With an eye on starting a clinical trial possibly as early
as 2014, a scientific team in San Diego is painstakingly working to design the
best drug possible to defeat Huntington’s disease.
For the past seven years,
Don Cleveland, Ph.D., of the
Ludwig Institute for Cancer Research at the University of California San Diego
(UCSD) and Frank Bennett, Ph.D., the senior vice
president for research at
Isis Pharmaceuticals, Inc.,
have envisioned treating HD with a revolutionary gene-silencing technology that,
if successful, would attack the disease at its genetic roots and perhaps even partially
reverse symptoms.
Since late 2007, the UCSD and Isis teams have partnered with
the
CHDI Foundation, Inc., the multi-million-dollar non-profit
biomedical organization dedicated to finding HD treatments. Together they aim
to develop what Dr. Bennett has described as a “laser-guided missile” to
prevent the damage to brain cells caused by the mutant huntingtin gene carried
by HD patients.
Dr. Cleveland and Isis senior
scientist Holly Kordasiewicz, Ph.D., were honored as the 2012 Researchers of
the Year by the San Diego Chapter of the Huntington’s Disease Society of
America (HDSA-San Diego) last night before some 500 attendees at the chapter’s
twelfth annual Celebration of Hope Gala.
Isis employs a cutting-edge technology known as antisense
oligonucleotides, or ASOs. DNA, the building block of life, runs our cells by
telling them which proteins to make. It does so by sending messages with
another molecule called messenger RNA.
As encoded by DNA, RNA has a very specific template,
somewhat akin to a unique electrical outlet into which a plug can fit. RNA is
known as a sense molecule, and Isis manufactures specific ASOs, artificial
strands of DNA, to act as antisense molecules, the plugs that control the RNA.
(Click
here and
here to read previous reports on the project.)
The ASOs accomplish two goals. First, they destroy the
huntingtin RNA and thus prevent the production of the huntingtin protein.
Second, eliminating the RNA removes it as a potential cause of other problems
in the cell.
Above, some of the Isis HD team members: (left to right) Michael Oestergaard, Punit Seth, Bethany Fitzsimmons, Curt Mazur, Amy Blackley, Eric Swayze, Holly Kordasiewicz, Frank Bennett, and Marco Giorgetti (photo by Gene Veritas) (click on image to enlarge). Below, Gene Veritas inside the Isis facility in Carlsbad, CA (photo by Amy Blackley, Isis).
Fine-tuning,
tailoring, and twiddling
Isis had originally hoped to begin a clinical trial as early
as late 2010, but has delayed the project in order to perform highly important
fine-tuning on several fronts.
As previously described by Dr. Bennett, Isis is searching
among the many “flavors” of ASOs it makes in order to find the best match for
treating HD. From an original pool of thousands, Isis has narrowed down the
candidate ASOs to just five, Bennett said in a recent interview.
Isis, CHDI, and other researchers have also made significant
advances on two other key research questions. First, how much of the huntingtin
protein should the drug remove? So far, the scientific consensus seems to have
settled on 50 percent lowering (also known as
knock-down) as the current target. However, this question will
ultimately be resolved through the clinical trials.
The second, related question is trickier but could
ultimately open the door to an even better drug. Because HD patients have both
mutant and normal huntingtin proteins in their brain cells, should the drug
lower both or just the mutant? In the early going, the ASOs did not distinguish
between the “good” and “bad” proteins. However, Isis has now developed a way to
knock down just the bad.
At least in theory, knock-down of just the bad is the safer
approach for patients, although the project’s experiments have also surprisingly
demonstrated that knock-down of both is not harmful, explained Dr. Kordasiewicz, the former head of the HD
project in Dr. Cleveland’s UCSD lab.
Dr. Holly Kordasiewicz in the lab at Isis (photo by Curt Mazur of Isis)
“The decision still hasn’t been made,” she said, referring
to the choice between the two types of ASOs. “It’s hedging your bets.
Everything’s on the table. The chemists are doing amazing things. It would be
irresponsible of us not to consider all of the options before making our final
decision.”
“You never know, once you get into a human, what’s going to
work,” she added. “So having everything ready to go, so you don’t have to wait
three more years to develop the next thing, if one doesn’t work, you try the
next.”
Using second-generation ASO technology, the Isis chemists found
ways to increase both the selectivity (the ability to bind to the mutant RNA as
opposed to the normal one) and the potency of the potential HD drug.
