The authors present a stereotypical case presentation of x-linked hypophosphatemia (XLH) and provide a review of the pathophysiology and related pharmacology of this condition, primarily focusing on the FDA-approved medication burosumab. XLH is a renal phosphate wasting disorder caused by loss of function mutations in the PHEX gene (phosphate-regulating gene with homologies to endopeptidases on the X chromosome). Typical biochemical findings include elevated serum levels of bioactive/intact Fibroblast Growth Factor 23 (FGF23) which lead to i) low serum phosphate levels, ii) increased fractional excretion of phosphorus, and iii) inappropriately low or normal 1,25-dihydroxyvitamin D (1,25-vitD). XLH is the most common form of heritable rickets and short stature in XLH patients is due to chronic hypophosphatemia. Additionally, XLH patients experience joint pain and osteoarthritis from skeletal deformities, fractures, enthesopathy, spinal stenosis, and hearing loss. Historically, treatment for XLH was limited to oral phosphate supplementation, active vitamin D supplementation, and surgical intervention for cases of severe bowed legs. In 2018, the United States Food and Drug Administration’s (FDA) approved burosumab for the treatment of XLH and this medication has demonstrated substantial benefit compared with conventional therapy. Burosumab binds circulating intact FGF23 and blocks its biological effects in target tissues, resulting in increased serum inorganic phosphate (Pi) concentrations and increased conversion of inactive vitamin D to active 1,25-vitD.

Phosphocalcium metabolism disorders have emerged as potential complications in individuals recovering from COVID-19. This abstract provides a concise summary of the current understanding of phosphocalcium metabolism disturbances post-COVID, their prevalence, clinical manifestations, and underlying mechanisms. The impact of COVID-19 on phosphorus, calcium, and parathyroid hormone levels is discussed, highlighting the importance of regular monitoring and appropriate management strategies. The abstract emphasizes the need for further research to enhance our understanding of these disorders and improve post-COVID patient care.

To maintain an optimal body content of phosphorus throughout postnatal life, variable phosphate absorption from food must be finely matched with urinary excretion. This amazing feat is accomplished through synchronised phosphate transport by myriads of ciliated cells lining the renal proximal tubules. These respond in real time to changes in the phosphate and composition of the renal filtrate, and to hormonal instructions. How they do this has stimulated decades of research. New analytical techniques, coupled with incredible advances in computer technology, have opened new avenues for investigation at a sub-cellular level. There has been a surge of research into different aspects of the process. These have verified long held beliefs and are also dramatically extending our vision of the intense, integrated, intracellular activity which mediates phosphate absorption. Already, some have indicated new approaches for pharmacological intervention to regulate phosphate in common conditions, including chronic renal failure and osteoporosis, as well as rare inherited biochemical disorders. It is a rapidly evolving field. The aim here is to provide an overview of our current knowledge, to show where it is leading, and where there are uncertainties. Hopefully this will raise questions and stimulate new ideas for further research.

Adult X-linked hypophosphatemia (XLH) patients present with specific symptoms, including enthesopathies (e.g., ossification of the longitudinal ligament (OPLL), osteophytes around the large joint, and enthesopathy in the Achilles tendon), the development of severe secondary and tertiary hyperparathyroidism (SHPT/THPT) and the subsequent progression of chronic kidney disease (CKD). In addition, these patients exhibit the typical phenotypes of osteomalacia, such as pseudofracture and fracture in weight-bearing bones, odontitis, and tooth abscess. The mechanism underlying enthesopathy development is unknown; however, a common underlying mechanism among XLH and autosomal recessive hypophosphatemic rickets, ARHR1.2, due to mutations in PHEX, DMP1, and ENPP1, is assumed. Clarification of the pathogenesis and drug discovery for this complication is an urgent issue to address, as many adult XLH patients suffer subsequent debilitating nervus symptoms or impingement syndrome, and existing treatments are ineffective. Severe SHPT and THPT are associated with conventional therapy, including active vitamin D and phosphate supplementation, and complicated and careful adjustment of the dosage by experienced clinicians is required to avoid SHPT/THPT. Burosumab is a very effective therapy without risk for the development of SHPT/THPT. However, the indication of this drug should be carefully considered along with the cost-effectiveness, guidelines or recommendations and health care system of each country.