ENFLONSIA Regulatory Review Period Determination

Content

ACTION:

Notice.

SUMMARY:

The Food and Drug Administration (FDA or the Agency) has determined the regulatory review period for ENFLONSIA and is publishing
this notice of that determination as required by law. FDA has made the determination because of the submission of an application
to the Director of the U.S. Patent and Trademark Office (USPTO), Department of Commerce, for the extension of a patent which
claims that human biological product.

DATES:

Anyone with knowledge that any of the dates as published (see
SUPPLEMENTARY INFORMATION
) are incorrect must submit either electronic or written comments and ask for a redetermination by June 2, 2026. Furthermore,
any interested person may petition FDA for a determination regarding whether the applicant for extension acted with due diligence
during the regulatory review period by September 30, 2026. See “Petitions” in the
SUPPLEMENTARY INFORMATION
section for more information.

ADDRESSES:

You may submit comments as follows. Please note that late, untimely filed comments will not be considered. The https://www.regulations.gov electronic filing system will accept comments until 11:59 p.m. Eastern Time at the end of June 2, 2026. Comments received
by mail/hand delivery/courier (for written/paper submissions) will be considered timely if they are received on or before
that date.

Electronic Submissions

Submit electronic comments in the following way:

• Federal eRulemaking Portal: https://www.regulations.gov. Follow the instructions for submitting comments. Comments submitted electronically, including attachments, to https://www.regulations.gov will be posted to the docket unchanged. Because your comment will be made public, you are solely responsible for ensuring
that your comment does not include any confidential information that you or a third party may not wish to be posted, such
as medical information, your or anyone else's Social Security number, or confidential business information, such as a manufacturing
process. Please note that if you include your name, contact information, or other information that identifies you in the body
of your comments, that information will be posted on https://www.regulations.gov.

• If you want to submit a comment with confidential information that you do not wish to be made available to the public, submit the comment as a written/paper submission and in the manner detailed (see “Written/Paper Submissions” and “Instructions”).

Written/Paper Submissions

Submit written/paper submissions as follows:

• Mail/Hand Delivery/Courier (for written/paper submissions): Dockets Management Staff (HFA-305), Food and Drug Administration, 5630 Fishers Lane, Rm. 1061, Rockville, MD 20852.

• For written/paper comments submitted to the Dockets Management Staff, FDA will post your comment, as well as any attachments, except for information submitted, marked and identified, as confidential, if submitted as detailed in “Instructions.” Instructions: All submissions received must include the Docket No. FDA-2025-E-3626 for “Determination of Regulatory Review Period for Purposes of Patent Extension; ENFLONSIA.” Received comments, those filed in a timely manner (see ADDRESSES ), will be placed in the docket and, except for those submitted as “Confidential Submissions,” publicly viewable at https://www.regulations.gov or at the Dockets Management Staff between 9 a.m. and 4 p.m., Monday through Friday, 240-402-7500.

• Confidential Submissions—To submit a comment with confidential information that you do not wish to be made publicly available,
submit your comments only as a written/paper submission. You should submit two copies total. One copy will include the information
you claim to be confidential with a heading or cover note that states “THIS DOCUMENT CONTAINS CONFIDENTIAL INFORMATION.” The
Agency will review this copy, including the claimed confidential information, in its consideration of comments. The second
copy, which will have the claimed confidential information redacted/blacked out, will be available for public viewing and
posted on https://www.regulations.gov. Submit both copies to the Dockets Management Staff. If you do not wish your name and contact information to be made publicly
available, you can provide this information on the cover sheet and not in the body of your comments and you must identify
this information as “confidential.” Any information marked as “confidential” will not be disclosed except in accordance with
§ 10.20 (21 CFR 10.20) and other applicable disclosure law. For more information about FDA's posting of comments to public
dockets, see 80 FR 56469, September 18, 2015, or access the information at: https://www.govinfo.gov/content/pkg/FR-2015-09-18/pdf/2015-23389.pdf.

Docket: For access to the docket to read background documents or the electronic and written/paper comments received, go to https://www.regulations.gov and insert the docket number, found in brackets in the heading of this document, into the “Search” box and follow the prompts
and/or go to the Dockets Management Staff, 5630 Fishers Lane, Rm. 1061, Rockville, MD 20852, 240-402-7500.

FOR FURTHER INFORMATION CONTACT:

Patrick Clouser, Office of the Commissioner, Food and Drug Administration, 12420 Parklawn Drive, Rockville, MD 20852, 240-402-5276.

