Wednesday, April 29, 2015

HBsAg Rules at International Liver Congress

Now that the HBV world has gathered extensive clinical experience with interferons and polymerase inhibitors (NUCs) and with the resolve to finally find a cure for a serious disease afflicting hundreds of millions worldwide, the hepatitis B surface antigen (HBsAg) has become recognized as the key determinant for treatment outcome.  This is being confirmed by trial after trial investigating combining the actions of both NUCs and interferons, either one after the other or together at once.

Some of these studies were presented at the International Liver Congress last week in Vienna (ILC2015).  Emerging from them are actionable HBsAg rules which can predict fairly well whether a patient will eventually seroconvert to (or at least lose) HBsAg.  No matter the excitement around CRISPR technologies, HBsAg seroconversion remains the gold standard outcome in HBV treatment in the foreseeable future.

These rules can be divided into pre- and post-IFN treatment onset.

In the pre-IFN setting, it is those patients that have below ~500 IU/ml serum HBsAg as a result of NUC treatment that will most likely respond to interferon treatment/immune stimulation with s-antigen seroconversion (see earlier blog entry).  Since NUCs alone hardly do anything to promote s-antigen seroconversion despite its dramatic lowering of viral HBV titers, it appears to be their slow impact on HBsAg levels ( 0.1 log per year HBsAg reduction) that has the synergistic effect with interferon: with HBsAg lowering you take off the foot on the immune brake, with interferon you step on the immmune gas pedal.

As such, HBsAg knockdown by RNA(i) Therapeutics would seem to do the same for interferons as NUCs do, only in a more rapid and potent manner.  Of course, both could be used concurrently as a run-in to IFN treatment.

However, once on IFNs (post-IFN onset), it is the relative HBsAg decline that has high positive predictive value in prognosticating who will seroconvert.  Of note, the HBsAg decline comes before any adaptive immunity can be detected.  This supports that HBsAg decline in itself contributes to seroconversion rather than it being a mere correlation.  In that setting, it is a 1 log decline in HBsAg the first few weeks after IFN treatment onset that separates the winners from the losers. 

It is uncertain to me, however, whether 1 log is a precondition to s-antigen seroconversion as the non-responders do not even come close to that (maybe 0.3log).  It is therefore possible that anything that pushes HBsAg below say -0.3-0.5log could have a dramatic effect on s-antigen seroconversion rates.

An RNAi Therapeutic for HBV used simultaneously with IFNs may therefore aim at helping IFNs to get to the  0.5-1log reduction threshold, and rapidly at that.

Ergo, there are now a number of obvious strategies that one can apply regarding the use of RNAi Therapeutics in HBV with various knockdown goals, both absolute and relative.  The exact strategy would depend on how the RNA agent is combined with polymerase inhibition/NUCs or immune stimulation. 

While a number of other HBV targets were reported at the conference such as core assembly and entry inhibitors, HBsAg (and HBV mRNA knockdown in general) lowering remains the most distinguished and the mechanism predicted to be most synergistic to existing treatment approaches.  As combination treatment is strongly predicted to be the future of HBV, HBsAg lowering should become a pillar of those treatment regimes.

Disclosure: long ARWR, looking for lower entry in TKMR.

Monday, April 27, 2015

Regulus Therapeutics RG-101 Continues to Have Potential in HCV Treatment Landscape

Having attended the International Liver Congress last week in Vienna, Austria, it has become clear to me that HCV is not going away soon.  Even in the US where progress towards its ‘eradication’ may be considered most advanced with about 1/5 of the known patient population treated last year alone (~250k), it will be an uphill battle to identify, treat, and pay for the millions more infected. Worse still, it is not enough to simply cure the existing pool of HCV patients, but also stop the cycle of re-infection (largely the result of injection drug use).

