If the cells don t die in vitro, they won t die in vivo, we presume. I personally have seen patients respond, though the assay shows extreme resistance to both drugs. Second, the drug-resistance assay does not test sensitivity, because the concentrations used in testing are 100 to 1,000 times higher than you would use on the patient. A drug that kills the tumor cells in the culture dish will not necessarily do so in the patient. The drug that attacks cells in tissue culture is not necessarily the same as the drug that reaches a patient s tumor.
Many drugs go through metabolic activation in the patient.
Complicating matters further, in a patient no tumor cell is more than four cells removed from a blood vessel, whereas in tissue culture, tumor cells often clump, protecting cells inside the mass from the drug. But drug-resistance testing may be useful for patients who fail chemotherapy, if the patient is followed closely. Unfortunately, in many cases, the tumor cells don t even grow in tissue culture. The drug-resistance assay may have efficacy, but to prove that we need randomized clinical trials of salvage therapies, in which half the patients take the drug the doctor chooses, and half are treated with a drug for which the assay predicts low resistance.