Update on Current Hormonal and Non-Hormonal Contraceptive Options in Non-Human Primates

Abstract

Reproductive success in captive non-human primates (NHPs) has increased the demand for safe, effective, and reversible population control methods. This review provides an overview of the current literature on hormonal and non-hormonal contraceptives as reproductive control methods in NHPs. Where available, behavioral and welfare implications, as well as drug efficacy, reversibility, and associated adverse effects, are considered. However, a paucity of data exists for NHPs, particularly in regard to non-hormonal contraceptives, emphasizing the need for institutions to share their experiences with reproductive management techniques in the species under their care.

1. Introduction

Improved husbandry and medical care have resulted in increased lifespan and breeding success in captive non-human primates (NHPs). Combined with the absence of predation, the number of births exceeds mortality rates in many NHP species kept in zoological institutions. Therefore, captive populations may quickly outgrow the available space and resources, posing a risk to animal welfare [1]. This welfare hazard is of particular importance in species where the offspring does not stay with the natal group [2]. In these species, resource scarcity is more likely to pose a significant concern. Moreover, depending on the NHP species, only the males, or sometimes females, may disperse from their natal group and territory at some stage. The lack of possibility for dispersal in captivity makes the management of this offspring even more challenging. Therefore, institutions may aim to prevent the birth of this so-called surplus offspring. Moreover, institutions may consider contraception to prevent reproduction in specific animal groups, such as family groups, or to extend the inter-birth interval of female NHPs.

In order to achieve sustainable population management, it is imperative that zoological institutions have a population management plan (PMP) to safeguard the welfare of their animals. Population management plans should be tailored to the behavioral and welfare needs of a specific NHP species. In general, PMPs may utilize selective breeding and reproduction control to regulate population size.

One way to control reproduction is separation of the sexes, which may be appropriate in species that naturally segregate in the wild. However, the ability to implement this management strategy largely depends on the ability of zoological institutions to house multiple groups. Although this method does not limit individual reproductive potential and allows a range of natural behaviors to be expressed, a range of social and sexual behaviors will be limited [3]. Moreover, bachelor groups or groups with unnatural sex ratios may behave aberrantly and can subsequently be difficult to manage.

Surgical techniques for reproductive management may be considered, although these methods are invasive and generally irreversible, with the potential exception of vasectomy [4]. Castration and ovariohysterectomy may have profound social and behavioral effects, resulting from the loss of sex hormones and secondary sex characteristics [3]. In addition, the loss of ovarian function in females may result in adverse health effects, such as osteoporosis in long-lived NHPs [5].

It can be argued that PMPs should facilitate reproduction, as they allow animals to exhibit alloparental, parental, and intricate social behaviors that are not exhibited in the absence of offspring [6]. Moreover, a lack of breeding restricts the behavioral experience of the group, which may negatively impact offspring survival rates when breeding is desired at a later time [6,7]. Therefore, if medical intervention is elected, reversible contraception techniques are preferred.

A variety of reversible hormonal and non-hormonal contraceptives are available (

Table 1. Overview of the literature describing hormonal and non-hormonal contraceptives in non-human primates.

Section			Active Substance
Section 2	Hormonal contraception
Section 2.1		GnRH agonists
Section 2.1.1			Deslorelin acetate
Section 2.1.2			Leuprolide acetate and buserelin
Section 2.2		GnRH antagonists
Section 2.3		Progestins
Section 2.3.1			Etonogestrel
Section 2.3.2			Levonorgestrel
Section 2.3.3			Medroxyprogesterone acetate
Section 2.3.4			Melengestrol acetate
Section 2.3.5			Progestin or combined progestin–estrogen contraceptive pills
Section 2.4		Progesterone receptor antagonists
Section 2.5		Testosterone derivates
Section 2.6		Phyto-estrogens
Section 3	Non-hormonal contraception
Section 3.1		Immunocontraception
Section 3.1.1			Zona pellucida vaccines
Section 3.1.2			EPPIN based vaccines
Section 3.1.3			GnRH vaccines
Section 3.2		EP055
Section 3.3		Triptonide
Section 3.4		Prostaglandin receptor antagonists
Section 3.5		Prostaglandin F2α analogs
Section 3.6		COX inhibitors
Section 3.7		Phosphodiesterase inhibitors

), with varying rates of reversibility, adverse effects, effectiveness, and effects on behavior. This review aims to compare contraceptive options in NHPs, considering species-specific behavioral needs, medical aspects, and contraceptive effectiveness. To this end, an initial literature search was performed across academic databases, such as Google Scholar, PubMed, and Web of Science. Relevant literature, including book chapters, peer-reviewed articles, conference proceedings, and newsletters, was obtained using keywords and phrases such as contraception, hormones, prevention of reproduction, and non-human primates. Subsequently, search results were assessed to identify reports deemed clinically relevant.

Some of the evaluated treatments are experimental and are not currently available to the veterinary clinician. Moreover, the availability of commercial contraceptive agents may be subject to change and may vary depending on the geographic area. As a result, we encourage readers to contact local manufacturers or distributors to obtain current information on product availability.

2. Hormonal Contraceptives

Hormonal contraceptives act on the hypothalamus–pituitary–gonadal (HPG) axis. The most widely used hormonal contraceptives act as gonadotropin releasing hormone (GnRH) agonists or synthetic progestins. Alternatives, such as antiprogestins or testosterone derivatives, are also described. Most hormonal contraceptives are commercially available in the form of implants or injectable depot preparations manufactured for human use that work for an extended period of time. Contraceptive pills are also frequently used, particularly in great apes, but have the disadvantage of requiring daily administration.

2.1. GnRH Agonists

Pulsatile GnRH secretion from the hypothalamus stimulates the secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from the pituitary gland. However, non-pulsatile GnRH secretion, such as that achieved through use of synthetic GnRH agonists, induces the downregulation of pituitary GnRH receptors, resulting in the suppression of LH and FSH secretion from the pituitary gland. As a result, the production of estrogen, progesterone, and testosterone from the gonads is suppressed [8], as demonstrated in Figure 1. A benefit of GnRH agonists is that they can be used in both male and female animals. Innate to its mechanism of action, initial stimulation of the reproductive system may result in estrus in females and temporary enhancement of testosterone and semen production in males [3]. In females, this initial stimulatory phase may be prevented by administering oral progestin birth control pills for seven days before and after injection or implant insertion [3].

2.1.1. Deslorelin Acetate (Suprelorin^®)

Deslorelin acetate is a synthetic GnRH agonist, marketed as either a 4.7 mg or 9.4 mg subcutaneous (SC) sustained-release implant (Suprelorin^®). Suprelorin^® can be easily implanted SC with the use of a large-bore needle. It is considered to be highly effective in various mammal species [9,10], including NHPs [3].

Before the downregulation of pituitary GnRH receptors occurs, an initial stimulatory phase is observed following implantation in most species. In great apes, the initial stimulatory effect may be prevented in females by oral progestin birth control treatment, such as megestrol acetate (MA), for at least 2–3 weeks following implantation. This is, however, not an option in male apes, and females should be contracepted for at least 6–8 weeks following implantation of the male to allow viable sperm to clear the males’ system [11].

