Flex Pharma Announces that ISS and Glass Lewis Recommend Shareholders Vote “FOR” All Agenda Items relating to Flex’s Strategic Merger with Salarius Pharmaceuticals

Flex
Pharma, Inc. (Nasdaq: FLKS) today announced that Institutional
Shareholder Services Inc. (“ISS”) and Glass Lewis recommend that
shareholders vote FOR all agenda items before or at Flex Pharma’s June
14 Special Shareholders Meeting. Flex’s Board of Directors also
unanimously recommends voting “FOR” all agenda items as the best way to
preserve future shareholder value.

ISS and Glass Lewis are leading U.S. institutional voting advisory
services. Salarius Pharmaceuticals, LLC, is a privately held
clinical-stage oncology company targeting the epigenetic causes of
cancers.

ISS stated:

• The proposed transaction with Salarius appears to offer FLKS
  shareholders a better potential outcome than FLKS’ standalone business.
• The Flex board also ran a reasonably thorough sale process before
  identifying this transaction as the best available option. The company
  contacted or was contacted by 184 parties to solicit interest and the
  company received proposals from 26 parties.
• The reverse stock split is necessary to avoid delisting of the
  company’s common stock from the Nasdaq Capital Market. As such,
  support for this proposal is warranted.
• In light of the upside potential for Salarius’ leading drug candidate,
  the reasonably thorough sale process used to evaluate alternative
  options, the positive market reaction, and the downside risk of
  rejecting this proposal and continuing with FLKS’ standalone business
  plan, support for this proposal is warranted.

ALL VOTES ARE IMPORTANT - URGENT ACTION NEEDED BEFORE JUNE 14, 2019

Votes received to date are overwhelmingly “FOR” the agenda items
relating to the merger. However, at this time, not enough votes have
been cast to allow Flex to complete the merger with Salarius
Pharmaceuticals.

All shareholders are urged to vote as soon as possible “FOR” all
agenda items by telephone, via the Internet or using the proxy
card they received with their proxy materials. For any questions, or
assistance in voting shares, or to receive additional copies of the
proxy materials, shareholders should call proxy solicitor Innisfree M&A
Inc., toll-free at 1 (888) 750-5834.

If Flex Pharma does not complete the merger, it likely
will wind-up, which could cause
shareholders to lose most of the value of their investment.

About Salarius Pharmaceuticals

Salarius Pharmaceuticals, LLC is a clinical-stage oncology company
targeting the epigenetic causes of cancers and is developing treatments
for patients that need them the most. The company’s lead candidate,
Seclidemstat, is currently in clinical development for treating Ewing
sarcoma, for which it has Orphan Drug designation and Pediatric Rare
Disease Designation by the U.S. Food and Drug Administration. Salarius
believes that Seclidemstat is one of only two reversible inhibitors of
the epigenetic modulator LSD1 currently in human trials, and that it
could have potential for improved safety and efficacy compared to other
LSD1-targeted therapies. Salarius is also developing Seclidemstat for a
number of cancers with high unmet need and expects to commence
additional clinical studies in 2019 targeting advanced solid tumors,
including prostate, breast and ovarian cancers. For more information,
please visit salariuspharma.com.

About Flex Pharma

Flex Pharma, Inc. is a biotechnology company that was founded in 2014
by National Academy of Science members Rod MacKinnon, M.D. (2003 Nobel
Laureate) and Bruce Bean, Ph.D., recognized leaders in the fields of ion
channels and neurobiology, along with Christoph Westphal, M.D., Ph.D.

