Kythera Biopharmaceuticals (a subsidiary of Allergan) is developing setipiprant, an oral prostaglandin D2 (PGD2/CRTH2) receptor antagonist, for the treatment of alopecia. Setipiprant was initially developed as a treatment for allergic rhinitis, but recent hair loss-related discoveries have made it a better candidate for hair loss reversal. As CRTH2 antagonist, setipiprant, blocks the effects of prostaglandin D2 (PGD2) role in inflammation and, in consequence, the amplification and maintenance of allergic reactions. It targets the allergic inflammation at the beginning of the cascade. In clinical trials for allergic rhinitis and asthma, the drug performed quite well in the treatment of allergen-induced airway responses in asthmatic patients. It was also well tolerated by participants. However, its results were similar to those of drugs already on the market, so further trials were discontinued. In 2012, researchers discovered a link between the PGD2 receptor and hair loss. More specifically, this receptor is seen at high levels in the scalps of men diagnosed with Androgenetic Alopecia (AGA). setipiprant steps in before PGD2 attaches to the receptors, and therefore prevents receptor activation and, as a result, hair loss. With the discovery of the possible link to PGD2 receptors, Kythera acquired the drug, and trials began to test the effects of setipiprant on hair loss.