Curigliano, Giuseppe

PURPOSE Long-term outcomes of patients with stage I human epidermal growth factor receptor 2 (HER2)–positive breast cancer receiving adjuvant trastuzumab emtansine (T-DM1) remain undefined, and prognostic predictors represent an unmet need. METHODS In the ATEMPT phase II trial, patients with stage I centrally confirmed HER2-positive breast cancer were randomly assigned 3:1 to adjuvant T-DM1 for 1 year or paclitaxel plus trastuzumab (TH). Coprimary objectives were to compare the incidence of clinically relevant toxicities between arms and to evaluate invasive disease-free survival (iDFS) with T-DM1. Correlative analyses included the HER2DX genomic tool, multiomic evaluations of HER2 heterogeneity, and predictors of thrombocytopenia. RESULTS After a median follow-up of 5.8 years, 11 iDFS events were observed in the T-DM1 arm, consistent with a 5-year iDFS of 97.0% (95% CI, 95.2 to 98.7). At 5 years, the recurrence-free interval (RFI) was 98.3% (95% CI, 97.0 to 99.7), the overall survival was 97.8% (95% CI, 96.3 to 99.3), and the breast cancer-specific survival was 99.4% (95% CI, 98.6 to 100). Comparable iDFS was observed with T-DM1 irrespective of tumor size, hormone receptor status, centrally determined HER2 immunohistochemical score, and receipt of T-DM1 for more or less than 6 months. Although ATEMPT was not powered for this end point, the 5-year iDFS in the TH arm was 91.1%. Among patients with sufficient tissue for HER2DX testing (n = 187), 5-year outcomes significantly differed according to HER2DX risk score, with better RFI (98.1% v 81.8%, hazard ratio [HR], 0.10, P = .01) and iDFS (96.3% v 81.8%, HR, 0.20, P = .047) among patients with HER2DX low-risk versus high-risk tumors, respectively. CONCLUSION Adjuvant T-DM1 for 1 year leads to outstanding long-term outcomes for patients with stage I HER2-positive breast cancer. A high HER2DX risk score predicted a higher risk of recurrence in ATEMPT.

TPS1127 Background: Patients with estrogen receptor-positive (ER+) metastatic breast cancer (mBC) may develop resistance to endocrine therapy (ET), particularly following treatment with a cyclin-dependent kinase 4/6 inhibitor (CDK4/6i), potentially driven by a mutation in the ERa-coding gene, ESR1. Lasofoxifene (LAS), an oral, next-generation ET and ER breast antagonist, was evaluated in two phase 2 studies of women with ER+/HER2- mBC and an ESR1 mutation who had disease progression on previous ET and CDK4/6i. The ELAINE 1 trial showed numerically greater progression-free survival (PFS, median ~6 vs 4 mos; P=0.138), objective response rate (ORR, 13% vs 3%; P=0.124), and clinical benefit rate (CBR, 37% vs 22%; P=0.117) with LAS monotherapy versus the ER degrader fulvestrant (fulv), with a favorable safety profile (1). The single-arm, ELAINE 2 trial demonstrated that LAS plus abemaciclib (Abema) was well tolerated with a median PFS of ~13 mos, ORR of 56%, and CBR of 66% (2). Based on these data, the phase 3, registrational, ELAINE 3 trial was initiated. Methods: ELAINE 3 (NCT05696626) is an open-label, phase 3, multicenter study evaluating the efficacy, safety, and tolerability of LAS plus Abema versus fulv plus Abema. Key inclusion criteria are pre- and postmenopausal women and men aged ≥18 yrs; ER+/HER2-, locally advanced and/or mBC (measurable and/or non-measurable disease); ≥1 acquired ESR1 mutation; progression on an aromatase inhibitor plus palbociclib or ribociclib as their first hormonal treatment for advanced/mBC; and ≤1 line of chemotherapy in the advanced/metastatic setting. Patients will be randomized 1:1 to receive LAS 5 mg/day plus Abema 150 mg BID, or fulv 500 mg IM on days 1, 15, and 29, then once monthly plus Abema 150 mg BID. Treatment will continue until progression, death, unacceptable toxicity, or withdrawal from the study. The primary endpoint is PFS; key secondary endpoints are ORR, overall survival, CBR, duration of response, and time to response. Time to cytotoxic chemotherapy, quality of life, and safety will also be evaluated. Blood samples for circulating tumor DNA (ctDNA) will be collected for genomic analyses at screening, at weeks 4 and 8 and every 8 weeks thereafter, and at the final visit. Outcomes with LAS/Abema and fulv/Abema will be compared using a stratified Cox proportional hazards model and stratified logrank test. Expected PFS is ≥10.3 mos for LAS/Abema and 7 mos for fulv/Abema (PFS hazard ratio of 0.68 at final analysis). The target sample size is 400, to achieve 90% power with a one-sided type I error rate of 0.025 after 285 PFS events. Full recruitment is expected to occur over 18 mos at approximately 125 global sites. 1. Goetz MP, et al. Ann Oncol. 2023;34:1141-1151. 2. Damodaran S, et al. Ann Oncol. 2023;34:1131-1140. Clinical trial information: NCT05696626 .

