7448 Endogenous ACTH But Not Intermittent Cosyntropin Prevents Dexamethasone-Induced Adrenal Suppression In Mice

Abstract Disclosure: L.S. Gaston: None. H.R. Friedman: None. J.A. Majzoub: None. Background: Iatrogenic adrenal insufficiency (IAI) can persist for up to a year after withdrawal of long-term glucocorticoids (GC) by unknown mechanisms. We developed a mouse model of IAI in which 8 weeks of dexamethasone (DEX) induced protracted adrenocortical dysfunction despite early ACTH recovery. We then asked if 1) adrenal secretory dysfunction after GC withdrawal exceeded the cellular defect, and 2) ongoing trophic ACTH signaling while on DEX (either exogenous cosyntropin [COS] or endogenous ACTH) prevented IAI. Methods: Adult, male, wild-type C56BL/6J (n=3-6) mice were treated for 8 weeks (long-term) or 4-days (short-term) with DEX ± daily, intraperitoneal COS. At 1-2-week intervals after DEX±COS withdrawal, basal and stimulated ACTH and CORT were measured (mean±SEM) followed by necroscopy for morphometric analysis of immunostained, equatorial adrenal sections. Experiments were repeated in mice with irrepressible, endogenous ACTH due to selective loss of the hypothalamic GC receptor (Sim1-Cre:GR fl/fl, or SGR) and their Cre-negative (WT) littermates. Results: Stimulated corticosterone (CORT) required 8 weeks to normalize after stopping long-term DEX. Notably, both basal (811±220 vs. 216±24 pg/mL) and stimulated (1708±85 vs. 876±44) ACTH increased above controls by 1-week post-withdrawal (p<0.001 for both). Adrenal weights (1±0.1 vs. 2.5±0.1 mg) and cortical areas (480±9 vs. 1290±222 mm2) were significantly lower in DEX-treated glands vs. controls (p≤ 0.0001 for both) but recovered by 1-week post-withdrawal. Paucicellular, lipofuscin-rich accumulations were present in DEX-treated glands and persisted through 8 weeks post-withdrawal. In a linear regression of cortical area minus these inclusions (functional cortical area) vs. stimulated CORT (r2=0.43), predicted CORT was >2x higher (10 and 11 mg/dL) than measured CORT (5 for both) at 1- and 2-weeks post-DEX. In subsequent experiments, short-term COS inhibited DEX-induced adrenocortical apoptosis (200±85 vs 1170±481 TUNEL+ cells/HPF, p<0.01). However, CORT recovered more slowly in mice treated with long-term DEX+COS vs. DEX alone (doubling time=5.2 vs. 2.5 weeks), and glands were atrophic with lipofuscin-rich inclusions. Meanwhile, SGR mice maintained normal ACTH levels on long-term DEX. At the time of DEX withdrawal, SGR basal (14±4 vs.1.5±1.7 mcg/dL, p<0.05) and stimulated CORT responses (33±9.6 vs. 1.1±1.1, p<0.001), adrenal weights (2.3±0.2 vs. 1.1±0.06 mg, p≤ 0.001), and functional cortical area (1249±45 vs. 324±102 mm2, p≤ 0.001) were all greater than WT littermates, and lipofuscin-rich inclusions were absent in DEX-treated SGR glands. Conclusions: The adrenal is the major anatomic locus of post-withdrawal IAI in mice, which may involve early defects in both cortical size and secretory function. Novel therapies that mimic endogenous ACTH signaling during the period of GC exposure may prevent IAI. Presentation: 6/2/2024