Erythromycin B

A method was developed for the large-scale bioconversion of novel 6-deoxyerythronolide B (6-dEB) analogs into erythromycin analogs. Erythromycin biosynthesis in Saccharopolyspora erythraea proceeds via the formation of a polyketide aglycone, 6-dEB, which is subsequently glycosylated, hydroxylated and methylated to yield the antibiotic erythromycin A. A modular polyketide synthase (PKS) directs 6-dEB synthesis using a dedicated set of active sites for the condensation of each of seven propionate units. Strategies based on genetic manipulation and precursor feeding are available for the efficient generation of novel 6-dEB analogs using a plasmid-based system in Streptomyces coelicolor. 6-dEB and 13-substituted 6-dEB analogs produced in this manner were fed to S. erythraea mutants which could not produce 6-dEB, yet retained their 6-dEB modification systems, and resulted in the generation of erythromycin A and 13-substituted erythromycin A analogs. Erythromycin B, C and D analogs were observed as intermediates of the process. Dissolved oxygen, temperature, the specific aglycone feed concentration, and pH were found to be important for obtaining a high yield of erythromycin A analogs. Cultivation conditions were identified which resulted in the efficient bioconversion of 6-dEB analogs into erythromycin A analogs, which this process demonstrated at the 100 l scale.

An S-adenosylmethionine synthetase (SAM-s) gene from Streptomyces spectabilis was integrated along with vector DNA into the chromosome of a Saccharopolyspora erythraea E2. Elevated production of SAM was observed in the recombinant strain Saccharopolyspora erythraea E1. The results from the bioassay showed that the titer of erythromycin was increased from 920 IU ml(-1) by E2 to approximately 2,000 IU ml(-1) by E1. High performance liquid chromatography (HPLC) analysis revealed that there was a 132% increase in erythromycin A compared with the original strain, while the erythromycin B, the main impurity component in erythromycin, was decreased by 30%. The sporulation process was inhibited, while the SAM-s gene was expressed. The addition of the exogenous SAM also inhibited sporulation and promoted an increase in erythromycin titers.

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.

Erythromycin A is normally formulated for children as its 2'-ethyl succinate, a taste-free prodrug. Unfortunately, the prodrug hydrolyzes at a measurable rate in the medicine bottle, leading to the vile-tasting erythromycin. We have prepared derivatives of erythromycin B as putative paediatric prodrugs, taking advantage of the much improved acid stability of erythromycin B relative to erythromycin A. Thus, erythromycin B enol ether ethyl succinate is very poorly soluble in water, and its hydrolysis is undetectable in conditions resembling the medicine bottle. In acid, however, it converts rapidly to erythromycin B 2'-ethyl succinate, and this is in turn hydrolyzed to erythromycin B in neutral and basic conditions. Derivatives of erythromycin B enol ether are therefore proposed as taste-free proprodrugs of erythromycin B.