Atovaquone
Atovaquone, an anti-malarial drug, is FDA-approved for the treatment of lung infection called pneumocystis pneumonia, and for toxoplasmosis [
85]. It is efficacious against a variety of protozoa such as
plasmodium spp.,
Toxoplasma gondii., P. carinii., and Babesia spp. [
128]. It was particularly developed to treat malarial infections and targeted the mitochondrial function in
Plasmodium spp. by inhibiting the mitochondrial complex III, also known as cytochrome bc1 complex [
85]. Atovaquone is ubiquinone (also known as coenzyme Q) analogue and blocks its binding site on cytochrome bc1 complex, therefore, inhibiting the transport of electrons into complex III [
128]. This leads to a significant decline in mitochondrial membrane potential (MMP), disruption of key enzymes linked to the ETC, suppression of OXPHOS, and consequently results in a complete failure of malarial mitochondrial function [
128]. Additionally, the anti-malarial activity of atovaquone is enhanced by synergizing it with proguanil [
128].
Atovaquone is observed to decrease the OCR and alleviate hypoxia in various cancer cell lines by targeting mitochondrial complex III [
84]. This effect has also been observed in animal models. Decreasing OCR is an attractive strategy to enhance the radiosensitivity of hypoxic tumors, and if reproduced in humans, it would render the tumors more sensitive to radiotherapy, thus improving clinical outcomes [
26]. Fiorillo et al. 2016 illustrated that atovaquone reduces cell proliferation and induces apoptotic cell death in breast CSCs by inhibition of mitochondrial complex III and OXPHOS [
85]. This resulted in a decrease in mitochondrial respiration, ATP levels, and MMP, along with a subsequent increase in cytotoxic ROS [
85]. Moreover, atovaquone was found to decrease OCR and diminish hypoxia in FaDu (hypopharyngeal carcinoma), HCT116 (colorectal carcinoma) and H1299 (lung carcinoma) cell lines. It also reduced hypoxia in FaDu and HCT116 mouse xenografts models and significantly decreased tumor growth in the FaDu xenograft model in combination with radiation [
84]. Other studies on the anti-cancer activity of atovaquone reveal that it induced apoptosis and inhibited cell growth in cervical cancer, thyroid cancer, retinoblastoma, and renal cell carcinoma (RCC), by suppressing mitochondrial respiration [
86‐
89]. It also enhanced the sensitivity of retinoblastoma and RCC to chemotherapy and immunotherapy [
88,
89]. Since mitochondrial biogenesis differs amongst cancers, the anti-cancer effect of atovaquone are more likely to eradicate those cancers with a higher dependency on mitochondrial biogenesis [
86].
Atovaquone also targets signal transducer and activator of transcription 3 (STAT3), which is actively expressed in many solid and hematological malignancies [
129]. Upregulated STAT3 contributes to tumorigenesis by increasing cancer proliferation, metastasis, drug resistance, and prevents apoptosis [
129]. Atovaquone is found to inhibit STAT3 in thyroid cancer, acute myeloid leukemia (AML), and glioblastoma, therefore decreasing cell viability and inducing apoptosis in these cancers [
87,
90,
91].
Ivermectin
Ivermectin is FDA-approved for the management of onchocerciasis, intestinal strongyloidiasis, pediculosis capitis, and inflammatory lesion – rosacea [
93]. It belongs to a family of macrocyclic lactone compounds called Avermectins, all of which bind to the glutamate-gated chloride ion channels (Glu-Cl) and interact with gamma-aminobutyric acid (GABA) gated Cl
− channels in the nerve and muscle cells of the parasites [
130]. This results in an increased build-up of Cl
− ions, causing hyperpolarization of parasitic cell membranes, and leads to muscle paralysis and cell death [
131]. The safety profile of ivermectin in mammals is confirmed by the presence of the intact BBB which secures GABA-sensitive neurons, therefore protecting against toxic effects of avermectins [
131]. Ivermectin exhibits limited BBB penetrability due to the plasma membrane efflux pump p-glycoprotein which limits the drug intake into the brain [
132]. However, a study found that avermectins, including ivermectin has the potential to inhibit p-glycoprotein efflux transporter and could penetrate the BBB at the appropriate concentrations [
94,
133,
134]. This provides support for further research into the dosages required for BBB penetrability of ivermectin.
