Offer Letter, dated as of November 19, 2012, by and between the Registrant and Jonathan P. Mow, as amended to date

Annual salary of $270,000

Eligibility for an annual bonus up to 20% of your base salary, based upon achievement of specified company and/or individual goals

Eligibility for a further bonus of $250,000 if the company completes a transaction of the type described in Article II, Sections 3(b)(1) or 3(b)(2), of the Fourth Amended and Restated Certificate of Incorporation (a “Liquidation Event”) on or before March 31, 2014. In the event of a Liquidation Event, the board of directors will cause company to seek in good faith to obtain approval of the bonus and any other payments that are considered “parachute payments” under section 280G of the Internal Revenue Code, in accordance with the shareholder approval requirements of the regulations under section 280G, subject to your taking such actions as may be necessary or appropriate under such regulations.

Subject to approval by the Board, you will be granted an option to purchase shares of PhaseBio Pharmaceuticals, Inc., common stock equal to 1.5% of the company’s outstanding shares at an exercise price equal to the fair market value of the stock at the date of grant. The option with respect to shares equal to 1% of the outstanding shares will vest 25% after one year of employment, with the balance vesting monthly over the subsequent three (3) years, so long as you continue to be employed by PhaseBio. In the event of a Liquidation Event, the option will become fully vested if the acquiring company does not offer you a comparable position (no diminution of duties, the same or greater base salary, and at the same geographic location), or if the acquiring company terminates your employment without cause, or you terminate for “good reason” (reduction in base salary, diminution of duties or geographic relocation), within one year after the Liquidation Event. The option with respect to shares equal to 0.5% of the outstanding shares will vest upon consummation of a Liquidation Event with a Net Present Value of at least $350 million, so long as you continue to be employed by PhaseBio. For purposes of determining Net Present Value, we will use a 15% cost of capital and risk adjustment factors of 90% for entry into a Phase 3 trial, 75% for an NDA, 65% for all commercial milestones, and 50% on any residual royalties based on the lower of: i) the Company’s forecast or ii) a forecast developed in the context of a fairness opinion by an investment bank.

In lieu of the option for shares equal to 1 % of the outstanding shares, you may specify upfront a number of shares that you wish to purchase as restricted stock, vesting as per the vesting schedule specified above. The

You will receive a severance payment equal to six months’ base salary (a) in the event of a Liquidation Event, if the acquiring company does not offer you a comparable position (no diminution of duties, the same or greater base salary, and at the same geographic location), or if the acquiring company terminates your employment without cause, or you terminate for “good reason” (reduction in base salary, diminution of duties or geographic relocation), within one year after the Liquidation Event, or (b) in the event your employment is terminated without cause at any time. The severance pay will be subject to your execution of a release of claims and will be paid in a lump sum payment 30 days after your termination date. You must provide written notice of termination for good reason within 30 days after the event constituting good reason, the acquiring company will have 30 days to cure the good reason event, and if it does not cure the event, you termination date will occur within five days after the end of the cure period.

Health, prescription and vision Insurance with Independence Blue Cross (as approved by the carrier), effective on the date of employment; employees currently contribute 20% for employee coverage, and 20% for coverage for eligible dependents on a pre-tax basis

Dental insurance; currently the policy for the employee only is paid in its entirety by the Company, and the employee currently pays 20% for coverage for eligible dependents

Long-term disability and life insurance (effective 30 days after date of employment); currently this is paid entirely by the Company (employee only)

Eligibility to participate in the PhaseBio 401 (k) Plan

Twenty (20) days paid vacation (prorated for employment of less than 1 year), two (2) personal days and nine (9) holidays per calendar year

Sick leave of up to forty (40) hours/five (5) days per calendar year, (prorated for employment of less than 1 year)