RNAi Also Conquers the Central Nervous System

In the span of a day, RNAi Therapeutics have gone from a mechanism widely viewed as being constrained to the liver only, to a major therapeutic modality for many targets and indications in a variety of tissues.  Due to the demonstrations in the liver, lung (yesterday), and today the central nervous system to potently and specifically knock down genes with infrequent dosing, RNAi will play a prominent role in today’s precision medicine-oriented drug development.

Employing C16 lipid-conjugated, chemically stabilized RNAi triggers, Alnylam and their partner in CNS drug development Regeneron achieved 84-90% maximal target gene knockdown with knockdown persisting at >70% for at least 3 months after a single dose.

Since chemical stability has been key to the successes in the lung, CNS, and also liver, it seems very likely that similar breakthroughs will be achievable for muscle, kidney, adipose tissues, and (in the words of Alnylam's President) 'even tumors' that Alnylam and Arrowhead are working on.

The initial target in the phase I study of ALN-APP was amyloid beta precursor protein (APP). Unlike the armada of antibodies that have targeted every known aggregation form of abeta for the treatment of Alzheimer’s, ALN-APP reduces them all and before they are even made thereby offering a unique angle to this important target.  An even more exciting near-term application of ALN-APP in my opinion is for cerebral amyloid angiopathy (CAA) where abeta accumulation near blood vessels can lead to intracerebral hemorrhage.  Studies with antibodies in Alzheimer’s have actually led to fatal damage to those very intracerebral blood vessels by causing local inflammation, and thereby make them a bad choice for CAA.

Beyond abeta and tau for Alzheimer’s, the CNS in particular abounds with otherwise difficult-to-drug important targets for diseases like Parkinson’s, Huntington’s, ALS, spinocerebellar ataxias for which RNAi is ideally suited.

The prolonged and robust knockdown observed is significantly better than what has been observed for previous RNaseH antisense candidates such as against SOD1 and tau (~50% target gene lowerings).  Safety also appears to be superior to the broadly phosphorothioated antisense molecules with no changes in neuronal markers of damage and inflammation seen with ALN-APP compared to placebo.

The US FDA though slapped a clinical hold on the multi-dose part of the trial based on findings in standard preclinical animal tox studies at doses well above what will be needed in the clinic.  The single dose exploration study, however, has been allowed to continue, and Canada has already allowed the multi-dose part to go ahead.  It therefore seems highly unlikely that the findings could derail ALN-APP or even this technology approach at this point.

With the recent news, the pharmaceutical landscape has changed and Big Pharmaceutical companies will have to think hard whether not having a stake in RNAi as a platform is viable.  The achievement is also one of delivery and stabilization chemistry which can be more broadly applied to other oligonucleotide therapeutics modalities in the CNS.

An RNAi Therapeutic Could Be Last Man Standing in PCSK9

PCSK9 has arguably become biotech’s hottest drug target as it promises to unlock much of the remaining unmet need in lowering ‘bad’ LDL cholesterol.  While monoclonal antibodies have been the predominant and most advanced modality to address PCSK9 (e.g  alirocumab Regeneron/Sanofi-Aventis and AMG145 by Amgen), RNAi Therapeutics have made tremendous, yet widely underappreciated progress in this area with predicted potencies equivalent to if not superior to monoclonal antibodies.

Alnylam’s ALN-PCS leads in this effort (preclinical 67% LDLc lowering without statins) and should have entered clinical development by the beginning of 2015.  It is partnered with The Medicines Company.

The news today that the FDA has become concerned about neurocognitive adverse events in the ‘PCSK9 class’ could mean that RNAi Therapeutics, despite their development delays, will carve out a nice junk of the PCSK9 market which is widely estimated to be a multi-billion dollar market in terms of annual sales.

But if it’s a ‘PCSK9 class’ issue, so how could this be positive for RNAi Therapeutics?  Wouldn’t this concern also apply to ALN-PCS?

Not necessarily.  Since by far the most clinical experience with PCSK9 has been obtained with monoclonal antibodies, the FDA may be inappropriately lumping all PCSK9 inhibitor agents in one bucket confusing a short-coming of a therapeutic target with a short-coming of monoclonal antibodies.

