Liposomal siRNA nanocarriers for cancer therapy

Abstract

Small interfering RNAs (siRNA) have recently emerged as a new class of therapeutics with a great potential to revolutionize the treatment of cancer and other diseases. A specifically designed siRNA binds and induces post-transcriptional silencing of target genes (mRNA). Clinical applications of siRNA-based therapeutics have been limited by their rapid degradation, poor cellular uptake, and rapid renal clearance following systemic administration. A variety of synthetic and natural nanoparticles composed of lipids, polymers, and metals have been developed for siRNA delivery, with different efficacy and safety profiles. Liposomal nanoparticles have proven effective in delivering siRNA into tumor tissues by improving stability and bioavailability. While providing high transfection efficiency and a capacity to form complexes with negatively charged siRNA, cationic lipids/liposomes are highly toxic. Negatively charged liposomes, on the other hand, are rapidly cleared from circulation. To overcome these problems we developed highly safe and effective neutral lipid-based nanoliposomes that provide robust gene silencing in tumors following systemic (intravenous) administration. This delivery system demonstrated remarkable antitumor efficacy in various orthotopic human cancer models in animals. Here, we briefly overview this and other lipid-based approaches with preclinical applications in different tumor models for cancer therapy and potential applications as siRNA-nanotherapeutics in human cancers.

Keywords: Cancer; Delivery; Gene silencing; Liposomes; Nanovectors; Targeted therapies; siRNA.

Figure 1. The process RNA-Interference in eukaryotic cells

Long precursor microRNA (miRNA), called primiRNA, is cleaved by RNase III endonuclease (Drosha) into pieces of approximately 70 nucleotides each (called pre-miRNA) in the nucleus. Following transportation into the cytoplasm by exportin 5 another RNase III endonuclease (Dicer) cleaves it into mature miRNA segments. Degradation of messenger RNA (mRNA) and translational repression occurs after miRNA binds to the RNA-induced silencing complex (RISC). Cytoplasmic long double-stranded RNA (dsRNA) is cleaved by Dicer into small interfering RNA (siRNA), which is incorporated into RISC, resulting in the cleavage and degradation of specific target mRNA. Synthetic double-stranded siRNA does not processed by Dicer and directly incorporated by RISC (Reprinted with permission from Meritt et al Copyright 2008).

Figure 2. In vivo therapeutic silencing of Bcl-2 by NL-Bcl-2 siRNA inhibits in vivo tumor growth of ER(–) MDA-MB-231 xeonografts in nude mice

(Tekedereli & Ozpolat, 2013). A) Mice were orthotopically (into mammary fat pat) injected with luciferase expressing MDA-MB-231 cells. About 2 weeks after tumor cell injection mice were imaged using IVIS-Xenogen live imaging system for tumor growth. Mice bearing tumors with equal size were randomly assigned into control and treatment groups (n=5 mice per group). Mice received either NL-Bcl-2 siRNA or NL-control siRNA treatments (0.15 mg siRNA/kg or 4µg/mouse, i.v, twice a week) from tail vein for 4 weeks (total of 8 injections). After 4-weeks of treatments mice were imaged by IVIS live-imaging system and tumors were removed and weighted. B) In vivo silencing of Bcl-2 by intravenously administered NL-Bcl-2 siRNA enhances the antitumor efficacy of chemotherapy. Mice were treated with NL-Bcl-2-siRNA or NL-control siRNA (0.15 mg siRNA/kg, i.v, twice a week) and also received doxorubicin (4 mg/kg, i.p, once a week) for 4 weeks. The tumor weights after 4 weeks of treatments were measured. Mice that received NL-Bcl-2 siRNA had significantly inhibited tumor weight compared with the control group that was treated with NL-Control siRNA and enhanced the anti-tumor efficacy of doxorubicine. C) Bcl-2 protein expression after 4 weeks of treatments in MDA-MB-231 tumors. After sacrificing mice tumors were removed (48h after the last NL-siRNA injection) and tumor lysates were analyzed for Bcl-2 expression by Western blot. NL-Bcl-2 siRNA treatment was well tolerated and did not have weight lose in mice, compared to those who received NL-control siRNA.