To date the only options men have for contraception are condoms or vasectomy. Studies have shown that many men would theoretically take a birth control pill if it were available to them. However, research into male reproduction has traditionally been geared towards treatment to reverse infertility not to induce it. Comparatively little effort has been spent on developing a male contraceptive analogous to the female birth control pill. Of the work that has been done, most of it has been on researching and developing hormonal methods of male contraception. Similar to the “pill”, which uses hormones to block ovulation in women, hormonal male contraceptives block the development of spermatozoa by inhibiting spermatogenesis; inducing a state of azoospermia.
Azoopermia is the absence of spermatozoa in the semen of adult males. Hormonal male contraception focuses mainly around modulating testosterone levels to produce the desired effect of diminished fertility in men. However, like their female counterpart, the use of hormonal methods have undesirable side effects that can negatively impact ones health. As a result, non-hormonal methods have been investigated, to develop safer and more practical contraceptives for men. Non-hormonal contraceptives are designed to directly impact either the development, motility, or capacity of spermatozoa.
One promising non-hormonal strategy that has been investigated is, immunocontraception, which is the use of immunization to temporarily and reversibly inhibit male fertility. Early studies into male fertility found that infertile men often had anti-sperm antibodies. Antibodies are proteins produces by immune cells in the body against pathogens like bacteria and viruses. This lead researches to postulate that sperm specific proteins found in the testis could be used for the development of a contraceptive for men. One protein, in particular, known as Eppin, has be shown to be a promising target for the development of a non-hormonal male contraceptive.
Eppin or Epididymal Protease Inhibitor, is a protein specific to the male reproductive system. Expressed in the testis and epididymis, it is found on the surface of sperm and is essential for male fertility. It forms a complex with another protein, known as Semenogelin, which is secreted by the seminal vesicles as the major component of seminal plasma coagulum.. Together they are essential for the proper function of ejaculated spermatozoa.
Eppin Targeted for the Development of Male Contraceptive
To study whether Eppin could successfully be used as an immunocontraceptive, scientist at the Indian Institute of Science conducted research to determine the effect of the immune response elicited by immunization with Eppin on spermatozoa. They tested whether antibodies to Eppin would cause infertility.
Adult male M. Radiate monkeys were immunized with human recombinant Eppin. Then their ability to impregnate several female monkeys was tested. Monkeys with a high antibody tither were unable to impregnate any females, and were therefore deemed infertile. Monkey’s that were not able to obtain a high antibody tither were excluded from the study. Over a year after immunization, 71% of the monkeys regained fertility. This showed that Eppin is essential to male fertility in humans and primates. In addition, immunization did not interfere with spermatogenesis (as is the case with hormonal methods). Neither testosterone levels nor sperm count were altered. Although safe and efficacious, anti-Eppin immunization was problematic mainly because infertility was not reversible in all subjects. In addition to irreversible infertility in some subjects receiving anti-Eppin, the antibody may elicit an adverse autoimmune responses, causing inflammation and infection, culminating in permanent infertility`. Antibodies are also degraded in the stomach, which would require that the method be delivered via injection.
At the time, the mechanism of action of many spermatozoa surface proteins like Eppin were just being studied, and the interplay of Eppin with Semenogelin and their role in sperm motility was not yet fully understood. It was determined that Eppins association with Semenogelin inhibited sperm motility. However, researchers observed that the infertile immunized monkeys lacked semen coagulum. The antibodies against Eppin, blocked Semenogelin from binding to Eppin on the surface of spermatozoa. Instead the anti-Eppin antibodies would bind Eppin in place of Semenogelin, thereby also retarding sperm motility. This showed that Semenogelin was also essential to male fertility.
The inhibition of sperm motility by the Eppin-Semenogelin complex differ from that of an Anti-Eppin-Eppin complex. One being necessary for fertility and the other inducing infertility. Figuring out the role that the Eppin-Semenogelin complex played in male fertility would be key to figuring out how induce infertility for the purpose of contraception.