The reason why I have a good feeling that this turn of events could be very positive for RNAi Therapeutics is that RNAi is a genetic tool that reduces PCSK9 expression before any protein is made.  The approach therefore resembles populations that under-express PCSK9 for genetic reasons and which enjoy improved cardiovascular health compared to the general population without any apparent negative consequences of their PCSK9 deficiencies.  It is for this very observation that PCSK9 has become such a sought-after target and RNAi, a gene knockdown approach, best mimics human genetics.

Monoclonal antibodies don’t.  One mechanism by which antibody-specific toxicity might arise from is the fact that they form complexes with their target.  Such complexes could elicit adverse immune reactions in organs where they form and accumulate.  While I am not familiar with the binding sites of the particular Regeneron and Amgen antibodies to PCSK9, it might also be possible that the immune complexes form directly on neuronal cells expressing LDL-receptor-like proteins (note: PCSK9 binds the LDL-receptor).

Beyond PCSK9, today’s events emphasize yet another hitherto under-appreciated value proposition of RNAi Therapeutics, namely providing differentiation value when going head-to-head with the small molecules and monoclonal drug establishments.

New Clinical Results Put ALN-PCS Firmly in Business in PCSK9 Race

Alnylam reported on Friday updated results from the phase I single-dose, dose-escalation trial of its hypercholesterolemia candidate ALN-PCS02.  Importantly, the data demonstrate that ALN-PCS can produce clinically meaningful 30-40% reductions in LDLc (the ‘bad’ cholesterol) for much of the 28 study days of the study after only a single infusion of the drug.  This bodes well that ALN-PCS may be useful for the millions of patients that cannot achieve their LDL cholesterol goals despite the availability of statins.

After presenting solid PCSK9 knockdown results in January from subjects receiving up to 0.25mg/kg of the PCSK9 siRNAs which are critically enabled by Tekmira's SNALP delivery technology, the latest data include the results from subjects receiving 0.4mg/kg PCSK9 siRNAs after the decision had been made to increase the top dose based on the favorable safety-efficacy profile thus far.  This was important because despite the solid plasma PCSK9 reductions seen up to 0.25mg/kg (roughly 50% mean reductions throughout the 28 days), both the peak and average mean LDLc reductions until then had been less than 30% and 20%, respectively.  Probably not enough to effectively compete in the hot PCSK9 field.  

Based on the latest data, which included impressive >80% target plasma PCSK9 reductions, it is reasonable that ALN-PCS will be able to achieve average (=sustained) ~45-50% LDLc reductions in a once-every-4-week dosing regime.  This would put it just slightly behind Regeneron’s PCSK9-targeting monoclonal antibody REGN727 in terms of LDLc lowering.  It may be more difficult for ALN-PCS to achieve the LDLc lowering efficacy of Amgen’s PCSK9-targeting antibody AMG145 which has been hailed at the recent ACCconference to produce LDLc lowering of ‘up to’ ~60-80% at apparently pristine safety and tolerability.  I should add, however, that the warts-and-all data on AMG 145 have yet to be made publicly available.

In light of the intense competition in the PCSK9 space and a possible potency disadvantage for this surrogate marker, it will be interesting whether, firstly, the pharmaceutical industry (note, a larger partner will have to be found to financially shoulder the unavoidable outcomes trial), and eventually the market-place will adopt this new class primarily for their absolute LDLc reducing ability or in order for patients to achieve their lipid goals.  Some LDLc proponents, of course, stress that the lower the LDLc, the better.  On the other hand, whether a drug can lower LDLc by 50% or 70% won’t matter for many patients when it comes to simply achieving standard LDLc goals of 100mg/dL or even 70mg/dL.  In that environment, the focus in making the choice between competing drugs would shift to factors beyond mere LDLc lowering potential, particularly safety.

While ALN-PCS is mainly suffering from the fact that it is given following pre-treatment with corticosteroids, something that also had important impacts on the PCSK9 and LDLc efficacy data, the once much-touted safety profile of the PCSK9 antibodies have taken a significant hit with the publication of phase II data from REGN727 (McKenney et al. 2012).  In addition to one serious adverse event, a case of potentially life-threatening inflammation of blood vessels (leukocytoclastic vasculitis) that, however, readily resolved after giving corticosteroids, there was an apparently dose-dependent, adverse event-related increase in the rate of discontinuations, with ~15-20% discontinuing treatment in the biweekly dose cohorts in study 11565.  Next to the case of vessel inflammation, the AE-related discontinuations involved cases of neutropenia, fatigue, injection site rashes, chest pain, and headache/nausea. 

