RT Journal Article SR Electronic T1 Circumvention of Inherent or Acquired Cytotoxic Drug Resistance In Vitro Using Combinations of Modulating Agents JF Anticancer Research JO Anticancer Res FD International Institute of Anticancer Research SP 4381 OP 4387 VO 33 IS 10 A1 DAVID CADAGAN A1 STEPHEN MERRY YR 2013 UL http://ar.iiarjournals.org/content/33/10/4381.abstract AB Background/Aim: Modulating agents are used to circumvent drug resistance in the clinical setting. However achievable serum concentrations are often lower than those which are optimal in vitro. Combination of modulating agents with non-overlapping toxicities may overcome this obstacle. We have investigated combinations of three modulating agents (quinine, verapamil, and cinnarizine) to circumvent inherent or acquired resistance to the cytotoxic drugs doxorubicin, vincristine and paclitaxel. Materials and Methods: Dose-response curves to cytotoxic drugs in the presence/absence of modulating agents were determined using colony formation and cell proliferation assays. Doxorubicin accumulation into cell monolayers was measured by fluorescence spectrophotometry. Results: Greater (1.9-fold) sensitisation to particular cytotoxic drugs was observed for certain combinations of modulating agents compared to individual effects. The most effective combination was quinine-plus-verapamil with the cytotoxic drug doxorubicin. This increase in sensitivity was associated with increased doxorubicin accumulation. Such enhanced activity was, however, not observed for all combinations of modulating agents or for all studied cytotoxic drugs. Conclusion: The findings of the present study suggest certain combinations of modulating agents to have a clinical role in circumventing drug resistance. Particular combinations of modulating agents must be carefully chosen to suit particular cytotoxic drug treatments.