Early change in tumour size predicts overall survival in patients with first-line metastatic breast cancer.

•We establish a drug-disease model for first-line metastatic breast cancer (mBC).•We model the tumour growth inhibitory effects of gemcitabine combotherapy.•We relate model-predicted change in tumour size (CTS) to overall survival.•CTS at week 8 predicts survival in mBC.•Late phase outcome may be predicted from early phase II data, facilitating timely go/no-go decision making.