麥角鈣化醇 (維生素D2)
**麥角鈣化醇 (Ergocalciferol, 維生素D2)** 是一種脂溶性維生素及**維生素D類似物**，主要用於獸醫學中治療與甲狀旁腺功能低下相關的低血鈣症。 * **具成本效益**：通常作為骨化三醇 (Calcitriol) 或雙氫速甾醇 (DHT) 的較廉價替代品。 * **起效較慢**：因為它需要肝臟和腎臟的酵素活化，與活性代謝物相比，需要較長時間（5-21天）才能對血清鈣產生最大影響。 * **毒性持久**：具有高度脂溶性並儲存於脂肪中。如果發生高血鈣症，即使停藥，毒性作用也可能持續長達18週。 > **臨床要點**：與膽鈣化醇 (維生素D3) 相比，貓無法有效地將麥角鈣化醇轉化為其25-羥基化形式。因此，在治療貓的低血鈣症時，通常首選骨化三醇。
作用機制: Ergocalciferol is an inactive prodrug that requires a two-step enzymatic activation process to regulate calcium homeostasis: 1. Ergocalciferol → **hepatic 25-hydroxylase** → 25-hydroxyvitamin D2 (calcidiol, partially active). 2. 25-hydroxyvitamin D2 → **renal 1-alpha-hydroxylase** → 1,25-dihydroxyvitamin D2 (fully active calcitriol equivalent). The active metabolite binds to intracellular **Vitamin D Receptors (VDR)**, acting as a hormone alongside parathyroid hormone (PTH) and calcitonin to: * Enhance intestinal absorption of calcium and phosphate. * Promote renal tubular reabsorption of calcium. * Increase the rate of accretion and osteoclastic resorption of minerals in bone, mobilizing calcium into the extracellular fluid.
各物種劑量
- Maintenance therapy of parathyroid failure after using parenteral calcium to control hypocalcemic tetany · Initially 4000-6000 Units/kg PO once daily. Maintenance doses usually range from 1000-2000 Units/kg PO once daily to once weekly. · PO · once daily to once weekly · Lifelong · Patient should be hospitalized initially. Adjust dosage to maintain serum calcium concentrations between 8-9.5 mg/dL. Goal is to prevent hypocalcemic tetany, but not induce hypercalcemia.
- Maintenance therapy of parathyroid failure after using parenteral calcium to control hypocalcemic tetany · Initially 4000-6000 Units/kg PO once daily. Maintenance doses usually range from 1000-2000 Units/kg PO once daily to once weekly. · PO · once daily to once weekly · Lifelong · Patient should be hospitalized initially. Adjust dosage to maintain serum calcium concentrations between 8-9.5 mg/dL. Goal is to prevent hypocalcemic tetany, but not induce hypercalcemia.
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給藥途徑
禁忌症
- Hypercalcemia
- Vitamin D toxicity
- Malabsorption syndrome
- Abnormal sensitivity to the effects of vitamin D
不良反應
- Hypercalcemia
- Hyperphosphatemia
- Nephrocalcinosis
- Soft tissue mineralization
藥物相互作用
- Corticosteroids · Can reduce the effects of vitamin D analogs by decreasing calcium absorption and increasing renal excretion.
- Digoxin · Patients are highly sensitive to the arrhythmogenic effects of hypercalcemia; intensified monitoring is required.
- Verapamil · Patients are sensitive to the effects of hypercalcemia; intensified monitoring is required.
- Mineral Oil · May reduce the amount of orally administered ergocalciferol absorbed from the GI tract.
- Thiazide Diuretics · May cause hypercalcemia when given in conjunction with Vitamin D analogs due to decreased renal calcium excretion.
監測
- Serum calcium concentrations (weekly initially, then monthly for 6 months, then every 2-3 months)
- Serum phosphorus concentrations
- Renal function parameters (BUN, Creatinine)
- Clinical signs of hypercalcemia (PU/PD, lethargy, anorexia, vomiting)
過量
### Acute Toxicity * The toxic acute oral dose of ergocalciferol in dogs is reported as **4 mg/kg (160,000 Units/kg)**. * **Decontamination**: Acute ingestions should be managed using established protocols for removal or prevention of GI absorption. Orally administered **mineral oil** may reduce absorption and enhance fecal elimination. ### Chronic Overdosage (Hypercalcemia) * **Immediate Action**: Temporarily discontinue ergocalciferol and any exogenous calcium therapy. * **Medical Management**: If hypercalcemia is severe, administer **0.9% normal saline** (calcium-free IV fluids), **furosemide** (to promote calciuresis), and **corticosteroids** (to decrease GI absorption and increase renal excretion of calcium). Urine acidification may also be employed. * **Prolonged Effect**: Because of the long duration of action (potentially up to 18 weeks), hypercalcemia may persist for weeks to months. Restart therapy at a reduced dosage only when calcium serum levels return to the normal range.
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