Mitochondria-Targeted Analogues of Metformin Exhibit Enhanced Antiproliferative and Radiosensitizing Effects in Pancreatic Cancer Cells

Abstract

Metformin (Met) is an approved antidiabetic drug currently being explored for repurposing in cancer treatment based on recent evidence of its apparent chemopreventive properties. Met is weakly cationic and targets the mitochondria to induce cytotoxic effects in tumor cells, albeit not very effectively. We hypothesized that increasing its mitochondria-targeting potential by attaching a positively-charged lipophilic substituent would enhance the antitumor activity of Met. In pursuit of this question, we synthesized a set of mitochondria-targeted Met analogs (Mito-Mets) with varying alkyl chain lengths containing a triphenylphosphonium cation (TPP+). In particular, the analog Mito-Met10, synthesized by attaching TPP+ to Met via a 10-carbon aliphatic side chain, was nearly 1,000 times more efficacious than Met at inhibiting cell proliferation in pancreatic ductal adenocarcinoma (PDAC). Notably, in PDAC cells Mito-Met10 potently inhibited mitochondrial complex I, stimulating superoxide and AMPK activation, but had no effect in non-transformed control cells. Moreover, Mito-Met10 potently triggered G1 cell cycle phase arrest in PDAC cells, enhanced their radiosensitivity and more potently abrogated PDAC growth in preclinical mouse models, compared to Met. Collectively, our findings show how improving the mitochondrial targeting of Met enhances its anticancer activities, including in aggressive cancers like PDAC in great need of more effective therapeutic options.

Modified Metformin as a More Potent Anticancer Drug: Mitochondrial Inhibition, Redox Signaling, Antiproliferative Effects and Future EPR Studies

“…We hypothesized that attaching the triphenylphosphonium cation (TPP + ) to metformin via an alkyl chain would greatly enhance its accumulation in mitochondria of cancer cells, leading to selective inhibition of proliferation of cancer cells. (7) Results obtained in this study substantiate the proposal that TPP + -modified metformins are more cytostatic to pancreatic cancer cells.…”

“…MiaPaCa-2 cells which were treated with 0.5 or 1 millimolar concentration of metformin, or 0.5 to 1 micromolar concentration of mito-metformin, showed the same effects in colony formation and cell proliferation experiments. (7) The IC 50 values for inhibiting 50% of colony formation for metformin and mito-metformin 10 were calculated to be 1.3 mM and 1.1 μM, respectively. Thus, mito-metformin 10 is ca.…”

“…Metformin's antitumor effects were attributed in part to its ability to inhibit mitochondrial complex I-dependent cell respiration and ATP generation. (7, 12) In contrast to rotenone, a non-selective inhibitor of complex I in both normal and cancer cells, metformin reversibly and more selectively inhibits mitochondrial complex I in cancer cells. (12) As mito-metformin 10 selectively accumulates into mitochondria, we compared the relative inhibitory effects of metformin and mito-metformin 10 on complex I activity in MiaPaCa-2 cells.…”

“…The mitochondrial respiration or OCR was monitored in MiaPaCa-2 cells treated with metformin and mito-metformin 10 using the Seahorse XF extracellular flux analyzer. (7) Figure 3 shows the OCR measurement of mitochondrial complex I activity of MiaPaCa-2 cells pretreated for 24 h with metformin or mito-metformin 10 . The IC 50 values for inhibition of mitochondrial complex I for metformin and mito-metformin 10 were determined to be ca.…”

“…Results show that the mito-metformin analogs are much more potent (100 to 1,000-fold higher) than metformin in inhibiting human pancreatic adenocarcinoma cells (PDACs). (7)…”