Amgen has yet to publicize more detailed data for AMG145.

Next to multi-dosing and the safety and efficacy impacts of omitting corticosteroids in future studies, another important question for ALN-PCS that remains to be answered in the upcoming trials will be testing the candidate in patients on statins.  REGN727 encountered problems here as patients concomitantly taking statins experienced an LDLc rebound effect after around 2 weeks.  Whether this is a class effect of agents merely blocking extracellular PCSK9 (such as antibodies), instead of inhibiting the synthesis of both intra- and extracellular PCSK9 such as ALN-PCS is an interesting question.  

This mechanistic difference could also be the big wildcard when it comes to the ultimate arbiter of clinical utility, the outcome trials: despite the clear evidence that LDLc levels are inversely related with cardiovascular events, with all the complicated feedback loops of lipid biology and related diseases, mechanistic differences could easily erase 20% differences in LDLc lowering.  An indication that these theoretical considerations are translating into the clinic may be deduced from the observation that (in Alnylam's words) ‘importantly, ALN-PCS demonstrated consistent clinical activity toward both PCSK9 and LDL-C independent of baseline levels of PCSK9, highlighting the unique mechanism of action for a PCSK9 synthesis inhibitor.’

For Alnylam, the near-term challenge will be thus to find a large partner, probably without a stake in PCSK9 yet, willing to take the risk with RNAi Therapeutics mainly for the differentiation it offers over the other PCSK9 approaches.  A stumble of the monoclonal antibody class could easily mean that ALN-PCS becomes the lead candidate in the PCSK9 market, estimated by many to be a multi-billion dollar one. 

New Antibody Data Indicate Tough Battle for RNAi Therapeutic PCSK9 Approach

The past week has been a busy one for the hypercholesterolemia field. New clinical data for PCSK9-targeting monoclonal antibodies from Amgen and Regeneron/Sanofi-Aventis were presented at the 2012 American College of Cardiology meeting, while ISIS/Sanofi-Aventis separately presented long-term efficacy, and especially safety data on the ApoB antisense compound mipomersen (aka KYNAMRO) for which marketing applications have now been submitted in Europe and the US. As hypercholesterolemia also represents a significant medical and commercial opportunity for RNAi Therapeutics due to the advances in knocking down genes in the liver, I will briefly summarize the new data and discuss some of the implications for RNAi approaches, including the phase I candidate ALN-PCS02 by Alnylam.

If there ever were doubts as to the commercial attractiveness of PCSK9 as a target for treating high cholesterol, last week dispelled them all. When it seems that all the thought leaders in the field, including famed cardiologist Steven Nissen, otherwise known for his ultra-critical views of certain medicines, and Wall Street (expected annual sales as a class of up to $20B thrown out) hail the data as proof that PCSK9 will be the final nail in the hypercholesterolemia coffin and thus throw their support behind the class, the commercial success of PCSK9-targeting therapeutics seems a foregone conclusion.

Although pretty much every large pharmaceutical company sports a PCSK9-targeting antibody, the excitement this time centered around the phase II data of Regeneron’s/Sanofi-Aventis’ REGN727 and phase I data of Amgen’s AMG145.

The REGN727 phase II 8-12 week multi-dose studies were conducted in close to 300 patients with elevated ‘bad’ LDL-cholesterol (LDLc) on statins. Depending on the amount and schedule of antibody administered, mean LDLc reductions (note: it is not entirely clear to me whether this refers to the LDLc reduction over time, or the peak LDLc reductions) of 40-73% were achieved. Interestingly, one press report said that ‘727 suffered from relatively short-lived activity such that subcutaneous administrations every 2 weeks would be required.

The phase I studies with ‘727, just published in NEJM (Stein et al.) indicated that the apparent rebound effect after 2 weeks may be due to the concomitant use of statins and that without statins, 50-55% persistent LDLc reductions can be achieved with close to 4-week dosing intervals. This is relevant for example for the statin-intolerant population. With statins, that type of persistent knockdown (more on the higher end of that range) would probably require every 2 week dosing.

