08 Apr 2008

Eight years ago, JoAnne McLaurin’s research suggested that a particular stereoisomer of a inositol sugar alcohol might make a respectable AD drug (McLaurin et al., 2000; McLaurin et al., 2006). Soon after, the Canadian biotech company Transition Therapeutics Inc. (in which McLaurin declared a financial interest) took on the approach, and then Elan Pharmaceuticals licensed it. The FDA has fast-tracked the drug candidate, variously called scyllo-inositol, AZT-103, or ELND005, and a Phase 2 trial comparing three different doses in 340 people began last December. While this three-year trial proceeds, much remains to be learned about this molecule. At the Keystone conference held 24-29 March in Keystone, Colorado, McLaurin presented some of her mechanistic studies. In brief, scyllo-inositol appears to bind oligomers of Aβ42, preventing them from damaging synapses. The small molecule readily crosses the blood-brain barrier thanks to active transport; its accumulation in CSF and brain was shown recently (Fenili et al., 2007).

McLaurin first summarized published data in mice. Adding scyllo-inositol to the drinking water of transgenic CRND8 mice (which are an aggressive mouse model of amyloidosis, tau hyperphosphorylation, cognitive deficit, and early death) returned the Morris water maze performance of these otherwise impaired mice to that of non-transgenic controls. The compounds gave normal mice no boost, suggesting it is no cognitive enhancer. This worked well into the advanced stages of disease in this model. McLaurin noted that an intact cholinergic system is a prerequisite for learning and remembering in the water maze. When looking at the indicator enzyme choline acetyltransferase (ChAT), McLaurin and colleagues found its levels in the requisite brain area rescued to that of non-transgenic controls. This hinted, again, at a connection between Aβ and the cholinergic system (see ARF Keystone story) The scyllo-spiked drinking water also increased synaptophysin staining in the CRND8 mice, reduced their plaque burden, soluble and insoluble Aβ40 and 42 levels, as well as their astrogliosis and CAA. Treated CRND8 mice survived longer than their untreated littermates, McLaurin said.

To find out how the compound works, McLaurin first ruled out effects on γ-secretase activity and Aβ clearance. Next, oligomer-specific Aβ antibodies indicated that scyllo-inositol appears to increase the number of monomers and trimers while reducing the amount of larger oligomeric species, such as 40mers. An even more aggressive mouse model of amyloidosis, where young mice show plaques by the time they wean, responded to scyllo-inositol treatment with decreased plaque load but increased soluble oligomers. To McLaurin, this suggested that this mouse makes too much Aβ for the brain to be able to clear oligomers after scyllo-inositol stops aggregation and deposition. Despite the accumulating Aβ oligomers, the synaptic deficits of this model improved. In reply to a question about that, McLaurin noted that collaborative experiments with Jim Cleary at the University of Minnesota, Minneapolis, suggested that scyllo-inositol added to drinking water of rats rescues errors they make in a lever-pressing task when injected with Aβ oligomers; data on mouse LTP exist, as well (Townsend et al., 2006). “We think scyllo-inositol binds to oligomers and prevents them from interacting with the neurons,” McLaurin said.

Recent collaborative mass spectrometry experiments with Austin Yang, now at the University of Maryland School of Medicine, Baltimore, suggest that one Aβ42 matches up with two scyllo-inositol molecules, whereas Aβ40 does not appear to bind the compound with any measurable stoichiometry. Prior in-vitro experiments already had suggested that scyllo-inositol inhibits aggregation of Aβ42 but not 40, McLaurin said. Scyllo-inositol does not bind membrane lipids, she added.

To explore the molecular mechanism of this compound, McLaurin and colleagues tinkered with the side groups sticking out from its six-carbon ring. These experiments showed that scyllo-inositol needs to be shaped “just so”—not a hydrogen may change. Removing a hydrogen atom from a hydroxyl group to create a double-bonded oxygen, or substituting a hydroxyl with a fluoro, chloro, or methyl group all wiped out the desired activity. Moreover, all six of scyllo-inositol’s hydroxyl groups are in an equatorial plane, and turning even one into an axial orientation extinguished its activity, indicating that the molecule’s stereochemistry is critical (Nitz et al., 2008).—Gabrielle Strobel.