“It improves potency quite a bit,” said Punit Seth, an Isis
senior research fellow in medicinal chemistry, in describing one of the key
chemical innovations. “You can get anywhere from three-fold to ten-fold
improvement, which then translates to lower costs in drugs and administering
less [of the] drug to the patients.”
Dr. Cleveland added that these improvements would also
produce a drug with potentially fewer side effects.
Eric Swayze, Ph.D., Isis’s vice president for medicinal
chemistry, summed up the fine-tuning as “tailoring” and “twiddling with the
number of different building blocks” that go into the ASO.
“It turns out to make a huge difference, which we didn’t
really expect,” he observed.
Dr. Eric Swayze explains the function of the Isis ASOs (photo by Gene Veritas).
Patient-friendly
delivery
Isis has also strived to simplify the delivery of the drug.
Originally, the company planned to direct the drug into the brain using a
device implanted in the abdomen and connected to a catheter running under the
skin to the skull.
Now, however, the researchers aim to introduce the ASO
directly into the cerebral spinal fluid (CSF, the fluid that bathes the brain)
by injecting it through a quarter-sized port implanted near the rib cage, with
the catheter running to the area of the spinal cord.
This method is “more convenient to the patient and
longer-term more commercially attractive,” Dr. Bennett observed.
Gene Veritas (left) with Dr. Bennett at a CHDI conference in February
Dr. Bennett noted that Isis gained valuable experience in
drug delivery through a trial of its ASO drug for spinal-muscular atrophy, a
childhood neurological disease. Isis also has conducted a Phase I ASO clinical
trial for amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig’s
disease.
With the improved delivery method, instead of continuous
infusion of the drug, patients will probably need only occasional injections,
each one lasting only a few minutes, Dr. Bennett added.
“There’s a long history of safety and efficacy using this
method,” he said.
Furthermore, the Isis approach avoids the potentially more
risky delivery methods used in two other HD gene-silencing approaches: the use
of a virus, or an operation on the skull to introduce the drugs into the brain.
Getting into the
brain
To improve the efficacy and safety of the ASOs, Isis and
CHDI have been testing them in mice and non-human primates.
One of the key mouse testing sites is Dr. Cleveland’s lab at
UCSD, where an HD team led first by Dr. Kordasiewicz and, after her departure
to Isis, by Clotilde Lagier-Tourenne, M.D., Ph.D.
In conjunction with experiments in other labs, the Cleveland
HD team has demonstrated surprisingly good results.
One major hurdle to treating the brain is the blood-brain
barrier, which shields the brain from foreign substances that might cause harm.
The barrier makes it difficult to get drugs into the brain.
Significantly, an article recently published in the journal
Neuron, with Dr.
Kordasiewicz as the lead author, suggested that the ASOs delivered via the CSF
reach a wide area of the non-human primate brains, including the regions known
as the cortex and the striatum, two areas critically damaged in HD.
As Dr. Cleveland explained, a decade ago scientists viewed
neurological diseases as the result of problems in a particular kind of neuron
(brain cell). Since then, they have developed a radically different view: the
various kinds of cells are linked together in a system – including connections
between the cortex and the striatum.
“It’s actually a disease not just of individual neurons but
of the whole system, a neuron and the cells surrounding it,” Dr. Cleveland said
of HD. “It’s such a simple message. It’s a little surprising that it took so
long to realize it. Neurons don’t live by themselves. They require their
partners, and the partners develop damage that drives and spreads disease. So,
in Huntington’s disease it’s now clear that there’s a partnership between
striatal neurons that send projections into the cortex and vice versa.”
Above, Dr. Cleveland in his office at the Ludwig Institute for Cancer Research on the UCSD campus. Below, Dr. Cleveland with lab scientists Jon Artates (middle) and Jihane Boubaker (photos by Gene Veritas).
A ‘Huntington’s
holiday’
The most stunning test results involved the amelioration of
symptoms.
“Because we are hitting the cortex to such a high level, my
prediction would be that we will have a very strong effect on things like
cognition and mood and anxiety,” said Dr. Kordasiewicz of the ASOs’ ability to
restore brain functions lost in HD. Chorea, the shaking and trembling that
occurs in HD, also could be ameliorated, she added.
By reducing the level of mutant huntingtin protein in the
mouse brains, the ASOs reversed the HD-like symptoms.
