19 Jul 2018

Though mutations in superoxide dismutase 1 (SOD1) have been a known cause of familial ALS for more than two decades, researchers have had little success suppressing the mutant protein in patients. While initial antisense nucleotides proved safe, poor efficacy sidelined further clinical development until more potent versions could be found. Now, in the July 16 Journal of Clinical Investigation, researchers led by Timothy Miller at Washington University in St. Louis publish the first preclinical data on those second-generation oligonucleotides. They report that these more potently quashed SOD1 expression and better prolonged survival in mouse and rat ALS models than did the first-generation SOD1-ASOs. Adopting a different anti-SOD1 strategy, researchers led by Hidenori Ichijo at the University of Tokyo screened for small molecules that block the dismutase’s interactions with Derlin1, a component of the ER-associated protein degradation apparatus. In the July 10 Nature Communications, they report that lead compounds prolonged survival in ALS mice. Clinicians have already begun testing the next-generation ASOs in SOD1-ALS patients.

About 15–20 percent of cases of ALS are familial, and of those, 20 percent are caused by mutations in SOD1. The prevailing view in the field is that SOD1 causes ALS via a toxic gain of function since overexpressing the protein in mice triggers many aspects of the human disease, including denervation of neuromuscular junctions and motor neuron death. As such, efforts to dampen SOD1 expression are in vogue.

ASOs represent one such strategy, and in fact, the first ASOs ever tested in humans, back in 2013, were to SOD1 (May 2013 news). Made by Ionis Pharmaceuticals in Carlsbad, California, these nucleotides proved safe in that small Phase 1 study, led by Miller. Ionis later teamed up with Biogen to move forward with SOD1-ASOs, but opted to generate more potent versions. After the 2013 trial, Ionis scientists went back to the drawing board, Miller said. They screened more than 2,000 ASOs, each designed to pair with different snippets of the SOD1 RNA sequence, to find those that most efficiently knocked down expression. The top two ASOs from the screen—dubbed ASO1 and ASO2—targeted the 3'UTR of SOD1. The current study describes the activity of those ASOs in mouse, rat, and nonhuman primate models.

First author Alex McCampbell of Biogen, Inc., in Cambridge, Massachusetts found a dose-dependent reduction in SOD1 mRNA following injection of ASO1 or ASO2 into the cerebrospinal fluid of transgenic mice or rats expressing SOD1-G93A. Reaching a maximum of 75 percent reduction in SOD1 transcripts, both ASOs knocked down SOD1 expression more potently than ASO-333611, Ionis’ previous lead ASO—sixfold less was needed to halve SOD1 expression. The SOD1 takedown held steady for 10 weeks after a single injection of the therapy.

The suppression came with improved symptoms. ASO1 or ASO2 delayed weight loss—a marker of disease onset—by 70 days in SOD1-G93A rats, and prolonged their survival by 53 and 64 days, respectively. In contrast, ASO-333611 did not delay onset and extended lifespan by only 29 days. In SOD1-G93A mice, the new ASOs had similar effects on onset and survival as in the rats.

At the cellular level, the ASOs staved off the erosion of neuromuscular junctions. SOD1-G93A mice treated with ASO1 at five weeks of age maintained innervation of the tibialis anterior muscles in the calf over the next 12 weeks, while 75 percent of junctions fell apart in control mice. Animals treated with the SOD1-ASOs also had a slower rise in CSF phospho-neurofilament heavy chain (pNHF)—a marker of disease progression. In a treatment paradigm, the ASO1 appeared to reverse loss of neuromuscular function when given to nine-week-old mice, and limited the uptick in pNHF. This hinted that perhaps the new ASOs might reverse neural dysfunction.

The researchers also tested ASO1 in cynomolgus monkeys. One week after intrathecal injection of ASO1, they found a dose-dependent reduction in SOD1 mRNA and protein in the CNS. SOD1 protein dropped by half at the highest dose. The drug was not as potent at knocking down SOD1 expression in monkeys as it was in the rodent models expressing human SOD1, likely because the ASO1 sequence has a one-base-pair mismatch with the monkey SOD1 sequence, they write.

In all, the findings suggest that the new ASOs make for promising therapeutic candidates for people with SOD1-ALS, Miller told Alzforum. In 2016, Ionis and Biogen initiated a Phase 1 trial with ASO1, also known as BIIB067 (see clinicaltrials.gov). The trial is slated for completion in February 2019, and participants are transferring to an ongoing open-label extension study (clinicaltrials.gov).

