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.