Background

This gene-silencing approach uses intrathecal injection of a small interfering RNA targeting the amyloid precursor protein mRNA as a potential treatment for Alzheimer’s disease and cerebral amyloid angiopathy. The goal is to reduce expression of APP in the central nervous system and thus depress production of substrates for amyloid deposition, e.g. Aβ42 in AD and Aβ40 in CAA.  

ALN-APP fuses a synthetic siRNA to a proprietary 2'-O-hexadecyl (C16) lipophilic conjugate, which was designed to enhance cellular penetration and distribution in the CNS. In published preclinical work, C16-siRNAs delivered by intrathecal or brain injection in mice spread broadly through the brain and targeted neurons, astrocytes and microglia. The half-life of gene suppression was three to four months. In nonhuman primates, a single intrathecal injection of ALN-APP knocked down APP mRNA in the spinal cord and brain, and drove down CSF levels of the APP fragments sAPPα and sAPPβ by about 75 percent. The reductions persisted for two to three months, and gradually recovered to normal after about nine months. Intracerebroventricular injection in the CVN transgenic mouse model of brain and vascular amyloid deposition resulted in 50 percent reduction of APP mRNA and sAPPα, reduced Aβ40 deposition and inflammation, and normalization of brain glutamate levels and open-field behaviors (Brown et al., 2022).

This is the first siRNA therapy to be tested in people for a CNS-targeted disease. Alnylam’s marketed siRNA drugs target liver and other peripheral tissues.

Findings

In February 2022, Alnylam began a first-in-human Phase 1 study of the safety and target engagement of ALN-APP. The single- and multiple-dose trial will enroll 60 adults with MCI or mild dementia due to early onset AD, as defined by symptoms appearing before age 65. Participants must have a CDR global score of 0.5 or 1.0, and MMSE greater than 20, with no recent treatment with an amyloid-targeting antibody. Participants will receive a single, escalating dose of ALN-APP or placebo by intrathecal injection, or multiple doses over one year, against a primary outcome of adverse events. Secondary endpoints are CSF levels of sAPPα and sAPPβ, and pharmacokinetics. The trial will run through July 2025 at six sites in the U.S., Canada, Netherlands, and the U.K. 

On April 26, 2023, the company reported single-dose data on 20 patients in three dose cohorts (press release). Adverse events were mild to moderate, the company said, without apparent elevation of CSF protein or white blood cells, and with neurofilament light chain readings comparable to placebo. ALN-APP caused dose-dependent reductions of CSF sAPPα and sAPPβ. The highest dose lowered sAPP proteins by up to 90 percent within two weeks, and maintained greater than 70 percent reduction for at least three months. Multiple dose evaluation is under a partial clinical hold in the U.S., due to FDA concerns about findings in animal toxicology studies. Multiple ascending doses are approved to begin in Canada.

At the October 2023 CTAD conference in Boston, more data on the first three single-dose cohorts were presented (news). Each enrolled six or eight participants, who received 25, 50, or 75 mg siRNA, or placebo. The 50 and 75 mg doses reduced CSF Aβ42 and Aβ40 by 50 and 70 percent, respectively, after two months. Adverse events were mild to moderate; the majority were deemed related to lumbar puncture. Consistent with previous reports, the two highest doses reduced CSF sAPPα and sAPPβ concentrations by 69 and 82 percent, respectively, after two weeks. In the 75 mg group, sAPPα and β were 33 and 30 percent lower 10 months after dosing. Testing of additional single doses is ongoing. Multiple dosing has begun in Canada, and is approved to start in the U.K. and the Netherlands.

For details on ALN-APP trials, see clinicaltrials.gov.