Zhu, Viola W.

Background: Rearranged during transfection ( RET ) alterations are targetable oncogenic drivers in thyroid cancer. Primary data from the open-label, phase 1/2 ARROW study demonstrated clinical activity and manageable safety with pralsetinib, a selective RET inhibitor, in patients with advanced/metastatic RET -altered thyroid cancer. We present an updated analysis with more patients and longer follow-up. Methods: Adult patients with advanced/metastatic RET -mutant medullary thyroid cancer (MTC) or RET fusion-positive thyroid cancer who initiated oral pralsetinib at 400 mg once daily were included. Primary endpoints were overall response rate (ORR) by blinded independent central review (per RECIST v1.1) and safety. Secondary endpoints included duration of response (DoR), progression-free survival (PFS), and overall survival. Responses were assessed in three cohorts of patients with baseline measurable disease: patients with RET -mutant MTC who had received prior cabozantinib and/or vandetanib (C/V), treatment-naïve patients with RET -mutant MTC, and patients with previously treated RET fusion-positive thyroid cancer. Patient-reported outcomes (PROs) were an exploratory endpoint. Results: As of October 18, 2021, the measurable disease population comprised of 61 patients with RET -mutant MTC and prior C/V, 62 treatment-naïve patients with RET -mutant MTC, and 22 patients with RET fusion-positive thyroid cancer who had received prior systemic therapy, including radioactive iodine. The ORR was 55.7% [confidence interval; 95% CI: 42.4–68.5] in patients with RET -mutant MTC and prior C/V, 77.4% [95% CI: 65.0–87.1] in treatment-naïve patients with RET -mutant MTC, and 90.9% [95% CI: 70.8–98.9] in patients with previously treated RET fusion-positive thyroid cancer. Median DoR and median PFS were both 25.8 months in patients with RET -mutant MTC and prior C/V, not reached in treatment-naïve patients with RET -mutant MTC, and 23.6 and 25.4 months, respectively, in patients with previously treated RET fusion-positive thyroid cancer. In the RET -altered thyroid cancer safety population ( N = 175), 97.1% of patients reported a treatment-related adverse event (TRAE); these led to discontinuation in 5.7% and dose reduction in 52.6% of patients. There was one death (0.6%) due to a TRAE. PROs improved or remained stable after pralsetinib treatment. Conclusions: In this updated analysis of the ARROW study, pralsetinib continued to show deep and durable clinical activity and a manageable safety profile in patients with advanced/metastatic RET- altered thyroid cancer. Clinical Trial Registration: NCT03037385

6080 Background: Alterations in RET are targetable oncogenic drivers in TC. Pralsetinib is a highly potent, selective RET inhibitor, with demonstrated efficacy in pts with RET-altered TC. In a previous analysis of the Phase I/II ARROW trial (NCT03037385; data cutoff: 22 May 2020), overall response rates [ORR; measurable-disease population] with pralsetinib at 400 mg once daily (QD) were 60% (33/55) and 71% (15/21) in pts with RET-mutant medullary TC ( RET-mutant MTC) who had received prior multikinase inhibitors cabozantinib and/or vandetanib (C/V), and those who were treatment naïve, respectively, and 89% (8/9) in pts with previously treated RET fusion-positive TC ( RET-fp TC). Here we report an updated analysis of these cohorts in the intention-to-treat (ITT) population. Methods: Adult pts with RET-altered locally advanced/metastatic TC who had enrolled in ARROW and initiated pralsetinib at 400 mg QD, were included. Phase II primary endpoints: ORR (by blinded independent central review, per RECIST v1.1) and safety. Efficacy endpoints for this analysis were assessed in the ITT population. Safety was assessed in all pts with RET-altered TC who initiated pralsetinib at 400 mg QD. Enrolment cutoff: 23 August 2020 for the ITT population; data cutoff: 12 April 2021. Results: The ITT population comprised 145 pts with RET-mutant MTC (with or without prior systemic therapy, including C/V) and 22 pts with RET-fp TC, 21 of whom had received prior systemic therapy (including multikinase inhibitor[s] and/or radioactive iodine). In pts with RET-mutant MTC who had received prior C/V (n = 67), ORR was 51% (34/67; 95% CI 38–63; 2 complete responses [CR]; 32 partial responses [PR]), median duration of response (DoR) was 25.8 months (95% CI 18.0–not reached [NR]) and median progression-free survival (PFS) was 24.9 months (95% CI 19.7–31.2). In treatment-naïve pts with RET-mutant MTC, ORR was 72% (48/67; 95% CI 59–82; 4 CR; 44 PR), and median DoR and median PFS were not reached. In pts with previously treated RET-fp TC, ORR was 86% (18/21; 95% CI 64–97; 3 CR; 15 PR), median DoR was 17.5 months (95% CI 16.0–NR) and median PFS was 19.4 months (95% CI 13.0–NR). Median overall survival (OS) was not reached in any of the cohorts. The safety population comprised 172 pts with RET-altered TC treated at 400 mg QD. The most common treatment-related adverse events (TRAEs) were increased aspartate aminotransferase (n = 67; 39%), anemia (n = 60; 35%), hypertension (n = 57; 33%) and decreased white blood cell count (n = 52; 30%). Serious TRAEs were reported in 27 pts (16%); the most frequent was pneumonitis (n = 5; 3%). Nine pts (5%) discontinued pralsetinib due to a TRAE and one patient ( < 1%) died due to a TRAE ( pneumocystis jirovecii pneumonia) following 44 days ( < 3 cycles) on pralsetinib. Conclusions: In this updated analysis including more pts, pralsetinib continues to be efficacious with a manageable safety profile in pts with RET-altered TC. Clinical trial information: NCT03037385.

