FK506 is superior to methotrexate in therapeutic effects on advanced stage of rat adjuvant-induced arthritis.

Objective and Design: The anti-arthritic properties of FK506 were compared with methotrexate (MTX) in established adjuvant-induced arthritis (AIA) in rats.
Material: Female Lewis rats.
Treatment: Arthritic rats were orally administered with FK506 (1-5.6 mg/kg) and MTX (0.1-1 mg/kg) from days 15-24.
Methods: Arthritis was induced by injection of Mycobacterium tuberculosis into the right hind footpad on day 0. Efficacy was determined on the basis of paw inflammation measured by paw volume and histological change, hyperalgesia and grip strength. Grip strength measurement was employed as an indication of function of paws in arthritic rats. Peripheral white blood cell (WBC) counts and thymus weights were measured, mainly as indicators of toxic side effects.
Results: FK506 suppressed paw inflammation and hyperalgesia without toxic effects on WBC and thymus in established AIA. MTX slightly suppressed paw inflammation and hyperalgesia at the highest dose (1 mg/kg). Toxic effects were observed at lower doses than the effective treatment dose with MTX. Grip strength was found to decrease during development of AIA. FK506, but not MTX, treated rats recovered grip strength loss.
Conclusions: The results show that FK506 is more effective and less toxic than MTX in treating established AIA in rats.