Arterial and venous blood samples from the arterial-venous (A-V) shunt (0.5 mL in BD Microtainer pink top K2EDTA tubes) for PK analysis will be collected from ESRD subjects at 0.5, 1.5, 2.5, and 3.5 hours into each of the two 4 hour hemodialysis sessions. Additionally, fingerprick blood samples (0.5 mL) will be collected from the hand contralateral to the arm used for the dialysis A V shunt at each of these specified time points. Dialysate from ESRD subjects will be collected during the two 4 hour hemodialysis sessions at 0 1 hour, 1 2 hour, 2 3 hour, and 3 4 hour. Two 10 mL samples from each of the 1-hour pooled dialysate collections will be aliquoted into suitable collection tubes.