In our endeavor towards the development of potent anticancer agents, two different sets of biphenylurea-indolinone conjugates, 5a-s and 8a,b were synthesized. The in vitro cytotoxicity of the synthesized compounds was examined in two human cancer cell lines, namely MCF-7 breast cancer and PC-3 prostate cancer cells using the sulforhodamine B (SRB) colorimetric assay. In particular, the MCF-7 cancer cell line was more susceptible to the synthesized compounds. Compound 5o (IC50 = 1.04 ± 0.10 μM) emerged as the most active member in this study against MCF-7, with 7-fold increased activity compared to the reference drug, doxorubicin (IC50 = 7.30 ± 0.84 μM). Compounds 5l, 5q and 8b also exhibited superior cytotoxic activity against MCF-7 with IC50 values of 1.93 ± 0.17, 3.87 ± 0.31 and 4.66 ± 0.42 μM, respectively. All of the tested compounds were filtered according to the Lipinski and Veber rules and all of them passed the filters. Additionally, several ADME descriptors for the synthesized compounds 5a-s and 8a,b were predicted via a theoretical kinetic study performed using the Discovery Studio 2.5 software.