13G Filing: James E. Flynn and Marinus Pharmaceuticals Inc (MRNS)

NAME OF REPORTING PERSONS

I.R.S. IDENTIFICATION NO. OF ABOVE PERSONS (ENTITIES ONLY)

Deerfield Mgmt, L.P.

CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*

(a)
☐

(b) ☒

SEC USE ONLY

CITIZENSHIP OR PLACE OF ORGANIZATION

Delaware

 NUMBER OF

SHARES

BENEFICIALLY

OWNED BY

EACH

REPORTING

PERSON

WITH

SOLE VOTING POWER

0

SHARED VOTING POWER

2,238,587 (1)

7.

SOLE DISPOSITIVE POWER

0

8.

SHARED DISPOSITIVE POWER

2,238,587 (1)

9.

AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

2,238,587 (1)

CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN
SHARES*

11.

PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9

9.98%

TYPE OF REPORTING PERSON*

PN

56854Q101

NAME OF REPORTING PERSONS

I.R.S. IDENTIFICATION NO. OF ABOVE PERSONS (ENTITIES ONLY)

Deerfield Management
Company, L.P.

CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*

(a)
☐

(b) ☒

SEC USE ONLY

CITIZENSHIP OR PLACE OF ORGANIZATION

Delaware

 NUMBER OF

SHARES

BENEFICIALLY

OWNED BY

EACH

REPORTING

PERSON

WITH

SOLE VOTING POWER

0

SHARED VOTING POWER

2,238,587 (2)

7.

SOLE DISPOSITIVE POWER

0

8.

SHARED DISPOSITIVE POWER

2,238,587 (2)

9.

AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

2,238,587 (2)

CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN
SHARES*

11.

PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9

9.98%

TYPE OF REPORTING PERSON*

PN

56854Q101

NAME OF REPORTING PERSONS

I.R.S. IDENTIFICATION NO. OF ABOVE PERSONS (ENTITIES ONLY)

Deerfield Special
Situations Fund, L.P.

CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*

(a)
☐

(b) ☒

SEC USE ONLY

CITIZENSHIP OR PLACE OF ORGANIZATION

Delaware

 NUMBER OF

SHARES

BENEFICIALLY

OWNED BY

EACH

REPORTING

PERSON

WITH

SOLE VOTING POWER

0

SHARED VOTING POWER

2,238,587

7.

SOLE DISPOSITIVE POWER

0

8.

SHARED DISPOSITIVE POWER

2,238,587

9.

AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

2,238,587

CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN
SHARES*

11.

PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9

9.98%

TYPE OF REPORTING PERSON*

PN

NAME OF REPORTING PERSONS

I.R.S. IDENTIFICATION NO. OF ABOVE PERSONS (ENTITIES ONLY)

James E. Flynn

CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*

(a)
☐

(b) ☒

SEC USE ONLY

CITIZENSHIP OR PLACE OF ORGANIZATION

United States

 NUMBER OF

SHARES

BENEFICIALLY

OWNED BY

EACH

REPORTING

PERSON

WITH

SOLE VOTING POWER

0

SHARED VOTING POWER

2,238,587 (3)

7.

SOLE DISPOSITIVE POWER

0

8.

SHARED DISPOSITIVE POWER

2,238,587 (3)

9.

AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

2,238,587 (3)

CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN
SHARES*

11.

PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9

9.98%

TYPE OF REPORTING PERSON*

IN

Marinus Pharmaceuticals, Inc.

170 N. Radnor Chester Road, Suite 250

Radnor, Pennsylvania 19087

James
E. Flynn, Deerfield Mgmt, L.P., Deerfield Special Situations Fund, L.P., and Deerfield Management Company, L.P.

