Researchers at the University of Kansas Medical Center in Kansas City had reason to hope that people with ALS might benefit from rasagiline. The drug, already in use for Parkinson’s disease, is thought to be neuroprotective and to, somehow, protect or bolster mitochondria (see Wu et al., 2015). However, as Richard Barohn announced at the April American Academy of Neurology meeting in Boston, there was no evidence of significant therapeutic benefit in their most recent Phase 2 ALS trial. Albert Ludolph of Germany’s Ulm University shared similar results, from his own Phase 2 rasagiline trial, at the May meeting of the European Network to Cure ALS in Ljubljana, Slovenia.
Over the last several years, mitochondria have proved a tempting but intractable target for ALS. In cells, mice, and people, the disease clearly creates trouble for the cell’s powerhouse (Atsumi, 1981; Igoudjil et al., 2011). Moreover, mutations in the gene for the mitochondrial protein CHCHD10 can cause the disease in people, suggesting breakdown of the organelle can lead to ALS (see June 2014, Oct 2014 news; Bannwarth et al., 2014; Chaussenot et al., 2014; Johnson et al., 2014; Muller et al., 2014). But rasagiline is the most recent in a series of mitochondria-targeted drugs that disappointed by not meeting endpoints in clinical trials.
So, are mitochondria still a worthwhile target for ALS researchers and clinicians to pursue?
“I certainly think so,” said neurologist Russell Swerdlow of the University of Kansas Medical Center, an investigator on the rasagiline study. So do others in the field. A Massachusetts General Hospital team, led by Sabrina Paganoni, is soon to begin a Phase 2 trial of Amylyx Pharmaceuticals’ AMX0035, a combination therapy they hope will promote the survival of motor neurons in ALS. One-half of the combo, the bile compound tauroursodeoxycholic acid (TUDCA), is thought to be neuroprotective in part by stabilizing mitochondria, increasing their turnover and reducing their generation of reactive oxygen species , as well as by preventing apoptosis (Rodrigues et al., 2003; Rosa et al., 2017). In a pilot study, people with ALS who took TUDCA had a 15% slower disease progression during a 54 week period (Elia et al., 2015).
Nonetheless, the idea of mitochondria as an ALS target remains immature, said Swerdlow: “We’re still not very sophisticated in terms of how we’re going to manipulate mitochondria [in people with the disease].”
Nor do researchers have a clear picture of what, precisely, is wrong with those powerhouse organelles, though it’s clear they’re relevant to the disease. People with ALS exhibit swollen mitochondria in the brain, spinal cord and skeletal muscle, according to ultrastructural studies of post-mortem tissue (reviewed in Smith et al., 2017). A SOD1 mouse model of ALS exhibits key signs of mitochondrial malfunction, including defects in the ability to buffer calcium ions and electron transport protein complex activity (reviewed in Tan et al., 2014). TDP-43 localizes to mitochondria where it affects the organelles’ size and distribution (Wang et al., 2017). Additionally, poly(GR) repeats encoded by another ALS gene, C9orf72, bind to mitochondrial ribosomes to incite oxidative stress and mitochondrial dysfunction (Lopez-Gonzalez et al., 2016).
“The majority of genetic forms of ALS, in one way or another, touch mitochondria,” said Giovanni Manfredi of Weill Cornell Medical College in New York. “But the ways they’re doing that are different.”
In 2014, researchers discovered a direct genetic link between mitochondria and ALS, mutations in CHCHD10 (see June 2014 news; Bannwarth et al., 2014). The finding suggested that a power drop in the CNS could result in the disease. But a recent study of cultured mammalian cells led by David Kang and colleagues at the University of South Florida in Tampa raises questions about exactly how the mutations instigate trouble. Kang’s team reported that CHCHD10 mutations caused the normally nuclear TDP-43 to move into the cytoplasm, a common feature of most forms of ALS. Conversely, wild-type CHCHD10 reduced TDP-43 toxicity (see June 2017 news; Woo et al., 2017).
The results suggest that CHCHD10 may protect neurons in the CNS by preventing TDP-43 from leaving the nucleus. When levels of CHCHD10 drop or mutations interfere, TDP-43 enters the cytoplasm, sickening the neurons. At the same time, mitochondria become destabilized, leading to energy deficits and increased levels of dangerous superoxide.
What, then, is the root problem in CHCHD10 cases—the damage to the mitochondria, or the movement of TDP-43 into the cytoplasm? Manfredi suspects that the defects in these powerhouses alone would not be sufficient to kill the cells. Both mitochondria and TDP-43 defects are likely involved, he suggested.
