Development of an RNA-based Cancer Therapeutic Restoring Tumor Suppressor microRNA

Based on the knowledge that miRNAs are dysregulated in diseases such as cancer, there have been multiple attempts to develop miRNA-based cancer therapeutics. However, clinical delivery of the processed or mature miRNA to cancer cells still remains a challenge due to lack of efficient vehicles. To overcome this challenge, innovative methods are being explored to increase the pools of therapeutically relevant miRNAs. This is especially true for the tumor suppressive miRNA let-7 family. Loss of mature let-7, often observed in cancer, induces deregulation of oncogenes including KRAS, MYC, and HMGA2 allowing the cells to become tumorigenic. Therefore, it is important to restore the levels of let-7 prolonging cellular differentiation and suppressing cellular proliferation. We propose that let-7 levels can be restored (1) by enhancing let-7 biogenesis by small molecule regulators or (2) through targeted and efficient delivery of let-7 using a ligand-mediated delivery approach. We have conducted a cell-based high-throughput screen to identify small molecule regulators that increase let-7 activity. The screen was performed against 23,680 compounds. Compounds identified as positive hits were further evaluated to identify the mechanism involved in increasing let-7 activity. Interestingly, one of the compounds that specifically increased let-7 activity also regulated transcription of MYC, a key transcription factor in cancer development. In addition, using a relevant ligand:receptor pair that is specific to tumor cells, in this case folate and the folate receptor, folate conjugated let-7 was delivered to ovarian cancer cells, which overexpress the folate receptor. Through receptor-mediated endocytosis folate conjugated let-7 was internalized into cancer cells and downregulated its target. We have generated a second generation folate conjugate by intramolecularly attaching a small molecule ionophore to overcome one of the major challenges of ligand delivery; endosomal entrapment. Results from our study are expected to have significant positive impact in cancer treatment by providing new opportunities to advance the next phase of miRNA-based therapeutics.