Interestingly, ivermectin has shown promising anti-cancer efficacy in various cancers by increasing mitochondrial dysfunction, inducing oxidative stress, and energy crisis [
130]. By selectively targeting the ETC complex I, it inhibits the electron transport to the subsequent ETC complexes and causes a decline in MMP [
130]. This results in a decrease in OCR, leading to suppression of mitochondrial respiration and ATP production, and also production of cytotoxic ROS which potentiate DNA damage [
130]. A study by Zhu et al. 2017 observed that by targeting mitochondrial function, ivermectin causes apoptosis and suppresses cellular proliferation in a variety of RCC cell lines and also significantly impairs tumor growth in RCC xenograft mouse model [
95]. Also, ivermectin-induced oxidative stress and mitochondrial dysfunction upregulated caspase-dependent apoptosis in chronic myeloid leukemia (CML) cells and inhibited tumor size in CML xenograft models [
96]. It also sensitized the CML cells to BCR-ABL tyrosine kinase inhibitors, particularly, nilotinib and dasatinib [
96]. Moreover, it exhibited preferential toxicity to both RCC and CML cells and spared the healthy cells due to the high reliance of these cancers on mitochondrial biogenesis [
95,
96]. The inhibitory effects of ivermectin were also visualized in both
in vitro and
in vivo subcutaneous glioblastoma models, where it inhibited proliferation, induced caspase-dependent apoptosis, and potentiated angiogenic inhibition [
97]. These responses were mediated via the role of ivermectin in depolarizing the MMP with an increase in ROS production, and inhibition of the capillary network formation [
97].
Furthermore, ivermectin was also found to induce oxidative stress in glioblastoma cells by inhibiting the Akt/mTOR pathway [
97]. The mTOR pathway is critical to mitochondrial function and acts as a cellular switch between glycolysis and mitochondrial respiration, such that mTOR inhibitors tend to decrease OCR and mitochondrial respiration directly and shift the cells towards a glycolytic phenotype [
79]. The cytotoxic profile of ivermectin was not observed in mitochondrial-deficient cells or cells exposed to antioxidants, thus validating the role of ivermectin in inhibition of mitochondrial function [
97]. Moreover, it induced caspase-mediated apoptosis and inhibited cell cycle progression in neuroglioma cells and suppressed tumor growth in
in vivo xenografts [
94].
Studies have also demonstrated that ivermectin induces cytostatic autophagy in breast cancer, and inhibits proliferation of ovarian cancer and oesophageal squamous cell carcinoma, by inhibition of PAK1 protein [
98‐
100]. Moreover, it induces apoptosis in various other cancer such as melanoma by inhibition of ROS-TFE3 dependent autophagy; in leukemia by increasing chloride ions influx and ROS production; and in colon cancer cells via blocking the WNT/TCF pathway [
101‐
103]. It also decreased cell viability of cervical cancer cells by mechanisms including mitochondrial dysfunction, increased ROS generation, DNA fragmentation, and chromatin condensation, and arresting cells in G1/S phase [
104].
Mefloquine
Mefloquine, a quinolinemethanol, is FDA-approved for the treatment of malarial infection caused by chloroquine-resistant
Plasmodium falciparum and is an effective blood schizonticide for
P. vivax [
135]. It has been clinically available for thirty years and known to concentrate in the lysosomes of
P. falciparum; however, the exact mechanism of action remains unknown. Recent investigation has found that mefloquine inhibits protein synthesis by interacting with the GTPase-associated center of the 80S ribosomal subunit in
P. falciparum and therefore kills the malarial parasite [
136].