467 Background: Recent tumor-agnostic drug approvals have resulted in a paradigm shift in cancer treatment away from organ/histology specific indications to biomarker-guided tumor-agnostic approaches. Pralsetinib is a potent and selective RET inhibitor, which has recently been approved by the U.S. Food and Drug Administration (FDA) for the treatment of adults with metastatic RET fusion–positive non-small cell lung cancer (NSCLC) and is under New Drug Application review for RET mutant thyroid cancers by the FDA. RET fusions occur in up to approximately 7‒8% of patients with gastrointestinal malignancies, including pancreatic, liver, and colorectal cancers. There are currently no approved selective RET inhibitors for patients with RET fusion–positive solid tumors other than NSCLC and thyroid cancer. Here, we present data on the clinical activity of pralsetinib in patients with RET fusion–positive solid tumor types other than NSCLC enrolled in the Phase I/II ARROW study (NCT03037385). Methods: ARROW consists of a phase I dose escalation (30–600 mg once [QD] or twice daily) followed by a phase II expansion (400 mg QD) in patients with advanced RET-altered solid tumors. Primary objectives are overall response rate (ORR), per RECICT v1.1 and safety. Results: A total of 13 patients with RET fusion–positive thyroid cancer (12 papillary, 1 poorly differentiated; enrollment cutoff July 11, 2019) and 14 patients with RET fusion–positive solid tumors other than NSCLC and thyroid (3 pancreatic, 3 colon, 2 cholangiocarcinoma, 6 other; enrollment cutoff November 19, 2019) were enrolled in ARROW and received pralsetinib. At the February 13, 2020, data cutoff, the ORR (blinded central review) in response-evaluable patients with RET fusion–positive thyroid cancer was 91% (10/11; 95% CI: 59‒100) and disease control rate was 100% (95% CI: 72‒100). Treatment was ongoing in 7 of 11 patients. In RET fusion–positive solid tumors other than NSCLC and thyroid, ORR (investigator’s assessment) was 50% (6/12; 95% CI: 21‒79) and responses were observed in all patients with pancreatic cancer (3/3) and cholangiocarcinoma (2/2). Treatment was ongoing in 6 of 12 patients, including 2 of 3 patients with pancreatic cancer and 1 of 2 patients with cholangiocarcinoma. Responses were observed across multiple fusion genotypes. In the 27 patients with RET fusion–positive tumors other than NSCLC, most frequent treatment-related adverse events (TRAEs) were grade 1–2, and included anemia (33%), increased aspartate aminotransferase (33%), decreased white blood cell count (33%), hypertension (30%), increased alanine aminotransferase (26%), hyperphosphatemia (19%), and neutropenia (19%). No patients discontinued due to TRAEs. Conclusions: Pralsetinib demonstrated broad and durable antitumor activity across multiple advanced solid tumor types, regardless of RET fusion genotype, and was well tolerated. The study is ongoing. Clinical trial information: NCT03037385.

109 Background: RET gene fusions are targetable oncogenic drivers in multiple tumor types, including up to 20% of papillary thyroid cancers (PTC). Pralsetinib is an investigational, highly potent, selective inhibitor of oncogenic RET alterations. In the registration-enabling Phase 1/2 ARROW study (NCT03037385), pralsetinib demonstrated an overall response rate (ORR; response-evaluable patients [REP], central review) of 73% (19/26) in treatment-naïve patients and 61% (49/80; 2 pending confirmation) in platinum-exposed patients with RET fusion+ non-small cell lung cancer (NSCLC) and was well tolerated (data cut-off November 18, 2019). We provide an update on the clinical activity of pralsetinib in other RET fusion+ solid tumor types. Methods: ARROW consists of a phase 1 dose escalation (30–600 mg once [QD] or twice daily) followed by a phase 2 expansion (400 mg QD) in patients with advanced RET-altered solid tumors. Primary objectives were ORR and safety. Results: As of November 18, 2019, 29 patients with metastatic solid tumor types other than NSCLC (16 PTC, 1 undifferentiated thyroid, 3 pancreatic, 3 colon, 6 other) bearing a RET fusion have received pralsetinib. Efficacy data are presented for REP enrolled by July 11, 2019. In patients with thyroid cancer that is RET fusion+, ORR (investigator assessment) was 75% (9/12; all confirmed). Median (range) duration of response (DOR) was 14.5 (3.7+, 16.8) months (mo), with 67% of responding patients continuing treatment. Two patients with stable disease were continuing treatment at 11.5+ and 19.3+ mo. In other RET fusion+ cancers, ORR was 60% (3/5; all confirmed) with partial responses in 2/2 patients with pancreatic cancer (DOR 5.5, 7.4+ mo) and 1 patient with intrahepatic bile duct carcinoma (DOR 7.5 mo). Two patients with colon cancer had stable disease for 7.3 and 9.3 mo. Responses were observed across multiple fusion genotypes. In the entire safety population (all patients treated with 400 mg QD pralsetinib, regardless of diagnosis; n = 354), most treatment-related adverse events (TRAEs) were grade 1-2, and included increased aspartate aminotransferase (31%), anemia (22%), increased alanine aminotransferase (21%), constipation (21%) and hypertension (20%). Only 4% of patients in the safety population discontinued due to TRAEs. Conclusions: Pralsetinib demonstrated broad and durable antitumor activity across multiple advanced solid tumor types, regardless of RET fusion genotype, and was well tolerated. The study is ongoing and still enrolling patients in this cohort. Clinical trial information: NCT03037385.