(St. enhanced the intracellular entrapment of DOX due to blocking the efflux activity of p-glycoprotein pump. In conclusion, RES and DID sensitize colorectal cancer cells to DOX Indolelactic acid via facilitating Indolelactic acid apoptosis and enhancing intracellular entrapment of DOX. Colorectal cancer (CRC) is the third most commonly diagnosed cancer in males and the second in females with an estimated 1.4 million cases and 693.000 deaths occurring in 2012, accounting for 8% of all cancer related deaths1. Despite recent advances in chemotherapy, currently used anticancer molecules are unable to improve the prognosis of advanced or recurrent colorectal cancer, which remains incurable2. The anthracycline, doxorubicin (DOX), is usually a widely used chemotherapy due to its efficacy in fighting wide range of cancers such as carcinomas, sarcomas and hematological neoplasias3,4. However, there are clinical limitations that arise from its susceptibility to multi-drug resistance5. Overexpression of ATP-dependent efflux pump p-glycoprotein (P-gp) and its related proteins is crucial to multidrug resistance (MDR) and chemotherapy failure in cancer treatment6. P-gp is usually encoded by gene; and is considered a member of the ATP-binding cassette (ABC) transporter superfamily. It is energy-dependent transporter pump that removes xenobiotics such as, DOX outward from cells and confer drug resistance in tumor cells7,8. Compounds of natural origin are very rich source for leads with potential anticancer properties as well as chemomodulatory effects such as, P-gp inhibitors7,9,10,11,12,13,14,15. Resveratrol (RES) is usually naturally occurring herb antibiotic known as phytoalexin, found in various plants, nuts, fruits and especially abundant in grapes and red wine16,17. It has been extensively studied for its antioxidant, anti-aging and anti-inflammatory activities18,19,20,21,22,23. In addition, and studies showed that RES possesses potential anti-tumor activity against several malignancies24,25,26,27. According to our previous study, RES potentiates the cytotoxic properties of DOX in MCF-7, HeLa and HepG2 cells via P-gp inhibition and downregulation of gene28. Didox (DID) is a synthetic polyphenolic compound which shares important biochemical targets with RES29. It is potent inhibitor for ribonucleotide reductase enzyme which interferes with DNA synthesis and repair30. Ribonucleotide reductase enzyme has been considered potential target for cancer chemotherapy31. DID showed anti-tumor effects in a variety of experimental systems, and several human tumor xenografts32,33,34. It may exert its anti-tumor effect via the activation of various apoptosis pathways35. According to our previous work as well as other research groups, Indolelactic acid DID and RES improve the cytotoxic profile of different anticancer agents and protect from their toxic effects28,36,37,38,39. DID and RES might be potential successful adjuvant candidates for combination with DOX33,40. Therefore, we investigated the potential improvement effects of RES and DID on DOX-anticancer properties and the possible underlying mechanisms in two colorectal cancer cell lines with different expression levels of gene. Results RES and DID improve the cytotoxicity of DOX LPP antibody in colorectal cancer cells To study the effect of RES and DID on the cytotoxic profile of DOX, the dose response curve of DOX alone was assessed relative to its combination with RES or DID in two colorectal cancer cell lines (Fig. 1ACD) (Table 1). In HCT 116 cells, DOX exerted gradient cytotoxic activity with increasing concentration; viability started to drop significantly (P? ?0.05) at concentration of 0.3?M. Cellular log kill was gradual in profile with IC50 of 0.96??0.02?M (Fig. 1A,B). Similarly, RES and DID single treatments exerted gradual cytotoxic activity with increasing concentration; viability started to drop significantly (P? ?0.05) at concentrations of 10?M and 100?M, respectively. Both RES and DID have steep cellular log kill profile with IC50s of 17.5??02?M and 105??1.5?M, respectively (Fig. 1A,B). Equitoxic combination of RES or DID with DOX significantly improved the cytotoxic profile of DOX (Fig. 1A,B). IC50s of DOX after combination with RES and DID were significantly (P? ?0.05) decreased from 0.96??0.02?M to 0.52??0.05?M and 0.4??0.06?M, respectively (Table 1). The calculated Indolelactic acid CI-values for DOX with RES and DID were 1.16 and 0.76, respectively. These CI-values are indicative of additive interaction of DOX with RES and synergistic interaction with DID in HCT 116 cell line (Table 1). Open in a separate window Figure 1 The effect of.