8 August 2008. An oral antihistamine first marketed 25 years ago in Russia has emerged as a contender in the race to become the first Alzheimer disease drug with stronger, more lasting benefit than any of the drugs currently available. Dimebon drew notice for its ability to improve AD patients for at least 12 months on all of five cognitive measures in a recently published Russian trial of the compound (see ARF related news story). It’s odd to consider how a drug that fights sniffles could also boost cognition, and fresh data for how that might be turned more than a few heads at the International Conference on Alzheimer’s Disease (ICAD), held 26-31 July in Chicago. New data from the Russian trial extension also came in.
The new work extends earlier evidence that Dimebon stabilizes mitochondria. This target is distinct from those of available AD drugs and has traditionally attracted scant attention alongside big guns amyloid and tau. Interest in mitochondrial mechanisms in AD is growing, however, and the lecture hall was packed when Andrew Protter presented details on Dimebon’s proposed mitochondrial mechanism of action. Protter works for Medivation, Inc., the San Francisco-based company that is developing Dimebon as a treatment for Alzheimer and Huntington diseases.
The current batch of AD drugs work by inhibiting acetylcholinesterases (enzymes that break down a key learning/memory neurotransmitter) or NMDA receptors (proteins whose hyperactivation may underlie the degeneration of cholinergic cells). Dimebon, too, acts on these molecules, as shown in earlier work by Russian scientists (Bachurin et al., 2001; Bachurin et al., 2003). It does so more weakly, however, suggesting that other mechanisms mediate the drug’s cognitive effects if indeed those benefits hold up in larger trials. Because potency is a critical factor in drug development, researchers at Medivation have confirmed the earlier acetylcholinesterase findings in two sets of experiments—one using recombinant enzyme and another using fresh blood samples from three volunteers. In these follow-up studies presented at ICAD, Dimebon blocked acetylcholinesterase activity with a half-maximal inhibitory concentration (IC50) in the 31-72 μMolar range—several thousand-fold lower than that of the widely prescribed anti-cholinesterase drug donepezil. In NMDA receptor binding assays, Dimebon’s inhibition constant (Ki—a related measure of potency, defined as the concentration of test compound required to bind 50 percent of receptors in the absence of agonist) was 97 μMolar, compared with 0.5 μMolar for the AD drug memantine, the Medivation scientists claimed.
Another hint that Dimebon functions differently from current AD drugs came from its side effect profile in human studies: less than 3 percent of clinical trial participants who took Dimebon have reported gastrointestinal problems. In contrast, Protter said, more than 20 percent of AD patients taking donepezil routinely experience such symptoms. “Just looking at the low side effect profiles says (Dimebon) is not working like the other guys,” he told ARF.
Then how might it work? The same earlier studies provided initial clues to mitochondrial effects, and these have been confirmed and extended in unpublished experiments Protter presented at ICAD. A search for Dimebon’s mechanism of action that ranged from enzyme inhibition and cell-based activity assays to kinase profiling to commercial compound screening services identified mitochondrial enhancement under cellular stress conditions as the most potent activity. Mitochondrial dysfunction has been linked to aging and neurodegenerative diseases, including Alzheimer’s, in various ways. As the primary power plants of the cell, mitochondria churn out toxic compounds that can accelerate Aβ production in AD mouse models (see ARF related news story). A more recent study (see ARF related news story) highlights the importance of mitochondrial waste management: in transgenic mice with a mitochondrial enzyme defect leading to buildup of harmful aldehydes, age-related neurodegeneration kicked in and led to early death. (For more on how mitochondrial dysfunction might tie in with sporadic AD, see ARF Live Discussion).
