The 2028 patent cliff is the defining event for Merck's next decade. Tulisokibart, ADCs, and subcutaneous Keytruda are the three bets management is making.
Can Merck replace $25 billion in annual Keytruda revenue before the patent cliff hits in 2028? That's it. That's the whole thesis. Every other data point — the pipeline, the acquisitions, the dividend, the buybacks — is subordinate to that single question.
At $303 billion market cap and roughly 15x forward earnings, the stock prices in moderate confidence that the answer is yes. The analyst consensus target of $129 suggests the Street agrees. We've spent the last month going through every pipeline asset, every recent acquisition, and every competitive threat. Our conclusion: the answer is probably yes, but the margin of safety is thinner than the market assumes.
Keytruda is the best-selling drug in the world. Annual revenue exceeds $25 billion across dozens of cancer indications, from lung cancer to melanoma to bladder cancer. It is, by any measure, one of the most commercially successful pharmaceutical products in history.
The problem is that Keytruda's composition-of-matter patent expires in 2028. Biosimilar competition will begin entering the market in 2029. Historical precedent from biologics like Humira suggests that within 3-4 years of biosimilar entry, the originator loses 40-60% of revenue. For Keytruda, that implies a $10-15 billion revenue hole by 2032.
Merck's entire strategic agenda revolves around filling that hole. The subcutaneous formulation of Keytruda, which received FDA approval, extends exclusivity for the new delivery mechanism through the early 2030s. But it won't prevent biosimilar competition for the existing IV formulation. At best, it preserves 30-40% of Keytruda revenue beyond the cliff. The rest must come from new products.
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Merck's pipeline has several assets capable of generating $3-5 billion in peak annual sales. None individually replaces Keytruda. Collectively, they might.
Lagevrio (molnupiravir) for COVID treatment has limited long-term potential as the pandemic becomes endemic. Lynparza, the PARP inhibitor partnered with AstraZeneca, is growing but faces its own competitive pressures. The Prometheus Biosciences acquisition brought PRA023 (now tulisokibart), an anti-TL1A antibody for inflammatory bowel disease that could reach $5-8 billion in peak sales — making it the most important pipeline asset after Keytruda.
The ADC (antibody-drug conjugate) platform, bolstered by the Daiichi Sankyo partnership and the acquisition of Harpoon Biosciences, represents Merck's bet on next-generation oncology. ADCs are the hottest category in pharma, with Enhertu (Daiichi/AstraZeneca) demonstrating that targeted chemotherapy can achieve Keytruda-like efficacy in certain tumours. Merck's ADC pipeline is early-stage but broad, with assets targeting HER2, Trop-2, and B7-H4.
The animal health business — Merck Animal Health — generates $6 billion annually and is often overlooked. It's the second-largest animal health company globally, behind Zoetis, and grows at a steady 7-8% annually. In a spin-off scenario, this business alone could be worth $40-50 billion.
Merck isn't the only pharma company facing a patent cliff. Bristol-Myers Squibb lost Revlimid exclusivity and saw its stock halve. AbbVie survived the Humira cliff largely because Rinvoq and Skyrizi ramped faster than expected. Pfizer's COVID revenue cliff — while not patent-driven — demonstrates how quickly pharma revenues can evaporate.
The lesson from these examples is instructive: survival through a patent cliff requires either a blockbuster replacement drug or a diversified enough portfolio that no single product's loss is fatal. Merck is attempting both strategies simultaneously — tulisokibart as the potential blockbuster, and the broader pipeline as the diversification play.
Against peers, Merck trades at 15x forward earnings versus Eli Lilly at 27x, AbbVie at 15x, and Pfizer at 9x. The multiple reflects moderate optimism — neither the premium of a company with visible growth (Lilly) nor the discount of one where the cliff has already hit (Pfizer).
The next 18 months will determine whether Merck navigates the cliff or falls off it. Key catalysts include: tulisokibart Phase 3 readouts in ulcerative colitis and Crohn's disease (expected late 2026), ADC pipeline data across multiple tumour types, and Keytruda subcutaneous formulation uptake data.
If tulisokibart delivers Phase 3 results in line with the Phase 2 data — which showed 49% remission rates versus 7% for placebo — it becomes a $5-8 billion asset and substantially narrows the Keytruda gap. Combined with the subcutaneous Keytruda retention, the ADC portfolio, and animal health, Merck could plausibly sustain $55-60 billion in annual revenue through the cliff period.
The bear case — tulisokibart disappoints, ADCs face clinical setbacks, and Keytruda biosimilars erode faster than expected — implies $40-45 billion in revenue by 2032 and a stock worth $80-90. That's 30% downside.
Drug development is binary. Tulisokibart either works in Phase 3 or it doesn't. ADC programmes either show sufficient efficacy-toxicity balance or they don't. There's no partial credit in pharmaceutical R&D.
Merck has further execution risk on the M&A front. The Prometheus acquisition cost $10.8 billion for a single-asset company. If tulisokibart fails, that write-down alone could wipe out two years of free cash flow. The Daiichi Sankyo ADC partnership carries a $22 billion total deal value. These are massive concentrated bets on clinical outcomes that are inherently uncertain.
The balance sheet, while manageable, is leveraged relative to five years ago. Long-term debt sits at $35 billion. The dividend consumes $7 billion annually. Share buybacks require another $6-8 billion. Without Keytruda's cash generation, these capital allocation programmes become unsustainable by 2030.
Our base case values Merck at $125-135 per share — roughly in line with the current price and analyst consensus. The bull case, where tulisokibart succeeds and ADCs deliver multiple $3 billion+ assets, implies $160-180. The bear case, where pipeline setbacks compound the Keytruda cliff, implies $80-90.
The range is wide because the outcomes are genuinely uncertain. At 15x forward earnings, the stock isn't pricing in either extreme. It's pricing in a muddle-through scenario where Merck loses $10-12 billion in Keytruda revenue and replaces $7-8 billion through pipeline assets and M&A.
We'd hold Merck for income investors — the 2.7% yield is secure through 2028 — and accumulate below $110. Above $130, the risk-reward tilts unfavourably until we have Phase 3 data on tulisokibart. This is a name where patience pays more than conviction.
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