A comparison of in vitro ADME properties and pharmacokinetics of azithromycin and selected 15-membered ring macrolides in rodents

The purpose of this study was to evaluate the impact of structural modifications on the 15-membered macrolactone ring and/or substituents on the in vitro ADME properties and in vivo pharmacokinetic (PK) profile for selected derivatives in rodents in comparison to azithromycin. Azithromycin and seven selected 15-membered macrolide derivatives, modified either by removal of the sugar moieties, replacement of the amine with a lactam, or addition of lipophilic substituents, were screened in several in vitro ADME assays and in vivo PK studies in rodents. In vitro ADME profiling included assessment of passive permeability and P-gp substrate, metabolic stability in liver microsomes and hepatocytes, as well as CYP direct inhibition measurements. In vivo PK studies were performed in rats (Sprague-Dawley), mice (Balb/c), and P-gp wild-type and deficient mice (CF-1™). Different structural modifications on the azithromycin scaffold resulted in substantial changes in disposition kinetics and oral bioavailability in both rodent species. However, these differences in vivo cannot be predicted based on in vitro results since most of these molecules are classified in the same category. Therefore, in the case of 15-membered ring macrolides, the in vitro ADME screens presented here seem to have low predictive value for in vivo prediction, making their use as routine in vitro screens prior to PK assessments questionable.