RNA-Based Therapeutics and the Future of Clinical Medicine: A Promising Part of the Emerging Story

Yves here. Many have become skeptical of mRNA vaccines, if nothing else due to the aggressive overselling of their effectiveness (preventing contagion, as opposed to worst outcomes; also overstating the duration of protection conferred by boosters). That may prejudice patients against other RNA-based approaches which use different channels and often have real promise.

KLG has managed to miss old people TV. He’s new to LEQVIO ads, which your humble blogger first saw well over a year ago. He’s singularly unimpressed with this mass-market cholesterol-targeting RNA-based therapeutic, but finds that others for rare metabolic diseases look effective and they also have great potential for otherwise untreatable cancers.

By KLG, who has held research and academic positions in three US medical schools since 1995 and is currently Professor of Biochemistry and Associate Dean. He has performed and directed research on protein structure, function, and evolution; cell adhesion and motility; the mechanism of viral fusion proteins; and assembly of the vertebrate heart. He has served on national review panels of both public and private funding agencies, and his research and that of his students has been funded by the American Heart Association, American Cancer Society, and National Institutes of Health

My family cut the cord on cable television many years ago, but there is nevertheless a 54-inch Samsung flatscreen directly behind me as I write this while gazing through the wavy panes of our old windows at the nearby intersection that includes a park, the public library, and the US Post Office, all less than 150 yards from where I sit.  We acquired this really quite amazing TV several years ago on a Thanksgiving Night at a Big Box store (mea culpa, mea maxima culpa), primarily as it turned out to allow yours truly to watch the occasional college football game, the World Series, and the major golf championships when they are broadcast on free television.  By the way, both baseball and Wolf Hallare much better on a screen larger than an iPad.  Earlier this fall while watching a football game I promised myself that I would pay attention to the commercials for prescription medicines that so often accompany sporting events (e.g., finasteride and sildenafil on golf telecasts).  I have nearly worn out the mute button on the remote control while silencing all ads and the more loquacious broadcasters, so this took a bit of willpower.  Most of the commercials were standard fare.  Nevertheless, we all have our priors, so I did perk up when this ad for LEQVIO appeared[1].  I had never heard of this drug, which is said to reduce LDL-C, or Low-Density-Lipoprotein-Cholesterol – i.e., “bad cholesterol” – when used in conjunction with a statin, of course.

I cannot remember a thing about that particular football game, but as I have read some of the scientific literature, LEQVIO, also called Inclisiran, has held my attention off and on for weeks because Inclisiran is an siRNA, a small interfering RNA.  We all now know how mRNA works as a vaccine, so this seems like a good time to consider some of the other RNA therapeutics that have a brighter future, as presented in this recent technical but relatively accessible public access review by Maik Freidrich and Achim Aigner of the University of Leipzig.[2]  In the late 1990s Craig Mello of the University of Massachusetts Medical School in Worcester and Andrew Fire of the Carnegie Institution of Washington in Baltimore discovered RNA interference (RNAi) in the nematode Caenorhabditis elegans.  RNAi is a naturally occurring defense mechanism against foreign nucleic acids and a mechanism for the control of gene expression in these worms,[3] and RNA interference is a general characteristic many organisms.  siRNAs were subsequently identified as the mediators of RNA interference in mammalian cells.

The mechanism of how siRNA works is straightforward if not particularly simple at the molecular level.  Basically, when delivered into the target cell the active half of the siRNA binds to its complementary mRNA, which then leads to the degradation of the mRNA.  The destruction of the siRNA-specific mRNA results in lower levels of the indirectly targeted protein.  If this protein causes disease or dysfunction, siRNA can be used as an effective intervention for a previously “undruggable” disease.  On a personal, professional, and scientific note, I was a research fellow when RNAi was discovered, across town as it happened, and it was immediately recognized as revolutionary.  This was confirmed in the relatively short time it took from discovery in 1998 to Nobel Prize in 2006.  Here was a naturally occurring mechanism by which a biologist in the laboratory might regulate gene expression in both whole organisms and cultured cells[4] while only gently disturbing the experimental system.  I remember well the first experiment I did using siRNA about 20 years ago, in which three different siRNAs to the mRNA of my favorite protein showed that down-regulation of this protein prevented cultured mouse muscle cells from forming a contractile “myotube,” which while not precisely a functional muscle is still an excellent model for studying skeletal and cardiac muscle assembly.  One of those rare days in the lab that carries a scientist for months.  If not years!

