プラゾシン
プラゾシンは高選択的な**α1アドレナリン受容体遮断薬**（交感神経抑制薬）です。 獣医療においては、かつてはうっ血性心不全（CHF）や全身性・肺高血圧症の補助治療として、前負荷と後負荷の両方を軽減する血管拡張薬として使用されていましたが、現在では主に**下部尿路疾患の管理**に広く用いられています。 **臨床上のポイント：** * **尿道の弛緩：** プラゾシンは内尿道括約筋の平滑筋を弛緩させるのに非常に有効です。機能的尿道閉塞の治療、特に「尿道閉塞を起こした猫」（猫特発性膀胱炎/FLUTD）や、反射性協調運動障害または前立腺疾患を持つ犬において定番の薬剤です。 * **心血管系への影響：** ヒドララジンとは異なり、プラゾシンは動脈と静脈の両方を拡張させ、通常、反射性頻脈を引き起こしたりレニン分泌を増加させたりすることはありません。
作用機序: **Prazosin** competitively binds to and blocks **post-synaptic alpha-1 adrenergic receptors**. * **Genitourinary Tract:** Blockade of alpha-1 receptors in the trigone of the bladder and the proximal urethra → **smooth muscle relaxation** → decreased urethral resistance → facilitates micturition and relieves functional obstruction. * **Cardiovascular System:** Blockade of alpha-1 receptors on vascular smooth muscle → **vasodilation** of both arterioles and veins → decreased peripheral vascular resistance (afterload) and decreased venous return (preload) → lowered systemic blood pressure and reduced right atrial pressure.
動物種別の用量
- Decrease urethral resistance · 0.5 mg (total dose) PO q8h or 0.03 mg/kg IV · PO/IV · q8h (PO)
- Decrease urethral resistance · 0.03 mg/kg IV · IV
- Functional urethral obstruction · 0.25-0.5 mg/cat (total dose) PO q12-24h · PO · q12-24h
- Urethral spasm · 0.25-1 mg/cat (total dose) PO q8-12h · PO · q8-12h · Initially give for 5-7 days then wean off if possible
- Reduce urethral tone · 0.5 mg/cat (total dose) PO once to two times a day · PO · q12-24h
- Adjunctive treatment of heart failure · 1 mg PO three times daily for dogs weighing less than 15 kg; 2 mg three times daily PO for dogs weighing more than 15 kg · PO · q8h
- Hypertension · 1-4 mg (total dose) PO q12-24 hours · PO · q12-24h
- Hypertension in a large dog · 1 mg (total dose) PO q8-12h · PO · q8-12h
- Decrease urethral resistance · 1 mg per 15 kg of body weight PO q8h · PO · q8h
- Functional urethral obstruction · 1 mg/15 kg of body weight PO q8-24h · PO · q8-24h
- Decrease urethral resistance · 1 mg per 15 kg of body weight PO q12-24h · PO · q12-24h
投与経路
禁忌
- Preexisting hypotensive conditions
- Pre-existing hypotension
- Hypovolemia
- Known hypersensitivity to alpha-blockers
有害事象
- Hypotension (including orthostatic hypotension/syncope)
- Lethargy
- Dizziness
- Nausea
- Vomiting
- Diarrhea
- Constipation
- Tachyphylaxis (drug tolerance)
- Hypotension (especially first-dose effect)
- Lethargy / Weakness
- Dizziness / Syncope
- Gastrointestinal upset (vomiting, diarrhea)
- Reflex tachycardia
薬物相互作用
- Beta-blocking agents (e.g., propranolol) · May enhance the postural hypotensive effects seen after the first dose of prazosin
- Clonidine · May decrease prazosin antihypertensive effects
- Sildenafil (and other PDE inhibitors) · May increase risk for hypotension
- Verapamil or Nifedipine · May cause synergistic hypotensive effects when used concomitantly with prazosin
- Amlodipine · Additive hypotensive effects · moderate
- ACE Inhibitors (e.g., Benazepril, Enalapril) · Increased risk of severe hypotension · moderate
- Beta-blockers (e.g., Propranolol) · Exacerbation of postural hypotension · moderate
- Sildenafil · Profound hypotension · major
モニタリング
- Baseline thoracic radiographs
- Mucous membrane color
- Capillary refill time (CRT)
- Arterial blood pressure
- Venous PO2 (if possible)
- Systemic blood pressure
- Heart rate
- Urine output and ease of urination (if used for urethral spasm)
過量投与
Based on ASPCA APCC data (2008-2009): * **Dogs:** Common findings include hyperactivity, tachycardia, hyperthermia, panting, and agitation. * **Cats:** Common findings include tachycardia. **Treatment:** * Evacuate gastric contents and administer activated charcoal using standard precautionary measures if ingestion was recent and cardiovascular status is stable. * Monitor heart rate and blood pressure. * Treat shock using volume expanders and pressor agents if necessary. * Monitor and support renal function.
VetSheet の薬剤リファレンスは、獣医療従事者向けの臨床意思決定支援を目的としており、専門的判断やメーカーの最新添付文書に代わるものではありません。