Three-Year Successful Cinacalcet Treatment of Secondary Hyperparathyroidism in a Patient with X-Linked Dominant Hypophosphatemic Rickets: A Case Report

Hypophosphatemic rickets (HR) is a rare inherited disorder characterized by a classic rickets phenotype with low plasma phosphate levels and resistance to treatment with vitamin D. Development of secondary hyperparathyroidism (SHPT) as a direct consequence of treatment is a frequent complication and a major clinical challenge, as this may increase risk of further comorbidity. Cinacalcet, a calcimimetic agent that reduces the secretion of PTH from the parathyroid glands, has been suggested as adjuvant treatment to SHPT in patients with HR. However, only two papers have previously been published and no data are available on effects of treatment for more than six months. We now report a case of 3-year treatment with cinacalcet in a patient with HR complicated by SHPT. A 53-year-old woman with genetically confirmed X-linked dominant hypophosphatemic rickets developed SHPT after 25 years of conventional treatment with alfacalcidol and phosphate supplements. Cinacalcet was added to her treatment, causing a sustained normalization of PTH. Ionized calcium decreased, requiring reduction of cinacalcet, though asymptomatical. Level of phosphate was unchanged, but alkaline phosphatase increased in response to treatment. Cinacalcet appeared to be efficient, safe, and well tolerated. We recommend close control of plasma calcium to avoid hypocalcemia.