recently published (Gu et al., 2009). In this study (Table 1), 500 mg of T undecanoate (U) i.m. (Fig. 5) was administered monthly for 30 months to 1045 men, of whom 733 (86%) completed the efficacy and follow-up phases. A total of 4.8% of the men failed to reach azoospermia

or severe oligozoospermia (<1 × 106 per mL) within six months. Post-suppression sperm rebound occurred in 1.3% of men, and nine pregnancies occurred in 1551 person-years of exposure during the 24-month efficacy phase. This resulted in a cumulative contraceptive failure rate of 1.1 per 100 men. The suppression of spermatogenesis was reversible in all but two subjects, and no serious side effects were reported. The excellent results of the Chinese trial may partly be due to the ethnicity of the study population, since good contraceptive efficacy of male hormonal regimens in South-East Asian populations has been a constant finding (Wenk and Nieschlag, 2006). Hence, the treatment was far superior to the 14% first-year failure rate of ‘typical use’ of condoms (Fu et al., 1999) and of 8% of female contraceptive pills (Kost et al., 2008; Trussell, 2004). It seems that T alone provides an efficacious method for male contraception in China, but in Caucasian populations, more effective means to suppress spermatogenesis are needed. Another key to the success of the Chinese study was apparently due to the T preparation used, i.m. TU dissolved in tea seed oil, which provided sustained release of hormone at injection intervals of 4

279

i.m. Testosterone enanthate

i.m. Testosterone undecanoate

Reference range

Serum testosterone

Peroral testosterone undecanoate

0

12 Hours

24

0

2 Weeks

4

0

2 Months

4

Fig. 6. Schematic presentation of pharmacokinetic profiles of the different modes of T administration. Peroral T undecanoate has to be administered several times a day to provide sufficient concentration. T enanthate has to be administered once a week for contraceptive purpose. T undecanoate in oil administered i.m. maintains constant androgen levels up to 12 weeks. Many side effects of T treatment have been ascribed to the unphysiologically high T levels immediately following administration of the short-acting regimens.

weeks. The earlier studies were carried out using T enanthate at one-week intervals (WHO, 1990, 1996). The 4-week interval of TU injections can be even extended up to 12 weeks if the hormone is dissolved in castor oil (Fig. 6), and an 8-week interval will be used in an ongoing WHO/CONRAD trial on TU and progestin.

Testosterone plus progestin Because of the sub-optimal efficacy of T alone in Caucasian men, several combination treatments have been tested to improve the level of suppression of gonadotropins and spermatogenesis. The most promising of them have been the combinations of T with progestins, in which case the T dose can be reduced and enhanced by antigonadotropic effect of progestins and possibly also through their direct testicular effects (El-Hefnawy et al., 2000). Multiple T and progestin combinations have been tested, and it can be concluded that they do provide better suppression of spermatogenesis than T alone (Page et al., 2008). The progestins tested have included oral and implant forms of levonorgestrel, oral

desogesterel, oral and implant etonogestrel (ENG), norethisterone enanthate, and medroxyprogesterone acetate (Fig. 7). The anti-androgenic effect of some progestins (cyproterone acetate, denogest) may further improve their anti-spermatogenic efficacy by suppressing action of the residual testicular levels of T (Meriggiola et al., 1996). The contraceptive effect of all progestins seems to be fairly similar, although the extent and spectrum of their side effects show some variance. The best androgen/progestin combinations reach about 90% contraceptive efficacy, and at the moment they seem as the most likely hormonal contraceptive applicable for Caucasian men. The seminal study on the T/progestin combination treatment was a pharmaceutical industry-driven male hormonal contraceptive trial conducted in collaboration with Organon-Schering-Plough and Merck-Schering (Mommers et al., 2008). A remarkable feature of this study was its doubleblind nature, which provided for the first time evidence-based information about the real nature and extent of side effects of hormonal contraceptives in men. The treatment entailed 354 Caucasian men who were treated with ENG implants combined with TU injections. Either low- or

