In vitro studies of ways to overcome resistance to VAMP--high dose melphalan in the treatment of multiple myeloma.

Myeloma colonies (MY-CFUc) from 7/24 patients undergoing treatment with VAMP (vincristine, adriamycin and methyl prednisolone) and high dose melphalan (HDM) were melphalan-resistant. It was not possible to conclude that VAMP induced melphalan resistance in MY-CFUc, but that resistance is endogenous in some myeloma cell populations. In 12/13 of the same patients of whom four had MY-CFUc which were melphalan resistant, the sensitivity of MY-CFUc and GM-CFUc to busulphan was similar. Thus resistance of MY-CFUc to melphalan did not confer resistance to busulphan. MY-CFUc from 1/7 of a second group of patients were adriamycin-resistant. This resistance was removed when the cells were treated with a combination of verapamil (3 micrograms/ml) and adriamycin. Verapamil also enhanced the toxicity of adriamycin to MY-CFUc from two patients where there was no evidence for adriamycin resistance. In these three patients the sensitivity of both MY-CFUc and GM-CFUc was similar after treatment with verapamil. Verapamil did not affect the uptake or efflux of 3H-daunorubicin in sensitive and resistant RPMI-8226 cells (myeloma) and peripheral blood mononuclear cells from a normal donor; neither did it affect the binding of 3H-daunorubicin to nucleic acid. It is concluded that verapamil may be a useful adjuvant to VAMP chemotherapy and that busulphan may provide an alternative to melphalan in patients whose myeloma cells are melphalan resistant.