Pfizer Study Clears Way for Broad Cardiomyopathy Label of TTR-lowering Drugs

TTR amyloidosis represents a major market opportunity for RNAi Therapeutics.  The pivotal trials underlying the expected approvals of Patisiran from Alnylam and antisense rival Inotersen from Ionis/Akcea were focused on the neuropathy aspect in the inherited form of the disease.  It is hoped, however, that approvals will be obtained that will also cover the cardiomyopathy spectrum of the disease. 

Nevertheless, the absence of a prospective study focused on cardiomyopathy raises the concern that the largely biomarker-related and post-hoc analyses conducted by Alnylam and Ionis/Akcea will not hold up when hard endpoints like mortality are considered.  This uncertainty could lead to resistance by payors to cover the drugs for cardiomyopathy uses.

Data released last week from Pfizer’s ATTR-ACT study should greatly aid in addressing this concern, paving the way for broad product labels and reimbursements not only for the familial form of TTR cardiomyopathy, but even to encompass those with wild-type TTR cardiomyopathy.

Evolving disease understanding

TTR amyloidosis is caused by the deposition and accumulation of misfolded tranthyretin protein in various tissues thereby poisoning them.  Historically, TTR amyloidosis was not considered a single disease, but either TTR neuropathy or TTR cardiomyopathy depending on where disease symptoms are most pronounced.

Over the last 5-10 years, however, it has become recognized that a given patient may suffer from a range of symptoms across organ systems.  Whether an individual patient suffers from largely neuropathic or cardiac symptoms or both to similar degrees is typically informed, but not entirely explained by the underlying mutation in the familial forms of the disease.

Cardiac symptoms can also be caused by wild-type TTR protein alone.  This is referred to as senile systemic amyloidosis (SSA).  This population has not been the subject of any rigorous, randomized trial in the development programs of TTR-lowering drugs, but based on my impressions from last November’s seminal Paris meeting on the disease, there is great anxiety in this particular patient community about access to TTR-lowering drugs.  

Tafamidis as an underappreciated TTR trailblazer

Pfizer, through its 2010 acquisition of Tafamidis, was the original trailblazer in this orphan disease.  Tafamidis falls in the class of (small molecule) TTR tetramer stabilizers (along with widely used off-label generic diflusinal) which prevent TTR tetramers to fall apart in the rate-limiting step to forming misfolded pathogenic TTR aggregates.

As is often the case in orphan disease drug development, being the first means that you have to do a lot of the heavy-lifting in terms of understanding the natural history of the disease to design adequately powered clinical trials with the appropriate endpoints.  Consequently, the first pivotal trial of Tafamidis in V30M early-stage neuropathy patients fell short of garnering FDA approval and only got a narrow label from European regulators. 

The problem was that, although the data strongly suggested efficacy, it turned out to be an underpowered study due to unexpectedly high drop-outs for patients undergoing liver transplants: starting from 125 intent-to-treat (ITT) patients, the efficacy evaluable (EE) number dropped to just 87 in this placebo-controlled study.  

Nevertheless, if you disregarded the liver transplant patients in the statistics (very reasonable in my opinion since a liver transplant throws everything off), the study would have met the quality of life and NIS-LL co-primary endpoints by greatly halting, although not stopping disease progression.  In addition, all key secondary endpoints were positive.

Given the improved understanding of TTR amyloidosis and a much more forgiving regulatory environment, this study, despite its limitations, would have ensured FDA approval today.       

Tafamidis succeeds in cardiomyopathy study

Last week's announcement by Pfizer represents another breakthrough for those living with TTR amyloidosis.  Its phase III ATTR-ACT trial in patients with pronounced cardiac symptoms, including those with wild-type TTR SSA with largely cardiac symptoms, has met the co-primary endpoints of reducing overall mortality (!) and cardiovascular-related hospitalizations.

Having learned their lesson, Pfizer went out of its way to make sure that this study would show a positive signal if the drug were active: instead of 18 months, 30; instead of 125 patients, 441; and instead of just one daily 20mg dose of tafamidis also 80mg.  Talk about not taking any chances!

Tetramer stabilizer and TTR lowering results mutually beneficial

When Pfizer announced late last trading week the ATTR-ACT results, the sponsors behind the TTR lowering RNAi and antisense drugs Alnylam, Akcea, and Ionis took it on the chin with 5-13% sell-offs in their stocks due to competitive concerns.

The main concern apparently is that while Tafamidis has now succeeded in a trial specifically targeted at the cardiomyopathy ‘population’, the APOLLO study of Patisiran and NEURO-TTR study of Inotersen have not specified this aspect as a primary endpoint.

This concern is lessened, however, due to the recognition of TTR amyloidosis as a single disease with the relative degree of various symptoms varying between patients.  Of course, let’s be frank and admit that this is also a self-serving agenda that has been mainly promoted by Alnylam so as to increase the market size of Patirisan without having to wait for another 3-4 years. 

On the other hand, since the root cause of the various manifestations is the same, TTR aggregation and tissue accumulation, a drug that works in addressing it should be beneficial for all these manifestations.  In fact, strong evidence on improved cardiac outcomes has come from the APOLLO and NEURO-TTR study as well as an open-label investigator-instigated study of Inotersen specifically in the cardiomyopathy indication (both mutant and wild-type forms; ‘Benson study’).

Similarly, since the mechanism of action of TTR stabilizers and TTR-lowering drugs are essentially the same, lowering the pool of aggregation-prone TTR, success in ATTR-ACT is highly supportive of the cardiac benefits of Patisiran and Inotersen as much as APOLLO and NEURO-TTR strengthen the case for Tafamidis use in addressing TTR-related neuropathy. 

All this mutually reinforcing data should ultimately help in Inotersen and Patisiran getting a very broad label and helping with reimbursement, perhaps even in the SSA indication which I believe the market could not have priced in yet.  Having said this, we have yet to see the SSA vs hereditary subgroup analysis from the Pfizer study.

Relative drug efficacy

Finally, in terms of drug efficacy, Tafamidis is unlikely to challenge Patisiran even in the cardiomyopathy indication, since Patisiran improved outcomes in the APOLLO study while Tafamidis stabilized or merely delayed disease progression.  

The efficacy of Inotersen based on Quality of Life data should end up being somewhat ahead of Tafamidis (QOL in EE population vs placebo of -9 for Tafamidis in the neuropathy study vs -12 for Inotersen in NEURO-TTR and -20 for Patisiran in APOLLO), although its safety profile appears to lag that of Tafamidis.  Because of the new data indicating efficacy similar to diflusinal, but with better safety, Tafamidis ought to replace generic diflusinal which has dominated the tetramer stabilizer market until now.  

Ultimately, if patient welfare was a top concern, TTR stabilizers probably ought to be used on top of TTR-lowering drugs to prevent any TTR protein that survived TTR knockdown from misfolding.