Serum protein binding of methotrexate was studied in 14 patients with various forms of malignant disease and in eight age- and sex-matched subjects (control group) attending outpatient clinics for various clinical conditions. 2. Protein binding was determined by continuous ultrafiltration and methotrexate concentrations by double-antibody radioimmunoassay. 3. Protein binding of the drug is critically dependent on albumin concentration, as shown by results in individual subjects and a significant regression of methotrexate binding on albumin concentration. Moreover, at high methotrexate concentrations drug binding becomes non-linear, resulting in disproportional elevation of free methotrexate levels. Both these findings have important implications for the treatment of hypoalbuminaemic patients. 4. Two classes of binding sites were observed in both groups of patients, viz a high-affinity, low-capacity group and a low-affinity group with higher capacity. 5. No significant difference was found between patient and control groups either in the percent bound drug or in the binding parameters. 6. In conclusion, while there appear to be no factors specific to malignant disease which perturb methotrexate's protein binding, it may be important to determine the extent of drug binding before methotrexate can be used judiciously, particularly when total drug level is related to likely toxicity and in the design of an appropriate folinic acid rescue regimen after high-dose therapy.
