Autosomal dominant hypocalcemia type 1 (ADH1) is a rare genetic disorder most often caused by gain-of-function (GOF) variants in the gene encoding the calcium sensing receptor, CASR (HGNC:1514), which regulates calcium homeostasis. Diagnosis of ADH1 can be challenging in part due to limited knowledge of which CASR variants result in GOF. To address these challenges, we integrated population-based biobank data and experiments to identify novel associations of CASR variants with ADH1.

ABSTRACT Autosomal dominant hypocalcemia type 1 (ADH1) is a rare form of hypoparathyroidism due to activating variants of the calcium-sensing receptor gene (CASR). Inherited or de novo activating variants of the CASR alter the set point for extracellular calcium, resulting in inadequate parathyroid hormone (PTH) secretion and inappropriate renal calcium excretion leading to hypocalcemia and hypercalciuria. Conventional therapy includes calcium and activated vitamin D, which can worsen hypercalciuria, resulting in renal complications. A systematic literature review, using published reports from 1994 to 2021, was conducted to catalog CASR variants, to define the ADH1 clinical spectrum, and to determine the effect of treatment on patients with ADH1. There were 113 unique CASR variants reported, with a general lack of genotype/phenotype correlation. Clinical data were available in 191 patients; 27% lacked symptoms, 32% had mild/moderate symptoms, and 41% had severe symptoms. Seizures, the most frequent clinical presentation, occurred in 39% of patients. In patients with blood and urine chemistries available at the time of diagnosis (n = 91), hypocalcemia (99%), hyperphosphatemia (59%), low PTH levels (57%), and hypercalciuria (34%) were observed. Blood calcium levels were significantly lower in patients with severe symptoms compared with asymptomatic patients (6.8 ± 0.7 versus 7.6 ± 0.7 mg/dL [mean ± SD]; p < 0.0001), and the age of presentation was significantly lower in severely symptomatic patients (9.1 ± 15.0 versus 19.3 ± 19.4 years; p < 0.01). Assessments for complications including nephrocalcinosis, nephrolithiasis, renal impairment, and brain calcifications in 57 patients on conventional therapy showed that 75% had at least one complication. Hypercalciuria was associated with nephrocalcinosis, nephrolithiasis, renal impairment, or brain calcifications (odds ratio [OR] = 9.3; 95% confidence interval [CI] 2.4–37.2; p < 0.01). In 27 patients with urine calcium measures before and after starting conventional therapy, the incidence of hypercalciuria increased by 91% (p < 0.05) after therapy initiation. ADH1 is a condition often associated with severe symptomatology at presentation with an increase in the risk of renal complications after initiation of conventional therapy. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Abstract Autosomal dominant hypocalcemia type 1 (ADH1), caused by gain-of-function variants in the gene encoding the calcium-sensing receptor (CaSR), is characterized by hypocalcemia, hyperphosphatemia, hypomagnesemia, low parathyroid hormone (PTH), and hypercalciuria. Conventional therapy (calcium and activated Vitamin D) worsens hypercalciuria and can lead to renal morbidity. Calcilytics (negative allosteric modulators of the CaSR) decrease the sensitivity of hyperactive receptors to extracellular calcium and normalize blood and urine abnormalities in ADH1 rodent models. Encaleret is an oral calcilytic under investigation as a potential treatment for ADH1. Thirteen adults (22-60y) with ADH1 due to 9 unique CASR variants participated in a 3-period, Phase 2b, open-label, dose-ranging study. Conventional therapy was discontinued prior to encaleret initiation. Period 1 (P1) was a 5-day inpatient dose-escalation course (n=6). Period 2 (P2) was a 5-day inpatient course (n=13) in which doses were individually titrated to normalize albumin-corrected blood calcium (cCa) and minimize hypercalciuria and hypophosphatemia. All 13 participants continued into Period 3 (P3), a 24-week outpatient maintenance period. Patients underwent serial 24hr blood and urine sampling in P1, P2, and in P3 at Weeks 8, 16, and 24, with additional outpatient laboratory timepoints. The mean±SD encaleret dose at the end of P2, Day 5 (P2D5) was 94±64mg BID (range: 10-180 BID); by P3, Week 24 (P3W24), the mean was 78±67mg BID (5-190 BID). Encaleret was well-tolerated with no serious adverse events reported; there were no treatment discontinuations or study withdrawals. Twenty-four hour mean±SD values from P2D5 (n=13) and P3W24 (n=12) compared to baseline are presented. Baseline PTH was low at 6.3±7.8 pg/mL (nl 10-65) and had normalized by P2D5 (40.5±37.5, p<0.01); this was sustained through P3W24 (31.3±20.8, p<0.01). Likewise, baseline hypocalcemia (cCa=7.1±0.4 mg/dL [nl 8.4-10.2]) corrected to 8.6±0.7 by P2D5 (p<0.01) and remained normal through P3W24 (9.0±0.6, p<0.01). Baseline urinary calcium was 395±216 mg/d (nl <250-300) and decreased to 179±108 by P2D5 (p<0.01) and 189±72 at P3W24 (p<0.05); urinary calcium excretion was normal in 10/12 patients at P3W24. Baseline phosphate decreased from 4.5±1.1 mg/dL (nl 2.3-4.7) to 3.2±0.7 on P2D5 (p<0.01) and was maintained through P3W24 (3.5±0.6, p<0.05). Magnesium increased from low- to mid-normal (baseline 1.7±0.2 mg/dL [nl 1.6-2.6]; P2D5 1.9±0.2 [p<0.01]; P3W24 2.0±0.2, [p<0.01]). 1,25-dihydroxy-Vitamin D increased from 19.5±4.4 pg/mL (nl 20-70) to 32.0±16.7 on P2D5 (p<0.05) and 30.2±14.0 on P3W24 (p<0.05). Bone turnover markers were not different between baseline (n=7, CTX=253±111 pg/mL; P1NP=34±10 mcg/L) and P2D5 (CTX=241±181, p=NS; P1NP=26±12, p=NS) but had increased by P3W24 (CTX=784±686, p<0.01; P1NP=102±87, p<0.01). In conclusion, this study represents a molecularly targeted, precision medicine approach to the treatment of ADH1. The consistent and sustained results from all periods of this Phase 2 study establish a clinically meaningful efficacy, tolerability, and safety profile for encaleret as a potential treatment of adults with ADH1. Presentation: Monday, June 13, 2022 11:15 a.m. - 11:30 a.m.

