*2.6.1 Eribulin*

*2.4.2 Pentamidine*

with malignant glioma.

**2.5 Antipsychotic drugs**

*2.5.1 Fluvoxamine*

*2.5.2 Fluspirilene*

**142**

Pentamidine is effective in the treatment of pneumonia caused by *Pneumocystis jirovecii*. This drug exerts its therapeutic effects via the inhibition of glucose metabolism, protein synthesis, amino acid transport, and ribonucleic acid (RNA) synthesis [47]. Previous studies have shown the therapeutic effects of pentamidine in various cancers [48]. One in vitro study revealed that pentamidine suppressed cancer activity via the inhibition of phosphatase of regenerating liver (PRL) [48] and the inhibition of PRL phosphatase suppressed the activation of Akt and ERK [49]. Based on these studies, we investigated the effect of pentamidine in glioma cells and GSCs. Pentamidine suppressed the proliferation of glioma cells and GSCs and reduced the stemness of GSCs. Additionally, there are clinical benefits to repurposing pentamidine as the therapeutic drug for malignant glioma, because the current chemoradiotherapy sometimes induces lymphopenia as a side effect and patients might suffer from pneumonia caused by *P. jirovecii*. Further research to investigate the molecular mechanism of pentamidine is in underway. In the future, clinical trials are warranted to determine the benefit of pentamidine for patients

*Drug Repurposing - Hypothesis, Molecular Aspects and Therapeutic Applications*

Fluvoxamine has been used as an antidepressant since 1986 and is widely applied in the treatment of anxiety disorders owing to its selective serotonin reuptake inhibitor activity, which helps maintain sufficient serotonin levels in the brain to function [50, 51]. Recently, a new screening method for the quantitative determination of actin polymerization showed that fluvoxamine inhibits the formation of F-actin, which induces lamellipodial protrusions, focal adhesions, and stress fibers at the edge of GBM and is essential for the migration and invasion of GBM cells into normal brain tissues [52–54]. The molecular signal changes in fluvoxamine-treated glioma cells are achieved by the suppression of the activity of actin polymerization regulators, focal adhesion kinases, and mTOR complex 2 [55, 56]. The daily administration of

fluvoxamine to an intracranial xenograft mouse model significantly prolongs survival and blocks the infiltration of tumor cells into normal brain tissues in vivo [57]. Therefore, fluvoxamine disrupts focal adhesion and actin depolymerization, blocks the migration and invasion ability of GBM cells, and prolongs patient survival. Fluvoxamine is a potentially effective anti-invasive drug for the treatment of glioma.

Fluspirilene, a member of the diphenylbutylpiperidine class of drugs, is an effective, traditional, long-acting antipsychotic [58, 59]. Fluspirilene displays an effective Ca2+ channel blocking activity [60] and inhibits synaptic transmission; thus, fluspirilene can mitigate a seizure [58]. However, recent studies have shown a new effect of fluspirilene against some incurable cancers, such as hepatocellular carcinoma [61] and GBM [62]. Fluspirilene has been identified as a potential anti-GSC drug. An in vitro investigation has shown that fluspirilene not only attenuates the cell viability, stemness, sphere-forming ability, and proliferation of GSCs but also suppresses the invasion of GBM cells via the inhibition of signal transducer and activator of transcription 3 (STAT3) activity and its nuclear reduction in GBM cells [62]. In vivo, fluspirilene significantly decreases tumor volume and prolongs

Eribulin, a non-taxane inhibitor of microtubule dynamics [63, 64], was approved by the US Food and Drug Administration (FDA) in 2010 for the treatment of stage 4 breast cancer [65]. Eribulin prevents the growth of tumor cells via the inhibition of microtubule activity during cell mitosis and induces M-phase arrest, which result in cell apoptosis (**Figure 3**) [66, 67]. Eribulin also reduces the aberrance of the vascular microenvironment of a tumor [68]. Based on these effects on various cancers, recent studies have demonstrated that eribulin sensitizes a tumor to radiation via eribulininduced M-phase arrest and causes more DNA damage than radiation alone. This induces an increase in cleaved caspase-3 and cleaved poly-ADP ribose polymerase levels and results in mitotic catastrophe (**Figure 3**) [69, 70]. An in vivo study of the concomitant administration of radiation with eribulin showed that this combination prolongs the survival of the intracranial xenograft GBM mouse model [71]. Eribulin also suppresses vascular remodeling and normalizes the radiation-induced aberrant vascular microenvironment in the xenograft mouse model [71]. A growing evidence indicates that a telomerase reverse transcriptase (TERT) promoter mutation, a common mutation in GBM [72], maintains telomerase activity to evade telomere shortening; thus, tumor cells overcome replicative senescence and proliferate infinitely [73] telomerase-independent RNA-dependent RNA polymerase (RdRP) activity [74, 75]. Eribulin has been identified as a specific inhibitor of TERT-RdRP through drug screening [76]. Thus, TERT-targeting therapies would be a novel direction to treat glioma (**Figure 3**). Both in vitro and in vivo experiments using eribulin to treat gliomas have shown that eribulin exerts an anticancer activity and suppresses glioma proliferation through its function as a TERT-RdRP inhibitor, in addition to its microtubule inhibitor activity. Now, eribulin is in a phase II doctor-led clinical trial in recurrent GBM (UMIN ID: 000030359).

#### **Figure 3.**

*Antitumor mechanisms of eribulin. The effect of eribulin against glioblastoma multiforme. Eribulin suppresses microtubule activity and induces M-phase arrest, which makes cells more radiosensitive and ends up with apoptosis. Eribulin also suppresses proliferation by inhibiting the TERT-RdRP activity.*
