The long march of antisense
>> Nature Reviews Drug Discovery 10, 401-402 (June 2011) | doi:10.1038/nrd3474
The long march of antisense
Dan Jones
Antisense therapy has generated waves of enthusiasm and disappointment. With a new drug about to be submitted for approval, is it on the cusp of becoming established as a platform technology?
In 1978, Paul Zamecnik and Mary Stephenson reported the first experiments on antisense mechanisms of gene silencing, using short synthetic antisense oligonucleotides to inhibit replication of the Rous sarcoma virus by binding and blocking the action of 35s RNA. In the following years, attention turned to the possible therapeutic applications of antisense technology — yet, more than three decades later, just one antisense therapy has reached the market: Isis Pharmaceuticals' fomivirsen, which was approved in 1998 for retinitis induced by cytomegalovirus, and discontinued in 2004 as the drug's market shrank.
Despite substantial efforts since this approval, clinical trials in this field have not delivered. Two first-generation Isis products — aprinocarsen, which lowers expression of protein kinase Ca (PKCa) for the treatment of various cancers, and alicaforsen, which lowers intercellular adhesion molecule 1 (ICAM1) expression to treat Crohn's disease — reached Phase II/III trial before being dropped because of lack of efficacy. Genta's oblimersen, which targets the mRNA encoding B cell lymphoma 2 (BCL2), was filed for approval in the United States for melanoma and chronic lymphocytic leukaemia (in 2003 and 2006, respectively), but was rejected for both indications.
As the early antisense buzz failed to generate additional products, large pharmaceutical companies lost interest in the approach and smaller biotechnology companies were left to carry the torch. Yet, there are some signs the tide could be ready to turn for 'classic antisense' drugs, which act by binding to target mRNA, blocking protein translation and initiating ribonuclease H-dependent degradation. Last year OncoGeneX and Teva pushed OGX-011, a second-generation antisense agent that was licensed from Isis and targets clusterin, into Phase III trials for prostate cancer. New Phase III data on oblimersen in melanoma — in a targeted patient population — are expected soon. And Isis is set to submit its mipomersen, which targets expression of apolipoprotein B100 (APOB100) and thereby reduces levels of low-density lipoprotein (LDL, or 'bad cholesterol'), for approval in both the European Union and the United States within months.
From product to platform
A key factor in the drawn-out history of antisense therapies, says Stanley Crooke, founder and CEO of Isis, has been the need to develop the antisense approach into a true platform technology. That is, despite the early success with fomivirsen, all the necessary elements were not in place at that time to make multiple products. Indeed, in recent decades, this has arguably only been achieved for one novel therapeutic modality — monoclonal antibodies (mAbs). Crooke therefore didn't expect a flood of new antisense drugs in the immediate years after fomivirsen's introduction, and nor did others in the industry. “Fomivirsen was widely seen as a one-off approval,” says Raymond Warrell, Chairman and CEO of Genta. “mAbs were widely seen as the magic bullets of the 1980s, yet failed to deliver for many years — so time-wise, I don't think the emergence of antisense therapies is peculiar”, he adds.
mAbs were widely seen as the magic bullets of the 1980s, yet failed to deliver for many years — so time-wise, I don't think the emergence of antisense therapies is peculiar.
To create a viable antisense platform, pioneers in the field have had to integrate new technical knowledge about antisense drugs with an increasing understanding of the biology of their targets, within the context of an evolving regulatory environment. And such a process naturally involves some early missteps, which, for drug development, are timely and costly. “You can't really learn about a technology until you get some drugs into development, because it's only then that you really find out what the issues are,” says Crooke. “This means making some bets on early candidates, which may not pan out, and then trying again.”
In developing antisense into a platform technology, much attention naturally focused on the design of antisense oligonucleotides. Sudhir Agrawal, CEO and President of Idera Pharmaceuticals, says that from day one there have been three fundamental issues: stability, delivery and off-target effects. With regard to stability, for instance, natural oligonucleotides have a phosphodiester backbone that is susceptible to degradation by nucleases, prompting Agrawal and Zamecnik to develop the first degradation-resistant synthetic oligonucleotides — phosphorothioate oligonucleotides (PSOs). Subsequently, the team developed the second-generation chemistry, which relies on 2'-O-substitutions to provide greater stability and minimize off-target effects. These have been licensed to Isis, which is now developing 'generation 2.5' molecules.
As oligonucleotide chemistry has advanced, new classes of antisense drugs — which modulate gene expression by harnessing routes other than the combination of ribonuclease H degradation and steric blocking — have also emerged (Box 1).
