Increased binding of chlorin e6 to lipoproteins at low pH values

It is well known that the extracellular pH in tumors is lower than that of normal tissue. This has been proposed to be one of the reasons for the tumor selective uptake of several photosensitizers. Photosensitizers like chlorin e(6) are bound to blood components and delivered to different sites in the organism. Thus, the effect of pH on their interaction with human plasma needs to be studied in order to understand a possible role of the acidic microenvironment in tumors for the drug distribution. Increasing amounts of human plasma in the sample resulted in a gradual red shift of the fluorescence emission maxima of chlorin e(6), indicating binding of the drug to some of the plasma components. Titration showed that the drug-plasma interaction was pH-dependent. The titration curve had an inflection point at 7.4+/-0.1. The relative distribution of the drug among plasma components, as found after ultracentrifugation of chlorin e(6)-doped plasma in a salt gradient, showed more binding of the drug to nonlipoproteins than to lipoprotein classes at both pH values studied (6.5 and 7.4). A decrease in the pH was connected with a significant increase in drug-lipoprotein binding. The pH of the environment affects chlorin e(6)-plasma interaction and the distribution of the drug among different plasma components. The results of this study indicate a possible role of the acidic microenvironment in tumors for the preferential uptake and retention of several photosensitiziers.