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TRAIL is a death receptor ligand that induces cell death preferentially in tumor cells. Recombinant soluble TRAIL, however, performs poorly as an anti-cancer therapeutic because oligomerization is required for potent biological activity. We previously generated a diabody format of tumor-targeted TRAIL termed Db αEGFR -scTRAIL, comprising single-stranded TRAIL molecules (scTRAIL) and the variable domains of a humanized variant of the EGFR blocking antibody Cetuximab. Here we define the bioactivity of Db αEGFR -scTRAIL with regard to both EGFR inhibition and TRAIL receptor activation in 3D cultures of Caco-2 colorectal cancer cells, which express wild-type K-Ras. Compared with conventional 2D cultures, Caco-2 cells displayed strongly enhanced sensitivity toward Db αEGFR -scTRAIL in these 3D cultures. We show that the antibody moiety of Db αEGFR -scTRAIL not only efficiently competed with ligand-induced EGFR function, but also determined the apoptotic response by specifically directing Db αEGFR -scTRAIL to EGFR-positive cells. To address how aberrantly activated K-Ras, which leads to Cetuximab resistance, affects Db αEGFR -scTRAIL sensitivity, we generated stable Caco-2tet cells inducibly expressing oncogenic K-Ras G12V . In the presence of doxycycline, these cells showed increased resistance to Db αEGFR -scTRAIL, associated with the elevated expression of the anti-apoptotic proteins cIAP2, Bcl-xL and Flip S . Co-treatment of cells with the Smac mimetic SM83 restored the Db αEGFR -scTRAIL-induced apoptotic response. Importantly, this synergy between Db αEGFR -scTRAIL and SM83 also translated to 3D cultures of oncogenic K-Ras expressing HCT-116 and LoVo colorectal cancer cells. Our findings thus support the notion that Db αEGFR -scTRAIL therapy in combination with apoptosis-sensitizing agents may be promising for the treatment of EGFR-positive colorectal cancers, independently of their  KRAS  status.

EGFR-Targeted TRAIL and a Smac Mimetic Synergize to Overcome Apoptosis Resistance in KRAS Mutant Colorectal Cancer Cells