NEWTON, MA – Dec. 12, 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
pricing of its underwritten public offering of 916,667 shares of its common stock at a public
offering price of $12.00 per share. The gross proceeds from the offering, before deducting
underwriting discounts and commissions and estimated offering expenses payable by Acer, are
expected to be approximately $11.0 million. In addition, Acer granted the underwriters a 30-
day option to purchase up to 137,500 additional shares of common stock at the public offering
price, less the underwriting discounts and commissions. All shares in the offering will be sold by
Acer.

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. is acting as sole book-running manager of the offering. H.C.
Wainwright & Co. is acting as lead manager of the offering.

The offering is expected to close on or about December 14, 2017, subject to customary closing
conditions.

The shares of common stock described above are being 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 is being 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, when available, 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 prospectus@williamblair.com.

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 www.acertx.com.

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 underwritten public offering, the expected closing date of the offering, 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, the satisfaction of customary closing conditions related to the public
offering, 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 http://www.sec.gov.

Investor Contact:
Hans Vitzthum
LifeSci Advisors
Ph: 617-535-7743
hans@lifesciadvisors.com