Monitoring drug efflux from sensitive and multidrug-resistant single cancer cells with microvoltammetry

Multidrug resistance (MDR) is the eventual cross-resistance of certain cancer cells to a series of chemically unrelated drugs. It is attributed to a number of possible biophysical processes, one of them being increased drug efflux from resistant cells which leads to a decreased intracellular drug accumulation and retention. In this work, a carbon fiber microdisk electrode was used to monitor directly doxorubicin efflux from single preloaded cancer cells. Electrochemical cleaning, adsorptive preconcentration, and an electrocatalytic effect due to ambient oxygen made it possible to detect eventually very low drug concentrations (down to 1 nM) at good temporal resolution (down to 30 s/measurement) very close (< or = 1 micron) to single cancer cells for the first time. The results from a sensitive (AUXB1) and a drug-resistant (CHRC5) version of Chinese hamster ovarian cancer cells show that resistant cells exhibit a much higher initial efflux rate and shorter efflux time constant when both cell lines are preloaded up to the same intracellular drug concentration. These observations are consistent with results obtained from populations of the same cells by conventional techniques, proving that microvoltammetry can be used to monitor doxorubicin efflux at the single-cell level. Compared with existing methodologies, however, whose data represent only average cell behavior at typically low temporal resolution, the technique described here can provide information on the microheterogeneity of cancer cell populations in terms of drug efflux at high temporal resolution. The actual driving force of efflux is obtained since concentrations are measured directly at individual cells. This approach may lead to important new information on the mechanisms and prospective treatments of MDR.