Male contraception: Another holy grail

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Dec 7, 2013 - est by big pharma with most of the support for male contraception provided by the ..... of germ cells from the circulation by a blood testis barrier.
Bioorganic & Medicinal Chemistry Letters 24 (2014) 419–424

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Bioorganic & Medicinal Chemistry Letters journal homepage: www.elsevier.com/locate/bmcl

BMCL Digest

Male contraception: Another holy grail q Fern E. Murdoch a, Erwin Goldberg a,b,⇑ a b

The Center for Reproductive Science Northwestern University, Evanston, IL 60208, United States Department of Molecular Biosciences Northwestern University, Evanston, IL 60208, United States

a r t i c l e

i n f o

Article history: Received 26 September 2013 Revised 27 November 2013 Accepted 2 December 2013 Available online 7 December 2013 Keywords: Contraception Spermatozoa Testes Fertility Isozymes Sertoli cells Blood-testis barrier

a b s t r a c t The idea that men should participate in family planning by playing an active role in contraception has become more acceptable in recent years. Up to the present the condom and vasectomy have been the main methods of male contraception. There have been and continue to be efforts to develop an acceptable hormonal contraceptive involving testosterone (T) suppression. However the off target affects, delivery of the analogs and the need for T replacement have proven difficult obstacles to this technology. Research into the development of non-hormonal contraception for men is progressing in several laboratories and this will be the subject of the present review. A number of promising targets for the male pill are being investigated. These involve disruption of spermatogenesis by compromising the integrity of the germinal epithelium, interfering with sperm production at the level of meiosis, attacking specific sperm proteins to disrupt fertilizing ability, or interfering with the assembly of seminal fluid components required by ejaculated sperm for acquisition of motility. Blocking contractility of the vas deferens smooth muscle vasculature to prevent ejaculation is a unique approach that prevents sperm from reaching the egg. We shall note the lack of interest by big pharma with most of the support for male contraception provided by the NIH. Ó 2013 The Authors. Published by Elsevier Ltd. All rights reserved.

Family planning, or contraception has generally been the responsibility of the female partner in a relationship. After all, it’s the woman who gets pregnant. This doesn’t mean that men can’t or shouldn’t accept the contraceptor’s role. In fact, in some partnerships this becomes a necessity, if the woman is unable for health reasons to use the birth control pill. Presently, there is a concerted research effort to develop a male pill. That man realized he provided the seed that produced the baby dates to ancient times as evidenced by the reference in Genesis 38:9 to Onan spilling his seed on the ground to avoid impregnating his brother’s widow. Awareness that some natural products could decrease male fertility also traces to antiquity. The use of plants stems from at least the first century CE when Dioscorides included that hemp seeds ‘extinguished conception’ in his extensive compilation of natural materials for medicinal use.1 This documents what may have been a long history of attempts to use plant seeds, leaves and extracts for birth control and perhaps the beginning of men taking an active role in this process. In fact, it is likely that contraception, whether used by women or men, was not a topic for public discussion in polite society until the advent of the ‘‘pill’’ for use by women. It was not long after this that sex came out from under the covers. And then the idea of sharing the contraceptive

q This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. ⇑ Corresponding author. E-mail address: [email protected] (E. Goldberg).