Treatment cost is one challenge and the current drug rationing approach leads to the counterproductive warehousing phenomenon where current medical intervention is focused on the patients with more advanced liver disease.  This is not only the hardest-to-treat population, but also allows the liver health of the previously less sick patients to deteriorate.  This obviously makes little sense also from a pharmaco-economical perspective when getting rid of HCV early on has now been shown over and over again to dramatically reduce HCV-related cirrhosis and liver cancer.

Another problem is the fractured treatment landscape that exists for the various patient populations (split up according to fibrosis/cirrhosis stage, genotype, co-morbidities, the rapidly growing concerns around drug-drug interactions, available/accessible meds etc) making it difficult for even the learned gastroenterologist to keep up with the latest developments and putting HCV treatment practically out-of-reach for the general practicioner.  

12-24 weeks remains the standard drug treatment duration with docs worrying about shorter treatment regimens being sub-optimal.  A triple regimen by Gilead after 4 weeks of treatment merely achieved a 27% SVR12 which in the DAA world is practically synonymous with a cure.  The best shot at shortening treatment duration may therefore come from Achillion with 6 week of DAAs achieving high cure rates in 'easy' patient populations.

While interferon is on the way out, it could make an at least transient comeback for the hard-to-treat genotype 3 where, in addition to cirrhosis, cure rates with the all-oral DAAs low (60-80%).

To sum it up, the liver community has expressed multiple times at ILC2015 that a short-acting pangenotypic regimen is an important goal in the development of new medications for HCV.   And if they paid attention at the oral late-breaker on Saturday, RG-101 is poised to play a critical role in filling this unmet treatment goal due to its long duration of antiviral activity following administration, regardless of genotype.

RG-101 update: more relapses, but thesis intact

The clinical investigators of Regulus Therapeutics presented a 20 week update on the phase I study in genotype 1, 3, and 4 patients with good to moderate liver health.  28 patients received study drug RG-101, 4 placebo.

At the primary endpoint on week 8 (reported in earlyFebruary), slightly more than half (15/28) of patients treated with RG-101 were below the level of quantitation (BLOQ).  According to the company, most of them were not only BLOQ, but undetectable (by sensitive PCR) at that.  This is a remarkable feat given that the GalNAc-conjugated phosphorothioate antisense molecule had been only given once.

According to the latest update, half of those patients eventually relapsed (7-8 depending on whether you count the patient that was lost to follow-up), most of them shortly after week 8.  Although the relapsers are slightly disappointing as in the short-acting DAA world undetectable virus for 8 (or better 12, SVR12) weeks following cessation of treatment is more or less equivalent to a full-blown cure.

Of course, the prognostic rules for a long-acting agent like RG-101 with a slower onset of antiviral knockdown ought to be different.  The notion that the viral rebounds were simply due to waning drug levels in the liver (and not due to viral escape mutations!), was supported by the biomarker analysis in the healthy volunteer part of the phase I study, also presented at ILC2015, where the trough in viral knockdown (~day 28) more or less coincided with maximal total cholesterol lowering as a predicted by miR-122 biology.

Of note, there was no apparent benefit of increasing the dose from 2 to 4mg/kg which was consistent with preclinical evidence that showed a declining liver/kidney drug ratio at 4mg/kg and maximal cholesterol lowering at 2mg/kg in humans.  This indicates that ASGPR receptor binding becomes saturated when too much GalNAc antisense is given at once.  The increased drug liver concentration at higher concentrations observed in earlier preclinical studies probably indicate uptake in non-productive compartments of the liver, including Kupffer and sinusoidal endothelial cells.
The hope is that with a second dose of RG-101 28 days after the first shot, maximal viral suppression can be maintained for at least another 4 weeks to stave off any viral comeback as seen in the single-dose study.  This is supported by both the healthy volunteer part of the study and the chimeric PXB mouse experiments presented which showed that such a second dose not only maintained drug potency, but in fact led to a step-up in efficacy.  I am therefore optimistic that with 2 doses of RG-101 monotherapy alone ~50% cure rates can be achieved in patient populations similar to that in the phase I study.