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Introduction: Autosomal dominant hypocalcemia type 1 (ADH1) is caused by gain-of-function variants of the calcium-sensing receptor (CaSR) gene, resulting in receptor proteins with increased sensitivity to extracellular calcium. Individuals with ADH1 exhibit hypocalcemia, hypomagnesemia, hyperphosphatemia, and inappropriately low levels of PTH. Hypocalcemia and hypomagnesemia can slow cardiac repolarization and potentially prolong the QT interval in patients with ADH1. Prolongation of the QT interval can lead to potentially malignant ventricular arrhythmias. Calcilytics are antagonists of the CaSR with the potential to restore normal CaSR sensitivity in ADH1 and thereby normalize blood levels of calcium and magnesium. This study investigated the biochemical and ECG effects of encaleret, an investigational oral calcilytic, in individuals with ADH1. Methods: CLTX-305-201 [NCT04581629] is a 3-period, open-label, dose-ranging study of encaleret in adults with ADH1. In Period 1, supplements were discontinued 1 or 2 days prior to administering sequential, increasing daily doses of encaleret over 5 days with frequent biochemical monitoring. Standard 12-lead surface ECGs were recorded at baseline (BL) and on Day 5 (D5). Results: Data (mean±SD) from the first 6 participants are presented. BL and D5 values were compared as follows: • PTH: BL 3.4±4.5; D5 64.8±49.6 (nl 10-65 pg/mL; p<0.01) • Albumin-corrected total calcium: BL 7.6±0.6; D5 9.0±0.5 (nl 8.4-10.2 mg/dL; p<0.01) • Magnesium: BL 1.6±0.4; D5 2.0±0.5 (nl 1.6-2.6 mg/dL; p<0.01) • Phosphate: BL 4.5±0.7; D5 2.9±0.5 (nl 2.3-4.7 mg/dL; p<0.01) • QT c B: BL 452±9; D5 433±11 (nl <440 msec; p<0.01) There were no important changes or trends in heart rate, blood pressure, or other ECG intervals. Encaleret was well-tolerated and without serious adverse events. Conclusion: Hypocalcemic ADH1 study participants experienced abnormal cardiac repolarization manifested as prolongation of the QT interval at baseline. The consistent responses following encaleret treatment ADH1 participants provide preliminary evidence that encaleret, by raising blood levels of calcium and magnesium, may normalize cardiac repolarization in patients with ADH1. Longer-term evaluation of encaleret in ADH1 is ongoing.

Abstract Autosomal dominant hypocalcemia type 1 (ADH1) is a rare form of hypoparathyroidism caused by gain-of-function pathogenic variants in the gene encoding the calcium-sensing receptor (CaSR). It is characterized by variable degrees of hypocalcemia, hyperphosphatemia, and hypomagnesemia, with inappropriately low levels of parathyroid hormone (PTH) and hypercalciuria. Conventional therapy includes oral calcium and activated Vitamin D supplementation, which can lead to or exacerbate hypercalciuria. As a result, patients may develop nephrolithiasis and/or nephrocalcinosis, which can progress to renal insufficiency. Calcilytics (antagonists of the CaSR) have demonstrated in in vitro and in vivo models of ADH1, as well as in a small clinical trial (Roberts et al, JBMR 2019), the ability to shift the dose-response relationship between extracellular calcium and the cellular response of cells bearing the mutant CaSR towards normal. This shift has the potential to increase endogenous PTH secretion which in turn may promote skeletal release of calcium into the bloodstream, production of endogenous calcitriol, renal excretion of phosphate, and renal reabsorption of calcium. Additionally, direct effects of calcilytics on renal CaSRs may further reduce renal calcium and magnesium excretion in ADH1. Taken together, this class of drugs has the capacity to restore normal mineral homeostasis, without calcium and activated vitamin D supplements and without attendant risks of iatrogenic hypercalciuria. This Phase 2b, open-label, dose-ranging study will evaluate the safety, tolerability, pharmacodynamics, and pharmacokinetics of the calcilytic encaleret (CLTX-305) in up to 16 participants with ADH1 (NCT04581629). The study will consist of 3 periods. In periods 1 and 2, participants will undergo a 1-week inpatient evaluation to study the safety and tolerability of daily and twice-daily doses of encaleret. Period 3 will follow participants for up to 24 weeks of continuous outpatient dosing, with periodic inpatient and outpatient assessments. The primary endpoint of period 3 is the change from baseline in albumin-corrected blood calcium concentration. Secondary endpoints of the study include the change in urine calcium (fractional and 24-hour excretion), 1,25-dihydroxy-Vitamin D, phosphate, magnesium, and other blood/urine biomarkers. Enrollment for this study at the National Institutes of Health (NIH) began in September 2020 with topline results expected in 2021. This study is supported by Calcilytix Therapeutics, Inc. and the NIH Intramural Research Program.