The most immediate and quantifiable threat to Regeneron Pharmaceuticals' margin structure and long-term revenue trajectory is the accelerating erosion of the EYLEA franchise by biosimilar competition and the high-dose EYLEA HD transition, which together caused total U.S. EYLEA net product sales to decline 27% year-over-year in 2025, falling from $5.97 billion in 2024 to $4.38 billion in 2025, even as EYLEA HD sales grew 36% to $1.64 billion. This $1.58 billion decline in the company's historically largest direct revenue stream is not merely a product lifecycle event but a structural challenge that threatens the 42% revenue concentration that has funded Regeneron's 41% R&D intensity for over a decade. The FDA approval of multiple aflibercept biosimilars, including Formycon and Bioeq's FYB201 and other entrants expected to launch at 15-25% discounts to the branded list price, is rapidly commoditizing the wet age-related macular degeneration market, where EYLEA once commanded premium pricing due to its superior efficacy and less frequent dosing compared to Genentech's Lucentis. While EYLEA HD was designed to defend the franchise through extended 12-to-16-week dosing intervals, the transition has been slower than projected, with the original EYLEA formulation still accounting for 63% of combined U.S. sales in 2025, suggesting physician and patient inertia that biosimilar competitors are exploiting to capture market share during the switching window. The second major challenge is the patent cliff facing Dupixent, which, despite achieving $17.8 billion in global sales for Sanofi in 2025 with 26% growth, faces composition of matter patent expirations in the late 2020s and early 2030s that will inevitably invite generic biologic competition, though the complexity of IL-4Rα blockade and the expanding indication portfolio may provide some defense through method-of-use patents and regulatory exclusivity. Regeneron's profit share from Dupixent, which reached $5.24 billion in 2025, is vulnerable to Sanofi's commercial decisions regarding pricing in competitive European markets, where national health technology assessment bodies have increasingly demanded discounts for dermatology and asthma biologics, and to any manufacturing disruptions at the shared supply chain. The third challenge is customer concentration, with Besse Medical and McKesson accounting for 74% of gross product revenue in 2024, creating a distribution bottleneck where inventory destocking by either wholesaler could artificially depress quarterly revenue by hundreds of millions of dollars, as occurred in prior quarters when channel inventory fluctuations impacted reported EYLEA sales by approximately $85 million in Q4 2024 alone. Regulatory risks compound these commercial pressures, as the FDA's increasing scrutiny of biologics manufacturing facilities, including the need for pre-approval inspections for new EYLEA HD syringe manufacturers with decisions expected in Q2 2026, could delay capacity expansion and create supply constraints during the critical biosimilar defense period. The company's $9 billion manufacturing infrastructure commitment, while strategically necessary, strains capital allocation and will not generate incremental revenue until facilities come online in 2027 and beyond, creating a multi-year cash flow drag. Finally, the pipeline diversification challenge remains acute: despite nearly 50 product candidates, no late-stage program has demonstrated the clear blockbuster potential required to replace EYLEA or Dupixent revenues, with candidates like itepekimab in COPD facing competitive markets already populated by GSK's Nucala and AstraZeneca's Fasenra, and odronextamab in lymphoma entering a bispecific antibody space crowded with Genmab's epcoritamab and Roche's glofitamab. The company's historical reliance on internally discovered antibodies may prove insufficient in cell therapy and gene editing, where Regeneron is effectively a newcomer despite the Decibel and 2seventy acquisitions.