This, however, is not even the goal.  The ultimate goal would be to establish a simple pan-genotypic 4-week treatment regimen.  Accordingly, the combination of RG-101 with a DAA(s) (sandwich regimen) can be expected to result in a very, very deep viral knockdown by week 4.  At this point, a second shot of RG-101 would be administered to give the immune system another 6 weeks or so to finish off the virus.  I believe a very realistic scenario, and depending on which patient populations you are looking at a very compelling alternative to current HCV medications and those in development. 

PS: Open questions

Unfortunately, given that cholesterol lowering was still close to maximal at week 8, it would have been comforting to show a mutation analysis from the clinical study to confirm that viral relapse was not explained by the virus successfully developing resistance against RG-101.  This is such an obvious question that one wonders why Regulus did not present the data (yet). 

Another question mark around the current data set is why Regulus is not disclosing the differential effect of RG-101 on good and bad cholesterol and instead is reporting total cholesterol lowering.  The study investigator said that they are still analyzing the data from the multi-dose healthy volunteer study  (months after completing dosing???) and are planning to publish those.  I am mildly optimistic that this could unexpectedly bring to the fore the cardiovascular potential of miR-122 targeting, although in this case (à mostly chronic treatments) the liver cancer concern around miR-122 inhibition may be more valid than it is for the 2-shot HCV treatment goal. 

Thursday, April 23, 2015

Absolute HBsAg Levels, Not Percent Decline May Be Goal of HBV RNAi

I just walked into an early morning session at the International Liver Congress in Vienna and may have learned the most important nugget of information regarding RNAi treatment for HBV.

During the discussion in the ‘Banishing B’ session, Dr. Joerg Petersen referred to clinical analyses to be presented this Saturday showing that patients who achieve HBsAg levels of less than 500, or even better 200 IU/ml and then are given an interferon have a much better chance of eventually losing HBsAg, i.e. being functionally cured, than those that do not.

Dr. Petersen has been involved in clinical studies combining nukes and an interferon, and data presented at last year’s American Liver meeting (AASLD) showed that 9% treated with the combination for 1 year lost HBsAg in the year following combination therapy compared to just 2.8% receiving interferon alone.

Note that currently only interferons are thought to give you a chance of functionally curing HBV by medication and that HBsAg seroconversion attributed to interferon treatment may occur in the years after cessation of interferon therapy.  It will therefore be important to see whether this increase in seroconversion with combination treatment continues to hold up in subsequent years. The nukes are ‘only’ thought to protect the liver from the ongoing damage from HBV replication and may have to be given indefinitely.

I am not sure whether the new insight comes from a fresh clinical study or are the result of a more detailed subgroup analysis of the previous study.  Regardless, it hammers home a message that can be heard again and again, even more so in Vienna this year than at the London meeting last year: it is all about HBsAg lowering.

Implications for HBV RNAi

The implications of the new analysis for RNAi approaches for the treatment of HBV is obvious: use RNAi to get HBsAg below the threshold. In other words, it may be less about getting a magical 1log knockdown, but more about getting patients below 500IU/ml. 

For this, Dr. Petersen recommended using 2 nukes so that the tiny and very slow HBsAg declines observed on nukes continue ( < 0.1 log per year). With RNAi agents, you would likely achieve this goal for a patient with an HBsAg at baseline of 1000IU/ml within a month, even with a single 2mg/kg dose of ARC520 from Arrowhead Research as reported last year. 

The absolute knockdown potency of an RNAi agent would determine which fraction of the HBV population fall within this sweet spot. With a 1log knockdown e.g. you could start RNAi with patients with as much as 5000IU/ml.

For the clinical development of RNAi agents for HBV, the first step will be to determine the HBsAg decline on a background of nukes (just what Arrowhead is doing right now).  For pivotal trials, an immunostimulatory agent such as an interferon should be added to the nukeà RNAi/nuke treatment regimen to finish off the virus.

