Scientists involved in cell-based therapy for bleeding disorders
provided updates on this exciting discipline of gene therapy Tuesday
morning. David Lillicrap, Canada, the chair of the session, “Gene
Therapy”, said the momentum of clinical gene therapy is remarkable.
Chritopher B. Doering, Atlanta, Georgia, USA, took the audience
through the process of using stem cells in gene therapy of hemophilia A.
Stem cells were first applied to T cells in the 1990s, but safety
concerns led the research back into academic laboratories, said Doering.
“Stem cells are rare populations of unspecialized cells that are
self-renewing and can become other cells,” began Doering. “Donor cells
from a non-affected individual (from the blood) are transplanted into
the patient. In order to apply to hemophilia, this may require gene
transfer.”
In order to implant stem cells, some of the patient’s cells must be
negated to make room for the new cells. Doering said that it is possible
to use the patient’s own cells harvested peripherally or from bone
marrow.
Challenges to using stem cells include the optimization of transgene
expression and producing biosynthesis, and also safe and effective
pre-transplantation conditioning and clinical vector manufacturing, said
Doering. “Stem cells can last for the life time of an individual so we
have a potential cure. We need only to target a few cells as each stem
cell will produce hundreds of daughter cells.”
A pilot clinical trial design has been approved the US Food and Drug
Administration with a single site trial at Emory University, USA, to
start.
Matthew Porteus, Stanford, California, USA, said that genome editing
is a method to correct disease causing variants. “This is a precise,
controlled mutagenesis of the genome. Creating a break in the DNA will
cause the cells look for this and make a repair. So we can stimulate
mutations at the site of the break.”
The repair could change the DNA sequence to one that already exists
in the genome or to something novel using synthetic biology. This second
option “Creates a new therapeutic phenotype in the cell. In hemophilia
it might be used to overly express a clotting factor,” said Porteus.
Using homologous recombination to change single nucleotide variants
can be delivered on an adeno-associated virus (AAV) nanoparticle. “In
research with sickle cell disease there is about a 20 percent success
rate. We can also insert a gene cassette into a safe harbor or single
location in the genome,” said Porteus.
“Targeting transgene addition without knocking out the target gene or
knocking it into a highly expressed gene has some exciting
applications,” noted Porteus. “There are opportunities and challenges
for
in vivo gene editing for hemophilia. There would be no need
to give patients conditioning agents and it is a potential method to
edit cells that naturally make clotting factors.” One drawback however,
is a lesser ability to monitor efficacy and off-target effects, he
added.
Porteus noted that an ethical concern of genome editing is equity and
distribution and how to take it to the parts of the world where most
people with hemophilia live.
Brigit E Riley, Sangamo BioSciences, USA, delivered new data on FVIII
using AAV delivery. “Using AAV in clinical and preclinical trials for
FIX has been successful, however there is a lag in the clinic for
FVIII.”
She said liver-directed AAV FVIII CDNA gene therapy is being explored
as liver cell DNA is separate from transgene DNA. Recombinant AAV is
efficient and stable long-term in tissues that do not divide such as the
liver, brain and muscle.
FVIII is not an ideal gene for AAV as it is constrained by gene size
and low efficiency of transcription/translation. Thus, it requires
multi-factorial modifications. “With the modifications, virus yield was
improved 8 to 10 fold,” said Riley.
She noted that data from
in vitro design saw good correlation between FVIII activity and levels along a range of doses.
In vivo wild type mouse data showed FVIII level 2 times normal.
In vivo
hemophilia A mouse model FVIII activity was 3 times normal and levels
were stable over time. A reduced bleeding time was also observed.
In vivo
non-human primate data FVIII levels were 4 to 6 times normal levels.
“Follow-up dose finding studies are aimed at determining minimal dose,”
said Riley.
The challenge of financial incentives is one area to still be
addressed. Porteus pondered, “With no established reimbursement model
for a one time gene therapy, it begs the question, ‘Are stakeholders
willing to take a chance on experimental curative therapies that have a
different conceptual basis when the current paradigm has transformed the
lives of hemophilia patients?’”
source:http://www.wfhcongressdaily.org/2016/07/options-in-delivery-of-gene-therapy-explored/