by Skolkovo Institute of Science and
Technology
Skoltech researchers and their colleagues
from Russia and the UK investigated the safety and efficacy of new chemistry in
antisense oligonucleotides used to treat spinal muscular atrophy (SMA), a
debilitating genetic disease. Their results may lead to the development of
drugs with less toxicity and fewer injections needed thanks to prolonged
action. The paper was published in the journal Nucleic Acid Therapeutics.
Antisense oligonucleotides are single
stranded chemically modified fragments of DNA that target pre-messenger RNA,
short bits of genetic information a ribosome reads to make a protein. Depending
on how a particular antisense oligonucleotide works, the target mRNA can either
be destroyed or undergo subtle changes in how it's spliced, i.e. how exons, the
coding regions, are excluded or included in the final mRNA.
Antisense oligonucleotides are good at
targeting so-called monogenic disorders, where the cause of the disease stems
from one particular gene/protein. A common example of such disease is spinal
muscular atrophy (SMA); people with this disease lose a functional protein
encoded by gene SMN1, and even though the human genome contains a nearly
identical copy, SMN2, the mRNAs transcribed from this gene lack just one
necessary exon, which leads to a poorly functioning protein.
To help the cells successfully use SMN2
instead of SMN1, an antisense oligonucleotide can interfere with splicing of
the precursor to mature mRNA that includes a particular exon. That is how
nusinersen, the clinically approved antisense oligonucleotide against SMA
marketed as Spinraza, works.
Timofei Zatsepin, Associate Professor at
the Skoltech Center of Life Sciences, CLS Senior Research Scientist Olga
Sergeeva and their colleagues studied alternatives to phosphorothioate groups
in splice switching oligonucleotides. Previously, Dr. Stetsenko from
Novosibirsk state university had developed oligonucleotides with
methanesulfonyl (mesyl, µ) or 1-butanesulfonyl (busyl, β) phosphoramidate
groups.
"Phosphorothioate is the key chemical
modification of nucleic acids developed by Prof. Fritz Eckstein in late 1960s
that is present in almost all oligonucleotide drugs approved so far. It
improves stability, pharmacodynamics, and pharmacokinetics of oligonucleotides,
but demonstrates significant toxicity that limits applications of
oligonucleotide drugs. During the last 30 years, many alternatives were
developed, but we do believe that mesyl phosphoramidates are superior to other
phosphate mimics in therapeutic oligonucleotides," Zatsepin said.
The new compounds are a bit unusual as
their structure is quite confusing when one sees the formula for the first
time. "You expect that such a bulk group should strongly interfere with
all intracellular interactions. However, our colleagues from Novosibirsk, led
by Dr. Dmitry Stetsenko, previously demonstrated that µ-oligonucleotides are much
less toxic than PS oligonucleotides, while duplexes of µ-oligonucleotides with
DNA are good substrates of RNAse H—a key enzyme for mRNA degradation via
antisense mechanism," Zatsepin noted.
The researchers were inspired by these
results and looked, among other things, for splice switching oligonucleotides
with potentially prolonged action. Nusinersen is administered several times a
year via an injection into the spinal canal, so fewer injections would improve
the quality of life for patients with SMA.
The team was able to test the activity of
novel antisense oligonucleotides in vitro in SMA patient-derived fibroblasts
and in vivo in a neonatal mouse model of SMA. "In our study we found that
µ-oligonucleotides were active in vitro, while in vivo the efficacy was lower
in comparison to nusinersen at the same dose. As µ-oligos are more stable and
less toxic in vivo than PS oligos, we propose that µ-oligos used in higher
doses can provide the same efficacy together with more prolonged action—this
study is under development now," Zatsepin explained.