The binding capacity of ABT-737 (solid lines) and ABT-263 (dotted lines) to human serum albumin (HSA) was investigated using a fluorescence polarization assay. A, to measure binding to site 2 on HSA-IIIA, dansyl sarcosine was used as a probe. In this assay the IC50 was 711 and 37 μM for ABT-737 and ABT-263, respectively. B, to measure binding to site 1 on HSA-IIA, dansyl L-glutamate was used as a probe. The IC50 was >1000 and 145 μM for ABT-737 and ABT-263, respectively.