Efficacy of Margetuximab vs Trastuzumab in Patients With Pretreated ERBB2-Positive Advanced Breast Cancer: A Phase 3 Randomized Clinical Trial

Key Points

Question  Does margetuximab plus chemotherapy prolong progression-free survival and/or overall survival of patients with pretreated ERBB2-positive advanced breast cancer, relative to trastuzumab plus chemotherapy?

Findings  In the SOPHIA phase 3 randomized clinical trial of 536 patients with pretreated ERBB2-positive advanced breast cancer, margetuximab plus chemotherapy generated a statistically significant 24% relative risk reduction in the hazard of progression vs trastuzumab plus chemotherapy. After the second planned interim analysis of 270 deaths, median OS was 21.6 months with margetuximab vs 19.8 months with trastuzumab, and final analysis of OS will be reported subsequently.

Meaning  This trial demonstrates a head-to-head advantage of margetuximab (an Fc-engineered ERBB2-targeted antibody) compared with trastuzumab in a pretreated ERBB2-positive advanced breast cancer population.

Importance  ERRB2 (formerly HER2)–positive advanced breast cancer (ABC) remains typically incurable with optimal treatment undefined in later lines of therapy. The chimeric antibody margetuximab shares ERBB2 specificity with trastuzumab but incorporates an engineered Fc region to increase immune activation.

Objective  To compare the clinical efficacy of margetuximab vs trastuzumab, each with chemotherapy, in patients with pretreated ERBB2-positive ABC.

Design, Setting, and Participants  The SOPHIA phase 3 randomized open-label trial of margetuximab plus chemotherapy vs trastuzumab plus chemotherapy enrolled 536 patients from August 26, 2015, to October 10, 2018, at 166 sites in 17 countries. Eligible patients had disease progression on 2 or more prior anti-ERBB2 therapies and 1 to 3 lines of therapy for metastatic disease. Data were analyzed from February 2019 to October 2019.

Interventions  Investigators selected chemotherapy before 1:1 randomization to margetuximab, 15 mg/kg, or trastuzumab, 6 mg/kg (loading dose, 8 mg/kg), each in 3-week cycles. Stratification factors were metastatic sites (≤2, >2), lines of therapy (≤2, >2), and chemotherapy choice.

Main Outcomes and Measures  Sequential primary end points were progression-free survival (PFS) by central blinded analysis and overall survival (OS). All α was allocated to PFS, followed by OS. Secondary end points were investigator-assessed PFS and objective response rate by central blinded analysis.

Results  A total of 536 patients were randomized to receive margetuximab (n = 266) or trastuzumab (n = 270). The median age was 56 (27-86) years; 266 (100%) women were in the margetuximab group, while 267 (98.9%) women were in the trastuzumab group. Groups were balanced. All but 1 patient had received prior pertuzumab, and 489 (91.2%) had received prior ado-trastuzumab emtansine. Margetuximab improved primary PFS over trastuzumab with 24% relative risk reduction (hazard ratio [HR], 0.76; 95% CI, 0.59-0.98; P = .03; median, 5.8 [95% CI, 5.5-7.0] months vs 4.9 [95% CI, 4.2-5.6] months; October 10, 2018). After the second planned interim analysis of 270 deaths, median OS was 21.6 months with margetuximab vs 19.8 months with trastuzumab (HR, 0.89; 95% CI, 0.69-1.13; P = .33; September 10, 2019), and investigator-assessed PFS showed 29% relative risk reduction favoring margetuximab (HR, 0.71; 95% CI, 0.58-0.86; P < .001; median, 5.7 vs 4.4 months; September 10, 2019). Margetuximab improved objective response rate over trastuzumab: 22% vs 16% (P = .06; October 10, 2018), and 25% vs 14% (P < .001; September 10, 2019). Incidence of infusion-related reactions, mostly in cycle 1, was higher with margetuximab (35 [13.3%] vs 9 [3.4%]); otherwise, safety was comparable.

Conclusions and Relevance  In this phase 3 randomized clinical trial, margetuximab plus chemotherapy had acceptable safety and a statistically significant improvement in PFS compared with trastuzumab plus chemotherapy in ERBB2-positive ABC after progression on 2 or more prior anti-ERBB2 therapies. Final OS analysis is expected in 2021.

Trial Registration  ClinicalTrials.gov Identifier: NCT02492711

Addition of ERBB2 (formerly HER2)–targeting monoclonal antibodies to chemotherapy improves progression-free survival (PFS) and overall survival (OS) in patients with ERBB2-positive advanced breast cancer (ABC).^1-4 Generally, patients with ERBB2-positive metastatic breast cancer (BC) receive multiple lines of therapy, yet with rare exceptions, ERBB2-positive metastatic BC remains incurable.^5,6

Margetuximab is a chimeric, Fc-engineered, immune-activating anti-ERBB2 immunoglobulin G1 (IgG1) monoclonal antibody that shares epitope specificity and Fc-independent antiproliferative effects with trastuzumab. Fc engineering of margetuximab alters 5 amino acids from wild-type IgG1 to increase affinity for activating Fcγ receptor (FcγR) CD16A (FcγRIIIa) and to decrease affinity for inhibitory FcγR CD32B (FcγRIIb).^6,7 These effects are proposed to increase activation of innate and adaptive anti-ERBB2 immune responses, relative to trastuzumab.

Three FcγRs (CD16A, CD32A, and CD32B) expressed on immune effector cells regulate cellular activation by antibodies.⁸ CD16A can trigger antibody-dependent cellular cytotoxicity (ADCC) by innate immune cells.^9,10 Two CD16A polymorphisms at amino acid 158 bind IgG1 with higher (valine [V]) or lower (phenylalanine [F]) affinity.¹¹ Clinical benefit of therapeutic antibodies, including trastuzumab,^11-15 appears greater for patients with the high-affinity VV genotype compared with the lower-affinity FV and FF genotypes (CD16A-158F carriers), although not all studies observed this effect.^16,17 Notably, most people (approximately 85%) are CD16A-158F allele carriers.^11,12 A key feature of margetuximab’s engineered Fc domain is increased binding to all CD16A-158 V/F variants, relative to wild-type IgG1.

In a phase 1 study¹⁸ of margetuximab monotherapy in 66 patients with pretreated ERBB2-positive carcinomas, 4 of 24 (17%) evaluable patients with ABC had a confirmed partial response. Three patients continued margetuximab monotherapy for 4 or more years.¹⁹ Here we report initial results of a phase 3 randomized clinical trial of margetuximab vs trastuzumab, each combined with single-agent chemotherapy, in pretreated patients with ERBB2-positive ABC. In addition, we present an exploratory analysis of PFS and OS by FcγR genotype.

This international, randomized, open-label, phase 3 study (SOPHIA; MacroGenics study protocol No. CP-MGAH22-04) enrolled patients at 166 centers in 17 countries. Eligible patients were aged 18 years or older with confirmed ERBB2-positive ABC by local or optional central testing of the most recent biopsy, following 2013 American Society of Clinical Oncology testing recommendations.²⁰ Patients must have had progressive disease after 2 or more lines of prior ERBB2-targeted therapy, including pertuzumab, and 1 to 3 lines of nonhormonal metastatic BC therapy. Prior brain metastases were allowed if treated and stable. Trial conduct was in accordance with Good Clinical Practice guidelines and the Declaration of Helsinki. An independent ethics committee approved the protocol at each participating site. All patients provided written informed consent. The trial protocol and statistical analysis plan are available online (Supplement 1). This study followed the Consolidated Standards of Reporting Trials (CONSORT) reporting guideline.