Stay tuned to learn about the fold benefit of HBV cures in patients falling below the 500IU/ml threshold, and the 3-4mg/kg single-dose results for ARC520 from Arrowhead Research to be reported later this quarter (note to Arrowhead PR department: next Monday may be a good time to do so).

Wednesday, April 22, 2015

First RNAi Therapeutic Nearing Finish Line

ALN-TTR02 for the treatment of TTR amyloidosis is the most advanced RNAi Therapeutic in clinical development and has been carrying the torch for the field as whole. Expectations are therefore high and a stumble in this program as a result of the therapeutic hypothesis underlying the program not panning out would likely trigger a temporary*, but steep sector-wide sell-off.  

* it should be clear, however, that with the current ability to robustly knock down genes in the liver, RNAi Therapeutics will result in a number of successful treatments.

It therefore came as a relief yesterday when Alnylam presented (press release here, data slides here) 12-month data from an open-label extension phase II study of ALN-TTR02 in the polyneuropathy form of the disease (FAP) showing

a)      continued robust gene knockdowns for more than a year (88-80% reductions peak/trough in 3-week cycle);

b)      disease stabilization (if not improvement) when the Natural History of the disease would have predicted marked deterioration (predicted mNIS+7 at 12 months of +18 in Natural History vs ~-2.5 on ALN-TTR02).

Furthermore, the safety of this liposomal formulation enabled by Tekmira seemed more than sufficient for a disease as severe as FAP TTR amyloidosis (5-15 year survival following diagnosis) with the most significant adverse events being related to the intravenous route of administration meaning that ALN-TTR02 should be given under trained medical surveillance.

There are some questions that remain open, some of which should be answered by the ongoing phase III APOLLO study which should complete sometime in 2016 (with an 18 month primary endpoint).

Firstly, it will be important to show disease stabilization to be strictly related to ALN-TTR02.  In the phase II study, most patients (20 of 27) were on tetramer stabilizers which have previously shown to result in very modest (tafamidis) to moderate (diflusinal) therapeutic benefits.  While neither tafamidis nor diflusinal have shown disease stabilization after 12 months, the concern remains that they could have contributed to the apparent therapeutic benefit seen in the phase II study.

To my surprise, the 7 patients not taking tetramer stabilizers on top of ALN-TTR02 seemed to do even better, at least numerically than those taking them (-6.5+/-9.2 vs -1.1+/2.5).  Although the number was quite small, the fact that this is the opposite result from what one might have expected, it is possible that a slight placebo effect may have played a role in this open-label study: those for which ALN-TTR02 was the only medical intervention might have had a greater ALN-TTR02-driven placebo effect.

Other more complicated explanations based on TTR lowering affecting the PK/PD relationship of tetramer stabilizers are also possible if this phenomenon is for real.

Fortunately, the blinded phase III APOLLO study will compare ALN-TTR02 to patients taking no tetramer stabilizers to treat their FAP.  Finally, it would be of interest to look at the effect of ALN-TTR02 on the spleen in the APOLLO study as spleen toxicity due to lipid stability might be the most important safety parameter with chronic dosing.    

In summary, yesterday’s 12-month data removed important overhangs over the RNAi sector and we are on track for the first commercial RNAi drug in 2017. Seeing is believing.

Sunday, April 19, 2015

Checkpoints Drugs Promise to Unlock RNA Therapeutics Cancer Potential

If you are involved in cancer drug development, you are probably thinking of ways to exploit checkpoint inhibitors for your purposes.  Checkpoints are the mechanisms whereby cancer cells avoid being recognized by the immune system as foreign and the initial clinical results of inhibiting them, e.g. by targeting CTLA4 and PD-1/PD-L1, has caused immune-oncology as a whole to take the cancer drug development space by storm.

Suddenly, every scientist and their technician believe that by taking checkpoints out of the equation, their immune approach to cancer, which in many cases was marked by failure after failure in the past, will work. 

Take e.g. cancer vaccines and the idea of grinding up tumors and use the cell mash to train dendritic cells.  Sounds compelling to me, but I also know that I lost a bunch much money on the very same idea 10 or so years ago with nothing, but some early promising data from the clinical Wild East to support it. 

