RNAi Arrives at the Bedside after a Mere Two Decades
Authors: Dirk Haussecker
Journal: Molecular Therapy (2018)

There were audible gasps in the audience at the European ATTR Amyloidosis Meeting for Patients and Doctors being held November 2 and 3, 2017 in Paris, France when Dr. David Adams of the Centre Hospitalier Universitaire (CHU) Hôpital Bicêtre unveiled the results of the pivotal clinical trial (APOLLO) of Patisiran in subjects with hereditary familial TTR amyloid polyneuropathy (hATTR FAP). By showing physiological and quality of life improvements in the majority of patients and a solid safety profile, the results exceeded the expectations of most observers of perhaps seeing a delay in disease progression. The APPOLLO data, which paved the way to the first marketing approvals of an RNAi drug in August of this year in both the US and Europe, was a moment the field of RNAi therapeutics had braced for over the years. After demonstrating in 2012 that substantial RNAi gene knockdown could indeed be achieved in WoMan, the anxiety over whether this finding would translate into a measurable clinical benefit was great. In particular, some RNAi drug candidates had stumbled previously owing to liver toxicity, apparently the result of the difficult-to-predict consequences of mild off-target gene modulation of often dozens of random genes. Luckily, Patisiran (now marketed as ONPATTRO) did not suffer from similar toxicity issues following administration to trial subjects every 3 weeks for 18 months. The treatment had been conceived at a time when the focus was on achieving any kind of meaningful gene knockdown in WoMan, and off-targeting was largely an after-thought at the time. Patisiran is a 19 base-pair small interfering RNA (siRNA) with partial 2′-O-methylation and internucleotide phosphorothioation with 2 nucleotide 3′ overhangs formulated in a cationic MC3 lipid-based nanoparticle (LNP). It targets the transthyretin (TTR) gene in hepatocytes. In patients with hATTR FAP, the mutant Transthyretin can misfold and deposit in various organ systems leading to disease symptoms ranging from polyneuropathy to cardiomyopathy. Death typically occurs within 2–5 years following diagnosis. A precursor candidate of Patisiran featuring the same siRNA, but formulated in prior-generation LNPs based on ionizable cationic lipid 1,2-dilinoleyloxy-3-dimethylaminopropane (DLinDMA), had been abandoned in 2012 because of poor potency. Despite this 3-year setback, overall, the speed of clinical translation from the seminal 1998 report in Caenorhabditis elegans that double-stranded RNA was the trigger of a mysterious gene silencing phenomenon seen across the tree of life to last month's marketing approvals was rapid. From there, it took 2–3 years of painstaking biochemistry for the eureka discovery that RNAi could also be effectively triggered in mammalian cells using short 19–23 bp double-strand RNAi triggers; another ∼2 years to show the same in adult mammals using a potentially clinically relevant method involving LNPs; another ∼2 years (in 2006) for scaling this up to non-human primates, which in turn led the way to a first clinical trial involving a systemic (DLinDMA-based) LNP-delivered RNAi drug candidate in 2009. This steady scientific progress was accompanied by considerable ups and downs in the public perception of RNAi as a therapeutic modality. The initial period was one of benign neglect and belittlement when RNAi was viewed merely as a fine laboratory tool, but, as was the case for gene therapy at the time, never to be accepted by the establishment as a serious therapeutic modality. This was followed by an RNAi gold rush in 2005–2008 when the first investments by large pharmaceutical companies triggered angst among their peers of losing out on access to a potentially transformational technology as the traditional large indication-focused small molecule screening-based drug discovery model became increasingly inefficient. When the big promises and quick money had been made and the dust had settled, big pharmaceutical companies suddenly got cold feet, realizing that money spent on acquiring intellectual property access did not address fundamental issues such as getting the RNAi triggers into cells in patients (delivery) and doing so without dangerous stimulation of the innate immune system. As quick as they came, big pharma and investors thus deserted the field. As funding dried up while the rest of the biotech industry was recovering from the 2008 financial crisis, there was a very real risk that the few remaining substantial pure-play RNAi companies, such as Alnylam, Arrowhead, Tekmira, and Dicerna, would disappear had Alnylam not shown first clinical gene knockdown in 2012 in the form of phase I results with ONPATTRO. Looking back, timing and a good portion of luck has been everything. Without the timely advent of genomic technologies, affordable quality oligonucleotide manufacturing and, critically, the acceptance of genomic modalities such as oligonucleotides and gene therapy by the medical establishment and society at large, even a technologically enabled RNAi platform would not have reached patients. Similarly, on the RNAi technology development side, who could have foreseen that certain LNPs naturally target ApoE-receptors in the liver, the GalNAc system is so efficient in taking up oligonucleotides, and simple oligonucleotide modifications can allow the RNAi trigger to evade the immune system without having to adversely impact knockdown efficacy? Using newer conjugation-based GalNAc-RNAi triggers, the arsenal of approved RNAi therapeutics is about to expand by 1–2 molecules a year, addressing both orphan diseases (e.g., acute hepatic porphyria, primary hyperoxaluria) to more wide-spread indications (e.g., dyslipidemias and chronic HBV infection) involving gene expression in the liver. Failure rates due to RNAi off-targeting should also come down because companies have become more cognizant thereof and are increasingly applying modification strategies to substantially minimize them. Enboldened by the success in the liver, companies such as Arrowhead and Alnylam Pharmaceuticals are serious about developing delivery platforms for new tissue targets, including the brain, lung, and cancer. The future is bright for RNAi therapeutics.