The development of new anti-cancer agents is expensive and time consuming. Generally, pharmaceutical companies use large-scale gene analysis, in silico analysis, and high-throughput screening for the identification of new drugs against new cancer targets; however, these techniques may require a large developmental budget and long-term development. In addition, once new cancer drugs reach the clinical field, unexpected side effects sometimes occur and, in some cases, the drugs must be withdrawn. Alternatively, when the use of a new cancer drug is expanded to malignant brain tumors, especially GBM, their distribution, pharmacokinetics, and BBB permeability become an issue.

The concept of drug repositioning has recently drawn attention and involves old clinical drugs being put to practical use for another disease and another target. In the present study, we report identification of the antidepressant fluvoxamine as a potent inhibitor of actin polymerization, which is essential for cancer cell migration and invasion. Consistent with this, fluvoxamine effectively inhibited the formation of lamellipodia, focal adhesions, and stress fibers, as well as the migration and invasion of human GBM cells in vitro. Fluvoxamine also suppressed FAK and Akt/mTOR signaling. Furthermore, daily administration of fluvoxamine to mice bearing hGICs blocked tumor cell invasion and prolonged the mouse survival period. Taken together with the fact that fluvoxamine has been safely used for a long time for the treatments of various mental disorders, our findings suggest that fluvoxamine is a promising candidate for anti-invasion therapy for GBM.

Specifically, we identified fluvoxamine, a clinically used antidepressant, as a potent anti-invasive drug for the treatment of GBM. Here we demonstrated that fluvoxamine effectively inhibits actin polymerization, which is essential for cancer cell migration and invasion. Consistent with this, fluvoxamine-treated GBM cells failed to form lamellipodia, focal adhesions, and stress fibers and displayed decreased migration and invasion in vitro. We also demonstrated that fluvoxamine suppressed FAK and Akt/mTOR signaling pathways, suggesting that inhibition of these signaling pathways results in disrupted actin polymerization. Furthermore, daily administration of fluvoxamine to mice bearing hGICs blocked tumor cell invasion and prolonged the survival of these mice. These results suggest that fluvoxamine may be a promising anti-invasive drug against GBM and that a strategy targeting the actin cytoskeleton has the potential to inhibit tumor cell motility and overcome the poor prognosis of GBM.

Although fluvoxamine was observed to inhibit actin polymerization in vitro (Fig. 1), it did not involve a direct effect on actin (Fig. S1). Moreover, fluvoxamine decreased autophosphorylation of FAK at Y397 (Fig. 5a, 5c) and disrupted focal adhesions and stress fibers (Fig. 5d). FAK has been shown to be overexpressed in various types of cancer, including GBM20,21,22, and to modulate actin polymerization and lamellipodial protrusion23, suggesting that FAK is a potential target for anti-invasive therapies for various cancers. In fact, Y15, a small-molecule inhibitor of FAK autophosphorylation, was shown to decrease the invasivity of human GBM cell lines24. Consequently, it is possible that the inhibition of actin polymerization by fluvoxamine is mediated via FAK signaling. However, the autophosphorylation state of FAK at Y397 is cell attachment-dependent25. Thus, we cannot rule out the possibility that decreased FAK autophosphorylation in fluvoxamine-treated GBM cells was caused by cell detachment.

We also observed that fluvoxamine decreased the phosphorylation of Akt at S473 and T308 (Fig. 5a). Akt-S473 is phosphorylated by mTOR complex 2 (mTORC2) and DNA-activated protein kinase26,27,28. Because mTORC2 has been shown to regulate actin polymerization29, it is possible that fluvoxamine directly and/or indirectly affects mTORC2 activity. Although T308 of Akt was previously shown to be phosphorylated by PDK1 through the PI3K pathway30,31, the activity of PDK1 (p-S241-PDK1) was not affected by FBS stimulation (Fig. 5a), suggesting that Akt-T308 is phosphorylated by unknown protein kinases in U87-MG cells, and fluvoxamine inhibits this activity. Unlike protein-protein interactions, it is difficult to examine direct interactions between small molecules and proteins. In addition, there are more than 100 proteins involved in the regulation of actin dynamics32. Thus, the molecular mechanism by which fluvoxamine inhibits actin polymerization has not yet been elucidated and further understanding of the precise mechanism requires additional studies.

We show that the effective dose of fluvoxamine required to inhibit lamellipodia formation and migration and invasion of GBM cells in vitro was approximately 20–30 μM (Figs 2 and 3). A clinical study using fluorine magnetic resonance spectroscopy demonstrated that the steady-state brain concentration of fluvoxamine exceeded 20 μM in some patients administered a clinical dose (100–300 mg/day) of fluvoxamine for the treatment of depression33. Accordingly, our study demonstrates that the clinically anti-depressive dose of fluvoxamine is sufficient to prevent invasion of GBM cells. Consistent with this, we observed that the anti-depressive dose of fluvoxamine significantly blocks tumor invasion and prolongs the survival of GBM-bearing mice without obvious side effects (Fig. 4). Fluvoxamine is selectively incorporated into the central nervous system and its brain concentration is 10- to 20-fold higher than its plasma concentration33,34, suggesting it can inhibit GBM invasion without severe peripheral side effects. Furthermore, it is reported that selective serotonin reuptake inhibitors, including fluvoxamine, can be safely used for the treatment of depression in patients with GBM35. Taken together, these findings demonstrate that fluvoxamine is a promising candidate for safe and effective GBM therapy.

Most anti-cancer drugs currently used are anti-proliferative, affecting cell division or DNA synthesis, whereas very few drugs effectively inhibit tumor cell invasion36. A recent study reported that imipramine-blue (IB), a derivative of the tricyclic anti-depressant imipramine, inhibits the invasion of GBM cells in vitro, and liposome-encapsulated IB prolongs the survival of tumor-bearing rats when combined with liposomal doxorubicin37. These findings suggest that a combination of standard therapies with anti-invasive drugs could be a useful approach for the treatment of GBM.

Finding new uses for existing clinically used drugs, termed drug repositioning or repurposing, is an alternative strategy for drug discovery and development12. This approach has been widely attempted and has been successful in some cases, such as the use of aspirin as an anti-platelet medication and sildenafil in erectile dysfunction12,13. Because the pharmacokinetics of most existing clinically used drugs have already been studied, the effective dose and possible side effects are well known, the cost and time required to bring these drugs to market can be reduced14. Given that fluvoxamine has been used safely for various mental disorders, it is a potential candidate for drug repositioning.

In conclusion, we demonstrated that a clinically used anti-depressant, fluvoxamine, effectively inhibits actin polymerization and blocks the invasion of GBM cells into normal brain tissues. Our findings and concepts suggest the therapeutic potential of fluvoxamine as an anti-invasive drug in the treatment of GBM and provide evidence that targeting actin dynamics is a novel therapeutic approach for the treatment of GBM.