whether enough tumor cells will grow [evaluability] in the test tube, 

excluding nonmalignant components [stromal and vascular and necrotic, usually present in tumor specimens] from the test tube, 

preserving cell-to-cell interactions present in the original tumor in the test tube, 

the drug's ability to get into the tumor in the body, 

structural factors related to the tumor in the body, 

variations of the micro-environment in and around the tumor in the body, 

whether the drug acts on hypoxic tissue [ parts of the tumor that don't get enough blood supply to be well oxygenated],

metabolism of the drug to its active metabolite within the body