New Exon 51 Skipping Therapy in Phase 1 Safety Trial Explained by Sarepta CEO in Webinar

Patricia Inácio, PhD avatar

by Patricia Inácio, PhD |

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Extracellular RNAs

Exondys 51 (eteplirsen), an RNA-targeted therapy marketed by Sarepta Therapeutics, is the first approved treatment targeting the underlying cause of Duchenne muscular dystrophy (DMD).

Sarepta is continuing to refine its RNA-targeted therapies, with a goal of being able to transport more of the treatment into damaged muscle cells. It is currently recruiting about 30 DMD patients amenable to exon 51 skipping — the same group that Exondys 51 treats — for a Phase 1 clinical trial (NCT03375255) testing a newer version of this therapy approach, called SRP-5051.

Doug Ingram, the company’s president and CEO, recently discussed the science behind this potential therapy and Phase 1 trial goals in a May 9 webinar arranged by Parent Project Muscular Dystrophy (PPMD).

Sarepta currently has 16 DMD programs, 11 of which are RNA-targeted therapies — first produced using a platform called PMO and now testing one using what’s called PPMO.

Others include therapies to modulate utrophin, gene therapies (three programs), and a gene-editing strategy. Utrophin is a protein functionally and structurally similar to dystrophin, and previous preclinical studies have shown it can improve muscle performance.

“PPMO is the program we are going to talk today,” said Ingram. Therapies based on it aim to treat 43 percent of children with DMD.

RNA-targeted therapy candidates for DMD are designed to target the so-called messenger RNA of dystrophin. The generation of the protein dystrophin starts with DNA, when the information encoded in the dystrophin gene is copied into an RNA molecule that, at first, is called precursor messenger RNA (pre-mRNA). This is because the pre-mRNA still contains the introns and exons found in the dystrophin gene.

Exons are the regions containing the information for protein production, while introns carry no relevant information for protein formation and are removed from the pre-mRNA. After their removal and all exons are linked together, a reading code is formed — the final messenger RNA molecule — that will be used to make the dystrophin protein.

In DMD, mutations in the dystrophin gene cause a deletion in an exon that affects its assembly. The exons no longer fit together, and the code becomes unreadable. As a result, very little or no dystrophin protein is made.

Sarepta RNA-targeted therapies are based on an exon-skipping strategy where a small molecule, an oligo, “hides” an exon so that the cellular machinery that processes the pre-mRNA is able to form the RNA molecule. As a result, a functional — although shorter  — dystrophin protein is made.

This is the mechanism underlying Sarepta’s PMOs, short for phosphorodiamidate morpholino oligomers, which are precise small RNA sequences that target the region in the pre-mRNA to be skipped.

One advantage of PMOs is their precision, as they’re created as a mirror image of the exon to be hidden, Ingram explained.

“They literally lay over that particular exon and they hide it from the cellular machinery, so that when [the machinery] is coming through to eliminate all the introns it doesn’t see that exon,” he said.

And they work 100% of the time, he added.

Their safety profile is also “fantastic,” Ingram said. This is in contrast with a lot of RNA molecules that, although with high therapeutic potential, are linked to toxicity.

But current PMOs — the technology on which Exondy’s 51 is successfully based — have a limitation. They are neutrally charged, which means they have a hard time getting inside cells.

“They do get in the cells,” Ingram said, especially since DMD children have leaky cells that facilitate their entry. But “they do not get into the cells as abundantly as we would like.”

Sarepta’s goal is a next-generation of PMOs with a higher penetrance into cells, and this is the promise of PPMOs.

PPMOs are basically PMOs with a new component, a cell-penetrating peptide — the reason for the extra in PPMO.

“What we’ve done is that we’ve taken the same sequence of PMO and we are attaching to it this cell-penetrating peptide,” Ingram said.

Data from animal models shows that adding this peptide brings the PMO at much greater abundance into the cell — overall, it works as a carrier molecule for PMOs. The intent is more effective exon skipping, more functional RNA, and more dystrophin protein production.

Preclinical data in non-human primates show a higher percentage of exon 51 skipping with increasing doses of PPMO — 20 mg/kg, 40 mg/kg and 80 mg/kg.  Animals tolerated the treatment, even at higher doses, and none had to stop the therapy due to safety issues.

