Mechanism of action and pharmacokinetics
Benzathine benzylpenicillin (Bicillin-1) belongs to the class of beta-lactam antibiotics. In classification, it is a natural penicillin for parenteral administration. The drug is acid-labile — it is completely destroyed under the influence of hydrochloric acid in the stomach. This fundamentally distinguishes it from drugs for enteral administration (such as phenoxymethylpenicillin, which has a phenoxymethyl group instead of a benzyl group and is not destroyed in the gastrointestinal tract).
In terms of duration of effect, this is a durant (prolonged) form — a depot drug. Unlike short-acting salts (sodium and potassium salts of benzylpenicillin) and benzylpenicillin procaine (active for 12–18 hours), Bicillin-1 is absorbed extremely slowly after intramuscular injection. This ensures the maintenance of a therapeutic concentration in the blood for 7–10 days. There is also a combined drug Bicillin-5 (a mixture with procaine salt), effective for up to 1 month.
Indications for use
Like all benzylpenicillin group drugs, this antibiotic is active against sensitive flora. However, its prolonged profile dictates specific indications. Benzathine benzylpenicillin is the drug of choice (first-line) for the following conditions:
- Syphilis. Penicillin-class drugs are effective at all stages of this disease and are prescribed in courses.
- Rheumatic diseases. Used for year-round bicillin prophylaxis (prevention of rheumatic recurrences).
- Streptococcal infections. Including tonsillopharyngitis (sore throat), scarlet fever, and various wound infections.
To understand the spectrum of the group: natural penicillins in general are used for a wide range of infections. These include ENT and respiratory tract lesions (focal and lobar pneumonia, lung abscess, diphtheria), skin, soft tissue, and bone infections (erysipelas, osteomyelitis, actinomycosis). They are also indicated for bacterial endocarditis, specific anaerobic infections (gas gangrene, tetanus), and vector-borne diseases (tick-borne borreliosis or Lyme disease).
Limitations and replacement strategy
The main disadvantage of depot forms is that they do not create high peak concentrations in blood plasma and practically do not cross the blood-brain barrier (BBB). Because of this, Bicillin-1 is strictly not used for severe acute infections (e.g., meningitis, where crossing the BBB is required).
Selection strategy and alternatives: Benzylpenicillin drugs are the first line. However, if the patient has an intolerance (allergic reactions) to penicillins, alternative agents (reserve drugs) must be used. These include:
- Macrolides and azalides: erythromycin, azithromycin (Sumamed).
- Cephalosporins: cephaloridine (Ceporin), ceftriaxone (Longacef).
Dosage Form and Prescription Writing
According to the prescription reference book, the drug is available in vials containing powder for suspension preparation. Available dosages: 300,000, 600,000, and 1,200,000 IU.
The standard route of administration is strictly intramuscular (IM). The usual dose is 300,000–600,000 IU administered once a week.
Prescription example: Rp.: Benzathini benzylpenicillini 600,000 ED D.t.d. N. 4 S. Intramuscularly 600,000 IU once a week.
Briefly: answers to topical questions
What is the mechanism of action of benzathine benzylpenicillin on the bacterial cell?
Benzathine benzylpenicillin exerts a bactericidal effect by binding to the transpeptidase enzyme (penicillin-binding protein), which disrupts cross-linking between peptidoglycan chains and halts the synthesis of a complete cell wall. Disruption of the balance between murein synthesis and its degradation leads to uncontrolled activation of autolysins, cell lysis, and a bactericidal effect.
Bicillin-ONE acts for ONE week (7–10 days), and Bicillin-FIVE acts for almost FIVE weeks (1 month).
Class and exam questions
Why is Bicillin-1 contraindicated in meningitis?
The drug is a depot formulation: it does not create high peak concentrations in the blood and practically does not cross the blood-brain barrier (BBB). The treatment of meningitis requires short-acting water-soluble salts.
Can Benzathine benzylpenicillin be prescribed in tablets?
No, this is an acid-labile drug that is completely destroyed by hydrochloric acid in the stomach. For oral administration (per os), acid-resistant phenoxymethylpenicillin is used.
What to do if a syphilis patient has a penicillin allergy?
As alternative agents (reserve drugs), antibiotics of other groups are prescribed: macrolides (e.g., erythromycin, azithromycin) or cephalosporins (ceftriaxone).
What else to study on this topic
- Differences between sodium, potassium, and novocaine salts of benzylpenicillin.
- Mechanism of development of allergic reactions to beta-lactam antibiotics.
- Regimens for rheumatic fever bicillin prophylaxis.
- Acid-resistant penicillins: pharmacokinetics of phenoxymethylpenicillin.
- Cephalosporins and macrolides as reserve drugs: spectrum of activity.
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