1105 Background: Approximately 2-5% of breast cancers harbor HER2 mutations, often occurring in hormone receptor-positive (HR+) disease. There are currently no approved treatment options for patients (pts) with HER2-mutated metastatic breast cancer (HER2-mut MBC). Tucatinib (TUC) is a tyrosine kinase inhibitor highly selective for HER2 and is approved for previously treated HER2+ MBC with or without brain metastases in combination with trastuzumab (Tras) and capecitabine. Here, we report the efficacy and safety results of TUC combined with Tras in pts with previously treated HER2-mut MBC. Methods: SGNTUC-019 (NCT04579380) is an open-label phase 2 basket study evaluating efficacy, safety, and tolerability of TUC and Tras in pts with metastatic HER2-altered solid tumors. Pts in the HER2-mut MBC cohort must have HER2 mutations determined locally by tissue- or blood-based next-generation sequencing, not be HER2+ per local testing, been previously treated with ≥1 systemic therapy in the locally advanced, unresectable, or metastatic setting, and if HR+, have received a CDK4/6 inhibitor. Pts were treated in 21-day cycles with TUC (300 mg orally twice a day) and Tras (8 mg/kg IV followed by 6 mg/kg every 3 wks). Pts with HR+ disease also received fulvestrant (F; 500 mg IM once every 4 wks starting from cycle 1 day 1, and cycle 1 day 15). Disease status was determined based on RECIST v1.1 with assessments performed every 6 wks for 24 wks and every 12 wks thereafter. The primary endpoint is cORR per investigator assessment. Secondary endpoints include OS, DCR, DOR, PFS, and safety. Results: As of Nov 1, 2023, 31 pts were enrolled in the HER2-mut MBC cohort. The median duration of follow-up for OS was 15.0 months. Twenty-seven (87%) pts had HR+ disease and 18 (58%) had lobular histology. The pts received a median of 4 prior lines of systemic therapy in any setting. cORR was 41.9% (90% CI, 26.9-58.2) with 13 responses including 2 complete responses. Median DOR was 12.6 months (90% CI, 4.7, not estimable), DCR was 80.6% (n=25; 90% CI, 65.3-91.2), median PFS was 9.5 months (90% CI, 5.4-13.8), and median OS was 20.1 months (90% CI, 15.9 to not estimable). The most common treatment-emergent adverse events (TEAEs) reported were diarrhea (64.5%) and nausea (35.5%). The most common grade ≥3 TEAEs reported were diarrhea (13%), alanine aminotransferase increase (10%), and hypertension (10%). Two (6.5%) pts discontinued TUC due to TEAEs, but these pts continued Tras. No deaths were due to TEAEs. Additional biomarker analyses will be included in the presentation. Conclusions: The investigational combination of TUC and Tras, plus F in pts with HR+ disease, was well tolerated and had clinical activity in pts with previously treated HER2-mut MBC. The results support further evaluation of the combination of TUC and Tras as a potential treatment option for this pt population. Clinical trial information: NCT04579380 .

