Novel Combinative Therapy and Integration of Circulating Tumour DNA(ctDNA)Analysis in the Management of Colorectal Cancer

Background: Drug resistance is the main factor limiting the efficacy of anticancer therapies. Combinative therapies are superior to monotherapies. Understanding the tumour heterogeneity and mechanisms that contribute toward resistance is a key element in personalised medicine. Herein, we aim to discover a novel combinative therapy and expand the applications of circulating tumour DNA (ctDNA) in colorectal cancer (CRC).

Methods: We explored the in vitro efficacy of two drug combinations [Palbociclib with Gedatolisib (P+G) and Palbociclib with PD0325901 (P+PD)]in five CRC cell lines with different mutational status. Moreover, we analysed ctDNA and tumour samples from 10 non-metastatic rectal cancer patients during neoadjuvant chemoradiotherapy (NACRT).

Results: A synergistic response to treatment with the combination of P+G is seen in all cell lines [CI range=0.10-0.85]. A synergistic response to treatment with the combination of P+PD is also seen in all cell lines [CI range=0.07-0.55], apart from LS411N [CI=not reached]. Using Reverse Phase Protein Arrays (RPPA), we observed a significant suppression of S6rp(S240/244) in all cell lines treated with the combination of P+G, without AKT reactivation. This indicated efficient inhibition of the PI3K/AKT/mTOR pathway, even in PIK3CA-mutated cell lines. The combination of P+G induced BAX and BCL-2 in PIK3CA-mutatedcell lines. Using ctDNA to identify and monitor changes in mutations over the course of NACRT, we identified additional nonsynchronous mutations which were not identified in tumour samples using next generation sequencing (NGS). Patients without pathological complete response (pCR) following NACRT had elevated KRAS levels in ctDNA during treatment and post-operatively, indicating the potential for ctDNA to monitor and evaluate response to standard and novel therapies.

Conclusion: Our data supports further in vivo evaluation of P+G combinative therapy for clinical development in chemorefractory metastatic CRC. S6rp(S240/244) may be a promising biomarker of response to this drug combination. Serial ctDNA is explorable and potentially impactful in the management of CRC.