Investigators chose 1 of 4 chemotherapies (capecitabine, eribulin, gemcitabine, or vinorelbine) for each eligible patient before 1:1 randomization by a permuted-blocks procedure. Stratification factors were metastatic sites (≤2, >2), lines of therapy for metastatic disease (≤2, >2), and chemotherapy choice. The trial was open label for patients and investigators but sponsor blinded with central blinded analysis (CBA) of PFS to prevent observer bias.

Margetuximab was given intravenously at 15 mg/kg over 120 minutes on day 1 of each 21-day cycle. Trastuzumab was given intravenously at 6 mg/kg (over 30-90 minutes) on day 1 of each 21-day cycle after a loading dose of 8 mg/kg (over 90 minutes). Capecitabine was given orally at 1000 mg/m² twice daily for 14 days followed by 7 days off. Eribulin, gemcitabine, and vinorelbine were given intravenously before antibody infusion at 1.4 mg/m², 1000 mg/m², and 25 to 30 mg/m², respectively, on days 1 and 8 of each cycle. Margetuximab premedication was recommended, if not already given with chemotherapy: standard doses of acetaminophen or ibuprofen, diphenhydramine, ranitidine, and dexamethasone, or equivalents. Disease assessment was performed at baseline, every 6 weeks for the first 24 weeks of therapy, and then every 12 weeks until disease progression, adverse event (AE) necessitating discontinuation, consent withdrawal, or death. Safety was assessed at each visit, including AEs from study therapy initiation through the end-of-treatment visit, or 28 days after last administration of the study drug. Investigators assessed both event severity, using Common Terminology Criteria for Adverse Events, version 4.03, and causality. Left ventricular ejection fraction (LVEF) was monitored every 6 weeks for 24 weeks and then every 12 weeks thereafter. Optional CD16A, CD32A, and CD32B genotyping was performed by polymerase chain reaction amplification of blood DNA, followed by DNA sequencing.

Primary end points were PFS by CBA, with the α entirely allocated to PFS, and OS. The PFS was defined as time from randomization to disease progression or death from any cause. Secondary end points included investigator-assessed PFS and CBA-assessed objective response rate (ORR). The PFS and ORR were assessed according to Response Evaluation Criteria in Solid Tumors, version 1.1. Additional end points included safety, clinical benefit rate (CBR; ORR plus stable disease lasting at least 6 months), investigator-assessed ORR, response duration, antidrug antibodies, and exploratory evaluation of FcγR allelic variation on efficacy.

For 90% power to detect median PFS improvement from 4 to 6 months (hazard ratio [HR], 0.67) at a 2-sided .05 significance level, 257 PFS events were needed. Primary PFS by CBA occurred after 257 PFS events or all patients were randomized, whichever occurred last. The OS was time from randomization to death from any cause and was to be assessed only if PFS was positive. For 80% power to detect a median OS improvement from 12 to 16 months (HR, 0.75) at a 2-sided .05 significance level, 385 OS events were needed. Three OS analyses were planned: first interim coincident with primary PFS analysis, second interim after 270 deaths, and final analysis after 385 events. All α was allocated to PFS, tested at a 2-sided .05 significance level. If PFS passed the test, then OS would be tested at the same significance level of 2-sided .05. The O’Brien-Fleming type Lan-DeMets α-spending function was applied for α allocation to each interim OS analysis.

The PFS and OS were assessed in the randomized, intention-to-treat population. Patients were censored at the last tumor assessment date for PFS and at the last time known to be alive for OS. The ORR and CBR were assessed in randomized patients with baseline measurable disease (response-evaluable population). For ORR analysis, if a patient’s response was missing, the patient was classified as not available. Safety and antidrug antibodies were assessed in randomized patients after any study treatment (safety population).

Kaplan-Meier methods were used to estimate median PFS, OS, and 95% CIs for each treatment group. The stratified log-rank test was used to compare time-to-event end points between groups. A stratified Cox proportional hazards model, with treatment as the only covariate, was used to estimate PFS and OS HRs and 95% CIs.

Prespecified PFS and OS subgroup analyses included chemotherapy choice, metastatic sites, lines of prior metastatic therapy, prior ado-trastuzumab emtansine use, hormone receptor status, ERBB2 status, Eastern Cooperative Oncology Group performance status, region, age, and race, as well as FCGR3A (FcγRIIIa/CD16A), FCGR2A (FcγRIIa/CD32A), and FCGR2B (FcγRIIb/CD32B) genotype. The HRs and 95% CIs for PFS in each subgroup were assessed using an unstratified Cox proportional hazards model with treatment as the only covariate.

If the primary PFS and OS were each positive, then secondary PFS and ORR end points were to be tested using the Hochberg step-up procedure for multiplicity adjustment. Investigator-assessed PFS was analyzed using the same methods as the primary PFS end point. The ORR was compared between groups by the stratified Mantel-Haenszel test. Data were analyzed from February 2019 to October 2019. Analyses were performed using SAS, version 9.4 (SAS Institute).

From July 2015 through October 2018, a total of 536 patients were enrolled at 166 centers in 17 countries and randomized to receive margetuximab plus chemotherapy (margetuximab group, n = 266) or trastuzumab plus chemotherapy (trastuzumab group, n = 270; Figure 1). The median age was 56 (27-86) years (55.0 [29-83] years for patients in the margetuximab group and 56.0 [27-86] years in the trastuzumab group); 266 (100%) women were in the margetuximab group, while 267 (98.9%) women were in the trastuzumab group. Investigator-selected chemotherapy choices were vinorelbine (n = 191, 35.6%), capecitabine (n = 143, 26.7%), eribulin (n = 136, 25.4%), and gemcitabine (n = 66, 12.3%). Patients received a median of 6 cycles of margetuximab vs 5 cycles of trastuzumab.

Baseline characteristics were balanced across groups (Table 1). All patients had received prior trastuzumab; all but 1 had received prior pertuzumab, and 489 (91.2%) had received prior ado-trastuzumab emtansine. One-third of patients in both groups received 3 or more prior therapies for metastatic BC (margetuximab, 92 of 266 [34.6%]; vs trastuzumab, 87 of 270 [32.2%]).

Primary PFS analysis was triggered by last randomization (October 10, 2018), after 265 PFS events. On that date, 79 (30%) vs 58 (22%) patients remained on margetuximab vs trastuzumab, respectively, including 13 (5%) and 5 (2%) remaining exclusively on margetuximab vs trastuzumab. Margetuximab plus chemotherapy prolonged centrally assessed PFS (HR, 0.76; 95% CI, 0.59-0.98; P = .03; median PFS, 5.8 [95% CI, 5.5-7.0] months vs 4.9 [95% CI, 4.2-5.6] months; Figure 2A), with a 24% PFS relative risk reduction over trastuzumab plus chemotherapy, meeting the primary end point of the study. The test of proportional hazards assumption indicated that the proportional hazards assumption was not violated. Investigator-assessed PFS based on 337 events was also greater with margetuximab than with trastuzumab (HR, 0.70; 95% CI, 0.56-0.87; P = .001; median, 5.6 vs 4.2 months; Figure 2B), with a 30% PFS relative risk reduction over trastuzumab. Coincident with the second interim OS analysis, updated investigator-assessed PFS based on 430 PFS events showed increased statistical significance in favor of margetuximab with a similar HR (HR, 0.71; 95% CI, 0.58-0.86; P < .001; median, 5.7 vs 4.4 months; Figure 2C).