280 CH3

CH3

C=O

O

HCOH H2C

O C CH3

Medroxprogesterone acetate

O

O CH

Levonorgestrel O

CH3

H3C H2C

H2C

CH3 OH

C=O C CH

OAc H2C

Desogestrel O

Cyproterone acetate

O Cl

Fig. 7. Examples of structures of progestins that have been used in male contraceptive trials.

high-release implants and two doses of TU were tested, including placebo implants and injections. Treatment duration was 42 or 44 weeks, with a 24-week post-treatment follow-up. Overall, gonadotropins (Fig. 2) and spermatogenesis (Fig. 4) were equally suppressed in all treatment groups, the latter reaching a sperm count £1 × 106 per mL at week 16 in 89% of men and in 94% of the high-release ENG groups. Only 3% of the participants never reached the levels of £1 × 106 per mL considered to provide effective contraception. Median recovery time of sperm to the levels of  20 × 106 per mL was 15 weeks. The treatment was well tolerated and the adverse effects included weight gain, mood changes, acne, sweating and libido change (see below). The findings of the Mommers et al. (2008) study were very important, because they were the first male hormonal contraceptive study where side effects were systematically monitored in a placebo-controlled fashion. It was concluded that the ENG implant/TU injection treatment was well tolerated and provided effective and reversible suppression of spermatogenesis. Admittedly, the implant/injection treatment regimen is cumbersome and unsuitable for wider use, but the

proof of principle was very important, that is, that sufficient contraceptive efficacy with a welltolerated hormonal treatment can be achieved. This study would have provided the stepping stone for further pharmaceutical development to achieve a more practical treatment mode with similar contraceptive efficacy. Unfortunately, both the pharmaceutical companies participating in the study were bought during the course of the study, and the new owners were not interested and/or motivated to develop the regimen further. It is at the moment uncertain how the promising concept can be developed into a final product in the absence of support from the industry. The remaining public sector agencies and committed academic researchers may not have the resources to complete the task.

Testosterone plus GnRH analogues GnRH agonists and antagonists have potent antigonadotropic action and are therefore logical agents to be tested for hormonal male contraception in combination with T. Agonists have not shown contraceptive efficacy, most likely because

281

of the rebound of FSH levels after initial suppression (Behre et al., 1992; Huhtaniemi et al., 1987). Although these compounds suppress circulating T to the castrate range, it is possible that the ‘addback’ level of T along with the residual constitutive T production and sustained FSH action provide an endocrine milieu able to support spermatogenesis. In contrast, GnRH antagonists achieving immediate and sustained suppression of both gonadotropins have proven efficacious in some (Pavlou et al., 1991) though not all (Bagatell et al., 1993) of the small trials so far conducted. A recent study with a newer GnRH antagonist, acyline, proved effective in providing prolonged and profound suppression of gonadotropins and T, and indicated that an injection twice a month may provide sufficient gonadotropin suppression for contraception (Herbst et al., 2004). Even longer-acting depot preparations of GnRH antagonists are now available for the treatment of prostate cancer (Crawford and Hou, 2009), and they might also be applicable for male contraception. The latest antagonist molecules have much reduced effect on histamine release and are therefore suitable for long-term treatments (Doehn et al., 2009). These compounds are potentially interesting alternatives for the progestin/T combination, but at least for the time being, their high price is prohibitive for their development into a widely accepted and affordable male contraceptive.

