Immunotoxins, Resistance and Cancer Stem Cells: Future Perspective

Cancer relapse or recurrence has been the greatest challenge in the treatment of this life threatening disease, which occurs due to resistance of cancer cells to drug or radiation therapy. Most often this resistance is developed during treatment, which makes it even more complicated, leading to the failure of chemo or radiation therapy in the majority of cases. To circumvent these problems associated with conventional therapies, newer strategies were adopted like targeted therapy using monoclonal antibodies, immunotoxins and antibody-drug conjugates. However, targeted therapy also showed failure in many in vitro and in vivo studies that was again attributed to the emergence of resistant cells. Here, we discuss the various factors and cellular mechanisms responsible for resistance against conventional therapies and targeted approaches like recombinant immunotoxins. Cancer stem cells (CSC's) were identified as the major reason for resistance and their role in cancer relapse has been proved convincingly in recent studies. Hence, resistance mechanisms involved in CSC's have been elaborated. We also summarize the strategies being adopted currently to overcome resistance and different means of targeting resistant cancer stem cells that could be used in the future.