Amid ALS Mania, Possible Drug Target Spotted
As, all around them, everyone from Derek Jeter to the Kennedy family was dousing themselves for the ALS “Ice Bucket Challenge,” Harvard University researchers announced last week they may have found an ALS therapy—or two. When they blocked the gene for prostanoid receptor DP1 in ALS-model glial cells in a dish, they found neurons made from human embryonic stem (ES) cells were “completely protected” from death.
When they created ALS (amyotrophic lateral sclerosis) mice with that same gene deleted, the mice lived 6.7 percent longer.
This helps validate the idea that neurons made from human stem cells— in a dish--can be drug screens, team leader and Harvard stem cell researcher Kevin Eggan told a press conference. The 6.7 percent survival increase may rise even more, he said, if/when their DP1 antagonist is given with a drug they earlier found has anti-ALS properties. And as both drugs are FDA-approved for other indications, clinical trials for them could, conceivably, move fast.
“We think this is a significant advance—both in terms of the use of stem cells for understanding disease, and with respect to understanding the degenerative process of ALS and how we might inhibit them,” said Eggan.
Leading ALS researcher Jeffrey Rothstein of Johns Hopkins University was more cautious. A trial he helped conduct blocking the same inflammatory pathway failed, said Rothstein (who was uninvolved in the Eggan study). One reason: Earlier stage patients in that trial did not have raised levels of prostaglandin, a key protein in the pathway.
“If you want to try it in humans, you have to make sure that pathway is activated, and data from that earlier trial suggests that it’s not really that activated,” said Rothstein in an interview. “It may be activated by the time you die, but who cares. You try to treat patients well before death.” So the team may want to seek patients with elevated prostaglandin levels, long before end stage.
“That would perhaps do it,” Rothstein said.
In vivo and in vitro studies agree
In Science Translational Medicine, the Eggan team reported creating motor neurons from green fluorescent protein (GFP)-expressing ES cells (which light up green when they become neurons). These were then co-cultured with either normal cells, or SOD1 mutant glial cells, from neonatal mice. (SOD-1 is a gene mutated in a subset of ALS patients.) After ten days, they found the glial cells from the SOD1 mice caused a 55 percent decrease in motor neurons— compared to neurons co-cultured with normal glial cells.
“Strikingly, chronic treatment” with a selective DP1 antagonist “completely protected” the neurons from toxic SOD1 glia, the team reported. The team then found an activator of DP1 caused a 49 percent decrease in motor neurons, similar to the effect of SOD1 mutant glia. The team also found the DP1 inhibitor acted on glial cells, which then impacted neurons.
As “it is still unclear to what extent findings from stem cell models will be predictive of outcomes in vivo,” the group also created SOD1 mice with an inoperative DP1 gene. They lived “significantly longer,” or a minimum of 6.7 percent longer, than mice without the disabled DP1 gene. This “provides in vivo validation that DP1 suppression is a relevant therapeutic strategy in ALS.”
“There are now many examples of the successful recapitulation of known phenotypes from patients in stem cell models of disease. However there have been few attempts to determine whether mechanistic insights gleaned by stem cell disease modeling can be validated in vivo…Here we report the DP1 receptor is a critical mediator of glial toxicity to motor neurons in a stem cell model of ALS. Elimination of even a single allele of the gene encoding this receptor significantly extended the life span of the most widely studied ALS mouse model…inhibition of the DP1 receptor is a rational strategy for slowing the negative impact of microgliosis in ALS,” the team reported.
At the presser, Eggan said prostanoid receptors like DP1 are “upregulated in the spinal cord of many ALS patients,” according, he said, to a recent biomarker study conducted by Massachusetts General Hospital. “This provides tantalizing evidence the same process could be at play in the spinal cord of many ALS patients.” He admitted the study did not find the receptor upregulated “in all ALS patients, so an important aspect of the study may be to develop a biomarker to allow us to discover which ALS patients” should try the drug— just as Rothstein suggested.
He also noted that “two major pharmaceuticals have significant development programs around this particular receptor for another indication, which is niacin induced flushing.” That is, a potential drug to block the PD1 receptor is already on the shelf.
Furthermore, Eggan said, in two previous papers, his group and a Children’s Hospital team reported finding that the anti-epileptic retigabine quells hyper-excitability of ALS motor neurons. GlaxoSmithKline is involved with that research and a clinical trial is expected next year. That drug, as noted, may eventually be tried in conjunction with a DP1 inhibitor. “In the future it could be very attractive to combine these two therapeutic approaches, as they act on different disease mechanisms that converge on motor neurons in ALS,” Eggan said.
Rothstein cautioned the above combo is “interesting,” but added that “we never do combinations of drugs until each is found individually effective. Putting two drugs together as a new combination can have bad [results]. Some trials of the FDA-approved riluzole with new drugs turned out worse” than riluzole alone, he said. “Even FDA-approved drugs when studied first can be toxic. Years ago, Columbia University studied the antibiotic Minocycline in ALS. Instead of helping, it actually spread the disease by 25 percent.”
Eva Feldman, head of the American Neurological Association, said in an email that combinations may be key. “Many cell types and a complex interplay of several mechanisms are likely involved.” Calling the Harvard study “definitely encouraging,” she listed caveats. “The authors cited one study which reported elevated DP1 in spinal cord tissue of ALS subjects; however, additional confirmation of alterations in DP1 expression at earlier disease stages in humans--and confirmation of the ability to reverse the phenotype of already sick motor neurons--may be warranted.”
Uninvolved with the Harvard study, she noted DP1 receptors are located elsewhere, so “consequences of DP1 inhibition in other cell types are important to understand to avoid off-target side effects." More preclinical work is needed, she said, if she agreed “preclinical studies based on repurposing available highly selective DP1 antagonists could accelerate the clinical translation.”
Overall, she said: “I definitely believe the combined approach, using both cellular models and established rodent models, confirms that the proposed pathways are pertinent and relevant.”