burden, at least in a stable relationship, motivated research into developing the ‘male pill’. Directed research in male contraceptive development is relatively new. It is also fraught with difficulties because the goal is to provide a drug that can be used safely by healthy individuals of reproductive age (the fact that the same is true for a woman doesn’t seem to have been a real problem). The burden for demonstration of safety, efficacy, and reversibility by regulatory agencies and the consumer is high. This burden may even include the assurance of no deleterious effects to the future offspring. This perhaps is a major reason that ‘Big Pharma’ rapidly lost interest in pursuing a birth control pill for men. As a consequence, the National Institutes of Health (NIH) is the major, if not sole source of funding to develop a male pill in the United States. Perusal of the 124 currently open clinical trials retrieved under the search term ‘contraception’ that are registered at the National Library of Medicine revealed only one that was concerned with the development of a male contraceptive drug, dimethandrolone undecanoate, discussed below.2,3 The lack of pharmaceutical support can be argued to have more to do with the slow progress in this field than anything else. In the United States, condoms and vasectomy are the only two methods of contraception available for use by men. Of these two methods, only condom use is reliably reversible. Condoms serve as exterior barriers to block fertilization and have in one form or another been used for perhaps thousands of years.4 Not only are condoms widely available, even in rather esoteric shapes and sizes, but nowadays packages are prominently displayed in pharmacies

0960-894X/$ - see front matter Ó 2013 The Authors. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.bmcl.2013.12.004

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compared to behind the counter location in the ‘old days’. The failure rate for typical use is as high as 18%.5 Condoms also serve the dual function of protecting against STD’s, and should be used with or without the pill in casual sexual relationships. Vasectomy, an invasive procedure, is used by only an estimated 6% of U.S. couples.6 Efficacy is extremely high and complications are relatively rare. Vasovasostomy (re-anastomoses of the vas) by skilled surgeons is relatively straight forward, with patency rates (return of sperm to semen) ranging from 70–95% dependent on the procedure and time elapse since vasectomy.7 Pregnancy rates of the partner are considerably lower at 30–76%, which may reflect a host of variables including the woman’s age. In addition, there is evidence that vasectomized men develop antibodies to sperm that lead to infertility which of course defeats the purpose of the vasovasostomy. Therefore, vasectomy is not an ideal choice for men who plan to have a family in the future. Cultural and other barriers also contribute to its low level of use in the United States.6 The prospect of developing a non-invasive, reversible, physically satisfying, and safe pharmaceutical contraceptive for use by men is worthwhile. But first we might ask why? Nearly half of all pregnancies in the United States are unintended.8 This statistic illustrates the urgent need to develop new methods of male contraception and overcome uncontrolled fertility. While a variety of options are available for female contraception, there is no safe, effective, and reversible contraceptive drug product in the market for men. There is however a rather startling array of instructions for the preparation of homemade contraceptive brews for men available on the Internet. This unsubstantiated collection must represent the need and interest of men seeking out reversible contraceptive choices. It is astonishing, that in the 21st century, the reversible contraceptive choices for a man differ little from the first century. Development of a male contraceptive targeting the hormonal control of spermatogenesis, consisting of a progestin and testosterone, is the most developed pharmaceutical option at this time, though nothing will be on the market in the US in the near future. The strategy is to down-regulate pituitary release of LH and FSH resulting in decreased production of testosterone in the testis and impaired spermatogenesis.9–12 Concerns about a hormonal male contraceptive are raised by studies that have revealed an array of physiological changes with short-term administration of testosterone derivatives that were undesirable. These included metabolic disturbances such as weight gain, decreased HDL/LDL ratios, and increased serum glucose levels. Acne, decreased libido, reduced testis size, and mood changes have also been commonly reported. In addition, a significant percentage of men do not achieve sufficient suppression of spermatogenesis for contraception and, although not understood, this is influenced by ethnic background.13 The need to routinely inject some formulations is likely to reduce acceptance by many men. There is a promising recent report of a clinical trial that combined delivery of testosterone (T) and nestorone (a nonandrogenic progestin) by transdermal gels for the suppression of spermatogenesis.14 Transdermal gels are a more acceptable method of delivery and efficacy across diverse ethnic groups was achieved with 88–89% of treated men achieving sperm concentrations below 1 million/ml. The authors concluded that a combination of daily NES+T gels suppressed sperm concentration to contraceptive levels, with minimal adverse effects, and may be further studied as a male transdermal hormonal contraceptive. The same research group has phase 1 studies underway to assess safety and tolerability of dimethandrolone undecanoate (DMAU); an androgen with progestin activity.2,15 In addition to approaches based on the hormonal control of spermatogenesis, there is a need to develop non-hormonal methods by identifying novel targets for contraceptive intervention. We have no bias as far as hormones are concerned, but the

