Tumor Selective Silencing Using an RNAi-Conjugated Polymeric

Feb 2, 2016 - Phone: 781-209-6380. ... Formulating siRNA into nanoparticles offers two advantages: enhanced siRNA stability against nuclease degradati...
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Tumor Selective Silencing Using an RNAi-Conjugated Polymeric Nanopharmaceutical Sonke Svenson, Roy I. Case, Roderick O. Cole, Jungyeon Hwang, Sujan R. Kabir, Douglas Lazarus, Patrick Lim Soo, Pei-Sze Ng, Christian Peters, Pochi Shum, Beata Sweryda-Krawiec, Snehlata Tripathi, Derek van der Poll, and Scott Eliasof* Cerulean Pharma Inc., 35 Gatehouse Drive, Waltham, Massachusetts 02451, United States S Supporting Information *

ABSTRACT: Small interfering RNA (siRNA) therapeutics have potential advantages over traditional small molecule drugs such as high specificity and the ability to inhibit otherwise “undruggable” targets. However, siRNAs have short plasma half-lives in vivo, can induce a cytokine response, and show poor cellular uptake. Formulating siRNA into nanoparticles offers two advantages: enhanced siRNA stability against nuclease degradation beyond what chemical modification alone can provide; and improved site-specific delivery that takes advantage of the enhanced permeability and retention (EPR) effect. Existing delivery systems generally suffer from poor delivery to tumors. Here we describe the formation and biological activity of polymeric nanopharmaceuticals (PNPs) based on biocompatible and biodegradable poly(lactic-co-glycolic acid) (PLGA) conjugated to siRNA via an intracellular cleavable disulfide linker (PLGA−siRNA). Additionally, these PNPs contain (1) PLGA conjugated to polyethylene glycol (PEG) for enhanced pharmacokinetics of the nanocarrier; (2) a cation for complexation of siRNA and charge compensation to avoid high negative zeta potential; and (3) neutral poly(vinyl alcohol) (PVA) to stabilize the PNPs and support the PEG shell to prevent particle aggregation and protein adsorption. The biological data demonstrate that these PNPs achieve prolonged circulation, tumor accumulation that is uniform throughout the tumor, and prolonged tumor-specific knockdown. PNPs employed in this study had no effect on body weight, blood cell count, serum chemistry, or cytokine response at doses >10 times the effective dose. PNPs, therefore, constitute a promising solution for achieving durable siRNA delivery and gene silencing in tumors. KEYWORDS: siRNA delivery, PLK1, drug conjugation, polymeric nanoparticles, PLGA, cancer therapy

1. INTRODUCTION Small interfering RNA (siRNA) therapeutics have the potential to inhibit otherwise “undruggable” targets by taking advantage of sequence-driven specificity and high potency. Many human diseases involve some form of aberrant gene overexpression that can serve as a potential siRNA target, including cancers, viral infections, and genetic and metabolic disorders. siRNAs and siRNA variants consist of two strands, generally 21−25 base pairs in length. The molecular weight of siRNA is around 13−16 kDa, and the strands carry nearly 40−50 negative charges due to the phosphodiester backbone. Once delivered to the cytoplasm, siRNAs degrade the targeted sequence through RISC (RNA-induced silencing complex) mediated silencing.1−3 siRNAs are very hydrophilic and water-soluble due to their sugar−phosphate backbone. However, siRNA has a short plasma half-life in vivo (10% of the injected dose in tumors following a single dose of siRNA PNPs. More recently, an in vitro study involving siRNA(GFP) and siRNA(LUC), both conjugated to PEG via disulfide bonds, was reported. Negatively charged poly(glutamic acid) was used as the polymer, and chitosan as the counterion for complex formation and charge compensation.54 Major differences from the study described here are (1) PEGylation of siRNA maintains or even increases siRNA hydrophilicity, requiring the very polar poly(glutamic acid) polymer as the carrier material; and (2) the use of this negatively charged polymer in conjunction with negatively charged siRNA requires large amounts of positively charged (cationic) chitosan for the polyplex formation. As mentioned before, cationic lipids and polymers are potentially cytotoxic. To reduce the need for cationic material we used siRNA conjugation to biocompatible PLGA in our study, taking advantage of hydrophobic interaction between PLGA−siRNA and the PLGA core of the nanoparticles to retain siRNA, while comparatively small quantities of spermine were employed only for charge compensation.

(Supplementary Table 4). A single dose was chosen in this experiment because repeated dosing tends to blunt cytokine responses. The endotoxin lipopolysaccharide was separately administered as a positive control. Statistically significant differences in IL-10, IL-6, IL-12, IFNγ, TNFα, and KC were observed in response to lipopolysaccharide relative to vehicle, but no statistically significant differences were observed in response to the administration of siRNA PNPs at either 2 or 6 h postadministration.