To get a look at Dimebon’s mitochondrial activities, the researchers treated SK-N-SH human neuroblastoma cells with ionomycin, which reduces the electrochemical gradient across mitochondrial membranes, triggering apoptosis. They assessed mitochondrial function using JC-1, a dye that fluoresces red when it accumulates in healthy mitochondria and green when forced to remain in the cytoplasm due to collapsed mitochondrial membrane potentials. This allowed the researchers to gauge mitochondrial health as a percentage of cells fluorescing red versus green. The red/green ratio for cells hit with ionomycin dropped to 70 percent of that seen for untreated cells, Protter said. Picomolar concentrations of Dimebon brought the ratio up to 85-90 percent of control cells in a concentration-dependent fashion. As measured in MTT (3-[4,5-dimethyl-2-thiazolyl]-2,5-diphenyl-2H-tetrazolium bromide) assays, ionomycin treatment reduced cell viability to 40 percent of control cells, and picomolar Dimebon restored survival to 75-85 percent. Separate experiments examined mitochondrial function in a related neuroblastoma line (SH-SY5Y) using a different probe (tetramethyl rhodamine methyl ester, or TMRM). With this, too, the researchers reported that pre-treatment with one nanoM Dimebon protected cells from ionomycin-induced stress. Together, these findings suggest that Dimebon’s mitochondria-stabilizing effects are potent and correlate with improved cell viability, Protter said.
Because dendritic mitochondria are important for synaptic plasticity, a process critical for learning and memory (see ARF related news story), the company researchers examined Dimebon’s effects on the growth of neurites. In primary rat cortical neurons cultured for three days with either brain-derived neurotrophic factor (BDNF) or Dimebon, picomolar concentrations of Dimebon increased neurite outgrowth as well as did BDNF applied at its maximally effective concentrations, Protter reported.
“Dimebon has shown quite encouraging results in the clinic,” Protter said in a post-ICAD phone conversation. “Combined with the pharmacology studies we’ve done thus far, we can say that Dimebon is working with a novel mechanism.” So far, signs point to the mitochondria, though researchers have yet to clearly demonstrate that Dimebon binds these organelles. Protter said Medivation has not been keen thus far on doing experiments with purified mitochondria. For starters, he said that it is hard to isolate the organelles cleanly. In addition, the literature is increasingly ascribing functional roles to proteins that are bound to mitochondria, yet these interactions could be disrupted in the purified preparations. Protter claimed the company has preliminary data showing that Dimebon “is affecting proteins that are bound to mitochondria” but did not show that data at ICAD. No independent confirmation of these data by other laboratories is available to date.
Also at ICAD, Jeffrey Cummings of the University of California, Los Angeles, presented clinical results of the six-month open-label extension of Dimebon’s Phase 2 trial in Russia, which brought exposure to the drug out to 18 months in these patients with mild to moderate AD. In this extension, patients who had been on drug during the blinded phase continued on Dimebon. Patients who had been randomized to placebo crossed over to Dimebon. This created, at the 18-month time point, a mixture of people who had been on drug for 18 months and had actually still enjoyed a slight improvement over baseline at 12 months, and of other people who been declining in the placebo group for 12 months. People who had been on drug all along declined somewhat after 12 months, but ended the 18-month time point with only an ADAS-Cog change of about minus 2 from the start of the trial. They appeared to hold their own, i.e., around baseline, on a psychiatric rating scale, as well. People who had previously been on placebo at 18 months remained stable at the lower level of cognitive function at which they had ended their 12 months on placebo.
Perhaps the most straightforward value of open-label extensions lies in safety and tolerability, because they give an indication of long-term use. On that score, “no safety or tolerability issues came up that would concern us,” Cummings said. The one side effect reported more frequently in the treatment group was depression. This depression was self-reported, not measured by depression scales, raising the question of whether people might become more depressed as their cognition improves slightly and they regain insight into their still-difficult condition. Cummings noted that a Phase 3 trial of Dimebon monotherapy held at 30 sites in the U.S., as well as in Europe and South America, is presently enrolling. Cummings added that the FDA had accepted the Phase 2 trial, which was conducted in Russia with the guidance of established clinical leaders including Rachelle Smith Doody, of Baylor College of Medicine in Houston, as a first pivotal trial in the process, and that the Phase 3 would be the second pivotal trial to support a New Drug Application. If all goes well, that is.—Esther Landhuis and Gabrielle Strobel.