Mallinckrodt Announces Interim Analysis Results for Therakos(R) Platform Phase 3 Clinical Trial in Treatment of Pediatric Patients with Steroid Refractory Acute Graft-versus-Host Disease

--Overall response rate exceeded specified study midpoint target (n=25); company elects to discontinue further patient enrollment as allowed by protocol --

STAINES-UPON-THAMES,United Kingdom,Feb. 21,2019 --Mallinckrodt plc (NYSE:MNK),a leading global specialty pharmaceutical company,today announced interim analysis results of its company-sponsored,Phase 3,single-arm,open-label,multi-center study assessing the efficacy of treating steroid-refractory,acute Graft-versus-Host Disease (aGvHD) in pediatric patients with the Therakos® Photopheresis platform in conjunction with liquid methoxsalen. Given the overall response rate exceeded the specified target at the study midpoint,the company is electing not to proceed with further enrollment of additional patients in the clinical trial.

The planned interim analysis occurred at the midway point of enrollment (n=25) when pediatric patients (ages 1-21 years) reached four weeks of therapy to assess response rates. The analysis found an overall response rate of 74%. The company also performed an ad hoc 12-week analysis showing an overall response rate of 48%. The study protocol allows for discontinuation of the study if the treatment overall response rate at the four-week interim analysis is at least 48%. Eleven serious adverse events were reported,leading to two subject withdrawals and two deaths; none were considered related to the Therakos photopheresis therapy. The safety and effectiveness of THERAKOS CELLEX Photopheresis System for use in treating pediatric patients with steroid-refractory aGvHD has not been evaluated by any regulatory body.

"Mallinckrodt is pleased to have been able to partner with the pediatric transplant community concerning this fragile patient population," said Steven Romano,M.D.,Executive Vice President and Chief Scientific Officer,Mallinckrodt. "We believe the full details from the study's interim analysis – which will be made public – will make a meaningful contribution to the body of data related to this critical condition."

More details on the study (NCT02524847) can be found hereon www.clinicaltrials.gov.

About Therakos Immunotherapy

Therakos immunotherapy is delivered through CELLEX systems to harness the power of the patient's immune system to treat the skin manifestations of CTCL,and is used by academic medical centers,hospitals,and treatment centers in more than 25 countries. Therakos photopheresis systems are fully integrated closed systems indicated for the administration of photopheresis.

CAUTION: READ THE THERAKOS UVAR XTS or THERAKOS CELLEX PHOTOPHERESIS

SYSTEMS' OPERATOR'S MANUAL PRIOR TO PRESCRIBING OR DISPENSING THIS MEDICATION.

For the THERAKOS® UVAR XTS®/CELLEX® Photopheresis Procedure:

INDICATIONS

The THERAKOS™ UVAR XTS™ and THERAKOS™ CELLEX™ Photopheresis Systems are indicated for the administration of photopheresis.

IMPORTANT SAFETY INFORMATION

Contraindications

THERAKOS™ Photopheresis is contraindicated in patients possessing a specific history of a light sensitive disease. THERAKOS™ Photopheresis is contraindicated in patients who cannot tolerate extracorporeal volume loss or who have white blood cell counts greater than 25,000 mm3. THERAKOS™ Photopheresis is contraindicated in patients who have coagulation disorders or who have previously had a splenectomy.

Warnings and Precautions

THERAKOS™ Photopheresis treatments should always be performed in locations where standard medical emergency equipment is available. Volume replacement fluids and/or volume expanders should be readily available throughout the procedure. Both men and women should take adequate contraceptive precautions both during and after completion of photopheresis therapy. Safety in children has not been established.

Adverse Events

Hypotension may occur during any treatment involving extracorporeal circulation. Closely monitor the patient during the entire treatment for hypotension.

Transient pyretic reactions,37.7–38.9°C (100–102°F),have been observed in some patients within six to eight hours of reinfusion of the photoactivated leukocyte-enriched blood. A temporary increase in erythroderma may accompany the pyretic reaction.

Treatment frequency exceeding labelling recommendations may result in anaemia.

Venous access carries a small risk of infection and pain.

Please see the appropriate THERAKOS Photopheresis System Operator's Manual for full product information.

ABOUT MALLINCKRODT

Mallinckrodt is a global business that develops,manufactures,markets and distributes specialty pharmaceutical products and therapies. Areas of focus include autoimmune and rare diseases in specialty areas like neurology,rheumatology,nephrology,pulmonology and ophthalmology; immunotherapy and neonatal respiratory critical care therapies; analgesics and gastrointestinal products. To learn more about Mallinckrodt,visit www.mallinckrodt.com.

Mallinckrodt uses its website as a channel of distribution of important company information,such as press releases,investor presentations and other financial information. It also uses its website to expedite public access to time-critical information regarding the company in advance of or in lieu of distributing a press release or a filing with the U.S. Securities and Exchange Commission (SEC) disclosing the same information. Therefore,investors should look to the Investor Relations page of the website for important and time-critical information. Visitors to the website can also register to receive automatic e-mail and other notifications alerting them when new information is made available on the Investor Relations page of the website.

Cautionary Statements Related to Forward-Looking Statements

This release includes forward-looking statements concerning Therakos including expectations with regard to the study described in this release. The statements are based on assumptions about many important factors,including the following,which could cause actual results to differ materially from those in the forward-looking statements: satisfaction of regulatory and other requirements; actions of regulatory bodies and other governmental authorities; changes in laws and regulations; issues with product quality,manufacturing or supply,or patient safety issues; and other risks identified and described in more detail in the "Risk Factors" section of Mallinckrodt's most recent Annual Report on Form 10-K and other filings with theSEC,all of which are available on its website. The forward-looking statements made herein speak only as of the date hereof andMallinckrodtdoes not assume any obligation to update or revise any forward-looking statement,whether as a result of new information,future events and developments or otherwise,except as required by law.

CONTACTS

Investor Relations

Daniel J. Speciale,CPA

Vice President,Investor Relations and IRO

314-654-3638

daniel.speciale@mnk.com

Media

Daniel Yunger

Kekst CNC

212-521-4879

mallinckrodt@kekstcnc.com

Mallinckrodt,the "M" brand mark and the Mallinckrodt Pharmaceuticals logo are trademarks of a Mallinckrodt company. Other brands are trademarks of a Mallinckrodt company or their respective owners. © 2019 Mallinckrodt. 02/19

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