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Abstract Disclosure: R. Sandooja: None. J. Saini: None. R. Gregg garcia: None. C. Zhang: None. S. Achenbach: None. E. Atkinson: None. J. Van Gompel: None. W.F. Young: None. I. Bancos: None. Hydrocortisone vs Prednisone During Recovery Following Surgical Cure for Endogenous Hypercortisolism: A Longitudinal Observational Study Introduction: Glucocorticoid withdrawal syndrome (GWS) is a constellation of symptoms that occurs after successful surgery for endogenous hypercortisolism. The objective of this study is to compare hydrocortisone versus prednisone on GWS symptoms. Methods: Study design: Single-center prospective longitudinal cohort study, 2019-2023. Participants: Adults with Cushing syndrome (CS) and mild autonomous cortisol secretion (MACS) with post-operative adrenal insufficiency treated with either hydrocortisone or prednisone for glucocorticoid replacement. Measurements: Clinical and biochemical severity scores for hypercortisolism were calculated. Quality of life (QOL) was assessed using Short Form-36 (SF-36) and Cushing QOL surveys. GWS was assessed using weekly AddiQoL surveys for the first 12 weeks after surgery. Results: Of 131 patients, 91 (69%) patients were treated with hydrocortisone and 40 (31%) with prednisone. No differences in age (median age 51 vs 53 years, P=0.62), sex (84 vs 85% women, P=0.83), etiology of hypercortisolism (ACTH-dependent: 44% vs 48%, P=0.26), clinical severity of hypercortisolism (median score 13 vs 12, P=0.97), or biochemical severity of hypercortisolism (median score of 6 in both, P=0.31) were noted between groups. When assessed at 4-, 8-, and 12-weeks post-surgery, the prevalence and trajectory of symptoms (fatigue, myalgias/arthralgias, sleep, headaches, nausea, weakness) were similar in patients treated with hydrocortisone and prednisone (P>0.05 for all). However, patients treated with prednisone were less likely to report mood changes (8.5% vs 20.9%, P= 0.004) compared to those treated with hydrocortisone. When compared to the presurgical baseline, the overall Cushing QOL score 12 weeks post-surgery improved in both groups (mean delta 8.9 vs 13.0, P=0.21). No differences in the hydrocortisone vs prednisone treatment were found in the SF-36 physical component score (mean delta of -1.7 vs 0.3, P=0.23) and SF-36 mental component score (mean delta 2.3 vs 5.0, P=0.22). Using multivariable analysis, age (estimate 0.22, P= 0.005), clinical severity score (estimate -0.73, P<0.001), female sex (estimate -5.45, P= 0.046) but not glucocorticoid type (estimate -0.06, P=0.98) were associated with the overall GWS symptom burden at 12 weeks. Conclusion: Use of prednisone after curative surgery for hypercortisolism was associated with lower mood symptoms, but did not impact the overall severity or trajectory of other symptoms associated with GWS. Baseline clinical severity of hypercortisolism, age, and female sex were associated with GWS burden. Presentation: 6/1/2024

Abstract Disclosure: K. Gunter: Employee; Self; Inozyme Pharma. Stock Owner; Self; Inozyme Pharma. R.A. Wermers: Research Investigator; Self; Inozyme Pharma. R. Fuhr: Employee; Self; Parexel GmbH. Research Investigator; Self; Inozyme Pharma. D. Schnabel: Research Investigator; Self; Inozyme Pharma. A. Besancon: Research Investigator; Self; Inozyme Pharma. M.A. Nour: Research Investigator; Self; Inozyme Pharma. D. Wenkert: Stock Owner; Self; Inozyme Pharma. Y. Sabbagh: Employee; Self; Inozyme Pharma. Stock Owner; Self; Inozyme Pharma. Background: ENPP1 Deficiency is a rare disorder due to inactivating mutations in the ENPP1 gene that result in low levels of inorganic pyrophosphate (PPi), a critical regulator of mineralization. Subsequent pathologic soft tissue calcification results in severe vascular calcification and ∼50% infant mortality. By early adolescence the majority of survivors develop FGF-23 mediated hypophosphatemic rickets, characterized by bone deformities, short stature, joint/ligament calcification and musculoskeletal