US researchers have produced a male "pill" capable of temporarily making rats infertile. Cheun-Yan Cheng, from the Centre for Biomedical Research in New York and his colleagues have found a way to block the maturation of sperm cells in the testis. Whilst sperm are forming they attach themselves to a class of supporting "Sirtoli cells", which nurture their development. Cheng's group have identified a compound, called Adjudin (1-(2,4-dichlorobenzyl)-1H-indazole-3-carbohydrazide) which can selectively inhibit this attachment process, preventing the sperm from developing and rendering recipient rats temporarily infertile. However, administered on its own, the agent caused liver and muscle damage in experimental animals, so the researchers have now found a way to selectively target the drug just to the testis alone. They have linked the Adjudin to an altered form of the hormone FSH, which normally locks onto receptors in the testis and triggers sperm formation. And because FSH receptors are not found anywhere else in the body only tiny doses of the agent are required to temporarily stop sperm production for over 4 weeks in rats given the new formulation. The effect wore off by 8 weeks after the drug was washed out from the body. The team are now exploring the possiblity of producing a skin-patch capable of releasing the drug through the skin to avoid the necessity to administer it by injection.