Breaking Up a Toxic Relationship
As described in their Nature Communications paper, first author Naomi Tsuburaya and colleagues took an entirely different approach to attack SOD1. The researchers previously reported that a Derlin-1 binding domain is exposed on mutant, but not wild-type, forms of SOD1, facilitating a toxic partnership between the two proteins (Nishitoh et al., 2008; Fukisawa et al., 2012). 

To see if they could free Derlin-1 from mSOD1’s grip, the researchers screened 160,000 small molecules from the public chemical library at University of Tokyo’s Drug Discovery Initiative. They capitalized on a high-throughput system they developed that uses iPSC-derived motor neurons from SOD1-ALS patients. To keep tabs on the SOD1-Derlin1 association, the researchers chose time-resolved fluorescence resonance energy transfer (TR-FRET), in which photons released by one fluorophore are absorbed by another close by, changing the emission spectrum. In this case, the donor and acceptor fluorophores were attached to SOD1 and Derlin1 antibodies. From the screen, the researchers found 44 compounds that specifically suppressed SOD1-Derlin FRET in a dose-dependent manner. They then took into account a multitude of other factors, including medicinal chemistry, to narrow the list down to one test compound called #56. The researchers subsequently generated multiple analogs of #56 to find more effective compounds. Notably, #56 analogs successfully thwarted SOD1-Derlin1 interaction in all 122 mutants of SOD1 the researchers tested.

Could #56 analogs stave off ALS? The researchers first tested the compounds in iPSC-derived motor neurons from SOD1-ALS patients. One derivative—#56-59—increased the survival of motor neurons harboring the L144FVX mutation in SOD1, but not the G93A mutation. However, in SOD1-G93A mice, intracerebroventricular infusion of #56-59 six weeks prior to typical disease onset pushed back the first signs of symptoms from 31 weeks to 35.5, and prolonged survival by about five weeks. This suggested the compound primarily worked by delaying the start of disease, but not by slowing its progression. The treatment also spared the death of motor neurons. At 31 weeks, #56-59-treated mice had more than twice as many motor neurons as their untreated counterparts.

Christine Vande Velde of the University of Montreal in Quebec expressed cautious optimism for the new class of inhibitors. “It will be interesting to see validation of this result in other mutant SOD1 mouse models with lower copy number/total accumulated protein levels,” she wrote. “Going forward, the work raises additional questions, such as how does the efficacy of this small molecule compare to the SOD1 ASO in development?”—Jessica Shugart

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References

News Citations

1. Paper Alert: Antisense Oligonucleotide Therapy Safe for ALS? 9 May 2013

Research Models Citations

1. SOD1-G93A (hybrid) (G1H)

Paper Citations

1. Nishitoh H, Kadowaki H, Nagai A, Maruyama T, Yokota T, Fukutomi H, Noguchi T, Matsuzawa A, Takeda K, Ichijo H. ALS-linked mutant SOD1 induces ER stress- and ASK1-dependent motor neuron death by targeting Derlin-1. Genes Dev. 2008 Jun 1;22(11):1451-64. PubMed.
2. Fujisawa T, Homma K, Yamaguchi N, Kadowaki H, Tsuburaya N, Naguro I, Matsuzawa A, Takeda K, Takahashi Y, Goto J, Tsuji S, Nishitoh H, Ichijo H. A novel monoclonal antibody reveals a conformational alteration shared by amyotrophic lateral sclerosis-linked SOD1 mutants. Ann Neurol. 2012 Nov;72(5):739-49. PubMed.

External Citations

1. clinicaltrials.gov
2. clinicaltrials.gov
3. Drug Discovery Initiative

Further Reading

Papers

1. Fujisawa T, Takahashi M, Tsukamoto Y, Yamaguchi N, Nakoji M, Endo M, Kodaira H, Hayashi Y, Nishitoh H, Naguro I, Homma K, Ichijo H. The ASK1-specific inhibitors K811 and K812 prolong survival in a mouse model of amyotrophic lateral sclerosis. Hum Mol Genet. 2016 Jan 15;25(2):245-53. Epub 2015 Nov 24 PubMed.

News

1. At AAN, Sights Set on Antisense Therapies for Diseases of the Brain 25 May 2018
2. Motor Neuron Vulnerability: The Case of the Missing Chaperone? 20 Mar 2015