Detection of Antagonistic Interactions Between Cannabidiol and Temozolomide in Various Neuroblastoma and Glioblastoma Cell Lines in the MTT Assay by Means of Synergy Finder 3.0 Software and Log-Probit Isobolographic Analysis

Despite progress in experimental neuro-oncology and advances in the treatment of brain cancers, glioblastoma multiforme is still the most aggressive primary brain tumor in adults. Cannabinoid-based add-on therapy is likely to be a treatment option for glioblastoma patients who have failed surgical excision and treatment with temozolomide (a classic chemotherapeutic drug). Cannabidiol is one of the naturally occurring compounds isolated from Cannabis, which displays the anti-cancer and anti-viability effects in several glioblastoma cell lines. In the MTT assay, the anti-viability effects of combined treatment with cannabidiol and temozolomide were assessed in two neuroblastoma (CHP-134, KELLY) and four glioblastoma (SF295, U-87MG, T98G, and C6) cell lines. The interactions between cannabidiol and temozolomide from the MTT assay were characterized by means of two independent methods: a classic log-probit isobolographic analysis and a Synergy Finder 3.0 web-application allowing automatic evaluation of interactions in five pharmacodynamic models, including the zero interaction potency (ZIP), Bliss, Loewe, highest single agent (HAS) and Bliss–Loewe consensus models. The impact of cannabidiol on several apoptotic-related proteins (p21, p18, cyclin D1, Bax, Bcl-2) and the cannabinoid CB1 receptor expressed on the glioblastoma SF295 and U-87MG cells was examined with the Western blot technique. Both cell migration and cell–cell interaction after treatment with cannabidiol, temozolomide and their combination were assessed in a wound healing assay. The log-probit isobolographic analysis of interactions revealed that cannabidiol combined with temozolomide exerted antagonistic interactions only in two neuroblastoma cell lines, CHP-134 and KELLY. In contrast, the combination of cannabidiol with temozolomide in all the glioblastoma cells, SF295, U-87MG, T98 and C6, produced (in the classic isobolography) additive interactions with a tendency toward antagonism in the MTT assay. With Synergy Finder 3.0 in the Bliss–Loewe model, the interactions between cannabidiol and temozolomide were antagonistic in all the tested cell lines (except for CHP-134) with synergy score values ranging from −19.64 (for T98G) to −15.07 (for U-87MG). Only the interaction between cannabidiol and temozolomide in the CHP-134 cell line was additive in the Bliss–Loewe model with a synergy score value of 0.95. This comparative study indicates that the combination of cannabidiol with temozolomide should not be recommended and used clinically because of antagonistic interactions reported in the MTT assay. The Bliss–Loewe consensus model can be recommended as a model of choice when screening and evaluating the types of interactions in in vitro studies using the Synergy Finder 3.0 free online available web-application.