Others meanwhile believe that their technologies may enable immune-oncology by e.g. being able to modulate the tumor microenvironment so as to permit better access of the tumor killing cells.

RNA Therapeutics are no exception.  RNA Therapeutics, due to its endless targeting opportunities, has always been considered promising for oncology, but has struggled to show clear-cut success either due to drug delivery issues or due to the difficulties of predicting immune activities based on rodent studies.

Emblematic of this reversal of fortunes is the toll-like receptor (TLR) field of activating the innate immune system with oligonucleotide stimulants.  The idea here is to provide an environment that is more conducive to tumor cell killing and/or to directly impact tumor cell survival (e.g. interferon stimulation).  Art Krieg, of TLR9 (CpG) fame, but who has been erring as its lost son in the (blooming) deserts of RNAi, RNaseH antisense, splice modulation, and RNA activation, is apparently re-energized enough to go back to TLR Therapeutics and has started Checkmate Pharmaceuticals which will likely try and harness TLR agonists for cancer immunotherapy.

Nanoparticle delivery may also see a revival.  This is because they have a propensity, usually undesired, to be taken up by phagocytic cells, some of which may be effective in antigen presentation (e.g. dendritic cells).  I have thus noticed that a number of mRNA cancer vaccine approaches involve nanoparticle delivery with the aim of expressing tumor antigens in the training camps of the immune system, the lymph nodes.  

Nanoparticles may also be a way to knock down the gate-keepers in the tumor microenvironment which inhibit tumor infiltration by cytotoxic T (incl. CAR T-cells) and other helpful immune cells.  Since gate-keepers should be most useful when positioned at the entrance, this might actually take advantage of another limitation of many nanoparticle delivery technologies, namely getting stuck close to the vasculature instead of penetrating deep into the tumor.

Once deeper into the tumor, the struggle may not be over for cytotoxic T cells and tumor eating cells due to potentially immunosuppressive activities in the tumor microenvironment.  This is the new positioning for the phosphorothioate-based antisense molecule ISIS-STAT3 by Isis Pharmaceuticals and AstraZeneca for which, like for other phosphorothioate-based approaches, the demonstration of robust uptake and gene knockdown activity in tumor cells themselves is lacking, but functional uptake in cells of the tumor microenvironment has been reported.

Finally, RNA Therapeutics such as CRISPR genome editing or straightforward self-delivering RNAi (see recent license of MirImmune from RXi Pharmaceuticals) can be tools for the ex vivo preparation of T- and dendritic cells.  A self-delivering approach may be advantageous here as it may function in normal cell culture media and thus not confound cell signaling pathways in the maturation of these cells.

Investor, tread carefully

Overall, I’m convinced that there are synergistic potentials to be exploited and checkpoint inhibition may open the door to certain that have failed in the past.  Nevertheless, one should be mindful that most immune oncology drugs only work in a fraction (maybe 25%) of patients and the current hype around immune oncology guarantees that there will be many bad apples for investors to avoid.  Trust me, I've seen it when RNAi was indiscrimately hyped and abused for short-term financial gain in 2006-8.  Just because you can contrive a link to immune oncology or because a desperate Big Pharma does a deal, does not mean that all the biological problems will dissolve.  The bubble will burst after which mostly only well-financed quality plays will recover.

Disclosure: I am fishing for short opportunities in the immune oncology space as a hedge against a (hopefully temporary) correction to what seems to be an in a number of areas (e.g. gene therapies, immune oncology, one-drug orphan wonders such as Alexion etc) overheated biotech space.

Wednesday, April 8, 2015

AstraZeneca Selects MicroRNA Development Candidate, Blazes Innovative Trail

When it is screaming into your face that your business model has failed you and the young competition is running circles around you, only then you might be compelled to change. 

