Josamycin (EN-141) is a macrolide antibiotic exhibiting antimicrobial activity against a wide spectrum of pathogens, such as bacteria. The dissociation constant K_d from ribosome for Josamycin is 5.5 nM. IC50 & Target: Kd: 5.5 nM (ribosome)^[1] In Vitro: Studies show that the average lifetime on the ribosome is 3 h for Josamycin and that the dissociation constants for Josamycin binding to the ribosome is 5.5 nM. Josamycin slows down formation of the first peptide bond of a nascent peptide in an amino acid-dependent way and completely inhibits formation of the second or third peptide bond, depending on peptide sequence at a saturating drug concentration, synthesis of fulllength proteins is completely shut down by Josamycin. At a saturating drug concentration, synthesis of fulllength proteins is completely shut down by Josamycin^[1]. In Vivo: Blood and tissue levels of Josamycin after oral administration are 200 mg/kg to rabbits. Tissue levels are generally much higher than the blood levels, and 3 h after the administration, when the blood level is very low, the tissue levels are rather higher than those 1 h after the dose. One hour after the medication, the level in the lungs is the highest of all the tissue levels^[2].