AMG145 seemed to attract even more excitement than '727. In the 6 to 8-week phase I studies in around 100 patients taking statins, mean LDLc reductions of 63-75% were observed. These numbers indicate more potent, and likely more persistent LDLc lowering compared to REGN727, although without having seen the LDLc response curves over time, it is difficult to conclude that for sure.

Route of administration and dosing frequency are often cited as important competitive criteria for this class of drugs. In the case of ALN-PCS02 which is enabled by Tekmira’s SNALP technology, the current data indicate that a range of 40-55% persistent LDLc reductions with intravenous dosing every 4 weeks are conceivable. This, however, would require further improvements in potency over the 0.25mg/kg dose level which was the highest dose for which data was presented by Alnylam at their early January update. For this indication, it will also be important to wean the SNALP formulation off the transient immune suppression currently used. This would not only address criticisms that such immune suppressions carry risks, but it also would likely get rid of the observed PCSK9 rebound effect that was apparently linked to it and would thus contribute to prolonging the efficacy.

Of course, an important wildcard in the competition between MAbs and RNAi pertains to the safety of each drug candidate, especially when they are used long-term. The monoclonal antibodies seem to perform quite well in this regard in the short-term, albeit multi-dose studies. RNAi, however, has the theoretical advantage in that it does not involve the formation of antibody-PCSK9 complexes which could eventually have an impact on both long-term safety and efficacy/dosing frequency.

In retrospect, RNAi has picked a tough battle here. As a target that acts extracellularly, readily accessible from the blood, it seems an ideal target for monoclonal antibodies and thus falls outside the undruggable target space that currently still drives the interest of large pharmaceutical companies in RNAi Therapeutic development. On the other hand, the PCSK9 field is populated with antibody approaches, and should there be antibody-specific class adverse effects, RNAi could be the last one standing (note: the antisense candidates by ISIS and Santaris have already dropped out of the race, at least for now), something that one of the few Big Pharma/Biotech that has not yet invested in a PCSK9 candidate may value from a strategic point of view alone (RE partnering).

Meanwhile, the mipomersen extension study data presented this week suggest that the high drop-out rates and liver fat accumulations* (both possibly linked to some extent) at modest ~30% LDLc reductions make it a niche drug for the very small indication of homozygous familial hypercholesterolemia in the US and Europe, and possibly severe hypercholesterolemia in Europe. Although ISIS continues to claim that the liver fat accumulations normalize after reaching maximum median increases of +13% over baseline at week 52 (note: according to the mipo-related Visser et al. 2009 study, 5.6% absolute intrahepatic triglyceride contents are considered the upper-limit of normal), the absolute values would call for caution, especially when there are outliers in whom almost half the liver consists of fat. In the related conference call, ISIS further claimed that the apparent normalizations were not the result of the patients that discontinued mipomersen (possibly due to high liver fat contents). However, since ISIS has a history of erring on the side of optimism when it comes to mipomersen safety and tolerability, I’d like to see the liver fat-related sub-group analyses that take into account the discontinuations, dose reductions and interruptions.

The liver fat data may also mean that ApoB is unlikely to be a suitable stand-alone target for an RNAi Therapeutic, especially if even more pronounced ApoB knockdowns can be achieved than with mipomersen. Consequently, in order to exploit the therapeutic utility of ApoB as the critical protein of atherogenic lipoproteins, an ApoB-targeting RNAi Therapeutic should involve at least one other target gene which will also reduce liver fat content. It remains to be seen whether ApoC-III can be that target.

* Note: Despite the caution expressed in this article, I should add that there is general controversy about the relevance of simply elevated liver fat (NAFLD) in the absence of inflammation.

ALN-PCS02 Results Provide Glimpse of First RNAi Therapeutics Blockbuster

If the experts at Roche and other Big Pharma companies got you convinced that RNAi Therapeutics was about to disappear from the scene by pulling the plug on the technology…think again. Alnylam this morning reported tantalizing insights into the safety and efficacy of ALN-PCS02, a SNALP-formulated RNAi Therapeutic targeting PCSK9 for the treatment of hypercholesterolemia. Given that PCSK9 is viewed as potentially the next biggest thing in cardiovascular drug development after statins have come off patent, and considering the results so far from the monoclonal antibody and antisense competition, this RNAi Therapeutic has the potential to become the field’s first blockbuster. As we know, a field does not need too many blockbusters to become more widely accepted, and it is acceptance, not so much lack of scientific progress, which has been lacking over the last 2-3 years. Together with the ALN-TTR01 results reported in November, I expect these results to dramatically change perceptions about RNAi Therapeutics in the pharmaceutical industry. And if I had to write the script, Alnylam will announce a share offering tomorrow or Friday afternoon in order to go hostile on Tekmira to remove the uncertainties arising from their use of Tekmira’s SNALP technology which critically enabled these results and on which most of Alnylam's $400M market cap depends on.

