Cancer chemotherapy uses toxic chemicals to kill cancer cells; but such potent agents often damage normal cells and systems as well. Such side effects seriously limit the prolonged use of effective concentrations of chemotherapeutic drugs. Researchers are thus avidly seeking more selective drug-delivery systems to limit damage outside the tumor site. One approach is to use a normally benign "prodrug," which is specifically activated by an enzyme catalyst found only in the tumor. The enzyme could be naturally produced by tumor cells, or it could be brought to the tumor by a targeting moiety such as an appropriate polymer. To use a "smart-bomb" analogy, a homing device (the polymer) delivers the explosive charge (the prodrug) to the target (the tumor) where it is triggered (by the enzyme). In such a scenario, non-specific toxicity to other tissues should be minimal.
Dr. Doron Shabat and his colleagues at the TAU School of Chemistry have prepared such a delivery system using a HPMA copolymer "homing device" and a catalytic antibody 38C2 "trigger." The two are joined (conjugated) by an amide bond between the -amino group of a lysine residue in the antibody molecule and a p-nitrophenyl ester in the polymer. After purification and isolation, one or two molecules of HPMA polymer were conjugated to each molecule of antibody, as determined by gel analysis (see Figure 1). The resulting conjugate retained 75-81% of the free antibody's catalytic activity, as determined using a fluorogenic substrate and a HPLC prodrug activation assay. The conjugate was evaluated in vitro for its ability to activate an etoposide anti-cancer prodrug using two different cancer cell lines. The growth of the tumor cells was inhibited equally well by the prodrug in the presence of the polymer-antibody conjugate or the free antibody. The new TAU conjugate can be adopted for the selective activation of a wide variety of prodrugs in the existing "polymer-directed enzyme prodrug therapy" approach, by simply replacing the enzyme component with the catalytic antibody 38C2.
The TAU group has also developed a novel drug delivery system based on a highly generic chemical "adaptor," that provides a ready linkage to a wide variety of drugs and chemical targeting devices. However, the highly versatile adaptor holds on tightly to the other two components, until its special triggering group is removed by highly specific enzymatic activity (Figure 2). The enzymatic substrate group can also readily be varied, as needed.
Cleavage of the enzyme's substrate generates a chemical intermediate that spontaneously rearranges itself to release the drug from the adaptor/targeting-device complex. This chemical adaptor system was designed with stable chemical linkages, in order to avoid nonspecific drug release in vivo. Proof of concept has already been demonstrated using etoposide as the drug, an HPMA-copolymer as the targeting device, and catalytic antibody 38C2 as the triggering enzyme. However, many other combinations are possible in this innovative modular approach to selective chemotherapy.
