Alnylam's Experimental RNA Therapy Lowers Huntington's Disease Protein by 60 Percent in Early Trial

A single spinal injection of an experimental gene-silencing drug has cleared out more than half of the toxic protein that destroys the brains of patients with Huntington's disease. In a Phase 1b clinical trial, the RNA interference therapy brezosiran reduced mutant huntingtin protein in the cerebrospinal fluid by an average of 60 to 64 percent after three months.[1][3]

The results represent some of the deepest protein reductions ever recorded in human trials for the fatal neurodegenerative disorder. One patient in the highest-dose cohort saw their mutant protein levels plummet by 83 percent, a magnitude of clearance that analysts called a best-in-class knockdown.[1][2]

"We plan to evaluate our differentiated huntingtin-lowering approach in a rigorous placebo-controlled study to determine whether it can alter the course of Huntington's disease," said Pushkal Garg, chief research and development officer at Alnylam Pharmaceuticals.[2][4]

The data, presented at the European Huntington's Disease Network Clinical Research Congress in Krakow, Poland, offer a rare bright spot in a field defined by clinical heartbreak. Huntington's disease is an inherited genetic condition that typically strikes between the ages of 30 and 50, causing a progressive loss of motor control, cognitive function, and psychiatric stability over 15 to 20 years.[2][4]

There are currently no approved treatments capable of slowing or stopping the disease. The condition is caused by a genetic mutation that forces cells to produce a misfolded, toxic version of the huntingtin protein, which clumps together and kills neurons.[1][4]

Silencing the genetic instructions

Brezosiran, previously known as ALN-HTT02, uses a mechanism called RNA interference to intercept the genetic instructions before the toxic protein can be manufactured. Developed in a partnership with Regeneron Pharmaceuticals that is worth upward of $1 billion, the drug is administered via an intrathecal injection directly into the fluid surrounding the spinal cord.[2][4]

The therapy utilizes Alnylam's proprietary C16 conjugate platform to deliver the RNA interference payload directly into the central nervous system. The Phase 1b trial enrolled 48 adults between the ages of 25 and 70 who have genetically confirmed Huntington's disease in Stage 2 or early Stage 3 of the disease's integrated staging system.[3][5]

Participants were divided across five cohorts receiving single, ascending doses of the drug, with doses increasing by no more than three times when escalating to the next cohort. This structure allowed researchers to track both safety and the dose-dependent biological response, confirming target engagement through two independent laboratory assays.[4][5]

Alnylam reported that the protein reductions were durable enough to suggest that patients might only need to receive the injection twice a year. The company is currently enrolling a sixth cohort of 12 patients to test an even higher dose of the medication.[1][4]

Crucially, the early data showed no dose-dependent increases in neurofilament light chain, a key biomarker of nerve cell injury. The trial also found no spikes in inflammatory markers or changes in brain ventricular volume over a 12-month follow-up period, clearing a major safety hurdle that has derailed previous candidates.[3][5]

Targeting a toxic fragment

The Huntington's drug development pipeline is littered with high-profile failures. In 2021, Roche halted a massive Phase 3 trial of its antisense oligonucleotide tominersen after the drug failed to outperform a placebo, and other gene therapies have seen their initial efficacy signals fade over time.[1][2]

Alnylam believes brezosiran's specific targeting strategy may help it avoid a similar fate. While previous drugs suppressed both the mutant and healthy forms of the huntingtin gene, brezosiran targets a specific region of the messenger RNA called exon 1.[1][4]

This approach is designed to halt the production of the full-length mutant protein as well as a shorter, highly toxic fragment known as HTT1a. Researchers increasingly suspect that this shorter fragment plays a critical role in driving the rapid progression of the disease, making it a vital target for intervention.[3][5]

The 60 to 64 percent reduction compares favorably to other experimental therapies currently in development. A drug developed by Wave Life Sciences reduced the mutant protein by 46 percent when given every eight weeks, while a candidate from Vico Therapeutics lowered it by 38 percent after four months.[1]

"The severity of HD, coupled with lack of treatments, confers a significant market opportunity," analysts at Oppenheimer wrote in a note to investors, describing the trial results as a "best case scenario" for the drug's biological activity.[2]

The long road to efficacy

Despite the striking biomarker data, the Phase 1b trial was not designed to measure whether the drug actually improves patients' symptoms or slows their decline. The follow-up period was too short, and the patient cohorts too small, to detect changes in motor function or cognitive preservation.[2][4]

Proving that a reduction in the mutant protein translates into a meaningful clinical benefit remains the ultimate test. Eligible participants from the current study are now transitioning into an open-label extension trial, where they will receive brezosiran every six months for up to three years to track long-term safety and exploratory disease progression metrics.[3][4]

Pending alignment with regulatory agencies, Alnylam plans to launch a pivotal, placebo-controlled efficacy trial in 2027. That larger study will definitively test whether clearing the toxic protein can finally alter the devastating trajectory of Huntington's disease.[2][3]