diverse targets available for non-steroidal suppression of spermatogenesis hold the promise of development of an efficacious drug with minimal side effects and acceptable routes of delivery. The remainder of this review will focus on the clinical and research progress towards development of non-hormonal contraceptive methods. There are a number of exciting, potential drug targets that are being pursued. The challenge is that the delivery must be simple and that the effects of the drug be non-toxic, specific to the target and reliably reversible in affecting fertility. Levels of attack are the testes, epididymis, and spermatozoa. The goal may be to either suppress spermatogenesis (i.e., reduce sperm counts) or to render sperm non-functional. That is, in order to reach the egg, sperm must be motile and in order to penetrate the outer coat, the zona pellucida of an egg, sperm must undergo a maturation process, called capacitation, in the female reproductive tract. All these steps must take place before fertilization can occur providing multiple targets for contraceptive intervention. Drug discovery is a complicated process requiring collaborations with medicinal chemists, structural biologists, practitioners in pharmacokinetics, and in this arena, reproductive biologists. Medicinal chemistry procedures are well established, but complex and costly. The process involves target identification and high throughput screens (HTS) to identify active compounds. Requirements for activity, selectivity, and physicochemical properties must be supported by bioassay and computational methods and, where possible, biophysical measurements and protein crystallography. HTS triage (selection of compounds for follow-up) requires that each compound that is screened must also be tagged as a confirmed active, a non-confirmed active, or an inactive compound. Because most of the data collected is proprietary the information about targeting described below is limited. Figure 1 shows the chemical structures of the non-steroidal compounds with potential for male contraception discussed below. The mammalian testis is a dual function organ responsible for maintaining the body’s hormonal homeostasis as well as being the site of sperm production. Testosterone (T) is the most familiar and important hormone for sexual behavior as well general physical well-being. There are no shortages of ads promoting T to not only improve men’s sex lives but also enhance their business acumen and social behavior. Usually T levels decline with age and there is research, which will not be addressed here, on such studies. The point in this context is that an endocrine method of birth control in men, which affects hormonal balance must be carefully adjusted and the difficulty in accomplishing the appropriate treatment regime is one of the major complications in hormonal intervention for male birth control. For this reason, an alternative approach aimed at discovery of a non-androgenic compound to disrupt sperm production or sperm function is attractive. Vitamin A metabolism has long been considered as a target for male contraception.16 Meiosis in both male and female germ cells requires retinoic acid.17,18 Vitamin A deficient rodents and retinoic acid receptor (RAR) alpha knock-out mice are infertile in the male.17,19 The phenotypic defects in spermiation are similar in both models. Recently, a RAR antagonist (BMS-189453) that binds all three RARs (a,b,c) has been studied for contraception in male mice.20 It causes a failure of spermatid alignment and sperm release and is extremely effective, highly reversible, and shows low off-target effects. This research is at the stage of trying to develop an RARa-specific antagonist to minimize off-target effects.21 The RARa-selective antagonists, BMS-189532 and BMS-195614 displayed selective activity in vitro, both compounds failed to suppress spermatogenesis with oral administration to mice. These investigators plan to build on these results, which they suggest are crucial for designing new RARa-selective antagonists for pharmaceutical application.

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O

O OH

O

N

O

OH

O

N H

BMS-189453

OH N H

BMS-189532

BMS-195614 Cl

Cl

O

O

O

H N

N

Cl

Cl

N H O

Cl N

S

Win 18,446

N

JQ1

N HO

O

N

O

NH 2

I

NH

N

N F

N

N

F F Cl

Cl

O

Cl

Cl

Adjudin

H2-gamendazole

O

CDB-4022 O

O

S

HO OH

S N+

N Cl

α-Chlorohydrin

O

O-

HC-056456

Figure 1. Structures of non-steroidal compounds under study for male contraception. Structures were obtained from the publications cited in discussion of each compound below or the PubChem Compound Database at the National Center for Biotechnology Information.