Eppin Physiological Function
It was later understood that Eppin functions as a protective shield, guarding spermatozoa against bacterial and proteolytic attack while in the female reproductive tract. Prior to ejaculation spermatozoa travel through the ducts of the male reproductive system where they are mixed with secretions from the seminal vesicles and prostate gland. During this process Eppin binds other proteins at the surface of sperm, such as Semenogelin. Semenogelin stops sperm from swimming, and is responsible for the gelatinous nature of the initial ejaculate. By the binding of Eppin and Semenogelen. sperm are kept in an immotile state which prevents their hyperactivation (before reaching their target).
In order for sperm to be capacitated, the seminal coagulum needs to undergo liquefaction. This process takes from five to 30 minutes, and involves the cleavage of Semenogelin by a protease called, Prostate Specific Antigen (PSA), from the Eppin-Semenogelin complex. Eppin inhibits the displacement of Semenogelin by PSA, and since Eppin itself is not hydrolyzed by PSA, it modulates the gradual removal of Senemogelin from the surface of spermatozoa, enabling the progressive release of motile sperm. The removal of the Eppin-Semenogelin complex is necessary for fertility, failure to remove this complex results in impaired spermatozoa. Essentially the complex incapacitates spermatozoa, and its removal allows for gradual hyperactivation of the cell, allowing it to migrate up through the female reproductive tract. Without its formation sperm expend their energy immaturely.
Eppin: Immunocontraceptive to Small Molecule Drugs
In 2016, researchers at the University of North Carolina at Chapel Hill (UNC) submitted a patent application for a non-hormonal male contraceptive. The premise for their application, that small molecules that mimic Eppin can be used to inhibit male fertility, is based on the previous immunocontraceptive study in monkeys from 2004.
To overcome the shortcomings of immunocontraception, UNC researchers developed a series of small organic molecules that act like anti-Eppin antibodies, and prevent sperm motility. Because these compounds occupy the same sites, they block Anti-Eppin antibodies and Semenegelin from binding to Eppin. Binding to Eppin by either of these substances triggers a drop in pH level within spermatozoa, which subsequently decreases intracellular calcium levels, through the prevention of extracellular calcium uptake. This prevents the formation of contractile force energy needed for motion. For normal physiological function the pH level within sperm needs to increase in order to facilitate hyperactivation, which takes place after ejaculation with the gradual enzymatic removal of Semenogelin by PSA from the Semenogelin-Eppin complex. Unlike with semenogelin and PSA, there is no gradual enzymatic removal of Anti-Eppin Antibodies or these small molecules from Eppin binding sites after ejaculation.
Since there is no semenogelin binding, there is no seminal coagulum formed. Therefore, sperm motility is inhibited not by the physical encapsulation of sperm within coagulum, but by the reaction that takes place when Eppin binding sites are occupied.
Using small molecule drugs to impair male fertility is advantageous to current male contraceptive options, as their action is safe, efficacious, and reversible. Condoms may fail due to user error, hormonal contraceptives and immunocontraceptives vary in efficacy by the individual and may produce unwanted side effects, and like vasectomies, they are not readily reversible.
Small molecule targeting Eppin represent a new class of therapeutic agents that specifically target spermatozoa without disrupting spermatogenesis or altering hormone levels. They can be used administered orally to inhibit sperm motility prior to ejaculation, or applied topically after ejaculation, to directly inhibit motility on contact with spermatozoa. Therefore, these compounds can be used as a sole means of contraception or when applied topically, in conjunction with condoms, diaphragms, and spermicidal jellies.
- The University of North Carolina at Chapel Hill Applies for Patent on Small Molecules for Inhibiting Male Fertility. (2017, Mar 04). Global IP News.Pharmaceutical .
- John K Amory, 2016. Male Contraception. Fertility and Sterility Vol 106:6 p 1303-1309.
O’Rand Michael G., Silva Erick J.R., Hamil G. Katherine, 2016. Non-hormonal male contraception: A review and development of an Eppin based contraceptive. Pharmacology & Therapeutics Vol 157 p105-111.
Silva, Erick J.R., Hamil Katherine G., O’Rand Michael G., 2013. Interacting Proteins on Human Spermatozoa: Adaptive Evolution of the Binding of Semenogelin I to EPPIN. PLoS One 8:12 e82014.
O’Rand, ,M.G., Widgren, E. E., Sivashanmugam, P., Richardson, R. T., & al, e. (2004). Reversible immunocontraception in male monkeys immunized with eppin. Science, 306(5699), 1189-90.