Pharmacokinetics, pharmacodynamics and toxicities of methotrexate in healthy and collagen‐induced arthritic rats

Methotrexate (MTX) is an anchor drug used to treat rheumatoid arthritis (RA), but responsiveness is variable in effectiveness and toxicity. Methotrexate and its polyglutamate conjugates (MTXPGn) in red blood cells (RBC) have been associated with patient response. In the current study, 13 collagen‐induced arthritic (CIA) rats and 12 healthy rats were given subcutaneous doses of either saline or 0.3 or 1.5 mg/kg per 2 days of MTX from day 21 to 43 post‐induction. Blood samples were obtained at various times to measure MTX in plasma, and MTX and MTXPGn in RBC. Effects on disease progression were indicated by body weight and paw size. After multiple‐doses, RBC MTX reached steady‐state (82.4 nm) within 4 days. The MTXPG2 and MTXPG3 in RBC kept increasing until the end of the study, attaining 12.5 and 17.7 nm. Significant weight loss was observed after dosing with 1.5 mg/kg/2 days, whereas moderate effectiveness was observed after dosing with 0.3 mg/kg/2 days. A pharmacokinetic/pharmacodynamic/disease (PK/PD/DIS) model with indirect mechanisms and transduction components incorporating plasma MTX, RBC MTX and RBC MTXPGn concentrations, and paw size was developed using naïve data pooling and ADAPT 5. The PK/PD in CIA rats dosed at 0.3 mg/kg/2 days were captured well by our proposed model. Methotrexate showed modest (Imaxd = 0.16) but sensitive (IC50d = 0.712 nm) effectiveness on paw edema. The higher dose produced toxicity. The proposed model offers improved understanding of the effects of methotrexate on rheumatoid arthritis. Copyright © 2013 John Wiley & Sons, Ltd.