The maximal therapeutic doses of the cytostatic drug doxorubicin (DOX) are strictly limited by the development of systemic toxicity, especially cardiotoxicity. The inhibition of DOX-metabolizing enzymes within cancer cells is possible strategy to improve DOX efficacy. In breast cancer cells (MCF7), DOX is effectively deactivated by carbonyl reduction. The aim of the present study was to test whether isoquinoline derivative oracin (ORC) is able to inhibit DOX reductases and to enhance DOX cytotoxic efficacy. The kinetics studies of DOX reduction in MCF7 cytosolic fractions were evaluated using high-performance liquid chromatography. The cytotoxicity of DOX, ORC, and DOX+ORC combinations was assayed using cell-viability tests and caspases activities and monitored using xCELLigence System for real-time cell analysis. ORC significantly inhibited DOX reduction in MCF7 cytosol. Competitive inhibition was found. The viability was significantly lower in cells treated with ORC+DOX combinations in comparison to cells treated with DOX alone. Significant enhancement of DOX cytotoxicity was achieved already with 0.5 µM ORC. DOX together with ORC was able to kill about 55% cells more than DOX alone. ORC significantly increases DOX efficacy in MCF7 cells probably due to the inhibition of DOX reductases.