Exhibit A

JOINT FILING AGREEMENT

The undersigned hereby agree that the Statement on this Schedule 13D, dated December 5, 2012 (the “Schedule 13D”), with respect to the Common Stock, par value $0.001 per share, of Supernus Pharmaceuticals, Inc. is filed on behalf of each of us pursuant to and in accordance with the provisions of Rule 13d-1(k) under the Securities Exchange Act of 1934, as amended, and that this Agreement shall be included as an Exhibit to this Schedule 13D. Each of the undersigned agrees to be responsible for the timely filing of the Schedule 13D, and for the completeness and accuracy of the information concerning itself contained therein. This Agreement may be executed in any number of counterparts, all of which taken together shall constitute one and the same instrument.

IN WITNESS WHEREOF, the undersigned have executed this Agreement as of the 5thday of December, 2012.

OrbiMed Advisors LLC
By:	/s/ Samuel D. Isaly
	Name:  Samuel D. Isaly
	Title:  Managing Member
OrbiMed Capital GP II LLC
By:	OrbiMed Advisors LLC, its Managing Member
By:	/s/ Samuel D. Isaly
	Name:  Samuel D. Isaly
	Title:  Managing Member
By:	/s/ Samuel D. Isaly
	Name:  Samuel D. Isaly