The in vitro monolayer proliferation assay (MP-assay) described here enables predictive determination of the efficacy of anticancer drugs considered for clinical application. The assay was designed (1) to achieve a high plating efficiency, (2) to adapt in vitro growth as close as possible to in vivo conditions, and (3) to prove that the cells in vitro correspond with the in vivo tumour cells they were derived from. From 452 freshly explanted or biopsied tumours, 321 (71%) proliferating cultures could be established. To prove malignant origin of the incubated cells each strain was characterised by DNA-cytophotometry for aneuploidy and by immunocytochemistry for marker proteins. Drug potency was determined by comparing the number of living cells in drug-treated cultures with non-treated controls. Drug concentrations in vitro corresponded with those achievable in tumour tissue and thus represented clinically relevant levels. Growth inhibition in vitro was correlated with in vivo tumour response. Two hundred in vitro/in vivo correlations were performed (50 retrospective, 150 prospective). Overall predictive accuracy of the MP-assay was 86%, with correct indication of resistance in 94.5% and of sensitivity in 75.8% (P<0.001). The results show that the proposed assay is capable of estimating the response probability of cytostatic drugs in individual tumours and thus can contribute to reducing the applications of non-effective drugs and, within limitations, to improving the basis of drug selection.
