Pediatric Erythromycins:  a Comparison of the Properties of Erythromycins A and B 2‘-Ethyl Succinates

The antibiotic erythromycin A is generally administered to children as a suspension of the pro-drug erythromycin A 2‘-ethyl succinate. The success of the pro-drug depends on (a) elimination of the unacceptably bitter taste of free erythromycin, (b) its stability against stomach acid, and (c) its smooth (base-catalyzed) hydrolysis in the body to yield active erythromycin. We have investigated the rates and pathways of acid-catalyzed degradation and base-catalyzed hydrolysis of the 2‘-ethyl succinates of erythromycins A and B. Esterification does not protect the drugs against acid-catalyzed degradation in solution; however, erythromycin B 2‘-ethyl succinate is much more stable than the corresponding erythromycin A ester, degrading nearly 40 times more slowly. The rates of base-catalyzed hydrolysis in conditions mimicking the blood stream are similar for the two pro-drugs. We conclude that erythromycin B 2‘-ethyl succinate is an attractive prospect as a pediatric erythromycin pro-drug.