NovGene-silencing oligonucleotides (oligos) offer exciting new prospects for novel therapeutics, including cancer, viral infections, autoimmune diseases, and cardiovascular disorders. But despite their potential role in treating diseases resulting from dysregulated gene expression, the clinical use of oligos and small interfering (siRNAs) remains extremely challenging.
Challenges for Oligo and siRNA Drug Development
Barriers to clinical use of siRNAs, including drug delivery, poor cellular uptake, instability under physiological conditions, off-target effects, and possible immunogenicity, have led some large pharma companies to cut their commitments to siRNA drug development.
Most recently Alnylam announced in October of 2016 that it had halted development of its revusiran, an investigational RNA interference (RNAi) therapeutic targeting transthyretin (TTR), for the treatment of hereditary ATTR amyloidosis with cardiomyopathy (hATTR-CM). Revusiran employs a siRNA targeting wild-type and all mutant forms of TTR and uses the company’s proprietary N-acetylgalactosamine (GalNAc)-conjugate delivery platform.
This life-threatening disease results from misfolded TTR proteins that accumulate as amyloid fibrils in multiple organs, but specifically in the peripheral nerves and heart. A Phase III trial of revusiran in the rare disease hATTR-CM, which can cause nerve and heart damage, showed that more patients died on the drug than on placebo.
The company continues clinical development of patisiran, a siRNA encapsulated in a lipid nanoparticle (LNP) and delivered intravenously. LNPs are opsonized by apolipoprotein E (ApoE) and are delivered to the liver through interaction with ApoE receptors expressed on hepatocytes. The company’s Phase III APOLLO trial testing patisiran in familial amyloidotic polyneuropathy (FAP) is ongoing, with top-line data anticipated by the middle of 2017.
And on October 20, 2016, Alnylam announced that the Data Monitoring Committee (DMC) for the Phase III APOLLO study of patisiran in patients with hereditary ATTR amyloidosis with polyneuropathy (hATTR-PN) met on October 7, 2016, and recommended continuation of the trial without modification.
FDA-Approved Oligo Drugs
To date, three oligo drugs have been approved. The FDA green-lighted the first therapeutic oligo, fomivirsen (VitraveneTM, Isis Pharmaceuticals) in 1998 for local treatment of cytomegalovirus (CMV) retinitis in AIDS patients via intravitreal injection. Vitravene oligonucleotide, complementary to the messenger RNA of the major immediate-early region proteins of CMV, acts as a potent and selective antiviral agent for CMV. But low demand for the drug, not its lack of efficacy, following the introduction of effective HIV therapies effectively eliminated Vitravene’s key market.
In 2013, the FDA approved Isis/Genzyme’s mipomersen (Kynamro®), an antisense oligonucleotide inhibitor to treat homozygous familial hypercholesterolemia. This oligo reduces apolipoprotein B100 (ApoB100) transcription and lowers blood levels of cholesterol, low-density lipoprotein cholesterol (LDL-C), and ApoB. Administered once weekly by subcutaneous injection, mipomersen was the first systemically delivered oligonucleotide.
And on September 19 of this year, the FDA approved Sarepta’s Duchenne muscular dystrophy (DMD) drug Exondys 51TM (eteplirsen), after a torturous process in which the FDA overruled its own scientific advisers. An independent advisory panel had voted against recommending approval in a 7-to-6 vote in April, saying that the company’s clinical trials for the treatment were poorly designed.
The underlying cause of DMD is a mutation, or error, in the gene encoding dystrophin, an essential protein involved in muscle fiber function. Sarepta says its investigational therapies for DMD are designed to skip an exon in the dystrophin precursor mRNA to enable the synthesis of a shortened, functional form of the dystrophin protein.
Cutbacks on Oligo Drugs
But big pharma has backed off oligo drugs, due to the risks inherent in using them clinically and poor clinical results.
In 2014, Novartis said that it was cutting back its RNAi research program, citing “ongoing challenges with formulation and delivery” of RNAi therapies and the possibility they would only work in only a few conditions.”
And in April, 2016, Ionis Pharmaceuticals announced that GlaxoSmithKline (GSK) had decided not to initiate a Phase III outcome study, CARDIO-TTR, planned to evaluate IONIS-TTRRx in patients with ATTR-CM.
