Triptonide, a natural compound purified from the vine plant Tripterygium wilfordii Hook F, has been shown to possess reversible contraceptive effects in male mouse and primate models. The findings were published in Nature Communications.
Male contraceptives – decades of research, why the delay?
Despite considerable effort to develop male contraceptives over the past few decades, there has been limited success. This failure, in part, could be due to inadequate knowledge of spermatogenesis and male reproductive biology.
There is also concern about the potential adverse effects that may manifest as a result of blocking spermatogenesis. Therefore, rather than trying to inhibit the production of sperm entirely, Dr Wei Yan and colleagues took an alternative approach in their latest study. They investigated if it could be possible to disable the sperm that are being produced.
"The more we know about spermatogenesis, the more ideas we can have for developing the male pill," – Wei Yan.
"Thanks to decades of basic research, which inspired us to develop the idea that a compound that targets a protein critical for the last several steps of sperm assembly would lead to the production of non-functional sperm,” said Yan, in a recent press release. This approach, Yan explained, avoids severely depleting the number of testicular cells.
"We first reached the conclusion that proteins essential for sperm assembly serve as ideal drug targets for developing the male pill because disruptions of these proteins will not cause severe cell depletion within the testis, but instead lead to sperm deformation and consequently nonfunctional sperm in the ejaculates. Therefore, if you have a compound that targets such a protein, all sperm produced will be deformed and non-functional although a normal number of sperm are produced," said Yan.
The authors noted that previous studies using mouse models of male infertility were motivation to explore this area further. The studies investigated genes encoding proteins that are exclusively expressed in elongating and elongated spermatids (these cells undergo morphologic changes during spermatogenesis). In these experiments, knockout (KO) male mice were confirmed as being completely infertile. However, their testis weight, sperm count and testicular histology were comparable with/to “normal” mice. The animals’ infertility, therefore, was not due to a lack of sperm, but instead caused by sperm deformation or a “missing” component due to the KO that was essential for the correct functioning of sperm.
The results from the studies using KO mice indicated that “late spermiogenesis appears to lack a stringent ‘checkpoint’ for eliminating defective late spermatids”. Could this lack of “quality control” be exploited? To explore this, the team sought to identify existing drug compounds with sperm deformation reported as an adverse effect – the search revealed triptonide.
A once-daily oral dose
A once-daily dose of triptonide was orally administered to mice and cynomolgus monkeys (Macaca fascicularis). This induced the production of deformed sperm with “minimal or no forward motility” and subsequently infertility occurred in three to four weeks for treated mice and five to six weeks in treated monkeys.
"For mice, we fed drops of liquid containing triptonide through a gavage feeding tube. For monkeys, their favorite foods (banana or bread) containing drops of the drug solution were served as breakfast every morning," said Yan.
The authors noted that to develop a viable oral male contraceptive two key properties should be considered – reversibility of treatment and adverse effects.
Long-term triptonide treatment using single daily oral doses (0.8 mg/kg body weight) was investigated in mice (N=16) for three (n=8) and six months (n=8). In both groups, infertility persisted in all male mice administered triptonide. No apparent adverse effects were observed in the mice that had undergone long-term treatment.
Long-term efficacy testing was also performed in male monkeys (N=4) administered a daily oral dose of triptonide (0.1 mg/kg body weight) for up to 126 weeks. The authors noted that: “no major health issues were observed except that all displayed oligo-asthenoteratozoospermia” (OAT). OAT is a term used to describe the presence of the following three factors: low sperm count, poor sperm motility and abnormal sperm shape. Infertility was observed within eight weeks (as a result of OAT) and the data indicated that infertility could be maintained indefinitely in adult male cynomolgus monkeys at the dose outlined above.
Fertility was regained for both species within ~ four to six weeks after termination of triptonide.
Identifying triptonide’s target
As part of the study, the researchers also attempted to identify triptonide’s therapeutic target and its mechanism(s) of action. They concluded that it does not induce DNA double-strand breaks, a mechanism that had previously been proposed. In the paper's conclusion, the authors determined that the drug most likely binds junction plakoglobin/gamma-catenin and by doing so disrupts its interactions with spermatid maturation protein 1 (SPEM1) – which is required for cytoplasm removal during spermatogenesis.
"Our data suggest that triptonide enters the bloodstream and gets to the testis, where it binds a protein called JUP [Junction plakoglobin] and prevents JUP from interreacting with SPEM1. When SPEM1 does not function properly, all sperm produced are deformed, these deformed sperm cannot swim properly and thus can never reach and fertilize eggs," explained Yan.
"We are very excited that the new idea worked and that this compound appears to be an ideal male contraceptive. Our results using non-injurious studies on lower primates suggest triptonide will be an effective treatment for human males as well.”
“Hopefully, we will be able to start human clinical trials soon to make the non-hormonal male contraceptive a reality," concluded Yan.
Reference: Chang Z, Qin W, Zheng H, et al. Triptonide is a reversible non-hormonal male contraceptive agent in mice and non-human primates. Nature Commun. 2021;21:1253. doi: 10.1038/s41467-021-21517-5
Wei Yan was speaking with Laura Elizabeth Lansdowne, Managing Editor for Technology Networks.