Selective interactions of verapamil with anthraquinones in adriamycin-sensitive and -resistant murine and human tumour cell lines in vitro.

Enhancement of the cytotoxicity of adriamycin (ADR) by addition of verpamil (VPM) was selective and, in part, concentration-dependent. In an ADR-resistant murine L5178Y lymphoma subline sensitivity was improved with ADR and 1 microM VPM, whereas the cytotoxic effects of mitoxantrone or esorubicin were not affected by VPM. In human ovarian SK-OV-3 tumour cells ADR cytotoxicity was only enhanced by 6.6 microM VPM and there was no selective advantage against an ADR-resistant subline. Circumvention of ADR resistance by VPM is not a universal phenomenon, appearing least effective against sublines expressing relatively low orders of resistance, which may be those most commonly encountered clinically.