フェノキシベンザミン
フェノキシベンザミンは、長時間作用型の非競合的**α-アドレナリン受容体拮抗薬**です。 獣医療では主に以下の目的で使用されます： - **小動物**：機能的尿道閉塞や排尿筋無反射を治療するための内尿道括約筋の緊張低下。また、**褐色細胞腫**の患者において、腫瘍の外科的操作中の致命的な高血圧クリーゼを予防するための重要な術前投与薬でもあります。 - **馬**：初期の蹄葉炎、分泌性下痢、膀胱不全麻痺の管理などの特殊な用途。 > **臨床のポイント**：フェノキシベンザミンは受容体に不可逆的に結合するため、その効果は長く続き、新しい受容体が合成されることによってのみ回復します。このプロセスには数日かかります。
作用機序: Phenoxybenzamine covalently and irreversibly binds to **alpha-1 and alpha-2 adrenergic receptors** → non-competitive blockade of circulating epinephrine and norepinephrine → profound vasodilation and relaxation of smooth muscle. - **Urinary Tract**: Blockade of alpha receptors in the trigone and internal urethral sphincter → decreased urethral resistance → facilitation of bladder emptying. - **Cardiovascular**: Blockade of vascular alpha receptors → decreased peripheral vascular resistance → lowered blood pressure (often causing reflex tachycardia). Because the binding is irreversible, the effect is often described as a 'chemical sympathectomy'. It has no effect on beta-adrenergic or parasympathetic receptors.
動物種別の用量
- Functional urethral obstruction · 2.5-7.5 mg/cat · PO · once to twice daily
- Functional urethral obstruction · 1.25-7.5 mg (total dose) · PO · q12-24h
- Functional urethral obstruction · 2.5-10 mg (total dose) · PO · q24h
- Short-term treatment of hypertension · 0.5 mg/kg · PO · q12h
- Short-term treatment of hypertension · 2.5 mg (total dose) increasing by 2.5 mg up to a maximum of 10 mg (total dose) · PO · q12h
- Short-term treatment of hypertension · 2.5-7.5 mg per cat · PO · q8-12h
- Decrease urethral sphincter tone in bladder paresis · 0.7 mg/kg · PO · 4 times a day · Used in combination with bethanechol at 0.25-0.75 mg/kg PO 2-4 times a day.
- Adjunctive treatment of laminitis (developmental phase) · 1 mg/kg · IV · q12h · for 2 doses
- Treatment of profuse, watery diarrhea · 200-600 mg · q12h
- Functional urethral obstruction · 0.25 mg/kg · PO · q12-24h · Alternative dose: 2.5-20 mg (total dose) PO q12-24h
投与経路
禁忌
- Conditions where hypotension is undesirable (e.g., shock, unless fluid replacement is adequate)
- Horses with clinical signs of colic
- Glaucoma (relative contraindication)
- Diabetes mellitus (relative contraindication)
有害事象
- Hypotension (postural/orthostatic)
- Hypertension (rebound)
- Miosis
- Increased intraocular pressure
- Tachycardia (reflex)
- Sodium retention
- Inhibition of ejaculation
- Nasal congestion
- Weakness or dizziness
- Gastrointestinal effects (nausea, vomiting)
- Constipation (specifically noted in horses)
- Hypotension
- Tachycardia
薬物相互作用
- Epinephrine · Epinephrine reversal: If used with drugs that have both alpha- and beta-adrenergic effects, increased hypotension, vasodilation, or tachycardia may result due to unopposed beta effects.
- Phenylephrine · Phenoxybenzamine will antagonize the effects of alpha-adrenergic sympathomimetic agents.
- Reserpine · Phenoxybenzamine can antagonize the hypothermic effects of reserpine.
- Beta-blockers · Increased risk of first dose hypotensive effect; if used for phaeochromocytoma, beta-blockers must only be started after alpha blockade is in place to avoid hypertensive crisis. · major
- Diuretics · Increased risk of first dose hypotensive effect. · moderate
- Alpha-adrenergic sympathomimetics (e.g., phenylephrine) · Phenoxybenzamine will antagonize the effects of these agents. · major
モニタリング
- Clinical efficacy (e.g., adequate urination, ease of voiding)
- Blood pressure (monitor for hypotension)
- Heart rate (monitor for reflex tachycardia)
- Blood pressure
- Heart rate and rhythm
- Urine output and ease of urination
過量投与
Overdosage may yield signs of **postural hypotension** (dizziness, syncope), tachycardia, vomiting, lethargy, or shock. **Treatment**: - Empty the gut if ingestion was recent and there are no contraindications. - Treat hypotension with aggressive intravenous fluid support. - **DO NOT USE EPINEPHRINE** (causes paradoxical further drop in blood pressure due to unopposed beta-adrenergic vasodilation). - Most standard vasopressors are ineffective due to the alpha-blockade. Intravenous norepinephrine (levarterenol) may be beneficial if clinical signs are severe.
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。