The response of tumors to chemotherapy is complex. Of the many factors that affect the in vivo response to drugs, cell proliferation kinetics and intrinsic chemosensitivity play important roles. The response of primary tumor cells to drugs in vitro is likewise complex. Data from this laboratory shows that quiescent reproductive cells from fresh tumors are recruited into the cell cycle within the first four days of culture. As quiescent cells are inherently chemoresistant, their recruitment into the cell cycle during incubation artifactually increases drug sensitivity and complicates the interpretation of sensitivity or resistance. It is suggested that the limited positive predictive accuracy and the poor impact on survival of chemosensitivity assays may be improved by considering cell proliferation kinetics in the interpretation of in vitro chemosensitivity measurements.

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