Investigating the Therapeutic Potential of Two Novel Protein Degraders Targeting STAT3 and CBP/p300 in Multiple Myeloma

Abstract

Background: Multiple myeloma (MM) remains an incurable plasma-cell malignancy in which disease persistence is sustained not only by tumour-intrinsic oncogenic programmes but also by protective cues from the bone marrow microenvironment (BMME). In particular, cytokine-driven STAT3 signalling and CBP/p300-dependent enhancer activity support malignant plasma-cell survival, proliferation, transcriptional plasticity and resistance to therapy. In parallel, myeloid populations such as tumour-associated macrophages contribute to an immunosuppressive, chemoprotective marrow niche. Although current MM therapies have improved patient outcomes, relapse remains common, highlighting the need for strategies capable of dismantling both cell-intrinsic and microenvironmental resistance mechanisms. Proteolysis-targeting chimeras (PROTACs) offer one such opportunity by inducing catalytic degradation of oncogenic proteins rather than transient inhibition alone. Aims and Hypothesis: Overall, this thesis aimed to investigate the therapeutic potential of two novel protein degraders, SD-36 and CBPD-409, targeting key oncogenic and microenvironmental dependencies in MM. The aims and hypotheses of the thesis can be addressed according to the three experimental chapters. Hypothesis 1/SD-36 project: PROTAC-mediated degradation of STAT3 by SD-36 would deplete active and total STAT3, suppress STAT3-dependent transcriptional outputs, including MYC, and reduce MM cell fitness alone and in combination with standard MM therapies. Aim 1: To determine the effects of SD-36 on STAT3 degradation, downstream signalling, proliferation, cell-cycle progression and viability in MM cell lines and primary CD138⁺ cells, and to assess its combinatorial activity with dexamethasone and lenalidomide. Hypothesis 2/CBPD-409 project: PROTAC-mediated degradation of CBP/p300 by CBPD-409 would induce deeper collapse of enhancer-dependent oncogenic transcriptional programmes than bromodomain inhibition, resulting in marked anti-myeloma activity, synergy with dexamethasone and tumour control in vivo. Aim 2: To evaluate the effects of CBPD-409 on CBP/p300 degradation, survival, cell-cycle dynamics and oncogenic transcriptional outputs in MM models, to compare its activity with CCS1477, and to assess its efficacy in combination with dexamethasone and in an MM1.S xenograft model. Hypothesis 3/Macrophage project: Targeted degradation of STAT3 and CBP/p300 would disrupt macrophage-mediated support of myeloma by suppressing IL-10-driven M2-like polarisation and overcoming macrophage-mediated dexamethasone resistance. Aim 3: To determine whether SD-36 inhibits IL-10-induced CD163 upregulation in THP-1-derived and primary CD14+-derived macrophages, and whether CBPD-409 restores dexamethasone sensitivity in MM1.S macrophage co-culture. Results: SD-36 induced efficient degradation of phosphorylated and total STAT3 in MM cell lines and primary CD138+ cells and suppressed downstream transcriptional outputs including MYC, BCL3 and JAK3. However, its biological activity was context dependent. In the IL-6-responsive U266 line and in primary MM cells, SD-36 produced a predominantly cytostatic phenotype characterised by reduced proliferation, G1 arrest and diminished viability, whereas MM1.S and RPMI8226 cells were comparatively refractory. In MM1.S cells, phospho-kinase profiling revealed a potentially adaptive activation of PI3K/AKT signalling, and SD-36 showed no meaningful synergy with lenalidomide or dexamethasone. In contrast, CBPD-409 showed broader and deeper anti-myeloma activity. It induced marked depletion of CBP/p300 and downstream effectors including IRF4 and MYC, reduced survival in U266, MM1.S, OPM-2 and primary MM cells, and triggered profound G1/S and G2/M cell-cycle arrest at sub-nanomolar doses. CBPD-409 also outperformed CCS1477/Inodromib in both cytostatic and apoptotic endpoints, synergised with dexamethasone in MM1.S cells, reduced tumour burden in the MM1.S-GFP-Luc xenograft model, and produced broader repression of oncogenic transcriptional programmes, including IRF4, MYC, CCND2, BCL2 and BCL2L1. In the microenvironment-focused studies, IL-10 was elevated in MM patient marrow samples, and SD-36 prevented IL-10-induced CD163 upregulation in both THP-1-derived and primary CD14+ monocyte derived macrophages, indicating that this polarisation programme depends on persistent STAT3 signalling. CBPD-409 further restored dexamethasone sensitivity in MM1.S macrophage co-culture without significantly compromising macrophage viability. Conclusion and Future Work: Collectively, the work in this thesis supports targeted protein degradation as a promising strategy to disrupt both tumour-intrinsic survival circuitry and microenvironment-mediated resistance in MM. SD-36 identifies a cytokine-driven, STAT3-dependent subset of myeloma that is selectively vulnerable to STAT3 degradation, while CBPD-409 demonstrates broader activity consistent with collapse of enhancer-addicted survival programmes and re-sensitisation under macrophage-protective conditions. These findings support a model in which degraders can be deployed against distinct but complementary biological dependencies within MM. Future work should validate the provisional “STAT3-addicted”, “enhancer-addicted” and dual-resistant classes in larger primary cohorts, define adaptive resistance mechanisms and degrader-specific liabilities, compare degradation directly against inhibitor-based approaches in matched in vitro and in vivo settings, and extend these studies into more physiologically relevant marrow models and rational combination strategies.