Abstract Background: Breast cancer (BC) in males represents a rare clinical entity, accounting about 1% of all breast malignancies diagnosed every year. Emerging evidence suggest that a gender medicine approach is critical in the research and treatment of cancer, based on the possibility that sex hormones, molecular differences, immune system, and other factors might play a crucial role in disease management. Here, we characterized molecular and immune differences between male BC (MaBC) and female BC (FeBC) across BC subtypes. Methods: 10,728 BC samples (male, n=137; female, n=10591) were analysed by next-generation sequencing (592, NextSeq; WES, NovaSeq), Whole Transcriptome Sequencing (WTS; NovaSeq) (Caris Life Sciences, Phoenix, AZ). Tumor mutational burden (TMB) totaled somatic mutations per tumor (high ≥10 mt/MB). Microsatellite-instability (MSI) was tested by IHC and NGS. Immune cell fractions were calculated by deconvolution of WTS: Quantiseq. Real world treatment-associated survival was extracted from insurance claims and calculated from sample collection to last contact using Kaplan-Meier estimates. Statistical significance was determined using chi-square and Mann-Whitney U test with p-values adjusted for multiple comparisons (q < 0.05). Results: MaBC represented higher HR+/HER2+ (6.61% vs 4.76%), HR+/HER2- (80.17% vs 60.66%) and lower HR-/HER2+ (1.65% vs 3.5%), TNBC (11.57% vs 31.08%) molecular subtypes compared to FeBC. Compared to HR+/HER2+ FeBC, MaBC had higher frequency of CHEK2, RAD51B, TSC1 mutation (Table 1) and RARα (100% vs 13.61%), DAXX (10% vs 0.16%) copy number alteration (all p < 0.05). HR+/HER2- MaBC had higher frequency of BRCA2, FOXA1, CREBBP, FLCN mutation (Table 1), and FGF3 (21.62% vs 14.31%), MRE11 (1.75% vs 0.23%), MEN1 (1.75% vs 0.13%) copy number alteration, but lower frequency of TP53, CHD1, ESR1 and AKT1 mutation (Table 1) (all p < 0.05). HR-/HER2+ MaBC had higher frequency of BRCA1 (p < 0.05) mutation (Table 1). Male TNBC had higher frequency of CDKN2A, KMT2D, STK11, ASXL1, MYC, NF2, PPM1D, WT1, EPHA2, AMER1, ARHGAP35 mutation (Table 1) and BCL6 (4.35% vs 0.05%) copy number alteration, but lower frequency of TP53 mutation (all p < 0.05) (Table 1). MaBC had higher AR protein expression (82.5% vs 60%) but lower frequency of fusion variant-AR (0.7% vs 3.9%) (all p < 0.05) compared to FeBC. There was no difference in TMB high (7.1% vs 8.4%, p = 0.5) and dMMR/MSI-H (1.8% vs 0.8%, p = 0.1). Analysis of inferred immune cell infiltrates showed that MaBC had increased immune cell infiltration of B cells (5.8% vs 5.3%) and M2 Mφ (4.9% vs 4.3%), but decreased infiltration of DC (2% vs 2.6%) (all p < 0.05). MaBC had increased expression of MHC class II gene HLA-DQB2 (FC: 1.3), but decreased expression of immune-related genes CD274 (FC: 1.2), IDO (FC: 1.4), IL1A (FC: 1.3) and IL12A (FC: 1.5) (all p < 0.05). Correlative analyses with survival will be presented at the meeting. Conclusions: These data indicate that MaBC has a differential mutational frequency, copy number alteration, immune gene expression and immune cell infiltration and overall survival compared to their FeBC counterparts. A better understanding of these sex-based differences with additional research may help inform disease outcomes, provide a rationale for tailored therapeutic approaches and design future treatments. Table 1. Mutation frequency in male and female breast cancer Citation Format: Dario Trapani, Sachin Deshmukh, Sharon Wu, Joanne Xiu, Phil Walker, Pooja Advani, Nancy Lin, Giuseppe Curigliano, Stephanie Graff, Chadi Nabhan, George Sledge Jr, Sara Tolaney, Jose Leone. Molecular and immunological landscape of sex-based differences in breast cancer: a distinct disease in men [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PS13-09.