ALN-PCS02 Phase I Data

This trial was a simple single-dose escalation trial in subjects with slightly elevated levels of bad LDL cholesterol. Dosing in the 5 dose cohorts commenced at 0.015mg/kg and went up to 0.250mg/kg with each cohort consisting of 3 subjects receiving an intravenous infusion of ALN-PCS02 and 1 receiving placebo. As could have been expected with this particular rationally designed '2nd gen' SNALP formulation, robust target protein knockdown was observed at the highest dose tested with a mean 60% reduction in plasma PCSK9 levels presumably 3-5 days after administration. In line with PCSK9 genetics, this type of knockdown entailed a mean 39% reduction in bad LDL cholesterol. Importantly, and again consistent with the wealth of non-human primate data, this knockdown was sustained for days and weeks. Given that ALN-PCS02 apparently was well tolerated, with the only noted adverse event being a rash that was likely to be related to the mode of administration and not the drug itself, Alnylam now plans further dose escalation to a) increase knockdown potency, but b) also extend the duration of PCSK9/LDLc suppression. This is particularly desirable in light of the competitive situation.

The PCSK9 Competition

Needless to say, having now two examples in short order where RNAi Therapeutics unambiguously hit their targets, proof-of-concept of technical success can be claimed and one can now focus on the medical and commercial potential of these therapies. In this regard, the competition in the PCSK9 space is fierce. Big Pharma and Big Biotech have fielded their A-teams to develop monoclonal antibodies against PCSK9. Since much of the LDL-related effect is due to PCSK9 in the serum, this target is a particularly amenable to MAb technology.

Sanofi-Aventis/Regeneron are most advanced with REGN727 which is in the middle of a number of phase II studies. Amgen also reported recently phase I results with their PCSK9 MAb. Studies with both compounds have shown more potent LDLc reductions of around 65% compared to the 39% with ALN-PCS02 so far. This, of course, is a good reason for Alnylam to go ahead with its plans and continue dose escalation. However, once you achieve the type of 60-70% reductions in LDLc, the next major focus becomes safety, and so far the RNAi Therapeutic looks quite competitive in that regard.

As discussed in October, the PCSK9 antisense compounds by Santaris and ISIS have fallen a bit behind due to probably both safety (Santaris) and efficacy (ISIS) issues, although I expect ISIS to reciprocate tomorrow by presenting TTR knockdowns at their Investor Day tomorrow that are more pronounced than Alnylam’s first attempt with ALN-TTR01.

[Update 5Jan12: ISIS reported 44% mean TTR reductions at 200mg per week, very similar to ALN-TTR01 at 1mg/kg; and 81% reductions at 400mg per week, a dose that I consider too high for tolerability and commercialization; ISIS also disclosed that the BMS-partnered PCSK9 candidate was dropped citing a slow partner and regulatory concerns about PCSK9 as a drug target].

Overall, we can now conclude that the ALN-TTR01 results were no fluke (I never expected that to be the case anyway) and that in theory all targets in the liver are fair game now for RNAi Therapeutics. Moreover, the data show that the preclinical data with SNALPs translate very well into humans, removing a considerable uncertainty and opening up the prospect of further improvements in SNALP delivery based on what Tekmira and Alnylam have presented over the last few months.

A biotech company does not need many these PCSK9-type products to rise into the ranks of Big Biotech. While there is still a long clinical way to go, I believe that sometimes you are allowed to cheat and look at hard preclinical data, and in doing so I believe that today we may have seen the first glimpse of an RNAi Therapeutics blockbuster. I’m wondering how the Roches and Pfizers will be feeling over the coming months and years as the field of RNAi Therapeutics matures at this rate. It is also an opportunity for new entrants to build strong positions in the space with relatively small investments (still). Am I a bit vindictive today? Yes, definitely.