[Reversal of multidrug resistance with (R)-verapamil in vitro and in vivo].

The clinical use of racemic (R/S)-verapamil (CAS 52-53-9) as resistance modifier is limited because of the cardiovascular activity of the substance. The stereoisomer (R)-verapamil shows significant less cardiovascular activity. Therefore we tested the resistance modifying abilities of (R)-verapamil and (R/S)-verapamil in murine multidrug-resistant L 1210 ascites-tumor-cells in vitro and in vivo. The present results demonstrate, that the (R)-isomer has the same resistance modifying effects as racemic verapamil. Both modifying substances have no effects in the parental (sensitive) and cytosine-arabinoside resistant L 1210 ascites-tumor-cells. Thus, (R/S)-verapamil and (R)-verapamil show their resistance modifying abilities only in multidrug-resistant tumor-cells.