Although the first clinical uses of siRNA were proposed immediately after discovery, only recently have siRNAs been approved for therapeutic use. The first was ONPATTRO/Patisiran (2018) as a treatment of transthyretin amyloidosis[5], which is a very rare, progressive, and fatal disease.  Previous treatments have not been effective.  Transthyretin, encoded by the TTR gene, normally transports thyroid hormone, thyroxine, and retinol but when mutated it forms insoluble amyloid fibrils[6] that result in cardiomyopathy and neuropathy characterized by sensory, motor, and autonomic nervous system dysfunction.  Phase I, Phase II and Phase III clinical trials of Patisiran have been successful.  TTR levels decreased by 81% over 18 month study period, but more importantly neuropathy decreased and other legitimate clinical endpoints such as motor strength, body mass index, level of disability, motility, and quality of life measurements improved and TTR siRNA was well tolerated with vitamin A supplementation.  So far, Patisiran works.  The siRNA Vutisiran has also been approved as a target of TTR (2022).  The ONPATTROwebsite is here and the Vutisiran/AMVUTTRA site is here.

Givosiran/GIVLAARI (2019) has been approved for treatment of acute hepatic porphyria[7] (AHP), which is another rare disorder of heme biosynthesis (remember that heme is the oxygen-binding molecule attached to hemoglobin in red blood cells).  In this case overproduction of the enzyme ALAS1 leads to the production of intermediates in the heme biosynthetic pathway, which are neurotoxic, with associated symptoms such as abdominal pain, nausea, vomiting, weakness, and psychiatric disturbances.  Conventional treatments are limited.  However, subcutaneous injection of Givosiran resulted in sustained decrease in ALAS1 levels in liver cells and a concomitant diminished level of circulating heme intermediates and lower rate of porphyria attacks compared with the placebo group.  Although liver and kidney function were affected in some patients, long-term administration showed an acceptable safety profile with significant benefits in AHP patients as measured by reduction in attack frequency and severity of worst pain with improving quality of life.  Even with the side effects Givosiran works, so far.  The obligatory company website is here.

Primary hyperoxaluria (PH) is a very rare inborn error of metabolism characterized by the accumulation of glyoxylate in the liver.  Glyoxylate can be converted into oxalate by several mechanisms, after which it forms kidney stones made of calcium oxalate.  Lumasiran/OXLUMO (2020) down-regulates glycolate oxidase and has been shown to work in children over the age of 6 years and adults, substantially reducing urinary oxalate concentrations and leading to normal or near-normal urinary and plasma oxalate levels.  The results are early, but indications are that these siRNAs also work as expected.

While these advances are significant for those with these rare inherited diseases, siRNA promises to be useful in the treatment of a host of other diseases, including several cancers that are particularly refractory to other pharmacological approaches.  In many of these the target is an oncogene[8] characteristic of the tumor.  For example, pancreatic ductal adenocarcinoma and metastatic pancreatic ductal adenocarcinoma are usually lethal, with few good options for treatment.  However, it may be possible to target the oncogene KRASG12D [pronounced K-RAS-G-12-D: G12D means that the glycine (G) at position 12 KRAS has been mutated to aspartic acid (D)] in these tumors with an siRNA.

