The emergence of sequencing technology has allowed for an unbiased approach to uncovering mechanisms of chemotherapeutic resistance. Furthermore, the investigation of resistance can reveal the underlying cell biology of therapeutic treatments, thereby highlighting novel targets for drug design and combination.
Our lab has pioneered research into targeting ribosome biogenesis directly using CX-5461, a first-in-class small molecule inhibitor of Pol I transcription. We established the proof of principle that ribosome biogenesis, once thought to be merely a “housekeeping” process, can be selectively targeted in tumour cells while leaving normal cell unharmed. This translates to a significant therapeutic advantage in the Eμ-Myc lymphoma mouse model. On the basis of this research, we commenced a Phase I clinical trial in haematologic malignancies with CX-5461.
To determine potential mechanisms of resistance, mice were transplanted with a single Eμ-Myc lymphoma clone and treated with either CX-5461 or vehicle. Upon sacrifice, the tumours were harvested and the DNA was sequenced and contrasted. The tumours that had become resistant to CX-5461 had acquired a range of mutations including multiple mutations across all mice in Top2a, suggesting Top2a as a potential biomarker for CX-5461 resistance.
We have demonstrated that Top2a mutant alleles are not expressed, and Top2a +/mut cells have a roughly 50% reduction in Top2a expression and activity. Furthermore, I have shown that Top2a knockdown is able to confer resistance to CX-5461 in vitro, while the corresponding in vivo experiments are ongoing. These data suggest the Top2a expression in a novel biomarker for CX-5461 sensitivity. We are continuing to investigate the mechanism of Top2a-mediated resistance to CX-5461, with the aim of identifying available therapeutic agents which may enable the Top2a +/mut cells to overcome resistance. This will highlight potential combination therapies for future clinical trials of CX-5461 and other next-gen Pol I inhibitors.