Press ReleaseDaiichi Sankyo and Merck & Co., Inc., Rahway, N.J., USAAnnounce Global Development and Commercialization
Collaboration for Three Daiichi Sankyo DXd ADCs••••Collaboration combines Daiichi Sankyo’s proven ADC expertise and DXd technology with Merck &Co., Inc., Rahway, N.J., USA’s deep experience in oncology and clinical development capabilities toadvance and expand the reach of ADCs for patients across multiple types of cancer
Daiichi Sankyo and Merck & Co., Inc., Rahway, N.J., USA to co-develop and co-commercialize
patritumab deruxtecan, ifinatamab deruxtecan and raludotatug deruxtecan worldwide except for Japan
where Daiichi Sankyo retains exclusive rights
Merck & Co., Inc., Rahway, N.J., USA to pay Daiichi Sankyo a $4 billion upfront payment inaddition to $1.5 billion in continuation payments over the next 24 months, and may make additional
payments of up to $16.5 billion contingent upon the achievement of future sales milestones, for a total
potential consideration of up to $22 billion
Daiichi Sankyo investor conference call scheduled for Friday, October 20 at 6 am EST / 7 pm JST

Tokyo and Basking Ridge, N.J. – (October 20, 2023) – Daiichi Sankyo (TSE: 4568) and Merck & Co.,
Inc., Rahway, N.J., USA (known as MSD outside of the United States and Canada) (NYSE: MRK) haveentered into a global development and commercialization agreement for three of Daiichi Sankyo’s DXd
antibody drug conjugate (ADC) candidates: patritumab deruxtecan (HER3-DXd), ifinatamab deruxtecan(I-DXd) and raludotatug deruxtecan (R-DXd). The companies will jointly develop and potentiallycommercialize these ADC candidates worldwide, except in Japan where Daiichi Sankyo will maintainexclusive rights. Daiichi Sankyo will be solely responsible for manufacturing and supply.

All three potentially first-in-class DXd ADCs are in various stages of clinical development for thetreatment of multiple solid tumors both as monotherapy and/or in combination with other treatments.
Patritumab deruxtecan was granted Breakthrough Therapy Designation by the U.S. Food and Drug
Administration in December 2021 for the treatment of patients with EGFR-mutated locally advanced ormetastatic non-small cell lung cancer (NSCLC) with disease progression on or after treatment with athird-generation tyrosine kinase inhibitor (TKI) and platinum-based therapies. The submission of abiologics license application (BLA) in the U.S. is planned by the end of March 2024 for patritumab
deruxtecan, which is based on data from the HERTHENA-Lung01 phase 2 trial recently presented at theIASLC 2023 World Conference on Lung Cancer and simultaneously published in the Journal of ClinicalOncology.
1Ifinatamab deruxtecan is currently being evaluated as monotherapy in IDeate-01, a phase 2 clinical trial inpatients with previously treated extensive-stage small cell lung cancer (SCLC). Updated results from asubgroup analysis of a phase 1/2 trial of ifinatamab deruxtecan in SCLC were recently presented at theIASLC 2023 World Conference on Lung Cancer. Raludotatug deruxtecan is currently being evaluated ina first-in-human phase 1 clinical trial and updated results in patients with advanced ovarian cancer will bepresented at the upcoming European Society for Medical Oncology (ESMO) Congress 2023.
Designed using Daiichi Sankyo’s proprietary DXd ADC technology to target and deliver a cytotoxic
payload inside cancer cells that express a specific cell surface antigen, each ADC consists of a
monoclonal antibody attached to a number of topoisomerase I inhibitor payloads (an exatecan derivative,DXd) via tetrapeptide-based cleavable linkers.
“The promising results from clinical trials of patritumab deruxtecan, ifinatamab deruxtecan andraludotatug deruxtecan continue to demonstrate the broad applicability of Daiichi Sankyo’s DXd ADC
technology across multiple targets, with each of these medicines having the potential to change clinicalpractice as has been already seen with ENHERTU®,” said Sunao Manabe, Representative Director,Executive Chairperson and CEO, Daiichi Sankyo Company, Limited. “As Daiichi Sankyo continues its
transformation into a global oncology leader by increasingly building our infrastructure and talent, werecognize that a collaboration with Merck & Co., Inc., Rahway, N.J., USA, a company with remarkableoncology experience and strong in-house development capabilities and resources, will help us deliver on
our obligation to deliver these potential new DXd ADCs to more patients as quickly as possible.”“At Merck & Co., Inc., Rahway, N.J., USA, we continue to augment and diversify our oncology pipeline
while building on our immuno-oncology foundation,” said Robert M. Davis, Chairman and Chief
Executive Officer, Merck & Co., Inc., Rahway, N.J., USA. “The pioneering work by Daiichi Sankyo
scientists has highlighted the far-reaching potential of ADCs to provide meaningful new options forpatients with cancer. We look forward to forging this collaboration to deliver the next generation ofprecision cancer medicines, driven by our mutual compassion for patients around the world.”