Mefloquine has demonstrated anti-cancer efficacy across various cancer cell lines and has been repurposed in the therapeutic targeting of several cancer signaling pathways. Yan et al. 2013 observed that mefloquine reduced the cell viability of prostate cancer cells by altering MMP and producing excess cytotoxic ROS, which decreased the phosphorylation of Akt and triggered JNK, ERK and AMPK signaling pathways [
105]. Interestingly, mefloquine did not inhibit the proliferation of normal human fibroblasts, thus indicating that it selectively targets the cancer cells [
105]. Another study reported that mefloquine prevents the proliferation of the gastric cancer cells and reduced tumor growth in xenograft mouse models by inhibition of PI3K/Akt/mTOR pathway [
106]. Similarly, it also displayed cytotoxicity against cervical cancer cells by targeting mitochondrial function and deactivating the mTOR pathway [
107]. An increase in levels of PARP cleavage protein, which inhibits PI3K/Akt system, was also observed [
107]. The PI3K/Akt pathway is critical for protecting MMP as it inactivates pro-apoptotic BAD protein and reduces cellular oxidative stress [
137]. Activation of the Akt system also phosphorylates various downstream regulatory proteins such as BAD, caspase-9, FKHR, and GSK3β, which protects against cellular stress [
137]. Similarly, the mTOR pathway is highly sensitive to mitochondrial dysfunction, and inhibition of this pathway results in a decrease in OCR, mitochondrial respiration, and overall mitochondrial activity [
79]. Hence, targeting the PI3K/Akt/mTOR pathway in cancer cells would directly influence the mitochondrial function and trigger cell death.
Mefloquine also stimulated apoptotic cell death in breast cancer by inhibiting autophagy and decreased the proliferative and self-renewal capacity of liver CSCs by targeting the β-catenin pathway [
108,
109]. It inhibited tumor proliferation and decreased tumor growth in colorectal cancer xenograft mouse models by targeting NF-κB and the downstream signaling pathways [
110]. It further prevented autophagic degradation of defective mitochondria in colorectal CSCs by suppression of RAB5/7, LAMP1/2, and PINK1/PARKIN, therefore resulting in increased mitochondrial dysfunction and cellular apoptosis [
111].
Furthermore, mefloquine was also found effective in inducing cell death in AML, chronic lymphocytic leukemia (CLL), and glioblastoma through disruption of lysosomes [
112‐
114]. Lysosomal disruption releases the catalytic proteases cathepsins into the cytosol, which activates mechanisms of apoptosis [
113]. Mefloquine stimulates the release of cathepsins from lysosomes in AML and initiates mitochondria-mediated cell death [
113]. In glioblastoma, mefloquine disturbed the lysosomal stability and increased the levels of caspase-3, inducing cell death regardless of the p53 status [
114]. It was also found to be more potent than chloroquine at inducing glioblastoma cell death and exhibited a better BBB penetrability profile [
114].
Proguanil
The biguanide proguanil was first developed as an anti-malarial drug for the management of acute
P. vivax malaria; it was initially used alone and later administered in combination with chloroquine or atovaquone and the combination was found to be highly effective [
138]. It is metabolized into cycloguanil, which regulates inhibition of parasite dihydrofolate reductase [
138]. Another synergistic combination of proguanil and atovaquone is effective against
P. falciparum malaria; both of these drugs target the parasite
P. falciparum’s pre-erythrocytic and erythrocytic stages and induces causal and suppressive prophylaxis [
139].
Proguanil enhances the activity of atovaquone in collapsing MMP and causing mitochondrial dysfunction [
139]. Despite of being a complex I inhibitor; proguanil does not inhibit cellular and mitochondrial respiration as its penetrability into the mitochondria is limited [
140]. However, proguanil still displayed the highest growth inhibition of colon and bladder cancer cell compared to other biguanides [
115]. This points to the intriguing possibility of the extramitochondrial cytotoxic activity of proguanil and also suggests its potential role as an anti-cancer agent [
115].
Quinacrine
Quinacrine, an acridine derivative, was initially used for the prophylaxis and management of the malarial infection [
141]. It is clinically available as quinacrine dihydrochloride and is effective against the treatment of various conditions, including malaria, giardiasis [
142], and tapeworm infection [
143]. Quinacrine’s effectiveness has also been reported in autoimmune inflammatory conditions, including systemic lupus erythematosus [
144], and rheumatoid arthritis [
145]. Moreover, it is efficacious in the treatment of pleural effusions in cancer patients [
146] and also prevents spontaneous pneumothorax in patients with increased recurrence risk [
147]. Quinacrine has also been utilized for safer female sterilization as it causes fibrosis and occlusion of fallopian tubes [
148], and is currently under investigation for managing Creutzfeldt-Jacob disease [
149].
