プリミドン
Primidone
5-ethyldihydro-5-phenyl-4,6(1H,5H)-pyrimidinedione
別名: Mysoline · Neurosyn · hexamidinum · primaclone · primidonum · Cyral · Epidona · Liskantin · Mylepsinum · Prysoline · Resimatil · Sertan
VetSheet 獣医チームが監修更新日 2026年4月5日エビデンスに基づく獣医学リファレンス
用量は獣医専門家向けの臨床参考資料です。必ず最新の医薬品情報および個別症例に基づいて確認してください。
動物種別用量
🌎 NA — North America🌍 EU — Europe
犬
| 適応 | 用量 | 経路 | 頻度 | 期間 | 地域 |
|---|---|---|---|---|---|
| Seizure control | 10-30 mg/kg per day divided into 2-3 doses | PO | divided into 2-3 doses | — | 🌎 NA |
| Seizure control | 10 mg/kg | PO | q8h | — | 🌎 NA |
- Seizure control: Initially
- Seizure control: Not recommended as first choice
猫
| 適応 | 用量 | 経路 | 頻度 | 期間 | 地域 |
|---|---|---|---|---|---|
| Seizure control | 20 mg/kg | PO | q12h | — | 🌎 NA |
- Seizure control: Extreme caution advised; many consider contraindicated in cats.
用量は獣医専門家向けの臨床参考資料です。必ず最新の医薬品情報および個別症例に基づいて確認してください。
概要
プリミドンはバルビツール酸系の抗てんかん薬であり、主にプロドラッグとして作用し、犬の体内では急速にフェノバルビタールおよびフェニルエチルマロンアミド (PEMA) に変換されます。
歴史的には犬のてんかん発作のコントロールに使用されてきましたが、現在、ほとんどの獣医神経科医は第一選択薬としての使用を推奨していません。
臨床のポイント: フェノバルビタールと比較して重篤な肝毒性の発生率が高いことが、長期治療における最大の制限要因です。
犬におけるプリミドンからフェノバルビタールへの変換率は約4:1です（プリミドン250mgはフェノバルビタール約60mgに相当）。PEMA代謝物がフェノバルビタールの抗てんかん作用を増強するという理論に基づき、フェノバルビタール単独では効果不十分な難治性症例に限定して使用する獣医師もいます。猫やウサギに対しては猛毒であると考えられています。
作用機序
Primidone and its active metabolites, phenylethylmalonamide (PEMA) and phenobarbital, exert anticonvulsant effects by raising seizure thresholds and altering seizure patterns.
Mechanistic Pathway:
- Phenobarbital (Primary active metabolite): Binds to the allosteric barbiturate site on GABA_A receptors in the CNS → prolongs the duration of chloride channel opening → increases intracellular chloride influx → hyperpolarizes the postsynaptic neuron → globally depresses CNS excitability and raises the seizure threshold.
- PEMA: Has weak intrinsic anticonvulsant activity but is believed to synergistically potentiate the effects of phenobarbital.
- Primidone (Parent drug): May have some independent action on voltage-gated sodium channels, though its primary efficacy in veterinary species is attributed to its phenobarbital metabolite.
安全性・警告
禁忌
- Severe liver disease
- Demonstrated previous hypersensitivity to primidone or barbiturates
- Nephritis (large doses contraindicated)
- Severe respiratory dysfunction (large doses contraindicated)
- Cats (considered contraindicated by many clinicians due to high toxicity risk)
有害事象
- Anxiety and agitation (especially during initiation)
- Elevated liver enzymes (ALT, ALP, GLDH)
- Decreased serum albumin
- Hepatic lipidosis
- Hepatocellular hypertrophy and necrosis
- Extramedullary hematopoiesis
- Depression and sedation
- Ataxia
- Polydipsia (PD)
- Polyuria (PU)
- Polyphagia
- Anorexia
- Tachycardia
- Dermatitis
- Episodic hyperventilation
- Urolith formation (primidone uroliths reported)
- Megaloblastic anemia (rare)
注意事項
Species Warnings: Use with extreme caution, if at all, in cats. Primidone is considered highly toxic to felines.
Patient Conditions: Use cautiously in patients who are hypovolemic, anemic, have borderline hypoadrenal function, or have cardiac or respiratory disease.
Drug Transition: When converting dogs from primidone to phenobarbital, it is suggested to do this slowly (tapering 1/4 of the dose each month) to prevent withdrawal seizures.
Laboratory Interference: Barbiturates may cause falsely elevated bromosulfophthalein (BSP) retention. Primidone/phenobarbital can alter thyroid testing (decreased total and free T4, normal T3, normal/increased TSH); wait at least 4 weeks after discontinuation to perform thyroid testing. May cause a false-positive low-dose dexamethasone suppression test.