Without getting too far into the weeds, antagonists of KRAS have been called a “holy grail” of cancer drug discovery.  If a specific siRNA can be shown to down-regulate KRASG12D, this previously undruggable cancer driver will have been targeted, specifically and possibly effectively.  Recurrent glioblastoma, which is always lethal, is another possible with the target being Bcl2L12 (BCL-2-L-12).  Bcl2 proteins make cells resistant to normal programmed cell death (apoptosis) and when overexpressed can lead to cancer.  From Friedrich and Aigner (p.564): “A single-arm, open-label, phase 0, first-in-human study in patients with recurrent glioblastoma demonstrated gold enrichment in the tumors (tumor cells, tumor-associated endothelium, macrophages) and significant reduction in BCL2L12 protein levels, with no significant treatment-related toxicity.”  Still early but this remarkable result may be the first step toward an effective and specific treatment for glioblastoma.  Other conditions appropriate for siRNA include macular degeneration and several other eye diseases, although early trials have been disappointing.

Several conclusions can be drawn from the success of siRNAs in the treatment of the metabolic diseases considered above, and the hopeful speculation about siRNAs in the treatment of pancreatic cancer and glioblastoma.  The first is that basic scientific research is absolutely essential if we are to make principled progress in medical research.  The importance of basic research was covered earlier in this series.  Would RNAi – RNA interference – have been discovered if Sydney Brenner, who as a close colleague of Francis Crick was a pioneer in modern molecular biology, had not been supported in his work on the obscure nematode worm Caenorhabditis elegans as an experimental organism?  Perhaps, but who knows?  As it happened, C. elegans allowed scientists to describe the first complete cell lineage of an animal and to discover programmed cell death, which was mentioned in the previous paragraph.  A second point is that virtually all biomedical research is incremental rather than revolutionary.

A long-standing dogma of the natural philosophy that became natural science is Natura non facit saltum – Nature does not make leaps.  Maybe, maybe not, depending on one’s stance on punctuated equilibrium in evolutionary biology and the uniformitarianism of Charles Lyell and others, but research in biology certainly does not make leaps very often.  The paths to cures for childhood leukemias and several other cancers was long and full of short hops and little sprints, but it was also sure.  And more importantly it was absolutely dependent on a wide and deep foundation of basic research.  siRNA is new to clinical medicine, but it has worked better than expected as a treatment for several otherwise intractable metabolic diseases.  As siRNA therapeutics continue to develop, its revolutionary potential is very real.  And finally, for our purposes, the more specific the drug, the more likely the therapy is to work without off-target side effects.  Although nonspecific effects have been associated with siRNA, they are uncommon.

And so now we come back to LEQVIO/Inclisiran (2020) and LDL-C.  The “markets” for porphyria and transthyretin amyloidosis are important to those patients but they are small.  The same is true for pancreatic cancer and glioblastoma.  However, not so much for LDL-C!  And as we have seen in the LEQVIO commercials and the scientific literature, this siRNA does lower LDL-C in patients.  The Phase 3 trials are described here[9], and the Novartis LEQVIO website is here.  The active siRNA in LEQVIO targets PCSK9 (pro-protein convertase subtilisin/kexin type 9).  That is a mouthful, but it has been known for a long time that knockdown of PCSK9 increases the recycling of the LDL receptor on the surface of liver cells.  This increases the uptake of LDL-C from the plasma and lowers circulating LDL-C, which is the so-called bad cholesterol.  Thus, it can be expected that reductions in PCSK9 will lead to reductions in plasma cholesterol associated with LDL, and this is the result, with a durable reduction of LDL-C of 51% at more than 500 days.  But “whether decreased PCSK9 and LDL-C levels will translate into improved cardiovascular outcomes remains to be seen. Considering previous studies on the monoclonal anti-PCSK9 antibody evolocumab, which was able to reduce LDL-C without improving cardiovascular mortality.  Indeed, in a pooled analysis of (the trials) ORION-9, ORION-10, and ORION-11, only a 2.5% decrease in major cardiovascular events upon inclisiran treatment was found.”(Friedrich and Aigner, p.559.).

