カルシトリオール
カルシトリオールは、ビタミンDの活性型ホルモンです。獣医学では、主に**低カルシウム血症**（多くの場合、副甲状腺機能低下症に続発）の管理や、慢性腎臓病（CKD）における**二次性副甲状腺機能亢進症**の抑制に使用されます。また、特定の犬種における原発性特発性脂漏症の治療にも有益な場合があります。 > **臨床のポイント：** 健康な動物では、腎臓が1-α-ヒドロキシラーゼという酵素を産生し、不活性なビタミンDを活性型（カルシトリオール）に変換します。慢性腎不全では、機能的な腎質量の喪失によりこの酵素が欠乏します。カルシトリオールを補充することでこの代謝欠陥を迂回し、副甲状腺ホルモン（PTH）の合成を直接抑制して、腎性二次性副甲状腺機能亢進症（腎性骨異栄養症）を予防または治療します。
作用機序: 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.
動物種別の用量
- 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.
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
- Hypercalcemia
- Hyperphosphatemia (Calcium x Phosphorus product > 70)
- Vitamin D toxicity
- Malabsorption syndromes
- Abnormal sensitivity to vitamin D effects
有害事象
- Hypercalcemia
- Hypercalciuria
- Hyperphosphatemia
- Polydipsia
- Polyuria
- Anorexia
- Tissue mineralization (if given with hyperphosphatemia)
薬物相互作用
- 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.
モニタリング
- 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)
過量投与
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.
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。