The efficacy of deslorelin varies between NHP species. For example, a 4.7 mg deslorelin acetate implant was unsuccessful in suppressing testosterone and spermatogenesis in male ring-tailed lemurs (Lemur catta), likely as a result of an insufficient dosage [12]. In female common marmosets (Callithrix jacchus), a half-implant dose of 4.7 mg deslorelin acetate failed to suppress ovarian cyclicity and ovulation. In common marmosets, ovarian activity was initially stimulated, such as occurs in most species, but no subsequent suppression of cyclic activity was observed. Whether this lack of efficacy is a result of inadequate dosing, possibly as a result of cutting the implants in half, or species differences in reproductive physiology is unclear [13].

In other species, deslorelin administration is considered to be more successful in controlling NHP fertility. A 9.4 mg deslorelin acetate implant reduced the annual birth rate of captive female Guinea baboons (Papio papio) (n = 8) in 86% [14]. The implantation of a SC 4.7 mg deslorelin acetate implant in female rhesus macaques (Macaca mulatta) (n = 6) demonstrated a transient increase in estradiol and progesterone one to three days after implantation, followed by a decrease to basal levels within six days of implantation. Reversibility was observed in two macaques that returned to cyclicity by 96 and 113 days after implantation, which is shorter than the manufacturers minimum duration of efficacy of six months for this product. The other four animals remained suppressed until at least eight months post implantation. The reported variation in implant duration of effect appears to be unrelated to age, weight or pairing status [15].

The variability in implant efficacy and duration of effect between individuals and between NHP species suggests that animals should be monitored to determine implant efficacy, and dosage and frequency of reimplantation should subsequently be adjusted to best fit the animal [15]. Although reversible, Suprelorin^® was not designed to be removed, as the rod becomes brittle and difficult to locate [11].

Observed adverse effects of deslorelin acetate include weight loss and loss of muscle condition in male lion-tailed macaques (Macaca Silenus) [16]. Moreover, the removal of gonadal hormones has profound implications for social and sexual behaviors in most species, which should be taken into consideration when selecting contraceptive agents.

The reported behavioral effects of GnRH agonists are similar to those of gonadectomy. As a result, deslorelin acetate has been used to reduce male aggression in chacma baboons (Papio ursinus), with effects observed as soon as three weeks post-implantation. In addition to reducing aggression, an increase in the median time spent engaging in affiliative behaviors was observed, indicating a positive effect on animal welfare. It has been hypothesized that the effectiveness of the implant in reducing male aggression is related to the males’ social status, as social ranking is often positively correlated with testosterone levels [17]. A transient decrease in testosterone levels and aggression has been reported in both male olive baboons (Papio anubis) [18] and lion-tailed macaques [19]. In male lion-tailed macaques, deslorelin acetate effectively reduced serum testosterone levels and was associated with an increase in grooming behavior between males. The subsequent decrease in male–male aggression allowed for the creation of a bachelor group in this species [16]. Interestingly, social roles were maintained in male lion-tailed macaques, in spite of the recovery of testosterone in some individuals. This suggests that the new social roles had been learned and became independent of androgen influence [19].

In Guinea baboons, deslorelin acetate decreased aggression in females, but also reduced overall affiliation within the group. In this group, deslorelin acetate was observed to reduce self-directed displacement behavior, suggesting improved animal welfare. Females in estrus may attract male aggression by participating in conflicts, as males frequently intervene. Subsequently, the observed decrease in male agonistic behavior in Guinea baboons could also be a knock-on effect of reduced female aggression [14]. In chacma baboons, it has been observed that pregnant and lactating female baboons exhibit increased aggression towards females in estrus, likely as a result of reproductive competition [20]. Therefore, it may also be possible that deslorelin acetate reduced aggression in females by reducing competition over males.

Whilst these behavioral effects are promising, care must be taken when extrapolating these data to other NHP species, as a species’ social system must be taken into account. Whereas Guinea baboons are highly tolerant and form strong bonds [21], this may not be the case for other, even relatively closely related, species, such as hamadryas baboons (Papio hamadryas).

Interestingly, no effect of deslorelin acetate on affiliative behavior has been observed in female coppery titi monkeys (Plecturocebus cupreus) kept in pairs (n = 17) when compared to either pairs containing untreated females (n = 9) or pairs in which the female was treated with monthly intramuscular injections of 15 mg medroxyprogesterone acetate (MPA) (n = 9) [22]. Although no differences in affiliative behaviors were reported, coppery titi monkeys are socially monogamous [23], and affiliative contact is usually initiated by the female [24]. Thus, any changes in male-driven interest may have gone unnoticed. This emphasizes the fact that species-specific social structures should be taken into consideration when evaluating the effects of contraceptive treatment.

2.1.2. Other GnRH Agonists

Leuprolide acetate is a reversible GnRH agonist that has rarely been used as a contraceptive tool in NHPs [3]. As a result, little is known about its efficacy and adverse effect profile in NHPs. It has been marketed as a depot suspension of 3.75 mg or 11.25 mg (Lupron^®), unlike deslorelin acetate, which is available as an implant. It is presumed to have an efficacy and side effect profile similar to that of deslorelin acetate [25], although more research is required to assess the safety, reversibility, and effectiveness of this drug in NHPs.

Another GnRH agonist implant, buserelin, was successful in suppressing hormone production and cyclicity in female stump-tailed macaques (Macaca arctoides) for at least 90 days at a dosage of 2.6 mg [26]. Future research may focus on assessing its safety, reversibility, and effectiveness in different NHP species. Moreover, future research may evaluate the efficacy of buserelin depot suspensions, as opposed to implants.

2.2. GnRH Antagonists

GnRH antagonists competitively inhibit the binding of GnRH, thereby suppressing the secretion of LH and FSH from the pituitary gland, with subsequent suppression of plasma testosterone and inhibin concentrations [27]. Similar to GnRH agonists, antagonists may be used in both males and females.

Deterelix^® is an injectable GnRH antagonist that was experimentally observed to reversibly induce azoospermia when combined with testosterone in male cynomolgus macaques (Macaca fascicularis) when SC injected at a dosage of 750 μg/kg daily. Testosterone was supplemented to suppress negative physiological and behavioral effects of long-term GnRH deficiency. Although azoospermia was achieved in all monkeys within 12 weeks, a significant reduction in body weight was observed in all animals receiving the antagonist only, and an insignificant decrease in bodyweight was observed in animals receiving Deterelix^® and testosterone [28]. Moreover, the need for daily injections is undesirable from both a management and an animal welfare perspective.

2.3. Progestins

Synthetic progestins are frequently used in NHPs and include melengestrol acetate (MGA), medroxyprogesterone acetate (MPA), etonogestrel, and levonorgestrel. Synthetic progestins may also be administered in the form of daily oral contraceptive pills, either combined with an estrogen or as a progestin-only pill (see Section 2.3.5). Whereas GnRH agonists may be administered to both male and female NHPs, the usage of progestin-based contraception is limited to females.