This certainly is true for Big Pharma which have lost sight that their business is to make a buck while increasing the health of their customers instead of wasting time and energy on challenges like turning a twice-a-pill into a once daily therapy.  In its quest to optimize their business processes, it has thus thrown out of the window revolutionary, innovative technologies that just would not fit into those loved models.

Case in point, Merck writing off their multi-billion dollar investment in RNAi Therapeutics and selling it to Alnylam for $175 in largely equity and some cash. Alnylam then turned around and made at least a 10x return on the RNAi trigger stabilization chemistry by Merck in little more than a year.

This is a rough estimation of how much the Merck RNAi assets have approximately contributed to increasing Alnylam’s market cap.

AstraZeneca leading the way for Big Pharma in RNA Therapeutics

Not long ago, AstraZeneca was widely vilified for being the worst of the worst in terms of R&D productivity.  Their labs just would not produce new compounds that mattered to patients.

After a corporate shake-up, things certainly have changed on the innovation front.  AstraZeneca has fully emerged as a real risk-taker when it paid Moderna $240M in upfront monies alone for access to a comparatively early-stage mRNA Therapeutics platform in 2013. 

Before that, however, it already got active in the RNA Therapeutics in a less visible manner, notably with a much smaller, but possibly more profitable deal with microRNA Therapeutics platform company Regulus Therapeutics.

In the 2012 deal, AstraZeneca made a $25M equity investment in addition to a token $3M cash hand-out in the then privately-held Regulus Therapeutics.  In exchange, AstraZeneca received 3 microRNA target picks in the cardiovascular, metabolic, and/or oncology areas.

The best part of the deal for AstraZeneca (and the reason why I took money off the table today at what I considered an outsized reaction) was that it only has to pay $2.5M per target/candidate pick and Regulus Therapeutics has to pay for part of the work involved in generating the candidate at that.  There would, of course, be the milestones and royalties, but they should also be modest, in-line with the $2.5M payment due now. 

Oh, those were the good old times of abusive (because they could) Big Pharma biotech business development deal right on par with the steal that The Medicines Company got from Alnylam with regard to the PCSK9 target.

But still, you have got to credit AstraZeneca that unlike its brethren they not only sealed the deal, but actually advanced one of the first clinical candidates involving a fundamentally new molecular target class.  It will be interesting whether they will do the same in mRNA Therapeutics.

Anti-miR103/107 antagonism for improving liver health in diabetes

Initially, the focus  of the partnership had been on what looked like a very promising HDL-augmentation strategy by inhibiting miR33 in the liver, but this candidate has apparently taken a backseat in favor of the insulin-sensitizing strategy by inhibiting miR103/107.

It had been known that in type II diabetes, there is an inverse correlation between insulin sensitivity and miR103/107 expression.  Supporting a causal involvement, inhibiting miR103/107 in mouse models of diabetes with (unconjugated) antisense oligonucleotides increased not only insulin sensitivity, but also had positive effects on a couple of other diabetes-related parameters not only in the liver (e.g. triglyceride levels), but also body fat (adipocyte size/differentiation).

One puzzling aspect, somewhat akin to Regulus’ Alport’s program (--> miR-21), in exploiting anti-miR103/107 for pharmacological intervention is that it was initially uncertain what the target cells ought to be: adipocytes and/or hepatocytes?  A role for miR103/107 expression in adipocytes was particularly supported by the observation that its steady-state level there is higher than in the liver and the fact that single-strand phosphorothioate oligonucleotides also distribute to body fat.

However, with the adoption of GalNAc conjugation technology where most of the oligonucleotides now accumulate in hepatocytes it seems that AstraZeneca and Regulus have come to the conclusion that it is the liver that once again is calling the shots here as it usually does in diabetes.  You can deduce this from the fact that a GalNAc version was selected as the clinical candidate (AZD4076) slated to enter the clinic later this year.

Taking advantage of the observation that anti-miR103/107 has positive effects on liver triglyceride levels, the clinical development of AZD4076 will at least initially be geared towards treating non-alcoholic steatohepatitis (NASH) in diabetes patients.

