NEWTON, MA – Dec. 11, 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 that it
intends to offer and sell shares of its common stock, subject to market and other conditions, in
an underwritten public offering. All shares being offered are to be sold by Acer. Acer intends to
grant the underwriters a 30-day option to purchase an additional 15% of the shares of common
stock offered in the public offering.

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.

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 preliminary 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 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 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 forwardlooking
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