James E. Flynn, Deerfield Mgmt, L.P., Deerfield
Special Situations Fund, L.P. and Deerfield Management Company, L.P., 780 Third Avenue, 37th Floor, New York, NY 10017

Deerfield
Mgmt, L.P., Deerfield Management Company, L.P. and Deerfield Special Situations Fund, L.P. – Delaware limited partnerships;

James E. Flynn – United States citizen

common
stock, par value $0.001

56854Q101

If This Statement is Filed Pursuant to
Rule 13d-1(b), or 13d-2(b) or (c), Check Whether the Person Filing is a:

Broker or dealer registered under Section 15 of the
Exchange Act.

Insurance company as defined in Section 3(a)(19) of
the Exchange Act.

Investment company registered under Section 8 of the
Investment Company Act.

An employee benefit plan or endowment fund in accordance
with Rule 13d-1(b)(1)(ii)(F);

A parent holding company or control person in accordance
with Rule 13d-1(b)(1)(ii)(G);

(h)

A savings association as defined in Section 3(b) of
the Federal Deposit Insurance Act;

A church plan that is excluded from the definition
of an investment company under Section 3(c)(14) of the Investment Company Act;

Provide the following information regarding the aggregate number and percentage of the class of securities of the issuer identified in Item 1.

Deerfield
Mgmt, L.P. – 2,238,587 shares

Deerfield Management Company, L.P. – 2,238,587 shares

Deerfield Special Situations Fund, L.P. – 2,238,587
shares

James E. Flynn – 2,238,587 shares

Deerfield
Mgmt, L.P. – 9.98%

Deerfield Management Company, L.P. – 9.98%

Deerfield Special Situations Fund, L.P. –
9.98%

James E. Flynn – 9.98%

Deerfield Mgmt, L.P. – 2,238,587

Deerfield Management Company, L.P. – 2,238,587

Deerfield Special Situations Fund, L.P. – 2,238,587

James E. Flynn – 2,238,587

Sole power to dispose or to direct the disposition of:

Shared power to dispose or to direct the disposition
of:

Deerfield Mgmt, L.P. – 2,238,587

Deerfield Management Company, L.P. – 2,238,587

Deerfield Special Situations Fund, L.P. – 2,238,587

James E. Flynn – 2,238,587

     If this statement
is being filed to report the fact that as of the date hereof the reporting person has ceased to be the beneficial owner of more
than five percent of the class of securities check the following ☐.

If any other person is known to have the right to receive or the power to direct the receipt of dividends from, or the proceeds
from the sale of, such securities, a statement to that effect should be included in response to this item and, if such interest
relates to more than five percent of the class, such person should be identified. A listing of the shareholders of an investment
company registered under the Investment Company Act of 1940 or the beneficiaries of employee benefit plan, pension fund or endowment
fund is not required.

Identification and Classification of the Subsidiary
Which Acquired the Security Being Reported on by the Parent Holding Company or Control Person.

If a parent holding company or Control person has filed this schedule, pursuant to Rule 13d-1(b)(1)(ii)(G), so indicate under
Item 3(g) and attach an exhibit stating the identity and the Item 3 classification of the relevant subsidiary. If a parent holding
company or control person has filed this schedule pursuant to Rule 13d-1(c) or Rule 13d-1(d), attach an exhibit stating the identification
of the relevant subsidiary.

If a group has filed this schedule pursuant to ss.240.13d-1(b)(1)(ii)(J), so indicate under Item 3(j) and attach an exhibit stating
the identity and Item 3 classification of each member of the group. If a group has filed this schedule pursuant to ss.240.13d-1(c)
or ss.240.13d-1(d), attach an exhibit stating the identity of each member of the group.

     Notice of dissolution
of a group may be furnished as an exhibit stating the date of the dissolution and that all further filings with respect to transactions
in the security reported on will be filed, if required, by members of the group, in their individual capacity. See Item 5.

    “By signing below I certify that,
to the best of my knowledge and belief, the securities referred to above were not acquired and are not held for the purpose of
or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held
in connection with or as a participant in any transaction having that purpose or effect, other than activities solely in connection
with a nomination under §240.14a–11.”