Strophanthus gratus. Photo: Wikimedia Commons

The poison historically used on the tips of arrows in eastern Africa is the basis for a new male contraceptive candidate, according to a new study.

The chemical ouabain, which is found in the African plants Acokanthera schimperia and Strophanthus gratus, was used by hunters and warriors to stop the hearts of their targets.

An analogue of the chemical is found to essentially bypass the heart effects, and instead cripples the motility of sperm cells, researchers report in the Journal of Medicinal Chemistry, published by the American Chemical Society.

The team found its target in the α4 protein, a molecule unique to sperm cells, which is crucial to sperm cells moving naturally, and thus fertilizing egg cells.

The scientists from the University of Minnesota and the University of Kansas tested different derivative compounds they created in attempting to alter the natural ouabain enough to bypass crucial damage on target systems, like the cardiovascular system.

Through chemistry, they removed one of the sugar groups in the natural ouabain, and also replaced a lactone group with a triazole group, the ACS reports.

The 25th analogue tested had the desired effect of nullifying the α4 protein, and leaving the heart and other vital molecules alone, they found.

“Compound 25 significantly reduced the hyperactivation accompanying sperm capacitation by approximately 70 percent,” they write. “These results show that compound 25 not only reduces sperm motility in general, but also specifically interferes with the hyperactivation that sperm acquire when capacitated.”

Tests were performed on rat sperm, both in vitro and in vivo trials. They found it blocked off the function of mature sperm without permanently affecting the reproductive system.

“After administration to rats, compound 25 decreased sperm motility as soon as three days after administration and with doses as low as 5 mg/kg,” they write. “This suggests that the compound is reaching target cells and that its effects persist even after sperm is isolated from the rat epididymis.”