pain. No targeted therapy exists for this disease. INZ-701 is a recombinant ENPP1-Fc investigational product. Purpose: To describe the safety, tolerability, immunogenicity, and exploratory efficacy (bone and mineral metabolism biomarkers and clinical outcomes) of INZ-701 in adults with ENPP1 Deficiency through the end of the phase 2 study period (week 48). Methods: Phase 1/2, multicenter, open-label, multiple ascending dose study including three adults in each of three dose cohorts (0.2, 0.6 and 1.8 mg/kg, total N=9) with genetic variants in ENPP1 and PPi <1300 nM (NCT04686175). Participants were dosed subcutaneously on Day 1, then twice weekly from Day 8 to end of study. Data through wk 48 as of July 6, 2023 are included; topline wk 48 data will be presented. Per protocol, participants may continue to receive INZ-701 beyond wk 48. Results: Rapid increase in mean PPi from baseline of 426±407 nM was noted in all patients, reaching the healthy volunteer range within 6 hrs of the first dose with sustained elevation through wk 48. Mean PPi across the 0.2, 0.6 and 1.8 mg/kg dose cohorts from day 32 to wk 48 was 1299±131 nM, 1356±136 nM, and 1282±81 nM, respectively. There were improvements from baseline to wk 48 in mean pooled FGF-23 (-22 pg/mL) and serum phosphate (+0.18 mg/dL), with dose-dependent changes in bone specific alkaline phosphatase consistent with bone healing. DEXA revealed improvements in spine BMD and BMC. There were trends towards improvement in mean % predicted change in 6MWT from baseline to wk 48: cohort 1, 76.7% (SE 10.5) to 85.0% (SE 3.0); cohort 2, 52.2% (SE 10.9 ) to 79.0% (SE 2.9); cohort 3, wk 48 data not available. Improvements were also observed in physician and patient global impression of change score (with no patients deteriorating) and PROMIS pain intensity score. INZ-701 was well tolerated with no drug-related serious or severe (> grade 2) adverse events. Low titers of non-neutralizing anti-drug antibodies (≤160) were observed in 7/9 patients. Long half-life of approximately 126 hours suggests the potential for once-weekly dosing. Conclusions: In adults with ENPP1 Deficiency, INZ-701 was well tolerated with no related serious or severe adverse events. INZ-701 demonstrated a rapid and sustained increase in PPi levels from baseline to week 48 with corresponding improvements in bone mineral biomarkers, functional performance, and patient and physician-reported outcomes. Presentation: 6/3/2024

Abstract Disclosure: B. Salama: None. J. Saini: None. V. Fell: None. E.J. Atkinson: None. S.J. Achenbach: None. Background: Mildly autonomous cortisol secretion (MACS) can be unilateral in 80% of cases, or bilateral in 20% of cases, due to either bilateral adrenal adenomas or bilateral macronodular adrenal hyperplasia. Scarce evidence demonstrates the differences in plasma steroid profiling of patients with bilateral versus unilateral MACS, however, these findings have not been confirmed using a more comprehensive 24h urine steroid profiling. Objectives: To characterize 24h urine steroid metabolome in patients with unilateral and bilateral MACS. Methods: We conducted a single-center cross-sectional study between 2019-2022. Eligible participants included consecutive patients with MACS who provided a 24h urine sample. Urine was analyzed for 25 steroids with a high-resolution mass spectrometry assay. MACS was defined as cortisol >1.8 mcg/dL following the overnight administration of 1 mg dexamethasone (DST-cortisol). Partial cohort analysis is presented. Results: Of 72 patients with MACS included in the interim analysis, 36 (50%) had bilateral MACS. The two groups (unilateral vs bilateral) were similar in age (median 58 vs 58 years), sex (women 67% vs 61%), BMI (median 33 vs 32 kg/m2), and menopausal status (36% vs 39%), P>0.05 for all. Prevalence of comorbidities was also similar in both groups: hypertension (89% vs 89%), hyperlipidemia (69% vs 78%), and hyperglycemia (69% vs 61%), P >0.05 for all. While the DST-cortisol (median 3.0 vs 3.2 mcg/dL, P= 0.51) and dehydroepiandrosterone