13D Filing: Caxton Corp and Strongbridge Biopharma PLC (SBBP)

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of
1934

(Amendment No. 1)

Strongbridge Biopharma plc

(Name of Issuer)

Ordinary Shares, $0.01 par value

(Title of Class of Securities)

G85347105

(CUSIP Number)

Mr. Heath N. Weisberg

Caxton Corporation

731 Alexander Road, Bldg. 2, Suite 500

Princeton, New Jersey 08540

(212) 205-6805

(Name, Address and Telephone Number of
Person Authorized to

Receive Notices and Communications)

April 29, 2017

(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 Section
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 Section

240.13d-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 (“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).

This Schedule 13D relates to the ordinary
shares (the “Ordinary Shares”), of Strongbridge Biopharma plc (the “Company”). The principal executive office
of the Company is located at 900 Northbrook Drive, Suite 200, Trevose, Pennsylvania 19053.

(a)
This statement is being filed by Caxton Corporation, a Delaware Corporation, CDK Associates, L.L.C., a Delaware limited
liability company and Mr. Bruce Kovner (each a “Reporting Person” and collectively the “Reporting
Persons”).

(d)          No
person filing this statement has during the past five years, been convicted in a criminal proceeding (excluding traffic violations
or similar misdemeanors).

(e)          No
person filing this statement has during the last five years been a party to a civil proceeding of a judicial or administrative
body of competent jurisdiction and as a result of such a 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.

As of the date hereof Caxton Corporation
may be deemed to beneficially own 7,614,994 Ordinary Shares.

As of the date hereof CDK Associates,
L.L.C. may be deemed to beneficially own 7,202,433 Ordinary Shares.

As of the date hereof Mr. Kovner may be
deemed to beneficially own 7,614,994 Ordinary Shares.

The funds for the purchase of the Ordinary
Shares beneficially owned by the Reporting Persons (other than Ordinary Shares owned by employees of an affiliate of Caxton Corporation)
came from working capital.

Each of the Ordinary Shares of the Company
that are indicated herein were acquired for investment purposes. Based on a review of the investment, certain of the Reporting
Persons may, in the open market or otherwise, acquire, cause to be acquired, dispose of, or cause to be disposed of, a material
portion of Ordinary Shares or other securities of the Company, or derivatives or other instruments related to the securities of
the Company.

Except as set forth in this Schedule 13D,
none of the Reporting Persons has any plans or proposals that relate to any of the matters referred to in paragraphs (a) through
(j) of the instructions to Item 4 of Schedule 13D.

(a) (i) Caxton Corporation is the Manager
of CDK Associates, L.L.C. and as such, has voting and dispositive power with respect to the 7,202,433 Ordinary Shares beneficially
owned by CDK Associates, L.L.C., and may be deemed to have voting and dispositive power with respect to the 412,561 Ordinary Shares
beneficially owned by employees of an affiliate of Caxton Corporation. The 7,202,433 Ordinary Shares beneficially owned by CDK
Associates, L.L.C. represent 5,102,433 Ordinary Shares and 2,100,000 Warrants (as described in Item 6 below). The 412,561 Ordinary
Shares beneficially owned by employees of an affiliate of Caxton Corporation represent 292,561 Ordinary Shares and 120,000 Warrants.
As a result, Caxton Corporation may be deemed to beneficially own 7,614,994 Ordinary Shares, representing approximately 18.1% of
the 39,812,344 Ordinary Shares outstanding as of October 2, 2017, as reported in the Prospectus filed by the Issuer on October
3, 2017. The percentages used herein and in the rest of this Schedule 13D are calculated based upon such number of outstanding
Ordinary Shares, as adjusted to reflect the exercise of warrants held by the Reporting Persons.

Caxton Corporation disclaims beneficial
ownership of the Ordinary Shares owned directly by CDK Associates, L.L.C., and employees of an affiliate of Caxton Corporation,
except to the extent of its pecuniary interest therein.

(ii)       CDK
Associates, L.L.C. beneficially owns 7,202,433 Ordinary Shares, representing approximately 17.2% of the total Ordinary Shares issued
and outstanding.

(iii)       Mr.
Kovner is the Chairman and sole shareholder of Caxton Corporation, the Manager of CDK Associates, L.L.C. and as a result may be
deemed to beneficially own 7,614,994 Ordinary Shares beneficially owned by CDK Associates, L.L.C. and employees of an affiliate
of Caxton Corporation, representing approximately 18.1% of the total Ordinary Shares issued and outstanding.

Mr. Kovner disclaims beneficial ownership
of the Ordinary Shares owned directly by CDK Associates, L.L.C. and employees of an affiliate of Caxton Corporation, except to
the extent of his pecuniary interest therein.

(b)       CDK
Associates, L.L.C. may be deemed to share voting and dispositive power over 7,202,433 Ordinary Shares. Caxton Corporation and Mr.
Kovner, as Chairman and sole shareholder of Caxton Corporation, the Manager of CDK Associates, L.L.C., may be deemed to share voting
and dispositive power over 7,202,433 Ordinary Shares beneficially owned by CDK Associates, L.L.C., and 412,561 Ordinary Shares
beneficially owned by employees of an affiliate of Caxton Corporation.

(c)       No
transactions in the Ordinary Shares were effected by the Reporting Persons during the past 60 days.

The Warrants held by the Reporting Persons
are exercisable at any time, and from time to time, until June 28, 2022 at an exercise price of $2.50 per share. The applicable
per share purchase price and the number of Ordinary Shares issuable upon exercise of the Warrants are subject to adjustment for
the occurrence of certain events, including stock dividends, splits, subdivisions, combinations, reorganizations and reclassifications.
The foregoing description of the Warrants is qualified in its entirety by reference to the form of Warrant, which is filed as an
exhibit to this Schedule 13D and is incorporated by reference herein.

