By a comparison of in vitro and in vivo effectiveness of cytostatic agents on a transplantable mouse osteosarcoma, a number of drugs were identified for which the in vitro response is not a good prediction for the in vivo response. For a number of such discrepancies, explanations have been identified or tentatively been put forward, namely: 1. (1) absence in vitro of drug metabolizing systems which are effective in vivo. 2. (2) too low tumour drug levels reached by safe drug doses; 3. (3) too short duration of effective drug levels after single dose in vivo. For a number of drugs, unidentified factors cause such discrepancies. However, there is another group of widely used cytostatic agents for which an in vitro test may give a good prediction of overall tumour response and further studies seem to confirm this for other types of tumours.
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