Progestins prevent pregnancy by suppression of the secretion of LH, thickening the cervical mucus, which prevents sperm from making contact with the oocyte, and causing atrophy and thinning of the endometrial lining, which prevents implantation. This results in the prevention of ovulation, a decreased penetration of spermatozoa into the uterus, and in the prevention of attachment of the blastocyst to the uterine wall [29,30,31].

Although no reports exist in primates, progestins have been associated with an increased incidence of type 2 diabetes in humans [32] and should be avoided in diabetic animals.

2.3.1. Etonogestrel (Implanon^®, Implanon NXT^®, Nexplanon^®)

Implanon^®, Implanon NXT^®, and Nexplanon^® implants can be inserted through a large-bore needle in a conscious, trained, cooperative animal [33]. Whereas one of the contraceptive mechanisms of action of progestins is the prevention of implantation through thinning of the endometrial lining, no effect of etonogestrel on endometrial thickness and uterine volume has been observed in macaques following implantation with Implanon^®. Nevertheless, etonogestrel appears to be effective in macaques, as even one-third or one-fourth dosages of Implanon^® (68 mg etonogestrel) in rhesus macaques (n = 140) and cynomolgus macaques (n = 70) achieved a contraceptive efficacy exceeding 99%. Significantly lower serum etonogestrel concentrations were observed in rhesus macaques compared to cynomolgus macaques, likely as a result of differences in body weight [34]. In female common marmosets (n = 52) receiving one-fourth or one-third Implanon^® implants, contraceptive efficacy exceeding 99.3% was described. The usage of partial implants supports the observation that implants remain efficacious, even after being divided into smaller parts and stored for up to six months in the refrigerator [35]. It is generally considered that callitrichids require a higher dose of steroid hormone-based contraceptives. This is likely the result of higher endogenous steroid levels than those observed in Old World primates and prosimians. High endogenous steroid levels likely result in a decreased binding affinity of hormone receptors [36]. It has been suggested that dosages up to ten- to twentyfold of Old World primates’ dosages should be used for callitrichids [35]. Implanon^® has also been observed to be effective in female Barbary macaques (Macaca sylvanus) for at least four years [37], similar to what has been observed in rhesus macaques (3.1–5.0 years) (n = 14) and cynomolgus macaques (3.2–4.0 years) (n = 8) [34].

Reversal of contraception has been observed both spontaneously and following implant removal. In three rhesus macaques, a breeding male was introduced with the Implanon^® implant still in situ, resulting in parturitions at 6.3, 6.4, and 6.5 years post-etonogestrel implantation [34]. This duration of action exceeds the three-year duration in humans that is suggested by the manufacturer. In female chimpanzees (Pan troglodytes) implanted with Nexplanon^®, a reversal rate of 57% with a mean time of 71.2 months (range: 30.2–126.3 months) was observed in cases where the implant was left in place. This mean time to reversal significantly exceeds the Nexplanon^® three-year duration of effect in humans that is proposed by the manufacturer. Following implant removal in female chimpanzees, a reversal rate of 67% and a mean time to reversal of 11.6 months (range: 1.2–36.6 months) was observed [11]. Implant removal effectively reversed contraception in common marmosets implanted with one fourth or one third Implanon^® implants, with five out of six marmosets regaining fertility [35]. In cynomolgus macaques, a 100% pregnancy rate (n = 14) was achieved following implant removal, of which 78.5% resulted in uneventful deliveries, as compared to 90% in the whole breeding colony. The earliest conception occurred approximately 18 days following removal of the implant in a cynomolgus macaque [34].

A significant increase in bodyweight was observed in both humans and common marmosets over a one-year period following implantation. This is likely due to multifactorial causes [35,38]. At the moment, there is no strong evidence to suggest a negative impact of etonogestrel implantation on established pregnancy [35,39].

Following Implanon^® administration, no behavioral abnormalities were reported in common marmosets [35]. In Barbary macaques, however, the administration of Implanon^® resulted in increased aggression, as well as increased reception of grooming and reduced grooming of conspecifics [37]. It has been suggested that anxious animals solicit grooming from others to regulate their emotional state. Furthermore, it has been observed that giving grooming is associated more strongly with lower stress levels than receiving grooming. As a result, reduced time spent grooming others may serve as a valuable indicator of increased stress in Barbary macaques [40]. Moreover, female Barbary macaques treated with Implanon^® displayed higher rates of anxiety-related behaviors, such as self-scratching and self-grooming [37]. These welfare implications should be considered when selecting an appropriate contraceptive strategy in this species.

2.3.2. Levonorgestrel (Norplant^®, Norplant-2^®, Jadelle^®)

Levonorgestrel is a long-acting, potentially reversible, synthetic progestin that has been issued most commonly in the form of a subdermal implant (Norplant^®, Norplant-2^®, Jadelle^®). Norplant^® implants can be inserted through a large-bore needle in a conscious, trained, cooperative animal [33]. Whereas Norplant-2^® and Jadelle^® consist of two rods, the Norplant^® implant consists of six separate rods. In chimpanzees, levonorgestrel implants were observed to prevent pregnancy by the direct inhibition of ovulation or by inducing luteal phase inadequacy for at least three years (n = 7). During the first three months, sexual cyclic swelling was most disrupted, but genital swelling resumed as levonorgestrel concentrations declined [41]. In white-faced saki (Pithecia pithecia), levonorgestrel was observed to completely block ovulation for at least eight months, and in some females for up to two years following implantation [42]. Levonorgestrel is considered to be reliable and effective in cotton-top tamarins (Sanguinus oedipus), as the administration of 68.44 ± 8.61 mg/kg levonorgestrel as a biodegradable gel formulation eliminated ovarian cycles for up to 19–50 weeks in this species. This gel formulation consisted of polylactic-co-glycolic acid, triethyl citrate, and N-methylpyrrolidone, and was injected subcutaneously between the scapulae. A benefit of the administration of levonorgestrel as a gel is that the implant cannot be removed by conspecifics [43]. Implant removal by conspecifics has been suggested to contribute to the reported low effectiveness of Norplant^® in female hamadryas baboons, where the implant only prevented pregnancy for more than a few months in 54% of animals. It is important to note that these implants were inserted between the scapulae, and that the location of implant placement may affect the implant removal rate [1].

Reversibility has been observed in one cotton-top tamarin, which resumed cycling at 117 days post-injection of a levonorgestrel containing biodegradable gel formulation, after which she successfully conceived. All other studied cotton-top tamarins similarly returned to cyclicity but were not allowed to breed subsequently [43]. Spontaneous return to cyclicity has been reported in a white-faced saki, who started sexual cycling 5.8 months after implant insertion, although she did not conceive. Following implant removal in two other white-faced sakis, both females conceived within three ovulatory cycles [42].

Few adverse effects of levonorgestrel administration have been reported, although a significant increase in bodyweight was observed in chimpanzees following the administration of Norplant^® (n = 7) [41]. Moreover, abscess formation at the injection and deposition sites have been observed in observed in cotton-top tamarins injected with a levonorgestrel-containing biodegradable gel [43].

