(P 130) MONITORING SOFT TISSUE SARCOMA RESPONSE TO IMMUNE CHECKPOINT INHIBITION AND CRYOTHERAPY WITH CIRCULATING TUMOR DNA ANALYSIS

Objective: Currently, there are few effective systemic therapies for metastatic soft tissue sarcoma (STS). Immune checkpoint inhibition has led to promising responses in a subset of STS, but the majority of patients do not respond to treatment. Therefore, new approaches to predict and track response to immunotherapy will be crucial to unlock the potential of immune checkpoint inhibitors in STS. In addition, combining immunotherapy with local therapies may augment systemic responses by releasing tumor antigens and triggering immune responses. Circulating tumor DNA (ctDNA) kinetics have been shown to correlate with response to immunotherapy in other cancers, but previous studies have reported low rates of ctDNA detection in STSs. We performed personalized ctDNA analysis in patients enrolled on a phase 2 clinical trial combining immune checkpoint inhibition and cryotherapy for STS to determine if ctDNA analysis can be used to identify patients who benefit from treatment.

Methods: We enrolled 30 adult patients with unresectable or metastatic STS treated with at least 1 prior systemic therapy to an open-label phase II clinical trial (NCT04118166). Patients received ipilimumab and nivolumab every 3 weeks for 4 doses followed by nivolumab every 4 weeks until progression or intolerance with cryoablation and biopsy of at least one tumor performed between cycles 1 and 2. Blood samples were collected prior to each immunotherapy administration for ctDNA analysis. Treatment response and progression were assessed radiographically by RECIST 1.1 criteria for lesions not treated with cryotherapy. We performed whole exome sequencing of tumor tissue and matched leukocytes to identify somatic single nucleotide variants and indels for each patient. Personalized hybrid capture panels were designed to track all of the mutations present within each patient’s tumor, and CAncer Personalized Profiling by deep Sequencing (CAPP-Seq) was utilized to analyze ctDNA levels over the course of treatment.

Results: In the 16 patients analyzed by CAPP-Seq to date, we tracked a median of 46 variants per patient (range 11-174). Circulating tumor DNA was detected in at least one sample for 15 of the 16 patients (94%). The patient without ctDNA detected experienced a deep partial response with no evidence of progression at 22 months, and the earliest time point available for ctDNA analysis was 2 months after starting therapy. In patients with both pre-treatment and cycle 2 plasma samples analyzed (n=9), the ctDNA concentration increased in all patients after cryotherapy from median of 87.3 haploid genome equivalents per milliliter (hGE/ml) to a median of 324.5 hGE/ml. In all patients with longitudinal samples (n=7), ctDNA subsequently decreased at least temporarily after the initial post-cryotherapy rise, suggesting cryotherapy temporarily increases ctDNA release due to tumor cell death. In the one patient with long-term stable disease, ctDNA increased after cryotherapy prior to decreasing and remaining detected at low levels 9 months after starting immunotherapy.

Conclusion: Personalized CAPP-Seq can detect circulating tumor DNA in nearly all patients with unresectable or metastatic STS, and longitudinal ctDNA changes reflect the response of STS to immunotherapy and cryotherapy. These results suggest that ctDNA analysis may allow personalization and monitoring of response to immunotherapy in STS.

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