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Background Prednisolone and prednisone have a longer plasma elimination half-life after oral administration than hydrocortisone: 2.1 to 3.5 vs ∼1.5hrs1,2. For this reason, prednis(ol)one has been used in Congenital Adrenal Hyperplasia (CAH) by giving a dose in the evening to try to prevent the overnight rise in adrenal androgens. Modified-release hydrocortisone (MRHC) capsules, (Efmody, Diurnal Ltd, Cardiff UK), replicate the cortisol diurnal rhythm and improve the control of CAH compared to standard glucocorticoid therapy3. This post-hoc sub-analysis examines CAH control in MRHC treated patients switched from prednis(ol)one. Methods We reviewed the data of all patients taking prednis(ol)one in the randomised study of standard treatment versus MRHC3: 39 classic CAH patients on prednis(ol)one (alone or combined with hydrocortisone) (36 prednisolone; 3 prednisone) were randomised either to continue prednis(ol)one or switched to MRHC at the same hydrocortisone dose equivalent (HDE=prednisolone dose x5). Patients were assessed after 4 weeks following which blinded dose titration according to 17OHP and A4 control was performed to bring 17OHP into the optimal range ( Results Median baseline dose (HDE) was 30mg (n=39), androgen control was 56%. In patients randomised to MRHC (n=18), 39% were controlled at baseline and 94% at 4 weeks without dose adjustment. At 24 weeks, after androgen-control guided titration, the HDE dose in the prednis(ol)one group was 34mg, and in the MRHC group 27.5mg, with control being 71% and 94%, respectively. Percentage inhibition of 9am 17OHP from the levels at baseline to that at 24 weeks in the prednis(ol)one group was 32% and in the MRHC group 91%. In this study period there were no adrenal crises in the MRHC group and one in the prednis(ol)one group. MRHC-treated patients in the extension study were further titrated according to clinician assessment, at interim analysis the median MRHC dose was 20mg (median reduction 10mg) and 82% of patients were controlled at 18 months (n=28). In the ongoing extension study of all patients on MRHC (221 patient years), there were 12 adrenal crises in 5 patients (5.4/100 patient years). Conclusions Control of CAH is better on MRHC than prednisolone: 94% vs 39%; and the dose of MRHC can be down titrated to an adrenal replacement dose whilst maintaining control in the majority (82%) of patients. References 1. Pickup ME. Clin Pharmacokinet 1979 4 111-128; 2. Toothaker RD. J Pharm Sci 1982 71 573-576; 3. Merke DP. JCEM 2021 106 e2063-e2077. Presentation: Saturday, June 11, 2022 1:00 p.m. - 1:05 p.m., Saturday, June 11, 2022 1:00 p.m. - 3:00 p.m.