Evaluation of synergism and combination effect of sulfasalazine and valproic acid on glioma cells using a software based on Median-Effect Equation and Combination Index Theorem

Glioblastoma (GB), the most common and aggressive primary brain tumor in adults, exhibit poor prognosis and limited efficacy of conventional therapies. Sulfasalazine (SAS), a well-known drug targeting xc⁻ system, has shown antitumor activity in GB models. However, its clinical application is hindered by poor oral bioavailability and limited penetration across blood-brain barrier (BBB). Strategies to enhance its therapeutic potential, such as achieving synergistic effects with other anti-glioma drugs, are highly desirable to increase its potency. Valproic acid (VPA), a well-established anticonvulsant/mood stabilizer, is also proposed as anti-glioma agent due to its histone deacetylase (HDAC) inhibition activity, playing a role regulating glioblastoma cell proliferation and survival. Drug repurposing, thus, has emerged as a promising strategy due to its cost-effectiveness and already established safety profiles. In this study, we evaluated potential synergistic interactions between SAS and VPA in GB treatment using human (U87MG, GBM02) and rat (C6) cell lines. Pharmacological data from previous studies were analyzed using CompuSyn software to construct isobolograms, fraction affected–combination index (Fa-CI) and dose-reduction index (DRI) plots. Our results demonstrated synergistic interactions between SAS + VPA in U87MG and GBM02 cells (CI values below 1 at higher Fa levels). In contrast, C6 cells exhibited reduced synergism (CI > 1 in most conditions). DRI analysis revealed significant dose reductions for SAS + VPA treatments across all cell lines. These findings provide evidence supporting the synergistic potential of SAS + VPA in GB therapy highlighting the utility of CompuSyn for detailed drug interaction analysis. Further studies are warranted to explore the clinical applicability of this combination.