8 April 2013. At the 11th AD/PD 2013 Conference held last month in Florence, Italy, the last day unfolded as is often the case at long conferences: Some of the most anticipated talks spilled their new data before sparse audiences. So it was with Bernd Bohrmann’s first public presentation on a preclinical treatment study that combined two active, clinical-stage anti-amyloid drugs. Bohrmann, of F. Hoffmann-La Roche Ltd. in Basel, Switzerland, reported that, when given together, Roche’s Phase 1 BACE inhibitor and its Phase 3 antibody reduced Aβ levels and plaque burden more strongly in a transgenic mouse overexpressing mutant human APP than did either treatment alone. “We can fairly say we saw evidence for additive efficacy,” said Bohrmann.

Researchers from competing companies expressed sentiments that amount to "wish we were doing this." “This is very interesting,” said Dale Schenk, who oversaw development of immunotherapies at Elan and recently founded a new biotech company in South San Francisco called Prothena. “We had an internal discussion about combination therapy,” said Eric Siemers of Eli Lilly and Company in Indianapolis, Indiana, which could conduct similar experiments with its BACE inhibitor and solanezumab or a preclinical anti-plaque antibody published last December (DeMattos et al., 2012). Schenk and Siemers chaired the session. Other scientists privately expressed their interest in this work.

Combination therapy has been a buzzword for some time. Researchers call for it from lecterns and in conversation (see ARF related news story; see ARF news story), while advocacy groups are rallying around it. Regulatory scientists at the U.S. Food and Drug Administration expressly encourage the development of two unapproved, investigational agents (see ARF related news story). Alas, few groups are working on this goal. Among the academics, Philip Wong of Johns Hopkins University has genetically reined in both γ-secretase and BACE activity in mice (Chow et al., 2010). Joanna Jankowsky at Baylor College of Medicine, Houston, Texas, articulated the promise of combination therapy when she throttled Aβ production genetically to hold its levels steady while simultaneously clearing existing plaque with a research vaccine made in Todd Golde’s lab (Wang et al., 2011). Even so, despite widespread acknowledgment that a disease as complex as Alzheimer’s will require multiple hits on goal—either on a single pathway or on several different pathways at once—past talk has been more lip service than action.

In an ARF Q&A, Luca Santarelli of Roche declared an active interest in combination therapy, and in Florence Bohrmann showed the first data. The BACE inhibitor R7129 reduces both Aβ42 and 40 dose dependently in cell-based assays, CSF, and brain of wild-type and some transgenic mice. The anti-Aβ antibody gantenerumab is more advanced and better known—it binds most strongly to aggregated forms and reduces plaques in people as measured by amyloid imaging (Bohrmann et al., 2012; Ostrowitzki et al., 2011). This antibody is given monthly at doses up to 225 milligrams, and is one of the drugs being tested in the DIAN secondary prevention trial.

Working with Helmut Jacobsen and others at Roche, Bohrmann ran a combination trial of the two medicines in a transgenic mouse by the contract research organization reMYND in Leuven, Belgium. This strain overexpresses the London mutation in human APP. The scientists did not use their in-house mouse models because those express the Swedish APP mutation. That mutation boosts the affinity of the BACE cleavage site on APP, such that the BACE inhibitor was unable to compete well with the mutated APP binding site and the mice responded poorly to the drug. The APP Swedish mutation is extremely rare; most people with Alzheimer’s express normal variants of APP.

In a chronic treatment study, 13.5-month-old—that is, middle-aged mice—with active amyloid deposition received either 30 or 90 mg/kg daily doses of the BACE inhibitor, weekly infusions of 20 mg/kg gantenerumab, or both, for four months. In that time, brain Aβ levels increased sevenfold, Bohrmann told the audience.

BACE inhibition alone reduced Aβ42 and Aβ40 in brain and CSF, and slightly reduced plaque burden. Gantenerumab alone reduced plaque burden and brain Aβ42 but not Aβ40 levels. As expected, the antibody did not change Aβ levels in CSF, as it does not bind strongly to soluble Aβ.

The combination led to much greater Aβ42 reduction and more Aβ40 reduction, Bohrmann said. This effect was driven largely by the inhibitor. The combination also cleared up plaques more strongly; this was driven largely by the antibody. The scientists analyzed total plaque area, as well as change in the number of small, medium-size, and large plaques counted separately, and found that the combination drove amyloid deposition back to below baseline. “No new plaques formed in this mouse model, and small plaques were cleared,” Bohrmann said. This is notable because this mouse model overexpresses the APP London mutation fivefold. “It is a very aggressive amyloidosis model,” Bohrmann said.

Curiously, a residual pool of Aβ40 remained in the treated mice. Scientists debated where that pool is—soluble inside neurons? inside membranes?—and what it means.

Overall, the combination was more effective than either treatment alone. “It makes a lot of sense to go into prodromal trials with a combination like this,” Bohrmann said. “Monotherapy in mild to moderate Alzheimer’s may likely be too little and too late, as was concluded from recent immunotherapy trials by Reisa Sperling.”

For such trials, CSF Aβ can serve as a pharmacodynamic marker for BACE inhibition. The mice responded consistently with advancing amyloidosis, and the researchers saw a CSF signal within two hours of giving the inhibitor. Amyloid PET, even though it may miss some forms of brain aggregated Aβ, still serves as a pharmacodynamic marker of antibody treatment.

Beyond that, however, further mouse studies will tell the scientists little. For example, gantenerumab clears aggregated Aβ via phagocytosis by macrophages/microglia, but this response may be different in humans and mice. Amyloid-induced phagocytosis is more voracious in people than in mice. The current mouse results may be limited in that sense, and human responses should be estimated in an Alzheimer’s disease computational model, Bohrmann said.

Finding the right dosing regimen for human combination trials is the next challenge. For that, too, mice will be of little help. Humans express less APP than these transgenic mice to begin with, and mouse Aβ-degrading enzymes are different. What’s more, rodent metabolism clears drugs faster than in people. For these reasons, scientists generally give mice higher doses. In this study, 30-90 mg/kg was an effective range, whereas in humans, this might end up being the total dose, Bohrmann said. In toto, rodents pharmacokinetics is a poor guide for human trials, and information for dose finding has to come from careful clinical studies, Bohrmann said. Exactly when those can begin depends on Phase 1 results for BACE inhibitor monotherapy.

How about additive versus synergistic? In theory, the hope for combination therapies is for a synergistic effect that neither drug alone can deliver. In practice, however, few combination regimens achieve that, and an additive effect will be good enough for Alzheimer’s, the FDA’s Bob Temple said at a combination trial conference last November (see ARF related news story). “In our mouse study, I would call the result of the combination an additive or enhanced effect. We think that could translate into better efficacy. Whether in humans there is also synergy remains to be seen,” Bohrmann said.

Other combinations, involving anti-inflammatory or anti-tau agents are “open for discussion.” Have the scientists tried an anti-tau antibody together with an anti-amyloid treatment? “Not yet,” Bohrmann said.—Gabrielle Strobel.