Tuesday, April 7, 2015

Time is running out for Benitec

Benitec announced today that almost 1 ½ years after filing an IND for its DNA-directed RNAi HCV candidate, it has now obtained liver biopsy data from first 3 of the 4 patients dosed so far.  Needless to say, the analysis was a resounding success confirming that the right AAV vector coding for the shRNAs against HCV was administered to the trial subjects.

Given that no details were provided on the methods, I assume that the evidence is based on PCR analysis which pretty much picks up almost any activity.

TT-034 also shined on safety with ‘no treatment-related serious adverse effects (SAEs) in any of the four patients dosed’.

To wit, the motivation behind the gene therapy ddRNAi HCV trial is to provide a one-shot cure from HCV infection.  The company, however, said that ‘the amount of shRNA produced will not result in reduction of hepatitis C viral load’.  

So while this statement almost makes it sound like they did not look for antiviral efficacy, but that there might well have been, we can safely assume that they did (standard blood test to look for HCV; plus RNA analysis from biopsies) and failed to see such.  

It is also curious that no results from PCR-based target mRNA cleavage assays were disclosed which, while still PCR, requires a certain amount of RNAi robustness to detect with confidence and would have been used to further tout trial success. 

At this point, Benitec has almost completed the first 2 of 5 planned dose cohorts.  According to my notes, the top dose is about 25x higher than dose group 2.  To get from no change in viral titer to undetectable while increasing dose by 25x seems quite optimistic to me.  And even if this highly unlikely scenario materialized, at this pace, it will be sometime in 2023-4 when it would even be considered for approval.

So please, Benitec, if you cannot see a knockdown at the next higher dose cohort, give it a rest.   

Friday, April 3, 2015

Hereditary Angioedema: A High-Value RNA Therapeutics Target Confirmed by Antibody Trial

This week, Dyax released early, but arguably impressive data from a phase Ib trial of monoclonal antibody DX-2930 for the prophylaxis of Hereditary Angioedema (HAE), a rare disease (incidence of 1 in 10-50k births) with a rapidly evolving and growing market.  The results confirm that plasma kallikrein and, by extension, its precursor prekallikrein (PKK), are highly effective and safe targets for the treatment of HAE.

Due to the expression of kallikrein and other pathway components in the liver, HAE therefore shapes up as yet another high-value indication after PCSK9/cardiovascular disease and CC5/complement disorders PNH and aHUS (à Soliris) where RNA Therapeutics will be directly pitted against monoclonal antibodies to answer the following pharmasophical question: 

What would you prefer? Turn off the gene underlying a disease with a defined, synthetic molecule harnessing an endogenous biological mechanism, or mop up the disease-mediating gene product (protein) with a gemisch of cell-derived proteins?

You may know my preference already, and indeed, a phase I study of ISIS-PKKRx by Isis Pharmaceuticals targeting PKK has just been completed and showed a 80-90% gene knockdown at 300-400mg weekly subcutaneous injections.

Disease pathway: promoters and inhibitors

HAE is caused by the genetic absence or insufficient activity of C1 esterase inhibitor (C1-INH).  This predisposes to regular, often weekly to monthly episodes of tissue swelling which when abdominal can cause severe pain and when affecting the throat is life-threatening.

Although C1-INH is involved in a few intersecting pathways such as coagulation and complement, the results by Dyax confirm the growing evidence that in the end it is all about plasma kallikrein and subsequent vasoactive bradykinin generation.  The preclinical evidence includes an elegant study by Isis Pharmaceuticals (Bhattacharjee et al 2013) where the power of RNA Therapeutics was harnessed to knockdown a number of players in the coagulation and kinin-kallikrein pathways to show that only inhibition of members of the kinin-kallikrein pathway (e.g. PKK and factor 12) could reverse the symptoms caused by C1-INH deficiency.

Importantly, human genetics show that PKK deficiency is without apparent medical adverse consequence, thus making it a drug developer’s dream.

