Growth rate of sensitive cell as a function of P-gp content. A, computer simulation giving the number of sensitive MCF-7 versus time during a chemotherapy regimen, cycling every 14 days, with 10 days of 1 μM doxorubicin treatment followed by 4 days of interruption, for 4 cycles. The initial simulated tumour consisted in 104 MCF-7 having growth rates corresponding to ρ(pmin). B, plot giving the growth rates ρ as a function of the P-gp-driven calceinAM efflux activity p for cells exposed to 1 μM doxorubicin. The function describing ρ(p) is used in the simulations presented Figure 8. All cells having a negative growth rate are considered as sensitive MCF-7.