Alnylam GalNAc Improvements Incremental, but Likely Enough to Beat PCSK9 Antibodies

Over the last week, Alnylam presented pre-clinical data for their new development candidates for the treatment of hypercholesterolemia (ALN-PCSsc) and liver disease related to forms of alpha-1 antitrypsin deficiency (ALN-AAT).  These candidates are based on second-generation GalNAc-conjugate chemistry that the company is now dubbing ESC (Enhanced Stabilization Chemistry).  They involve the increased use of chemical nucleic acid modifications for stability with attendant improvements in knockdown potency and duration over first-generation GalNAc conjugates such as ALN-TTRsc.  ALN-TTRsc is the lead GalNAc candidate and currently in phase II development.

Borderline first-generation GalNAcs

In a phase I study of ALN-TTRsc, potent knockdowns were achieved with about:

-60% knockdown at 2.5mg/kg;

-80% knockdown at 5.0mg/kg (ED80), and

-90% knockdown at 10.0mg/kg.

At the risk of insulting medical geneticists for oversimplifying, assume that an RNAi technology that can safely achieve an 80% target gene knockdown provides for a solid platform.  In the case where a subcutaneous route of administration is desired and/or necessary, this should ideally also fit into a 1ml injection volume which in the case of GalNAc conjugates would correspond to a 2.5mg/kg dose.

There is some controversy around acceptable injection volumes and as often is the case, increased standards are applied to RNAi Therapeutics.  I say this because drugs and drug candidates such as expected mega-blockbuster PCSK9 antibody from Amgen, AMG-145, has been administered at 2ml volumes.  In fact, to achieve once-every-4-week dosing, 6ml (3x2ml) have been administered (see Giugliano et al. 2012).

In light of this, ALN-TTRsc has failed the 1ml test, but 80% are certainly possible with this first-generation GalNAc chemistry.  There is therefore room, and in some cases a competitive need (àcompetition with more potent delivery technologies such as Tekmira’s SNALP LNPs and Arrowhead’s single molecule DPCs) for improvements in GalNAc conjugation technology.    

Second-generation data could indicate progress

In agreement with this, Alnylam is now advertising the ESC second-generation GalNAc and has presented critical non-human primate data for their new development candidates ALN-PCSsc and ALN-AAT.  Non-human primate data are ‘critical’ because the RNAi knockdown observed in monkeys typically closely predicts what will be seen in humans based on the experiences with ALN-PCS (SNALP), ALN-TTR (SNALP), and ALN-TTRsc. 

In interpreting the newly presented data, it should be noted that in the case of ALN-TTRsc, an 80% knockdown was already observed at 2.5mg/kg (and 5mg/kg) in non-human primates, but that this shifted to 5mg/kg in humans.  Numerically a relatively small difference, in practical terms an important one.   

ALN-PCSsc achieved 80% PCSK9 target gene knockdowns at 2mg/kg in a weekly multi-dose study in non-human primates.  This resulted in a highly competitive 60% LDLc lowering in the absence of statins.  Similarly impressive in light of the monoclonal antibody competition was that 80% PCSK9 knockdown and 50-60% LDLc reductions were achieved and sustained for over 3 months (!) when a single dose of 10mg/kg (2x2ml) was given.  Compare this to 57% LDLc lowering with AMG145 in a once-every-4 week regimen in the MENDEL-2 phase III study of AMG145:

-ALN-PCSsc (RNAi): 50-60% LDLc reduction, 2x2ml subcutaneous, once-every-3-months

-AMG145 (monoclonal antibody): 57% LDLc reduction, 3x2ml subcutaneous, once-every-4-weeks (phase III MENDEL-2 monotherapy study)

(Yes, I do keep an eye on cash-rich, sub-$2B market cap The Medicines Company, Alnylam’s licensee for ALN-PCSsc for this reason).

Before I get carried away with all the advantages of the RNAi platform over monoclonal antibodies for even extracellular targets such as PCSK9, in terms of GalNAc improvements, these knockdown results are very much in line with what was seen for TTR in non-human primates. 

The same applies for ALN-AAT where a single dose of 3mg/kg translated into a 60% knockdown which, based on TTR and PCSK9, will likely translate into a weekly multi-dose ED80 of 2.0-2.5mg/kg in non-human primates.

However, with the caveat of different half-lives for different target genes, the durability of the PCSK9 knockdown is quite impressive and more than what is typically seen with e.g. SNALP LNPs which have historically utilized minimally modified RNAi triggers.  Such triggers may be turned over more rapidly in liver cells.

Similarly, the RNAi trigger sequence in the TTR development candidate is an extraordinarily potent one with low single-digit picomolar EC50 potency in tissue culture.  It is therefore possible that equivalent animal potencies have now been achieved with less potent sequences meaning that the underlying delivery technology has improved.

It’s been fascinating to watch the various delivery platforms, and indeed RNaseH antisense, compete over the years and pushing each other to new heights.  In this context, I am anxiously waiting for Arrowhead’s presentation at next week’s TIDES meeting with the intriguing title:  “Next Generation Dynamic Polyconjugates for siRNA Delivery in vivo,”

Market commentary: I remain mostly on the sidelines.  Years ago, these scientific data would have made me jump head-first into the market with me buying all I could in the opening minutes.  These days, however, I view them as ensuring the long-term value of RNAi stocks, but fail to see how they will support share prices in the current, growth-to-value rotating trading environment for more than a trading week or so.  I am therefore speculating on a capitulation event before some of the important clinical data read-outs roll in.  

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 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.