氨苄西林
**氨苄西林 (Ampicillin)** 是一種廣效、時間依賴型、具殺菌作用的氨基青黴素類抗生素。 主要藥理特徵包括： * **抗菌譜：** 對許多革蘭氏陽性菌（包含對青黴素敏感的腸球菌如 *E. faecium*）、厭氧菌（如梭狀芽孢桿菌）及部分革蘭氏陰性需氧菌（*E. coli*、*Klebsiella*、*Haemophilus*、*Proteus mirabilis*）有效。 * **抗藥性：** 與天然青黴素相似，極易被**產生β-內醯胺酶的細菌**（如金黃色葡萄球菌）破壞而失效。對綠膿桿菌、沙雷氏菌、腸桿菌及非典型病原（黴漿菌、立克次體、黴菌）無效。 * **臨床應用：** 由於口服吸收率較差，小動物的口服途徑已大多被阿莫西林 (Amoxicillin) 取代，但注射型氨苄西林仍是重要的基礎療法。 * **鹽類劑型：** * **氨苄西林鈉 (Sodium)：** 水溶性高，適合靜脈注射 (IV)，可迅速達到高血清峰值濃度。 * **氨苄西林三水合物 (Trihydrate)：** 儲積型長效製劑，供肌肉 (IM) 或皮下 (SC) 注射，吸收較慢且血清峰值較低（約為鈉鹽的一半）。若全身性感染需要較高的最低抑菌濃度 (MIC) 時，不宜使用此劑型。
作用機制: 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).
各物種劑量
- 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
給藥途徑
禁忌症
- 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
不良反應
- 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)
藥物相互作用
- 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
監測
- 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
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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