Source: Health Products Regulatory Authority (ZA) Revision Year: 2021 Publisher: Strides Pharma SA (Pty) Ltd., 106 16th Road, Building 2, Midrand, 1685
Pharmacological classification: A 22.1.4 Vitamins: Other Pharmacotherapeutic group: Vitamin D and analogues
ATC code: A11CC03
1α-hydroxyvitamin D3 (1α-OHD3) is converted in the liver to 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3), the metabolite through which vitamin D is considered to express most of its effects on calcium and phosphorus metabolism. Impaired endogenous production of 1,25-(OH)2D3 by the kidney appears to contribute to the disturbance in mineral metabolism found is several disorders including renal bone disease, hypoparathyroidism and pseudo-deficiency rickets. These disorders, termed “vitamin D resistant” since they usually require high doses of vitamin D for their correction, respond to small physiological doses of 1α-OHD3.
The delay in response and high dosage required in treating these disorders with parent vitamin D makes dosage adjustment difficult. This can result in unpredictable hypercalcaemia which may take weeks or months to reverse. Should inadvertent hypercalcaemia occur it can be reversed by stopping treatment.
In patients with renal failure, 1-5 microgram/day of 1α-hydroxyvitamin D (1α-OHD3) increased intestinal calcium and phosphorus absorption in a dose related manner. This effect was observed within 3 days of starting the drug and conversely, it was reversed within 3 days of its discontinuation.
In patients with nutritional osteomalacia, increases in calcium absorption were noted within 6 hours of giving 1 μg 1α-OHD3 orally and usually peaked at 24 hours. 1α-OHD3 also produced increases in plasma inorganic phosphorus due to increased intestinal absorption and renal tubular re-absorption. This latter effect is a result of PTH suppression by 1α-OHD3. The effect of the drug on calcium was about double its effect on phosphorus absorption.
Patients with chronic renal failure have shown increased serum calcium levels within 5 days of receiving 1α-OHD3 in a dose of 0,5-1,0 microgram/day. As serum calcium rose, PTH levels and alkaline phosphatase decreased toward normal.
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