“We see two extraordinary things — one is safety and the second is the exon skipping in these non-human primates,” Ingram said. “At 20mg/kg we start seeing a good exon skipping. At 40 mg/kg we really see amazing exon skipping, by historical standards, and at 80 mg/mg they have almost 100% of exon skipping RNA meaning that the therapy has gotten to the right place.”

Exon 51 skipping was detected not only in skeletal and smooth muscle, but also in the heart. Here, about 60 percent of exon skipping was found but only at the high 80 mg/kg dose, he reported.

“The PMO has really struggled to get into the heart and so when we look at the PPMO we see at the highest doses a significant amount of exon skipping that is very encouraging,” Ingram added.

PPMOs’ apparent safety was established in “four separate non-human primate studies … at very high doses, and much more frequently that it would be given to humans,” he added.

Researchers at Sarepta  also investigated a PPMO created specifically for a DMD mouse model, the mdx mice, which makes no dystrophin protein.

Results presented at the webinar showed that mice seven days after a single injection of PPMO had a marked amount of dystrophin in skeletal muscle quadriceps, and that protein expression was maintained at 30 and 60 days. Dystrophin protein began to decrease only after 90 days in these mice.

“There’s two things to notice here,” Ingram said. “The first that really gets us hopeful is that we are creating a significant amount of dystrophin protein, at least in a mouse model of DMD.”

The second is that a single injection induces dystrophin formation for 90 days before it begins to deteriorate.

“This means that we may not only create a therapy that leads to more dystrophin that we already had with PMO, but also that we can create a better dosing regimen for patients,” he said.

Exondy’s 51 is currently administered as a weekly infusion. These results suggest that PPMO-based treatments may be monthly infusions.

“That would be spectacular,” said Pat Furlong, president and CEO of Parent Project Muscular Dystrophy, who moderated the webinar.

In mdx mice, the increase in dystrophin expression after a single dose of PPMO, at either 40 mg/kg or 80 mg/kg, led to significant improvements in muscle function similar to that of mice without the disease.

The Phase 1 trial is now recruiting DMD patients — at seven sites in six U.S. states for now — to test SRP-5051, Sarepta’s first PPMO. SRP-5051 is designed to bridge the missing exon 51, a deletion mutation found in about 13 percent of all DMD patients.

The trial is open to those amenable to exon 51 skipping, ages 12 years or older, who are not being treated with Exondys 51 or drisapersen for at least six months before joining the study.

Ingram emphasized that patients on Exondys 51 should stay on it, as this is an early safety and tolerability study.

It will evaluate for potential side effects five escalating doses of SRP-5051 administered once, intravenously (directly into the blood). The treatment’s pharmacokinetics at each dose level, or how SRP-5051 moves through the body, will also be studied. Signs of efficacy will be measured by assessing the level of exon-skipping and the amount of dystrophin produced through a needle muscle biopsy, but effectiveness is not a main goal.

Enrolled patients will be sequentially assigned an SRP-5051 dose, starting with the lowest dose. A two-week screening period precedes the treatment, followed by five study visits over the next 12 weeks. A needle biopsy will follow a physical exam at week four post-treatment.

If all goes well, participants will be invited to continue treatment in an open-label extension study, Ingram said, in which all will receive SRP-5051 as monthly injections.

“As we learn more about the therapy and the doses go up, the dose administered in the open-label phase goes up as well,” Ingram said.

Four additional U.S. sites are planned, and Sarepta hopes to open sites in other countries.

“Our goal is to move as rapidly as possible hoping to get an approval from the FDA,” he said.

The company will is also working to develop PPMOs for patients with other deletion mutations amendable to this treatment approach, including exon 44, 45, 50, 52 and 53 — raising the potential to treat around 43 percent of all DMD patients. It plans to file requests with the FDA to open additional clinical trial later this year and early next year.

“Sarepta exists for one reason: that is to develop therapies that bring hopefully a better and longer life to those who are suffering from genetic diseases and very specifically DMD,” Ingram said. “We are a DMD company.”