Two compounds normally found in wild plants prevented human sperm from propelling into a woman’s egg and fertilizing it, according to a new study in the journal Proceedings of the National Academy of Sciences.

The study results show that the plant chemicals could potentially be offered as alternatives to hormone-based contraceptives commonly used now.

The first chemical, pristimerin, comes from the plant Tripterygium wilfordii or “thunder god vine.” The plant’s leaves have already been used as an antifertility drug in traditional Chinese medicine, and have also been used to help treat rheumatoid arthritis.

The other chemical, lupeol, is found in mango, aloe and dandelion root plants, and has been tested as an anticancer agent.

Researchers from UC Berkeley reported that both chemicals acted like “molecular condoms” to prevent fertilization in a human egg, but did not harm the egg or sperm in the process. They also did not present any harmful side effects, unlike some hormone-based contraceptives.

According to the study, human sperm move in a steady, rhythmic pattern until they reach the protective circle of cells that surround a woman’s egg, known as the zona pelucida. At this point, sperm’s movement switches to more of a whip-like motion to drive through the cell cluster and into the egg for fertilization.

This final “power kick” is initiated by a calcium channel known as CatSper, which floods the tail of the sperm with calcium and triggers the extra boost.

In previous research, the UC Berkeley researchers discovered that the hormone progesterone is key to opening the calcium channel. They determined that progesterone binds to a protein called ABHD2, which then opens the calcium channel.

Following up on this discovery, the researchers then searched for other chemicals that may bind to ABHD2 and either open the channel, or block it.

The team also scanned through books on natural contraceptives used by indigenous people and pinpointed pristimerin and lepeol as non-steroid chemicals that could be isolated from anti-fertility plants.

In tests, the chemicals effectively blocked progresterone from binding to ABHD2, which prevented the sperm’s final “power kick.”  

According to a release on the findings, human sperm take five to six hours to mature once they have entered the female reproductive system, which would be enough time for the chemicals to take effect if used as contraceptive agents.

The chemicals could serve as an emergency contraceptive taken either before or after intercourse, or as a permanent contraceptive via a skin patch or vaginal ring, the researchers said.

“Because these two plant compounds block fertilization at very, very low concentrations – about 10 times lower than levels of levonorgestrel in Plan B – they could be a new generation of emergency contraceptive we nicknamed ‘molecular condoms,'” said Polina Lishko, an assistant professor of molecular and cell biology, who led the team that discovered the anti-fertility properties of the two chemicals. “If one can use a plant-derived, non-toxic, non-hormonal compound in lesser concentration to prevent fertilization in the first place, it could potentially be a better option.”

The UC Berkeley team now plans to collaborate with researchers in Oregon to test the effectiveness of these chemicals in preventing primate in vitro fertilization. They also hope to find an inexpensive source of the chemicals as concentrations in wild plants are too low for cost-effective extraction.