WIN 18,446 is a bis-dichloroacetyldiamine that has long been known to inhibit spermatogenesis in human beings upon oral administration, however severe side effects occur when mixed with alcohol consumption.22,23 The mechanism of action has recently been shown to be at the level of inhibition of the aldehyde dehydrogenase 1a2 (ALDH1a2) required for the conversion of vitamin A to retinoic acid.24 The challenge in this research is to synthesize novel derivatives of WIN 18,446 that are specific for the testis aldehyde dehydrogenase while inactive against ALDH2. Whether this research is progressing to product development is unknown because of the proprietary nature of data on analog research. Achieving specificity between testis and somatic isoforms is a common obstacle to almost any approach to a non-steroidal male contraceptive that could be taken orally. The impact of serendipity is apparent from the finding during research for an anti-cancer drug that BRDT (murine, NCBI accession #EDL20168.1) targets spermatogenesis. This is an exciting and relatively recent observation. BRDT is a bromodomain protein member of the BET family, and it binds acetylated histone 4 associated with meiotic and post-meiotic chromosomes in sperm.25,26 Although named as testis-specific, it is also found in oocytes.27 Male mice homozygous for mutations in the first bromodomain

region of BRDT were infertile, but otherwise appeared normal indicating the specificity of this protein.28 Female mice appeared normal and were fertile. The small molecule, JQ1, a thieno-triazolo-1, 4-diazepine, was designed as a prototype for binding to the bromodomain in the BET protein family and is highly selective for this specific bromodomain.29 There is recently published work on the use of JQ1 as an i.p. daily injection, male contraceptive in mouse.30 Complete and reversible contraception was achieved in a small group of male mice and histochemical analysis indicated that effects were at the level of spermatocyte to spermatid maturation with a concomitant decrease in testicular volume. Off target effects were not observed and circulating levels of FSH, LH and testosterone were normal. There were no observed effects on offspring sired by males after treatment. Enthusiasm for this approach is tempered, since the severe decrease in testicular size (40–70%) could be unacceptable to many men.31 Further research is necessary to achieve increased selectivity based on structure-function insights from molecular recognition of bromodomain proteins by natural ligands. Sertoli cells comprise the somatic cell component of the germinal epithelium and are responsible for maintaining the separation of germ cells from the circulation by a blood testis barrier. Adjudin