sulfate (DHEAS) (median 37.5 vs 55.0, P = 0.21) were similar between the 2 groups, patients with bilateral MACS had a lower ACTH (median 12.4 vs 8.2 pg/mL, P = 0.012) when compared to patients with unilateral MACS. Median adrenal mass size was similar in patients with both unilateral MACS (2.7 cm, IQR 1.7-3.9) and bilateral MACS (3.0 cm, IQR 2.2-3.6) P = 0.54. Of the 25 steroids measured, only tetrahydrocorticosterone was higher in patients with unilateral MACS (median 245 mcg/24h, IQR 165-416 vs median 184 mcg/24h, IQR 121 -278) P = 0.039. No differences in androgens (median 2229 vs 2036 mcg/24h), cortisol metabolites (median 6566 vs 5191 mcg/24h), cortisone metabolites (median 10300 vs 9429 mcg/24h), or glucocorticoid/androgen ratio (median 7.9 vs 8.9) was demonstrated between the unilateral vs bilateral MACS groups (P>0.05 for all). Conclusion: In this interim analysis of patients with unilateral vs bilateral MACS, we have found that patients had a similar post-DST cortisol and similar tumor bulk. Despite a lower ACTH in patients with bilateral MACS, no differences in androgens were found between the two groups. In addition to expanding steroid profiling analysis to the whole cohort, subgroup analyses will need to distinguish between patients with bilateral adenomas and macronodular adrenal hyperplasia, consider aldosterone co-secretion, and employ a more accurate measurement of adrenal tumor bulk. Presentation: 6/3/2024

Abstract Disclosure: J. Saini: None. C.D. Zhang: None. V. Fell: None. E. Atkinson: None. S. Achenbach: None. I. Bancos: None. Background: Several studies employing computed tomography (CT) scans for body composition measurements have reported lower muscle mass and a higher proportion of visceral fat in patients with mild autonomous cortisol secretion (MACS) compared to referent subjects. While a CT scan assesses visceral adipose tissue (VAT) at a single slice level (L3), no studies have evaluated the overall VAT in patients with MACS using Dual-Energy X-ray absorptiometry (DXA). Objective: To determine the impact of MACS on VAT using DXA scan as compared to referent subjects. Methods: We conducted a single-center cross-sectional study of adults with 75 MACS and 75 referent subjects. MACS was defined as serum cortisol>1.8 mcg/dL after 1 mg dexamethasone suppression test in patients with adrenal adenomas and no features of overt hypercortisolism. Referent subjects were included if they had abdominal imaging and had no adrenal disorders. All the participants underwent a full body DXA scan between 04/2019 and 07/2022. We applied the CoreScan software to estimate the (1) total lean and fat mass (2) visceral fat mass and volume (3) lean android, fat android, and android mass, and compared measurements of patients and referent subjects using linear regression models adjusted for sex and BMI. Results: A total of 75 patients with MACS (median age 57.0 years, IQR 47.8-65.6, 67% women) and 75 referent subjects (median age 57.6 years, IQR 49.1-65.8, 67% women) were included. Patients had a higher BMI compared to referent subjects (median 31.7 kg/m2, IQR 27.4-37.9 vs 28.7, IQR, 25.4-33.8, P=0.004). The median post-DST cortisol in patients was 3.1 mcg/dL (IQR, 2.2-5.2). When compared to referent subjects, and after adjustment for sex and BMI, patients had a similar total body fat (estimate=846, p-value=0.28) and lean mass (estimate=-201, p-value=0.83), similar android fat mass (estimate=59, p-value=0.64) but lower gynoid fat mass (estimate=400, 0.007). Notably, sex and BMI-adjusted analysis also showed that patients had a higher VAT fat mass (estimate= -225, p-value=0.084) and a higher VAT fat mass/total mass ratio (estimate= -0.007, p-value=0.026). Conclusions: When adjusted for BMI and sex, patients with MACS do not demonstrate differences in overall fat and lean mass. However, we showed that MACS is associated with increased visceral adiposity, a known cardiovascular risk factor. Body composition assessment by DXA could be a safer and less expensive alternative in the evaluation and follow-up of patients with MACS. Presentation: 6/2/2024