The OS analysis after 270 deaths (70% of 385 final required events) occurred on September 10, 2019, after 131 (49.2%) and 139 (51.5%) OS events in the margetuximab and trastuzumab groups, respectively. Median OS was 21.6 months with margetuximab and 19.8 months with trastuzumab (HR, 0.89; 95% CI, 0.69-1.13; P = .33; Figure 3). The stopping threshold was not reached; final OS analysis will occur after 385 deaths (anticipated in 2021).

Among 524 response-evaluable patients, margetuximab recipients had higher blinded ORR (22% vs 16%; P = .06) and CBR (37% vs 25%; P = .003) than trastuzumab recipients (eTable 1 in Supplement 2). These rates were similar at the September 2019 cutoff when investigator-assessed ORR and CBR were 25% vs 14% (P < .001) and 48% vs 36% (P = .003), respectively (eTable 1 in Supplement 2). Median response duration was similar between treatment groups: 6.1 vs 6.0 months (October 10, 2018, CBA) and 6.9 vs 7.0 months (September 10, 2019, investigator assessed).

Prespecified exploratory subgroup analyses of primary PFS (October 10, 2018, cutoff) and second interim OS (September 10, 2019, cutoff) by CD16A genotype are shown in eFigures 1 and 2 in Supplement 2, respectively. Genotyping was available for 506 patients (94%). Study groups were in Hardy-Weinberg equilibrium for all 3 FCGR genotypes (eTable 2 in Supplement 2). Baseline characteristics of patients assigned to margetuximab vs trastuzumab by FcγR genotype are shown in eTable 3 in Supplement 2. The interim OS per treatment group by CD16A genotype is shown in eFigure 3 in Supplement 2. Efficacy outcomes by CD32A and CD32B are shown in eFigure 4 and eFigure 5 in Supplement 2.

As of April 10, 2019, which provided 6 additional months of safety follow-up after the primary PFS analysis, the safety population included 264 margetuximab and 266 trastuzumab recipients. Common AEs (≥20% of patients), regardless of causality, included fatigue, nausea, diarrhea, and neutropenia in both groups (Table 2), as well as vomiting (margetuximab group) and anemia (trastuzumab group). Grade 3 or greater AEs in at least 5% of patients included neutropenia and anemia in both groups, as well as fatigue in the margetuximab group and febrile neutropenia in the trastuzumab group. Discontinuations owing to AEs were similar (margetuximab, 8 of 266 [3.0%]; trastuzumab, 7 of 270 [2.6%]; eTable 4 in Supplement 2). Adverse events leading to death were reported in 5 patients (margetuximab, n = 3 [1.1%]; trastuzumab, n = 2 [0.8%]; eTable 4 in Supplement 2); none were considered treatment related.

Adverse events of special interest included infusion-related reactions (IRRs) and left ventricular (LV) dysfunction. All-grade IRRs were more common with margetuximab than with trastuzumab (35 [13.3%] vs 9 [3.4%], respectively; Table 2). Most IRRs were grade 1 or 2, occurred on cycle 1, day 1, and resolved within 24 hours. Grade 3 IRRs on cycle 1, day 1 were reported in 4 (1.5%) margetuximab-treated patients. Of these 4 patients, 2 (0.8%) continued therapy for 5 or more cycles and 2 (0.8%) discontinued owing to IRRs (eTable 4 in Supplement 2). No trastuzumab recipients had grade 3 IRRs. Adverse events of LV dysfunction occurred in 7 patients (3%) in each treatment group (eTable 4 in Supplement 2). Grade 3 LV dysfunction AEs were observed in 3 margetuximab recipients (1.1%) and 1 trastuzumab recipient (0.4%). Monitoring of LVEF led to dose delay or discontinuation in 4 margetuximab-treated (1.5%) vs 6 trastuzumab-treated patients (2.3%). All LVEF reductions detected by monitoring were asymptomatic. Reductions in LVEF were reversible for all patients with complete follow-up.

The phase 3 SOPHIA trial compared margetuximab, a novel chimeric Fc-engineered anti-ERBB2 antibody, to trastuzumab, each with chemotherapy, in patients with pretreated ERBB2-positive ABC. This randomized clinical trial was positive for its PFS primary end point. Margetuximab plus chemotherapy led to an independently assessed PFS benefit vs trastuzumab plus chemotherapy, with a 24% relative risk reduction. Investigator-assessed PFS complemented primary blinded PFS with a 29% relative risk reduction. No conclusion can be drawn at this time about OS based on the 2 OS interim analyses conducted after 40% and 70% of target OS events (immature data); final analysis of the effect of margetuximab vs trastuzumab on survival will occur after 385 deaths (anticipated in 2021).

Margetuximab plus chemotherapy had acceptable safety, comparable with control trastuzumab plus chemotherapy. Although IRRs were increased with margetuximab, almost all occurred during the first infusion only, and the observed margetuximab IRR rate aligns with that in published literature on trastuzumab first exposure (16%).²¹ In a nonrandomized infusion safety substudy, margetuximab was administered over 30 minutes from cycle 2 onward and appeared well tolerated with no increase in IRR risk, supporting 30-minute margetuximab infusions after cycle 1.²² There was no increase in cardiac toxic effects with margetuximab compared with trastuzumab.

The SOPHIA study also tested the hypothesis that altering Fc–FcγR interactions can drive clinical benefit. Trastuzumab triggers ADCC²³ via activation of FcγRIIIa (CD16A).⁸ Associations between efficacy and CD16A polymorphism in trastuzumab-treated patients with early and advanced BC suggest lower immune activation in CD16A-158F allele carriers compared with VV homozygotes.^10-12,14,24 Diminished clinical response to trastuzumab for these CD16A-158F carriers suggests these patients may benefit from an antibody with enhanced Fc-dependent immune activation.^11,12,24 Margetuximab Fc engineering increases affinity for both CD16A allotypes, enhances ADCC potency over trastuzumab with effector cells of all CD16A genotypes (FF, FV, VV), albeit proportionally more for CD16A-158F carriers under certain conditions, and boosts activity against ERBB2-positive BC xenografts in mice transgenic for human CD16A-158F.^6,25 Exploratory PFS analysis by CD16A genotype suggested that presence of a CD16A-158F allele may predict margetuximab benefit over trastuzumab. Early OS results showed a similar pattern. Conversely, there was no margetuximab benefit in the smaller CD16A-158VV group in this study of heavily pretreated patients. There is no clear biological explanation for the observation that margetuximab provided no clinical benefit in CD16A-158VV homozygotes compared with trastuzumab, although there was an imbalance in poor prognostic features between the 2 groups.

