There's a great paper just published in NRDD on the analysis of potential drug targets using an objective, evidence-based approach, it makes extensive use of ChEMBL data, alongside specific cancer-related datasets, pathway and interaction data and so forth. There is also some good discussion of drug-repurposing informatics approaches.
A link to the pdf of the paper is here.
%T Objective assessment of cancer genes for drug discovery %A M.N. Patel %A M.D. Halling-Brown %A J.E. Tym %A P. Workman %A B. Al-Lazikani %J Nature Reviews Drug Discovery %V 12 %P 35-50 %D 2013 %O doi:10.1038/nrd3913
Disclosure - one of the authors of the above paper (Al-Lazikani) is my wife.