Targeting Tumour-Initiating Cells with TRAIL Based Combination Therapy Ensures Complete and Lasting Eradication of Multiple Myeloma Tumours In Vivo

Multiple myeloma (MM) remains an incurable disease despite improvements to available treatments and efforts to identify new drug targets. Consequently new approaches are urgently required. We have investigated the potential of native tumour necrosis factor-related apoptosis-inducing ligand (TRAIL), in combination with doxorubicin, to induce apoptotic cell death in phenotypically distinct populations of myeloma cells in vitro and in vivo. The cytotoxic potential of TRAIL alone, and in combination with DOX, was assessed in vitro in purified CD138(+) and CD138(−) cells from the MM cell lines and samples from patients with MM. Mouse xenografts obtained by implanting CD138(−) MM cells were used to assess the efficacy of TRAIL, alone and in combination with DOX, in vivo. CD138(−) cells were shown to be more resistant to the cytotoxic activity of TRAIL than CD138(+) cells and have reduced expression of TRAIL death receptors. This resistance results in preferential killing of CD 138(+) cells during exposure of MM culture to TRAIL. Furthermore, prolonged exposure results in the appearance of TRAIL-resistant CD138(−) cells. However, when TRAIL is combined with doxorubicin, this results in complete eradication of MM cells in vivo. Most importantly, this treatment successfully eliminates CD138(−) cells implicated in tumour initiation and growth maintenance. These findings may explain the failure of current therapies and offer a promising new approach in the quest to cure MM and disseminated cancers.