Researchers Tweak Experimental Therapy for MMD1

Adding a cell-penetrating peptide to a compound that reduces clump formation in cell nuclei increased its effectiveness in type 1 myotonic dystrophy research mice

Article Highlights:
  • A compound known as ASO CAG25, developed in part with MDA support, is designed to reduce formation of abnormal clumps in the nuclei of MMD1-affected cells and to free cellular proteins from entrapment.
  • Adding a muscle-cell-penetrating peptide to ASO CAG25 appears to allow the modified ASO, at least in mice, to enter muscle tissue when given intramuscularly or intravenously, and to counteract the effects of the underlying defect that causes MMD1 without apparent toxicity.
by Margaret Wahl on February 27, 2013 - 3:51pm

Scientists at the biopharmaceutical company Genzyme, working with mice, say they have modified and improved an existing experimental strategy to treat type 1 myotonic muscular dystrophy (MMD1, also known as DM1).

The existing experimental therapy — known as antisense oliogonucleotide (ASO) CAG25 — was developed in part with MDA support. It sticks to and "neutralizes" the molecular defect — an elongated stretch of genetic material (RNA) that accumulates in the cell nucleus — that underlies MMD1.

When elongated strands of RNA accumulate in the nuclei of MMD1-affected cells, they ensnare proteins, particularly one called MBNL1, keeping them from doing their regular jobs, such as assisting in the synthesis of other proteins — including one that forms a chloride channel, a cellular pore through which chloride ions flow. It's the malfunction of the chloride channel that underlies myotonia, the inability to relax muscles at will, which is a hallmark of myotonic dystrophy.

The modification made by Genzyme scientists was to improve the ability of the CAG25 ASO to penetrate muscle fibers, thereby increasing its effectiveness.

"We're delighted to see that researchers from Genzyme have shown that they can effectively deliver an oligonucleotide that targets the mutation that causes myotonic dystrophy," said Jane Larkindale, MDA's vice president of research. "Delivery of such ASOs has long been a challenge, although the therapeutics have looked promising in terms of efficacy for some time."

Muscle injections of modified ASO dispersed tangles

Andrew Leger and colleagues at Genzyme conducted a series of experiments in mice with a disease mimicking human MMD1, publishing their findings online Jan. 11, 2013, in Nucleic Acid Therapeutics.

They attached a peptide (small protein) to the ASO and its backbone, creating two experimental compounds, which they dubbed PPMO-B and PPMO-K.

When they delivered PPMO-B and PPMO-K into leg muscles of the MMD1 mice by direct injection, they found that:

  • there was dispersal of the abnormal clumps in the cell nuclei;
  • the MBNL1 protein was freed from the clumps; and
  • a process called splicing — needed for proper synthesis of proteins — corrected the synthesis of a protein called SERCA1, which plays a role in calcium handling in muscle cells.

Intravenous treatment also effective

The researchers then evaluated whether PPMO-B and PPMO-K could be effective in the mice if they were delivered intravenously, which is a generally desirable form of drug administration when large areas of tissue (such as all the muscles) are targeted.

When the MMD1 mice were treated with six weekly intravenous injections of the experimental compounds, the researchers found that:

  • the mice tolerated the dose well and showed no overt signs of toxicity, including in the liver or kidney, throughout the study period;
  • the splicing process involved in synthesis of the SERCA1 protein was normalized in almost all the treated animals — something that did not happen with the unmodified ASO; and
  • the splicing process involved in synthesis of three other proteins, including the chloride channel that is improperly spliced in MMD1-affected cells, also was normalized.

"These results suggest," the researchers say, "that systemically delivered PPMO-B and PPMO-K were able to penetrate into the skeletal muscle, enter into the muscle [nuclei] … and liberate an amount of sequestered MBNL1 protein sufficient to restore … splicing."

Additional tests conducted in mice treated with intravenous PPMO-K detected no myotonia.

The researchers note that the test agent administered to mice in the studies is the same therapeutic candidate that could be evaluated for use in people with MMD1.

MMD therapy development

MDA has long supported the development of ASOs and other experimental therapies for MMD1 and the related MMD2.

Charles Thornton and Thurman Wheeler, both MDA research grantees at the University of Rochester in New York state, are acknowledged on the paper from Genzyme as having provided guidance and materials.

For more information

To learn more about MMD1 research strategies, read:

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