Abstract Disclosure: S.V. Rupani: None. J. Sfeir: None. L. Graeff-Armas: None. Bone microarchitecture is an important factor that impacts bone strength. Unlike other factors that affect bone strength, such as bone mineral density and bone turnover, assessment of bone microarchitecture is not yet widely accessible in clinical practice. Computed tomography is the primary method of non-invasively assessing bone microarchitecture. High-resolution peripheral quantitative CT (HR-pQCT) is an emerging imaging modality that provides a three-dimensional assessment of bone densitometry, morphometry, and microarchitecture with a low effective radiation dose. It provides a quantitative and qualitative assessment of cortical and trabecular structural and mechanical properties. We present two similar cases of osteoporosis in males, where knowledge of bone microarchitecture through HR-pQCT helped formulate two distinct and targeted therapeutic approaches. The first case is of a 46-year-old male with a history of idiopathic osteoporosis with multiple nontraumatic vertebral fractures. Given his presentation and age, anabolic therapy such as teriparatide was being considered. He underwent HR-pQCT scanning, which showed average radial trabecular parameters, but his cortical porosity was significantly above average, with similar findings in his tibial parameters. Since teriparatide has been shown to increase cortical porosity, this would not be the best option for our patient, given the risk of further weakening of the microarchitecture of his cortical bone. Therefore, denosumab, which has been shown to decrease cortical porosity, was chosen. Our second patient is a 52-year-old male with idiopathic osteoporosis with multiple nontraumatic vertebral fractures, whose HR-pQCT showed average cortical parameters at both the radius and tibia but low trabecular number and increased trabecular separation at both sites. Since teriparatide increases trabecular thickness and mass, we could confidently choose teriparatide for this patient, with less concern for significant deleterious effects on his cortical bone parameters. These case reports highlight the impact that the assessment of qualitative properties of bone, such as microarchitecture, can have in the management of osteoporosis. Currently, the lack of sufficient validated normative data for HR-pQCT limits its widespread use in the diagnosis of osteoporosis and fracture prediction. However, given that HR-pQCT can characterize the properties of trabecular and cortical bone, it may have a role in helping guide osteoporosis treatment, especially in challenging and atypical cases. Presentation: 6/3/2024

Abstract Disclosure: S.W. Ing: Advisory Board Member; Self; Ultragenyx, Amolyt. Grant Recipient; Self; Alexion Pharmaceuticals, Inc.. Research Investigator; Self; BridgeBio Pharma, Takeda, Amolyt, Radius Health, Inc, Ultragenyx. P. Harmatz: Research Investigator; Self; BridgeBio Pharma. S. Mora: Research Investigator; Self; BridgeBio Pharma. E.A. Imel: Research Investigator; Self; BridgeBio Pharma. P.J. Tebben: Research Investigator; Self; BridgeBio Pharma. M.L. Warren: Research Investigator; Self; BridgeBio Pharma, Ascendis, Amolyt. Speaker; Self; Ascendis. N. Ma: Research Investigator; Self; Calcilytix Therapeutics, Ultragenyx, Inozyme. Other; Self; Up To Date. A.A. Khan: Research Investigator; Self; BridgeBio Pharma, Ascendis. A. Palermo: Research Investigator; Self; BridgeBio Pharma. B. Decallonne: Research Investigator; Self; BridgeBio Pharma. S. Lemoine: Research Investigator; Self; BridgeBio Pharma, Kyowa Kirin. G. Mantovani: Research Investigator; Self; BridgeBio Pharma. A. Linglart: Consulting Fee; Self; Alexion, Novo Nordisk, Merck, Pfizer, Inc.. Research Investigator; Self; BridgeBio Pharma, Kyowa Kirin. H. Wasserman: Research Investigator; Self; BridgeBio Pharma. A.P. Barbosa: Research Investigator; Self; BridgeBio Pharma. C. Cardot-bauters: Research Investigator; Self; BridgeBio