Skeletal Disorders and Mineral Homeostasis Section

National Institute Of Dental & Craniofacial Research

Investigators

Linked publications & trials

Abstract

Hypoparathyroidism: Modulation of the function of the CASR by small molecule allosteric modulators of the CASR has represented a major breakthrough in the field of mineral homeostasis. Based on our earlier proof of principle study (PMID: 31063613), we partnered with BridgeBio Pharma to develop the calcilytic encaleret to treat ADH1. The results of our ongoing phase 2 study conducted by the Principal Investigator, Rachel Gafni, were recently presented at the annual meeting of the ASBMR, where the abstract achieved the Best Clinical Abstract of the Year award make clear that our hypothesis was correct. These early results showing a 100% response rate, and correction of all abnormal parameters of mineral homeostasis have given the greenlight for a multicenter phase 3 study of encaleret in ADH1 that is underway. The results of the phase 2b study will be published in the New England Journal of Medicine on Sep. 28, 2023. PTH therapy can effectively manage hypocalcemia; however, the effects of PTH treatment on QOL are unclear. Thirty-one patients with hypoparathyroidism were treated in an open-label study with full replacement subcutaneous PTH 1-34 twice daily for up to 5.3 years. The 36-Item Short Form (SF-36) Health Survey, Fatigue Symptom Inventory (FSI), and 6-minute walk test (6MWT) were assessed at PTH start (baseline), every 6 months on PTH, and after PTH discontinuation. The SF-36 assesses physical function (PF), physical role limitations (RP), bodily pain (BP), general health (GH), vitality (VT), emotional role limitations (RE), social function (SF), and mental health (MH). Patients at baseline had lower scores in RP, GH, VT, and MH (p < 0.05), consistent with impaired QOL. With PTH therapy, only GH at 6 months and VT at 12 months improved (p < 0.05). At the last treatment time point, RP, VT, and SF improved compared to baseline (p < 0.05). Follow-up scores were unchanged from baseline, except for SF, which had decreased at follow-up compared to on-PTH (p < 0.05). On the FSI, there were no changes in fatigue frequency; perceived interference was improved at 12 and 18 months and composite severity was improved only at 60 months (p < 0.05). The 6MWT measures did not change. In conclusion, hypoparathyroidism is associated with decreased QOL. Despite the bias in open-label studies to predict improvements in QOL, PTH therapy had limited and non-sustained effects on QOL, inconclusive changes in fatigue experience, and no change in the6MWT. The effects of PTH 1-34 on QOL appear to be minimal. Tumor-induced osteomalacia (TIO): Based on our previous success of infigratinib we conducted on open label, phase 2 study in patients with benign TIO. The primary endpoint was persistent normalization of blood phosphate and FGF23 after discontinuation. Four adults with TIO (two nonlocalized, two nonresectable PMTs) were treated with daily infigratinib for up to 24 weeks. All patients had a favorable biochemical response that included reduction in intact FGF23, and normalization of blood phosphate and 1,25D. However, these effects disappeared after drug discontinuation with biochemistries returning to baseline; no patients entered biochemical remission. Patients experienced mild to moderate, treatment-related, dose-limiting adverse events (AEs), but no serious AEs. Infigratinib treatment lowered FGF23, increased blood phosphate, and suppressed PMT activity, confirming the role of FGFR signaling in PMT pathogenesis. However, treatment-related AEs at efficacy doses and disease persistence on discontinuation support restricting the use of infigratinib to patients with life-limiting metastatic PMTs (PMID: 32888946). FGF23 measurement is a critical tool in the evaluation of patients with disordered phosphate homeostasis. Laboratory reference ranges for FGF23 were developed from normophosphatemic individuals. Reliance on such values can lead to misdiagnosis in patients with FGF23-mediated hypophosphatemia. To determine FGF23 levels that are diagnostic for identification of patients with FGF23-mediated hypophosphatemic disorders, we studied 149 patients with various disorders of FGF23-mediated and FGF23-independent hypophosphatemia and defined cut-off levels for both intact FGF23 (iFGF23) and C-terminal FGF23 (cFGF23) that can accurately distinguish between FGF23-mediated and FGF23-independent hypophosphatemia; we also assessed FGF23 levels and phosphate in 434 patients with various forms of hypophosphatemia, hyperphosphatemia, and normophosphatemia. An iFGF23 cut-point of 27 pg/ml was 100%sensitive and specific in distinguishing FGF23-mediated from FGF23-independent hypophosphatemia, and a cFGF23 cut-point of 90 RU/ml was 100% sensitive and specific in distinguishing specifically TIO from FGF23-independent hypophosphatemia. There was overlap in the cFGF23 range of 45-90 RU/mL between genetic forms of FGF23 excess and FGF23-independent hypophosphatemia, supporting the superiority of iFGF23 over cFGF23 in making the diagnosis of FGF23-mediated hypophosphatemia. Using the laboratory upper limit of normal for cFGF23 (180 RU/ml) would result in a misdiagnosis in more than half of patients with FGF23-mediated hypophosphatemia. In this, the largest study of FGF23 in chronic hypophosphatemia to date, we established iFGF23 and cFGF23 cut-off values to assist in the evaluation and diagnosis of hypophosphatemic conditions. Cutaneous Skeletal Hypophosphatemia Syndrome: CSHS is a multisystem disorder that prominently includes skin lesions, a skeletal dysplasia, and FGF23 excess due to somatic, mosaically-expressed gain-of-function variants in H or NRAS variants. The source of FGF23 has been debated, with some suspecting it was the skin lesions. Patients frequently undergo painful excision of skin lesions in an effort to correct FGF23 excess. The literature does not support this is successful and it was our hypothesis that the bone lesions were the source of FGF23 excess. To test this hypothesis, confirm bone is the source, and relieve patients from being subjected to futile, painful skin surgeries, we made mouse models of CSHS in which the RAS variants were expressed in either bone or skin. The findings, recently published in the Journal of Clinical Investigation, clearly demonstrated bone was the source of FGF23 (PMID: 36943390). FGF23 and PTH physiology: Parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF23) both influence blood phosphate levels by regulating urinary phosphate reabsorption. Clinical data suggest that adequate renal phosphate handling requires the presence of both FGF23 and PTH. To investigate whether the phosphaturic effects of PTH and FGF23 are interdependent, 11 patients with hypoparathyroidism and 1 patient with hyperphosphatemic familial tumoral calcinosis (HFTC), characterized by deficient intact FGF23 action and resulting hyperphosphatemia, were treated with synthetic human PTH 1-34(hPTH 1-34). Biochemical parameters, including blood phosphate, calcium, intact FGF23 (iFGF23), nephrogenic cAMP, 1,25(OH) vitamin D (1,25D), and tubular reabsorption of phosphate (TRP), were measured at baseline and after hPTH 1-34 treatment. In patients with hypoparathyroidism, administration of hPTH 1-34 increased nephrogenic cAMP, which resulted in serum phosphate normalization followed by a significant decrease in iFGF23. TRP initially decreased and returned to baseline. In the patient with HFTC, hPTH 1-34 administration also increased nephrogenic cAMP, but this did not produce changes in phosphate or TRP. No changes in calcium were observed in any of the studied patients, although prolonged hPTH 1-34 treatment did induce supraphysiologic 1,25Dlevels in the patient with HFTC. Our results indicate that PTH and FGF23 effects on phosphate regulation are interdependent and both are required to adequately regulate renal phosphate handling.