骨化三醇
骨化三醇（Calcitriol）是維生素D的活性荷爾蒙形式。在獸醫學中，主要用於治療**低血鈣症**（通常繼發於副甲狀腺機能低下），以及抑制慢性腎病（CKD）中的**繼發性副甲狀腺機能亢進**。它也可能對治療某些犬種的原發性特發性皮脂漏有益。 > **臨床要點：** 在健康的動物中，腎臟會產生1-α-羥化酶，將無活性的維生素D轉化為活性形式（骨化三醇）。在慢性腎衰竭中，功能性腎臟質量的喪失導致此酵素缺乏。補充骨化三醇可繞過此代謝缺陷，直接抑制副甲狀腺素（PTH）的合成，從而預防或治療腎性繼發性副甲狀腺機能亢進（腎性骨病）。
作用機制: Calcitriol binds to the intracellular **Vitamin D Receptor (VDR)** in target tissues (intestine, bone, kidney, and parathyroid gland) to regulate calcium and phosphorus homeostasis. * **Intestine:** Upregulates the synthesis of calcium-binding proteins (e.g., calbindin) → significantly enhances the GI absorption of calcium and phosphorus. * **Kidney:** Promotes renal tubular reabsorption of calcium. * **Parathyroid Gland:** Directly binds to VDRs on the parathyroid gland → inhibits the transcription of the **PTH** gene, reducing PTH synthesis and secretion. * **Bone:** Works synergistically with PTH to stimulate osteoclast activity, mobilizing calcium and phosphorus into the extracellular fluid. Unlike other forms of vitamin D (like cholecalciferol or ergocalciferol), calcitriol does **not** require hepatic or renal activation, resulting in a rapid onset of action (approximately 1 day) and a short half-life.
各物種劑量
- To suppress secondary hyperparathyroidism in CRF · 1.5-3.5 nanograms/kg PO daily given separately from meals (or 2.5-3.5 nanograms/kg PO once daily) · PO · q24h · Long-term · Remove oil from capsule, dilute in corn oil, then give the appropriate volume.
- Long-term maintenance in animals with hypoparathyroidism · 0.03-0.06 micrograms/kg/day · PO · q24h · Long-term · Combine with oral calcium to reduce vitamin D dose requirements.
- To suppress secondary hyperparathyroidism in CRF · 2.5-3.5 nanograms/kg/day PO (Dogs with refractory hyperparathyroidism may require up to 6 nanograms/kg/day). Alternatively, pulsed-dosing: 20 nanograms/kg twice weekly PO at bedtime on an empty stomach. · PO · q24h or twice weekly · Long-term · Confirm CRF (creatinine >2 mg/dL) and reduce hyperphosphatemia to <6 mg/dL before starting.
- Subacute and chronic maintenance treatment of hypocalcemia · Initially, 20-30 nanograms/kg/day PO divided twice a day for 3-4 days, then 5-15 nanograms/kg/day divided twice a day · PO · q12h · Initial 3-4 days, then maintenance
- Long-term maintenance in animals with hypoparathyroidism · 0.03-0.06 micrograms/kg/day · PO · q24h · Long-term · Combine with oral calcium to reduce vitamin D dose requirements.
- Primary idiopathic seborrhea (especially in spaniel breeds) · 10 nanograms/kg PO once daily · PO · q24h · Give as far away from the main meal as possible.
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給藥途徑
禁忌症
- Hypercalcemia
- Hyperphosphatemia (Calcium x Phosphorus product > 70)
- Vitamin D toxicity
- Malabsorption syndromes
- Abnormal sensitivity to vitamin D effects
不良反應
- Hypercalcemia
- Hypercalciuria
- Hyperphosphatemia
- Polydipsia
- Polyuria
- Anorexia
- Tissue mineralization (if given with hyperphosphatemia)
藥物相互作用
- Calcium-containing phosphorus binding agents (e.g., calcium carbonate) · Use with calcitriol may induce hypercalcemia.
- Corticosteroids · Can nullify the effects of vitamin D analogs.
- 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.
- Phenytoin, Barbiturates, Primidone · May induce hepatic enzyme systems and increase the metabolism of Vitamin D analogs, thus decreasing their activity.
- Thiazide diuretics · May cause hypercalcemia when given in conjunction with Vitamin D analogs.
監測
- Serum calcium (baseline, 1 week, then every 2-4 weeks/6 months depending on protocol)
- Serum phosphorus
- Serum creatinine
- Serum PTH levels (especially in cats or refractory cases)
- Clinical efficacy (improved appetite, activity level, slowed progression of disease)
過量
Overdosage can cause **hypercalcemia**, **hypercalciuria**, and **hyperphosphatemia**. * **Acute Ingestion:** Manage using established protocols for GI decontamination. Orally administered mineral oil may reduce absorption and enhance fecal elimination. * **Chronic Overdosage:** Hypercalcemia secondary to chronic dosing should be treated by first temporarily discontinuing (not just dose reduction) calcitriol and exogenous calcium therapy. * **Severe Hypercalcemia:** May require treatment with furosemide, calcium-free IV fluids (e.g., normal saline), urine acidification, and corticosteroids.
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