Kazuko Hirabayashi

Antitumor Activity of Small Interfering RNA/Cationic Liposome Complex in Mouse Models of Cancer

Purpose: The RNA interference effect is an alternative to antisense DNA as an experimental method of down-regulating a specific target protein. Although the RNA interference effect, which is mediated by small interfering RNA (siRNA) or micro-RNA, has potential application to human therapy, the hydrodynamic method usually used for rapid administration of oligonucleotides is unsuitable for use in humans. In this study, we have investigated the antitumor activity of a synthetic siRNA, B717, which is sequence specific for the human bcl-2 oncogene, complexed with a novel cationic liposome, LIC-101. Experimental Design: In a mouse model of liver metastasis, we administered B717/LIC-101 by bolus intravenous injection, adjusting the rate and volume of administration to what is feasible in human therapy. In a mouse model bearing prostate cancer in which the cells were inoculated under the skin, B717/LIC-101 was administered subcutaneously around the tumor. Results: The B717/LIC-101 complex inhibited the expression of bcl-2 protein and the growth of tumor cell lines in vitro in a sequence-specific manner in the concentration range of 3 to 100 nmol/L. Furthermore, the complex had a strong antitumor activity when administered intravenously in the mouse model of liver metastasis. B717 (siRNA) was shown to be delivered to tumor cells in the mouse liver, but only when complexed with LIC-101. The complex also inhibited tumor cell growth in the mouse model bearing prostate cancer. Conclusions: By combining siRNA with our cationic liposome, we overcame the difficulty of administering siRNA to animals in ways that can be applied in human therapy. Although our siRNA/liposome complex is not yet in clinical trials, it is expected to provide a novel siRNA therapy for cancer patients.

Tumor regression in mice by delivery of Bcl-2 small interfering RNA with pegylated cationic liposomes.

The pharmacokinetics and antitumor activity of pegylated small interfering RNA (siRNA)/cationic liposome complexes were studied after systemic administration to mice. We designed pegylated-lipid carriers for achieving increased plasma concentrations of RNA and hence improved accumulation of RNA in tumors by the enhanced permeability and retention effect. We compared the pharmacokinetics of siRNA complexed with liposomes incorporating pegylated lipids with longer (C-17 or C-18), shorter (C-12 to C-16), or unsaturated (C-18:1) acyl chains. When longer acyl chains were used, the plasma concentrations of siRNA obtained were dramatically higher than when shorter or unsaturated chains were used. This may be explained by the higher gel-to-liquid-crystalline phase-transition temperature (Tc) of lipids with longer acyl chains, which may form more rigid liposomes with reduced uptake by the liver. We tested a siRNA that is sequence specific for the antiapoptotic bcl-2 mRNA complexed with a pegylated liposome incorporating a C-18 lipid (PEG-LIC) by i.v. administration in a mouse model of human prostate cancer. Three-fold higher accumulation of RNA in the tumors was achieved when PEG-LIC rather than nonpegylated liposomes was used, and sequence-specific antitumor activity was observed. Our siRNA/PEG-LIC complex showed no side effects on repeated administration and the strength of its antitumor activity may be attributed to its high uptake by the tumors. Pegylation of liposomes improved the plasma retention, uptake by s.c. tumors, and antitumor activity of the encapsulated siRNA. PEG-LIC is a promising candidate for siRNA cancer therapy.

The role of IFN regulatory factor-3 in the cytotoxic activity of NS-9, a polyinosinic-polycytidylic acid/cationic liposome complex, against tumor cells

NS-9 is a complex of polyinosinic-polycytidylic acid and a novel cationic liposome, LIC-101. The complex has strong cytotoxic activity against tumor cells derived from epithelial or fibroblastic cells. We have investigated the mechanism of the cytotoxic activity of NS-9 using knockdown cells in which the expression of proteins of interest was inhibited by RNA interference. NS-9 showed strong cytotoxic activity against knockdown cells with reduced expression of double-stranded RNA-dependent protein kinase, RNase L, or IFN-α/β receptor, but showed no cytotoxic activity against IFN regulatory factor-3 (IRF3) knockdown cells. In IRF3-knockdown cells, NS-9 also did not induce either the DNA fragmentation or the rRNA degradation observed in negative control cells. We conclude that IRF3 plays a crucial role in the cytotoxic activity of NS-9 against tumor cells, whereas RNA-dependent protein kinase, RNase L, or type I IFNs are not important for its activity.

Development of NS-187, a potent and selective dual Bcr-Abl/Lyn tyrosine kinase inhibitor

Imatinib mesylate (Gleevec™) has improved the treatment of Bcr-Abl-positive leukemia. However, resistance is often reported in patients with advanced-stage disease. Chemical modifications of imatinib made with the guidance of molecular modeling have yielded several promising compounds that could override imatinib resistance. Among them, we selected a compound denoted NS-187. The most striking structural characteristic of NS-187 is its trifluoromethyl group at position 3 of the benzamide ring, which strengthens the hydrophobic interactions and fixes the conformation of the compound. NS-187 was 25–55 times more potent than imatinib against wild-type Bcr-Abl in vitro. At physiological concentrations, NS-187 also inhibited the phosphorylation and growth of all Bcr-Abl mutants tested except T315I. In addition to Bcr-Abl, NS-187 also inhibited Lyn, which might be involved in imatinib resistance, without affecting the phosphorylation of Src, Blk, or Yes. This indicates that NS-187 acts as a dual Bcr-Abl/Lyn inhibitor. Our proposed docking models of the NS-187/Abl complex support the notion that NS-187 is more specific for Lyn than for Src. In Balb/c-nu/nu mice, which were injected subcutaneously with Bcr-Abl-positive KU812 cells, NS-187 showed at least tenfold more potency than imatinib. We also tested the ability of NS-187 to suppress tumor growth in another murine tumor model, namely, Balb/c-nu/nu mice intravenously transplanted with BaF3 cells harboring wild-type or several mutations of Bcr-Abl (M244V, G250E, Q252H, Y253F, E255K, T315I, M351T, and H396P). NS-187 prolonged the survival of mice injected with leukemic cells expressing wild-type or all mutated Bcr-Abl except T315I, and its efficacy correlated well with its in vitro effects. NS-187 also inhibited leukemic cells harboring wild-type Bcr-Abl growth in the central nervous system, which sometimes becomes a sanctuary for leukemic cells under imatinib treatment. These results suggest that NS-187 may be a potentially valuable novel agent to combat imatinib-resistant Bcr-Abl-positive leukemia. A phase I study of NS-187 will start in 2006.

In Vivo Antitumor Activity of a New Cationic Liposome siRNA Complex

A cationic liposome such as LIC-101, which can mediate the rapid entry of siRNA into a majority of cells, would appear to be the ideal vector for in vivo delivery of siRNA. Our siRNA/LIC-101 complex has bcl-2-inhibitory and antiproliferative activity against various tumor cell lines. Furthermore, siRNA/LIC-101 can be safely administered to animals and it shows strong antitumor activity in mouse models of cancer. This in vivo result takes cancer therapy that includes siRNA one step closer to clinical use, with potential for application to various gene-targeting therapies. Finally, we hope that our cationic liposome can be developed as a uniform drug delivery system for various RNA medicines of the future, including siRNA, siRNA expression vectors, and ribozymes, in non-viral RNA delivery.