Reduced influx is a factor in accounting for reduced vincristine accumulation in certain verapamil-hypersensitive multidrug-resistant CHO cell lines.

The rates of accumulation, influx and efflux of vincristine have been examined in a series of multidrug-resistant Chinese hamster ovary cell lines which show exceptionally high levels of hypersensitivity (collateral sensitivity) to several resistance modifiers. The more highly resistant members of the series show significantly reduced levels of vincristine influx compared to the control cell line from which they were derived. It is possible that resistance modifier hypersensitivity and reduced vincristine influx may be due to a common change in membrane composition which has arisen during prolonged selection for vincristine resistance in these cell lines.