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(AF-2364) is a derivative of the chemotherapeutic compound lonidamine.22,32,33 It disrupts the bridges between spermatids and Sertoli cells thus perturbing sperm maturation. Although highly effective and reversible in rodents, there are off-target effects in other organs such as liver that have essentially halted investigations of the use of this formulation. Conjugation with a portion of the FSHß subunit allows more specific targeting to the testis, but increases cost. Delivery (injection) is less than optimal. H2-gamendazole is another lonidamine derivative that has been investigated as a non-steroidal male contraceptive with some promise, but there are significant concerns about toxicity and reversibility in rodents.34,35 CDB-4022 is a chemically distinct compound, an indenopyridine that also works at least in part in the seminiferous epithelium to disrupt germ cell adhesion.36,37 Reversibility is species dependent; high in primate models, but low in rats.33 The spermatozoan can be described as the DNA containing cell driven by a molecular motor toward an oocyte in the female reproductive tract. As such it contains tightly packaged genetic material in a nucleus propelled by a sleek flagellum, which houses the energy generating reaction mechanism for motility. The sperm of many species including rodent and man rely on aerobic glycolysis for ATP production. Just as the sperm is a unique cell, it also contains sperm specific isoforms of many of the glycolytic enzymes. Mature spermatozoa exhibit high levels of aerobic glycolysis and convert glucose into pyruvate with concomitant production of ATP. Targeting sperm specific glycolytic enzymes represents a novel approach to male contraception. Several in vitro studies provided evidence that glucose metabolism has an essential role in production of the ATP required for sperm motility, hyperactivation, and capacitation.38–41 This conclusion is consistent with the severe defects in sperm progressive motility and in male infertility shown to result from the targeted disruption of glycolytic enzymes. Inactivation of the gene for the sperm-specific glycolysis pathway enzyme glyceraldehyde 3-phosphate dehydrogenase (Gapdhs) dramatically reduced the level of ATP in sperm.42 Early studies of a-chlorohydrin provided the first experimental evidence that GAPDHS can be selectively inhibited. Those studies showed that the GAPDH step of glycolysis in sperm was inhibited by S-3-chlorolactaldehyde, the active metabolite of a-chlorohydrin and related compounds, even though it was not known that the sperm have a unique isozyme. Although there were side effects at high doses, sperm glycolytic activity and motility were inhibited at concentrations that did not inhibit the somatic isozyme in other tissues. Targeting this isozyme became knowledge-based with the highresolution crystal structures available for human GAPDHS and GAPDH. Comparison of these structures revealed that there are two regions near the active site that are different between the isozymes and may contribute to differences in catalytic properties.43 These regions may be exploited for developing selective inhibitors. Compounds showing partial selectivity by inhibiting sperm glycolysis (as measured by lactate production) and sperm motility, particularly hyperactivation may provide viable leads to drug development.44 The essential role of glucose metabolism in sperm function is also demonstrated by the severe defects in sperm progressive motility and in male infertility shown to result from the targeted disruption of Ldhc.45 LDH-C4 was thought to be the only member of the LDH family active in spermatozoa.46,47 A series of studies have shown that LDH-A4 also is present and localized to the principal piece of the sperm flagellum along with LDH-C4 and other glycolytic enzymes.45,48–51 However, LDH-A4 is bound tightly to the fibrous sheath while LDH-C4 is abundant throughout the cytoplasm of the sperm principle piece.48,50 These differences in localization suggest that LDH-C4 may be required for the initial ATP consuming steps of glycolysis while LDH-A4 completes at least