Abstract Background: Patients with estrogen receptor-positive (ER+) metastatic breast cancer (BC) may develop resistance to endocrine therapy (ET), particularly following treatment with a cyclin-dependent kinase 4/6 inhibitor (CDK4/6i), potentially driven by a mutation in the ERa-coding gene, ESR1. Lasofoxifene, an oral, next-generation ET and ER breast antagonist, had potent activity in mBC xenograft models expressing ESR1 mutations, either as a single agent or combined with a CDK4/6i. Subsequently, two phase 2 studies evaluated lasofoxifene in women with ER+/HER2- metastatic BC and an ESR1 mutation who had disease progression on previous ET and CDK4/6i. The ELAINE 1 trial showed numerically greater progression-free survival (PFS, median ~6 vs 4 months; P=0.138), objective response rate (ORR, 13% vs 3%; P=0.124), and clinical benefit rate (CBR, 37% vs 22%; P=0.117) with lasofoxifene monotherapy versus the ER degrader fulvestrant, with a favorable safety profile (Goetz MP, et al. Ann Oncol. 2022;33:S1387). The single-arm, ELAINE 2 trial demonstrated that lasofoxifene combined with abemaciclib was well tolerated with a median PFS of ~13 months, ORR of 56%, and CBR of 66% (Damodaran S, et al. J Clin Oncol. 2023;41:suppl 16; abstr 1057). Based on these data, the phase 3, registrational, ELAINE 3 trial was initiated. Methods: ELAINE 3 (NCT05696626) is an open-label, phase 3, multicenter study evaluating the efficacy, safety, and tolerability of lasofoxifene plus abemaciclib versus fulvestrant plus abemaciclib. Key inclusion criteria are pre- and postmenopausal women and men aged ≥18 years; ER+/HER2-, locally advanced and/or metastatic BC (measurable and/or non-measurable disease); ≥1 acquired ESR1 mutation; progression on an aromatase inhibitor plus palbociclib or ribociclib as their first hormonal treatment for advanced/metastatic BC; and ≤1 line of chemotherapy in the advanced/metastatic setting. Patients will be randomized 1:1 to receive lasofoxifene 5 mg/day plus abemaciclib 150 mg BID, or fulvestrant 500 mg IM on days 1, 15, and 29, then 4 weekly plus abemaciclib 150 mg BID. Treatment will continue until progression, death, unacceptable toxicity, or withdrawal from the study. The primary endpoint is PFS; key secondary endpoints are ORR, overall survival, CBR, duration of response, and time to response. Time to cytotoxic chemotherapy, quality of life, and safety will also be evaluated. Blood samples for circulating tumor DNA (ctDNA) will be collected at screening, at weeks 4 and 8 and every 8 weeks thereafter, and at the final visit for genomic analyses. Outcomes with lasofoxifene/abemaciclib and fulvestrant/abemaciclib will be compared using a stratified Cox proportional hazards model and stratified logrank test. Expected PFS is ≥10.3 months for lasofoxifene/abemaciclib and 7 months for fulvestrant/abemaciclib (PFS hazard ratio of 0.68 at final analysis). The target sample size is 400, to achieve 90% power with a one-sided type I error rate of 0.025 after 285 PFS events. Full recruitment is expected to occur over 18 months at 125 global sites. Citation Format: Matthew Goetz, Seth Wander, Thomas Bachelot, Gerald Batist, Javier Cortés, Massimo Cristofanilli, Giuseppe Curigliano, Alexandre de Nonneville, Einav Nili-Gal Yam, Komal Jhaveri, Cynthia Ma, Heather Parsons, Hope Rugo, Sarah Sammons, Daniel Stover, Chris Twelves, Aditya Bardia, Paul V. Plourde, David J Portman, Senthil Damodaran. Trial in progress: Open-label, randomized, multicenter, phase 3, ELAINE 3 study of the efficacy and safety of lasofoxifene plus abemaciclib for treating locally advanced or ER+/HER2- metastatic breast cancer with an ESR1 mutation [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO1-18-12.