Increasing evidence implicates adaptive immunity in the clinical activity of anti-ERBB2 monoclonal antibodies.²⁶ ERBB2-specific T-cell and antibody responses were observed in 50% to 78% and 42% to 69%, respectively, of trastuzumab-treated patients with ERBB2-positive BC.^10,24,26-30 Correspondingly, increases in ERBB2-specific T-cell and antibody responses were observed in 98% and 94%, respectively, of pretreated phase 1 study patients receiving margetuximab monotherapy.³¹

This trial demonstrates a small but statistically significant PFS benefit of margetuximab plus chemotherapy over trastuzumab plus chemotherapy in patients with ERBB2-positive ABC who progressed after treatment with trastuzumab, pertuzumab, and ado-trastuzumab emtansine.^1,2,4,32 Alternatives for this patient population include neratinib, tucatinib, and trastuzumab deruxtecan, which have emerged as active regimens, albeit with different levels of effectiveness, and all with notable toxic effects. Margetuximab may have a role for patients in this setting who are unable, or unwilling, to tolerate toxic effects of these novel therapies.

Limitations of this trial include that the primary end point did not allocate α to the CD16A analysis and that patients with active brain metastases were not included. An ongoing neoadjuvant investigator-sponsored trial is comparing margetuximab vs trastuzumab in patients with the low-affinity CD16A genotype (the MARGetuximab Or Trastuzumab trial, known as MARGOT; NCT04425018). Immune-mediated therapies, such as margetuximab, may be more effective in the earlier disease setting where the immune system is relatively intact.

The chimeric antibody margetuximab shares ERBB2 specificity with trastuzumab but incorporates an engineered Fc region to optimize immune activation. This phase 3 randomized clinical trial demonstrates improvement in PFS of margetuximab in combination with chemotherapy vs trastuzumab plus chemotherapy in patients with pretreated ERBB2-positive ABC, which remains typically incurable.

Accepted for Publication: November 18, 2020.

Published Online: January 22, 2021. doi:10.1001/jamaoncol.2020.7932

Open Access: This is an open access article distributed under the terms of the CC-BY-NC-ND License. © 2021 Rugo HS et al. JAMA Oncology.

Corresponding Author: Hope S. Rugo, MD, University of California San Francisco Helen Diller Family Comprehensive Cancer Center, 1825 Fourth St, 3rd Floor, PO Box 1710, San Francisco, CA 94158 (hope.rugo@ucsf.edu).

Author Contributions: Drs Rugo and Rock had full access to all of the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis.

Concept and design: Rugo, Im, Cardoso, Cortés, Curigliano, Pegram, Jakobsen, Koenig.

Acquisition, analysis, or interpretation of data: Rugo, Im, Cardoso, Curigliano, Musolino, Pegram, Wright, Saura, Escrivá-de-Romaní, De Laurentiis, Levy, Brown-Glaberman, Ferrero, de Boer, Kim, Petráková, Yardley, Freedman, Kaufman, Yerushalmi, Fasching, Nordstrom, Bonvini, Koenig, Edlich, Hong, Rock, Gradishar.

Drafting of the manuscript: Rugo, Curigliano, Musolino, Pegram, Yerushalmi, Nordstrom, Bonvini, Rock, Gradishar.

Critical revision of the manuscript for important intellectual content: Rugo, Im, Cardoso, Cortés, Curigliano, Musolino, Pegram, Wright, Saura, Escrivá-de-Romaní, De Laurentiis, Levy, Brown-Glaberman, Ferrero, de Boer, Kim, Petráková, Yardley, Freedman, Jakobsen, Kaufman, Fasching, Nordstrom, Bonvini, Koenig, Edlich, Hong, Rock, Gradishar.

Statistical analysis: Curigliano, Saura, Hong.

Obtained funding: Koenig.

Administrative, technical, or material support: Rugo, Im, Curigliano, Musolino, Petráková, Yardley, Kaufman, Fasching, Edlich, Rock, Gradishar.

Supervision: Curigliano, Musolino, Wright, Saura, Freedman, Koenig, Rock.

