Increased understanding of the molecular mechanisms underlying cancer development has shifted drug discovery towards target driven drug development the last decades, but the development of effective cancer drugs has been hampered by the lack of predictive preclinical models. 3-D cultures, considered to more accurately reflect solid tumors in vivo, have been proposed as one way to increase the predictability of clinical efficacy in cancer drug discovery and development. The aims of this thesis were to improve preclinical models for cancer drug development, with focus on colorectal cancer (CRC) and use of multicellular tumor spheroids (MCTS), and also to mechanistically characterize some potentially new anticancer drugs (papers I – IV). The most important technical improvement was the development of direct measurement of green fluorescent protein (GFP) marked cells in spheroids, simplifying live collection of viability data and enabling high-throughput screening (HTS) in the MCTS model (paper I). In paper III and IV, the 3-D model was adapted to enable studies on the interaction between drugs and radiation. Two potentially new anticancer drugs, VLX50 and VLX60, were mechanistically characterized. VLX60, a novel copper containing thiosemicarbazone, induced reactive oxygen species (ROS) formation, was selectively active against BRAF mutated colon cancer cells and exhibited anticancer activity in vivo (paper II). Furthermore, two potentially new anticancer drugs were found suitable for further development for use in combination with radiation (papers III and IV). In paper III, synergy with radiation in spheroids compared to monolayer cultured colon cancer cells was shown with the novel iron-chelating inhibitor of oxidative phosphorylation, VLX600. In paper IV, the antiprotozoal drug nitazoxanide was shown to sensitize quiescent clonogenic colon cancer cells to radiation. In conclusion, introduction of measurement of fluorescence of GFP marked cells in spheroids makes clinically relevant 3-D models feasible for HTS experiments and characterization of candidate drugs and radiosensitizers in early cancer drug discovery and development. VLX60 has several characteristics suitable for further development into a cancer drug, notably against BRAF mutated colorectal cancer cells. VLX600 and nitazoxanide show radiosensitizing properties making them promising for further development for use as cancer drugs in combination with radiation.