A TRAIL-R1-specific ligand in combination with doxorubicin selectively targets primary breast tumour cells for apoptosis

Introduction

Although the majority of tumour cell lines, including breast cancer cell lines, are sensitive to the death-inducing ligand and potential cancer biotherapeutic TNF-related apoptosis-inducing ligand (TRAIL), most primary tumours are TRAIL-resistant. Importantly, doxorubicin, a chemotherapeutic agent commonly used in breast cancer, has previously been shown to sensitize TRAIL-resistant breast cancer cell lines to TRAIL. Furthermore, using receptor-selective ligands (patent filed by MRC Technology) specific for the TRAIL death receptors, TRAIL-R1/TRAIL-R2, we have previously shown that primary leukaemic cells isolated from patients with chronic lymphocytic leukaemia can be selectively sensitized to apoptosis by combining an a histone deacetylase inhibitor (HDACi) with a TRAIL-R1-specific form of TRAIL/TRAIL-R1 mAb.

Methods and results

To examine the potency of TRAIL-R1/TRAIL-R2-specific ligands in breast cancer, a panel of breast tumour cell lines was employed, which included the TRAIL-resistant breast cancer cell line, T47D. In addition, a modified approach of culturing primary breast tumour explants ex vivo to maintain their three-dimensional architecture provided a more clinically relevant breast tumour model. Importantly, all TRAIL-sensitive breast tumour cell lines responded only to a TRAIL-R1-specific form of TRAIL. Despite expressing TRAIL-R1/TRAIL-R2, the T47D cell line required initial sensitization by doxorubicin and again exhibited selectivity towards apoptosis induced by a TRAIL-R1-selective ligand. Crucially, we show that doxorubicin can also sensitize TRAIL-resistant primary breast tumour explants to TRAIL-induced apoptosis, while having no effect on normal breast tissue. Furthermore, in this ex vivo model, TRAIL combined with doxorubicin induced significantly more apoptosis via TRAIL-R1 than TRAIL-R2.

Conclusions

Our results have important implications for the potential treatment of breast cancer with TRAIL-based therapeutic agents. We propose that using a TRAIL-R1-specific ligand/mAb combined with subtoxic concentrations of doxorubicin will selectively target tumour cells and minimise potential side effects, such as triggering of TRAIL-induced prosurvival pathways in TRAIL-resistant primary tumour cells or cardiotoxicity induced by higher concentrations of doxorubicin used in monotherapy.