ペニシリンG
**ペニシリンG**（ベンジルペニシリン）は、*Penicillium chrysogenum*から得られる狭域スペクトルの天然β-ラクタム系抗生物質です。ほとんどの**グラム陽性好気性菌**（レンサ球菌、ペニシリナーゼ非産生ブドウ球菌など）、一部のグラム陰性菌（パスツレラなど）、および多くの**嫌気性菌**（クロストリジウムなど）に対して高い有効性を示します。 > **臨床のポイント：** ペニシリンGの塩の形態により、投与経路と薬物動態プロファイルが決定されます： * **水溶性塩（ナトリウムまたはカリウム）：** 静脈内（IV）または筋肉内（IM）投与用。急速に高い血清中ピーク濃度に達しますが、半衰期が非常に短いため、頻回投与（例：4〜6時間ごと）が必要です。 * **持続性塩（プロカインおよびベンザチン）：** 懸濁液として製剤化されており、**筋肉内（IM）または皮下（SC）投与専用**です（静脈内投与は厳禁）。プロカインは12〜24時間持続するデポ効果をもたらし、ベンザチンは数日間持続する非常に低く延長された濃度を提供します。 ペニシリンGは、多くのブドウ球菌やグラム陰性腸内細菌が産生するβ-ラクタマーゼ（ペニシリナーゼ）酵素による分解を受けやすいです。
作用機序: Penicillin G is a **time-dependent bactericidal** antibiotic. * It covalently binds to **Penicillin-Binding Proteins (PBPs)** located on the inner membrane of the bacterial cell wall. * This binding → **inhibits the transpeptidation enzyme** responsible for cross-linking peptidoglycan strands. * Failure of cell wall synthesis → activation of autolytic enzymes within the cell wall → **osmotic lysis** and bacterial death. Because it targets cell wall synthesis, it is most effective against actively dividing bacteria.
動物種別の用量
- Soft tissue, systemic infections · 40,000 Units/kg · PO · q6-8h · for as long as necessary · Penicillin G potassium
- Soft tissue infections · 20,000 Units/kg · IM, SC · q12h · for as long as necessary · Penicillin G procaine
- Orthopedic infections · 20,000-40,000 Units/kg · IM · q8h · for as long as necessary · Penicillin G procaine
- Resistant organisms (Actinomyces) · 50,000-100,000 Units/kg · IM, SC · q12h · for as long as necessary · Penicillin G procaine
- Susceptible infections · 50,000 Units/kg · IM · q5 days · Penicillin G benzathine
- Susceptible infections · 20,000-40,000 Units/kg · IM · once a day to twice daily · Procaine Pen G
- Susceptible infections · 20,000 Units/kg · SC, IM or IV · q4h · Sodium or potassium Pen G
- Susceptible infections · 40,000 Units/kg · PO · three times daily · Sodium or potassium Pen G
- Susceptible infections · 25,000 Units/kg · IM or SC · once per day · Penicillin G procaine. For moderately susceptible bacteria give twice daily.
- Clostridial abomasitis and enteritis in calves · 10,000-20,000 Units/kg · PO · q12-24h · for 1-4 days · Procaine Penicillin G. Oral preferred to provide activity in intestinal lumen.
投与経路
禁忌
- Known hypersensitivity to penicillins or cephalosporins
- Intravenous administration of Procaine or Benzathine suspension formulations
- Oral administration in horses and hindgut fermenters (rabbits, guinea pigs) due to risk of fatal dysbiosis (unless specifically indicated, e.g., calves with clostridial enteritis)
有害事象
- Hypersensitivity reactions (anaphylaxis, urticaria, rash)
- Gastrointestinal upset (anorexia, vomiting, diarrhea) with oral use
- Procaine toxicity (CNS excitement, seizures) in small birds and horses if inadvertently given IV
- Pain or tissue reaction at IM injection sites
薬物相互作用
- Bacteriostatic Antibiotics (e.g., Tetracyclines, Macrolides) · May antagonize the bactericidal activity of penicillins, which require actively dividing cells to be effective.
- Aminoglycosides (e.g., Amikacin, Gentamicin) · In vivo synergy against certain bacteria; however, physically incompatible if mixed in the same syringe or IV line (inactivation of the aminoglycoside).
- Probenecid · Competitively inhibits renal tubular secretion of penicillins, significantly prolonging their half-life and increasing serum concentrations.
モニタリング
- Clinical efficacy (resolution of infection signs)
- Signs of toxicity or hypersensitivity (anaphylaxis, rash)
- Electrolytes (if using high doses of Na or K salts IV)
過量投与
Because penicillins usually have minimal toxicity associated with their use, monitoring for efficacy is usually all that is required unless toxic signs develop. > **Clinical Pearl:** Massive overdoses, particularly of aqueous salts given rapidly IV, can lead to neuromuscular hypersensitivity, seizures, or electrolyte imbalances (hyperkalemia with potassium salt, hypernatremia with sodium salt). Treatment is supportive.
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