Development of small molecules to impair and image mitochondria in cancer cells

Abstract

Mitochondrion, a central metabolic organelle is known as the powerhouse of the cells, required for bioenergetics, biosynthesis, and signaling. Overall, collective finding suggests that these functions of mitochondria are mainly responsible for tumorigenesis. Hence, illuminating and targeting the mitochondria in cancer cells with novel small molecules have immense potential for the next generational anticancer therapy. However, there are three significant challenges that need to be considered to develop small molecules which can impair and or illuminate mitochondria: (a) selectively impairing of mitochondria of cancer cells over normal cells, (b) selectively illuminate mitochondria without toxicity and (c) small molecule has to traverse not only the cell membrane but also the two mitochondria membranes. To address these, first, we have developed a library of hydrazide-hydrazone-based small molecules and identified novel compound in the panel of three colon cancer cell lines. The compound induces mitochondrial outer membrane permeabilization by inhibiting anti-apoptotic B-cell CLL/lymphoma 2 (Bcl-2) family proteins followed by sequestration of pro-apoptotic cytochrome C and triggered programmed cell death in HCT-116. Cell viability studies have verified that the small molecule rendered toxicity in a panel of colon cancer cells, keeping non-cancerous L929 fibroblast cells unharmed. Subsequently, we have developed an easy and short synthesis of novel hydrazide-hydrazone-based small molecules, which demonstrate for the first time remarkable aggregation-Induced emission (AIE) property in water as well as in solid state. These molecules internalized selectively into the HeLa cervical cancer cells over non-cancerous L929 fibroblast cells. One of the molecules was tagged with triphenylphosphine (TPP) moiety which successfully localized into mitochondria. Finally, we have developed a novel lipophilic positively charged, cyanine-based red fluorescence small molecules library. All the compounds were found extremely cytotoxic in the HeLa cells with IC50 values close to 1 µM. Two molecules in the library were able to home into mitochondria at 150 nM concentration within 20 mins. These compounds induced mitochondrial impairment validated with the formation of mitochondrial permeability transition pore (MPTP) upon inhibiting the anti-apoptotic Bcl-2 and Bcl-xl proteins and enhanced the expression of pro-apoptotic BAX protein which leads to triggering mitochondrial-dependent apoptosis. We anticipate that this finding will be helpful to explore the future mitochondria-targeted therapeutics in cancer.