Levonorgestrel has strong progestational effects and lacks estrogen-like activity, which may aid in maintaining species-typical sexual behaviors [41,44]. It has been suggested that levonorgestrel does not mask physical and behavioral signs of estrus, and females are observed to undergo more consecutive estrus cycles, resulting in more females being in estrus at any one time. When female hamadryas baboons exhibit signs of estrus, group stability may suffer as aggression and other agonistic behaviors may increase. However, no change in the frequency of self-directed behaviors was observed in these baboons, suggesting a minimal impact on stress levels [1]. Effects on estrus behavior have, however, been reported in female chimpanzees, where a decrease in sexual behavior has been observed animals treated with levonorgestrel-containing pills (Ovranette^®) or implants (Norplant^®) [45].

2.3.3. Medroxyprogesterone Acetate (Depo-Provera^®)

Whereas most contraceptives are available as an implant, the most frequently used form of MPA is a slow-release suspension (Depo-Provera^®). This suspension has the advantages of being delivered by injection, without the need for anesthesia or surgery. Subsequently, there is no action required for implant removal in case treatment is terminated. MPA can also be administered as an implant, however, implant failure has been reported following implant loss due to exacerbated grooming behavior in chimpanzees [33].

MPA appears to block follicular maturation but does not completely inhibit follicular growth, as suggested by observed decreased peak plasma estradiol concentrations following treatment in cynomolgus macaques [46]. MPA treatment may be effective when administered during any phase of the ovarian cycle [46]. In seasonally breeding NHPs, such as prosimians, it has been recommended to provide the first injection at least one month prior to the breeding season [47].

Reversible seasonal contraception was achieved in seven female black lemurs (Eulemur macaco) injected with either 10 mg/kg MPA at 90-day intervals or 2.5 mg/kg at 30-day intervals, with only one animal showing signs of estrus at 53 days post-injection with the 10 mg/kg dose [48]. As a result, it is recommended to use a 5 mg/kg at 36-day interval dosage regimen in this species [48]. Interestingly, a lower dosage of 7.5 mg/kg was ineffective in preventing conception in female cynomolgus macaques [46]. However, a single SC MPA injection dosed at 15 mg/kg reversibly suppressed ovulation in this species (n = 5) for up to 161 ± 17 days [46]. In stump-tailed macaques, a single intramuscular injection of 30 mg (3.7–4.5 mg/kg) MPA was observed to inhibit ovulation for at least three months [49]. In hamadryas baboons, injection of 3.5 mg/kg MPA successfully prevented cycles of perineal tumescence for 51–98 days, with younger animals resuming regular sexual cycling earlier than older females [50]. In great apes, the administration of 150 mg (estimated 3.2–4.7 mg/kg) Depo-provera^® to chimpanzees every two months yielded disappointing results, as this dosage effectively blocked sexual cycles in some individuals but failed to do so in others. This may have been the result of inadequate dosing, which could be the result of differences in MPA metabolism between chimpanzees and women, in which the 150 mg dose is reported to inhibit ovarian activity for 4–6 months. High individual variability in efficacy was observed, as depot MPA administration once every two months blocked sexual cycles in some chimpanzees, while monthly administration failed to do so in others. It has, however, been suggested that MPA implants may be more effective in chimpanzees than injectable MPA depots and may be efficacious for up to two years in this species [33].

MPA is also reported as a reliable contraceptive in female white-faced marmosets (Callithrix geoffreyi) (n = 10), with single 20 mg/kg intramuscular injections given every 30 days, resulting in the inhibition of ovarian cyclicity; although two pregnancies were observed, these may have been the result of contraceptive failure [51].

Depo-provera^® appears to be readily reversible in chimpanzees, with first conception occurring as soon as two months following the last injection [33]. The time to reversal may, however, be as long as two years in chimpanzees, but is considered to be three months on average [11].

Several reported cases of stillbirths in chimpanzees have been associated with the use of Depo-provera^® during pregnancy [33]. Weight gain, as compared to untreated male (n = 15) and female (n = 3) conspecifics on the same diet, was also noted following Depo-provera^® treatment [33]. It is worth mentioning, however, that no significant difference in weight gain was observed when the weight of the same females was compared to the period during which they were contracepted with intrauterine devices [33]. Moreover, it should be noted that no strong correlation between the usage of progestin-based contraceptives and weight gain has been established in humans, and only limited evidence is available for an effect of MPA on bodyweight [52]. In prosimians, no effect on bodyweight was observed in female black lemurs, although this may be the result of the treatment duration being limited to the breeding season [48]. MPA contraception in seasonally breeding species may prolong the breeding season, as has been observed in black lemurs. Nonetheless, potential adverse effects may be reduced by limiting contraceptive treatment to the extended breeding period. An interesting effect of MPA treatment in female black lemurs was darkening of the pelage, which reverted after cessation of treatment [48]. This shift towards male-type coloration is likely the result of MPA’s androgenic activity [53].

Increased aggression following MPA treatment has been observed in different NHP species. This increased aggression has been attributed to the androgenic activity of MPA. Female Depo-provera^®-treated stump-tailed macaques engaged in more subordinate behaviors and antagonistic behaviors toward males, and they directed more aggression towards younger animals. In spite of the effects of MPA on agonistic behavior, no effect on the dominance hierarchy was observed [54]. In cynomolgus macaques, an increase in male-male aggression was observed with weekly injections of 40 mg MPA, although a decline in aggression towards females was observed at the same time. Both findings accompanied a decline in male sexual activity. Neither the decline in sexual activity nor the changes in antagonistic behaviors were altered when macaques were supplemented with testosterone, suggesting that the behavioral effects of MPA are not mediated by the suppression of circulating testosterone [55]. Similar effects were observed in ovariectomized Southern pig-tailed macaques (Macaca nemestrina) (n = 6), in which MPA was associated with aggressive behavior as quickly as 3–7 days following MPA administration [56].

Decreased sexual activity has been observed in multiple NHP species following MPA treatment. In stump-tailed macaques, MPA injection in females resulted in a preference of males for untreated females, but no impact on female proceptive behavior was observed [49,54]. Male ring-tailed lemurs similarly preferred the scent of untreated females over that of treated females, likely because contracepted females had reduced chemical distinctiveness compared to non-contracepted females [57]. It has been demonstrated that lemurs maintain semiochemical patterns across years, facilitating the long-term recognition of individuals [58], which suggests that MPA interference with semiochemical patterns may have implications for a broader range of odor-mediated social behaviors [57]. Olive baboons are reported to alter their vaginal odor pattern during the sexual cycle, which could play a role in fertility signaling [59]. Although this has not yet been investigated in hamadryas baboons, male hamadryas baboons were observed to copulate less frequently with Depo-provera^®-treated females (3.5 mg/kg), likely as a result of the females exhibiting reduced visual signs of estrus. No effects on social interactions such as grooming, aggression, and affiliation were observed in this species [50]. In chimpanzees, MPA causes a near-total inhibition of cyclic sexual swelling, possibly impacting sexual behavior [33].

