Background Methotrexate (MTX) is an anchor drug for the treatment of rheumatoid arthritis (RA) because of efficacy and safety. Although MTX is used in monotherapy generally, combination therapy with biologics or another disease-modifying antirheumatic drugs (DMARDs) is recommended for RA patients refractory to MTX monotherapy. Tacrolimus (TAC) is used as immunosuppressant agent for control of graft rejection in hepatic or renal transplantation. Additionally, there are some reports that the combination of MTX and low dose TAC is effect for RA patients refractory to MTX. However the precise mechanism for the effect of this combination therapy is still unknown. On the other hand, increased efflux of MTX by ATP binding cassette transporter such as multidrug resistance protein 2 (MRP2) has been considered to be potential mechanism of drug resistance in carcinoma cell.

Objectives We first clarify the efficacy of combination therapy of MTX and low dose TAC. Additionally we investigate the mechanism which the combination therapy.

Methods At the first, we examined fifty MTX refractory RA patients who added on TAC retrospectively. Tender joint counts (TJC), swollen joint counts (SJC), serum C-reactive protein (CRP) levels and Disease activity score (DAS) 28 CRP were used to evaluate the disease activity before and after additional administration of TAC. To investigate the mechanism of the combination therapy, we used the human monocytic cell line THP-1 stimulated with lipopolysaccharide (LPS). Inflammatory cytokines such as Tumor necrosis factor (TNF)-α, Interleukin (IL)-6 and IL-1β in culture supernatant by ELISA. The effect of MTX and/or TAC on the expression of these cytokines from LPS stimulated THP-1 cells was investigated. The change of MRP2 mRNA and protein expression was analyzed by RT-PCR and Western blot respectively.

Results Forty-nine patients (98%) were enrolled to this study for one year. One patient withdrew TAC due to lack of efficacy, and five patient reduced TAC because of adverse event. 34 patients (68%) were treated with low dose TAC (< 1.5mg/day) combined with MTX after 52 weeks. All subjects (TJC, SJC, serum CRP levels and DAS28 CRP) were significantly improved after 12 weeks from add on TAC. Five patients (10%) achieved DAS28 CRP remission at 12 weeks, and 15 patients (30%) achieved DAS28 CRP remission at 52 weeks. Of the 50 patients, 27 (54%) attained to moderate or good response at 12 weeks according to the EULAR improvement criteria. Production of inflammatory cytokines from THP-1 cells such as TNF-α, IL-6 and IL-1β were inhibited synergistically by combination MTX and low dose TAC treatment significantly. Expression of MRP2 mRNA was increased by MTX or low dose TAC treatment. However the treatment of MTX combined with low dose TAC reduced MRP2 expression significantly.

Conclusions Our results indicate that the treatment of MTX combined with low dose TAC to MTX is useful for RA patients refractory to MTX. We suggest that the inhibition of MRP2 expression by TAC play an important role of the effect this combination therapy.

Disclosure of Interest None Declared