Therefore, unlike the trials with Patisiran (TTR amyloidosis), Givosiran (acute hepatic porphyria), and Lumasiran(primary hyperoxaluria), in which the target protein level was reduced significantly by siRNA and accompanied by other clinical endpoints indicating improvement in patient wellbeing, LEQVIO/Patisiran did lower LDL-C as expected, but without significant improvement in clinical outcomes.  This is not new, either with PCSK9 knockdown or with statins, which reduce plasma cholesterol by inhibiting the first committed step in cholesterol biosynthesis (the enzyme HMG-CoA reductase).

We have seen this before, and I see no reason to go much deeper here.  But others have made these points about LEQVIO/Patisiran.  Paula Byrne et al. (paywall) expressed concern that the UK government was rushing into a deal with Novartis for a drug that has not really been shown to work as intended.  Andrew N. Bamji, who is a retired rheumatologist asked (another paywall, sorry) “whether PCSK9 inhibitors do anything more than reduce LDL cholesterol,” and concludes that it does not.  Others make the same case,  Dr. Malcom Kendrick, for example, but YMMV.

However, the argument was conceded outright by Novartis in their own television commercials, here and here[10].  At about 47 seconds into each, the line in the purple banner reads: “IT IS NOT KNOWN IF LEQVIO CAN DECREASE PROBLEMS RELATED TO HIGH CHOLESTEROL, SUCH AS HEART ATTACKS AND STROKE.” (emphasis added)

To which one can only sit back and ask Novartis, “Then why should any of your target audience ask for, or their doctor prescribe, LEQVIO?”  Which is what I did the first time I saw the commercial and is why I was prompted to look into this a little further.  Yes, siRNA has great potential for previously undruggable diseases.  “Bad” cholesterol, however, is just cholesterol, which remains an essential component of the membranes in all the cells of our bodies and the starting material for the synthesis of steroid hormones.  As one headline put it, we have spent a trillion dollars on statins and yet cardiovascular disease is still the leading cause of death in the West…

This saga continues.  In a future post, I hope to do more on how basic research has led to effective treatments that should be both inexpensive and widely distributable to all who need them.

_________

[1] Novartis has also run this similar ad for LEQVIO.

[2] Not that mRNA-based vaccines do not have a bright future, but so far, their past and present have been rather dismal, including mRNA vaccines for Zika virus as well as those currently available for COVID-19.

[3] Rather than refer to all of the primary references related to siRNA individually, all of the background material for this post can be found in the review by Friedrich and Aigner mentioned above, which was published in August 2022.

[4] The mechanism of siRNA-mediated gene regulation is illustrated in Friedrich and Aigner, Figure 1.  The optimization of siRNA structure, synthesis , and delivery has many features in common with that required of mRNA as the active component of vaccines.

[5] For future reference, OMIM is the authoritative link for inherited diseases.  It has replaced the 4-volume, 6,000-page The Metabolic & Molecular Bases of Inherited Disease, 8^th edition (2001), which was initially published in one volume in 1960 as The Metabolic Basis of Inherited Disease.

[6] Amyloidosis can be defined as the “insidious deposition of protein fibrils (amyloid) in tissues accompanied by localized or widespread organ failure of in the cardiovascular system, brain and peripheral nerves, kidneys, liver, spleen, skin, endocrine glands, or intestines.”  Alzheimer’s disease is an amyloidosis, but whether insoluble amyloid-beta peptide is cause, correlation, or consequence remains unknown.

[7] Porphyria, which has multiple causes, has been linked to the legend of Dracula.

[8] An oncogene is the mutant gene of a normal cellular protein that in its mutant form causes and/or leads to the progression of cancer from the original neoplasm to the metastatic state, which is what usually kills the patient.  The mutant KRASG12D, which is discussed briefly here, is locked in the “on” position and drives cancer progression.  A current article is here.  TMI, but the rationale for targeting KRASG12D can be skimmed in the Abstract.

[9] I will not list, as I have before, the authors’ acknowledgments of support in this paper from NEJM, but they can be found at the end of the manuscript on p. 1518.  It is a long list of Big Pharma and Little Pharma, and the final sentence reads: “No other potential conflict of interest relevant to this article was reported.”  Okay, then.

[10] Deconstructing these ads could be a project in and of itself.