2.3.4. Melengestrol Acetate

The administration of an MGA implant requires a small surgical incision, necessitating anesthesia. The implant takes effect after one or two weeks [3]. Implant loss has been reported, most frequently as a result of grooming of the implant site. It was observed that intramuscular implant placement, as opposed to subcutaneous placement, significantly reduced implant loss in hamadryas baboons. Additionally, intramuscular placement reduced the migration of implants away from the insertion site, facilitating implant removal later on [60].

MGA successfully prevented pregnancy in eight common marmosets during both 6–8 month and 19–21-month treatments. Two pregnancies were observed during the treatment period, although conception had likely already occurred before MGA implantation. One pregnant female carried to term, and the other one aborted, likely due to vaginal infection. This suggests that MGA is only effective when implanted before ovulation occurs [61]. MGA is reportedly effective in lemurs, although implant loss with subsequent unwanted pregnancy has been reported [62]. The large subcutaneous protruberance caused by the implant increases the risk of implant removal or local infection with subsequent implant loss due to allogrooming of the implant site [3].

A marked difference in reversibility has been observed between female golden-headed tamarins (Leontopithecus chrysomelas) and the closely related golden lion tamarins (Leontopithecus rosalia). Following implant removal, a return to reproduction of 28.6% (n = 2 out of 7) within two years was observed in golden-headed tamarins. In golden lion tamarins, a return to reproduction of 61.9% (n = 13 out of 21) within 2.5 years of implant removal was observed [63,64]. In female Colobus monkeys (Colobus guereza), MGA implants were observed to be readily reversible if removed. However, reproductive suppression was maintained beyond the 2-year minimum duration of efficacy in individuals in which the implant was left in place. No significant difference was observed in the probability of reproduction between non-implanted and implant-removed groups, but the probability of reproduction was significantly lower for the implant-expired group [65]. In female hamadryas baboons, MGA implants may be reversible upon removal. A study in female hamadryas baboons discovered that the probability of return to reproduction demonstrated a plateau phase at 63% in this species, which was reached after 40 months. In non-contracepted females, however, a plateau of 93% after 84 months was observed. In females with implants left in place, continued reproductive suppression was observed in all individuals for at least 5.75 years, after which the study discontinued [61].

Marked adverse effects on the reproductive system have been observed in New-World monkeys. All Goeldi’s marmosets (Callimico goeldii) (n = 17) implanted with 88–267 mg/kg MGA were observed to have enlarged, firm, and distended uteri, with desiccated, friable luminal content. Three marmosets required ovariohysterectomy due to clinical symptoms related to these uterine lesions [66]. Similar observations have been made in squirrel monkeys (Saimiri sciureus) (n = 3) implanted with 109–376 mg/kg MGA, which demonstrated similar uterine lesions to the Goeldi’s marmosets, but with less necrotic debris and suppuration, and only slight invasion of the uterine wall. This excessive decidualization is thought to result from the progesterone-like action of MGA on endometrial stroma cells. Excessive decidualization may be reversible upon implant removal in cases where there is no luminal distension with necrotic debris and limited inflammation. However, it is highly unlikely that animals will return to fertility once extreme uterine distension has led to clinical symptoms [66]. Endometrial hypertrophy and decidualization without concurrent depression of follicular development has similarly been observed as early as one month following implantation in common marmosets (n = 7). In this species, uterine changes disappeared within three weeks following implant removal, and all females conceived within four months [61]; however, this does not necessarily imply that uterine changes are readily reversible following longer treatment periods or in other species.

Decreased litter size and increased infant mortality have been observed in female golden-headed lion tamarins following implant removal [63]. In golden-lion tamarins, an adverse effect on litter size was only observed in females in which the implant was not removed [64]. There is little evidence to suggest an effect of MGA on uterine pathology in golden-lion tamarins, as has been reported in Goeldi’s monkeys and squirrel monkeys, but infrequent reports exist for the closely related golden-headed lion tamarin [63,64,66].

The proportion of stillbirths was similar for implant-removed and implant-expired groups of Colobus monkeys. There was, however, a significantly higher stillbirth rate for contracepted females compared to untreated females. It is possible that this observation is, however, the result of a confounding factor, as the average age was higher in the contracepted groups as compared to the untreated group [65].

Another adverse effect observed in common marmosets is substantial weight gain following treatment [61]. This has also been reported in hamadryas baboons, where body weight was observed to increase anywhere between 17.3 and 47.6% during the treatment period, in spite of a concurrent increase in time spent foraging [67].

Behavioral effects appear to be limited with the use of MGA, as no significant alteration in social dynamics has been observed in hamadryas baboons when dosed at $\ge$8 mg/kg. Baboons treated with MGA displayed higher rates of sexual activity, yet less affiliative interaction was received and a tendency towards increased agonistic behavior was observed. Moreover, MGA reduced cyclic perineal swelling in all females, but did not completely eliminate it in any but the youngest individuals [67]. No effects on behavior were observed in chimpanzees following the administration of MGA implants [45].

2.3.5. Progestin or Combined Progestin–Estrogen Contraceptive Pills

Oral contraceptive pills (OCPs) constitute a reversible method of contraception that may be administered as a daily tablet. Combined oral contraceptives (COCs) are most frequently used and consist of a synthetic progestagen with an added estrogen. Alternatively, progestin-only pills (POPs) are available. Although daily administration may be seen as a drawback, pills are easily crushed up in a treat or food item for administration. The efficacy of oral contraceptives is high, particularly in great apes [68], although failure of daily delivery could lead to unwanted pregnancy. Whereas OCPs are most frequently administered to apes, they may also be broken up into smaller pieces and administered to other NHPs.

Oral contraceptive pills may be administered continuously, or a placebo period may be incorporated into the dosing regimen. Whereas POPs require daily administration, a placebo period may be considered with the use of COCs. With COC usage, no increase in endogenous progesterone was observed during a hormone-free week in Western lowland gorillas (Gorilla gorilla gorilla) [69], suggesting that there appears to be no need to implement placebo weeks at all from a reproductive physiological point of view [70,71]. Placebo periods may, however, be incorporated into the regimen to allow for a periodic expression of estrus behavior. This may be desirable in some species but could contribute to increased aggression among competing males in others [3]. For example, the expression of estrus behavior is of importance in establishing group hierarchy and regulating social tension in bonobos (Pan paniscus) [72]. Moreover, bonobos are observed to receive more grooming during periods of cyclic perineal skin swelling [73]. When oral contraceptives are administered synchronously to multiple females in a group, for instance in Western lowland gorillas, the implementation of a placebo period may increase the risk of multiple females exhibiting sexual behavior concurrently, with a subsequent risk of social disruption in the group [74]. Moreover, if a placebo week is desired, it must be recognized that the menstrual cycle length of some NHP species differs from that of humans, and OCP regimens incorporating a placebo week may be adapted accordingly to best suit the natural cycle length of the NHP species in question. For example, a regimen accounting for an inter-menstrual interval 34 [75] or 35 days [76] may be considered for bonobos [68].

