(P 339) SARCOMAS EXPRESS EXTRA DOMAIN B OF FIBRONECTIN AND PRECLINICAL PATIENT-DERIVED XENOGRAFT MODELS OF SEVERAL SARCOMA SUBTYPES ARE SENSITIVE TO THE STROMA TARGETING ANTIBODY DRUG CONJUGATE PYX-201

Objective: Sarcomas are a mesenchymally-derived, rare, broad population of cancers rich in extracellular matrix (ECM) that affect bones and connective tissues. Sarcomas exhibit high tumor heterogeneity, complicating treatment options which can often be ineffective. Thus, the need is high to develop novel therapeutics that could be effective across different sarcoma subtypes, such as those targeting the ECM. PYX-201, an investigational drug, is an antibody drug conjugate (ADC) that targets extra domain B of fibronectin (EDB+FN), an ECM protein that is abundant in the stroma of many solid tumors. Sarcoma is an ideal disease for the ADC PYX-201 because it targets the stroma thereby differentiating it from other ADCs that target proteins expressed on the surface of cancer cells. The objective of this study was to expand the understanding of EDB+FN expression and prevalence across human sarcoma subtypes, as well as evaluate the anti-tumor activity of PYX-201 in patient-derived xenograft (PDx) preclinical mouse models of sarcoma.

Methods: An immunohistochemistry (IHC) assay was developed to assess EDB+FN protein expression in the stroma of tumor tissues and was validated in a variety of solid tumor indications including sarcoma. A scoring algorithm was developed that weights both the intensity and distribution of EDB+FN expression in the stroma compartment and in tumor cells (stroma H- and tumor H-scores). In addition to traditional pathologist-scoring, a digital pathology algorithm was developed for this assay to streamline scoring of nonclinical and PDx samples. PDx models of sarcoma were pre-selected based on different levels of EDB+FN protein expression. Mice were dosed with PYX-201 Q4Dx4 at 3 mg/kg (treated group) or saline buffer (vehicle group). PYX-201 anti-tumor activity was determined by calculation of Tumor Growth Inhibition (TGI) at study end comparing the percentage change in mean tumor volume in the treated group divided by mean tumor volume in the vehicle group.

Results: The EDB+FN IHC assay specifically detected EDB+FN protein expression across a broad dynamic range of intensity in 10 solid tumor indications with little to no expression in normal tissues. The tumor sections evaluated in the sarcoma validation set included leiomyosarcoma, myxofibrosarcoma, liposarcoma, carcinosarcoma, osteosarcoma, rhabdosarcoma and synovial sarcoma and the median stromal EDB+FN H score was 35 (range 0 – 200, n=20). Sarcoma subtypes were further evaluated using tissue microarrays and a digital H score (dH) method, revealing a broad range of expression across subtypes. Sarcoma PDx models including 9 different subtypes were evaluated for antitumor activity with PYX-201 and 61% of the models (11 of 18) showed strong tumor growth inhibition (TGI>70%) with treatment. Ewing sarcoma was the least sensitive while osteosarcoma, myxofibrosarcoma and rhabdomyosarcoma were the most sensitive. Notably, PYX-201 induced tumor regression across different sarcoma subtypes.

Conclusion: EDB+FN is upregulated in sarcoma with a wide range of expression intensity within and across subtypes. EDB+FN is thus an intriguing target for drug delivery across sarcoma tumor subtypes. PYX-201 is well tolerated in PDx models of sarcoma with strong anti-tumor activity seen across a variety of subtypes. The potential for PYX-201 to demonstrate clinical benefit for patients with sarcoma and other solid tumors is currently being evaluated in a global phase 1 clinical trial (NCT05720117) with sites in the US, Spain and Belgium.