The drug is currently being evaluated in the company’s NEURO-TTR study, in which some patients receiving the drug have experienced dangerously low platelet counts, or thrombocytopenia. GSK will consider options for ATTR-CM once additional clinical data is available from the ongoing studies, Ionis said.
In September 2016, Alnylam presented interim results from its ongoing Phase I clinical trial with an investigational RNAi therapeutic targeting aminolevulinic acid synthase 1 (ALAS1) that also uses its stabilization chemistry for the treatment of acute hepatic porphyrias (AHPs). Alnylam, currently conducting Part C of the Phase I study in symptomatic acute intermittent porphyria (AIP) patients with recurrent porphyria attacks, says it plans to present initial porphyria biomarker data from Part C in late 2016, with potential clinical efficacy data on the frequency and severity of recurrent attacks expected in 2017.
Spherical Nucleic Acids as a Game Changer
But remarkably clever chemists may have come up with a game-changing approach to delivering therapeutic oligonucleotides using spherical nucleic acids (SNAs). These are nucleic acids that are arrayed radially on the surfaces of nanoparticles.
SNAs are being commercialized by Exicure (Skokie, IL), spun out of Dr. Chad A. Mirkin’s laboratory at Northwestern University and currently headed by David A. Giljohann, Ph.D., formerly the company’s principal scientist. Exicure is developing Spherical Nucleic Acid (SNA™) constructs as gene regulatory and immunotherapeutic agents.
Exicure raised $42 million from some sophisticated, deep-pocketed investors that included Bill Gates, David Walt (a co-founder of Illumina and a company director), and Boon Hwee Koh (Agilent Technologies Board of Directors and former Chairman of Singapore Airlines), who bet that the technology developed by the company might provide an enabling technology for nucleic acid delivery.
Exicure has taken the approach of constructing its nucleic acid arrays “inside out,” said Dr. Giljohann, who describes the densely packed arrays as “resembling Koosh balls.” Their three-dimensional architecture, he said, endows SNAs with chemical and physical properties that differ substantially from linear nucleic acids.
Dr. Giljohann explained to GEN that the particles are endocytosed via binding to scavenger receptors that occur on almost all cell types, without the toxicities associated with lipids or polymers. This architecture overcomes, he noted, a significant obstacle to nucleic acid-based therapeutics and provides safe and effective delivery into cells and tissues of therapeutic importance without the need for additional physical or chemical methods or components. The SNAs are designed to be extremely potent and highly targeted gene regulation and immune-modulatory agents.
In an early proof-of-principle study (“Topical Selivery of siRNA-based Spherical Nucleic Acid Nanoparticle Conjugates for Gene Regulation,” Proc Natl Acad Sci USA 2012;109:11975–11980), scientists at Northwestern University evaluated whether the SNAs could penetrate a variety of skin cells to determine whether the conjugates could provide topically delivered gene therapy to treat cutaneous tumors, skin inflammation, and genetic skin disorders.
The constructs freely penetrated almost 100% of keratinocytes in vitro, mouse skin, and human epidermis within hours after application, the investigators reported. The SNA structures could be delivered in a commercial moisturizer or phosphate-buffered saline, with no requirement for barrier disruption or transfection agents, such as liposomes, peptides, or viruses.
Exicure has since reported initial trial results from its first clinical trial using its compound AST-005 to treat chronic psoriasis. AST-005 is a SNA-based antisense drug designed to selectively reduce tumor necrosis factor (TNF), a proinflammatory cytokine that acts as a key mediator of psoriasis. The trial evaluated the safety, tolerability, and disease-related biomarkers in chronic plaque psoriasis patients following topical application of three different strengths of AST-005 formulated as a topical gel.
At the American Chemical Society (ACS) meeting in August 2016, Exicure founder Chad Mirkin said results suggest that SNAs are safe and noted the appearance of a dose-dependent response in knocking down TNF-α, raising hopes that doctors will be able to find a curative dose.
While a long way from proving SNAs are an effective cure against psoriasis, Mirkin said trial results are a “a hopeful first step.” The technology may also boost the utility oligonucleotides to treat a variety of human diseases.