partially the ATP generating reactions necessary for motility and capacitation. LDHC has been proposed as a valid target for a contraceptive drug. Despite the negligible structural diversity between the LDH isozymes, there are very significant dissimilarities in the kinetic properties of each form that become even more pronounced for the sperm-specific LDH-C4. LDH-C4 has kinetic properties very different from the five isozymes that are derived from the association of A and B subunits.52 LDH-C4 has high thermostability and a broad substrate specificity, including the ability to metabolize other aketoacids almost as efficiently as lactate; a property that could be exploited in drug development.53–56 These differences in catalytic properties, in abundance in germ cells, and in localization in spermatozoa support the hypothesis that LDH-C4 has unique structural and functional properties that are amenable to production of isozyme specific inhibitors. The discovery that LDH-C4 is required for male fertility in mice parallels observations that LDH-C4 was undetectable in semen samples from a subset of men presenting at a fertility clinic.45,57 These men produced normal numbers of sperm that initially were motile, but soon became poorly motile or non-motile. Large-scale gene expression studies in humans also correlate with reduced LDHC expression with infertility.58 Hence, inactivating mutations in the human LDHC gene might negatively impact human sperm function. Chemical inhibition of LDH-C4 as a target for male contraception by disabling sperm fertilizing capacity needs to be explored. The recent establishment of patch-clamp methodology in whole sperm has been essential for identification of functional ion channels and has opened up a new class of targets to inactivate sperm.59 A voltage-gated calcium channel restricted to sperm is constructed using four gene products: CatSper (sperm-associated cation channel) 1–4. Three other gene products are associated with the CatSper complex. This channel is regulated by pH, prostaglandin, and the progesterone produced by cumulus cells associated with the egg in the oviduct in human.60,61 All four of the CatSper proteins 1-4 are required for fertility in the male and the channel is important for sperm capacitation, hyperactivation, and egg penetration.62,63 The specificity to sperm, the lack of redundancy, and the function at the final stages of sperm maturation make these proteins ideal targets for contraceptive development and studies for both vaccine and pharmaceutical development have been reported.64 HC-056456 was reported to specifically block CatSper channel activity in whole sperm, using a high-throughput adaptable assay.65 The drug inhibited hyperactivation of the sperm. The high-blood pressure medication and calcium channel blocker, nifedipine, has been shown to cause reversible infertility in some men.66 It is not clear if this is by action on the CatSper channel, however its lack of specificity for sperm exclude it from development for contraception. A spermatozoa-specific pH-dependent potassium channel is encoded by the Slo3 gene and gene knock-out render mice infertile.64,67 This channel represents another potential target for a pharmacological contraceptive. The sperm Na+/H+exchanger (sNHE, Slc9c1 gene in mouse) is essential for sperm motility as demonstrated in knockout mice and has been widely discussed as a candidate target for male contraception.68 Although the Na,K-ATPase is ubiquitously expressed, the polypeptide responsible for the ATPase activity has a sperm-specific isoform, termed Na,K-ATPase a4.69 Male knockout mice for the Atp1a4 gene encoding a4 are infertile and produce sperm with defective morphology and motility. The specificity of the a4 polypeptide to differentiated spermatozoa makes it an excellent target for reversible contraception. EPPIN, an epididymal protease inhibitor, was identified in a human epididymal cDNA library and is the target of compound screening as well as an antigen for vaccine development.70 EPPIN

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is a spermatozoan surface protein that functions in the binding of the semen coagulating protein semenogelin I (SEMGI) and thus modulates sperm motility. The protein-protein interaction sites on EPPIN have been dissected and specific amino acid residues required for SEMGI binding have been identified, providing evidence that EPPIN is a prime target for contraceptive compound development.71 A wealth of data from expression profiling, knock-out mice, endocrine disrupters, and RNAi screens is yielding identification of many new candidate targets for male contraceptive development.72–77 The testis-specific protein adenine nucleotide translocase (ANT4) is a member of the ANT protein family involved in ADPATP translocation to mitochondria. Disruption of Ant4 leads to meiotic arrest at the leptotene stage during spermatogenesis.78 Chromatin modifying enzymes, including Dnmt3 and the PRC2 complex, play roles in the regulation of Ant4 gene expression that possibly could be exploited as a contraceptive approach.79,80 Further, through an evolutionary genomics approach, Ant4 has been proposed to have a specialized function in mammalian male fertility beyond meiosis.81 The goal is to identify candidate compounds that can selectively inhibit ANT4 activity over the other ANT family members. One approach might be to disrupt the protein-protein interaction of ANT4 and LDHC.82 The testis specific serine threonine kinases 1 & 2 are closely related genes that have recently been validated as male contraceptive targets.83 These proteins are expressed selectively in the testis in post-meiotic spermatids and knockout of Tssk 1 & 2 causes male infertility. A search for inhibitors of these proteins is underway. The observation of testis/sperm specific expression is useful only for the most preliminary identification of a protein as a candidate. Validation of candidates is laborious and expensive–knockout in the mouse being the most rigorous proof of a role in fertility. Development of compounds that affect the activity of these validated candidate proteins and studies to determine contraceptive efficacy, reversibility, and safety are essential. This conceptual approach has been in place for many years now, however little progress on moving compounds to clinical trials has been made. An attack on the delivery system has been proposed that would prevent or reduce the number of sperm deposited into the vagina. Sperm propulsion from the cauda epididymis to the urethra in sufficient numbers for fertilization requires autonomic nervous system control of smooth muscle contractions in the walls of the vas deferens. Contraction of the smooth muscle cells surrounding the vas deferens is mediated by P2X-purinoceptors in response to ATP and alpha1A-adrenoceptors in response to noradrenaline. 84–86 The recent report that male mice homozygous for null alleles of genes encoding both of these proteins are infertile suggests a unique contraceptive target.87 Male mice were rendered infertile without effects on sexual behavior or adverse system-wide physiological consequences. The alpha1A-adrenoceptors and P2X1-purinoceptors are widely distributed and particularly important in vascular control. Reduction in resting blood pressures of the mutant mice was observed but not considered a serious off target effect by the investigators of this study. The fact that a combined inhibition of both proteins is required for near 100% efficacy makes the search for a safe drug combination, as well as approval by regulatory agencies for human use, complicated at best. While libido (as in the mouse model) may not be reduced, the potential for decreased ejaculate may impact perception of orgasmic satisfaction for some men. This indeed was suggested in a pilot study administering an alpha1A-adrenoceptors blocker to healthy men.88 Nevertheless this is an innovative approach in which neither sperm production nor the sperm itself is targeted.

