NEWTON, MA – Dec. 14, 2017 – Acer Therapeutics Inc. (Nasdaq: ACER), a pharmaceutical
company focused on the acquisition, development and commercialization of therapies for
serious rare and ultra-rare diseases with critical unmet medical need, today announced the
closing of the previously announced underwritten public offering of 916,667 shares of its
common stock at a price to the public of $12.00 per share. The gross proceeds to Acer from this
offering were $11.0 million, before deducting the underwriting discount and other estimated
offering expenses. All of the shares in the offering were sold by Acer. In addition, Acer has
granted the underwriters a 30-day option to purchase up to an additional 137,500 shares of
common stock.

Acer intends to use the net proceeds from this offering to fund its research and development
efforts, to seek regulatory approval for EDSIVO™, to invest in pre-commercial activities for
EDSIVO™ and for general corporate purposes, including working capital and other general and
administrative purposes.

William Blair & Company, L.L.C. acted as sole book-running manager of the offering. H.C.
Wainwright & Co. acted as lead manager of the offering.

The shares of common stock described above were offered by Acer pursuant to its shelf
registration statement on Form S-3 previously filed and declared effective by the Securities and
Exchange Commission. The offering was made only by means of a prospectus supplement and
an accompanying prospectus. Copies of the final prospectus supplement and the accompanying
prospectus may be obtained from William Blair & Company, L.L.C., Attention: Prospectus
Department, 150 North Riverside Plaza, Chicago, IL 60606; Telephone: (800) 621-0687 or by
email at

This press release shall not constitute an offer to sell or the solicitation of an offer to buy these
securities, 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 any such state or jurisdiction.

About Acer Therapeutics
Acer, headquartered in Newton, MA, is a pharmaceutical company focused on the acquisition,
development and commercialization of therapies for patients with serious rare and ultra-rare
diseases with critical unmet medical need. Acer’s late-stage clinical pipeline includes two
candidates for severe genetic disorders for which there are few or no FDA-approved treatments:
EDSIVO™ (celiprolol) for vEDS, and ACER-001 (a fully taste-masked, immediate release
formulation of sodium phenylbutyrate) for urea cycle disorders (UCD) and Maple Syrup Urine
Disease (MSUD). There are no FDA-approved drugs for vEDS and MSUD and limited options for
UCD, which collectively impact more than 4,000 patients in the United States. Acer’s product
candidates have clinical proof-of-concept and mechanistic differentiation, and Acer intends to
seek approval for them in the U.S. by using the regulatory pathway established under section
505(b)(2) of the Federal Food, Drug, and Cosmetic Act, or FFDCA, that allows an applicant to rely
for approval at least in part on third-party data, which is expected to expedite the preparation,
submission, and potential approval of a marketing application.

For more information, visit

Forward-Looking Statements
This press release contains “forward-looking statements” that involve substantial risks and
uncertainties for purposes of the safe harbor provided by the Private Securities Litigation
Reform Act of 1995. All statements, other than statements of historical facts, included in this
press release regarding strategy, future operations, future financial position, future revenue,
projected expenses, prospects, plans and objectives of management are forward-looking
statements. Examples of such statements include, but are not limited to, statements relating to
the expected use of the net proceeds from the offering, and the development and future
potential of Acer’s product candidates. Acer may not actually achieve the plans, carry out the
intentions or meet the expectations or projections disclosed in the forward-looking statements
and you should not place undue reliance on these forward-looking statements. Such
statements are based on management’s current expectations and involve risks and
uncertainties. Actual results and performance could differ materially from those projected in
the forward-looking statements as a result of many factors, including, without limitation, risks
and uncertainties associated with market conditions, unexpected cash requirements, changes
in Acer’s business plan, and the process of developing, obtaining regulatory approval for and
commercializing drug candidates that are safe and effective for use as human therapeutics.
Acer disclaims any intent or obligation to update these forward-looking statements to reflect
events or circumstances that exist after the date on which they were made. You should review
additional disclosures we make in our filings with the Securities and Exchange Commission,
including our Quarterly Reports on Form 10-Q and Annual Report on Form 10-K and the
Registration Statement on Form S-3 (File No. 333-208314) for the public offering. You may
access these documents for no charge at

Investor Contact:
Hans Vitzthum
LifeSci Advisors
Ph: 617-535-7743