A new approach for enhancing differential selectivity of drugs to cancer cells.

The degree to which anticancer agents selectively target cancer cells is a key determinant in successful therapeutic outcomes. Inhibitors of the Hsp90 molecular chaperone represent an important new class of anticancer agents. We propose here a novel mechanism by which physiochemical properties of Hsp90 inhibitors can be optimized to increase selectivity towards cancer cells. The basis for this approach relies on differential intracellular pH gradients that have been shown to exist between normal and transformed cells. Five Hsp90 inhibitors containing basic or neutral properties were evaluated in antiproliferation assays using cells with variable lysosomal pH. Inhibitors with basic functionalities had reduced activity in cells with normal (low) lysosomal pH but showed significantly greater activity in cells with abnormally elevated lysosomal pH (similar to what has been recorded in many types of cancer cells). Conversely, such selectivity enhancement was not observed for neutral inhibitors. The mechanistic basis for the observed selectivity was demonstrated quantitatively by determining the concentration of inhibitors within relevant intracellular compartments. Collectively, these findings suggest that Hsp90 inhibitors with optimal basicity and physicochemical properties have enhanced selectivity toward cancer cells than their neutral counterparts. It is anticipated that these findings may be applicable to other classes of anticancer agents for improvement of differential selectivity.

A chemical strategy to manipulate the intracellular localization of drugs in resistant cancer cells.

A number of multidrug-resistant (MDR) cancer cells have been shown to have acquired an increased capacity to sequester weakly basic anticancer drugs in their lysosomes relative to drug-sensitive counterparts. In this report we have comparatively evaluated the concentrations of the anticancer agent daunorubicin (DNR) in intracellular compartments of drug-sensitive and MDR HL-60 cell lines, both of which do not express common efflux transporters such as P-glycoprotein at the plasma membrane. Our results suggest that lysosomal sequestration plays a significant role in the emergence of MDR since it effectively limits the drug's ability to interact with target molecules located in the nucleus. Using a series of weakly basic structural isomers with variable basicity, we illustrate that the magnitude of the pKa value correlates with the degree of lysosomal sequestration. Accordingly, a series of structurally modified forms of DNR with reduced basicity were synthesized, and their intracellular distribution was evaluated. Consistent with model compounds, derivatives of DNR with lowered pKa values showed visibly reduced lysosomal sequestration in two separate MDR cell lines. Collectively, this work highlights the importance of understanding the intracellular localization of drugs and proposes a rational strategy to manipulate it.

Fertilin beta peptidic liposomes inhibit fertilization by steric blockage.

Liposomes presenting fertilinbeta peptides are inhibitors of mouse in vitro fertilization. We undertook a study of the relationship between IC(50) and mole fraction of fertilinbeta in the liposome, and the size dependence of inhibition of fertilization to understand the mechanism of their inhibition. Our results indicate that a small number of multivalent contacts are required for efficient attachment of inhibitor to receptor on the target membrane, and that, as designed, the liposomes target the egg membrane but not the sperm membrane. The size dependence of the liposome inhibition demonstrates that the liposomes physically block access of the sperm to the egg membrane thereby preventing sperm binding to all egg receptors not just the fertilinbeta receptor.