ドキシサイクリン
ドキシサイクリンは、オキシテトラサイクリンから誘導された広域スペクトルの半合成**テトラサイクリン系抗生物質**です。古いテトラサイクリン系と比較して良好な薬物動態プロファイルを持つため、獣医療で広く使用されています。 **主な臨床的利点：** * **高脂溶性：** 中枢神経系（CSF）、前立腺、眼などの組織への移行性が非常に優れています。 * **腎臓への安全性：** 他のテトラサイクリン系とは異なり、主に不活性なキレートとして糞便中に排泄されます。腎機能障害のある患者でも蓄積**しない**ため、高窒素血症の患者に対する第一選択のテトラサイクリン系薬となります。 * **犬糸状虫（フィラリア）の補助治療：** 現代の犬糸状虫治療プロトコルにおいて重要な役割を果たします。糸状虫の生存と繁殖に不可欠な内部共生細菌である***Wolbachia***を排除し、成虫を弱体化させ、成虫駆除後の肺の病理学的変化を軽減します。 ボレリア（ライム病）、レプトスピラ、リケッチア、クラミジア、マイコプラズマ、バルトネラ、ボルデテラなどの非定型病原体に対して高い有効性を示します。
作用機序: Doxycycline is a **bacteriostatic** antibiotic. * **Primary Mechanism:** It reversibly binds to the **30S ribosomal subunit** of susceptible organisms → prevents the binding of aminoacyl transfer-RNA to the mRNA-ribosome complex → **inhibits bacterial protein synthesis**. * **Secondary Mechanisms:** It may also reversibly bind to 50S ribosomes and alter cytoplasmic membrane permeability. * **Anti-inflammatory/Immunomodulatory Effects:** Beyond its antimicrobial properties, doxycycline inhibits matrix metalloproteinases (MMPs) and scavenges reactive oxygen species, which contributes to its efficacy in treating chronic inflammatory conditions (e.g., idiopathic rhinitis, arthritis).
動物種別の用量
- Hemotropic mycoplasmosis · 5-10 mg/kg PO once daily for 14 days · PO · q24h · 14 days · Round dose to nearest whole tablet or capsule.
- Bartonellosis · 50 mg (total dose) PO q12h for 14-28 days · PO · q12h · 14-28 days
- Systemic infections, bacteremia · 5-11 mg/kg PO or IV q12h as long as necessary · PO/IV · q12h · As long as necessary
- Ehrlichiosis or Anaplasmosis · 5-10 mg/kg PO q12h for 21 days · PO · q12h · 21 days
- Clinical hemoplasmosis or bartonellosis · 10 mg/kg PO q12-24h · PO · q12-24h
- Toxoplasma gondii · 5-10 mg/kg PO q12h for 4 weeks · PO · q12h · 4 weeks
- Susceptible mycobacterial, L-Forms, or mycoplasma infections · 5-10 mg/kg PO q12h · PO · q12h
- Nocardia (N. nova) infections · 5 mg/kg or higher PO twice daily · PO · q12h · Combination therapy with amoxicillin and clarithromycin.
- Feline chlamydial infections (C. felis) · 10 mg/kg PO once daily for a minimum of 3-4 weeks · PO · q24h · 3-4 weeks
- Susceptible bacterial infections · 10 mg/kg · PO · q24h · As directed · Administer with food or a water bolus to prevent oesophageal ulceration.
- Feline chlamydiosis · 10 mg/kg · PO · q24h · 3-4 weeks · Administer with food or a water bolus.
投与経路
禁忌
- Hypersensitivity to tetracyclines
- Intravenous administration in horses (associated with fatal cardiac arrhythmias)
- First half of pregnancy (can retard fetal skeletal development and discolor deciduous teeth, though less likely than other tetracyclines)
- Pregnant animals
- Animals with evidence of oesophagitis or dysphagia
有害事象
- Vomiting
- Diarrhea
- Anorexia
- Elevated liver enzymes (ALT, ALP)
- Esophageal strictures (especially in cats if dry-pilled)
- Superinfections (non-susceptible bacteria or fungi)
- Photosensitivity (reported in humans)
- Cardiac arrhythmias, collapse, and death (if given IV to horses)
- Nausea
- Diarrhoea
- Oesophagitis
- Oesophageal ulceration/stricture (especially in cats)
- Tooth discoloration (if given during tooth development, though risk is lower than other tetracyclines)
薬物相互作用
- Oral Antacids (Aluminum, Calcium, Magnesium, Zinc, Bismuth) · Chelates with divalent/trivalent cations, significantly decreasing doxycycline absorption. Give at least 1-2 hours apart.
- Bismuth Subsalicylate, Kaolin, Pectin · May reduce gastrointestinal absorption of doxycycline.
- Oral Iron · Decreases tetracycline absorption. Give iron 3 hours before or 2 hours after doxycycline.
- Penicillins / Cephalosporins / Aminoglycosides · Bacteriostatic drugs (doxycycline) may theoretically interfere with the bactericidal activity of these antibiotics.
- Phenobarbital · May decrease doxycycline half-life and reduce serum levels. · moderate
- Warfarin · Tetracyclines may depress plasma prothrombin activity; anticoagulant dosage may need adjustment.
- Antacids · Reduced absorption of doxycycline · moderate
- Calcium salts · Reduced absorption of doxycycline · moderate
- Magnesium salts · Reduced absorption of doxycycline · moderate
- Iron salts · Reduced absorption of doxycycline · moderate
- Phenytoin · May increase doxycycline metabolism, decreasing plasma levels · moderate
- Antacids (Calcium, Magnesium, Iron salts) · Reduced absorption of doxycycline (though less marked than with water-soluble tetracyclines) · moderate
モニタリング
- Clinical efficacy
- Adverse effects (GI upset)
- Liver enzymes (ALT, ALP) with prolonged use
- Clinical response to therapy
- Signs of dysphagia, regurgitation, or oesophagitis (especially in cats)
- Gastrointestinal upset
過量投与
Doxycycline is generally quite safe in mild overdose situations (except for IV administration in horses). * **Oral Overdose:** Most likely associated with gastrointestinal disturbances (vomiting, anorexia, diarrhea). * **Treatment:** Oral administration of divalent or trivalent cation antacids (e.g., calcium, magnesium, aluminum) can bind the drug in the gut and reduce GI distress. Monitor and replace fluids/electrolytes if severe emesis or diarrhea occurs. * **IV Overdose:** Rapid intravenous injection has induced transient collapse and cardiac arrhythmias in several species, presumably due to chelation with intravascular calcium ions.
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