The occurrence of resistance to chemotherapeutic drugs is a major problem for successful cancer treatment. This resistant phenotype of cancer cell frequently reveals a broad spectrum to structurally and/or functionally unrelated anticancer drugs, termed multidrug resistance (MDR). Overexpression of P-glycoprotein (P-gp), a transmembrane drug efflux pump, is a major mechanism of MDR. Accordingly, considerable effort has been directed towards to development of compounds that inhibit P-gp, reverse the MDR phenotype and sensitize cancer cells to conventional chemotherapy without undesired toxicological effects. In an effort to search for novel MDR reversal agent, we tested the cytotoxicity of paclitaxel, a well-known substrate of P-gp, against P-gp-expressing HCT15 and HCT15/CL02 human colorectal cancer cells in the presence or absence of benzotriazepin analogues, as well as against P-gp-negative A549 human non-small cell lung and SK-OV-3 human ovarian cancer cells in vitro. Among the compounds tested, the agents that have phenyl amide moiety at 3 position remarkably increased the cytotoxicity of paclitaxel against P-gp-expressing cancer cells, but not against P-gp-negative cancer cells. BTZ-15 and BTZ-16 at $4;{mu}M$ revealed similar MDR reversal activity to $10;{mu}M$ verapamil, a well-known MDR reversal agent.