Abstract Background The current first-line (1L) standard of care (SOC) for human epidermal growth factor receptor 2-positive (HER2+) metastatic breast cancer (MBC) is trastuzumab (T) plus pertuzumab (P) and a taxane. Despite advances in 1L SOC, most patients progress during maintenance therapy with T+P. Tucatinib is a tyrosine kinase inhibitor (TKI) approved in combination with T and capecitabine for adults with HER2+ MBC, with and without brain metastases (BM). In HER2CLIMB, addition of tucatinib significantly prolonged progression-free survival (PFS) and overall survival (OS) in patients with HER2+ MBC and was well tolerated. Adding tucatinib also reduced the risk of disease progression or death in patients with untreated and/or active BM (Murthy et al. NEJM 2020, Curigliano et al. Ann Oncol 2022). HER2CLIMB-05 investigates whether adding tucatinib to 1L SOC as maintenance therapy will extend PFS while maintaining quality of life (QOL). Methods HER2CLIMB-05 (NCT05132582) is a phase 3, randomized, double-blind study evaluating tucatinib plus T+P as maintenance therapy for HER2+ MBC. Approximately 650 patients will be enrolled. Eligible patients will have advanced HER2+ disease, no progression on 4–8 cycles of prior 1L SOC, Eastern Cooperative Oncology Group Performance Status of 0 or 1, and no or asymptomatic BM. Exclusion criteria include prior treatment with anti-HER2 and/or anti-epidermal growth factor receptor TKI (prior SOC for early BC is permitted) or inability to undergo contrast magnetic resonance imaging of the brain. Patients will be randomized 1:1 to receive either tucatinib or placebo twice daily, with T+P once every 21 days. Patients with hormone receptor-positive disease may receive endocrine therapy. The primary endpoint is investigator-assessed PFS. Secondary endpoints include OS, time to deterioration of health-related QOL, central nervous system PFS, safety, and pharmacokinetic (PK) parameters. PFS and OS will be compared using a two-sided stratified log-rank test between treatment groups. Time-to-event endpoints will be summarized using the Kaplan–Meier method. PK and safety data will be summarized using descriptive statistics. Enrollment is ongoing in Austria, Chile, Greece, Italy, Japan, Poland, Portugal, Spain, the US, and several other European, APAC, and LATAM countries. This abstract was previously presented at ESMO-BC 2022, FPN (Final Publication Number): 415, by Veronique Dieras (reused with permission). Citation Format: Erika Hamilton, Junji Tsurutani, Giuseppe Curigliano, Miguel Martín, Ciara O'Sullivan, Joo Hyuk Sohn, Konstantinos Tryfonidis, Libero Santarpia, Shan Yang, Véronique Diéras. Phase 3 Study of Tucatinib or Placebo in Combination With Trastuzumab and Pertuzumab as Maintenance Therapy for HER2+ Metastatic Breast Cancer (HER2CLIMB-05, Trial in Progress) [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO1-19-11.

