Mechanisms of the in vivo resistance to adriamycin and modulation by calcium channel blockers in mice.

A sensitive fluorometric assay using Hoechst 33258 and a modified alkaline elution procedure were used to quantitate DNA single-strand breaks following an in vivo drug treatment of mice bearing P-388/S and P-388/R cells. After an i.p. treatment of mice with 1 to 20 mg/kg Adriamycin (DOX), the following differences between sensitive and resistant P-388 cells were observed: (a) at 2 h following drug treatment the net intracellular accumulation of Adriamycin in sensitive cells was 2- to 3-fold higher than resistant cells at all doses tested; (b) utilizing a therapeutic dose of DOX (10 mg/kg), the amount of single-strand breaks of DNA in sensitive and resistant cells was significantly different, K x 10(2) = 13.6 +/- 1.1 (SD) versus 3.6 +/- 0.9, respectively; (c) the 10 and 50% lethal doses for verapamil (VEP) were 10 and 23 mg/kg and for a tiapamil analogue, N-(3,4-dimethoxyphenethyl)-N-methyl-2-(2-naphthyl)-m-dithiane-2-propylam ine hydrochloride (DMDP), were 107 and 126 mg/kg, respectively; (d) while the in vivo intracellular accumulation and retention of DOX in sensitive cells were not affected by DMDP or VEP treatment, complete restoration of DOX accumulation and retention was achieved in resistant cells treated with well-tolerated doses of DMDP of 30 and 60 mg/kg. In contrast, utilizing the optimally tolerated dose of VEP (5 mg/kg), only partial restoration of DOX accumulation and retention in resistant cells was achieved; (e) DMDP or VEP did not alter the high level of DNA single-strand breaks induced by DOX in sensitive cells; in resistant cells, however, an increase in single-strand breaks of DNA was observed following treatment with DOX in combination with DMDP and to a lesser extent with VEP; and (f) the rapid DNA repair in resistant cells was inhibited by DMDP but not by VEP. These data demonstrate that DMDP but not VEP can effectively restore the in vivo intracellular accumulation of DOX in resistant cells at achievable nontoxic plasma concentrations. Previous studies have demonstrated that the in vitro intracellular concentrations and retention of DOX by resistant cells can be restored by VEP. The results reported herein demonstrated that similar effects can be achieved, however, in vivo by using a new calcium channel blocker, DMDP, with less in vivo toxicity and more efficacy than VEP in restoring cellular drug concentration, retention, and repair of DNA damage in the resistant cells.