The anti-cancer potential of quinacrine has been extensively explored, revealing various therapeutic targets. Due to its low toxicity profile, minimal side effects, and interference with many cancer signaling pathways, quinacrine offers compelling evidence as a promising antineoplastic agent [
116]. The cytotoxic mechanisms of quinacrine include its ability to intercalate into DNA and insert between adjacent base pairs resulting in DNA damage [
150]. It also prevents DNA damage repair by interfering with the nuclear proteins and inhibits both DNA and RNA polymerases, topoisomerases, telomerase, and upregulates tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) therefore promoting apoptosis [
116]. Quinacrine stimulates TRAIL binding with death receptors 4 and 5 (DR4 and DR5) and upregulates mitochondrial intrinsic apoptotic cascade [
116]. It abrogates the arachidonic acid pathway by directly targeting the enzyme phospholipase A2 (PLA2), resulting in a decline in production of prostanoids (COX), leukotrienes (LOX), and eicosanoids (MOX/CYP450), as these prevent cell death and sustain cancer cell survival [
150]. Moreover, quinacrine intercalates into DNA and alters the chromatin structure by inducing FACT (facilitates chromatin transcription) complex chromatin trapping; this leads to suppression of the NF-κB pathway and phosphorylation of the tumor suppressor protein p53 [
116]. Hence, quinacrine has the potential to be effective against various cancers by targeting single or multiple signaling pathways and inducing apoptosis. The DNA damaging potential of quinacrine makes it an effective candidate to be combined with radiotherapy and could significantly enhance the efficacy of radiotherapy in HGGs.
A study by Preet et al. 2012 demonstrated that quinacrine targets breast cancer cells and induces cell death, cell cycle arrest, and decreases cell migration through inhibition of topoisomerases and increased DNA damage [
117]. It stimulates apoptotic pathways in colon cancer cells by increasing the activity of p53 and p21 and the associated pathways [
118]. Research has also found that quinacrine induces autophagic and apoptotic cell death and suppressed tumor growth in ovarian cancer by downregulating p62/SQSTM1 [
119]. Furthermore, quinacrine was found to be effective in enhancing apoptosis in diffuse large B-cell lymphoma by downregulating MSI2 NUMB signaling pathway, suppressing c-Myc and arresting cells in S phase [
120]. It exhibited a significant tumoricidal effect against RCC by inhibiting NF-κB and activating p53; reduced cell viability and decreased tumor volume in melanoma xenograft mouse models; induced apoptosis in leukemia cells by depolarization of mitochondria, oxidative stress and downregulation of Bcl-2 and Bcl-2L; and inhibited cell growth and cell cycle progression in gastric cancer cells via activating p53 and caspase-3 dependent apoptotic pathways [
121‐
124].
Quinacrine has also been found to act synergistically with chemotherapy drugs such as carmustine, oxaliplatin, and carboplatin, or inhibitors including cisplatin, 5-fluorouracil, paclitaxel, and sorafenib, across various cancer types [
119,
125,
151‐
155]. It is useful in overcoming resistance against tyrosine kinase inhibitor erlotinib in non-small cell lung cancer via inhibition of NF-κB, FACT, and induction of cell cycle arrest [
126]. Moreover, quinacrine inhibited autophagy and reduced the cell viability of human glioblastoma cells both alone and in combination with the kinase inhibitor SI113 by upregulating p62 [
127]. Another study by Wang et al. 2017 discovered that quinacrine induced apoptosis in GSCs and enhanced the efficacy of curcumin in eradicating glioma cells both
in vitro and
in vivo animal models [
156]. Due to its ability to penetrate the BBB, induce DNA-damage and apoptosis in cancers, and its efficacy in gliomas, quinacrine seems a promising therapeutic agent for treatment of glioblastoma and DIPG. The DNA damaging role of quinacrine indicates that combining it with radiotherapy could be an effective approach in prolonging patient survival.
Quinacrine was later replaced by chloroquine for the treatment of malaria, however, it continued to be used in the treatment of diseases. Chloroquine, as an anti-malarial agent, has also shown promising anti-cancer potential. It is found to disrupt autophagy by blocking the formation of autolysosomes and activating the GRP78/BiP chaperone which enhances endoplasmic reticulum (ER) stress [
157,
158]. Impairment of autophagy results in apoptosis and this could therefore sensitize cancer cells to chemo- and radiotherapy [
157]. Interestingly, chloroquine has been shown to act synergistically with TMZ by inducing ER stress [
158].