Treatment of resistant human colon cancer xenografts by a fluoxetine–doxorubicin combination enhances therapeutic responses comparable to an aggressive bevacizumab regimen

Pre-clinical studies of multidrug resistance (MDR) usually address severe resistance, yet moderate MDR is already clinically-impeding. The purpose of this study was to characterize moderate drug resistance in human colon cancer, and it’s modulation by fluoxetine. In vitro fluoxetine enhanced doxorubicin’s cytotoxicity (10-fold), increased doxorubicin’s intracellular accumulation (32%) and decreased efflux of intracellular doxorubicin (70%). In vivo, mild treatment with a doxorubicin–fluoxetine combination slowed-down tumor progression significantly (p < 0.001 vs. doxorubicin alone), comparable to aggressive treatment with bevacizumab. Collectively, our results suggest that combinations of fluoxetine with chemotherapeutic drugs (P-glycoprotein substrates) are worthy of further pursuit for moderate MDR in the clinic.