Oral contraceptive pills are readily reversible, with 64% of great apes resuming to cyclicity within six months following the cessation of OCP treatment, and all individuals taking longer than one month to resume cycling. On average, bonobos gave birth 2.84 years after cessation of oral contraceptive pill treatment [68].

Common progestins used in human COCs and POPs are norethindrone, desogestrel, and drospirenone [77]. It is recommended to be cautious when selecting a contraceptive pill containing drospirenone, as conflicting evidence exists surrounding the potential risk of venous thromboembolism with the use of drospirenone-containing OCPs [78,79]. Most COCs administered to great apes contain ethinyl estradiol (EE) as opposed to mestranol, estradiol valerate, or 17β-estradiol [68]. In contrast to the progestins, it appears that the concentration of the estrogen rather than the derivative itself is the most important factor influencing contraceptive efficacy. High estrogen doses are associated with cardiovascular risks, such as venous thromboembolism, whereas low doses are associated with bleeding irregularities [80,81]. Due to the associated adverse effects, it has been recommended to use combined oral contraceptives with the lowest dose of estrogen that is effective in that individual [25].

No significant adverse health effects have been observed in bonobos with long-term oral contraceptive use [68]. Nevertheless, the European Association of Zoos and Aquaria (EAZA) Reproductive Management Group (RMG) does not currently recommend COCs during the first year of lactation, as it is thought that the estrogen component may have a suppressive effect on milk production, similar to what has been observed in humans [82]. In contrast to some other progestins, no effects on body weight were observed in bonobos during OCP treatment [68].

In Western lowland gorillas, the use of COCs was associated with a shift in sexual and reproductive behavioral timing away from the mid-cycle, towards the first week of the cycle. It is important to note that the used contraceptive regimen included a placebo week [69,83]. No changes in the exhibition of affiliative or aggressive behavior was observed during the treatment period [83]. Behavioral changes were, however, observed in bonobos, including a reduction in sexual behavior, a perceived negative effect on emotional state, as well as the observation of dominant females becoming subordinate [68]. Some degree of perineal swelling was maintained in bonobos during the treatment period [68]. In chimpanzees, COC treatment consisting of estradiol and norethindrone was associated with a complete elimination of mating behavior in male–female pairs with weak social bonds, and it was observed to reduce mating behavior in those with strong social bonds [84]. In cynomolgus macaques, no effect of COC treatment on aggression has been observed, although the usage of norgestrel as a progestin has been associated with a reduction in affiliative behavior as opposed to the usage of ethynodiol diacetate. Moreover, COC treatment was observed to reduce the frequency at which dominant females used aggression to terminate the sexual actions of subordinates [85].

Triphasic pills attempt to mimic cyclic rising and falling estrogen and progesterone concentrations [86]. There is currently, however, insufficient evidence to favor the use of triphasic pills over monophasic pills and, as a result, monophasic pills are recommended as the first choice of OCPs in great apes [68,86]. Triphasil^®, a triphasic pill containing levonorgestrel and ethinyl estradiol, has been associated with an increase in the reception of contact aggression in cynomolgus macaques. However, these females also spent more time in close proximity to conspecifics [87].

Progestin-only pills are considered to be safer than some COCs due to concerns about estrogen-related adverse effects. However, POPs are reported to be less effective than COCs [88] and are primarily recommended over COCs in women who are nursing. Although POPs are currently rarely used in great apes, they may be used to prevent pregnancy during the lactation period. Moreover, due to a presumedly reduced risk of thrombosis and heart disease, POPs are also suggested to be used in pubertal, juvenile, and older females [68].

2.4. Progesterone Receptor Antagonists

In spite of the implementation of contraceptive techniques, pregnancies may still occur due to contraceptive failure. In these instances, the termination of unwanted pregnancies may be considered and antiprogestins may offer a route of pregnancy termination, which is the main indication of these drugs. Additionally, antiprogestins may be used as emergency contraceptives.

Mifepristone (RU-486) is a competitive progesterone and glucocorticoid receptor antagonist that is used to terminate early intrauterine pregnancy in humans [89,90]. Mifepristone affects the epithelial and vascular physiological homeostasis of the endometrium, rendering it hostile to blastocyst implantation [91]. Moreover, mifepristone causes decidual necrosis and detachment of the conceptus, as well as regression and necrosis of the endometrial capillaries [89].

Whereas mifepristone is dosed orally in humans, this route did not result in the reliable termination of pregnancy in cynomolgus macaques. However, it was demonstrated that daily intramuscular administration of 1 mg/kg mifepristone in ethanol successfully inhibited ovulation in five out of six cynomolgus macaques [92]. As a result, mifepristone may be used as an emergency contraceptive. It has been observed that daily SC administration of 2 mg mifepristone during days 16–18 of the menstrual cycle provided protection against pregnancy establishment in rhesus macaques [93], and a 100% efficacy in the prevention of pregnancy establishment was observed in rhesus macaques treated with a single dose of 31.25 mg SC mifepristone 78 h post-mating (n = 31) [91]. Once pregnancy has been established, however, mifepristone may still be used to terminate the pregnancy. The efficacy of intramuscular mifepristone in terminating established pregnancy has been observed to be similar to that of oral mifepristone in human studies when dosed at 20 mg/kg as a single injection [94]. Mifepristone may be more effective when combined with a prostaglandin, such as misoprostol, which may induce myometrial contractions resulting in fetal expulsion [89]. When used as an abortifacient, mifepristone should be used during the early stages of pregnancy. In cynomolgus macaques, the abortive success rate was observed to decrease with gestational age [94].

No effects on hormonal profiles or bleeding patterns were observed in rhesus macaques treated within five days after ovulation, although no vaginal bleeding was detected when mifepristone was administered to rhesus macaques 5–8 days after ovulation [93]. Mifepristone treatment has been associated with adverse effects, such as pain, diarrhea, vomiting, hyporexia, and dehydration in cynomolgus macaques [94]. No effects were observed in two infant cynomolgus macaques who survived attempts to terminate pregnancy with mifepristone [95].

Another antiprogestin, ZK 137 316, was demonstrated to successfully prevent pregnancy in rhesus macaques when administered daily as an intramuscular injection dosed at 0.03 mg/kg [96]. ZK 137 316 suppresses menstruation while maintaining normal follicular phase hormonal profiles. Moreover, this antiprogestin was observed to be reversible with comparable pregnancy rates between treated and control animals after a year of treatment [96]. The requirement for daily injections, however, poses a significant practical challenge for the use of this compound in the management of captive NHPs. So far, the use of ZK 137,316 in NHPs has been entirely experimental, and no commercial formulations are available.