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An important question to raise is whether the search for a male contraceptive is a cost-effective way to spend limited research dollars? Although the approach is expensive, research to block fertility will also contribute to our understanding of the causes of male infertility, which are multiple and complex. A greater understanding of the mechanism of male fertility is essential in order to address the medical needs of the 1 in 20 men of reproductive age who are infertile. This will, in turn, provide for a more mechanistic approach to contraception. The current increase in information about the cellular and molecular mechanisms involved in spermatogenesis, which will be further accelerated by advances in genomics and proteomics can lead to identification of novel targets for contraceptive intervention. There are a great variety of targets and strategies that have already been proposed for non-steroidal male contraception and more can be developed.22,66,89–91 Many of these approaches have run into difficulties with toxicity, lack of reliable reversibility, unacceptable delivery methods, lack of efficacy, and/or expense, however progress is being made as demonstrated by the studies cited. A number of targets have been identified and the challenge remains to develop compounds that fulfill the demanding requirements for a broadly acceptable male contraceptive. Any pharmaceutical male contraceptive will need to be reversible in order to appeal to the largest cross-section of men that will make this a marketable product. Although several of the compounds described above show reversibility in studies using small numbers of animals, these studies were all done over a relatively small percentage of the reproductive life span of the test animal. In human populations, these drugs would potentially be used for years before the subject might want to regain fertility. Compounds (and proposed targets) that function early in the spermatogenesis pathway can, or may be expected to, cause significant tissue remodeling in the testis. The long-term consequences of such remodeling must be of concern. In addition to efficacy and reversibility, long-term safety for the patient, and perhaps to potential future offspring, is also required. The shotgun approach with small studies aimed at multiple targets, perhaps because of limited funding, is not likely to advance the cause in a timely fashion. It appears that most pharmaceutical industry research stopped by 2009.23 The complexity of the research needed perhaps provided too little return (‘bang for the buck’) economically. The fact that NIH has a Contraceptive Discovery and Development Branch suggests an awareness of the need for basic research into new methods of birth control. Unfortunately, contraception is not considered a disease and therefore such research is underfunded at best. There remains a critical need for additional contraceptive options, including those that target male gametes. It is not unreasonable to suggest that men and women share the contraceptive burden. The significant issue addressed by these various research projects is the urgent need to develop new methods of male contraception and overcome uncontrolled fertility, one of the most pressing public health challenges of the 21st century. Acknowledgements We are grateful to our colleagues in the field all of whom responded to our request for an update on their research much of which is cited here. We thank John Amory, David Clapham, Gunda George, John Herr, Marty Matzuk, Debbie O’Brien, Mike O’Rand, Naohiro Terada and Debra Wolgemuth. We also thank Pauline Bentley and Sarah Stein for assistance with manuscript preparation.

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