Financial Highlights
Under the terms of the agreement, Merck & Co., Inc., Rahway, N.J., USA will pay Daiichi Sankyo
upfront payments of $1.5 billion for ifinatamab deruxtecan due upon execution; $1.5 billion for
patritumab deruxtecan, where $750 million is due upon execution and $750 million is due after 12months; and $1.5 billion for raludotatug deruxtecan, where $750 million is due upon execution and2

$750 million is due after 24 months. Merck & Co., Inc., Rahway, N.J., USA also will pay Daiichi Sankyo
up to an additional $5.5 billion for each DXd ADC contingent upon the achievement of certain sales
milestones. When combined with the additional refundable upfront payment of $1 billion described
below, total potential consideration across the three programs is up to $22 billion.

Merck & Co., Inc., Rahway, N.J., USA may opt out of the collaboration for patritumab deruxtecan andraludotatug deruxtecan and elect not to pay the two continuation payments of $750 million each that aredue after 12 months and 24 months, respectively. If Merck & Co., Inc., Rahway, N.J., USA opts out ofpatritumab deruxtecan and/or raludotatug deruxtecan, the upfront payments already paid will be retainedby Daiichi Sankyo and rights related to such DXd ADCs will be returned to Daiichi Sankyo.

As referenced above, Merck & Co., Inc., Rahway, N.J., USA will pay an additional upfront payment of$1 billion ($500 million each for patritumab deruxtecan and ifinatamab deruxtecan), a pro-rated portionof which may be refundable in the event of early termination of development with respect to eachprogram. For raludotatug deruxtecan, Merck & Co., Inc., Rahway, N.J., USA will be responsible for 75%
of the first $2 billion of R&D expenses. Except as outlined above with respect to R&D expenses, thecompanies will equally share expenses as well as profits worldwide, except for Japan where Daiichi
Sankyo retains exclusive rights and Merck & Co., Inc., Rahway, N.J., USA receives a royalty based onsales revenue. Daiichi Sankyo will generally book sales worldwide.

In aggregate, the three programs have multi-billion dollar worldwide commercial revenue potential foreach company approaching the mid-2030s.
The impact on Daiichi Sankyo’s consolidated results for the fiscal year ending March 31, 2024 will beannounced at an appropriate time in the future. The collaboration is expected to contribute to enhancing
the corporate and shareholder value of Daiichi Sankyo over the medium to long term.

In conjunction with this transaction, Merck & Co., Inc., Rahway, N.J., USA will record an aggregate
pretax charge of $5.5 billion, or approximately $1.70 per share, reflecting the $4 billion upfront payment
and the $1.5 billion in continuation payments. The impact of this charge will result in a reduction in both
fourth-quarter and full-year 2023 GAAP and non-GAAP results. In addition, Merck & Co., Inc., Rahway,N.J., USA will invest in the pipeline assets and incur costs to finance the transaction, resulting in anegative impact to EPS of approximately $0.25 in the first 12 months following the close of thetransaction.
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About the DXd ADC Portfolio of Daiichi Sankyo
The DXd ADC portfolio of Daiichi Sankyo currently consists of six ADCs in clinical development acrossmultiple types of cancer. ENHERTU, a HER2 directed ADC, and datopotamab deruxtecan (Dato-DXd),a TROP2 directed ADC, are being jointly developed and commercialized globally with AstraZeneca.
Patritumab deruxtecan (HER3-DXd), a HER3 directed ADC, ifinatamab deruxtecan (I-DXd), a B7-H3
directed ADC, raludotatug deruxtecan (R-DXd), a CDH6 directed ADC, are being jointly developed andcommercialized globally with Merck & Co., Inc., Rahway, N.J., USA. DS-3939, a TA-MUC1 directedADC, is being developed by Daiichi Sankyo.
Designed using Daiichi Sankyo’s proprietary DXd ADC technology to target and deliver a cytotoxic
payload inside cancer cells that express a specific cell surface antigen, each ADC consists of a
monoclonal antibody attached to a number of topoisomerase I inhibitor payloads (an exatecan derivative,DXd) via tetrapeptide-based cleavable linkers.