医薬品相互作用
Increased risk for hepatotoxicity, particularly with large or chronic doses of barbiturates.
Oral administration may decrease the GI absorption of primidone.
May prolong phenobarbital effects.
Barbiturates may affect phenytoin metabolism, and phenytoin may alter barbiturate levels; therapeutic monitoring indicated.
May induce enzymes that increase the metabolism of barbiturates.
May increase the CNS depressant effects of phenobarbital.
May increase the effects of phenobarbital; phenobarbital may also decrease chloramphenicol levels.
May increase the CNS depressant effects of phenobarbital.
May increase the effects of phenobarbital; phenobarbital may decrease phenothiazine serum concentrations.
May increase the effects of phenobarbital.
Phenobarbital may decrease anticoagulant effects by lowering serum concentrations.
Phenobarbital may decrease effects by lowering serum concentrations.
Phenobarbital may decrease effects by lowering serum concentrations.
Phenobarbital may decrease effects by lowering serum concentrations.
Phenobarbital may decrease effects by lowering serum concentrations (effect may persist for weeks after barbiturate is discontinued).
Phenobarbital may decrease effects by lowering serum concentrations.
監視
- Anticonvulsant efficacy (seizure frequency and severity)
- Adverse effects (CNS depression, PU/PD, weight gain, signs of liver disease)
- Serum phenobarbital levels (therapeutic range in dogs thought to be 15-40 mcg/mL) if lack of efficacy or adverse reactions are noted
- Routine CBCs and liver enzyme panels at least every 6 months during chronic therapy
薬物動態
半減期
吸収
Slowly absorbed after oral administration in the dog, with peak levels occurring 2-4 hours after dosing. Bioavailability in humans is reported as 60-80%.
分布
Appears in maternal milk in substantial quantities.
代謝
Rapidly converted to phenylethylmalonamide (PEMA) and phenobarbital in the dog. Induces hepatic microsomal enzymes, increasing the rate of metabolism of itself and other drugs.
消失
Eliminated primarily via hepatic metabolism to active metabolites, which are subsequently cleared.
過剰投与
Clinical Signs: Because primidone is rapidly metabolized to phenobarbital in dogs, signs of acute toxicity mirror barbiturate overdose: sedation progressing to coma, anorexia, vomiting, ataxia, and nystagmus.
Treatment:
- Decontamination: Removal of ingested product from the gut if appropriate (emesis or gastric lavage).
- Adsorbents: Activated charcoal is of considerable benefit in enhancing the clearance of phenobarbital (acts as a 'sink' for the drug to diffuse from the vasculature back into the gut), even if the drug was administered parenterally.
- Supportive Care: Provide respiratory and cardiovascular support.
- Enhanced Elimination: Forced alkaline diuresis can augment elimination in patients with normal renal function. Peritoneal dialysis or hemodialysis may be helpful in severe intoxications or anuric patients.
製品
製剤
- Tablets
- Oral suspension
動物用医薬品
- Primidone Tablets: 50 mg and 250 mg (Neurosyn®)
ヒト用医薬品
- Primidone Tablets: 50 mg and 250 mg (Mysoline®, generic)
規制ステータス
規制データなし: 🇺🇸 US · 🇪🇺 EU · 🇬🇧 UK · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
保管・安定性
Tablets should be stored in well-closed containers, preferably at room temperature. Oral suspension should be stored in tight, light-resistant containers at room temperature; avoid freezing. Commercially available products generally have a 5-year expiration date.
飼主向け情報
治療成功の鍵: てんかん治療を成功させるためには、指示通りの投薬を厳守することが不可欠です。毎日必ず同じ時間に薬を与えてください。
- 行動の変化: 投薬開始時、ペットが不安そうにしたり、興奮したり、異常に眠そうにすることがあります。これらの症状は通常一時的なもので、体が慣れるにつれて改善します。
- 飲水量と食欲の増加: 水をたくさん飲む、尿量が増える、食欲が増すなどの変化が見られることがあります。体重を監視し、食事管理について獣医師に相談してください。
- 獣医師への連絡の目安: 極度の無気力、食欲不振、嘔吐、白目や歯茎の黄染（黄疸）、歯茎の蒼白が見られる場合、または発作がうまくコントロールできない場合は、直ちに獣医師に連絡してください。
- 急な休薬の禁止: 獣医師の指示なしに突然投薬を中止しないでください。重度で生命を脅かす発作を引き起こす危険があります。
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