Targeted drug delivery platforms rely on the availability of high affinity ligands against tumor-specific or tumor-associated antigens to specifically deliver a cytotoxic payload to a subset of cells or tissue. In the case of cancer, there is a substantial inter- and intra-tumoral heterogeneity in the pattern of tumor-antigen expression that makes targeting one antigen of limited value. Furthermore, the ideal tumor-antigen should be rapidly internalized to allow for the efficient uptake of targeted nanoparticles that recognize these antigens for maximal therapeutic efficacy.
We have developed targeted nanoparticles that are efficiently taken up by prostate cancer cells in vitro and in vivo, resulting in remarkable efficacy against this cancer using mouse models of prostate cancer. We demonstrated that these bioconjugates can efficiently target and get taken up by the prostate LNCaP epithelial cells, which express the PSMA protein. In contrast to LNCaP cells, the uptake of these particles is not enhanced in cells that do not express the prostate-specific membrane antigen protein. Next, we performed an aptamer (nucleic acid ligand) selection strategy to isolate aptamers that bind to and get taken up by prostate cancer cells making them suitable for targeted delivery of nanoparticles. We developed an RNA library (1016 distinct RNA molecules) and incubated it with healthy prostate epithelial cells to eliminate the RNA aptamers that bound to healthy prostate cells and used the remaining pool to select for RNA aptamers that had a high affinity to prostate cancer markers. As the RNA pool evolved through reiterative rounds of selection and amplification, the affinity for the pool for the cancer cells increased and the number of PCR cycles that were required to amplify the selected pool decreased. We completed 12 rounds of selection, and cloned the aptamers that bound to prostate cancer cells specifically. Sequence alignment revealed homology regions among families of aptamers suggesting the enrichment of distinct ligands against several prostate tumor-antigens.In conclusion, the body of data generated allows for the development of multi-antigen targeting drug encapsulated nanoparticles for the treatment of prostate cancer. By the virtue of having distinct ligands against more than one antigen, these vehicles may overcome inter- and intra-tumoral heterogeneity that is characteristic of prostate cancer. We believe that a similar approach may be used to develop targeted nanoparticles against other important human cancers.
98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA