Platinum group metallodrug-protein binding studies by capillary electrophoresis - inductively coupled plasma-mass spectrometry: A further insight into the reactivity of a novel antitumor ruthenium(III) complex toward human serum proteins

Biochemical speciation analysis has become a hot area of CE research due largely to growing emergence of inductively coupled plasma (ICP)‐MS as a proper detection technique. A benefit of CE–ICP‐MS coupling in species‐selective analysis of anticancer metal‐based drugs is the possibility of distinguishing the signals of the intact drug and its metabolites and hence of quantifying them independently. This advantage (over CE with UV‐vis detection) was exploited here in order to gain better knowledge about the rate and degree of the transformation of indazolium [trans‐tetrachlorobis(1H‐indazole)ruthenate(III)] (KP1019), a promising tumor‐inhibiting agent that successfully finished phase I clinical studies, upon its binding toward individual serum transport proteins. At increasing the KP1019/protein molar ratio, the reaction rate expressed by an evolving peak of the protein adduct became faster, with the equilibrium state being reached after about 40 and 60 min of incubation at 37°C for transferrin and albumin, respectively. The binding reaction was shown to obey the first‐order character that enabled for reliable calculation of the corresponding rate constants as (28.7 ± 1.5)×10−4 and (10.6 ± 0.7)×10−4/s, respectively. When incubated with a ten‐fold excess of KP1019, albumin and transferrin bound, respectively, up to 8 and 10 equiv. of ruthenium (Ru). Relative affinity of KP1019 toward different proteins under simulated physiological conditions was also characterized in terms of the overall binding constants (5600 and 10 600/M, respectively). To emphasize the difference in the protein‐binding behavior, a competitive interaction of KP1019 was followed by CE–ICP‐MS at the actual molar ratio of proteins in blood, i.e. a ten‐fold excess of albumin over transferrin. The fact that KP1019 binds to albumin stronger than to transferrin was manifested by finding almost all ruthenium (98–99%) in the albumin fraction.