Purpose d,l-methadone (MET), an analgesic drug used for pain treatment and opiate addiction, has achieved attention from oncologists and social media as possible chemoensitizing agent in cancer therapy, notably brain cancer (glioblastoma multiforme, GBM). MET has been reported to enhance doxorubicin-induced cytotoxicity in GBM cells via activation of the µ-opioid receptor (MOR). Here, we extended this work and quantified the toxic effect of MET in comparison to other opioids alone and in combination with doxorubicin and the clinically more relevant alkylating drug temozolomide (TMZ), using a set of GBM cell lines and primary GBM cells. Methods MOR expression in GBM cells was investigated by immunofluorescence and immunoblotting. Resistance to drugs alone and in combination with anticancer drugs was assessed by MTT assays. Concentration effect curves were fitted by nonlinear regression analysis and IC50 values were calculated. Apoptosis and necrosis rates were determined by annexin V/propidium iodide (PI)-flow cytometry. Results MET alone was cytotoxic in all GBM cell lines and primary GBM cells at high micromolar concentrations (IC50 ~ 60–130 µM), observed both in the metabolic MTT assay and by quantifying apoptosis and necrosis, while morphine and oxycodone were not cytotoxic in this concentration range. Naloxone was not able to block MET-induced cytotoxicity, indicating that cell death-inducing effects of MET are not MOR-dependent. We recorded doxorubicin and TMZ concentration- response curves in combination with fixed MET concentrations. MET enhanced doxorubicin-induced cytotoxicity in only one cell line, and in primary cells it was observed only in a particular MET concentration range. In all assays, MET was not effective in sensitizing cells to TMZ. In two cell lines, MET even decreased the cell's sensitivity to TMZ. Conclusion MET was found to be cytotoxic in GBM cells in vitro only at high, clinically not relevant concentrations, where it was effective in inducing apoptosis and necrosis. Sensitizing effects were only observed in combination with doxorubicin, but not with TMZ, and are dependent on cell line and the applied drug concentration. Therefore, our findings do not support the use of MET in the treatment of GBM in combination with TMZ, as no sensitizing effect of MET was observed.

Background D,L-methadone (MET), an analgesic drug used for pain treatment and opiate addiction has achieved attention from oncologist and social media as possible chemosensitizing agent in glioblastoma multiforme (GBM) treatment. MET has been reported to enhance doxorubicin-induced cytotoxicity in GBM cells via activation of the µ-opioid receptor (MOR) and subsequent apoptosis induction. Here, we further aimed at quantifying MET effects in comparison to other opioids alone and in combination with doxorubicin and clinically more relevant temozolomide (TMZ) in a set of GBM cell lines and primary GBM cells. Methods MOR expression in GBM cells was investigated by immunofluorescence and immunoblotting. Resistance to drugs alone or in combination was assessed by MTT assays. Concentration effect curves were fitted to data points by nonlinear regression analysis and IC50 values were calculated. Apoptotic rates were determined by Annexin V staining. Results We found that MET alone was cytotoxic to GBM cells at high micromolar concentrations in MTT assays by induction of apoptosis and necrosis while morphine and oxycodone were hardly cytotoxic. Naloxone was not able to block MET-induced cytotoxicity, indicating that cell death inducing effects of MET are not MOR dependent. We recorded doxorubicin and TMZ concentration response curves by MTT assays in combination with fixed MET concentrations. MET only enhanced doxorubicin cytotoxicity in one cell line and in part in primary cells at certain MET concentrations. MET was not effective in sensitizing cells towards TMZ. Contrarily, in two cell lines MET even decreased sensitivity towards TMZ. Conclusions MET can be considered cytotoxic to GBM cells only at clinically not relevant concentrations by induction of apoptosis and necrosis. Sensitizing effects are only observed in combination with doxorubicin but not with TMZ and are highly dependent on cell line and applied drug concentrations.