Selective modulation of vinblastine sensitivity by 1,9-dideoxyforskolin and related diterpenes in multidrug resistant tumour cells.

The ability of 1,9-dideoxyforskolin (DDF), 1-deoxyforskolin (DF) and forskolin to modulate cellular sensitivity to vinblastine (VBL) was examined in drug-sensitive parental KB-3-1 cells and a multidrug-resistant subline, KB-GRC1, derived by transfection of mdr1. Fifty microM DF and forskolin enhanced the 1 h uptake of VBL by 8.0 +/- 0.7 (s.d.) and 4.7 +/- 2.5-fold, respectively, with 50 microM DDF producing a 13.6 +/- 1.9-fold increase. The greater effect of DDF relative to forskolin indicated that the effect was independent of activation of cAMP, and this was supported by a lack of effect of dibutyryl cAMP on the uptake. The effect of these agents on uptake were < or = 1.4-fold in KB-3-1 cells. DDF selectively inhibited initial efflux in cells expressing a functional P-glycoprotein (PGP), but both forskolin and DDF inhibited the terminal phase of efflux irrespective of PGP expression. Neither agent affected membrane permeability of polarisation and forskolin did not enhance the uptake of VBL in protein-free liposomes. At a non-toxic concentration of 20 microM, DDF and forskolin decreased the IC50 of VBL from 18.9 to 2.7 and 13 nM in KB-GRC1 cells, respectively, and DDF acted synergistically with VBL as shown by median effect analysis [combination index = 0.20 +/- 0.05 (s.d.)]. In contrast, these diterpenes did not affect VBL sensitivity in KB-3-1 cells. These results indicate that the diterpenes modulate VBL sensitivity predominantly by inhibiting PGP-mediated efflux activity.