No Relevant Activity Of Olaratumab In Patient-Derived Soft Tissue Sarcoma Xenografts Selected For Expression Of Platelet-Derived Growth Factor Receptor A.

e22556 Background: Soft tissue sarcoma (STS) comprises a heterogeneous family of rare tumors of mesenchymal origin. Single-agent doxorubicin (DOX) remains the standard-of-care for advanced and inoperable STS, but response rates are < 15% and survival is generally poor. In 2016, Tap et al. reported impressive results of a phase Ib/II trial combining DOX and the platelet-derived growth factor receptor a (PDGFRA)-directed antibody olaratumab (OLA), suggesting an unprecedented survival advantage of the combination over DOX alone, without providing a mechanistic rationale for the observed activity (Lancet 2016;388:488-97). We decided to evaluate the efficacy of OLA in a panel of patient-derived STS xenografts (PDX). Methods: NMRI nu/nu mice were transplanted bilaterally with tumor tissue of models expressing PDGFRA, including PDX of leiomyosarcoma (UZLX-STS22), malignant peripheral nerve sheath tumor (STS39), myxofibrosarcoma (STS59) and undifferentiated pleomorphic sarcoma (STS84). Mice were randomly divided into 4 treatment groups: (1) control, (2) DOX (3 mg/kg once weekly), (3) anti-PDGFRA [OLA (60 mg/kg twice weekly) + mouse anti-PDGFRA antibody 1E10 (20 mg/kg twice weekly)] and (4) the combination of DOX and anti-PDGFRA (same dose/schedule as single treatment arms). Tumor volume, histopathology and Western blotting were used to assess treatment efficacy. Results: Anti-PDGFRA treatment as single agent did not reduce tumor growth and did not result in significant anti-proliferative or pro-apoptotic activity. Combining DOX and anti-PDGFRA did not reduce tumor burden, though a mild inhibition of proliferation was observed in models STS39 and STS59 and a pro-apoptotic effect was seen in all models except STS22. Antitumor effects on histology were not significantly different comparing DOX and the combination treatment. Anti-PDGFRA treatment, both as a single agent as well as with DOX, did not inhibit downstream MAPK and PI3K/AKT signaling pathways. Conclusions: In vivo findings in PDX models selected for PDGFRA expression support negative findings of the phase III trial NCT02451943 with OLA reported at this meeting.