Pharma. M. Roberts: Employee; Self; BridgeBio Pharma. A. Mathew: Employee; Self; BridgeBio Pharma. S. Adler: Employee; Self; BridgeBio Pharma. M.C. Zillikens: Research Investigator; Self; BridgeBio Pharma. R.J. Clifton-Bligh: Consulting Fee; Self; Ipsen, Kyowa Kirin, Amgen Inc, Eisai. Research Investigator; Self; BridgeBio Pharma. L. Rejnmark: Research Investigator; Self; BridgeBio Pharma, Ascendis, Amolyt, Takeda, Kyowa Kirin. Autosomal dominant hypocalcemia type 1 (ADH1), caused by gain-of-function calcium-sensing receptor gene (CASR) variants, is characterized by low parathyroid hormone (PTH) concentrations, hypocalcemia, hypercalciuria, hyperphosphatemia and hypomagnesemia. While a rare disease, ADH1 is one of the more frequently identified causes of genetic hypoparathyroidism. Conventional therapy includes calcium (Ca) and/or active vitamin D, but this regimen incompletely corrects the hypocalcemia and is associated with persistent hypercalciuria, which may result in renal complications including nephrocalcinosis (NC), nephrolithiasis (NL), and chronic kidney disease (CKD). The CLARIFY disease monitoring study [NCT05227287] is a global, multicenter, longitudinal study to understand disease burden, management, and progression in children and adults with ADH1 over a 5-year period. Here we report data on the characteristics of adult participants at study entry. As of November 2023, 45 adults (≥18 years) with ADH1 were enrolled, with a mean±SD age of 42.1±16.5 years (range 18-80). The mean±SD age of a hypocalcemia diagnosis was 19.1±19.1 years, while the mean±SD age for a diagnosis of ADH1 was 28.2±20.6 years. As reported on medical history, in decreasing order of prevalence, 36% (16) had NC, 22% (10) had intracranial calcifications, 11% (5) had history of seizures, 11% (5) had CKD, 9% (4) had cataracts, 7% (3) had NL, and 4% (2) had undergone renal transplant. Treatment data were available for 43 participants and included the following: 74% (32) Ca and active vitamin D, 9% (4) Ca alone, 9% (4) active vitamin D alone, 37% (16) magnesium, 33% (14) thiazide diuretics, 26% (11) potassium, 7% (3) phosphate binder, 7% (3) PTH, and 5% (2) no treatment. Mean±SD fasting values collected prior to conventional therapy dose are presented. PTH concentrations (10.1±8.2 pg/mL [nl 15-65]) and albumin-corrected calcium ([cCa]=7.5±1.0 mg/dL [nl 8.5-10.5]) were low. Despite the low mean cCa, the mean 24-hr urine calcium was elevated (268±183 mg/d, [nl <250 women, <300 men]). Blood phosphate was 4.8±0.8 mg/dL [nl 2.5-4.8] while blood magnesium was 1.8±0.2 mg/dL [nl 1.8-2.4]. 25-OH vitamin D was 35.0±13.5 ng/mL [nl 30-80]. Renal function as assessed by CKD-EPIcr_R showed eGFR of 86±23 mL/min/1.73m² (range 36-123). This study represents the largest cohort of adults with ADH1 described to date. These data highlight variability in therapeutic approaches in a real-world setting with some participants receiving up to 6 different medications/supplements. Despite being followed in expert centers, and treated with available therapies, patients on average have low cCa with relatively high 24-hr urine calcium excretion. The CLARIFY study provides an opportunity to better understand the progression and burden of disease in participants with ADH1. Presentation: 6/3/2024

Abstract Disclosure: S.W. Ing: Research Investigator; Self; BridgeBio Pharma. P. Harmatz: Research Investigator; Self; BridgeBio Pharma. S. Mora: Research Investigator; Self; BridgeBio Pharma. E.A. Imel: Research Investigator; Self; BridgeBio Pharma. P.J. Tebben: Research Investigator; Self; BridgeBio Pharma. M.L. Warren: Research Investigator; Self; BridgeBio Pharma, Ascendis, Amolyt. Speaker; Self; Ascendis. N. Ma: Research Investigator; Self; Calcilytix Therapeutics, Ultragenyx, Inozyme. Other; Self; Up To Date. A.A. Khan: Research Investigator; Self; BridgeBio Pharma, Ascendis. A. Palermo: Research Investigator; Self; BridgeBio Pharma. B. Decallonne: Research Investigator; Self; BridgeBio Pharma. S. Lemoine: Research Investigator; Self; BridgeBio