Datopotamab deruxtecan, ifinatamab deruxtecan, patritumab deruxtecan, raludotatug deruxtecan andDS-3939 are investigational medicines that have not been approved for any indication in any country.
Safety and efficacy have not been established.

About Daiichi Sankyo
Daiichi Sankyo is an innovative global healthcare company contributing to the sustainable developmentof society that discovers, develops, and delivers new standards of care to enrich the quality of life aroundthe world. With more than 120 years of experience, Daiichi Sankyo leverages its world-class science andtechnology to create new modalities and innovative medicines for people with cancer, cardiovascular andother diseases with high unmet medical need. For more information, please visit www.daiichisankyo.com.

About Merck & Co., Inc., Rahway, N.J., USAAt Merck & Co., Inc., Rahway, N.J., USA, known as MSD outside of the United States and Canada, weare unified around our purpose: We use the power of leading-edge science to save and improve lives
around the world. For more than 130 years, we have brought hope to humanity through the developmentof important medicines and vaccines. We aspire to be the premier research-intensive biopharmaceuticalcompany in the world – and today, we are at the forefront of research to deliver innovative healthsolutions that advance the prevention and treatment of diseases in people and animals. We foster a diverseand inclusive global workforce and operate responsibly every day to enable a safe, sustainable and4healthy future for all people and communities. For more information, visit www.msd.com and connect
with us on Twitter, LinkedIn and YouTube.

Forward-Looking Statement of Merck & Co., Inc., Rahway, N.J., USA
This news release of Merck & Co., Inc., Rahway, N.J., USA (the “company”) includes “forward-lookingstatements” within the meaning of the safe harbor provisions of the U.S. Private Securities Litigation
Reform Act of 1995. These statements are based upon the current beliefs and expectations of thecompany’s management and are subject to significant risks and uncertainties. There can be no guarantees
with respect to pipeline candidates that the candidates will receive the necessary regulatory approvals orthat they will prove to be commercially successful. If underlying assumptions prove inaccurate or risks or
uncertainties materialize, actual results may differ materially from those set forth in the forward-lookingstatements.

Risks and uncertainties include but are not limited to, general industry conditions and competition;
general economic factors, including interest rate and currency exchange rate fluctuations; the impact of
the global outbreak of novel coronavirus disease (COVID-19); the impact of pharmaceutical industry
regulation and health care legislation in the United States and internationally; global trends toward healthcare cost containment; technological advances, new products and patents attained by competitors;
challenges inherent in new product development, including obtaining regulatory approval; the company’s
ability to accurately predict future market conditions; manufacturing difficulties or delays; financial
instability of international economies and sovereign risk; dependence on the effectiveness of thecompany’s patents and other protections for innovative products; and the exposure to litigation, including
patent litigation, and/or regulatory actions.

The company undertakes no obligation to publicly update any forward-looking statement, whether as aresult of new information, future events or otherwise. Additional factors that could cause results to differ
materially from those described in the forward-looking statements can be found in the company’s Annual
Report on Form 10-K for the year ended December 31, 2022 and the company’s other filings with theSecurities and Exchange Commission (SEC) available at the SEC’s Internet site (www.sec.gov).

5Media Contacts:
Daiichi Sankyo
Global/US Media:
Jennifer Brennan
Daiichi Sankyo, Inc.
jbrennan2@dsi.com+1 908 900 3183 (mobile)

Japan Media:
Koji Ogiwara
Daiichi Sankyo Co., Ltd.
ogiwara.koji.ay@daiichisankyo.co.jp+81 3 6225 1126 (office)Investor Relations Contact:
DaiichiSankyoIR@daiichisankyo.co.jp

Merck & Co., Inc., Rahway, N.J., USAMedia:
Robert Josephson+1 203 914 2372

Investors:
Peter Dannenbaum+1 732 594 1579

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