葉酸
**葉酸**（維生素 B9）是一種重要的水溶性維生素，對於**核蛋白合成**和**正常的紅血球生成**至關重要。 在獸醫學中，它主要用於治療葉酸缺乏症，這通常繼發於胃腸道疾病。 **臨床要點與診斷效用：** - **吸收部位：** 葉酸專門在**近端小腸**吸收，而鈷胺素（維生素 B12）則在遠端小腸吸收。 - **診斷指標：** 測量血清葉酸和鈷胺素有助於定位腸道疾病。葉酸偏低通常暗示近端小腸疾病。 - **細菌過度生長：** 患有胰腺外分泌不足 (EPI) 或小腸細菌過度生長 (SIBO) 的犬隻，其血清葉酸通常會*升高*，因為腸道細菌會合成葉酸。 - **抗葉酸藥物救援：** 它也用於預防或治療因長期使用二氫葉酸還原酶抑制劑（例如：甲氧苄啶、乙胺嘧啶）所引起的缺乏症。
作用機制: 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.
各物種劑量
- 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.
劑量為持牌獸醫專業人員的臨床參考。請務必對照最新藥品說明書及個別病患確認。
給藥途徑
禁忌症
- 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)
不良反應
- 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)
藥物相互作用
- 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.
監測
- 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
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.
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