2.5. Testosterone Derivatives

Testosterone is one of the few options available for contraceptive management in male NHPs. It has been observed that exogenous testosterone suppresses gonadotropins and intratesticular steroid production [97]. In cynomolgus macaques, intramuscular injections of 20 mg/kg testosterone buciclate managed to reversibly abolish serum LH bioactivity in an experimental setting. Serum concentrations of FSH and inhibin-α were suppressed in a dose-dependent manner, resulting in severe oligozoospermia and transient azoospermia [98]. In male common marmosets, however, testosterone undecanoate and norethisterone enanthate failed to suppress spermatogenesis and did not decrease fertility. It has been hypothesized that this lack of efficacy may be because androgens play a smaller role in the regulation of reproductive organs in callitrichids compared to other NHPs and are primarily used for the maintenance of social status. Furthermore, a low serum concentration of testosterone and chorionic gonadotropin may be sufficient to maintain spermatogenesis in this species [97].

2.6. Phyto-Estrogens

Pueraria mirifica is a Thai herb that contains phyto-estrogens. A dose-dependent increase in menstrual cycle length was observed in female cynomolgus macaques following the daily oral administration of Pueraria mirifica, which corresponded with a dose-dependent decrease in serum profiles of FSH and LH. At the highest treatment dose (1000 mg), the complete suppression of FSH and LH serum profiles was observed for the 90-day treatment period and 60-day post-treatment period. No return to cyclicity has been reported in animals treated with 1000 mg during the 60-day post-treatment period. It is hypothesized that Pueraria mirifica works directly through interaction with estrogen receptors at the levels of the pituitary gland and/or ovaries, resulting in negative feedback on pituitary FSH and LH secretion. The active compounds are yet to be determined, but isoflavones are suspected to be responsible for the observed effects of Pueraria mirifica on reproductive physiology [99]. To date, Pueraria mirifica has only been used within an experimental setting. Moreover, although no adverse effects were reported, it is generally advisable to keep the dose of the estrogen component of a contraceptive regimen as low as possible, in order to minimize the risk of estrogen-related adverse effects, such as cancer and cardiovascular disease [100]. As a result, we do not currently recommend the use of phyto-estrogens as contraceptive agents in NHPs.

3. Non-Hormonal Contraception

3.1. Immunocontraception

Immunocontraception has been effectively applied in a wide range of mammal species, including wild, zoo, and domestic animals [101]. Contraceptive vaccines may vary in their mechanism of action. Some, such as Zona pellucida (ZP) vaccines or sperm-associated protein epididymal protease inhibitor (EPPIN)-based vaccines directly target gametocytes, whereas GnRH vaccines work through suppression with the HPG axis (Figure 1). Depending on the mechanism of action, immunocontraception may be utilized in females (ZP vaccines), males (EPPIN based vaccines), or both sexes (GnRH vaccines).

3.1.1. Zona Pellucida Vaccines

Zona pellucida vaccines induce circulating antibodies that inhibit sperm binding to oocyte ZP. The ZP serves to enable the acrosome reaction for the successful adhesion and penetration of the oocyte by the spermatozoa [101]. Zona pellucida antigens are tissue-specific and noncirculatory and, as a result, systemic immune-related complications may be avoided [102]. Nevertheless, adverse effects have been reported, such as ovarian pathology, which includes oophoritis in cynomolgus macaques [103], and the irreversible, premature loss of ovarian function due to demise of the primordial follicle pool in common marmosets [104]. No effects on reproductive hormones have been reported, however, allowing females to continue to cycle and display sexual behaviors [105].

The immunogenic and antigenic properties of the limited repertoire of ZP glycoproteins are highly complex and vary among species [106]. As a result, it is likely that species-specific interactions play an important role in ZP vaccine efficacy, making observations difficult to extrapolate to other species.

The successful inhibition of ovarian cyclicity was observed in cynomolgus macaques immunized with rabbit 75-kd ZP recombinant protein, but not in animals immunized with 55-kd ZP recombinant protein. The 75-kd ZP protein was observed to inhibit sperm binding to the ZP without reported effects on endocrine function or reproductive morphology. A benefit of recombinant proteins, as opposed to isolated (porcine) ZP proteins, is the absence of a risk of contamination with eukaryotic proteins, potentially impacting vaccine efficacy [92].

Zona pellucida proteins from other NHP species are similarly promising, as three monthly immunizations with recombinant bonnet macaques (Macaca radiata) ZP glycoprotein-B expressed in Escherichia coli managed to induce a high antibody titer in female olive baboons (n = 4). Immunized animals demonstrated no alterations in cyclicity but failed to conceive. The immunization protocol was observed to be reversible, as animals conceived four to seven cycles after the last immunization, concurrent with a decrease in antibody titers [107].

Currently available porcine ZP vaccines have not been shown to be effective in primates [104]. The successful immunization of NHPs using ZP proteins derived from rabbits or other NHP species emphasizes the potential of ZP vaccines as reversible contraceptive methods with a minimal impact on reproductive physiology and animal behavior, although these vaccines have only been used experimentally thus far. Future research is required to identify proteins that reliably induce contraceptive immunoglobulins in specific target NHP species.

3.1.2. EPPIN Based Vaccines

Another target for immunocontraception constitutes EPPIN, which is also targeted by the contraceptive drug EP055 [108]. Immunization with human recombinant EPPIN was observed to induce titers >1:1000 in 78% (7 out of 9) male bonnet macaques. None of these animals managed to impregnate a female during the treatment period, in which booster vaccinations were given every three weeks. Following the cessation of booster immunization, 71% (5 out of 7) of animals returned to fertility, with the time to return to fertility ranging from 15 to 180 days [109]. To date, EPPIN-based vaccines have only been used experimentally in NHPs, and future research may focus on evaluating the safety, efficacy, and reversibility in different NHP species.

3.1.3. GnRH Vaccines

Neutralizing GnRH antibodies work by inhibition of GnRH-mediated LH and FSH secretion. Subsequent decreased sex hormone concentrations may result in behavioral and reproductive physiological alterations [110], similar to those described for GnRH agonists and antagonists. In contrast to GnRH agonists, however, no initial stimulation phase occurs. Although GnRH is non-immunogenic, immunization may be achieved when it is conjugated with an appropriate carrier protein [101]. Whereas a large variety of structural forms of GnRH have been identified, mammalian GnRH-1 is typically used in contraceptive vaccines [111]. Because the GnRH receptor has >85% sequence conservation among species, the mammalian GnRH-I sequence may be used for contraceptive immunization in a wide range of mammals [101]. As a result, GnRH vaccines have been successfully utilized in a wide range of mammalian species, such as Asian elephants (Elephas maximus) [112], elk (Cervus elaphus) [113], swine (Sus scrofa) [114], domestic cats (Felis catus) [115], and domestic dogs (Canis lupus familiaris) [116]. However, little information exists on its usage in NHPs, although experimental trials have been performed in rhesus macaques in which a decrease in peripheral gonadotropin levels was observed following immunization [117]. The vaccine used in this trial, however, was never marketed. The use of commercially available GnRH vaccines has only been reported anecdotally captive baboons [118].

Although the reversibility of GnRH vaccines has been reported in other species following a gradual decrease in antibody titres [119,120], no information exists on the reversibility of GnRH vaccines in NHPs.

