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

SCHEDULE 13D
Under the Securities Exchange Act of 1934

(Amendment No. 5)

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 Rule 13d-1(e), 13d-1(f) or 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 Rule 13d-7 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 (the “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)

SCHEDULE 13D

(1) Includes shares of common stock issuable upon exercise of the Warrant (as defined in Item 5 below), subject to the limitations on exercise described in Item 5 below.

(2) Based on 21,493,405 shares of common stock outstanding as of November 6, 2013, as reported in the Issuer’s Form 10-Q filed with the SEC on November 8, 2013.

SCHEDULE 13D

(1) Includes shares of common stock issuable upon exercise of the Warrant (as defined in Item 5 below), subject to the limitations on exercise described in Item 5 below.

(2) Based on 21,493,405 shares of common stock outstanding as of November 6, 2013, as reported in the Issuer’s Form 10-Q filed with the SEC on November 8, 2013.

SCHEDULE 13D

(1) Includes shares of common stock issuable upon exercise of the Warrant (as defined in Item 5 below), subject to the limitations on exercise described in Item 5 below.

(2) Based on 21,493,405 shares of common stock outstanding as of November 6, 2013, as reported in the Issuer’s Form 10-Q filed with the SEC on November 8, 2013.

SCHEDULE 13D

(1) Includes shares of common stock issuable upon exercise of the Warrant (as defined in Item 5 below), subject to the limitations on exercise described in Item 5 below.

(2) Based on 21,493,405 shares of common stock outstanding as of November 6, 2013, as reported in the Issuer’s Form 10-Q filed with the SEC on November 8, 2013.

Amendment No. 5 to Schedule 13D

This Amendment No. 5 to Schedule 13D amends and supplements the previously filed Schedules 13D filed by Baker Bros. Advisors (GP) LLC (the “Adviser GP”), Baker Bros. Advisors LP (the “Adviser”, and together with the Advisor GP, Julian C. Baker and Felix J. Baker, the “Reporting Persons”), Julian C. Baker and Felix J. Baker. Except as set fourth below, the Amendment No. 5 does not supplement, restate or amend any of the information disclosed in the Schedule 13D as previously filed.

Pursuant to the amended and restated management agreements, as amended, among the Adviser, Baker Brothers Life Sciences, L.P. (“Life Sciences”), 14159, L.P. (“14159”), and 667, L.P. (“667”, and together with Life Sciences and 14159, the “Funds”), and their respective general partners, the Adviser has complete and unlimited discretion and authority with respect to the Funds’ investments and voting power over investments.

Item 5. Interest in Securities of the Issuer.

Item 5 of this Schedule 13D is hereby amended and restated in its entirety as follows:

(a) and (b) Items 7 through 11 and 13 of each of the cover pages of this Schedule 13D are incorporated herein by reference. Set forth below is the aggregate number of shares of Common Stock of the Mei Pharma, Inc. (the “Issuer”) directly held by each of the Funds, which may be deemed to be indirectly beneficially owned by the Reporting Persons, as well as shares of Common Stock that may be acquired upon exercise of the Warrant (as defined below) at an exercise price of $3.12 per share as of the date hereof by the Funds, subject to the limitation on exercise described below.

Pursuant to the Warrants to Purchase Common Stock by each Fund dated November 4, 2012 (the “Warrant”), if at any time the beneficial ownership of the Funds and their affiliates exceeds 9.99% of the shares of Common Stock outstanding immediately after giving effect to the exercise of the Warrant, then the number of shares of common stock that may be acquired by the Funds and their affiliates upon any exercise of the Warrant will be limited to the extent necessary to ensure that, following such exercise, the total number of shares of common stock then beneficially owned by the Funds and their affiliates does not exceed 9.99% of the total number of then issued and outstanding shares of Common Stock (including for such purpose the shares of Common Stock issuable upon exercise of the Warrant) (the “Maximum Percentage”). At any time that the Funds and their affiliates do not beneficially own in excess of the Maximum Percentage, the Funds may, by written notice to the Issuer, from time to time increase or decrease the Maximum Percentage to any other percentage not in excess of 9.99% specified in such notice. Pursuant to the Warrant, any such increase will not be effective until the 61st day after such notice is delivered to the Issuer.