CDK Associates, L.L.C. has entered into
a registration rights agreement with the Company, as described in the Reporting Persons Schedule 13D filed on December 30, 2017.

Exhibit A: Joint Filing
Agreement.

Exhibit B: Power of Attorney
of Bruce Kovner

Exhibit C: Form of Warrant
to purchase Ordinary Shares (incorporated by reference to Exhibit 10.3 to the Issuer’s Form 6-K filed on December 23, 2016).

Signatures

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.

The original statement shall be signed by each person
on whose behalf the statement is filed or his authorized representative. If the statement is signed on behalf of a person by his
authorized representative (other than an executive officer or general partner of the filing person), evidence of the representative’s
authority to sign on behalf of such person shall be filed with the statement: provided, however, that a power of attorney for this
purpose which is already on file with the Commission may be incorporated by reference. The name and any title of each person who
signs the statement shall be typed or printed beneath his signature.

* The Reporting Persons disclaim beneficial ownership
over the securities reported herein except to the extent of the reporting persons’ pecuniary interest therein.

ATTENTION: Intentional misstatements or omissions
of fact constitute Federal criminal violations (See 18 U.S.C. 1001).

Exhibit A

AGREEMENT

Pursuant to Rule 13d-1(k)(1) under the Securities
Exchange Act of 1934, as amended, each of the undersigned hereby agrees 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 Ordinary Shares, $0.01 par value of Strongbridge
Biopharma plc.

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

* The Reporting Persons disclaim beneficial ownership
over the securities reported herein except to the extent of the reporting persons’ pecuniary interest therein.

Exhibit B

POWER OF ATTORNEY

Know all by these presents, that the undersigned
hereby constitutes and appoints Heath Weisberg, signing singly, with full power of substitution, the undersigned’s true and
lawful attorney-in-fact to:

(1) execute for and on behalf of the undersigned
any applications, reports, or documents required or deemed appropriate by the attorney-in-fact to file pursuant to (i) the United
States Securities Exchange Act of 1934, as amended, or any rule, or regulation thereunder, including, without limitation, Schedules
13D, 13G, 13F, and 13H, and Forms 3, 4, and 5, (ii) the Securities Act of 1933, as amended, or any rule or regulation thereunder,
including, without limitation, Form 144, (iii) the U.S. Commodity Exchange Act, as amended or any rule or regulation thereunder,
or (iv) the statutes, rules or regulations of any other domestic or foreign governmental or self-regulatory authority;

(2) do and perform any and all acts for and
on behalf of the undersigned which may be necessary or desirable to complete and execute any such applications, reports, or documents;

(3) take any other action of any type whatsoever
in connection with the foregoing which, in the opinion of such attorney-in-fact, may be of benefit to, in the best interests of,
or legally required by the undersigned, it being understood that the documents executed by such attorney-in-fact on behalf of the
undersigned pursuant to this Power of Attorney shall be in such form and shall contain such terms and conditions as such attorney-in-fact
may approve in such attorney-in-fact’s discretion.

The undersigned hereby grants to each such
attorney-in-fact full power and authority to do and perform any and every act and thing whatsoever requisite, necessary, or proper
to be done in the exercise of any of the rights and powers herein granted, as fully to all intents and purposes as the undersigned
might or could do if personally present, with full power of substitution or revocation, hereby ratifying and confirming all that
such attorney-in-fact, or such attorney-in-fact’s substitute or substitutes, shall lawfully do or cause to be done by virtue
of this power of attorney and the rights and powers herein granted. The undersigned acknowledges that the foregoing attorney-in-fact,
in serving in such capacity at the request of the undersigned, are not assuming, nor is the Company assuming, any of the undersigned’s
responsibilities to comply with the applicable statutes, rules and regulations.

This Power of Attorney shall remain in full
force and effect until revoked by the undersigned in a signed writing delivered to the foregoing attorney-in-fact or otherwise
terminated by my death or other event described in section 5-1511 of the New York General Obligations Law.

SIGNATURE AND ACKNOWLEDGMENT:

In Witness Whereof I have hereunto signed my
name on the 9th day of May, 2013.

/s/ Bruce Kovner

Bruce Kovner

STATE OF NEW YORK     )

                                         ) ss:

COUNTY OF NEW YORK  )

On the 9th day of May 2013, before me, the
undersigned, personally appeared Bruce Kovner, personally known to me or proved to me on the basis of satisfactory evidence to
be the individual whose name is subscribed to the within instrument and acknowledged to me that he/she executed the same in his/her
capacity, and that by his/her signature on the instrument, the individual, or the person upon behalf of which the individual acted,
executed the instrument.

/s/ Cynthia Rosel Rivera

Notary Public

AGENT’S SIGNATURE AND ACKNOWLEDGMENT OF APPOINTMENT:

I, Heath N. Weisberg, have read the foregoing
Power of Attorney. I am the person identified therein as agent and attorney-in-fact for the principal named therein.

I acknowledge my legal responsibilities.

/s/ Heath N. Weisberg

Heath N. Weisberg

STATE OF NEW YORK     )

                                         ) ss:

COUNTY OF NEW YORK  )

On the 9th day of May, 2013, before me, the
undersigned, personally appeared Heath N. Weisberg, personally known to me or proved to me on the basis of satisfactory evidence
to be the individual whose name is subscribed to the within instrument and acknowledged to me that he/she executed the same in
his/her capacity, and that by his/her signature on the instrument, the individual, or the person upon behalf of which the individual
acted, executed the instrument.

/s/ Cynthia Rosel Rivera

Notary Public