Other androgens The key questions in the selection of the androgen component to the contraceptive regimen are the intrinsic activity of the compound, its spectrum of effects, the duration of action and the route of administration. T is in principle the optimal molecule because it is the natural androgen and therefore the most logical ‘addback’ androgen to maintain the extragondal androgen effects. The drawback of T is its relatively low intrinsic bioactivity. T is produced by the testes at a rate of about 6 mg per day, and therefore rather high amounts of T have to be administered to maintain the normal circulating levels. This poses practical problems, because the solubility of T into the

injectable oil vehicles is limited, and for instance the TU injections in the hormonal contraception treatments have to be 4 mL (250 mg/mL castor oil), which makes them rather painful. It would be advantageous if the androgen treatment could be provided in a smaller volume of a more potent compound. T is not bioavailable through the peroral route because of its rapid metabolism in the alimentary tract and during the first passage through the liver. The 17a-alkylated T derivatives (old anabolic steroids) are orally active, but their hepatotoxicity excludes them from contraceptive use. The short duration of action of peroral TU (Fig. 6) makes it an inefficient and impractical alternative. New orally active steroidal and nonsteroidal androgens are being currently developed, and they may offer alternatives for the selection of contraceptive androgens (Jones et al., 2009). Selective androgen receptor modulators (SARMs), besides potentially having higher intrinsic biopotency, could provide the benefit of a desirable spectrum of tissue-specific actions. Because of the potential of long-term side effects of T especially on the prostate, prostate-sparing androgens with no 5a-reduction would be desirable. While maintaining their inhibitory action on gonadotropins and spermatogenesis, and supporting effects on potency, libido, and muscle and bone health, they would not have the potential side effect of promoting prostate growth. In contrast, the possibility of aromatization is considered an advantage because of its positive action on the bone. One potentially useful SARM is 7a-methyl-19-nortestosterone (MENT) (Fig. 5), which is aromatized but not 5a-reduced (LaMorte et al., 1994; von Eckardstein et al., 2003). Moreover, its biological activity exceeds that of T by about 10-fold, meaning that doses about 10% of the dose of T would be sufficient for the maintenance of androgenic activity in a contraceptive regimen. This would be a real advantage in the development of long-acting formulations. Promising preliminary findings on the possibility to replace T with MENT in contraceptive regimens have been obtained in two studies (von Eckardstein et al., 2003; Walton et al., 2007).

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Prostate-sparing effects in animal experiments have been documented for some other SARMs, such as desoxymethyltestosterone and tetrahydrogestinone, and for some recently developed orally active steroidal (dimethandrolone) and non-steroidal SARMs (reviewed by Bhasin & Jasuja, 2009; Blithe, 2008; Page et al., 2008). An extra advantage of these compounds could be their partial progestin agonism, which might potentiate their anti-gonadotropic actions. Besides increased potency and/or favourable functional profile of the androgen, its duration of action is another important factor. The early hormonal contraceptive studies were carried out using T enanthate (Figs. 5 and 6), which had to be administered once a week to achieve sufficient concentrations for the contraceptive efficacy (WHO, 1990, 1996). T decanoate increases the injection intervals to 4–6 weeks (Brady et al., 2006). Sub-dermally implanted T pellets can be inserted every four to six months (Turner et al., 2003). Another treatment option is T gels, possibly combined with progestin. They seem to be acceptable to men despite the need of daily application (Page et al., 2006). A clear improvement was the advent of TU in injectable form (Figs. 5 and 6), which was used in the recent Chinese study at four-week intervals, but when dissolved in suitable oil it may be possible to prolong the injection intervals to 8–12 weeks (Zitzmann and Nieschlag, 2007). WHO has been developing another T ester, T buciclate, which showed excellent long-term depot effect suitable for male contraception (Rajalakshmi and Ramakrishnan, 1989). Unfortunately, its development is currently at halt because of lack of interest of industry to develop it further.