Pharma, Kyowa Kirin. G. Mantovani: Research Investigator; Self; BridgeBio Pharma. A. Linglart: Consulting Fee; Self; Alexion, Novo Nordisk, Merck, Pfizer, Inc.. Research Investigator; Self; BridgeBio Pharma, Kyowa Kirin. H. Wasserman: Research Investigator; Self; BridgeBio Pharma. A.P. Barbosa: Research Investigator; Self; BridgeBio Pharma. C. Cardot-bauters: Research Investigator; Self; BridgeBio Pharma. M. Roberts: Employee; Self; BridgeBio Pharma. A. Mathew: Employee; Self; BridgeBio Pharma. S. Adler: Employee; Self; BridgeBio Pharma. M.C. Zillikens: Research Investigator; Self; BridgeBio Pharma. R.J. Clifton-Bligh: Consulting Fee; Self; Ipsen, Kyowa Kirin, Amgen Inc, Eisai. Research Investigator; Self; BridgeBio Pharma. L. Rejnmark: Research Investigator; Self; BridgeBio Pharma, Ascendis, Amolyt, Takeda, Kyowa Kirin. Autosomal dominant hypocalcemia type 1 (ADH1), caused by gain-of-function calcium-sensing receptor gene (CASR) variants, is characterized by low parathyroid hormone (PTH) concentrations, hypocalcemia, hypercalciuria, hyperphosphatemia and hypomagnesemia. While a rare disease, ADH1 is one of the more frequently identified causes of genetic hypoparathyroidism. Conventional therapy includes calcium (Ca) and/or active vitamin D, but this regimen incompletely corrects the hypocalcemia and is associated with persistent hypercalciuria, which may result in renal complications including nephrocalcinosis (NC), nephrolithiasis (NL), and chronic kidney disease (CKD). The CLARIFY disease monitoring study [NCT05227287] is a global, multicenter, longitudinal study to understand disease burden, management, and progression in children and adults with ADH1 over a 5-year period. Here we report data on the characteristics of adult participants at study entry. As of November 2023, 45 adults (≥18 years) with ADH1 were enrolled, with a mean±SD age of 42.1±16.5 years (range 18-80). The mean±SD age of a hypocalcemia diagnosis was 19.1±19.1 years, while the mean±SD age for a diagnosis of ADH1 was 28.2±20.6 years. As reported on medical history, in decreasing order of prevalence, 36% (16) had NC, 22% (10) had intracranial calcifications, 11% (5) had history of seizures, 11% (5) had CKD, 9% (4) had cataracts, 7% (3) had NL, and 4% (2) had undergone renal transplant. Treatment data were available for 43 participants and included the following: 74% (32) Ca and active vitamin D, 9% (4) Ca alone, 9% (4) active vitamin D alone, 37% (16) magnesium, 33% (14) thiazide diuretics, 26% (11) potassium, 7% (3) phosphate binder, 7% (3) PTH, and 5% (2) no treatment. Mean±SD fasting values collected prior to conventional therapy dose are presented. PTH concentrations (10.1±8.2 pg/mL [nl 15-65]) and albumin-corrected calcium ([cCa]=7.5±1.0 mg/dL [nl 8.5-10.5]) were low. Despite the low mean cCa, the mean 24-hr urine calcium was elevated (268±183 mg/d, [nl <250 women, <300 men]). Blood phosphate was 4.8±0.8 mg/dL [nl 2.5-4.8] while blood magnesium was 1.8±0.2 mg/dL [nl 1.8-2.4]. 25-OH vitamin D was 35.0±13.5 ng/mL [nl 30-80]. Renal function as assessed by CKD-EPIcr_R showed eGFR of 86±23 mL/min/1.73m² (range 36-123). This study represents the largest cohort of adults with ADH1 described to date. These data highlight variability in therapeutic approaches in a real-world setting with some participants receiving up to 6 different medications/supplements. Despite being followed in expert centers, and treated with available therapies, patients on average have low cCa with relatively high 24-hr urine calcium excretion. The CLARIFY study provides an opportunity to better understand the progression and burden of disease in participants with ADH1. Presentation: 6/3/2024

