Overcoming multidrug resistance of small-molecule therapeutics through conjugation with releasable octaarginine transporters

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Abstract
Many cancer therapeutic agents elicit resistance that renders them ineffective and often produces cross-resistance to other drugs. One of the most common mechanisms of resistance involves P-glycoprotein (Pgp)-mediated drug efflux. To address this problem, new agents have been sought that are less prone to inducing resistance and less likely to serve as substrates for Pgp efflux. An alternative to this approach is to deliver established agents as molecular transporter conjugates into cells through a mechanism that circumvents Pgp-mediated efflux and allows for release of free drug only after cell entry. Here we report that the widely used chemotherapeutic agent Taxol, ineffective against Taxol-resistant human ovarian cancer cell lines, can be incorporated into a releasable octaarginine conjugate that is effective against the same Taxol-resistant cell lines. It is significant that the ability of the Taxol conjugates to overcome Taxol resistance is observed both in cell culture and in animal models of ovarian cancer. The generality and mechanistic basis for this effect were also explored with coelenterazine, a Pgp substrate. Although coelenterazine itself does not enter cells because of Pgp efflux, its octaarginine conjugate does so readily. This approach shows generality for overcoming the multidrug resistance elicited by small-molecule cancer chemotherapeutics and could improve the prognosis for many patients with cancer and fundamentally alter search strategies for novel therapeutic agents that are effective against resistant disease.