Gonadotropin-releasing hormone vaccines constitute an interesting target for contraception in both male and female NHPs. However, in spite of the availability of different commercial vaccines (Improvac^®, Improvest^®), no literature exists on the usage of these more novel vaccines in NHPs. Further research evaluating the efficacy, adverse effects, and reversibility of GnRH vaccines in NHPs is required before any claims can be made about its potential as a population management tool.

3.2. EP055

EP055 is a small organic compound that inhibits sperm motility through interaction with EPPIN. Four cynomolgus macaques were infused with a 75–80 mg/kg dose, followed by a 125–130 mg/kg dose. A complete cessation of sperm motility was observed within 30 h of treatment. The recovery of sperm motility was observed in all animals by 18 days post-infusion, although the recovery of sperm velocity required more than three weeks. No adverse effects were observed, although a rise in ALT and glucose levels was observed during treatment. This agent has solely been used in an experimental setting. Future research may focus on the efficacy and adverse effects of long-term EP055 administration [108].

3.3. Triptonide

Triptonide is a natural diterpenoid compound purified from the Chinese herb Tripterygium wilfordii. This compound has thus far only been used in experimental settings. Triptonide has been demonstrated to induce reversible male infertility in five to six weeks when dosed orally at 0.1 mg/kg daily in cynomolgus macaques through the induction of sperm deformity associated with minimal or no forward motility. Although the exact mechanism of action is unknown, triptolide, a structurally similar compound extracted from the same herb, is thought to act through the suppression of the spermatogenesis-related genes spe-10, spe-15, fer-1, and folt-1 to inhibit morphological development of spermatids [121]. Following the cessation of treatment, fertility may be regained in four to six weeks, with no reported lasting effects on testicular histology, sex hormone levels, or sexual behavior. Although no toxic effects were reported in this study, further pharmacokinetic and toxicological studies may be conducted to assess safety and efficacy at different dosages and with longer treatment regimens [122].

3.4. Prostaglandin Receptor Antagonists

A prostaglandin E2 receptor (PTGER2) antagonist was observed to block periovulatory follicular activity without affecting the menstrual cycle or steroid hormone patterns in female cynomolgus macaques (n = 9) when administered at 10 mg/kg subcutaneously twice daily for five months. No adverse health effects were observed during the six-month trial, and fertility was recovered as soon as one month after the cessation of treatment. While the PTGER2 antagonist significantly reduced pregnancy rates, two pregnancies occurred during the latter half of the treatment period, possibly as a result of the upregulation of metabolic pathways in response to the drug [123]. The use of PTGER2 antagonists has not been reported outside of an experimental setting.

3.5. Prostaglandin F2α Analogues

Cloprostenol is a synthetic prostaglandin F2α analogue that prevents pregnancy by inducing luteolysis, with a subsequent inhibition of progesterone, estradiol, and inhibin A synthesis and release [124,125].

In white-faced marmosets, luteolysis was observed when cloprostenol was administered after day 5 post-ovulation, but it failed to induce luteolysis during the early luteal phase (days 1–4) [51], at which point the corpus luteum is still considered to be immature [126]. In common marmosets, it has been suggested that administration between days 10 and 17 of the ovarian cycle is most effective in inducing luteolysis [125]. When administered during pregnancy (days 21–64) in common marmosets, luteolysis was successfully induced in 87% of the conceptive cycles (n = 111), with successful treatments being administered earlier on average than unsuccessful treatments [126]. In white-faced marmosets, the administration of 0.75 μg intramuscular cloprostenol every 20–30 days, coinciding with ovulation, resulted in shorter cycle durations, but ovulation was detectable. Nevertheless, conception was effectively prevented in all animals, and ovarian cycles demonstrated similar hormonal properties to normative nonconceptive ovarian cycles observed in this species [51].

Cloprostenol treatment is readily reversible, and females are reported to conceive immediately following the cessation of treatment, with no reported adverse effects on infant health or embryo morphology [125,126].

No adverse behavioral or health-related effects of cloprostenol treatment have been described in common marmosets during long-term treatment [126]. However, high dosages (>30 mg) have been associated with short-term adverse effects, such as vomiting, defecating, straining, and tachypnea [125].

Although cloprostenol could be utilized as a long-term contraceptive agent in common and white-faced marmosets, monitoring of the ovarian cycle by means of endocrine monitoring, uterine palpation, and/or ultrasonography is required to appropriately time treatment [51,126]. Moreover, the use of synthetic prostaglandin F2α analogues as long-term contraceptive agents should be questioned in light of available alternatives, as side effects, such as nausea, vomiting, and diarrhea, have been reported in domestic species at dosages as low as 1 μg/kg [127,128]. Even as abortive agents, these drugs should be combined with an antiprogestin so that a lower dose may be used in order to reduce adverse effects.

3.6. COX-Inhibitors

Cyclooxygenase-2 activity is required for successful ovulation, as prostaglandins produced by COX-2 play an important role in the release of the oocyte at the time of ovulation [129]. As a result, the administration of the COX-2 inhibitor meloxicam was observed to reduce the rate of oocyte release without alteration in reproductive hormones or menstrual cycle length in female cynomolgus macaques when administered orally for five days around the time of ovulation at a dosage of 0.5 mg/kg [130]. In another study, oral administration of 0.5 mg/kg meloxicam was demonstrated to be 80% effective when administered once daily for five days following breeding in cynomolgus macaques (n = 11) [131].

Interestingly, it was observed that meloxicam failed to prevent pregnancy in cynomolgus macaques that were allowed to cohabit freely with male macaques in a monthly contraceptive model. This is likely the result of the mechanism of action of meloxicam, as it merely delays ovulation and does not prevent it [131].

Meloxicam may be suitable as an emergency contraceptive agent, although it is not registered for this indication and is not effective for long-term contraceptive management in NHPs. As meloxicam is frequently used in veterinary medicine for its analgesic and anti-inflammatory properties, the veterinary clinician should consider the aforementioned effects of meloxicam on the reproductive physiology when using this drug in NHPs.

3.7. Phosphodiesterase Inhibitors

The phosphosdiesterase 3 inhibitor ORG 9935, a carboximidamide derivative, reversibly inhibits oocyte maturation without affecting ovulation in female rhesus macaques at serum concentrations greater than 300 nmol/L [132]. ORG 9935 is hypothesized to act by preventing nuclear maturation and interfering with the resumption of meiosis in dominant follicles [133]. The contraceptive effectiveness of this compound is, however, yet to be proven, as no significant difference in pregnancy rates was observed in female cynomolgus macaques during a seven-month treatment period [134]. This compound has not been evaluated outside of an experimental setting, and no commercial formulations are currently available. Future research may focus on assessing the safety, reversibility, and effectiveness of this drug in different NHP species.

4. Conclusions

Both hormonal and non-hormonal contraceptives are viable tools in PMPs. There is a paucity of data for many NHP species commonly kept in captivity, underscoring the need for ongoing research into both hormonal and non-hormonal contraceptive agents. Furthermore, institutions are encouraged to share their PMPs and their experiences with reproductive management techniques.