Background: Mutations in PIK3CA occur in 18% of colorectal cancers (CRC) and result in constitutive activation of the PI3K pathway. Previous work from our lab has determined that MTORC1/2 inhibition, with copanlisib (cop), a PI3K/MTOR inhibitor, is sufficient to induce a treatment response. We hypothesized that the BCL2 family inhibitor navitoclax (nav), a BCL2/BCLxL inhibitor, would enhance the therapeutic response of cop and increase the induction of apoptosis.

Methods: Murine derived cancer organoids (MDCOs) were generated from adenocarcinomas arising in Apc and Pik3ca mutant transgenic mice ((FVB x B6) F1 Apcfl/+ Pik3caH1047R/+). MDCOs were plated and matured for 24 hours. MDCOs were treated for 48 hours. Change in diameter was used to assess response. Additionally, human 2D isogenic cell lines SW48 and SW48PIK3CA-H1047R (SW48PK) were plated and treated for 48 hours. WST-1 proliferation assay was used to determine response. In vivo response was assessed as median relative change (MRC) in tumor volume of SW48 and SW48PK xenografts in immunocompromised Rag2-Il2rg-/- (R2G2) mice. Induction of apoptosis was determined with immunoblotting (IB) for PARP cleavage.

Results: MDCOs were treated with cop (200nM) or nav (250nM) alone and in combination. Single agent cop resulted in -16% MRC (p<0.001). Nav alone did not have an effect (+95% p<0.07). Enhanced therapeutic response was seen in the combination therapy compared to cop alone (cop -16% vs combo -100%; p<0.001). In SW48 and SW48PK with cop (1nM or 10nM) or nav (250nM) alone and in combination, a greater response was seen in the combination therapy in both cell lines compared to either single agent alone (p<0.005). SW48 and SW48PK xenograft studies demonstrated significant activity of this combination in vivo. SW48 tumor growth was limited with either cop (10mg/kg), nav (80mg/kg), or the combination compared to control (control +5.0 MRC, cop +3.4, nav +2.6, combo +2.6; control vs combo p<0.01; cop vs combo p=0.29). Enhanced activity of this combination was observed in SW48PK tumors compared to SW48. Growth was significantly more limited with the combination (control +5.0 MRC, cop +3.4, nav +4.5, combo +0.7; control vs combo p<0.01; cop vs combo p=0.06). IB of cleaved PARP showed induction of apoptosis was the highest in the combination therapy in all in vitro models and seen as early as 6 hours post treatment. To identify which BCL2 family member nav was primarily targeting, cop was combined with WEHI539 (BCLxL inhibitor; 250nM) or ABT199 (BCL2 inhibitor; 250nM). BCL2 inhibition did not enhance the efficacy of cop in the MDCOs or isogenic lines. However, BCLxL inhibition did enhance the efficacy of cop in both in vitro models.

Conclusion: These data indicate that navitoclax, through BCLxL inhibition, enhances the efficacy of copanlisib with a greater induction of apoptosis in several models of PIK3CA mutant CRC.

Citation Format: Rebecca Anna DeStefanis, Alyssa DeZeeuw, Gioia Sha, Autumn Olson, Samantha J. Anderson, Christopher P. Babiarz, Cheri A. Pasch, Linda Clipson, Dustin A. Deming. Navitoclax enhances the efficacy of copanlisib predominantly through inhibition of BCLxL in PIK3CA mutant colorectal cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1944.