Abstract Disclosure: M. Bassiony: None. P. Dogra: None. L. Šambula: None. J. Saini: None. U. Ambroziak: None. D. Kastelan: None. I. Bancos: None. Background: Frailty is characterized by a multi-system decline and increased vulnerability to adverse health outcomes. We have recently shown that patients with mild autonomous cortisol secretion (MACS) demonstrate a higher prevalence of frailty compared to patients with nonfunctioning adrenal adenomas (NFA). Objective: To determine the impact of adrenalectomy versus conservative management in patients with MACS, as compared to patients with NFA. Design: Multi-center, prospective cohort study (2019-2023). Methods: Adults with NFA and MACS were assessed for frailty at baseline and 1-year post-adrenalectomy, or conservative follow up. Frailty was assessed by frailty index (FI) that encompasses 47 health variables (comorbidities, symptoms, daily living activities). Patients with FI >0.25 were classified as frail. MACS was defined as post-dexamethasone cortisol>1.8 mcg/dL. Results: Paired frailty assessments were performed in 226 patients (MACS:117 and NFA:109). At baseline, patients’ median age was 63 years (interquartile range, IQR 55-78) and 113 (50%) were women, without differences between patients with MACS vs NFA. In conservatively treated patients with NFA (n=106) and MACS (n=61), baseline FI (median 0.15 vs 0.16, p=0.190) and frailty prevalence (18% vs 20%, p=0.846) were similar between groups. At follow up, patients with MACS demonstrated an increase in FI (mean of 0.013 vs -0.018 in patients with NFA, p=0.009). In conservatively treated patients, and after excluding outliers, post-dexamethasone cortisol positively correlated with a higher delta FI at follow up (estimate 0.015, p=0.01). Compared to conservatively treated, patients with MACS treated with adrenalectomy (n=56) were younger (median age of 58 vs 67 years, p=0.0002), had a similar post-dexamethasone cortisol (median 3.4 vs 2.8 mcg/dL, p=0.853), a higher FI (median 0.26 vs 0.15, p value <0.0001) and frailty prevalence (52% vs 20% P=0.0003) at baseline. At follow up, patients with MACS treated with adrenalectomy showed improved FI when compared to conservatively treated (mean delta FI of -0.025 vs 0.014, p=0.017). Conclusions: Patients with MACS treated conservatively demonstrate an accelerated frailty when compared to patients with NFA. In contrast, frailty improves in patients with MACS treated with adrenalectomy. Presentation: 6/2/2024

Abstract Disclosure: L. Rahimi: None. F. Al-Khazal: None. N. Becker: None. L. Maher, III: None. Carriers of loss-of-function alleles in genes encoding the four succinate dehydrogenase (SDH) subunits exhibit a heightened risk for pheochromocytoma/paraganglioma (PPGL), a rare neuroendocrine tumor of chromaffin cells. Tumorigenesis begins with the somatic loss of the remaining functional gene copy, resulting in a cell devoid of functional SDH. Compared to wild-type immortalized chromaffin cells (WT imCCs), SDHB-KO immortalized chromaffin cells (SDHB-KO imCCs) are larger, grow more slowly, display deformed mitochondria, show strikingly heightened succinate accumulation, and altered metabolism. Among several mechanisms proposed to link SDH deficiency with PPGL tumorigenesis, the accumulation of succinate as an oncometabolite is a leading hypothesis. We describe two attempts to suppress succinate accumulation in SDHB-KO imCCs. First, riboflavin has previously been shown to enhance residual SDHA flavinylation and enzymatic function, possibly reducing succinate levels by supporting conversion of accumulated succinate to fumarate in the absence of SDHB. We tested this by treating WT and SDHB-KO imCCs with 5 µM riboflavin for 2 weeks. Metabolomic analysis did not show decreased succinate, suggesting that enzyme activity of any remaining SDHA subunit was not rescued by increased flavinylation. Second, we envisioned a gene therapy approach to replace defective SDHB with a rescuing transgene. SDHB-KO imCCs received DNA encoding SDHB or SDHC cDNAs via transposon technology, confirmed by PCR and analyzed by western blot analyses. These studies showed that whereas WT imCCs successfully expressed processed transgene products in mitochondria, SDHB-KO imCCs could not process or transport rescuing SDH subunits into mitochondria. This shows that transposed SDHB-KO imCCs cannot properly express a rescuing SDHB subunit