Additional Information and Where to Find It.
This communication may be deemed to be solicitation material in respect
of the proposed transaction. In connection with the proposed
transaction, Flex Pharma filed with the Securities and Exchange
Commission (SEC) a registration statement on Form S-4 (File
No. 333-229666) containing a proxy statement / prospectus / information
statement, which was declared effective by the SEC on April 29, 2019. On
April 30, 2019, Flex Pharma filed a definitive proxy statement /
prospectus / information statement with the SEC. On or around May 3,
2019, Flex Pharma began mailing the definitive proxy statement /
prospectus / information statement to Flex Pharma stockholders of record
as of the close of business on April 17, 2019 and members of Salarius. FLEX
PHARMA URGES INVESTORS AND EQUITYHOLDERS OF FLEX PHARMA AND SALARIUS TO
READ THE DEFINITIVE PROXY STATEMENT / PROSPECTUS / INFORMATION STATEMENT
REGARDING THE PROPOSED TRANSACTION, AS WELL AS OTHER DOCUMENTS FILED OR
THAT WILL BE FILED WITH THE SEC, BECAUSE THEY CONTAIN OR WILL CONTAIN
IMPORTANT INFORMATION ABOUT FLEX PHARMA, SALARIUS AND THE PROPOSED
TRANSACTION. This communication is not a substitute for the
registration statement, definitive proxy statement / prospectus /
information statement or any other documents that Flex Pharma has filed
or may file with the SEC or send to Flex Pharma or Salarius
equityholders in connection with the proposed transaction.

You may obtain free copies of the registration statement, definitive
proxy statement / prospectus / information statement and all other
documents filed or that will be filed with the SEC regarding the
proposed transaction at the website maintained by the SEC, www.sec.gov.
Once they are filed, copies of the registration statement and definitive
proxy statement / prospectus / information statement will be available
free of charge on Flex Pharma’s website at www.flex-pharma.com
or by contacting John McCabe at [email protected].

Participants in Solicitation. Flex
Pharma, Salarius and their respective directors or managers and
executive officers may be deemed to be participants in the solicitation
of proxies from the holders of Flex Pharma common stock in connection
with the proposed transaction. Information about Flex Pharma’s directors
and executive officers is set forth in Flex Pharma’s Annual Report on
Form 10-K for the period ended December 31, 2018, which was filed with
the SEC on March 6, 2019, and amended on April 16, 2019. Other
information regarding the interests of such individuals, as well as
information regarding Salarius’ managers and executive officers and
other persons who may be deemed participants in the proposed
transaction, is set forth in the definitive proxy statement / prospectus
/ information statement. You may obtain free copies of these documents
as described in the preceding paragraph.

Non-Solicitation. This communication
shall not constitute an offer to sell or the solicitation of an offer to
sell or the solicitation of an offer to buy any securities, nor shall
there be any sale of securities in any jurisdiction in which such offer,
solicitation or sale would be unlawful prior to registration or
qualification under the securities laws of any such jurisdiction. No
public offer of securities in connection with the merger shall be made
except by means of a prospectus meeting the requirements of Section 10
of the Securities Act of 1933, as amended.

Forward-Looking Statements. Certain
statements in this communication constitute “forward-looking statements”
within the meaning of Section 27A of the Securities Act and Section 21E
of the Securities Exchange Act. These forward-looking statements reflect
Flex’s current views about its plans. Although Flex believes that its
plans as reflected in or suggested by those forward-looking statements
are reasonable, it can give no assurance that the plans, intentions,
expectations or strategies will be attained or achieved. Furthermore,
actual results will be affected by a variety of risks and uncertainties
that are beyond its control. Risks and uncertainties include, but are
not limited to: inability to complete the proposed transaction; the
ability of the proposed transaction to increase stockholder value; and
other risks and uncertainties detailed in the risk factors section of
Flex Pharma’s registration statement
on Form S-4 (File No. 333-229666), Form 10-K and Forms 10-Q filed with
the SEC, as well as other filings Flex Pharma makes with the SEC
from time-to-time. Many of these factors that will determine actual
results are beyond Flex Pharma’s ability to control or predict. Flex
Pharma disclaims any obligation to update information contained in these
forward-looking statements, whether as a result of new information,
future events or otherwise, except as required by law.

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