PhaseRx Announces Closing of Initial Public Offering

May 23, 2016

SEATTLE, WA – PhaseRx, Inc. (NASDAQ:PZRX), a biotechnology company
focused on developing mRNA therapeutics to treat orphan liver diseases, today
announced the closing of its initial public offering of 3,700,000 shares of common stock
at a public offering price of $5.00 per share, resulting in aggregate gross proceeds of
$18.5 million, before underwriting discounts and commissions and offering related
expenses. In addition, PhaseRx has granted the underwriters a 30-day option to
purchase up to an additional 555,000 shares of common stock at the initial public
offering price to cover over-allotments, if any. The company’s common stock began
trading on the NASDAQ Capital Market on May 18, 2016 under the symbol “PZRX.”

Laidlaw & Company (UK) Ltd. and Roth Capital Partners are acting as joint book-running
managers for the offering.

A registration statement relating to these securities was declared effective by the
Securities and Exchange Commission on May 17, 2016. The offering will be made only
by means of a prospectus. A copy of the final prospectus related to the offering may be
obtained from the offices of Laidlaw & Company (UK) Ltd., Attention: Syndicate
Department, 546 Fifth Avenue, New York, NY 10036, by telephone at (212) 953-4900 or
by email at syndicate@laidlawltd.com or from Roth Capital Partners, LLC, 888 San
Clemente Drive, Newport Beach, CA 92660, Attn: Equity Capital Markets, by phone at
(800) 678-9147 or by email at rothecm@roth.com.

This press release shall not constitute an offer to sell or the solicitation of an offer to buy, nor shall there be any sale of these securities in any state or jurisdiction in which such offer, solicitation or sale would be unlawful prior to registration or qualification under the securities laws of such state or jurisdiction.

ABOUT PHASERX

PhaseRx is a preclinical biopharmaceutical company dedicated to developing products for the treatment of inherited enzyme deficiencies in the liver using intracellular enzyme replacement therapy (i-ERT). The Company’s i-ERT approach is enabled by its proprietary Hybrid mRNA TechnologyTM platform. PhaseRx is headquartered in Seattle, Wash.

CONTACTS:
Media Contact
Julie Rathbun
Rathbun Communications
julie@rathbuncomm.com
206.769.9219

Investor Contact:
Robert H. Uhl
Westwicke Partners, LLC
Managing Director
robert.uhl@westwicke.com
(858) 356-5932