$ONTX Two Presentations at 2018 American Chemical Society

• Product stability studies for rigosertib, a Phase 3 clinical stage product in development for Myelodysplastic Syndromes (MDS)
• Discovery of a novel kinase inhibitor, ON 150030, a Type 1 inhibitor of FLT3 and Src for relapsed and refractory Acute Myeloid Leukemia (AML)

NEWTOWN, Pa., Feb. 20, 2018 — Onconova Therapeutics, Inc. (NASDAQ:ONTX), a Phase 3 clinical-stage biopharmaceutical company focused on discovering and developing novel products to treat cancer, today announced that the Company’s scientists, and collaborators from the Mount Sinai School of Medicine, will present two abstracts at the American Chemical Society National Meeting & Expo. The meeting is being held in New Orleans, LA, on March 18-22, 2018.

The first abstract relates to the analysis of stability of the clinical product during storage and describes the synthesis and characterization of a de-sulfonylated product of rigosertib, Onconova’s lead phase 3 stage clinical candidate. The second abstract describes a new chemical entity ON 150030, which is a type 1 novel pre-clinical stage inhibitor of FLT3 and Src pathways, believed to be important for targeted therapy of relapsed and refractory AML.

Details for the poster presentations are listed below.

Title: Synthesis and characterization of de-sulfonylated product of rigosertib, a late phase III clinical candidate

Date: 18^th March, 2018

Presentation Time: 7.00-9.00 PM CDT

Presenter: Dr. M.V. Ramana Reddy & Dr. Muralidhar Mallireddigari

Location: Ernest N. Morial Convention Center, Hall E.

Title: Discovery of a novel kinase inhibitor ON 150030, a type 1 inhibitor for relapsed and refractory AML

Date: 18^th March, 2018

Presentation Time: 7.00-9.00 PM CDT

Presenter: Dr. M.V. Ramana Reddy

Location: Ernest N. Morial Convention Center, Hall E.

About Onconova Therapeutics, Inc.

Onconova Therapeutics, Inc. is a Phase 3-stage biopharmaceutical company focused on discovering and developing novel small molecule drug candidates to treat cancer, with a primary focus on Myelodysplastic Syndromes (MDS).  Rigosertib, Onconova’s lead candidate, is a proprietary Phase 3 small molecule agent, which we believe blocks cellular signaling by targeting RAS effector pathways.  Using a proprietary chemistry platform, Onconova has created a pipeline of targeted agents designed to work against specific cellular pathways that are important in cancer cells.  Onconova has three product candidates in the clinical stage and several pre-clinical programs.  Advanced clinical trials with the Company’s lead compound, rigosertib, are aimed at what the Company believes are unmet medical needs of patients with MDS.  For more information, please visit http://www.onconova.com.

About IV Rigosertib

The intravenous form of rigosertib has been employed in Phase 1, 2, and 3 clinical trials involving more than 800 patients, and is currently being evaluated in a randomized Phase 3 international INSPIRE trial for patients with higher-risk MDS, after failure of hypomethylating agent, or HMA, therapy.

About INSPIRE

The INternational Study of Phase III IV RigosErtib, or INSPIRE, was finalized following guidance received from the  U.S. Food and Drug Administration and European Medicines Agency and derives from the findings of the ONTIME Phase 3 trial.  INSPIRE is a multi-center, randomized controlled study to assess the efficacy and safety of IV rigosertib in HR-MDS patients who had progressed on, failed to respond to, or relapsed after previous treatment with an HMA within the first 9 months or nine cycles over the course of one year after initiation of HMA treatment.  This time frame optimizes the opportunity to respond to treatment with an HMA prior to declaring treatment failure, as per NCCN Guidelines.  Following interim analysis in early 2018, the independent Data Monitoring Committee recommended that the trial continue with an expansion in enrollment to 360 patients based on a pre-planned sample size re-estimation.  Patients are randomized at a 2:1 ratio into two treatment arms: IV rigosertib plus Best Supportive Care versus Physician’s Choice plus Best Supportive Care.  The primary endpoint of INSPIRE is overall survival. Full details of the INSPIRE trial, such as inclusion and exclusion criteria, as well as secondary endpoints, can be found on clinicaltrials.gov (NCT02562443).

About Oral Rigosertib

The oral form of rigosertib was developed to provide more convenient dosing for use where the duration of treatment may extend to multiple years. This dosage form may also support many combination therapy modalities.  To date, 368 patients have been treated with the oral formulation of rigosertib.  Initial studies with single-agent oral rigosertib were conducted in hematological malignancies, lower-risk MDS, and solid tumors.  Combination therapy of oral rigosertib with azacitidine and chemoradiotherapy has also been explored.  Currently, oral rigosertib is being developed as a combination therapy together with azacitidine for patients with higher-risk MDS who require HMA therapy.  A Phase 1/2 trial of the combination therapy has been fully enrolled and the preliminary results were presented in 2016. This novel combination is the subject of an issued US patent with earliest expiration in 2028.

Forward Looking Statements

Some of the statements in this release are forward-looking statements within the meaning of Section 27A of the Securities Act of 1933, as amended, Section 21E of the Securities Exchange Act of 1934, as amended, and the Private Securities Litigation Reform Act of 1995, and involve risks and uncertainties. These statements relate to Onconova Therapeutics, Inc.’s expectations regarding the INSPIRE Trial. Although Onconova believes that the expectations reflected in such forward-looking statements are reasonable as of the date made, expectations may prove to have been materially different from the results expressed or implied by such forward-looking statements. Onconova has attempted to identify forward-looking statements by terminology including “believes,” “estimates,” “anticipates,” “expects,” “plans,” “intends,” “may,” “could,” “might,” “will,” “should,” “approximately” or other words that convey uncertainty of future events or outcomes. These statements are only predictions and involve known and unknown risks, uncertainties, and other factors, including Onconova’s ability to continue as a going concern, the need for additional financing and current plans and future needs to scale back operations if adequate financing is not obtained, the success and timing of Onconova’s clinical trials and regulatory approval of protocols, and those discussed under the heading “Risk Factors” in Onconova’s most recent Annual Report on Form 10-K and quarterly reports on Form 10-Q.

Any forward-looking statements contained in this release speak only as of its date. Onconova undertakes no obligation to update any forward-looking statements contained in this release to reflect events or circumstances occurring after its date or to reflect the occurrence of unanticipated events.

General Contact

http://www.onconova.com/contact/

Investor Relations Contact

Katja Buhrer, Affinity Growth Advisors on behalf of Onconova Therapeutics

Katja.Buhrer@affinitygrowth.com / (212) 661-7004