Improvements in methods of treating cancer with weakly basic anti-cancer compounds are provided. In one aspect, the invention provides an improvement in a method of treating cancer cells whose extracellular environment contains 1 8 mM urea, by exposing the cells to a weakly basic anti-cancer compound which is effective in inhibiting the growth of the cells. The improvement includes (a) exposing the cells to a urease enzyme composition and, (b) by step (a), reducing the amount of anti-cancer compound required to produce a given extent of inhibition in the growth of the cells when the cells are exposed to the anti-cancer agent. Methods of potentiating the specific therapeutic activity of a weakly basic anti-cancer compound in the treatment of a given mammalian cancer which is responsive to the compound are provided as are pharmaceutical compositions for use in intravenous administration to a subject are also provided.