Tumour
necrosis factor-related apoptosis-inducing
ligand (TRAIL) induces apoptosis in
cancer cells while sparing normal tissues. Despite promising preclinical results, few patients responded to treatment with recombinant TRAIL (Apo2L/
Dulanermin) or TRAIL-R2-specific
antibodies, such as
conatumumab (
AMG655). It is unknown whether this was due to intrinsic TRAIL resistance within primary human
cancers or insufficient agonistic activity of the TRAIL-receptor (TRAIL-R)-targeting drugs. Fcγ receptors (FcγR)-mediated crosslinking increases the
cancer-cell-killing activity of TRAIL-R2-specific
antibodies in vivo. We tested this phenomenon using FcγR-expressing immune cells from patients with
ovarian cancer. However, even in the presence of high numbers of FcγR-expressing immune cells, as found in
ovarian cancer ascites, AMG655-induced apoptosis was not enabled to any significant degree, indicating that this concept may not translate into clinical use. On the basis of these results, we next set out to determine whether
AMG655 possibly interferes with apoptosis induction by endogenous TRAIL, which could be expressed by immune cells. To do so, we tested how
AMG655 affected apoptosis induction by recombinant TRAIL. This, however, resulted in the surprising discovery of a striking synergy between
AMG655 and non-tagged TRAIL (Apo2L/TRAIL) in killing
cancer cells. This combination was as effective in killing
cancer cells as highly active recombinant
isoleucine-zipper-tagged TRAIL (iz-TRAIL). The increased killing efficiency was due to enhanced formation of the TRAIL death-inducing signalling complex, enabled by concomitant binding of Apo2L/TRAIL and
AMG655 to TRAIL-R2. The synergy of
AMG655 with Apo2L/TRAIL extended to primary
ovarian cancer cells and was further enhanced by combination with the
proteasome inhibitor bortezomib or a second mitochondrial-derived activator of
caspases (SMAC) mimetic. Importantly, primary human hepatocytes were not killed by the
AMG655-Apo2L/TRAIL combination, also not when further combined with
bortezomib or a SMAC mimetic. We therefore propose that clinical-grade non-tagged recombinant forms of TRAIL, such as
dulanermin, could be combined with
antibodies such as
AMG655 to introduce a highly active TRAIL-R2-agonistic
therapy into the
cancer clinic.