Suramin is a novel anti-cancer drug currently being investigated as a treatment for brain cancer. The objective of this study was to characterize the pharmacokinetics and intersubject variability of suramin in patients with brain cancer. Plasma samples were collected from 16 patients and data was analyzed using nonlinear mixed effects modeling. The data were best described by a two compartment model, with elimination from the central compartment with gender and body surface area (BSA) as covariates. Volume of distribution was estimated to be 4.16 L/hr/m 2 ( 0.31) in male patients and in 4.16 L/hr/m 2 (0.23) females in the first compartment. Volume of distribution in the second compartment was 21.2 ( 2.26) in males and 12.6 ( 3.8) in females. Elimination was from the central compartment. The results of this study showed that the pharmacokinetic parameters of suramin in brain patients were similar to those observed in studies examining use of suramin in prostate cancer. The study also showed that using a model that included gender as a covariate decreased the amount of inter- and intra-subject variability. Plasma protein binding studies were performed utilizing equilibrium dialysis to fully characterize the in vitro binding of suramin to human serum albumin, \ -p1-acid glycoprotein, and human plasma serum over wide range of drug concentrations. Suramin binds to two classes of binding sites on albumin, a high affinity saturable site and a low-affinity nonsaturable site (N1=3.5, K1=1.8 X 10 4 M -1 , N2K2=3.7 X 10 3 M -1 ). Suramin binds to a single low-affinity nonsaturable site on \ p1-acid glycoprotein (N3K3=1.5 x 10 5 M -1 ). The fraction of suramin bound to plasma proteins predicted from the in vitro binding to human serum albumin and p\ 1-acid glycoprotein was identical to that observed in human plasma (95% 0.015). Thus, the plasma protein binding of suramin can be accounted for by the binding to these two proteins.