Himanshu Brahmbhatt, Jennifer MacDiarmid and colleagues previously showed that minicells can deliver chemotherapy to tumour cell xenografts with the aid of bispecific antibodies (one arm is bound to the outer membrane of the minicell and the other is specific for a tumour antigen). Given the potential therapeutic use of small interfering RNAs (siRNAs) and the difficulties associated with targeting these agents to tumours, the authors investigated whether minicells could be effective siRNA vectors. They established that siRNAs can passively accumulate in minicells and that plasmids containing short hairpin RNAs (shRNAs) can be transfected into minicells. In vitro, these minicells, which are coated with epidermal growth factor receptor (EGFR)-bispecific antibodies, are taken up by EGFR-expressing human tumour cells through receptor-mediated endocytosis. Degradation of the minicell through the lysosomal pathway results in the release of active siRNAs. Indeed, minicells containing siRNAs that target the cell cycle genes polo-like kinase 1 or cyclin-dependent kinase 1 resulted in cell death in vitro. These minicells also proved effective in mouse xenograft models, in which they substantially suppressed tumour growth. Importantly, mice bearing tumours treated with minicells containing scrambled siRNA or minicells with CD33-bispecific antibodies did not respond, indicating that non-specific siRNA effects were not important and that tumour targeting using the EGFR-bispecific antibody was essential.
A major clinical problem for cancer therapy is drug resistance, so the authors investigated, as an example, whether shRNAs could be used to suppress expression of multidrug resistance protein 1 (MDR1). The authors treated a variant of human colon cancer Caco-2 cells that express MDR1 (and EGFR) with EGFR-targeted minicells containing shRNAs to MDR1. They then treated the cells with EGFR-targeted minicells containing irinotecan or 5-fluorouracil, which are exported from the cell through MDR1. Cells treated sequentially with these agents showed significant (P<0.0001) levels of cytotoxicity compared with untreated cells. Similar results were evident in vivo: Caco-2–MDR1 xenografts completely regressed, as did MDR1 breast cancer xenografts treated first with EGFR minicells containing MDR1 shRNAs and then with EGFR minicells containing doxorubicin.
This is a preview of subscription content, access via your institution