Are there databases or resources that contain expression of proteins or genes before and after cancer treatment?
One such database/tool is “[PharmacoGx][1]: an R package for analysis of large pharmacogenomic datasets.” Bioinformatics (Oxford, England).

They brought together a large compendium of experimental data (cell lines and the like if I remember correctly from a talk), standardized these and made available within a special object class. I think there is genomic and transcriptomic data, not sure if there is proteomic data though.