Conflict of Interest Disclosures: Dr Rugo reported receiving grants from MacroGenics during the conduct of the study; and grants from Roche, Pfizer, Novartis, Lilly, Merck, Seattle Genetics, Odonate Therapeutics, Eisai, Sermonix Pharmaceuticals, Immunomedics, and Daiichi Sankyo and personal fees from Puma and Samsung outside the submitted work. Dr Im reported receiving grants from Daewoong Pharmaceutical; grants and personal fees from AstraZeneca, Pfizer, and Roche; and personal fees from Amgen, Eisai, Hanmi Pharmaceutical, Novartis, Lilly, and Merck Sharp & Dohme outside the submitted work. Dr Cardoso reported receiving personal fees from MacroGenics during the conduct of the study; and personal fees from Amgen, Astellas/Medivation, AstraZeneca, Celgene, Daiichi Sankyo, Eisai, GE Oncology, Genentech, GlaxoSmithKline, MacroGenics, Medscape, Merck Sharp & Dohme, Merus BV, Mylan, Mundipharma, Novartis, Pfizer, Pierre Fabre, prIME Oncology, Roche, Sanofi, Samsung Bioepis, Seattle Genetics, and Teva outside the submitted work. Dr Cortés reported receiving personal fees from Roche, Celgene, AstraZeneca, Cellestia, Biothera Pharmaceuticals, Merus, Seattle Genetics, Daiichi Sankyo, Erytech Pharma, Athenex, Polyphor, Lilly, Servier, Merck Sharp & Dohme, GlaxoSmithKline, Leuko, Bioasis Technologies, Clovis Oncology, Boehringer Ingelheim, Novartis, Eisai, Pfizer, and Samsung Bioepis; honoraria from Samsung Bioepis, Pfizer, Eisai, Novartis, Roche, Celgene, Daiichi Sankyo, Lilly, Merck Sharp & Dohme; serving on advisory boards for Boehringer Ingelheim, Clovis Oncology, Bioasis, Leuko, GlaxoSmithKline, Merck Sharp & Dohme, Servier, Lilly, Polyphor, Athenex, Erytech, Daiichi Sankyo, Seattle Genetics, Merus, Biothera, Cellestia, AstraZeneca, Celgene, and Roche; and owning stock from MedSIR outside the submitted work; and travel, accommodation, and expenses from Roche, Novartis, Eisai, Pfizer, and Daiichi Sankyo. Dr Curigliano reported receiving travel grants from Roche; serving on advisory boards for Pfizer, Seattle Genetics, Novartis, AstraZeneca, Lilly, Merck Sharp & Dohme, Daichi Sankyo, Genomic Health, and Ellipsis; and serving as a steering committee member for Bristol Myers Squibb outside the submitted work. Dr Musolino reported receiving personal fees from MacroGenics during the conduct of the study; and grants and personal fees from Roche and Eisai and personal fees from Novartis and Lilly outside the submitted work. Dr Pegram reported receiving personal fees from MacroGenics and Roche during the conduct of the study. Dr Wright reported receiving payment to Sarah Cannon Research Institute for conduct of clinical trial from MacroGenics during the conduct of the study; and payment to Sarah Cannon Research Institute for conduct of clinical trial from AbbVie, Incyte, Genentech, Novartis, Lilly, Janssen, Astellas, Celgene, AstraZeneca, Bristol Myers Squibb, Boehringer Ingelheim, G1 Therapeutics, Medivation, Merrimack, Pfizer, Tesaro, and Odonate Therapeutics outside the submitted work. Dr Saura reported receiving personal fees from AstraZeneca, Celgene, Daiichi Sankyo, Eisai, F. Hoffmann-La Roche Ltd, Genomic Health, Merck, Sharp and Dohme España SA, Novartis, Odonate Therapeutics, Pfizer, Philips HealthWork, Pierre Fabre, prIME Oncology, Puma, Synthon, and Sanofi Aventis outside the submitted work. Dr Escrivá-de-Romaní reported receiving grants from MacroGenics during the conduct of the study; and grants from Synthon and Zymeworks; grants, personal fees, and nonfinancial support from Roche; personal fees and nonfinancial support from Pierre Fabre; personal fees from Eisai, Kyowa Kirin, and Esteve; and nonfinancial support from Daiichi Sankyo outside the submitted work. Dr De Laurentiis reported receiving grants from MacroGenics during the conduct of the study; and grants from Daiichi Sankyo; grants and personal fees from Roche, Novartis, Pfizer, Eli Lilly, Pierre Fabre, and AstraZeneca; personal fees from Celgene, Eisai, Genomic Health, and Amgen outside the submitted work. Dr Brown-Glaberman reported receiving personal fees from Novartis, Eisai, Biotheranostics, and Seattle Genetics outside the submitted work. Dr de Boer reported receiving grants from Parexel during the conduct of the study; and grants from Novartis, Pfizer, and Eli Lilly and grants and personal fees from Roche outside the submitted work. Dr Kim reported participation as a consultant in advisory boards for Novartis, AstraZeneca, Lilly, Enzychem Lifesciences, Dae Hwa Pharmaceutical Co Ltd, ISU Abxis, and Daiichi Sankyo and receiving research funding (paid to institution) from Novartis, Sanofi-Aventis, and DongKook Pharmaceutical Co Ltd outside the submitted work. Dr Petráková reported receiving personal fees from Novartis, Roche, Merck Sharp & Dohme, and Pfizer outside the submitted work. Dr Yardley reported a consultant/advisory role (paid to institution) for Bristol Myers Squibb, Genentech/Roche, Eisai, Biotheranostics, Daiichi Sankyo/Lilly, Celgene, Novartis, NanoString Technology, and MacroGenics; research funding (paid to institution) from Daiichi Sankyo, Clovis Oncology, AstraZeneca, Genentech/Roche, InventisBio, Syndax, Lilly, MedImmune, Pfizer, Eisai, Novartis, Medivation, Tesaro, Immunomedics, AbbVie, Merck, and Oncothyreon; and participation in speakers bureaus and travel/accommodations/expenses for Novartis and Genentech/Roche outside the submitted work. Dr Jakobsen reported receiving financial support from Cascadian Therapeutics during the conduct of the study; and grants from Cascadian Therapeutics and grants and personal fees from Roche, Novartis, Lilly, and Pfizer outside the submitted work. Dr Kaufman reported receiving personal fees from Pfizer and AstraZeneca outside the submitted work. Dr Fasching reported receiving personal fees from MacroGenics during the conduct of the study; and grants from Cepheid and personal fees from Novartis, Pfizer, Daiichi Sankyo, AstraZeneca, Eisai, Merck Sharp & Dohme, Lilly, Pierre Fabre, and Seattle Genetics outside the submitted work. Dr Bonvini reported receiving salary, stocks, and stock options from MacroGenics as compensation as a condition of employment. Dr Koenig is an employee of MacroGenics and reported holding a patent to US20180298100 pending and a patent to US20160257761 pending. Drs Nordstrom, Hong, and Rock and Mr Edlich are or were employees of MacroGenics. No other disclosures were reported.

Funding/Support: This study was supported by MacroGenics, Inc.

Role of the Funder/Sponsor: This study was designed by academic investigators and sponsor representatives. The sponsor participated in regulatory/ethics approval, safety monitoring, data cleaning/collection, and statistical analysis. The sponsor and coauthors analyzed data. MacroGenics, Inc, paid for medical writing support to generate the manuscript. All authors reviewed and approved manuscript submission.

