PROTEIN-BINDING OF METHOTREXATE BY THE SERUM OF NORMAL SUBJECTS

The protein binding of methotrexate by serum from eight normal volunteers was assessed by continuous ultrafiltration at pH 7.4 and 37°C. Methotrexate concentrations were measured by radioimmunoassay and the data analysed by the method of Scatchard. The major binding protein was albumin which bound 87.3% of the drug in serum. Analysis of the Scatchard plots indicated two distinct groups of binding sites. Class I was found to have 0.16±0.05 (S D) binding sites with an intrinsic association constant of 71.15±35.98 (S D)×104 M-1: Class II had 2.01±0.93 (S D) binding sites and an affinity of 0.18±0.15×104 M-1. No great change in the percentage of methotrexate bound occurred until the total concentration of the drug exceeded 50 μMol 1-1. © 1979 Springer-Verlag.