X0306 4 2015-01-02 0 0001160308 Mast Therapeutics, Inc. MSTX 0001600117 Parsley Edwin L. 12390 EL CAMINO REAL, SUITE 150 SAN DIEGO CA 92130 0 1 0 0 Chief Medical Officer & SVP Stock Option (Right to Buy) 0.58 2015-01-02 4 A 0 938300 0.00 A 2025-01-02 Common Stock 938300 938300 D This option will vest and become exercisable as to 156,383 shares on October 1, 2015, which is the first anniversary of the reporting person's employment with the issuer, and the remaining shares will vest and become exercisable in 48 substantially equal monthly installments on each monthly anniversary of October 1, 2015 such that all of the shares subject to this option will have fully vested and become exercisable as of October 1, 2019, subject to the reporting person's continued service. /s/ Brandi L. Roberts, Attorney-in-Fact for Edwin L. Parsley 2015-01-02