Purpose: Recently the Connective Tissue Oncology Society published consensus guidelines for recognizing Ultrarare Sarcomas (URS), defined as sarcomas with an incidence <1 per 1,000,000. We assessed the outcomes of 56 patients with soft tissue, and 21 with bone sarcomas, enrolled in phase 1 trials. Experimental Design: In this SArcoma Matched Biomarker Analysis (SAMBA-102 study), we reviewed records from patients on Phase 1 trials at the University of Texas MD Anderson Cancer Center between January 2013 and June 2021. Results: Among 587 sarcomas, 106 (18.1 %) were classified as URS. 50 (47%) were male, and the median age was 44.3 years (range 19-82). The most common subtypes were alveolar soft part sarcoma, chordoma, dedifferentiated chondrosarcoma, and sclerosing epithelioid fibrosarcoma. Compared to common sarcomas (CS), mOS was similar 16.1 months (95% CI: 13.6-17.5) versus 16.1 (95% CI: 8.2-24.0) in URS (p=0.359). Objective response to treatment was higher in URS 13.2% (n=14/106) compared to CS 6.9% (n=33/481) (p=0.029). Median OS for those treated on matched trials was 27.3 months (95% CI: 1.9-52.7) compared to 13.4 months (95% CI: 6.3-20.6) for those not treated on matched trials (p=0.291). 8 of 33 (24%) molecularly matched treatments resulted in an objective response, whereas 6 of 73 unmatched treatments (8.2%) resulted in an objective response (p=0.024). Clinical benefit rate was 36.4% (12/33) in matched trials versus 26.0 % (19/73) in unmatched trials (p=0.279). Conclusion: The results demonstrate the benefit of genomic selection in Phase 1 trials to help identify molecular subsets likely to benefit from targeted therapy.

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