“It was better than we could have imagined. In the sickest
animals, we stopped further brain loss,” said Dr. Cleveland “In other mice, a
single treatment led to partial reversal of symptoms. And what’s more, the
improvements lasted more than six months after a single treatment. And even
then, the disease process did not start back up. It was amazing.”
Dr. Cleveland observed that, unlike other kinds of
substances the ASOs are made of DNA that isn’t rapidly degraded by enzymes the
way many other drugs are affected.
“Once they get intracellular, they’re intracellular acting
to catalyze the destruction of the target RNA for, not just hours, not just
days, not just weeks, but actually months,” he continued. Just a single injection
of the ASO leads to a month of huntingtin RNA suppression in mice. A two-week
infusion brings four months of suppression.
The scientists refer to the as yet unexplained symptom-free
period after the ASO treatment is gone as a “Huntington’s holiday.”
Dr. Cleveland speculated that “since it takes 30-40 years
for HD symptoms to develop. If you could introduce a Huntington’s holiday,
maybe you could reset the pathogenic process so that it might take a
considerable time to build back up.”
As he and others have observed, success with this approach
means people might need to take an ASO HD drug only a few times per year.
As a preventive remedy, a future generation of ASOs might
even be prescribed early in life for individuals like me who have tested positive
for HD but remain asymptomatic, Dr. Cleveland added.
Watch Drs. Cleveland and Kordasiewicz receive their HDSA-San Diego awards and speak about the promise of their work for an HD treatment in the video below.
In the final run-up to the proposed clinical trial, the
Isis-UCSD-CHDI team and its collaborators are seeking the answer to two more
crucial questions: how can the efficacy of the ASO be measured when humans
participate in trials? And what is the proper size and frequency of the dose?
The impact of the ASOs on mice and non-human primate brains
is fairly easily measured. However, the scenario is different for humans, who
cannot be manipulated, sampled, or subjected to the other kinds of experiments
done with animal models.
To answer these questions, the scientists are seeking to
develop “biomarkers” for the ASO effects.
As Dr. Cleveland explained, the researchers are hoping to
find “signatures” in the cerebral spinal fluid of the trial participants that
would indicate the impact of the ASO. Those signatures could be related to both
to alterations in genes and the secretion of proteins.
“It’s a very big experiment,” Dr. Cleveland said. “We need a
partner like CHDI with deep pockets to do this. It’s an expensive experiment,
but we absolutely have to do it. Can we find biomarkers? I’m an optimist. We’ll
know the answer over the next six months.”
If successful, this experiment will help the scientists
determine the amount of drug to give to the patients and provide specific
measures of drug impact.
The pharmaceutical firm Novartis has found a way to measure
the huntingtin protein in the bloodstream and is seeking to do so in the CSF. The
Isis-UCSD-CHDI project also has at its disposal the valuable data from
long-term natural history studies of HD patients (TRACK-HD), and it will also probably
rely on brain imaging of the trial participants.
Light in the tunnel
In 2013, Isis hopes to select the final ASO drug candidate
to move into pre-clinical testing. If that testing is successful, then the
company will need another 12-18 months to obtain approval from the Food and
Drug Administration to initiate the Phase I human trial.
Planning for Phase I will involve not only the ASO
researchers, but toxicologists (who check for safety), pharmacokineticists (who
measure the penetration and exit of the drug), and clinicians (who work with
and care for the trial participants).
“They’re already starting to engage in the project, because
they can see the light at the end of the tunnel,” said Dr. Bennett. “They’re
becoming involved in thinking through the strategy of how we’re going to
develop this drug.”
Dr. Bennett emphasized that Phase I effort’s main purpose is
to measure safety and tolerability – not drug efficacy – although the
researchers will also take note of the effects. If Phase I is successful,
efficacy comes into play in the potential Phase II and III trials.
“We’re committed to try to do our best to bring that drug
forward,” said Dr. Bennett, who noted that the Isis HD team has worked many
nights and weekends to speed the project. “There’s still a lot of caveats in
there. The best-laid plans sometimes run into roadblocks. But we are very
enthusiastic. We’re in this to help patients.”
“For patients and their families, I know it’s too slow, but
I don’t think it could be done any faster,” concluded Dr
.
Cleveland. “I think everyone’s working absolutely flat out.”
Bringing hope to the HD community: Dr. Cleveland at the Gala with advocate Amy Anderson, wife of Craig Anderson, a former pilot afflicted with HD (photo by Gene Veritas)