Remaining questions and problems Why do not all men suppress to azoospermia Effective suppression of intra-testicular (IT) T is the key to the anti-spermatogenic efficacy of a hormonal male contraceptive. ITT concentration is normally 2–2.5 mmol/L (Huhtaniemi et al., 1985; Matthiesson et al., 2005), which is 100-fold higher

than in peripheral circulation (Fig. 2). When gonadotropin secretion is blocked, ITT is suppressed by about 98%. The remaining 2%, about 50 nmol/ L, is still twice the normal peripheral concentration. If the normal ITT level is necessary for spermatogenesis, it is paradoxical why its suppression by 98% does not block spermatogenesis in more than two-thirds of Caucasian men. A logical explanation is that much lower testicular T level than normal is sufficient for spermatogenesis. The ITT level may be so high only because of the mundane reason that the testis is the site of T synthesis in the body. If much lower than normal levels of ITT can support spermatogenesis, then it is possible that the suppression of gonadotropins alone does not reduce T production enough to bring about azoospermia. There is in fact experimental evidence to support this contention. In LH receptor knockout mice, totally devoid of LH-stimulated T production, the constitutive gonadotropin-independent ITT level can maintain qualitatively full spermatogenesis (Zhang et al., 2003). These mice have very low, albeit measurable level of ITT, about 10 nmol/L, which is about 3% of that of control animals. As young adults the knockout mice are azoospermic, with spermatogenesis stopping at the round spermatid stage and failing to enter the androgen-dependent transit to elongating spermatids (Plant and Marshall, 2001). When reexamined at 12 months of age, surprisingly, qualitatively full spermatogenesis up to elongating spermatids could be observed in the knockout testes. However, if the mice were treated with anti-androgen between 6 and 12 months, to block action of the residual androgen, the progression of spermatogenesis beyond round spermatids was totally blocked. Hence, the gonadotropinindependent low T production can maintain spermatogenesis. The finding prompts the question whether more profound T suppression than achieved with gonadotropin blockage would be the key to consistent azoospermia in contraceptive treatments. In fact, we do not exactly know how low the human ITT has to be before spermatogenesis stops completely. Rodent data suggest that there may not be a threshold concentration at all, but a

283

linear correlation prevails between ITT and sperm production (Singh et al., 1995; Zirkin et al., 1989). The 50 nmol/L ITT level in gonadotropin-suppressed men (GnRH agonist for prostatic carcinoma and T for contraception) (Huhtaniemi et al., 1985; Matthiesson et al., 2005) is about twice the normal serum concentration. A similar level in rats in proportion to normal testicular T (20 nmol/L) clearly stimulates spermatogenesis (Zirkin et al., 1989), and in gonadotropin-deficient hpg mice, T treatment can induce qualitatively complete spermatogenesis at doses that do not produce measurable elevation in ITT (Singh et al., 1995). The crucial question whether simultaneous inhibition of spermatogenesis and maintenance of peripheral androgen actions is possible has never been addressed. Elimination of the residual ITT could be the key to consistent and complete suppression of spermatogenesis. One hypothesis is that non-suppressor men might more effectively convert their residual ITT to biologically more potent 5a-dihydrotestosterone (DHT), which would then maintain spermatogenesis even at low concentrations (Anderson et al., 1996). However, combination of 5a-reductase inhibitor to the T treatment was found ineffective in non-suppressor men (Kinniburgh et al., 2001; Matthiesson et al., 2005). Another possibility is that the residual gonadotropin-independent T production remains higher in non-suppressors, but measurements of ITT levels have not proven this (Page et al., 2007). Incomplete suppression of gonadotropins in nonresponders would be a logical explanation, but no proof for this has been obtained (Handelsman et al., 1995; Wallace et al., 1993). Differential dependence of spermatogenesis on hormonal stimulation (Handelsman et al., 1995) and pharmacogenetic differences in the rate of gonadotropin suppression (McLachlan et al., 2004) have also been suggested. In an integrated analysis of all clinical trials so far conducted, only two factors appeared clinically significant, that is, the use of progestins and ethnicity affected the treatment response (Liu et al., 2008). The latter study also concluded that conventional laboratory analyses are unlikely to identify the men who do not