Box 1 | The many faces of antisense therapy
Full box
More than just chemistry
Despite the advances in oligonucleotide design, the importance of these new chemistries for turning antisense into a successful platform technology remains debatable. “The older chemistries may be adequate if you fool around with them enough,” says Sidney Altman, a molecular biologist at Yale University, Connecticut, USA. Warrell, whose company uses the early PSO technology, similarly argues that oligonucleotide chemistry has not been the fundamental obstacle within the field. “I don't view the generations of antisense chemistries as very important,” says Warrell. “We think that the first-generation PSO chemistry is fine and has acceptable targeting and binding characteristics.”
I don't view the generations of antisense chemistries as very important.
Agrawal points out, however, that the off-target effects of some antisense compounds can be problematic. “PSO compounds interact with a number of cellular factors, including Toll-like receptors (TLRs), which detect free-floating DNA or RNA and induce an immune response that manifests as flu-like symptoms,” says Agrawal. “If you're working with an antisense compound, you need to be sure that its activity is only related to its antisense mechanism of action, and not because of interactions with TLRs.”
Others in the industry point to more general — rather than antisense-specific — issues as the major obstacles to success. One fundamental challenge, says Warrell, has been getting a better handle on the underlying biology of diseases. “If you're going to have an effect with a single antisense agent, which attacks only a single gene product, you have to be sure that your target has an absolutely central role in the complex biology of the disease you're trying to treat,” says Warrell.
Target criticality — rather than general problems of off-target effects and/or toxicity — appears to have played a crucial part in the failure of some of Isis's early antisense compounds as well. Crooke says that aprinocarsen, Isis's anti-PKCa drug, could be given at very high doses as its side effects were modest compared with other anticancer agents. “But it failed because PKCa was a bad choice of target,” says Crooke. “It's just not very significant in the malignancies that we looked at.”
The usual problems with clinical trial design and regulatory requirements have also held up the field. The failure of alicaforsen, Isis's ICAM1-targeting drug for Crohn's disease, probably relates to the clinical trials Isis ran, says Crooke. “We learned afterwards that we probably ended up with patients who had irritable bowel syndrome, rather than Crohn's.”
The history of Genta's oblimersen also underscores the importance of the regulatory environment in explaining some late-stage setbacks. Oblimersen, a first-generation PSO, blocks the production of BCL2, a central regulator of apoptosis. BCL2 is overexpressed in numerous cancers and protects cancer cells against the cytotoxic effects of various therapies, so giving cancer patients oblimersen ahead of these therapies should, in theory, maximize their cell-killing power.
Despite preclinical validation for this approach, oblimersen failed to clear clinical and regulatory hurdles. Oblimersen missed its primary end point of overall survival in melanoma, but was nevertheless filed in this setting in 2003. “Today, in 2011, no one would submit having missed their primary end point, but in 2003 it was not clear that this would be a fatal issue,” says Warrell.
Genta has nonetheless learnt some important lessons about their patient population and is running an additional Phase III trial that used a biomarker to select patients who are most likely to benefit. Results are expected in the near future.
Light at the end of the tunnel?
Another product leading the field is Isis's mipomersen, a second-generation oligonucleotide that targets the expression of APOB100 — an essential protein for the formation of LDL, which is a major contributor to coronary heart disease. “We picked APOB100 as it is a genetically validated target and it is also made in the liver, an organ where we knew that about 12% of any dose we give accumulates,” says Crooke.
Mipomersen is being developed in collaboration with Genzyme (now Sanofi) for patients with severely high LDL despite receiving maximum lipid-lowering therapy, including patients with familial hypercholesterolaemia (FH). So far, late-stage clinical data from the drug have looked promising. In a Phase III trial in 51 patients with homozygous FH (HoFH), for instance, the drug reduced LDL levels by 25%, whereas placebo only reduced levels by 3% (Lancet 375, 998–1006; 2010). Although increased levels of liver enzyme in this and other trials have raised concern among some analysts, Crooke points out that most of these cases resolved with ongoing treatment and all resolved after treatment discontinuation. After several delays to its timeline — driven in part by regulatory requests for more data to support a submission — the drug is due to be filed for approval with European regulators this summer for HoFH and severe heterozygous FH, and with the US Food and Drug Administration later in the year for HoFH.
If mipomersen is approved, it may not only validate antisense as a therapeutic modality, but also may be the first step to demonstrating the commercial viability of such products. Although the initial focus for mipomersen is on patients with very high-risk HoFH, the company hopes to eventually expand the drug's market to include the much larger population of patients with high LDL levels. Similarly, if the recent trials of oblimersen and OGX-011 produce positive results, antisense may finally have come of age as a genuine platform technology. << |
