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Although papillary thyroid carcinoma (PTC) is the most frequent endocrine tumor with a generally excellent prognosis, a patient developed a clinically aggressive PTC eleven years after receiving platinum chemotherapy for ovarian endometrioid adenocarcinoma. Germline and somatic analyses of multi-temporal and multi-regional molecular profiles indicated that ovarian and thyroid tumors did not share common genetic alterations. PTC tumors had driver events associated with aggressive PTC behavior, an RBPMS-NTRK3 fusion and a TERT promoter mutation. Spatial and temporal genomic heterogeneity analysis indicated a close link between anatomical locations and molecular patterns of PTC. Mutational signature analyses demonstrated a molecular footprint of platinum exposure, and that aggressive molecular drivers of PTC were linked to prior platinum-associated mutagenesis. This case provides a direct association between platinum chemotherapy exposure and secondary solid tumor evolution, in specific aggressive thyroid carcinoma, and suggests that uniform clinical assessments for secondary PTC after platinum chemotherapy may warrant further evaluation.

6513 Background: Treatment options for aggressive TC are limited. Pre-clinical data suggests efficacy of CTLA-4 plus PD-1 blockade in aggressive RAIR TC. Methods: This investigator initiated phase II study tested N (3mg/kg every 2 weeks) plus I (1mg/kg every 6 weeks) until disease progression or completion of 24 mo of treatment in RAIR differentiated TC including poorly differentiated TC (PDTC) with exploratory cohorts in anaplastic (ATC) and medullary TC (MTC). Radiographic response rate by RECIST v1.1 (CR+PR) was primary endpoint. At least 6 pts with disease response among n=32 DTC provided 84% power to distinguish between a 10% and a 25% RR (one-sided 9% binomial test). Results: Accrual is complete with n=32 patients with DTC, 10 with ATC and 7 with MTC enrolled between October 2017 and May 2019. Thirty-two DTC included: n=17 papillary, n=7 Hurthle, n=4 follicular TC, n=4 PDTC. Among n=49, median (range) age was 65 (30-88), 51% (25/49) were female. To date, in DTC, 3/32 achieved a PR (n=2 Hurthle and n=1 PDTC), 9.4% RR (.95CI:2%-25%). One near complete response has been observed. Among pts w ATC, 3/ 10 profound PR by RECIST occurred (30% RR, .95CI: 7%-65%). Among them, two remain without clear evidence of disease at 26 and 13 mo after treatment start. No PR's were observed in MTC. Most frequent grade 3-4 TRAEs were as expected and included increased lipase (n=8), increased serum amylase (n=4). There was an unexpected number of treatment related adrenal insufficiency (AI) (n=4) which was associated with long PFS (range 10.1—16.4+mo). Conclusions: N+I appears to have considerable activity in ATC. In unselected RAIR DTC, activity was low but responses were seen in PDTC and Hurthle cell TC. Exceptional responses with prolonged remissions were observed. Clinical trial information: NCT03246958 .