suppress during contraceptive treatments. Progestin co-administration allows the use of smaller doses of T, which may reduce the ITT level below a threshold level needed in some men to maintain spermatogenesis. Of the different treatment regimens tested, the combination of T with the antiandrogen cyproterone acetate has been one of the most effective ones (Meriggiola et al., 2003), which suggests that elimination of the residual androgen action in the testis may really be critical. However, this particular regimen is not feasible because it may make the men ‘peripherally’ hypogonadal. A very recent study suggests that the higher fat mass in Caucasian men in comparison to Chinese men may after all contribute to their poorer gonadotropin suppression (Kornmann et al., 2009). Studies on genetic differences between suppressors and non-suppressors, for example, in the CAG repeat length of the androgen receptor exon 1 and CYP3A4, have yielded no reproducible findings (Eckardstein et al., 2002; Yu and Handelsman, 2001). One preliminary observation reported higher insulin-like factor 3 levels in nonsuppressor men (Amory et al., 2007). All in all, the reason for the variable suppression of spermatogenesis in the hormonal contraceptive trials remains enigmatic.

What are the short-term and long-term side effects The commonest short-term side effects of the contraceptive treatments include acne, weight gain, reduced testis size, increased haematocrite, adverse changes in blood lipids and changes in mood and sexual drive (Page et al., 2008). The anecdotal effects of androgens on aggression have not been observed. Potential long-term effects include atherogenic effects and increased risk of prostate cancer. However, no evidence for them exists, albeit the observation time in the treatment trials, usually up to 1–1.5 years, is too short. The longterm effects of progestins in men are even less well delineated, but effects on weight gain, suppression of high-density lipoprotein (HDL)-cholesterol, and increases in pro-inflammatory cytokines with increased cardiovascular risk may occur (Page et al., 2008). Altogether, the risks of androgen

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and progestin treatment of men remain at the moment hypothetical, and the real ones can only be verified by long-term follow-up studies. Besides the hormones used, the route of their administration (peroral vs. injection/ transdermal) makes a difference in the side effect profile. The real drug-related side effects are difficult to decipher from most of the studies, because placebo controls have rarely been used. An exception is the recently conducted pharmaceutical industry-driven male contraceptive trial using T and progestin in a double-blind placebo-controlled fashion (Mommers et al., 2008). The drug-related, statistically significant adverse effects included acne (26 vs. 10%, active treatment vs. placebo), night sweating (27 vs. 8%), libido changes (usually increased; 13 vs. 0%) and weight gain (24 vs. 10%). No clinically relevant differences were found between active and placebo treatments in haematology, biochemistry, lipid parameters and prostate-specific antigen (PSA). These may reflect the route of administration of the hormones (sub-cutaneous and intra-muscular), which does not burden the liver. The study was not designed to monitor long-term safety, where potential adverse effects on the cardiovascular and prostate health are the main concerns. Whether the above, apparently objective side effects of male hormonal contraception are acceptable and/or possible to reduce by different hormone combinations remains to be studied. Conversely, we can also envision potential beneficial side effects of androgens in young men, which include increased lean body mass and decreased fat mass (Bhasin et al., 2001). They could in the long term prevent obesity, metabolic syndrome, osteoporosis and cardiovascular diseases. Because the contraceptive treatments will be given to young men for limited periods of time, it is not certain whether the temporary adverse lipid changes would have long-term harmful effects on diseases that normally appear tens of years later. However, this is a concern that needs due attention because in general very little is known about effects of progestin on men.

A small study confirmed the adverse effect of progestin on blood lipids in men (Herbst et al., 2003). They are in particular related to oral administration of progestins, due to their firstpass metabolism in the liver, which information may be important when developing the final contraceptive product. Concerning the effects of contraceptive steroids on inflammatory markers in men, associated with coronary heart disease, one study has reported that T þ progestin combination increased the level of the pro-inflammatory interleukin-6 (IL-6) and T alone decreased it (Zitzmann et al., 2005). A potentially important serious adverse effect in the long term is the increased incidence of prostate cancer. Although the growth of existing prostate cancer is dependent on androgens, the opinion whether the endogenous level of T or exogenously administered T correlates with the appearance of prostate cancer is controversial; no solid evidence for such a connection exists. Administration of even high levels of T to young men has not been found to increase significantly PSA levels (Cooper et al., 1996). Likewise, no consistent effects on prostate volume or PSA levels have been observed in the male contraceptive trials (Meriggiola et al., 2005). The information is still limited, and only long-term studies will reveal the real side effects of the hormonal contraceptive treatments.

