Novel Bcl-2/Bcl-xL inhibitor, LP-118, enhances apoptosis induction by the CDK2/9 inhibitor, fadraciclib, in BRAF(V600E) human colorectal cancer cells

Background: The BRAF(V600E) mutation in colorectal cancer (CRC) is associated with treatment resistance and a poor prognosis. CDK2/9 is frequently activated in human cancers, and we have previously shown that BRAF V600E can upregulate MCL-1, which can be suppressed by the selective CDK2/9 inhibitor fadraciclib. Herein, we determined whether targeting CDK2/9 by fadraciclib can be enhanced by concurrent Bcl-2/Bcl-xL inhibition using LP-118, which minimizes platelet toxicity in patients, compared to the Bcl-2 inhibitor venetoclax. Effects of the drugs on apoptosis, colony formation and tumor growth were studied in a zebrafish model. Methods: We utilized isogenic human RKO ( BRAF V600E/V600E/WT ) and T29 ( BRAF WT/−/− ) CRC cell lines in addition to HCT-116 ( BRAF WT ) and Bax knockout (HCT-116 Bax-/- ) CRC cells. Cells were treated for 24h with fadraciclib (500nM) plus encorafenib (500nM) in the presence or absence of LP-118 (1µM) or venetoclax (1µM). Apoptosis and cell growth inhibition were assessed using annexin V/PI staining and colony formation assays, respectively. Expression of MCL-1, Bcl-xL, Bcl-2, PARP, and cleaved caspase-3 (Asp175) proteins was analyzed. In a zebrafish tumor xenograft model, the effects of the drugs on caspase-3 cleavage and tumor growth were then evaluated. Results: Fadraciclib suppressed MCL-1 expression, induced apoptosis and significantly inhibited colony formation in all CRC cell lines. However, this effect was enhanced by encorafenib only in RKO ( BRAF V600E/V600E/WT ) cells. The addition of LP-118 to fadraciclib plus encorafenib potently enhanced apoptosis induction, shown by annexin V and cleavage of PARP, caspase-3, in RKO and HCT-116 cell lines, which was synergistic and Bax dependent. Treatment with LP-118 combined with fadraciclib and encorafenib suppressed both Bcl-xL and MCL-1 expression in RKO cells. The addition of LP-118 to fadraciclib plus encorafenib enhanced apoptosis (annexin V) to a greater extent than did venetoclax. In a zebrafish tumor xenograft model, the addition of LP-118 to fadraciclib plus encorafenib significantly enhanced caspase-3 activation and reduced the tumor xenograft size. Conclusion: LP-118 synergistically enhanced the antitumor activity of fadraciclib plus encorafenib in BRAF V600E human CRC cells, and to a greater extent than did venetoclax. Mechanistically, targeting Bcl-2/Bcl-xL concurrent with inhibition of MCL-1 and CDK2/9 by fadraciclib potently induced apoptosis and suppressed tumor cell growth. These findings suggest a novel and effective therapeutic strategy for BRAF V600E human CRCs.