Group Information for the SOPHIA Study: Austria – Daniel Egle, MD, Universitätsklinik Innsbruck, Innsbruck, Alois Lang, MD, and Holger Rumpold, MD, Landeskrankenhaus Rankweil, Vorarlberg; Belgium – Sevilay Altintas, MD, PhD, UZ Antwerpen – Oncologie, Edegem, Annelore Barbeaux, MD, CHR Verviers East Belgium, Verviers, Jean-Francois Baurain, MD, PhD, Cliniques Universitaires Saint-Luc – Oncology, Bruxelles, Marleen Borms, MD, AZ Groeninge – Campus Loofstraat, Kortrijk, Nele Claes, MD, AZ Sint-Jan Brugge – Oostende AV – Campus Sint-Jan, Brugge, Caterina Confente, MD, INDC Entité Jolimontoise – CH de Jolimont-Lobbes, Haine-Saint-Paul, Ines Deleu, MD, AZ Nikolaas – Campus Sint-Niklaas Moerland, Sint-Niklaas, Luc Dirix, MD, PhD, GZA Ziekenhuizen – Campus Sint-Augustinus, Wilrijk, Christel Fontaine, MD, UZ Brussel – Campus Jette, Bruxelles, Marie-Pascale Graas, MD, CHC MontLégia, Liège, Stephanie Henry, MD, Donatienne Taylor, MD, and Peter Vuylsteke, MD, CHU UCL Namur – Site Sainte-Elisabeth, Namur, Jeroen Mebis, MD, PhD, Jessa Ziekenhuis – Campus Virga Jesse, Hasselt, Renaud Poncin, MD, Clinique Saint Pierre Ottignies, Ottignies, Isabelle Spoormans, MD, AZ Damiaan, Oostende; Canada – Orit Freedman, MD, MSc, RS McLaughlin Durham Regional Cancer Centre, Lakeridge Health, Oshawa, Skander Ghedira, MD, Dr Leon Richard Oncology Centre, Moncton, Ravi Ramjeesingh, MD, PhD, The Atlantic Clinical Cancer Research Unit, Halifax; Czech Republic – Zdenek Kral, MD, CSc, Fakultni nemocnice Brno, Brno, Bohuslav Melichar, MD, PhD, Fakultni nemocnice Olomouc, Olomouc, Katarína Petráková, MD, PhD, Masaryk Memorial Cancer Institute, Brno, Jana Prausova, MD, PhD, MBA, Fakultni nemocnice v Motole, Praha 5; Denmark – Vesna Glavicic, MD, Næstved Sygehus, Næstved, Erik H Jakobsen, MD, Vejle Hospital, Vejle, Julia Kenholm, MD, Regionshospitalet Herning, Herning Centralsygehus, Herning, Sven Langkjer, MD, PhD, Aarhus Universitets Hospital, Aarhus C; Finland – Johanna Mattson, MD, PhD, Helsinki University Central Hospital – Meilahden Sair, Helsinki, Minna Tanner, MD, PhD, MSc, Tampere University Hospital, Tampere; France – Thomas Bachelot, MD, PhD, Centre Leon Berard, Lyon Cedex 08, Etienne Brain, MD, PhD, Institut Curie – Hôpital René Huguenin, Saint-Cloud, Mario Campone, MD, PhD, Centre de Lutte Contre le Cancer – Institut de Cancer, Saint Herblain, Bruno Coudert, MD, and Audrey Hennequin, MD, Centre Georges Francois Leclerc, Dijon, Veronique Dieras, MD, MSc, Centre De Lutte Contre Le Cancer – Institut Curie, Paris, Jean-Marc Ferrero, MD, University Côte d’Azur, Centre Antoine Lacassagne, Nice, Cyril Foa, MD, and Robert Herve, MD, Hôpital Privé Clairval, Marseille, Christelle Levy, MD, Centre François Baclesse, Institut Normand du Sein, Caen Cedex 5, Marie-Ange Mouret-Reynier, MD, Centre Jean Perrin, Clermont-Ferrand Cedex 1, Francesco Ricci, MD, PhD, Centre De Lutte Contre Le Cancer – Institut Curie, Paris; Germany – Bahriye Aktas, MD, PhD, and Oliver Hoffmann, MD, PhD, Universitaetsklinikum Essen – Klinik fuer Frauenheilkunde, Essen, Nikola Bangemann, MD, Campus Charité Mitte, Berlin, Malgorzata J Banys-Paluchowski, MD, and Gerhard Gebauer, MD, MBA, Katholisches Marienkrankenhaus gGmbH – Frauenklinik, Hamburg, Wolfgang Eiermann, MD, Interdisziplinäres Onkologisches Zentrum München, München, Peter A. Fasching, MD, Erlangen University Hospital, Comprehensive Cancer Center Erlangen-EMN, Friedrich Alexander University of Erlangen-Nuremberg, Erlangen, Aristoteles Giagounidis, MD, Marien Hospital Düsseldorf, Düsseldorf, Eva-Maria Grischke, MD, Universitaetsklinikum Tuebingen, Tuebingen, John Hackmann, MD, Marien Hospital, Witten, Meinolf Karthaus, MD, Städtisches Krankenhaus München – Neuperlach, München, Anita Prechtl, MD, Praxis für Frauenheilkunde, München, Andreas Schneeweiss, MD, University Hospital Heidelberg, Heidelberg, Pauline Wimberger, MD, Medizinische Fakultät Carl Gustav Carus, Dresden; Israel – Noa Efrat, MD, Kaplan Medical Center, Rehovot, David Geffen, MD, and Margarita Tokar, MD, Soroka University Medical Center, Beer Sheva, Goldberg Hadassah, MD, Galilee Medical Center, Nahariya, Natalya Karminsky, MD, PhD, The Edith Wolfson Medical Center, Holon, Bella Kaufman, MD, Chaim Sheba Medical Center, Breast Oncology Institute, Ramat Gan, Iryna Kuchuk, MD, Meir Medical Center, Kfar Saba, Michelle Leviov, MD, Clalit Health Services – Lin Medical Center, Haifa, Larisa Ryvo, MD, and Ido Wolf, MD, Tel Aviv Sourasky Medical Center, Tel Aviv, Beatrice Uziely, MD, Hadassah Medical Center, Jerusalem, Rinat Yerushalmi, MD, Davidoff Cancer Center, Rabin Medical Center, Beilinson Hospital, Petah Tikva; Italy – Antonio Ardizzoia, MD, Presidio Ospedaliero “Alessandro Manzoni”, Lecco, Rossana Berardi, MD, Azienda Ospedaliero-Universitaria Ospedali Riuniti Umberto I, GM Lancisi, G Salesi, Ancona, Antonio Bernardo, MD, and Lorenzo Pavesi, MD, Istituti Clinici Scientifici Maugeri, Pavia, Laura Biganzoli, MD, Nuovo Ospedale di Prato Santo Stefano, Prato, Roberto Bordonaro, MD, Presidio Ospedaliero Garibaldi-Nesima, Catania, Marco Colleoni, MD, European Institute of Oncology, Milan, Giuseppe Curigliano, MD, PhD, European Institute of Oncology, University of Milano, Milan, Mauro D’Amico, MD, Ente Ospedaliero Ospedali Galliera, Genova, Bruno Daniele, MD, and Vincenza Tinessa, MD, Azienda Ospedaliera San Pio, Presidio Gaetano Rummo, Benevento, Michelino De Laurentiis, MD, PhD, Istituto Nazionale Tumori “Fondazione Pascale”, Naples, Alfredo Falcone, MD, Azienda Ospedaliero-Universitaria Pisana Santa Chiara, Pisa, Gabriella Farina, MD, and Nicla Maria La Verde, MD, Azienda Ospedaliera Fatebenefratelli e Oftalmico, Milano, Guido Francini, MD, Azienda Ospedaliero Universitaria Senese, Siena, Antonio Frassoldati, MD, Azienda Ospedaliero-Universitaria “Arcispedale Sant’Anna”, Ferrara, Daniele Generali, MD, Presidio Ospedaliero di Cremona, Cremona, Donatella Grasso, MD, and Paolo Pedrazoli, MD, Fondazione IRCCS Policlinico San Matteo, Pavia, Vito Lorusso, MD, Istituto Oncologico Giovanni Paolo II, Bari, Gabriele Luppi, MD, Azienda Ospedaliero-Universitaria Policlinico di Modena, Modena, Paolo Marchetti, MD, Azienda Ospedaliero-Universitaria