Verapamil Reversal of Doxorubicin Resistance in Multidrug-resistant Human Myeloma Cells and Association with Drug Accumulation and DNA Damage

Verapamil reversed resistance to doxorubicin in a human multiple myeloma cell line selected for multiple drug resistance. The drug-resistant cell line 8226/DOX40 is known to have reduced intracellular drug accumulation associated with the overexpression of P-glycoprotein when compared to the sensitive parent cell line 8226/S. Verapamil alone was minimally cytotoxic in both cell lines, but reversed doxorubicin resistance in a dose-related manner in 8226/DOX40. A similar dose-response relationship was observed for verapamil in increasing net intracellular doxorubicin accumulation. This increased net accumulation was secondary to block of enhanced doxorubicin efflux by verapamil from resistant cells. In contrast, verapamil did not alter initial doxorubicin accumulation over the first 60 s when incubated with resistant cells. Addition of verapamil to the 8226/DOX40 cells enhanced the formation of doxorub-icin-induced DNA single strand breaks, double strand breaks, and DNA-protein cross-links. Verapamil had no effect on these lesions in the drug-sensitive cells. In addition, verapamil did not affect chemotherapeutic cytotoxicity or transport in the drug-sensitive cell line. Verapamil appears to reverse doxorubicin resistance in this human myeloma cell line by blocking enhanced drug efflux, leading to increased drug accumulation and enhanced DNA damage.