The global impact of Alzheimer's disease (AD) continues to increase, and focused efforts are needed to address this immense public health challenge. National leaders have set a goal to prevent or effectively treat AD by 2025. In this paper, we discuss the path to 2025, and what is feasible in this time frame given the realities and challenges of AD drug development, with a focus on disease-modifying therapies (DMTs). Under the current conditions, only drugs currently in late Phase 1 or later will have a chance of being approved by 2025. If pipeline attrition rates remain high, only a few compounds at best will meet this time frame. There is an opportunity to reduce the time and risk of AD drug development through an improvement in trial design; better trial infrastructure; disease registries of well-characterized participant cohorts to help with more rapid enrollment of appropriate study populations; validated biomarkers to better detect disease, determine risk and monitor disease progression as well as predict disease response; more sensitive clinical assessment tools; and faster regulatory review. To implement change requires efforts to build awareness, educate and foster engagement; increase funding for both basic and clinical research; reduce fragmented environments and systems; increase learning from successes and failures; promote data standardization and increase wider data sharing; understand AD at the basic biology level; and rapidly translate new knowledge into clinical development. Improved mechanistic understanding of disease onset and progression is central to more efficient AD drug development and will lead to improved therapeutic approaches and targets. The opportunity for more than a few new therapies by 2025 is small. Accelerating research and clinical development efforts and bringing DMTs to market sooner would have a significant impact on the future societal burden of AD. As these steps are put in place and plans come to fruition, e.g., approval of a DMT, it can be predicted that momentum will build, the process will be self-sustaining, and the path to 2025, and beyond, becomes clearer.

The amyloid hypothesis has been the dominant framework for Alzheimer's disease (AD) research, including the development of anti-AD therapies. However, none of the phase III clinical trials conducted to date that targeted amyloid β (Aβ) production, aggregation, or clearance demonstrated a statistically significant treatment effect in patients with AD. This includes the approach of using monoclonal antibodies that recognize various Aβ epitopes and display different binding selectivity. While some monoclonal antibodies have failed in phase III trials, several are still in development. Aducanumab (BIIB037) is a human antibody that selectively targets aggregated forms of Aβ, including soluble oligomers and insoluble fibrils. In PRIME (NCT01677572), an ongoing phase Ib trial (N=196 patients dosed), aducanumab was shown to reduce Aβ plaques and slow decline in clinical measures in patients with prodromal or mild AD, with acceptable safety and tolerability. The main safety finding was amyloid-related imaging abnormalities (ARIA), a side effect associated with removal of Aβ, which was dose-dependent and occurred more often in ApoE ε4 carriers than non-carriers. ENGAGE (NCT02477800) and EMERGE (NCT02484547), the ongoing phase III trials of aducanumab in early AD, have been designed based on the outcomes of PRIME and on lessons from past clinical trials in patients with AD. Those study design features include patient selection with confirmed Aβ pathology, ensuring sufficient target engagement, and conducting clinical trials in patients at earlier symptomatic stages of AD.