Sant’Andrea, Roma, Filippo Montemurro, MD, Istituto di Candiolo, Candiolo, Antonino Musolino, MD, PhD, MSc, University of Parma – University Hospital of Parma, Parma, Andrea Rocca, MD, Istituto Scientifico Romagnolo per lo Studio e la Cura die Tumori, Meldola, Elena Rota Caremoli, MD, Azienda Ospedaliera Papa Giovanni XXIII, Bergamo, Enzo Ruggeri, MD, Ospedale Belcolle, Viterbo, Armando Santoro, MD, Istituto Clinico Humanitas, Rozzano, Giuseppe Tonini, MD, Policlinico Universitario Campus Bio-Medico, Roma; Korea – Seock-Ah Im, MD, PhD, Seoul National University Hospital, Cancer Research Institute, Seoul National University College of Medicine, Seoul, Young-Hyuck Im, MD, PhD, Samsung Medical Center, Seoul, Sung-Bae Kim, MD, PhD, Asan Medical Center, Seoul, Joo Hyuk Sohn, MD, PhD, Severance Hospital, Yonsei University Health System, Seoul; the Netherlands – Maaike de Boer, MD, PhD, Maastricht University Medical Center, GROW-School of Oncology and Developmental Biology, Maastricht, Franciscus Erdkamp, MD, Stichting Zuyderland Medisch Centrum, Sittard-Geleen, Daniel Houtsma, MD, and Johanna Portielje, MD, PhD, Haga Ziekenhuis, Leiden, Robbert van Alphen, MD, Elisabeth-Tweesteden Ziekenhuis, Tilburg; Poland – Barbara Bauer-Kosinska, MD, Mazowiecki Szpital Onkologiczny, Wieliszew, Dorota Garncarek-Lange, MD, PhD, Wojewódzki Szpital Specjalistyczny we Wrocławiu, Wroclaw, Bartosz Itrych, MD, and Tomasz Sarosiek, MD, PhD, Magodent Sp. z o.o. Szpital Elblaska, Warszawa, Tomasz Jankowski, MD, PhD, Centrum Onkologii Ziemi Lubelskiej im. sw. Jana z Dukli, Lublin, Zbigniew Nowecki, MD, PhD, Narodowy Instytut Onkologii im. Marii Skłodowskiej-Curie – Państwowy Instytut Badawczy, Warszawa, Tadeusz Pieńkowski, MD, PhD, Radomskie Centrum Onkologii, Warszawa, Piotr Wysocki, MD, PhD, Spzoz Szpital Uniwersytecki w Krakowie, Uniwersyteckie Lecznictwo Szpitalne, Krakow; Portugal – Miguel Abreu, MD, Instituto Português Oncologia Francisco Gentil do Porto, Porto, Fatima Cardoso, MD, and Maria Rita Dionisio, MD, Champalimaud Clinical Center – Champalimaud Foundation, Lisbon, Luis Costa, MD, PhD, Hospital de Santa Maria, Centro Hospitalar Lisboa Norte, Lisbon; Puerto Rico – Mirelis Acosta, MD, Fundacion de Investigacion de Diego, San Juan; Spain – José Alés Martínez, MD, PhD, Hospital Nuestra Señora de Sonsoles, Avila, Begoña Bermejo de las Heras, MD, PhD, Hospital Clínico Universitario de Valencia, Valencia, Beatriz Cirauqui, MD, Institut Català d’Oncologia-Hospital Universitari Germans Trias i Pujol, Badalona, Javier Cortes Castan, MD, PhD, IOB Institute of Oncology, Quironsalud Group, Madrid & Barcelona – Vall d’Hebron Institute of Oncology, Barcelona, Joan Dorca Ribugent, MD, ICO-Hospital Universitari de Girona Dr Josep Trueta, Girona, María Fernández Abad, MD, PhD, Hospital Universitario Ramón y Cajal, Madrid, Laura García Estévez, MD, and Elena Sevillano Fernández, MD, Hospital Madrid Norte Sanchinarro, Madrid, José García Sáenz, MD, Hospital Clínico San Carlos, Madrid, Joaquin Gavilá Gregori, MD, Fundación Instituto Valenciano de Oncología, Valencia, Antonio Gonzalez Martin, MD, PhD, and Raúl Márquez Vázquez, MD, MD Anderson Center Madrid, Madrid, Santiago González Santiago, MD, Hospital San Pedro de Alcántara, Cáceres, José Juan Illarramendi Manas, MD, Hospital de Navarra, Pamplona, Mireia Melé Olivé, MD, Hospital Universitari Sant Joan de Reus, Reus, Serafín Morales Murillo, MD, Hospital Universitari Arnau de Vilanova, Lleida, Laura Palomar Abad, MD, and Ana Santaballa Bertrán, MD, Hospital Universitari i Politècnic La Fe, Valencia, José Pérez García, MD, PhD, Hospital Quirón Barcelona, Barcelona, José Ponce Lorenzo, MD, Hospital General Universitario de Alicante, Alicante, Manuel Ruiz Borrego, MD, Hospital Universitario Virgen del Rocio, Sevilla, Cristina Saura Manich, MD, PhD, Vall d’Hebron University Hospital and Vall d’Hebron Institute of Oncology, Barcelona, Miguel Angel Segui Palmer, MD, PhD, Hospital Universitari Parc Tauli, Sabadell, Sonia Servitja Tormo, MD, PhD, Hospital del Mar, Barcelona; United Kingdom – Pavel Bezecny, MD, DVM, Blackpool Victoria Hospital, Blackpool, Steve Chan, MBBS, DM, Nottingham University Hospital, Nottingham, Amandeep Dhadda, MBChB, MSc, Castle Hill Hospital, Hull, Janine Graham, MBChB, South Tees NHS Foundation Trust, Middlesbrough, Catherine Harper-Wynne, MBBS, MD, CCST, Maidstone Hospital – Kent Oncology Centre, Maidstone, Martin Hogg, MBBS, Royal Preston Hospital, Blackburn, Chiara Intrivici, MD, PhD, United Lincolnshire Hospitals NHS Trust, Boston, Janine Mansi, MBBS, MD, Guys and Saint Thomas NHS Foundation Trust, London; Christopher J. Poole, MBBChir, University Hospital Coventry, Coventry; United States – Apurv Agrawal, MD, New Jersey Hematology Oncology Associates, Brick, NJ, Eugene Ahn, MD, and Dennis Citrin, MBChB, PhD, Cancer Treatment Centers of America at Midwestern Regional Medical Center, Zion, IL, Sramila Aithal, MD, Eastern Regional Medical Center Inc, Philadelphia, PA, Eleni Andreopoulou, MD, and Linda Vahdat, MD, MBA, Breast Center at Weill Cornell Medicine, New York, NY, Shakeela Bahadur, MD, Banner MD Anderson Cancer Center, Gilbert, AZ, Samuel Bailey, MD, and Hassan Ghazal, MD, ARH Cancer Clinic, Hazard, KY, Reema Batra, MD, Sharp Memorial Hospital, San Diego, CA, Chiara Battelli, MD, PhD, New England Cancer Specialists, Scarborough, ME, Thaddeus Beeker, MD, and Michael Meshad, MD, Southern Cancer Center, Mobile, AL, Christiana M. Brenin, MD, University of Virginia Cancer Center, Charlottesville, VA, Ursa Brown-Glaberman, MD, University of New Mexico Comprehensive Cancer Center, Albuquerque, NM, Adam Brufsky, MD, PhD, University of Pittsburgh Medical Center, Pittsburgh, PA, Daniel Bruetman, MD, and Ebenezer Kio, MD, IU Health Goshen Center for Cancer Care, Goshen, IN, Jennifer Carney, MD, MA, Kaiser Permanente Hawaii Moanalua Medical Center, Honolulu, HI, Helen Chew, MD, UC Davis Medical Center – UC Davis Comprehensive Cancer, Sacramento, CA, Marc Citron, MD, ProHealth Care Associates, LLP, Lake Success, NY, Melody Cobleigh, MD, Rush University Medical Center, Chicago, IL, Suzanne Cole, MD, Mercy Clinic Oncology and Hematology, Oklahoma