SAT-295 Development of Isoform-Specific HDAC Degraders as a Novel Treatment Option for Corticotroph Tumors

Abstract Disclosure: D. Zhang: None. A. Lee: None. C. Santana: None. M. Lee: None. E. Rahbarlayegh: None. S. Conway: None. M. Bergsneider: None. M.B. Wang: None. W. Kim: None. H.V. Vinters: None. A.P. Heaney: None. We and others have previously demonstrated the potential utility of histone deacetylase inhibitors (HDACis) as a treatment for pituitary corticotroph tumors. Their ability to simultaneously inhibit corticotroph tumor proliferation and adrenocorticotrophin (ACTH) secretion sets them apart from many of the therapies that are either approved or in development for Cushing disease. In further studies, we are developing proteolysis-targeting chimeras (PROTACs) to HDAC using a ubiquitin E3 ligand connected to an HDAC inhibitor by an alkyl linker. To determine the optimal HDACi to use in our PROTAC design, we first compared the antiproliferative effects of 11 HDAC inhibitors, 4 of which are FDA-approved, namely vorinostat, romidepsin, belinostat, and panobinostat using human and murine corticotroph tumor (AtT20) cells. The benzamide HDACi, tacedinaline (CI-994), exhibited a potent anti-proliferative effect with an IC50 in the low micromolar range and was chosen as the structural backbone to make our selective HDAC degraders. Following in vitro treatment of murine tumors with the HDAC PROTAC for 48 h at 10μM, we used western blot analysis to confirm HDAC isoform degradation. By comparing to vehicle control (1.0±0.01), we demonstrated that a better HDAC degradation was achieved toward HDAC1 & 3 (HDAC1: 0.2±0.05, p<0.001; HDAC3: 0.16±0.06, p<0.005) than that of HDAC2 (0.95±0.1, n.s.). The HDAC PROTAC exhibited comparable antiproliferative (IC50=4.45μM) and ACTH suppression (IC50=3.2μM) to the parental HDAC inhibitor compound (IC50=2.35μM for antiproliferation effect, IC50=1.18μM for ACTH inhibition, n.s.), but offers the advantage of sustained HDAC inhibition because it destroys the HDAC target and can be recycled to continue its effects in the corticotroph tumor cells. In a further refinement of our HDAC PROTAC, we have incorporated a hypoxia-activated moiety in the structure. This ensures the HDAC PROTAC is only activated in relatively hypoxic conditions, such as that present in corticotroph pituitary tumors and will limit potential adverse effects of the molecule. In summary, we have designed and synthesized a hypoxia-activated HDAC isoform-specific PROTAC; we are optimistic about developing novel, safe, efficacious targeted therapy for patients with Cushing Disease. Presentation: Saturday, July 12, 2025