C16-siRNAs in Focus: Development of ALN-APP, a Promising RNAi-Based Therapeutic for Alzheimer's Disease

This review examines a small interfering RNA (siRNA) designed for intrathecal (IT) injection, which reduces the formation of amyloid beta precursor protein (APP), a critical factor in the pathology of Alzheimer's disease (AD). The siRNA, designated ALN-APP, incorporates a 16-carbon chain (C16-siRNA) to enhance its delivery to the central nervous system (CNS) while leveraging advancements in specificity and duration of action based on previously approved drugs by the Food and Drug Administration. The development of ALN-APP involved a comprehensive analysis of the optimal carbon chain length and its conjugation position to the siRNA. Preclinical studies conducted on male Sprague Dawley rats, mice, and non-human primates (NHPs) demonstrated the efficacy of ALN-APP. In rats, an IT injection of C16-siRNAs at a concentration of 30 mg/mL, delivering a dose of 0.9 mg, resulted in cranial distribution via cerebrospinal fluid and led to a 75% reduction in copper-zinc superoxide dismutase 1 (SOD1) mRNA levels. These effects were dose-dependent and persisted for three months across multiple brain regions. Furthermore, studies in NHPs indicated that soluble APP levels were reduced to below 25%, sustained for two months. In the cerebrovascular amyloid Nos2^-/- (CVN) mouse model of AD, administration of 120 µg of siRNA via the intracerebroventricular route produced reductions in APP expression, with mRNA levels remaining suppressed for 60 days in the ventral cortex. Indeed, ALN-APP controlled neuropathology in 5xFAD mice by significantly reducing amyloid levels and brain neuroinflammation, with improved behaviors in the elevated plus maze. Following these promising results in animal models, ALN-APP advanced to a Phase 1 trial, designated ALN-APP-001, which assessed its safety and efficacy in 12 participants with early-onset Alzheimer's disease (EOAD). Initial findings revealed a 55% reduction in soluble APPα and a 69% reduction in APPβ by day 15. These exploratory findings require further validation with larger cohorts and proper statistical analysis. In a subsequent cohort of 36 patients, administration of the 75 mg dose via IT injection led to mean reductions of 61.3% in soluble APPα (sAPPα) and 73.5% in soluble APPβ (sAPPβ) after one month. These silencing effects persisted for six months and were associated with important decreases in Aβ42 and Aβ40 levels. These results highlight the potential of ALN-APPs to address Alzheimer's pathology while maintaining a favorable safety profile. Whether ALN-APP succeeds in further clinical trials, key challenges include ensuring accessibility and affordability due to treatment costs, the need for specialized intrathecal administration, and establishing infrastructure for large-scale production of siRNAs. In conclusion, advancements in ALN-APP represent a promising strategy to reduce beta-amyloid formation in AD, with substantial biomarker reductions suggesting potential disease-modifying effects. Continued development may pave the way for innovative treatments for neurodegenerative diseases.