The Adviser GP, Felix J. Baker and Julian C. Baker as principals of the Adviser GP, and the Adviser may be deemed to be beneficial owners of securities of the Issuer directly held by the Funds, and may be deemed to have the power to vote or direct the vote of and the power to dispose or direct the disposition of such securities.

The Reporting Persons disclaim beneficial ownership of the securities held by each of the Funds, and this Schedule 13D shall not be deemed an admission that the Reporting Persons are the beneficial owners of such securities for purposes of Section 13(d) or for any other purpose.

(c) The following transactions in the Issuer’s Common Stock were effected by the Funds noted below during the sixty days preceding the filing of this statement. All transactions were effected in the over-the-counter market directly with a broker-dealer. Except as disclosed herein or in any previous amendments to this Schedule 13D, none of the Reporting Persons or their affiliates has effected any other transactions in securities of the Issuer during the past 60 days.

(1) The reported price is a weighted average price. These shares were traded in multiple transactions at prices ranging from $8.50 to $8.57. The Reporting Persons undertake to provide the staff of the Securities and Exchange Commission (the “Staff”), upon request, full information regarding the number of shares traded at each separate price within the ranges set forth in this footnote.

(2) The reported price is a weighted average price. These shares were traded in multiple transactions at prices ranging from $8.50 to $8.57 The Reporting Persons undertake to provide the Staff, upon request, full information regarding the number of shares traded at each separate price within the ranges set forth in this footnote.

(3) The reported price is a weighted average price. These shares were traded in multiple transactions at prices ranging from $8.00 to $8.41 The Reporting Persons undertake to provide the Staff, upon request, full information regarding the number of shares traded at each separate price within the ranges set forth in this footnote.

(4) The reported price is a weighted average price. These shares were traded in multiple transactions at prices ranging from $8.25 to $8.59 The Reporting Persons undertake to provide the Staff, upon request, full information regarding the number of shares traded at each separate price within the ranges set forth in this footnote.

(5) The reported price is a weighted average price. These shares were traded in multiple transactions at prices ranging from $8.25 to $8.36 The Reporting Persons undertake to provide the Staff, upon request, full information regarding the number of shares traded at each separate price within the ranges set forth in this footnote.

(6) The reported price is a weighted average price. These shares were traded in multiple transactions at prices ranging from $8.20 to $8.41 The Reporting Persons undertake to provide the Staff, upon request, full information regarding the number of shares traded at each separate price within the ranges set forth in this footnote.

(7) The reported price is a weighted average price. These shares were traded in multiple transactions at prices ranging from $8.20 to $8.36 The Reporting Persons undertake to provide the Staff, upon request, full information regarding the number of shares traded at each separate price within the ranges set forth in this footnote.

(d) Certain securities of the Issuer are held directly by 667, a limited partnership the sole general partner of which is Baker Biotech Capital, L.P., a limited partnership the sole general partner of which is Baker Biotech Capital (GP), LLC. Julian C. Baker and Felix J. Baker are the controlling members of Baker Biotech Capital (GP), LLC.

Certain securities of the Issuer are held directly by Life Sciences, a limited partnership the sole general partner of which is Baker Brothers Life Sciences Capital, L.P., a limited partnership the sole general partner of which is Baker Brothers Life Sciences Capital (GP), LLC. Julian C. Baker and Felix J. Baker are the controlling members of Baker Brothers Life Sciences Capital (GP), LLC.

Certain securities of the Issuer are held directly by 14159, a limited partnership the sole general partner of which is 14159 Capital, L.P., a limited partnership the sole general partner of which is 14159 Capital (GP), LLC. Julian C. Baker and Felix J. Baker are the controlling members of 14159 Capital (GP), LLC.

(e) As of the close of business on December 26, 2013, each Reporting Person ceased to be the beneficial owner of more than five percent of MEI Pharma, Inc.’s Common Stock.

SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

December 30, 2013

Exhibit 99.1

JOINT FILING AGREEMENT

Pursuant to Rule 13d-1(k)(1) under the Securities Exchange Act of 1934, as amended, the undersigned hereby agree that only one statement containing the information required by Schedule 13D need be filed with respect to the ownership by each of the undersigned of the shares of common stock of MEI Pharma, Inc..

This Agreement may be executed in any number of counterparts, each of which shall be deemed an original.

EXECUTED as of this 30^th day of December, 2013.