C16-siRNAs in Focus: Development of ALN-APP, the First RNAi-Based Therapeutic for Alzheimer’s Disease

This review examines ALN-APP, a small interfering RNA (siRNA) formulated for intrathecal injection, intended to reduce the formation of beta-amyloid precursor protein (APP), a critical factor in the pathology of Alzheimer’s disease (AD). ALN-APP incorporates a 16-carbon chain (C16-siRNA) to enhance its delivery to the central nervous system (CNS) while integrating advancements in specificity and duration of action based on previous FDA-approved drugs. The development of ALN-APP involved a thorough analysis of the optimal carbon chain length and its conjugation position to the siRNA. Preclinical studies conducted in male Sprague Dawley rats, mice, and non-human primates demonstrated the efficacy of ALN-APP. In rats, intrathecal (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 over three months across multiple brain regions. Furthermore, studies in non-human primates indicated reductions in soluble APP levels (sAPPα and sAPPβ) to below 25% sustained for two months. In the 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. Following these promising results in animal models, ALN-APP advanced to a Phase 1 trial, designated ALN-APP-001, assessing 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. In a subsequent cohort of 36 patients, the administration of the 75 mg dose via IT injection produced mean reductions of 61.3% in sAPPα and 73.5% in sAPPβ after one month. These silencing effects persisted over a six-month period and were associated with important decreases in Aβ42 and Aβ40 levels. These results highlight ALN-APP's potential to address Alzheimer’s pathology while maintaining a favorable safety profile. Collectively, advancements in ALN-APP represent a promising strategy to reduce beta-amyloid formation in AD, with significant biomarker reductions suggesting potential disease-modifying effects. Continued development may pave the way for innovative treatments for neurodegenerative diseases.