Inducing apoptosis has become an important approach in the development of new anti-cancer treatments. Tumour necrosis factor apoptosis inducing ligand (TRAIL) based therapies have emerged as one of the most promising examples of this as they selectively induce apoptosis in tumour cells. However, many primary tumours are inherently resistant to TRAIL-mediated apoptosis and require additional sensitisation. Here we review apoptotic and non-apoptotic TRAIL-signalling, and the therapeutic effects of TRAIL-based treatments both as monotherapy and in combination with sensitising agents.

TRAIL induces apoptosis in cancer cells whilst sparing normal tissues. Despite promising pre-clinical 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-R-targeting drugs. 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 (DISC), 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 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.

Background: Approximately 20-25% of ovarian cancers are attributable to germline or somatic BRCA1/2 mutations, resulting in defects in the homologous recombination pathway. Inactivation of these genes can also be mediated by epigenetic changes, e.g., hypermethylation of CpG islands in the promoter regions. In such homologous recombination deficient tumors, platinum based chemotherapy is in general effective, however, loss of hypermethylation might lead to refractory disease. The aim of this study was to evaluate the stability of BRCA1 promoter hypermethylation in recurrent disease after platinum based chemotherapy.Methods: Tumor tissue from 76 patients with primary and 48 patients with platinum-sensitive recurrent high-grade ovarian cancer was collected. In a subgroup of 12 patients, 'paired' tumor tissue from primary and recurrent surgery was available. BRCA1 promoter methylation status was assessed using methylation specific polymerase chain reaction and was verified by Sanger Sequencing.Results: 73.7% (56/76) of primary and 20.8% (10/48) of recurrent tumors displayed BRCA1 promoter hypermethylation. BRCA1 promoter methylation status was not associated with progression-free-or overall survival. In the paired subgroup 83.3% (10/12) of the primary vs. 16.7% (2/12) of the recurrent tumors showed hypermethylation. In eight patients loss of BRCA1 hypermethylation was observed, whereas two patients had stable methylation status.Conclusions: Loss of BRCA1 promoter methylation may be a mechanism to restore BRCA1 function in recurrent disease. However, currently the clinical significance is still unclear and should be evaluated in prospective clinical trials.

ZUSAMMENFASSUNGVor 10 Jahren konnten die Autoren erstmals zeigen, dass der "tumor necrosis factor-related apoptosis-inducing ligand" (TRAIL) Apoptose spezifisch in Tumorzellen in vivo auslösen kann, ohne normale Zellen zu schädigen. Auf Grundlage dieser Ergebnisse ist die Biologie von TRAIL im zurückliegenden Jahrzehnt Gegenstand intensiver Forschung geworden. Dabei sind teils über-raschende Erkenntnisse im Bereich der Signaltransduktion zutage getreten. Die Einbindung dieser aus der Krebsgrundlagenforschung stammenden Ergebnisse wird für die erfolgreiche klinische Entwicklung von TRAIL und agonistischen Antikörpern gegen die beiden apoptosevermittelnden TRAIL-Rezeptoren 1 und 2 von größter Bedeutung sein. In diesem Beitrag werden die Mechanismen TRAIL-induzierter apoptotischer und nichtapoptotischer Signaltransduktion zusammengefasst sowie Resistenzmechanismen und der derzeitige Entwicklungsstand von klinischen Studien, die einen TRAIL-Rezeptor-Agonisten beinhalten, diskutiert. ABSTRACT TRAIL Receptor Agonists, a Novel Class of Pro-Apoptotic Agents in Cancer Therapy10 years ago the authors could demonstrate for the first time that the tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) causes cell death in tumor cells without harming normal tissue. This finding resulted in the biology of TRAIL becoming the focus of immense research and development efforts during the past decade. The results of this research, at least in some instances, led to surprising insights into the signal transduction processes triggered by TRAIL. The integration of these results from basic cancer research will be crucial for the successful clinical development of TRAIL and antibodies targeting the two apoptosis-inducing TRAIL receptors 1 and 2. This review summarizes the current knowledge of the biochemical mechanisms of TRAIL-induced apoptotic and nonapoptotic signal transduction, and discusses resistance mechanisms und the current development status of the various clinical studies undertaken with the different TRAIL receptor agonists.