Mezlocillin Sodium (Baycipen) is a penicillin beta-lactam antibiotic used in the treatment of bacterial infections caused by susceptible, usually gram-positive, organisms.

Mezlocillin sodium has in vitro activity against gram-positive and gram-negative aerobic and anaerobic bacteria. The bactericidal activity of mezlocillin results from the inhibition of cell wall synthesis and is mediated through mezlocillin binding to penicillin binding proteins (PBPs). Mezlocillin is stable against hydrolysis by a variety of beta-lactamases, including penicillinases, and cephalosporinases and extended spectrum beta-lactamases. Mezlocillin inhibits the third and last stage of bacterial cell wall synthesis by binding to specific penicillin-binding proteins (PBPs) located inside the bacterial cell wall. Cell lysis is then mediated by bacterial cell wall autolytic enzymes such as autolysins; it is possible that mezlocillin interferes with an autolysin inhibitor. Unlike many other penicillins, mezlocillin is either extensively metabolized or is subject to biliary excretion, as only about 50% of the dose is accounted for in normal urine. [1]