UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

(Amendment No. 1)*

RHYTHM PHARMACEUTICALS, INC.

(Name of Issuer)

COMMON STOCK

(Title of Class of Securities)

76243J105

(CUSIP Number)

OrbiMed Advisors LLC
OrbiMed Capital GP V LLC

601 Lexington Avenue, 54th Floor
New York, NY 10022
Telephone: (212) 739-6400

(Name, Address and Telephone Number of Person
Authorized to Receive Notices and Communications)

January 24, 2018

(Date of Event Which Requires Filing of this Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of § 240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box ☐.

Note: Schedules filed in paper format shall include a signed original and five copies of the schedule, including all exhibits. See § 240.13d-7(b) for other parties to whom copies are to be sent.

*The remainder of this cover page shall be filled out for a reporting person’s initial filing on this form with respect to the subject class of securities, and for any subsequent amendment containing information which would alter disclosures provided in a prior cover page.

The information required on the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).


CUSIP No. 76243J105
1	NAME OF REPORTING PERSON OrbiMed Advisors LLC
2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP			(a) ☐ (b) ☐
3	SEC USE ONLY
4	SOURCE OF FUNDS AF
5	CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO ITEMS 2(d) or 2(e)			☐
6	CITIZENSHIP OR PLACE OF ORGANIZATION Delaware
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH		7	SOLE VOTING POWER 0
		8	SHARED VOTING POWER 2,901,295
		9	SOLE DISPOSITIVE POWER 0
		10	SHARED DISPOSITIVE POWER 2,901,295
11	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 2,901,295
12	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES (See Instructions)			☐
13	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11) 10.63% (1)
14	TYPE OF REPORTING PERSON (See Instructions) IA

(1)

This percentage is calculated based upon 27,284,140 outstanding shares of common stock, par value $0.001 per share (the “Shares”), of Rhythm Pharmaceuticals, Inc. (the “Issuer”), as set forth in the Issuer’s Form 10-Q, filed with the Securities and Exchange Commission on November 14, 2017.


CUSIP No. 76243J105
1	NAME OF REPORTING PERSON OrbiMed Capital GP V LLC
2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP			(a) ☐ (b) ☐
3	SEC USE ONLY
4	SOURCE OF FUNDS AF
5	CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO ITEMS 2(d) or 2(e)			☐
6	CITIZENSHIP OR PLACE OF ORGANIZATION Delaware
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH		7	SOLE VOTING POWER 0
		8	SHARED VOTING POWER 2,901,295
		9	SOLE DISPOSITIVE POWER 0
		10	SHARED DISPOSITIVE POWER 2,901,295
11	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 2,901,295
12	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES (See Instructions)			☐
13	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11) 10.63% (1)
14	TYPE OF REPORTING PERSON (See Instructions) OO

(1)

Item 1.

Security and Issuer

This Amendment No. 1 (“Amendment No. 1”) to Schedule 13D supplements and amends the Statement on Schedule 13D originally filed by OrbiMed Advisors LLC, OrbiMed Capital GP IV LLC and Samuel D. Isaly with the Securities and Exchange Commission (the “SEC”) on October 20, 2017 (the “Statement”). The Statement relates to the common stock, par value $0.001 per share (the “Shares”), of Rhythm Pharmaceuticals, Inc., a corporation organized under the laws of Delaware (the “Issuer”), with its principal executive offices located at 500 Boylston Street, 11^th floor, Boston, MA 02116. The common stock is listed on the NASDAQ Global Market under the ticker symbol “RYTM”. Information given in response to each item shall be deemed incorporated by reference in all other items, as applicable.

This Amendment No. 1 is being filed to report that Samuel D. Isaly, who was previously identified as a reporting person, has ceased to be the beneficial owner of more than five percent of the outstanding Shares.

Item 2 of the Statement is amended to read in full as set forth below.

Item 2.

Identity and Background

(a) This Statement is being filed by OrbiMed Advisors LLC (“Advisors”), a limited liability company organized under the laws of Delaware and OrbiMed Capital GP V LLC (“GP V”), a limited liability company organized under the laws of Delaware (collectively, the “Reporting Persons”).

(b) – (c), (f) Advisors, a registered investment adviser under the Investment Advisers Act of 1940, as amended, is the sole managing member of GP V, which is the sole general partner of OrbiMed Private Investments V, LP (“OPI V”). OPI V holds Shares as described herein. Advisors has their principal offices at 601 Lexington Avenue, 54th Floor, New York, New York 10022.

GP V has its principal offices at 601 Lexington Avenue, 54th Floor, New York, New York 10022.

The directors and executive officers of Advisors and GP V are set forth on Schedules I and II, attached hereto. Schedules I and II set forth the following information with respect to each such person:

(i)

name;

(ii)

business address;

(iii)

present principal occupation or employment and the name, principal business and address of any corporation or other organization in which such employment is conducted;

(iv)

citizenship.