and do not revert to a wild-type phenotype. It is likely that the severely damaged mitochondria of SDHB-KO imCCs are incompetent to process and import rescuing SDHB subunits, so unprocessed protein accumulates without effect. This defect may lie in the mitochondrial targeting peptide recognition system because the targeting peptide of rescuing SDHB protein is not removed in SDHB-KO imCCs as judged by western blotting. In summary, we show the failure of two conceptually simple tactics to suppress succinate accumulation in SDHB-KO imCCs with highly defective mitochondria: rescue of SDHA by supporting flavinylation, or rescue of SDHB with a replacement gene copy. Further research may allow screening for small molecules that suppress succinate accumulation by other mechanisms. It remains to be determined if blocking succinate accumulation would be therapeutic in SDH-loss PPGL tumors. Presentation: 6/1/2024

Abstract Disclosure: J. Yang: None. J. Burrello: None. J. Goi: None. T.A. Williams: None. W.F. Young: Consulting Fee; Self; Arena Pharmaceuticals, Inc.. Other; Self; Crinetics Pharmaceuticals. P.J. Fuller: None. P. Investigators: None. Primary aldosteronism (PA) is the most common endocrine cause of hypertension. Mineralocorticoid receptor antagonists, at adequate doses, lower blood pressure and cardiovascular risk in patients with PA. However, there is a lack of a standardized approach to medical therapy and subsequent outcomes assessment. We aimed to: 1) establish criteria for assessing outcomes of targeted medical treatment of PA, and 2) evaluate outcomes using these criteria across an international cohort. A panel of 31 PA experts from 31 centres across four continents used the Delphi method, including three rounds of online questionnaires, to reach consensus. Clinical data were then collected from consecutive patients with PA who started targeted medical treatment between 2016 and 2021 at the participating centres. Consensus was reached for six outcomes, including complete, partial, or absent biochemical response, and complete, partial, or absent clinical response. “Complete biochemical response” was defined as “correction of hypokalemia without potassium supplementation, and normalization of plasma renin activity or concentration”; while “absent biochemical response” was defined as “persistent hypokalemia and/or persistently suppressed or low plasma renin activity or concentration without change from baseline”. “Complete clinical response” was defined as “normal blood pressure without additional antihypertensive medications”; while “absent clinical response” was defined as “the same or increased blood pressure compared to before targeted treatment with same or more antihypertensive medications, not counting targeted therapy”. Of 1483 patients (mean age 52y, 48% female) from 28 centers, 232 (16%) had a complete clinical response, and 719 (49%) had a complete biochemical response at the most recent follow-up (median 29 months after treatment initiation). Compared to those with partial or absent clinical response, patients with complete clinical response had a significantly higher proportion of women and lower proportion with diabetes, stroke or left ventricular hypertrophy. They also had a significantly shorter duration of hypertension, lower BMI, higher eGFR and lower defined daily dose of antihypertensives at baseline. In contrast, these differences were not observed in those with a complete, partial or absent biochemical response. The defined daily dose of targeted therapy did not differ across the various response categories. In conclusion, we have established an international expert consensus for standardized criteria to evaluate the response to targeted medical treatment of PA, facilitate comparisons of outcome data and guide clinicians who manage PA. In a retrospective international cohort, less than half of medically-treated patients reached the established biochemical treatment targets. Further work is needed to increase the rate of complete response. Presentation: 6/3/2024