Mechanism of Action and Pharmacokinetics
Benzathine benzylpenicillin (Bicillinum-1) belongs to the class of beta-lactam antibiotics. Classically, it is a natural penicillin formulated for parenteral administration. The drug is acid-labile—it is completely destroyed by hydrochloric acid in the stomach. This fundamentally differentiates it from enteral formulations (such as phenoxymethylpenicillin, which has a phenoxymethyl group instead of a benzyl group and is stable in the GI tract).
In terms of duration, it is a prolonged-release (depot) formulation. Unlike short-acting salts (sodium and potassium benzylpenicillin) and procaine benzylpenicillin (which acts for 12–18 hours), Bicillin-1 is absorbed extremely slowly following intramuscular injection. This maintains therapeutic blood concentrations for 7 to 10 days. There is also a combination product, Bicillin-5 (a mixture with procaine penicillin), which acts for up to 1 month.
Indications for Use
Like all benzylpenicillin drugs, this antibiotic is active against susceptible flora. However, its prolonged pharmacokinetic profile dictates specific clinical indications. Benzathine benzylpenicillin is the first-line agent of choice for the following conditions:
- Syphilis. Penicillin-class drugs are effective at all stages of this disease and are administered in standardized courses.
- Rheumatic diseases. Used for year-round secondary prophylaxis of rheumatic fever (preventing recurrences).
- Streptococcal infections. Including tonsillopharyngitis (strep throat), scarlet fever, and various wound infections.
To understand the broader spectrum of the group: natural penicillins are generally used for a wide range of infections. These include upper and lower respiratory tract infections (focal and lobar pneumonia, lung abscess, diphtheria), skin, soft tissue, and bone infections (erysipeloid/erysipelas, osteomyelitis, actinomycosis). They are also indicated for infective endocarditis, specific anaerobic infections (gas gangrene, tetanus), and vector-borne diseases (Lyme disease/borreliosis).
Limitations and Alternative Therapy
The primary disadvantage of depot formulations is that they do not achieve high peak plasma concentrations and practically do not cross the blood-brain barrier (BBB). Consequently, Bicillin-1 is strictly not used for severe acute infections (e.g., meningitis, which requires crossing the BBB).
Selection and Alternatives: Benzylpenicillin preparations are first-line therapy. However, if a patient has a penicillin allergy (hypersensitivity reactions), alternative agents (reserve drugs) must be used. These include:
- Macrolides and azalides: erythromycin, azithromycin.
- Cephalosporins: cefaloridine, ceftriaxone.
Formulations and Prescription
The drug is supplied in vials as a powder for suspension preparation. Available dosages include 300,000, 600,000, and 1,200,000 IU.
The standard route of administration is strictly intramuscular (IM). The typical dose ranges from 300,000 to 600,000 IU administered once weekly.
Prescription Example: Rp.: Benzathini benzylpenicillini 600,000 IU D.t.d. No. 4 S. Intramuscularly, 600,000 IU once weekly.