Inside Merck's Post-Keytruda Pipeline Engine

Keytruda (pembrolizumab) is the single largest-selling drug in the pharmaceutical industry. 2025 revenue was approximately $29.5 billion, representing roughly 45% of Merck's top line. The drug is approved across more than 40 indications, spanning non-small cell lung cancer, triple-negative breast cancer, melanoma, Hodgkin's lymphoma, renal cell carcinoma, urothelial carcinoma, and several other solid tumours plus a portfolio of MSI-high indications.

The post-2028 dynamic is not a cliff in the literal sense. Biologic erosion, including from biosimilars of anti-PD-1 antibodies, has historically been slower and less complete than small-molecule generic erosion. The relevant comparables are Humira (AbbVie), where US revenue retention five years post-LoE was still north of 50%, and Rituxan (Roche), where Europe saw faster biosimilar uptake but the US retained higher share for longer. Our base case for Keytruda: 2028 US LoE triggers a 25-30% revenue erosion over three years, stabilising at ~65% of peak. International patents expire at staggered intervals through 2034, meaning the revenue erosion curve is more gradual than the cliff framing suggests.

The subcutaneous formulation of Keytruda is the critical defensive asset. It has a separate patent life that extends into the early 2030s and has shown convenience-driven adoption in other biologics (Darzalex Faspro, Rituxan Hycela). Merck has signalled the subQ formulation could retain 40-50% of IV prescriptions. That is not replacement of LoE revenue, but it substantially blunts the erosion curve.

The replacement revenue framework for Merck is not a single blockbuster; it is a portfolio. Walking through the significant 2028-2032 contributors:

Winrevair (sotatercept) for pulmonary arterial hypertension launched in 2024 and is tracking to a peak sales estimate of $4-6 billion. The drug is first-in-class, has a differentiated mechanism (activin inhibition), and is priced in line with specialty cardiology norms. The 2025 run-rate of roughly $800 million puts it firmly on the trajectory toward the $4 billion lower bound.

Enflonsia (clesrovimab) for RSV prophylaxis in infants received EU approval in 2025 and launches broadly in 2026. The peak sales opportunity is moderate, approximately $1.5-2.5 billion depending on seasonal uptake and pricing, but the gross margin profile is excellent.

MK-1084 (KRAS G12C inhibitor, combination with Keytruda) is in multiple pivotal trials across lung and colorectal cancer. This is the platform story: Merck's ability to stack its own pipeline assets against Keytruda to extend franchise life in combination. Peak sales estimate of $2-4 billion.

Doravirine/islatravir (HIV) and the broader HIV portfolio are the sleeper. The Merck HIV franchise is small today (~$1.5 billion) but the long-acting injectable combinations represent a market-share recapture story against Gilead's Biktarvy. Peak sales opportunity: $2-3 billion incremental by 2030.

The ADC portfolio built through the Daiichi Sankyo partnership and ambrxbio acquisition is the highest-upside piece. Patritumab deruxtecan for NSCLC has completed phase 3. Ifinatamab deruxtecan targets B7-H3 in small cell lung cancer. These two assets alone could contribute $3-5 billion of revenue by 2030 if the combination data supports broad first-line use.

And the miscellaneous: Capvaxive (pneumococcal vaccine adults), expanded oncology vaccines, and the Prometheus IBD portfolio collectively add another $2-3 billion of revenue by 2030.

Sum the lower bounds: Winrevair $4B + Enflonsia $1.5B + MK-1084 $2B + HIV $2B + ADCs $3B + Other $2B = $14.5B of new revenue by 2030. The Keytruda LoE hit, under our 30% erosion assumption, is roughly $8-9 billion by 2031. The pipeline more than covers the cliff. That is the central data point the bear case refuses to price.

Merck's balance sheet carries roughly $35 billion of gross debt against $13 billion of cash. Net debt of $22 billion is comfortable for a business generating $12-18 billion of annual free cash flow. The dividend payout of approximately $8 billion is covered 1.5-2.0x by cash generation across the cycle.

The capital allocation dynamic is notable. Management has deployed roughly $25 billion in business development and acquisitions over the past three years, anchored by Prometheus ($10.8B) and the Daiichi Sankyo collaboration ($5.5B upfront plus milestones). This is capital being redirected from share repurchases into pipeline assets, explicitly to bridge the Keytruda post-2028 transition. Share count has been flat to modestly up, reflecting the deliberate choice to over-invest in late-stage assets rather than shrink the float.

That is the right choice. A pharma facing a patent cliff that buys back shares instead of pipeline assets at this juncture has historically destroyed long-term value. Merck's reallocation toward BD is the capital allocation choice every analyst covering big pharma should want to see.

The relevant peer set is Pfizer, Bristol-Myers Squibb, AbbVie, Eli Lilly, and AstraZeneca. Within that set, Merck sits middle-of-the-pack on current multiple (22.9x forward PE vs. Lilly at 35x, Pfizer at 9x, BMY at 10x, AbbVie at 17x, AZN at 18x) and top-decile on 2025 operating margin.

The key differentiator is the R&D productivity measured in pipeline NPV per R&D dollar. Merck spent roughly $17.5 billion on R&D in 2025. The late-stage pipeline NPV based on the assets enumerated above is in the $150-200 billion range using standard risk-adjusted discount rates. That NPV/R&D ratio of 8-11x is above the peer median of 6-8x and reflects the advantage of starting from a Keytruda-anchored immunology platform that supports combination economics.

AbbVie is the closest historical comparable. Humira LoE was projected in 2014 as a catastrophic event; AbbVie delivered a roughly 150% total return from 2015 to 2023 as Skyrizi and Rinvoq replaced and exceeded the Humira cash flow. The Merck setup is structurally similar in that the LoE asset is mature, the pipeline is broad, and the management track record of execution under pressure is established.

Three growth drivers, ranked by probability-adjusted revenue contribution through 2030:

First: Keytruda subQ adoption and combination expansion. The IV-to-subQ transition and the combination studies with internal assets (MK-1084, ADCs) keep the pembrolizumab franchise productive through 2032-2033. Probability-adjusted revenue contribution: $8-12 billion of retained revenue by 2030. This is the single most important growth driver because it works inside the existing commercial franchise.

Second: Winrevair commercial ramp. The PAH market is underserved, the clinical data is best-in-class, and the pricing environment is accommodating. Probability-adjusted peak sales: $4.5 billion by 2029. Probability of reaching peak: 75% based on Phase 3 STELLAR and ZENITH data.

Third: HIV franchise recapture. Long-acting injectables (oral islatravir combinations) represent the single largest addressable market in the pipeline. Probability-adjusted peak sales: $2.5 billion by 2030. Execution risk is higher given the clinical history of islatravir, which has had development pauses.

The ADC portfolio is the highest-upside, highest-variance driver. Patritumab deruxtecan alone could hit $3 billion if the combination data with Keytruda supports first-line NSCLC use. If it does not, the asset is still viable as third-line but at materially reduced peak.