Resistance-breaking profiling and gene expression analysis on an organometallic ReI-phenanthridine complex reveal parallel activation of two apoptotic pathways

Emerging resistances of tumors against multiple anti-cancer agents are a major concern in the chemotherapeutical treatment of various cancers. Clearly, this raises the need for novel therapeutics with new modes of action. Herein, we report on the favorable in vitro anti-proliferative properties of a phenanthridine-containing Re-I(CO)(3) complex (compound 1, also abbreviated LR-166) and identify major contributions to its mode of action. The complex induces apoptosis in low micromolar concentrations even in drug-resistant Burkitt-like lymphoma (BJAB) and leukemia (Nalm-6) cell lines with known overexpression of p-glycoproteins as was confirmed by measuring the amount of hypodiploid DNA via FACS Scan analysis. Importantly, a gene expression analysis in combination with toxicity studies on a number of modified cell lines (leukemia: NALM-6, lymphoma: BJAB, melanoma: MelHO) and the reduction of mitochondrial membrane potential (determined by adding JC-1 dye, followed by FACS analysis) confirmed the activation of both, the extrinsic and the intrinsic apoptotic pathway. Finally, the mechanism of action was shown not to be influenced by overexpression of the anti-apoptotic factor Bcl-2 in Mel-HO cells which are known to be resistant to a variety of drugs. All taken together, our experiments underscore the unique opportunities inherent in this novel lead structure of Re complexes to act as an effective chemotherapeutic agent in a combination therapy to overcome documented drug resistances in tumors.