Enhancement of adriamycin-induced cytotoxicity by increasing retention and inhibition of DNA repair in DOX-resistant P388 cell lines with new calcium channel blocker, DMDP.

The effect of DMDP, N-(3,4-dimethoxyphenethyl)-N-methyl-2-(2-naphthyl)-m-dithiane-2-propylam ine hydrochloride, on DOX-induced cytotoxicity, drug uptake, DNA damage and repair was investigated in adriamycin sensitive and resistant P388 cell lines. In vitro, the DOX-resistant P388 cells used are about 300-fold more resistant than the sensitive cells. Resistant cells were characterized by lower DOX accumulation, rapid drug efflux, significant decrease of DNA single and double strand breaks and rapid repair of the induced single strand breaks. DMDP, a calcium channel blocker, is an effective modulator of DOX resistance in P388 cells. This modulation was found to be highly dependent of the concentration of the modulators, optimal at the maximally moncytotoxic concentrations of 1-4 microM, and the duration of exposure to the modulator, optimal under conditions of continuous exposure to the modulator. Under the optimal conditions in the presence of the modulator, DMDP, both intracellular concentration and retention of DOX were restored in the resistant P388 cells to the value comparable to those found in DOX sensitive P388 cells. Even though DOX accumulation and retention were at a comparable level in both the sensitive and resistant cells in the presence of DMDP, the amount of DNA single strand breaks achieved in the resistant cells was only about 30% of the amount of damage observed in the sensitive cells. The data indicate that if P388/R cells were only exposed to DOX for about 2 h, the induced DNA single strand breaks were repaired rapidly within 8 h thereafter, while no significant repair was seen in the resistant cells exposed to DOX in combination with DMDP. In contrast, the repair of the extensive DNA single strand breaks induced by DOX in P388/S cells was not effected by DMDP. These data clearly demonstrated that resistance to DOX in P388 cells are multifactorial. Restoration of intracellular accumulation and retention of DOX by DMDP in the resistant cells are although necessary but not sufficient for complete restoration of the sensitivity of the highly resistant cells.