Methotrexate binding to human plasma proteins.

The percentage of methotrexate (MTX) binding to plasma protein was 50.4 +/- 1.9 in healthy subject, and 32.3 +/- 3.6 in patients with cancer (breast carcinoma in most cases). Drugs used in combination with MTX in therapy (vincristine, vinblastine, cyclophosphamide, 5-fluorouracil, prednisone) depress the MTX binding to albumins. MTX binds to two kinds of albumins binding sites, with different binding constants: K1 = 8.88 mM-1 and K2 = 1.76 mM-1.