City, OK, Jessica Croley, MD, Saint Joseph Hospital, Lexington, KY, Christopher Croot, MD, Jewish Medical Center Northeast, Louisville, KY, Brooke Daniel, MD, Tennessee Oncology PLLC, Chattanooga, TN, Robert Dichmann, MD, Central Coast Medical Oncology, Santa Maria, CA, Alfred DiStefano, MD, Arlington Cancer Center, Arlington, TX, Tracy Dobbs, MD, Tennessee Cancer Specialists, PLLC, Dowell Springs, Knoxville, TN, Robert Droder, MD, and Arielle Lee, MD, HOPE Cancer Center of East Texas, Tyler, TX, Erin Ellis, MD, Swedish Cancer Institute, Seattle, WA, John Erban, MD, and Rachel Buchsbaum, MD, Tufts Medical Center Cancer Center, Boston, MA, Louis Fehrenbacher, MD, and Jennifer Suga, MD, Kaiser Permanente Medical Center, Vallejo, CA, Trevor Feinstein, MD, Piedmont Cancer Institute, PC, Atlanta, GA, Erin Fleener, MD, Saint Joseph Health Cancer Center, Bryan, TX, William Fusselman, MD, Physicians Clinic of Iowa, Cedar Rapids, IA, Nashat Gabrail, MD, Gabrail Cancer Center Research, Canton, OH, Christopher Gallagher, MD, MedStar Washington Hospital Center, Washington, DC, William J. Gradishar, MD, Northwestern University, Chicago, IL, Deena Graham, MD, Hackensack University Medical Center – John Theurer Cancer Center, Hackensack, NJ, Maria Grosse-Perdekamp, MD, Carle Cancer Center, Urbana, IL, Barbara Haley, MD, University of Texas Southwestern Medical Center, Dallas, TX, Kathleen Harnden, MD, and Mary Wilkinson, MD, Inova Schar Cancer Institute, Fairfax, VA, Lowell Hart, MD, Florida Cancer Specialists & Research Institute, Fort Myers, FL, John Hrom, MD, Hattiesburg Clinic, PA, Hattiesburg, MS, Sara Hurvitz, MD, UCLA Hematology Oncology, Ventura, Los Angeles, CA, Nicholas Iannotti, MD, Hematology/Oncology Associates of the Treasure Coast, Port Saint Lucie, FL, Sujith Kalmadi, MD, Ironwood Cancer & Research Center, Chandler, AZ, Edward Kaplan, MD, Hematology Oncology of the North Shore, Skokie, IL, Peter Kaufman, MD, and Gary Schwartz, MD, Dartmouth-Hitchcock Norris Cotton Cancer Center, Lebanon, NH, Mary Kemeny, MD, Queens Hospital Center, Jamaica, NY, Stephen Kendall, MD, Utah Cancer Specialist, Salt Lake City, UT, Elisa Krill-Jackson, MD, Mount Sinai Comprehensive Cancer Center, Miami Beach, FL, Bradley Lash, MD, Guthrie Medical Group, PC, Corning, NY, Andrew Brown, MD, and Anna Litvak, MD, Cancer Center of Saint Barnabas Medical Center, Livingston, NJ, Philip Lowry, MD, Guthrie Medical Group, PC, Sayre, PA, Kit Lu, MD, UPMC Pinnacle Health Cancer Institute, Harrisburg, PA, Cynthia Lynch, MD, Western Regional Medical Center, Inc, Goodyear, AZ, Ajit Maniam, MD, Pacific Cancer Medical Center, Inc, Anaheim, CA, Monte Martin, MD, Flaget Cancer Center, Bardstown, KY, Samuel McCachren, MD, Suratha Murali, MD, MS, and Wenqing Zhang, MD, PhD, Thompson Cancer Survival Center, Knoxville, TN, Diana Medgyesy, MD, and Ann Stroh, DO, Poudre Valley Health Care, Inc, Fort Collins, CO, Susan Melin, MD, Wake Forest University Baptist, Winston-Salem, NC, Raul Mena, MD, East Valley Hematology and Oncology Medical Group, Burbank, CA, Kathy Miller, MD, Indiana University Health Melvin and Bren Simon Cancer Center, Indianapolis, IN, Aldemar Montero, MD, Gwinnett Medical Center – Center for Cancer Care, Lawrenceville, GA, Mahvish Muzaffar, MD, ECU – Leo W Jenkins Cancer Center, Greenville, NC, Bichlien Nguyen, MD, Long Beach Memorial Medical Center, Bakersfield, CA, Damien Hansra, MD, and Mary Ninan, MD, Southeastern Regional Medical Center, Newnan, GA, Yelena Novik, MD, New York University Clinical Cancer Center, New York, NY, Brian O’Connor, MD, Frederick Memorial Hospital, Frederick, MD, Ira Oliff, MD, Orchard Healthcare Research Inc, Skokie, IL, Raul Oyola, MD, Northwest Georgia Oncology Centers, PC, Marietta, GA, Mark D Pegram, MD, Stanford Comprehensive Cancer Institute, Stanford University School of Medicine, Stanford, CA, Alejandra Perez, MD, University of Miami Sylvester Comprehensive Cancer Center, Plantation, FL, Timothy Pluard, MD, Saint Luke’s Cancer Institute, Kansas City, MO, David Riseberg, MD, Mercy Medical Center, Baltimore, MD, Angel Rodriguez, MD, Austin Cancer Center, Austin, TX, Hope S. Rugo, MD, University of California San Francisco Helen Diller Family Comprehensive Cancer Center, San Francisco, CA, Lupe Salazar, MD, University of Washington – Seattle Cancer Care Alliance, Seattle, WA, Nikita Shah, MD, UF Health Cancer Center at Orlando Health, Orlando, FL, Sagun Shrestha, MD, Southwestern Regional Medical Center Inc, Tulsa, OK, Bethany Sleckman, MD, Mercy Hospital St. Louis, St. Louis, MO, Robert Somer, MD, Cooper University Hospital, Camden, NJ, Scott Sonnier, MD, Touro Infirmary Hospital, Marrero, LA, Elizabeth Tan-Chiu, MD, Florida Cancer Research Institute, Plantation, FL, Saritha Thumma, MD, Cancer Care Northwest, PS, Spokane, WA, Michaela Tsai, MD, Virginia Piper Cancer Institute, Minneapolis, MN, Sonia Varghese, MD, MBBS, Mercy Clinic Oncology and Hematology, Dallas, TX, Sumithra Vattigunta, MD, Palm Beach Cancer Institute, West Palm Beach, FL, Pramvir Verma, MD, Fort Belvoir Community Hospital, Fort Belvoir, VA, Jeanine L Werner, MD, Anne Arundel Medical Center, Annapolis, MD, Gail S. Wright, MD, Florida Cancer Specialists & Research Institute, New Port Richey, FL, Denise A. Yardley, MD, Sarah Cannon Research Institute and Tennessee Oncology PLLC, Nashville, TN, Robyn Young, MD, The Center for Cancer and Blood Disorders, Fort Worth, TX, Andrew Zahalsky, MD, Monongahela Valley Hospital, Monongahela, PA.

Meeting Presentations: This study was presented at the American Society of Clinical Oncology Annual Meeting (May 31-June 4, 2019; Chicago, Illinois) and the San Antonio Breast Cancer Symposium (December 10-14, 2019; San Antonio, Texas).

Data Sharing Statement: See Supplement 3.

Additional Contributions: The authors thank all of the patients, their families, and the entire staff who participated in this trial. Professional medical writing support was provided by Francesca Balordi, PhD, and Emily Cullinan, PhD, of The Lockwood Group, Stamford, Connecticut, USA, in accordance with Good Publication Practice, GPP3 guidelines, with funding by MacroGenics, Inc, Rockville, Maryland, USA.