Importance The association of tumor-infiltrating lymphocyte (TIL) abundance in breast cancer tissue with cancer recurrence and death in patients with early-stage triple-negative breast cancer (TNBC) who are not treated with adjuvant or neoadjuvant chemotherapy is unclear. Objective To study the association of TIL abundance in breast cancer tissue with survival among patients with early-stage TNBC who were treated with locoregional therapy but no chemotherapy. Design, Setting, and Participants Retrospective pooled analysis of individual patient-level data from 13 participating centers in North America (Rochester, Minnesota; Vancouver, British Columbia, Canada), Europe (Paris, Lyon, and Villejuif, France; Amsterdam and Rotterdam, the Netherlands; Milan, Padova, and Genova, Italy; Gothenburg, Sweden), and Asia (Tokyo, Japan; Seoul, Korea), including 1966 participants diagnosed with TNBC between 1979 and 2017 (with follow-up until September 27, 2021) who received treatment with surgery with or without radiotherapy but no adjuvant or neoadjuvant chemotherapy. Exposure TIL abundance in breast tissue from resected primary tumors. Main Outcomes and Measures The primary outcome was invasive disease-free survival [iDFS]. Secondary outcomes were recurrence-free survival [RFS], survival free of distant recurrence [distant RFS, DRFS], and overall survival. Associations were assessed using a multivariable Cox model stratified by participating center. Results This study included 1966 patients with TNBC (median age, 56 years [IQR, 39-71]; 55% had stage I TNBC). The median TIL level was 15% (IQR, 5%-40%). Four-hundred seventeen (21%) had a TIL level of 50% or more (median age, 41 years [IQR, 36-63]), and 1300 (66%) had a TIL level of less than 30% (median age, 59 years [IQR, 41-72]). Five-year DRFS for stage I TNBC was 94% (95% CI, 91%-96%) for patients with a TIL level of 50% or more, compared with 78% (95% CI, 75%-80%) for those with a TIL level of less than 30%; 5-year overall survival was 95% (95% CI, 92%-97%) for patients with a TIL level of 50% or more, compared with 82% (95% CI, 79%-84%) for those with a TIL level of less than 30%. At a median follow-up of 18 years, and after adjusting for age, tumor size, nodal status, histological grade, and receipt of radiotherapy, each 10% higher TIL increment was associated independently with improved iDFS (hazard ratio [HR], 0.92 [0.89-0.94]), RFS (HR, 0.90 [0.87-0.92]), DRFS (HR, 0.87 [0.84-0.90]), and overall survival (0.88 [0.85-0.91]) (likelihood ratio test, P < 10e-6). Conclusions and Relevance In patients with early-stage TNBC who did not undergo adjuvant or neoadjuvant chemotherapy, breast cancer tissue with a higher abundance of TIL levels was associated with significantly better survival. These results suggest that breast tissue TIL abundance is a prognostic factor for patients with early-stage TNBC.

PURPOSE To develop recommendations for germline mutation testing for patients with breast cancer. METHODS An ASCO–Society of Surgical Oncology (SSO) panel convened to develop recommendations based on a systematic review and formal consensus process. RESULTS Forty-seven articles met eligibility criteria for the germline mutation testing recommendations; 18 for the genetic counseling recommendations. RECOMMENDATIONS BRCA1/ 2 mutation testing should be offered to all newly diagnosed patients with breast cancer ≤65 years and select patients >65 years based on personal history, family history, ancestry, or eligibility for poly(ADP-ribose) polymerase (PARP) inhibitor therapy. All patients with recurrent breast cancer who are candidates for PARP inhibitor therapy should be offered BRCA1/ 2 testing, regardless of family history. BRCA1/ 2 testing should be offered to women who develop a second primary cancer in the ipsilateral or contralateral breast. For patients with prior history of breast cancer and without active disease, testing should be offered to patients diagnosed ≤65 years and selectively in patients diagnosed after 65 years, if it will inform personal and family risk. Testing for high-penetrance cancer susceptibility genes beyond BRCA1/ 2 should be offered to those with supportive family histories; testing for moderate-penetrance genes may be offered if necessary to inform personal and family cancer risk. Patients should be provided enough pretest information for informed consent; those with pathogenic variants should receive individualized post-test counseling. Variants of uncertain significance should not impact management, and patients with such variants should be followed for reclassification. Referral to providers experienced in clinical cancer genetics may help facilitate patient selection and interpretation of expanded testing, and provide counseling of individuals without pathogenic germline variants but with significant family history. Additional information is available at www.asco.org/breast-cancer-guidelines .

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