Pyriproxyfen is photostable and is available in a variety of formulations that permit persistence in the coats of treated dogs and cats. An analytical study of pyriproxyfen administered orally to cats at doses up to 50 mg/kg was unable to detect any drug in samples of hair. However, dogs and cats treated topically on the dorsal midline of the neck with and mg/kg as a 1% solution of pyriproxyfen had sustained levels of pyriproxyfen in their hair at concentrations above mg/kg for more than 48 d. Not unexpectedly, there was a clear concentration gradient from the site of application to the hair of the hindquarters and considerable interanimal variation in hair concentration. The minimum concentration causing inhibition of development of flea eggs has been shown to be mg/kg in hair.
The ecdysteroid-binding pocket is located in the ligand binding domain of the EcR subunit , but EcR must be dimerised with a USP (or with an RXR) for high-affinity ligand binding to occur. In such circumstances, the binding of an agonist ligand triggers a conformational change in the C-terminal part of the EcR ligand-binding domain that leads to transcriptional activation of genes under ECRE control.  There is also a ligand-binding pocket in the corresponding domain of USP. Its natural ligand remains uncertain, and USPs appear to be locked permanently in an inactive conformation .