葉酸
**葉酸**（ビタミンB9）は、**核タンパク質合成**および**正常な赤血球形成**に不可欠な重要な水溶性ビタミンです。 獣医学では、主に胃腸疾患に続発する葉酸欠乏症の治療に使用されます。 **臨床的特徴と診断の有用性：** - **吸収部位：** 葉酸は**近位小腸**で専ら吸収されますが、コバラミン（ビタミンB12）は遠位小腸で吸収されます。 - **診断指標：** 血清葉酸およびコバラミンの測定は、腸疾患の局在化に役立ちます。葉酸の低下は近位小腸疾患を示唆します。 - **細菌異常増殖：** 膵外分泌不全（EPI）または小腸内細菌異常増殖（SIBO）の犬では、腸内細菌が葉酸を合成するため、血清葉酸が*上昇*することがよくあります。 - **抗葉酸薬のレスキュー：** ジヒドロ葉酸還元酵素阻害薬（トリメトプリム、ピリメタミンなど）の長期投与によって引き起こされる欠乏症の予防または治療にも使用されます。
作用機序: Folic acid is a precursor that must be enzymatically reduced to become biologically active. **Mechanism Pathway:** **Folic Acid** → reduced by **dihydrofolate reductase (DHFR)** → **Tetrahydrofolate (THF / 5-methyltetrahydrofolate)**. - **THF** acts as a crucial coenzyme and methyl donor in the synthesis of purine and pyrimidine nucleotides. - This pathway is absolutely necessary for **DNA synthesis**, cellular division, and the maintenance of normal **erythropoiesis**. - Drugs like trimethoprim and methotrexate competitively inhibit DHFR, blocking this pathway and potentially causing megaloblastic anemia or leukopenia, which is why folic acid supplementation is used as a rescue therapy.
動物種別の用量
- Severe folate deficiency · 0.5-2 mg (total dose) PO once daily · PO · q24h · 1 month
- Folate deficiency secondary to exocrine pancreatic insufficiency · 400 micrograms (0.4 mg) PO once daily · PO · q24h
- Long-term use of high dose trimethoprim/sulfa (for treating Nocardia) · 2 mg (total dose) PO once daily · PO · q24h
- Adjunctive therapy in cats with inflammatory bowel disease · 0.5-1 mg PO q24h (once daily) · PO · q24h · 4-6 weeks · Given with cobalamin at 125-250 micrograms SC or IM once a week.
- Prolonged therapy with antifolate medications (e.g., trimethoprim, pyrimethamine) · 20-40 mg (total dose) PO per day · PO · q24h · Pregnant mares should routinely receive folic acid supplementation during treatment with antifolates.
- Severe folate deficiency · 0.5-2 mg (total dose) PO once daily · PO · q24h · 1 month
- Folate and cobalamin deficiency secondary to inflammatory bowel disease · 5 mg (total dose) PO once daily · PO · q24h · 1-6 months · Given with cyanocobalamin 750 micrograms parenterally once per month.
用量は獣医療従事者向けの臨床リファレンスです。必ず最新の添付文書と個々の患者で確認してください。
投与経路
禁忌
- Known intolerance or hypersensitivity to folic acid
- Undiagnosed anemias (folic acid therapy may mask the hematologic signs of pernicious anemia/cobalamin deficiency while neurologic damage progresses)
有害事象
- Hypersensitivity reactions (rare)
- Gastrointestinal effects (rare)
- CNS effects such as difficulty sleeping, excitement, or confusion (seen only at very high oral doses in humans, e.g., 15 mg/day)
薬物相互作用
- Chloramphenicol · May delay the hematologic response to folic acid.
- Methotrexate · Inhibits dihydrofolate reductase; interferes with folic acid utilization.
- Trimethoprim · Inhibits dihydrofolate reductase; interferes with folic acid utilization.
- Pyrimethamine · Inhibits dihydrofolate reductase; interferes with folic acid utilization.
- Phenytoin · Folic acid may decrease serum phenytoin levels, potentially requiring a dosage increase; increased seizure frequency can occur.
- Sulfasalazine · May increase the risk for folate deficiency.
- Barbiturates · May increase the risk for folate deficiency.
- Nitrofurantoin · May increase the risk for folate deficiency.
- Primidone · May increase the risk for folate deficiency.
モニタリング
- Serum folate and cobalamin levels (before and after treatment)
- Clinical signs associated with deficiency (e.g., chronic diarrhea, weight loss)
- CBC (baseline and ongoing if abnormal, to monitor for anemia)
過量投与
Folic acid is relatively **non-toxic**. If an inadvertent overdose occurs, no specific treatment should be required. Excess drug is rapidly metabolized or excreted unchanged in the urine.
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。