Azithromycin Composition
Azithromycin is formulated as azithromycin dihydrate, an azalide antibiotic structurally derived from erythromycin with a methyl-substituted nitrogen incorporated into a 15-membered lactone ring. 1
Chemical Structure and Classification
Azithromycin is an azalide, a subclass of macrolide antibiotics, distinguished from traditional macrolides by the incorporation of a nitrogen atom into the lactone ring structure. 2, 1, 3
The chemical name is (2R,3S,4R,5R,8R,10R,11R,12S,13S,14R)-13-[(2,6-dideoxy-3-C-methyl-3-O-methyl-α-L-ribo-hexopyranosyl)oxy]-2-ethyl-3,4,10-trihydroxy-3,5,6,8,10,12,14-heptamethyl-11-[[3,4,6-trideoxy-3-(dimethylamino)-β-D-xylo-hexopyranosyl]oxy]-1-oxa-6-azacyclopentadecan-15-one. 1
The molecular formula is C38H72N2O12 with a molecular weight of 749 (anhydrous form). 1
As the dihydrate form (C38H72N2O12•2H2O), the molecular weight is 785. 1
Pharmaceutical Formulations
Oral tablets contain azithromycin dihydrate equivalent to either 250 mg or 500 mg of azithromycin base. 1
Inactive Ingredients in Tablets
The film-coated tablets include: 1
- Dibasic calcium phosphate dihydrate
- Hydroxypropyl cellulose
- Croscarmellose sodium
- Magnesium stearate
- Opadry II white coating (containing hypromellose, lactose monohydrate, titanium dioxide, and triacetin)
Physical Characteristics
- Azithromycin dihydrate appears as a white crystalline powder. 1
- Tablets are white, oval-shaped, and biconvex with film coating. 1
Structural Derivation from Erythromycin
Azithromycin differs from erythromycin by having a methyl-substituted nitrogen atom incorporated into the lactone ring, converting the traditional 14-membered macrolide ring into a 15-membered azalide ring. 1, 3, 4
This structural modification provides azithromycin with improved acid stability and enhanced tissue penetration compared to erythromycin. 5, 6
Mechanism of Action Related to Structure
The azalide structure allows azithromycin to bind to the 50S subunit of bacterial ribosomes, inhibiting RNA-dependent protein synthesis by blocking the polypeptide exit region. 2, 3, 4
Despite lower serum concentrations than erythromycin, the structural modifications enable extensive intracellular accumulation with tissue-to-plasma ratios exceeding 100:1 in lung and tonsillar tissue. 7, 6