When Biomarin late last year bought Prosensa for its experimental
exon skipper drisapersen for the treatment of Duchenne Muscular Dystrophy (for $680M plus potential milestones), it
exuded confidence about the likelihood of getting approval for the 2’-O-methyl
phosphorothioate antisense molecule.
This, despite of the fact that drisapersen failed in a pivotal phase III trial of
186 patients which prompted the old partner GSK to dump the drug and walk away.
Tenuous early evidence for drisapersen in earlier trials
The confidence is largely based on some supposedly successful earlier
trials, especially a multi-center, randomized, blinded 53-patient phase II
study which had seen improvements in the 6 minute walk distance (6MWD) at week
25, the primary endpoint of the study (Voit et al. 2014).
This, however was statistically significant only the case in the subgroup of patients that received drisapersen continuously (à
treatment in 10 out of 10 weeks with 6mg/kg),
but not in patients which were treated identically, except for the small
difference in skipping the last week in a 10-week treatment cycle.
At week 49, the difference with placebo failed to
reach statistical significance and Prosensa had to resort to pooling both
subgroups to claim victory for that time point. Similarly, drisapersen failed in obtaining
statistically significant outcomes for other muscle function endpoints.
Since the mechanism of action for the DMD exon skipping
candidates is to change splicing of the mutated dystrophin transcript to a form in which the reading frame is restored with recovery of partial activity, it is important
to understand the relationship between drug treatment and dystrophin production.
Here, too, the evidence was less than robust. For example, even when applying the sensitive
immunofluorescence technique, no increase or even a decrease in dystrophin was seen in almost half of treated subjects.
With the less sensitive Western blot, an increase in dystrophin was seen in only a
third of treated subjects (0 for placebo).
Therefore, given the failed phase III trial and the less than robust earlier evidence in
favor of the drug, I struggle to understand Biomarin’s confidence in obtaining approval in 2016.
The importance of dystrophin as a surrogate endpoint
Part of the difficulty of obtaining statistically
significant results for muscle function endpoints is most likely due to the
small patient size (orphan disease affecting ~1 in 3500 male births) and the
consequent need to pool boys at various stages of the disease together in a given trial. It would thus not be surprising if say obtaining 10%
levels of normal or Becker-type dystrophin will translate into very meaningful
clinical benefit in some, but not other boys.
This will be an even more challenging problem for the DMD
subgroups that are not amenable to exon 51-based exon skipping which is
targeted by drisapersen. Probably
insurmountable for first-generation chemistries like drisapersen.
Accordingly, in both the drisapersen
and the competitive PMO-based eteplirsen trials, it
has not been possible to correlate dystrophin production with functional
outcomes.
For that reason, I strongly support the importance of
establishing reliable, quantitative methods to measure dystrophin in clinical
trials (there was an FDA workshop related to this last week). Dystrophin-dependent markers may
also be acceptable if they can be measured by means that do not involve taking painful muscle biopsies. For example, serum-based
microRNAs as developed by Rosetta Genomics and Marina Biotech would be of
interest here.
Eteplirsen before drisapersen
I thus find it difficult to grasp the notion of rejecting the current crop of exon
skippers like drisapersen or eteplirsen should they be found to produce
functional dystrophin with few side effects.
After all, it is the loss of dystrophin function that causes Duchenne
Muscular Dystrophy and one has to wonder how generating additional dystrophin
cannot be beneficial to patients, especially since the principle behind
drisapersen and eteplirsen is strongly supported by human genetic evidence (à Becker’s Muscular Dystrophy).
In this world, it has got to be eteplirsen that should
be first in line for regulatory approval.
This is because there is overwhelming evidence (e.g. Heemskerk et al., 2009; Sarepta's Barclays presentation March 12, 2015) that the PMO-based drug
is much more potent than drisapersen which, let’s face it, is based on
stone-age antisense chemistry (2’-O-methyl phosphorothioate). Such chemistry is characterized by
minimal efficacy and dose-limiting toxicities, especially renal in the case of
drisapersen.
In a paper comparing 2’-O-methyl to PMO chemistry for DMD
exon skipping conducted by researchers close to eteplirsen, it was found that
at same doses in mice, PMO chemistry is moderately to vastly more potent than 2’-O-methyl phosphorothioate antisense compounds of a size comparable to
drisapersen. The extent of the difference depended on whether the human or mouse dystrophin were targeted and the target sequence. Unsurprisingly given the acrimonious competition between the two parties, Sarepta has also picked up
on this and continued along these lines by showing that in addition to chemistry, eteplirsen has the edge over drisapersen in terms of the targeted sequence:
Sure, there is the theoretical caveat that PMO and 2’-O-methyl
scale differently from mice to humans and that what is the most potent target
sequence for one chemistry does not necessarily have to be the most potent one for
the other. Intuitively, however, the
differences are too big for these factors to compensate the preclinical
evidence. Also, keep in mind that in the
clinic, eteplirsen is being given at 5 to almost 10-fold increased doses than drisapersen
and, on top of that, is much safer and better tolerated than drisapersen.
Because of this and the competition, it is not surprising and disingenuous when Biomarin would now suddenly like to de-emphasize the importance of dystrophin as a surrogate biomarker (see last week's workshop).
Dear regulatory agency,
if you approve drisapersen, you cannot deny eteplirsen. Sure, drisapersen has been tested in more patients than eteplirsen and Sarepta has conducted a clinical
trial in the worst possible manner and probably ‘embellished’/overstated some of their results, including
the dystrophin evidence. However, given
that eteplirsen almost certainly generates more dystrophin than drisapersen,
the highly favorable side effect profile of eteplirsen (also in comparison to
drisapersen), and in light of the 6MWD issue that applies to both drug
candidates, the question is whether the bureaucratic application of rules
should trump scientific evidence and patient interests.
Disclosure: I am long SRPT based on the notion that Biomarin, with its orphan disease savvy, will turn out to be the biggest supporter of eteplirsen getting approval this time around. Additionally, the agency is partly responsible for the long duration of the ongoing eteplirsen trial (close to 4 years soon) and the repeated taking of muscle biopsies, and after all this taking away hope from patients and their close ones is difficult to fathom.
