Various factors most likely to influence the plasma protein binding of IQO4, a new isoquinolinedione derivative, to 4% human serum albumin (HSA) were evaluated using an equilibrium dialysis technique at an initial IQO4 concentration of 5 microg/ml. It took approximately 12 h incubation to reach an equilibrium between 4% HSA and isotonic Sørensen phosphate buffer at pH 7.4 containing 3% dextran ('the buffer') using a Spectra/Por 2 membrane (molecular weight cut-off, 12000-14000) in a water-bath shaker kept at 37 degrees C and at a rate of 50 oscillations per min. IQO4 was stable both in 4% HSA and in 'the buffer' for up to 24 h incubation at 37 degrees C. The binding of IQO4 was constant (89.9 +/- 1.40%, mean +/- standard deviation) at IQO4 concentrations ranging from 1 to 100 microg/ml. However, the extent of binding was dependent on HSA concentrations. The values were 32.5, 62.0, 79.1, 84.9, 90.9, 91.2, and 91.7% at HSA concentrations of 0.5, 1, 2, 3, 4, 5, and 6%, respectively; on incubation temperature, 96.7, 93.8, and 91.0% when incubated at 4, 22, and 37 degrees C, respectively; and on the buffer pHs, 84.4, 87.2, 88.2, 90.9, and 92.3% for the buffer pHs of 5.8, 6.4, 7, 7.4, and 8, respectively. The free fraction of IQO4 increased with the addition of sulfisoxazole (0-300 microg/ml), and salicylic acid (0-300 microg/ml). The protein binding of IQO4 was independent of the quantity of alpha-1-acid glycoprotein (up to 0.32%), chloride ion (up to 0.546%) and heparin (up to 40 units/ml).