(PA-232) Investigating Ribosome-targeting Therapies for the Treatment of Relapsed/refractory Multiple Myeloma

Resistance to targeting protein degradation by the standard-of-care proteasome inhibitors (PI) in multiple myeloma (MM) remains a challenge. Recent studies implicate altered ribosome synthesis and mRNA translation activity in driving MM disease progression and therapy resistance. The selective inhibitor of nuclear export selinexor, which represses mRNA export and translation, exhibited therapeutic benefit as a single agent and in combination with PIs in several clinical studies. Our group developed a selective inhibitor of RNA polymerase (Pol I) transcription CX-5461, which inhibits ribosome synthesis. We showed that CX-5461 is effective in preclinical MM models and provided a signal of therapeutic benefit in 3/6 patients with relapsed/refractory MM (RRMM) in a phase I trial. We thereby propose that targeting ribosome biogenesis and mRNA translation is a promising therapeutic strategy for RRMM.


Methods:

We examined the effects of selinexor and CX-5461 on ribosomal assembly and the association of mRNAs with ribosomes in PI-resistant MM models using polysome profiling experiments. To define biomarkers of response to selinexor and CX-5461, genome-wide CRISPR-Cas9 knockout screens of PI-resistant MM cells were performed and novel mediators of sensitivity and resistance to these treatments were identified. In parallel, we characterised the bone marrow (BM) microenvironment in response to CX-5461 in MM mouse models and RRMM patient samples through immunophenotyping and spatial proteomic studies.

Results:

Both selinexor and CX-5461 reduce the formation of functional ribosomes and alter mRNA translation activity in PI-resistant MM cells, confirming their action in targeting the ribosome. The screen revealed antigen presentation and processing, adaptive immune response, and activation of NF-κB promote resistance to selinexor. In particular, we found 3 candidate genes encoding subunits of a phosphatase complex, whose deletion contributes to selinexor resistance via NF-κB signalling. We also discover defects in DNA damage response and repair processes confer sensitivity to CX-5461, which is consistent with its dual role in inhibiting Pol I and topoisomerase II activity. In addition, we noted a decrease in CD8+ T cell population expressing exhaustion markers CTLA4, PD-1, LAG3, and TIGIT in mice treated with CX-5461. In agreement with this finding, analyses of BM trephines indicated immune activation in patients with stable disease after CX-5461 treatment. We are currently conducting single-cell RNA sequencing to examine the interaction between immune and MM cells induced by selinexor and CX-5461 in vivo, and performing spatial transcriptomics on patient BM trephines from the phase III SeaLAND study which assesses the combination of selinexor and lenalidomide in NDMM.

Conclusions:

Our findings provide insights into molecular mechanisms underlying the disruption in protein homeostasis in MM, and strong evidence to promote the clinical development of therapies targeting the ribosome for RRMM.