What alternatives are there to improve the efficacy of hormonal contraception Increasing the androgen dose is clearly not the solution to improve the contraceptive efficacy because it would lead to increased side effects, and the concomitant increase in intra-testicular androgen levels could in fact reduce the effect. Therefore, combination of another anti-gonadotropic compound with minimal dose of T to maintain the extragonadal androgen effects is more likely to be successful, as the T þ progestin combinations demonstrate. But other alternatives should also be tested. As discussed above, a promising one is GnRH antagonist, but inhibiting the metabolism of T to the more active androgen,

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DHT, did not bring improvement. Neither did combination of estrogen to T increase the azoospermia rate (Handelsman et al., 2000). A third alternative, not yet tested, is to block the residual testicular T production using an inhibitor of the 17b-hydroxysteroid dehydrogenase (17b-HSD) type III enzyme. This enzyme catalyzes the final distal step of T biosynthesis, that is, the conversion of androstenedione to T. It is envisioned that inhibition of this step would bring testicular T production to a complete halt, and would then have a more profound inhibiting effect on spermatogenesis than gonadotropin suppression alone. The author’s laboratory is currently testing this principle experimentally.

Conclusions and future perspectives Further innovations are needed to release the current stagnation in the field where androgen alone or androgen/progestin combinations are practically the only approaches being pursued in the development of a male contraceptive. One possibility is to test whether more effective suppression of ITT levels by enzyme inhibitors (see above) improves the contraceptive efficacy and reduces side effects. The availability of new targets through advances in genomic, proteomic and bioinformatics fields may reveal totally new and unexpected concepts, and finally a non-hormonal method may turn out to offer the best solution (Kopf, 2008). The number of genes with a crucial role in male fertility is vast, offering unlimited possibilities for intervention; 4% of the mouse genome are expressed in post-meiotic germ cells (Schultz et al., 2003). A very recent review identified 57 gene knockouts affecting spermatogenesis and 10 affecting fertilization (Naz et al., 2009). Knockout and transgenic RNAi mouse models are useful methods for target validation. The epididymis is a promising target, expressing numerous specific genes with crucial roles in sperm maturation (Sipila et al., 2009). The potential advantages of epididymal contraception are that it does not affect testicular endocrine or gametogenic function and its initiation and reversibility may be

much faster than those of the hormonal methods. If an epididymal gene product is ‘druggable’, that is, it is an ion channel, receptor, exchanger or enzyme, it may be possible to inhibit its function with small pharmacological molecules. Such a concept would lead to a very specific post-testicular contraceptive without the need to influence testicular function, either its hormone production or gametogenesis. Identification of epididymis-specific genes and unravelling of their significance in sperm maturation using knockout mouse models are likely to be important in this research (Naz et al., 2009; Sipila et al., 2009). Although biological therapeutics of larger molecular size, such as vaccines, antibodies, most peptides and proteins, might be effective, their high development and manufacturing cost would preclude them from becoming a widely available option. Despite the recognized need of novel contraceptives, also for men, as a global health issue, many practical hurdles besides the scientific ones still exist. From the industrial perspective, the current market of contraceptives is highly fragmented with several low-cost products easily available. With the development of a new product, patent protection, marketing issues, product liability and pricing have to be weighed against the researchand-development cost. They form the key practical equation that has to be solved between industry, academia and government in order that tangible progress can be made, and we would finally have the long sought-after ‘male pill’.

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