4. DISCUSSION The clinical PLK1 candidate by Arbutus Biopharma (formerly Tekmira Pharmaceuticals, TKM-PLK1) was developed using stable nucleic acid lipid particles (SNALP). These particles displayed antitumor efficacy in both hepatic and subcutaneous tumor models.52 This important early study demonstrated that systemic administration of nanoparticle-formulated siRNA can trigger RNAi-mediated cleavage of mRNA within solid tumors, silencing target expression at a magnitude sufficient to induce the mitotic disruption and apoptosis of tumor cells. A side-by-side comparison of the siRNA knockdown results reported here with other studies employing PLGA nanoparticles is not straightforward. Most of these studies only show in vitro knockdown data, and in vitro−in vivo correlations are uncertain. However, two recent studies employed PLGA nanoparticles as the carriers for in vivo siRNA delivery. The study by Yang et al. reported the application of PLGA nanoparticles made by the double emulsion solvent evaporation technique.53 These particles were larger (170−200 nm) than the PNPs described here made via nanoprecipitation (Zave ∼85 nm). Loading of the emulsion particles with either luciferase (LUC) or polo-like kinase 1 (PLK1) siRNAs gave in vitro knockdown similar to what was reported here for PLK1 PNPs. Unfortunately, in vivo knockdown was only measured in the liver using an orthotopic HepG2 xenograft model. The second study, by Zhou et al., described the systematic development of optimized octafunctional PLGA nanoparticles for targeted siRNA delivery.41 PLGA nanoparticles made via the double emulsion process (∼150 nm in diameter) contained PLGA-poly(lysine) copolymers to improve intracellular siRNA release, PEG8000 for surface PEGylation, and an iRGD (CRGDKGPDC) peptide for ligand-mediated tumor delivery. siRNA(PLK1) was physically entrapped into these nanoparticles for in vivo knockdown and tumor growth inhibition studies, using subcutaneous A549 tumor cells. Treatment was administered every 3 days using 2 mg of siRNA(PLK1) formulation per mouse (∼10 mg/kg). The reported knockdown efficiency was around 50%, and therefore in the same order as the efficiency observed here. While the study by Zhou provided important insights into the factors affecting knockdown efficiency, the complexity of the nanoparticle formulation could raise questions about reproducibility and cost of goods during scale-up when translating into the clinic. In addition, two recent studies employed micelles for in vivo delivery of antiangiogenic siRNA(VEGF), further illustrating that siRNAs can be efficiently delivered to tumor cells using nanocarriers. In one case, 50% knockdown was observed in subcutaneous HeLa tumors after repeat dosing of targeted micelles containing 1 mg/kg siRNA, but plasma PK decayed quite rapidly (half-life ∼10 min), similar to LNPs.15 In another instance, 86% knockdown was observed in small PC3 subcutaneous tumors after repeat dosing of micelles containing ∼1 mg/kg siRNA, despite the fact that only 0.5% injected dose

5. CONCLUSIONS Formulating siRNA into nanoparticles offers four important advantages: (1) enhanced siRNA stability against nuclease degradation beyond the stability increase that chemical modification alone can provide; (2) prolonged circulation times due to the PEGylated particle surface; (3) improved tumor accumulation, taking advantage of the EPR effect in tumor tissues; and (4) tumor-specific knockdown at doses that were well tolerated. In an attempt to reduce the need for a potentially cytotoxic cationic polymer, we conjugated siRNA via a disulfide bond to the biocompatible and biodegradable polymer PLGA. Hydrophobic PLGA interaction between PLGA−siRNA and the PLGA core of the PNPs retains the siRNA inside the carrier, while the cation is mainly needed for charge compensation to maintain the zeta potential of the PNPs between ±10 mV. PEGylation of the nanoparticle surface using PEG−PLGA provides stealth properties for sufficient circulation time and tumor tissue accumulation. While this study employed siRNA(AHA1), we have observed a very similar degree and duration of knockdown with siRNA directed against polo-like kinase 1 (PLK1) and vascular endothelial growth factor (VEGF) in this same model, and with PLK1 in the A2780 subcutaneous ovarian xenograft model and the Hep3B subcutaneous hepatocellular carcinoma xenograft model, and with green fluorescent protein (GFP) in two orthotopic breast xenograft models (MDA-MB-231 and MDA-MB-468) that constitutively expressed GFP. Variations in the siRNA chemistry and the formulation are also possible: similar knockdown was observed whether we used 2′-OMe modifications or siSTABLE modifications from siRNA, and whether we used spermine, cationic poly(vinyl alcohol) (PVA), or PLGA-polylysine as the charge-neutralizing cation. The PNPs described here offer a compelling solution for the challenges siRNA delivery faces in therapeutic applications: (1) delivery of intact siRNA into tumor tissue; (2) delivery of siRNA into tumor cells; and (3) durability of the gene silencing effect. Lipid-based nanocarriers accumulate mainly in the liver, rendering them excellent liver treatment options, but show little accumulation in tumor tissues elsewhere. Polymeric nanoI

DOI: 10.1021/acs.molpharmaceut.5b00608 Mol. Pharmaceutics XXXX, XXX, XXX−XXX

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Molecular Pharmaceutics

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pharmaceuticals described here, on the other hand, provide an alternative carrier with improved tumor targeting potential.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.molpharmaceut.5b00608.

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Synthetic protocols and experimental details (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 781-209-6380. Fax: 844-894-2378. Notes

The authors declare the following competing financial interest(s): All authors are current or former employees of Cerulean Pharma Inc.

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DOI: 10.1021/acs.molpharmaceut.5b00608 Mol. Pharmaceutics XXXX, XXX, XXX−XXX