SUPPLEMENTARY INFORMATION:

I. Background

The Drug Price Competition and Patent Term Restoration Act of 1984 (Pub. L. 98-417) and the Generic Animal Drug and Patent
Term Restoration Act (Pub. L. 100-670) generally provide that a patent may be extended for a period of up to 5 years

  so long as the patented item (human drug or biologic product, animal drug product, medical device, food additive, or color
  additive) was subject to regulatory review by FDA before the item was marketed. Under these acts, a product's regulatory review
  period forms the basis for determining the amount of extension an applicant may receive.

A regulatory review period consists of two periods of time: a testing phase and an approval phase. For human biological products,
the testing phase begins when the exemption to permit the clinical investigations of the biological product becomes effective
and runs until the approval phase begins. The approval phase starts with the initial submission of an application to market
the human biological product and continues until FDA grants permission to market the biological product. Although only a portion
of a regulatory review period may count toward the actual amount of extension that the Director of USPTO may award (for example,
half the testing phase must be subtracted as well as any time that may have occurred before the patent was issued), FDA's
determination of the length of a regulatory review period for a human biological product will include all of the testing phase
and approval phase as specified in 35 U.S.C. 156(g)(1)(B).

FDA has approved for marketing the human biologic product ENFLONSIA (clesrovimab-cfor). ENFLONSIA is indicated for the prevention
of respiratory syncytial virus (RSV) lower respiratory tract disease in neonates and infants who are born during or entering
their first RSV season. Subsequent to this approval, the USPTO received a patent term restoration application for ENFLONSIA
(U.S. Patent No. 9,963,500) from Merck Sharp & Dohme LLC, and the USPTO requested FDA's assistance in determining this patent's
eligibility for patent term restoration. In a letter dated October 15, 2025, FDA advised the USPTO that this human biological
product had undergone a regulatory review period and that the approval of ENFLONSIA represented the first permitted commercial
marketing or use of the product. Thereafter, the USPTO requested that FDA determine the product's regulatory review period.

II. Determination of Regulatory Review Period

FDA has determined that the applicable regulatory review period for ENFLONSIA is 2,956 days. Of this time, 2,713 days occurred
during the testing phase of the regulatory review period, while 243 days occurred during the approval phase. These periods
of time were derived from the following dates:

1. The date an exemption under section 505(i) of the Federal Food, Drug, and Cosmetic Act (21 U.S.C. 355(i)) became effective: May 8, 2017. FDA has verified the applicant's claim that the date the investigational new drug application became effective
   was on May 8, 2017.

2. The date the application was initially submitted with respect to the human biological product under section 351 of the Public
   Health Service Act (42 U.S.C. 262): October 10, 2024. FDA has verified the applicant's claim that the biologics license application (BLA) for ENFLONSIA (BLA 761432)
   was initially submitted on October 10, 2024.

3. The date the application was approved: June 9, 2025. FDA has verified the applicant's claim that BLA 761432 was approved on June 9, 2025.

This determination of the regulatory review period establishes the maximum potential length of a patent extension. However,
the USPTO applies several statutory limitations in its calculations of the actual period for patent extension. In its application
for patent extension, this applicant seeks 955 days of patent term extension.

III. Petitions

Anyone with knowledge that any of the dates as published are incorrect may submit either electronic or written comments and,
under 21 CFR 60.24, ask for a redetermination (see
DATES
). Furthermore, as specified in § 60.30 (21 CFR 60.30), any interested person may petition FDA for a determination regarding
whether the applicant for extension acted with due diligence during the regulatory review period. To meet its burden, the
petition must comply with all the requirements of § 60.30, including but not limited to: must be timely (see
DATES
), must be filed in accordance with § 10.20, must contain sufficient facts to merit an FDA investigation, and must certify
that a true and complete copy of the petition has been served upon the patent applicant. (See H. Rept. 857, part 1, 98th Cong.,
2d sess., pp. 41-42, 1984.) Petitions should be in the format specified in 21 CFR 10.30.

Submit petitions electronically to https://www.regulations.gov at Docket No. FDA-2013-S-0610. Submit written petitions (two copies are required) to the Dockets Management Staff (HFA-305),
Food and Drug Administration, 5630 Fishers Lane, Rm. 1061, Rockville, MD 20852.

Grace R. Graham, Deputy Commissioner for Policy, Legislation, and International Affairs. [FR Doc. 2026-06479 Filed 4-2-26; 8:45 am] BILLING CODE 4164-01-P

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