Afflicting one in 4,000 boys and about half as many girls, Fragile X syndrome arises from disruptions in a single X-chromosome gene, Fmr1, reducing expression of Fragile X mental retardation protein (FMRP). “Excess” characterizes many Fragile X features—among them excessive excitability, body growth, and synaptic connectivity. Many FXS symptoms have been linked to overactive mGlu5 receptors, which drive synthesis of synaptic proteins. That led Bear to propose that FMRP acts to suppress mGlu5-triggered protein synthesis. According to his theory, mGlu5 activity goes unfettered without FMRP, producing the diverse problems seen in Fragile X (Bear et al., 2004). A genetic study by Bear and colleagues supported this idea—Fmr1 knockout (KO) mice with only one copy of the Grm5 gene and half normal mGlu5 activity were spared a slew of abnormalities seen Fmr1 KO controls (ARF related news story on Dölen et al., 2007). While that work established proof of principle that dampening mGlu5 could prevent FX deficits, it did not show that they could be reversed later in life. Other groups improved symptoms in adult mouse and fly Fragile X models using a mGlu5 antagonist (MPEP) (Yan et al., 2005; ARF related news story on McBride et al., 2005), but it was only partial relief, and some animals developed tolerance to the compound. The current paper shows “we can intervene pharmacologically after symptom onset and correct many aspects of Fragile X,” Bear told ARF.

Led by co-first authors Aubin Michalon of Roche and Michael Sidorov of MIT, the researchers treated Fmr1 knockout mice with a different mGlu5 blocker, a pyridine derivative called CTEP (Lindemann et al., 2011). Though discovered at Roche, CTEP is not being developed by the company as a human drug, but “has ideal properties for studies in rodents,” Lindemann said. The compound not only has great pharmacokinetics and oral bioavailability, but also is considerably more potent and long-lasting than commercially available mGlu5 inhibitors, for example, MPEP and another Roche compound, fenobam (Porter et al., 2005). In mouse studies, those molecules need to be given four to five times a day because their half-life is a mere hour or two. By comparison, CTEP acts for about 18 hours, allowing researchers to see some symptoms improve in Fmr1 knockout mice after just a single subcutaneous dose (2 mg/kg body weight). For chronic dosing experiments, the mice were given this amount of the inhibitor every other day for four to 17 weeks.

With the exception of macroorchidism (medical speak for enlarged testes), which was only partially rescued, all nine phenotypes analyzed in the study returned to normal with CTEP treatment. Fmr1-deficient mice become startled easily and have trouble acclimating to their environment. They also have problems remembering where danger signals (e.g., foot shocks) are. CTEP rescued these behavioral deficits. Fmr1 knockout mice respond to loud noise with seizures, and have exaggerated hippocampal long-term depression. These, too, were fully corrected by the compound. Moreover, CTEP restored the elevated spine density and protein synthesis rates to normal, and abolished hyperactive ERK kinase and mTOR signaling typically seen in Fmr1 knockouts.

“This article demonstrates that the kind of therapeutic intervention we can realistically implement in patients is extraordinarily effective in reversing the major Fragile X phenotypes at cellular, synaptic, neural circuit, and behavioral levels,” noted Michael Tranfaglia of FRAXA Research Foundation, which funds Fragile X research (see full comment below).

Before the current paper came out, Roche and several other companies had begun developing mGlu5 inhibitors for Fragile X. Roche completed a six-week Phase 2 trial of its compound, R04917523, but results have not been reported. “I am unfortunately unable to disclose data,” Lindemann noted. “We can say, though, that the data obtained so far support continuation of the clinical development of the molecule.” A larger, 12-week Phase 2 study is in the works, with recruitment to begin “any day,” Lindemann said.

Seaside Therapeutics, Inc., of Cambridge, Massachusetts, a company Bear co-founded in 2005, has several Fragile X compounds in its clinical pipeline. One of them—a mGluR antagonist (STX107) licensed from Merck—survived Phase 1 but has languished on the backburner while the company pours resources into another compound—a GABA B receptor agonist (STX209). By ramping up inhibitory activity, this molecule achieves the same effect as mGlu5 blockers—diminished glutamate receptor signaling—but has a faster path to regulatory approval, Bear told ARF. It is the active enantiomer of a drug (racemic baclofen) already in use for cerebral palsy and gastroesophageal reflux. The company reported at meetings that the GABA B agonist improved several global measures in an open-label Phase 2 study of autistic patients (see news release). In the Fragile X Phase 2 trial, the compound showed some benefit in participants with severe social avoidance (see news release). Recruitment is underway for a Phase 3 trial of the GABA B agonist in children with Fragile X syndrome.

But the frontrunner may be Novartis’ AFQ056, which has headed into Phase 3 testing. In a Phase 2 trial of 30 men with FXS, the mGlu5 blocker seemed to help a subset of participants with strong Fmr1 gene silencing (Jacquemont et al., 2011).

Might the present findings apply to neurodegenerative disease? Conceptually, it may be hard to see a connection, because in Alzheimer’s and Parkinson’s, for example, neurodegeneration occurs in discrete brain regions, whereas Fragile X deficits are widespread and primarily synaptic, noted Gül Dölen, now at Stanford University, California, but who obtained her Ph.D. under Bear. Still, there is some evidence that these disparate disorders could share molecular underpinnings. APP translation appears to be regulated by Fmr1 through mGlu receptors (see Westmark and Malter, 2007 and ARF Webinar), and people with Fragile X have unusually high brain Aβ levels (see Malter et al., 2010). On the clinical front, Novartis’ mGlu5 antagonist looked promising for PD patients who have developed dyskinesias as a side effect of dopamine-boosting drugs (Berg et al., 2011). Excess glutamate has been blamed for these disabling motor problems, and a drug (amantadine) that blocks glutamate signaling through AMPA receptors is currently used to treat PD patients with dyskinesias.—Esther Landhuis.

Reference:
Michalon A, Sidorov M, Ballard TM, Ozmen L, Spooren W, Wettstein JG, Jaeschke G, Bear MF, Lindemann L. Chronic Pharmacological mGlu5 Inhibition Corrects Fragile X in Adult Mice. Neuron. 12 April 2012;74:49-56. Abstract