(d) – (e) During the last five years, neither the Reporting Persons nor any Person named in Schedule I or II have been (i) convicted in a criminal proceeding (excluding traffic violations or similar misdemeanors) or (ii) a party to a civil proceeding of a judicial or administrative body of competent jurisdiction and as a result of such proceeding was or is subject to a judgment, decree or final order enjoining future violations of, or prohibiting or mandating activities subject to, federal or state securities laws or finding any violation with respect to such laws.

Item 5 of the Statement is amended to read in full as set forth below.

Item 5.

Interest in Securities of the Issuer

(a) — (b) As of the date of this filing, the Reporting Persons may be deemed, for purposes of Rule 13d-3 of the Act, directly or indirectly, including by reason of their mutual affiliation, to be the beneficial owners of 2,901,295 Shares held of record by OPI V. Based upon information contained in the Issuer’s Form 10-Q, filed with the Securities and Exchange Commission (the “SEC”) on November 14, 2017, such Shares constitutes approximately 10.63% of the issued and outstanding Shares. Advisors, pursuant to its authority as the sole managing member of GP V, which is the sole general partner of OPI V, may be deemed to indirectly beneficially own the Shares held by OPI V. GP V, pursuant to its authority as the general partner of OPI V, may be deemed to indirectly beneficially own the Shares held by OPI V. As a result, Advisors and GP V share the power to direct the vote and the disposition of the Shares held of record by OPI V. Advisors exercises this investment and voting power through a management committee comprised of Carl L. Gordon, Sven H. Borho and Jonathan T. Silverstein, each of whom disclaims beneficial ownership of the Shares held by OPI IV.

In addition, Advisors and GP V, pursuant to their authority under the limited partnership agreement of OPI V, prior to the date of this filing, caused OPI V to enter into the agreements referred to in Item 6 below.

(d) Not applicable.

(e) As of January 24, 2018, Samuel D. Isaly, who was previously identified as a reporting person, has ceased to be the beneficial owner of more than five percent of the outstanding Shares.

Item 7.

Materials to Be Filed as Exhibits

Exhibit	Description
1.	Joint Filing Agreement among OrbiMed Advisors LLC and OrbiMed Capital GP V LLC.
2.	Lock-Up Agreement.
3.	Amended and Restated Investors’ Rights Agreement by and among the Issuer and each of the persons listed on Schedule A thereto, dated as of August 21, 2017 (incorporated by reference to Exhibit 4.2 to the Issuer’s Registration Statement on Form S-1 (SEC 333-220337), filed with the SEC on September 5, 2017).

OrbiMed Advisors LLC

By:	/s/ Jonathan T. Silverstein
	Name:	Jonathan T. Silverstein
	Title:	Member of OrbiMed Advisors LLC

By:	/s/ Sven H. Borho
	Name:	Sven H. Borho
	Title:	Member of OrbiMed Advisors LLC

By:	/s/ Carl L. Gordon
	Name:	Carl L. Gordon
	Title:	Member of OrbiMed Advisors LLC

OrbiMed Capital GP V LLC

By:	OrbiMed Advisors LLC its Managing Member

By:	/s/ Jonathan T. Silverstein
	Name:	Jonathan T. Silverstein
	Title:	Member of OrbiMed Advisors LLC

By:	/s/ Sven H. Borho
	Name:	Sven H. Borho
	Title:	Member of OrbiMed Advisors LLC

By:	/s/ Carl L. Gordon
	Name:	Carl L. Gordon
	Title:	Member of OrbiMed Advisors LLC

Name	Position with Reporting Person	Principal Occupation
Samuel D. Isaly	Managing Member	Managing Member OrbiMed Advisors LLC
Carl L. Gordon	Member	Member OrbiMed Advisors LLC
Sven H. Borho German and Swedish Citizen	Member	Member OrbiMed Advisors LLC
Jonathan T. Silverstein	Member	Member OrbiMed Advisors LLC
W. Carter Neild	Member	Member OrbiMed Advisors LLC
Geoffrey C. Hsu	Member	Member OrbiMed Advisors LLC
Evan D. Sotiriou	Chief Financial Officer	Chief Financial Officer OrbiMed Advisors LLC

Exhibit	Description
1.	Joint Filing Agreement among OrbiMed Advisors LLC and OrbiMed Capital GP V LLC.
2.	Lock-Up Agreement.
3.	Amended and Restated Investors’ Rights Agreement by and among the Issuer and each of the persons listed on Schedule A thereto, dated as of August 21, 2017 (incorporated by reference to Exhibit 4.2 to the Issuer’s Registration Statement on Form S-1 (SEC 333-220337), filed with the SEC on September 5, 2017).