Background Methotrexate (MTX) is the anchor drug in rheumatoid arthritis treatment, either as monotherapy or in combination with other antirheumatics, e.g. chloroquine (CHQ).

Objectives Does resistance to MTX influence sensitivity to CHQ, and if so, by what mechanism?

Methods In vitro CHQ sensitivity was evaluated in human CEM (T) cell lines, wild type CEM (CEM^wt) and 3 CEM sublines resistant to MTX due to reduced cellular uptake (CEM/MTX-transport^low), elevated levels of dihydrofolate reductase (CEM/MTX-DHFR^up) or impaired polyglutamylation (CEM/MTX-FPGS^low).

Results CEM/MTX-transport^low cells were 1.5-fold more sensitive to CHQ than CEM^wt cells (IC₅₀: 25 μM vs 39 μM, respectively, after 72 h drug exposure). However, CEM/MTX-DHFR^up and CEM/MTX-FPGS^low cells were 2.3-fold (IC₅₀: 89 μM) and 1.9-fold (IC₅₀: 74 μM) resistant to CHQ, respectively, compared to CEM^wt cells. The mechanism of CHQ resistance in CEM/MTX-DHFR^up and CEM/MTX-FPGS^low cells is likely due to overexpression of multidrug resistance protein 1 (MRP1). This was suggested by a low level (1.5–2-fold) cross-resistance to doxorubicin and daunorubicin, and observations that an inhibitor of MRP1, MK571, was able to reverse CHQ resistance. The same phenotypic changes were noted in CEM/MTX-transport^low cells with acquired resistance to CHQ (4-fold) after in vitro exposure to stepwise increasing extracellular CHQ concentrations.

Conclusion Development of MTX resistance can be associated with up regulated expression of MRP1, which, concomitantly, can confer cross-resistance to CHQ. Analysis of MRP1 expression in inflammatory cells should therefore be considered as an important parameter determining the efficacy of anti-rheumatic (combination) therapy.