アンピシリン
**アンピシリン**は、広域スペクトルを持つ時間依存性の殺菌性アミノペニシリン系抗生物質です。 主な薬理学的特徴： * **抗菌スペクトル：** 多くのグラム陽性菌（ペニシリン感受性の腸球菌 *E. faecium* など）、嫌気性菌（*Clostridium* 属など）、および一部のグラム陰性好気性菌（*E. coli*、*Klebsiella*、*Haemophilus*、*Proteus mirabilis*）に対して有効です。 * **耐性：** 天然ペニシリンと同様に、**ベータラクタマーゼ産生菌**（黄色ブドウ球菌など）によって不活化されやすいです。緑膿菌、セラチア、エンテロバクター、および非定型病原体（マイコプラズマ、リケッチア、真菌）には無効です。 * **臨床的有用性：** 小動物における経口投与は、生体利用率がより優れたアモキシシリンにほぼ取って代わられていますが、注射用アンピシリンは依然として重要な治療薬です。 * **塩の形態：** * **アンピシリンナトリウム：** 水溶性が高く、静脈内（IV）投与に適しており、急速かつ高い血清中ピーク濃度をもたらします。 * **アンピシリン三水和物：** 筋肉内（IM）または皮下（SC）用の持続性製剤で、吸収が遅く、血清中ピーク濃度が低くなります（ナトリウム塩の約半分）。全身感染症で高い最小発育阻止濃度（MIC）が必要な場合には使用すべきではありません。
作用機序: Ampicillin is a **beta-lactam antibiotic** that exerts its bactericidal effect by interfering with bacterial cell wall synthesis. * **Mechanism:** Ampicillin covalently binds to specific **Penicillin-Binding Proteins (PBPs)** (primarily transpeptidases) located inside the bacterial cell wall. * **Pathway:** Binding to PBPs → inhibition of the final transpeptidation step of peptidoglycan cross-linking → weakening of the cell wall structure → activation of endogenous autolytic enzymes (autolysins) → **osmotic lysis and bacterial cell death**. * **Pharmacodynamics:** Efficacy is **time-dependent**, meaning the free drug concentration must remain above the Minimum Inhibitory Concentration (MIC) for a significant portion of the dosing interval (typically >40-50% of the interval).
動物種別の用量
- Gram-positive infections · 10-20 mg/kg PO twice daily; 5 mg/kg IM, SC twice daily; 5 mg/kg IV three times daily · PO/IM/SC/IV · q8h-q12h
- Gram-negative infections · 20-30 mg/kg PO three times daily; 10 mg/kg IM, SC three times daily; 10 mg/kg IV four times daily · PO/IM/SC/IV · q6h-q8h
- Susceptible UTI's · 12.5 mg/kg PO q12h for 3-7 days, 6.6 mg/kg IM or SC q12h for 3-7 days · PO/IM/SC · q12h · 3-7 days
- Susceptible soft tissue infections · 10-20 mg/kg PO, IM or SC q8h for 7 days · PO/IM/SC · q8h · 7 days
- Pneumonia, systemic · 22 mg/kg PO, IV or SC q8h for 7-14 days · PO/IV/SC · q8h · 7-14 days
- Meningitis, orthopedic infections · 22 mg/kg PO, IV, IM, SC q6-8h as long as necessary · PO/IV/IM/SC · q6-8h · As long as necessary
- Susceptible sepsis, bacteremia · 20-40 mg/kg IV, IM or SC q6-8h for as long as necessary · IV/IM/SC · q6-8h · As long as necessary
- Neonatal sepsis · 50 mg/kg IV or intraosseous q4-6h as long as necessary · IV/Intraosseous · q4-6h · As long as necessary
- Sepsis · 20-40 mg/kg IV q6-8h · IV · q6-8h
- Susceptible UTI's · 25 mg/kg PO q8h · PO · q8h
- To eliminate the leptospiremic phase of leptospirosis · 22 mg/kg q6-8h IV during the acute illness until patient is eating, then amoxicillin 22 mg/kg PO q8h · IV · q6-8h · During acute illness · Followed by oral amoxicillin
投与経路
禁忌
- Patients with a known hypersensitivity to penicillins
- Oral administration in patients with septicemia, shock, or grave illness (due to delayed/diminished GI absorption)
- Oral or parenteral use in small hindgut fermenters (rabbits, guinea pigs, chinchillas, hamsters) due to risk of fatal clostridial enterotoxemia
有害事象
- Gastrointestinal upset (anorexia, vomiting, diarrhea)
- Antibiotic-associated diarrhea and superinfections (alteration of gut flora)
- Hypersensitivity reactions (rashes, fever, eosinophilia, anaphylaxis)
- Neurotoxicity (ataxia, seizures) at very high doses or prolonged use
- Elevated liver enzymes (rare)
- Tachypnea, dyspnea, edema, and tachycardia (reported in dogs)
薬物相互作用
- Bacteriostatic Antimicrobials (e.g., chloramphenicol, macrolides, tetracyclines) · Potential in vitro antagonism; clinical significance is debated but concurrent use is traditionally discouraged.
- Methotrexate · Ampicillin may decrease the renal excretion of methotrexate, leading to increased levels and potential toxicity.
- Probenecid · Competitively blocks the tubular secretion of penicillins, increasing serum levels and prolonging half-life.
- Aminoglycosides (e.g., gentamicin, amikacin) · In vitro inactivation of aminoglycosides if mixed in the same syringe or fluid bag. May also occur in vivo in patients with severe renal failure.
- Tetracycline · Antagonism of bactericidal effect due to bacteriostatic action · moderate
- Erythromycin · Antagonism of bactericidal effect due to bacteriostatic action · moderate
- Chloramphenicol · Antagonism of bactericidal effect due to bacteriostatic action · moderate
- Aminoglycosides · In vitro inactivation if mixed in the same syringe; however, exhibits synergistic antimicrobial effects when used concurrently in vivo · major
モニタリング
- Clinical efficacy (resolution of infection signs)
- Adverse effects (GI signs, hypersensitivity reactions)
- Therapeutic drug monitoring is not routinely required due to the wide therapeutic index
過量投与
Acute oral penicillin overdoses are unlikely to cause significant problems other than **gastrointestinal distress** (vomiting, diarrhea, anorexia). In humans, very high dosages of parenteral penicillins, particularly in patients with underlying renal disease, have induced **CNS effects** (e.g., seizures, ataxia). Treatment is generally supportive and symptomatic.
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