Christine Vande Velde of the University of Montreal thinks that TDP-43 may sense the damage in mitochondria caused by CHCHD10 mutations, and migrates to the cytoplasm as a result.
At this point, much remains to be discovered about CHCHD10’s mechanism of action. Scientists still need to work out what CHCHD10 does in neurons. Plus, Manfredi pointed out, researchers disagree on whether the ALS-linked mutations simply block the protein’s normal function, or if they cause CHCHD10 to adopt some new, toxic ability. That could have important implications for possible treatments. For example, if mutant CHCHD10 is toxic, antisense oligonucleotides could be a potential treatment for this particular kind of ALS. But if the problem is when the CHCHD10 disappears, that would be the wrong approach.
Trials and Tribulations
Clinical trials have tested a handful of experimental and existing medications, thought to help mitochondria via somewhat murky mechanisms, against ALS. Olesoxime, now licensed to Hoffman-La Roche in Basel, Switzerland, interacts with the mitochondrial permeability transition pore and is thought to, in some way, mediate oxidative stress and uptake of metabolites (Bordet el al., 2007). The drug delayed disease onset and death in ALS SOD1–G93A model mice (Bordet el al., 2007). Olesoxime, however, did not significantly affect their survival of people with ALS, according to a Phase 3 clinical study (Dec 2011 News).
Meanwhile, scientists at Knopp Biosciences in Pittsburgh, Pennsylvania turned their attention to dexpramipexole as a potential treatment. They suspected that dexpramipexole would assist stressed mitochondria, in part, by boosting their efficiency (Alavian et al., 2011). Early studies of SOD1-G93A mice, with treatment starting at day 45, reported it prolonged motor function and lifespan by 10 days, a notable extension in mice that typically don’t last beyond 4.5 months (Danzeisen et al., 2006). In a Phase 2 trial, it seemed to slow disease (Cudkowicz et al., 2011; Rudnicki et al., 2013). But no benefit was seen in Phase 3, as the developers from Cambridge, Massachusetts-based Biogen, who licensed the drug, reported in 2013 (see Jan 2013 News). Subsequent analysis by the ALS-Therapy Development Institute, also in Cambridge, indicated that dexpramipexole was ineffective in SOD1-G93A mice, treated from 50 days, and in neural cultures expressing human TDP-43 (Vieira et al., 2014).
Rasagiline was the next mitochondrial hope. It’s a monoamine oxidase type-B inhibitor taken by people with Parkinson’s disease, but it’s also thought to have neuroprotective potential (Youdim et al., 2001). Early cell culture experiments hinted it might help stressed mitochondria by inhibiting apoptosis and minimizing oxidative damage (Chen and Swope, 2005; Maruyama et al., 2001; Maruyama et al., 2001). Subsequent analysis indicated that treatment with rasagiline improved motor abilities and survival in SOD1-G93A mice (Waibel et al., 2004). Barohn and his team, therefore, tested it ALS in an open-label pilot study of 36 patients (Macchi et al., 2015). There was no change in the rate of decline on the ALS-Functional Rating Scale. But, Swerdlow and colleagues measured mitochondrial function in the subjects’ white blood cells, and they found signs of target engagement. The organelle’s membrane potential went up, while markers of apoptosis such as Bcl-2 dropped. That convinced researchers to further evaluate the potential treatment approach by performing two new Phase 2 clinical trials. However, they found no significant benefit.
What’s the difficulty?
“In the haystack of things that can go wrong with the mitochondria, we don’t know what the needle is,” said Swerdlow. Vande Velde and Manfredi also believe these cellular powerhouses remain a valid potential target, if drugs could affect them more precisely.
“I don’t think any of these clinical trials have really addressed the problem head-on,” said Manfredi. “None of these drugs has a very clear mechanism of action.” In fact, he noted, there are no drugs for any mitochondrial disease, so the organelle is clearly a difficult target. Plus, just because a drug helps ALS model mice does not mean it will benefit people, particularly since human patients more often have sporadic disease, he pointed out.
Another question, Swerdlow said, is when in the course of ALS the mitochondria play a key role. Medications that help motor neurons power up have the best shot at also helping patients if mitochondria are an early player in the disease.
It remains possible that medications that target mitochondria could help, if given to people with ALS mutations before the disease ever manifests according to Vande Velde. She suspects these pre-onset folks are the ones who are most likely to benefit from such drugs. Indeed, many of the preclinical studies in mouse models of the disease that showed benefit were administered at least a month before the animals showed symptoms.
But Swerdlow is confident that the right medication should do the job for later-stage patients, too. “If we could figure out what the true target that needs to be modified is, and we could effectively modify that target, I would like to think that intervention would show efficacy at any point in disease,” he said.
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