Glioblastoma is a highly aggressive brain cancer commonly found in adults. The current treatment includes surgical resection followed by a combination of radiotherapy and temozolomide chemotherapy. However, the development of intrinsic or acquired resistance to temozolomide (Strobel, 2019, Mukherjee and Pillai, 2022) results in poor efficacy of treatment with a median survival rate of approximately 15 months for patients. In addition, numerous side effects are associated with these treatments (Mukherjee and Pillai, 2022).

Between 50% and 70% of cancer patients who undergo chemotherapy receive a platinum derivative treatment. Cisplatin (PtCl2(NH3)2), the lead drug, is commonly used for, ovarian, uterine, lung, testicular and bladder cancers (Rancoule, 2017). Alternatively, oxaliplatin is commonly used to treat metastatic colorectal, colon, pancreas and stomach cancers (Martinez-Balibrea, 2015). However, these two drugs, which are alkylating agents of nuclear DNA and cytosolic molecules leading to cell death, remain associated with severe systemic toxicity and resistance phenomena (Rosenberg et al., 1965, Wisher and Martindale, 2012, Dasari and Bernard Tchounwou, 2014, Enriquez Perez, 2019). Therefore, developing new platinum-based drugs that have reduced toxicity, higher efficacy and no drug resistance is an active area of research, including for glioblastoma (Ferrari, 2021). In this context, using N-heterocyclic carbene (NHC) ligands has emerged as an interesting strategy to develop new metal complexes to effectively combat cancer (Bourissou, 2000, Herrmann, 2002, Cesar et al., 2004, Mercs and Albrecht, 2010, Benhamou, 2011, Bellemin-Laponnaz and Dagorne, 2014, Teyssot, 2009). Platinum NHC complexes and other metal NHC complexes (Au and Ag in particular (Monticelli, 2017, Baron, 2014, Malik, 2021) have successfully been investigated for the development of potential metallodrugs (Gasser et al., 2011, Gautier and Cisnetti, 2012, Hartinger et al., 2012, Liu and Gust, 2016, Zou, 2018, Porchia, 2018, Mora et al., 2019, Hussaini et al., 2019, Bellemin-Laponnaz, 2020). They offer high stability of transition metal complexes, easy functionalization, and have been observed to be more cytotoxic than the platinum molecules currently used in vitro as well as in vivo (Chekkat, 2016, Chtchigrovsky, 2013, Skander, 2010, McCartin, 2022, McCartin, 2022, McCartin, 2022).

Resistance phenomena in cancer treatments are common, and several hypotheses have been put forward to explain them. A growing body of experimental and clinical evidence indicates that rare populations of cells, called cancer stem cells (CSCs), play an important role in the development and therapeutic resistance of several cancers, including glioblastoma (Reya, 2001, Singh, 2004), leading to invasive tumour growth and relapse (Venere, 2011, Safa, 2015). This is the consequence of the low proliferation capacity of CSCs, among other things, which renders them resistant to common treatments that target the replication cell systems. Thus, in addition to eradicating cancer cells, the elimination of CSCs is of crucial importance for the effective and definitive treatment of glioblastoma and other cancers in general.

Several studies have indicated that mitochondria are involved in drug resistance of CSCs due to their role in the energy metabolism and susceptibility to cell death. In agreement with these observations, it has been demonstrated that CSCs have a higher mitochondrial capacity (Vlashi, 2011, Farnie et al., 2015) and results obtained with a proteomic approach showed that CSCs overexpress>60 mitochondria-related proteins. This suggests that CSCs require significant mitochondrial biogenesis for their survival and proliferation. Importantly, Lamb et al. have shown that inhibition of mitochondrial biogenesis by drugs, such as several classes of FDA-approved antibiotics, can efficiently kill CSCs (Lamb, 2014).

Recently, our group has shown that N-heterocyclic carbene iridium(III) complexes (McCartin, 2022) as well as polyethylenimine (PEI) and a PEI-NHC-Pt conjugate (McCartin, 2022, McCartin, 2022) are effective in killing glioblastoma cancer stem cells. The iridium complex (McCartin, 2022) and the PEI-NHC-Pt (Chekkat, 2016) both localize at the mitochondria of the cells. Therefore, the hypothesis is that developing anticancer drugs that better target the mitochondria of CSCs could further increase their killing efficacy.

So, the objective of this study was to design, synthetize and evaluate novel NHC-based platinum complexes that target mitochondria to simultaneously eradicate glioblastoma cancer cells as well as glioblastoma CSCs. Herein, we propose the synthesis and characterization of N-heterocyclic carbene-platinum complexes with, or without, a mitochondria-targeting moiety, namely a triphenylphosphonium cation (TPP+) which is the most widely used mitochondriotropic carrier (Zielonka, 2017).

Our in vitro studies showed that these complexes are cytotoxic against a human glioblastoma cell line (U87) as well as against a human glioblastoma stem cell line (NCH421K). Additional studies indicate that these molecules alter mitochondria function leading to their death. The cell death mechanism is non-classical as caspase 3 and 7 activation was observed but not apoptosis.