Onconova Therapeutics, Inc. announced that the second of two evaluable participants in an investigator-initiated Phase 2 program evaluating rigosertib monotherapy in advanced squamous cell carcinoma complicating recessive dystrophic epidermolysis bullosa (RDEB-associated SCC) achieved a complete response of all cancerous skin lesions following 4 treatment cycles and remains on oral rigosertib. The Company previously announced that the RDEB-associated SCC program's first evaluable participant achieved a RECIST-defined complete response. Both of the program's evaluable participants remain on therapy, with the first participant in complete remission with no signs of metastatic disease for more than 18 months.

Rigosertib continues to demonstrate a favorable safety profile in this indication that is similar to that displayed in prior studies in other indications. RDEB is caused by insufficient expression of type VII collagen protein, which is responsible for anchoring the skin's inner layer to its outer layer. This leads to extreme skin fragility as well as chronic blistering and wound formation with recurrent infections in RDEB patients, many of whom go on to develop metastatic squamous cell carcinoma driven by over expression of polo like kinase 1 (PLK1).

RDEB-associated SCC tumors show a highly aggressive and early metastasizing course that makes them the primary cause of death for these patients, with a cumulative risk of death of 70% and 78.7% by ages 45 and 55, respectively. RDEB-associated SCC can appear in pediatric patients or in young adults. Currently available treatments such as targeted therapies and conventional chemo- and/or radiotherapy have demonstrated limited response rates and poor durability in RDEB-associated SCC.

Onconova and a program investigator plan to present more detailed data on the first two evaluable participants from the Phase 2 RDEB-associated SCC program at a future medical meeting. In addition, preclinical data on rigosertib's mechanism of action have been accepted for presentation at the American Association for Cancer Research (AACR) Targeting RAS Conference, which is taking place in Philadelphia, Pennsylvania from March 5 – 8, 2023.