The Foundation: Erythromycin Base
Erythromycin is a macrolide antibiotic that functions by inhibiting protein synthesis in susceptible bacteria [1.6.2]. It binds to the 50S ribosomal subunit, preventing bacteria from growing and multiplying [1.6.2]. The active component is erythromycin base, but it is easily inactivated by gastric acid [1.2.4, 1.2.5]. This instability led to the development of various derivatives to ensure the drug reaches the intestines for absorption or can be administered through other routes [1.2.4]. Standard-grade erythromycin is primarily composed of four related compounds: erythromycins A, B, C, and D, with erythromycin A having the most antibacterial activity [1.2.3].
Oral Formulations: Salts and Esters
To overcome destruction by stomach acid, oral erythromycin is available in several forms, which must be hydrolyzed to the active base after absorption [1.3.5]. These are generally either enteric-coated tablets or more stable salts and esters [1.2.4].
- Erythromycin Stearate: This is a salt formulation that comes in tablet form [1.2.1, 1.3.5]. It dissociates in the intestine, allowing the free base to be absorbed [1.3.3]. Studies show it may achieve higher peak serum concentrations compared to other forms like ethylsuccinate [1.3.1].
- Erythromycin Ethylsuccinate (EES): An ester formulation available as tablets, chewable tablets, and an oral suspension, making it suitable for pediatric use [1.2.1, 1.3.5]. It is absorbed partially intact and then converted to the active base by plasma esterases [1.3.3]. While generally well-tolerated, it may result in lower peak serum concentrations than the stearate form [1.3.1, 1.2.9]. Dosing differs, with 400 mg of EES producing similar serum levels to 250 mg of the base or stearate form [1.3.5].
- Erythromycin Estolate: This is the lauryl sulfate salt of a propionyl ester [1.2.5]. Its absorption is not affected by food [1.2.5]. Historically, it was thought to be solely responsible for liver injury among erythromycin types, but it's now understood that other forms can also cause hepatotoxicity [1.2.1].
- Erythromycin Base: Available in delayed-release capsules and enteric-coated tablets to protect the drug from stomach acid [1.2.2, 1.2.3].
Intravenous (IV) Formulations
For severe infections where high serum levels are needed immediately or when oral administration is not feasible, intravenous erythromycin is used [1.4.2].
- Erythromycin Lactobionate: A water-soluble salt prepared as a lyophilized powder for injection [1.2.5, 1.4.2]. It is administered via slow intravenous infusion over 20 to 60 minutes, as rapid injection is painful and can cause venous irritation [1.4.2, 1.4.6]. It's used for severe infections like Legionnaires' disease and acute pelvic inflammatory disease [1.4.2].
- Erythromycin Gluceptate: Another water-soluble salt for intravenous use, though it is now rarely available [1.2.4, 1.2.5].
Topical and Ophthalmic Formulations
Erythromycin is also effective when applied directly to the skin or eyes.
- Topical Preparations: Available as gels, solutions, pads, and powders, topical erythromycin is primarily used to treat acne vulgaris [1.5.1, 1.5.2, 1.5.6]. It works by stopping the growth of acne-causing bacteria [1.5.1]. It is sometimes combined with benzoyl peroxide to enhance its effectiveness [1.5.3, 1.5.4].
- Ophthalmic Ointment: A 0.5% erythromycin ointment is used to treat bacterial conjunctivitis and to prevent eye infections in newborns caused by Neisseria gonorrhoeae or Chlamydia trachomatis [1.2.5, 1.5.9, 1.6.7].
Comparison of Common Erythromycin Types
| Type | Administration Route | Common Formulations | Key Feature |
|---|---|---|---|
| Erythromycin Stearate | Oral | Film-coated tablets [1.2.1] | A salt that dissociates in the intestine [1.3.3]. |
| Erythromycin Ethylsuccinate | Oral | Tablets, oral suspension [1.2.1] | An ester often used in liquid form for children [1.2.5]. |
| Erythromycin Lactobionate | Intravenous | Powder for injection [1.4.2] | Used for severe infections requiring high serum levels [1.4.2]. |
| Erythromycin Base | Oral, Topical | Delayed-release capsules, gel, ointment [1.2.2, 1.2.7] | The active form, formulated to bypass stomach acid or for direct application [1.2.4]. |
Conclusion
Erythromycin is not a single entity but a family of formulations adapted for different clinical needs. The development of various salts, esters, and delivery systems allows this foundational macrolide antibiotic to be used effectively via oral, intravenous, and topical routes. These different types are designed to maximize absorption, ensure stability, and deliver the active drug to the site of infection, treating everything from respiratory infections and acne to serious conditions like Legionnaires' disease [1.6.2]. Understanding these variations is key to appreciating the versatility and continued relevance of erythromycin in modern medicine.
Authoritative Link: For more detailed drug information, consult MedlinePlus, a service of the National Library of Medicine. [1.2.2]