Therefore, although HAE is caused by a protein deficiency, a therapeutic knockdown approach is conceivable due to the presence of promoters and inhibitors in the pathway of disease. This is thus similar to antithrombin and hemophilia for which Alnylam is currently advancing a promising RNAi Therapeutics clinical candidate (ALN-AT3).

Poor pharmacokinetics of approved drugs leaves large unmet need

There are a handful of drugs approved for HAE.  Most of these are approved only for the mitigation of an acute HAE attack.  They have shown modest efficacy with kallikrein inhibitor Ecallantide/KALBITOR (a small protein by Dyax) and C1-INH protein replacement therapeutic CINRYZE (by Shire) approximately halving the severity of an attack or attack frequency, respectively. 

In addition to some potentially severe side effects like anaphylaxis (KALBITOR) and infusion reactions (CINRYZE), it is the poor pharmacokinetics necessitating frequent administration that render them impractical for prophylactic use.

For example, CINRYZE, the only agent approved for prophylactic use, has to be intravenously infused twice a week (and some investors/companies moan when Tekmira's RNAi LNP products have to be infused every 3-4 weeks), and still attack rates were reduced by only about a half.  This supports that despite frequent i.v. infusions, C1-INH levels cannot be kept above the needed threshold for long enough.

Given that once set in motion, the swelling cascade is probably difficult to stop cold, it is, however, chronic prophyxis that has the prospect of bringing most benefit to patients, in addition to filling the coffers of biotech companies and their investors (prophylactic CINRYZE: $300k per year).

DX2930 shaping up to be first satisfactory treatment

In light of the above, it is apparent that new drugs for HAE should have both increased potency and be sufficiently convenient for chronic prophylactic use.  In terms of dosing frequency, this means at a minimum once-a-quarter intravenous infusions, weekly subcutaneous injections, or daily oral pills.

As a subcutaneously administered antibody, DX-2930, of course, almost naturally satisfies the dosing frequency requirement.  Moreover, when 2 doses were given 14 days apart, the attack frequency for the 300 and 400mg cohorts dropped by ~10x from baseline during the time when plasma concentrations of the antibody were deemed above the threshold needed to keep the kinin-kallikrein pathway in check (~6 weeks after 2nd dose).  13/15 patients (87%) were thus attack free from days 8-50 compared to only 3/11 (27%) in the placebo group.

Attack frequencies for the 30mg and 100mg cohorts were not reported.  This probably means that these were not successful and support the notion that whether an attack can occur or not is a rather sharp threshold effect.  Also consistent with this was the fact that the 300mg cohort numerically performed better than the 400mg cohort in this small sample size. 

[correction] Attack frequencies for the 30 and 100mg cohorts were also dramatically reduced from days 8-50 although these doses had been predicted by Dyax to yield suboptimal plasma antibody levels, possibly raising questions about the patient population chosen.

Nevertheless, DX-2930 appears overall very promising and has further validated kallikrein/PKK as targets, but larger patient numbers are required to determine the more precise efficacy benefit over existing drugs, the dose response relationshipo, and, of course, safety.

RNA Therapeutics closing in

In addition to 2nd generation RNaseH antisense compound ISIS-PKKRx, I expect a number of additional RNA Therapeutics candidates to join the HAE fray due to the attractive economics of the market and the anticipated high rate of development success.  The latter is largely a function of the validated nature of the kinin-kallikrein pathway.

Although the DX-2930 study validates kallikrein, it should be cautioned that RNA Therapeutics cannot actually target kallikrein directly, but merely its precursor, PKK, as kallikrein is generated from PKK by enzymatic processing.  This means that the therapeutic threshold in terms of percent inhibition will be shifted either to the left or right depending on the enzymology of PKK processing.

In summary, HAE and the kinin-kallikrein pathway should be one to put on your RNA Therapeutics radar, also because it may offer new avenues towards important large markets such as diabetic macular edema and inflammatory bowel diseases.
By Dirk Haussecker. All rights reserved.

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