13D Filing: JFL Capital Management and Merrimack Pharmaceuticals Inc (NASDAQ:MACK)

UNITED STATES

SECURITIES AND EXCHANGE
COMMISSION

Washington, D.C. 20549

SCHEDULE 13D

(Rule 13d-101)

INFORMATION TO BE INCLUDED
IN STATEMENTS FILED PURSUANT

TO § 240.13d-1(a)
AND AMENDMENTS THERETO FILED PURSUANT TO

§ 240.13d-2(a)

(Amendment No.  )¹

Merrimack Pharmaceuticals, Inc.

(Name
of Issuer)

Common Stock, $0.01 par value per share

(Title of Class of Securities)

590328209

(CUSIP Number)

JOSEPH F. LAWLER, M.D., PH.D.

JFL CAPITAL MANAGEMENT LLC

2110 Ranch Road 620 S, #341732

Lakeway, Texas 78734

(512) 761-4500

STEVE WOLOSKY

RYAN NEBEL

OLSHAN FROME WOLOSKY LLP

1325 Avenue of the Americas

New York, New York 10019

(212)
451-2300

(Name, Address and Telephone Number of Person

Authorized to Receive Notices
and Communications)

October 23, 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 for other parties to whom copies are to be sent.

1
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).