Tumor Size Is Not Everything: Advancing Radiomics as a Precision Medicine Biomarker in Oncology Drug Development and Clinical Care. A Report of a Multidisciplinary Workshop Coordinated by the RECIST Working Group.

Radiomics, the science of extracting quantifiable data from routine medical images, is a powerful tool that has many potential applications in oncology. The Response Evaluation Criteria in Solid Tumors Working Group (RWG) held a workshop in May 2022, which brought together various stakeholders to discuss the potential role of radiomics in oncology drug development and clinical trials, particularly with respect to response assessment. This article summarizes the results of that workshop, reviewing radiomics for the practicing oncologist and highlighting the work that needs to be done to move forward the incorporation of radiomics into clinical trials.

Pembrolizumab plus chemotherapy versus chemotherapy in untreated advanced pleural mesothelioma in Canada, Italy, and France: a phase 3, open-label, randomised controlled trial.

BACKGROUND: Pleural mesothelioma usually presents at an advanced, incurable stage. Chemotherapy with platinum-pemetrexed is a standard treatment. We hypothesised that the addition of pembrolizumab to platinum-pemetrexed would improve overall survival in patients with pleural mesothelioma. METHODS: We did this open-label, international, randomised phase 3 trial at 51 hospitals in Canada, Italy, and France. Eligible participants were aged 18 years or older, with previously untreated advanced pleural mesothelioma, with an Eastern Cooperative Oncology Group performance status score of 0 or 1. Patients were randomly assigned (1:1) to intravenous chemotherapy (cisplatin [75 mg/m2] or carboplatin [area under the concentration-time curve 5-6 mg/mL per min] with pemetrexed 500 mg/m2, every 3 weeks for up to 6 cycles), with or without intravenous pembrolizumab 200 mg every 3 weeks (up to 2 years). The primary endpoint was overall survival in all randomly assigned patients; safety was assessed in all randomly assigned patients who received at least one dose of study therapy. This trial is registered with ClinicalTrials.gov, NCT02784171, and is closed to accrual. FINDINGS: Between Jan 31, 2017, and Sept 4, 2020, 440 patients were enrolled and randomly assigned to chemotherapy alone (n=218) or chemotherapy with pembrolizumab (n=222). 333 (76 %) of patients were male, 347 (79%) were White, and median age was 71 years (IQR 66-75). At final analysis (database lock Dec 15, 2022), with a median follow-up of 16·2 months (IQR 8·3-27·8), overall survival was significantly longer with pembrolizumab (median overall survival 17·3 months [95% CI 14·4-21·3] with pembrolizumab vs 16·1 months [13·1-18·2] with chemotherapy alone, hazard ratio for death 0·79; 95% CI 0·64-0·98, two-sided p=0·0324). 3-year overall survival rate was 25% (95% CI 20-33%) with pembrolizumab and 17% (13-24%) with chemotherapy alone. Adverse events related to study treatment of grade 3 or 4 occurred in 60 (27%) of 222 patients in the pembrolizumab group and 32 (15%) of 211 patients in the chemotherapy alone group. Hospital admissions for serious adverse events related to one or more study drugs were reported in 40 (18%) of 222 patients in the pembrolizumab group and 12 (6%) of 211 patients in the chemotherapy alone group. Grade 5 adverse events related to one or more drugs occurred in two patients on the pembrolizumab group and one patient in the chemotherapy alone group. INTERPRETATION: In patients with advanced pleural mesothelioma, the addition of pembrolizumab to standard platinum-pemetrexed chemotherapy was tolerable and resulted in a significant improvement in overall survival. This regimen is a new treatment option for previously untreated advanced pleural mesothelioma. FUNDING: The Canadian Cancer Society and Merck & Co.

Association of Molecular Profiles and Mutational Status With Distinct Histological Lung Adenocarcinoma Subtypes. An Analysis of the LACE-Bio Data.

BACKGROUND: Adjuvant chemotherapy (AC) is indicated for stage II and stage III lung adenocarcinomas (ADC). Using the LACE Bio II database, we analyzed the distribution of various mutations across the subtypes of ADCs and studied the prognostic and predictive roles of PD-L1, TMB, and Tumor Infiltrating Lymphocytes (TILs). MATERIALS AND METHODS: Clinical and genomic data from the LACE Bio II data were extracted. Patients were divided into ADC subtypes, in which the grouping was done based on their known clinical behavior (Lepidic [LEP], Acinar/Papillary [ACI or PAP], Micropapillary/Solid [MIP or SOL], Mucinous [MUC] and Others). Kaplan-Meier (KM) and log-rank test were used to compare survival based on PD-L1, TMB, TILs and combinations of TMB with PD-L1 and TILs. Adjusted Hazard Ratios (HR) were analyzed with Overall Survival (OS), Disease-Free Survival (DFS) and Lung Cancer-Specific Survival (LCSS) as endpoints. RESULTS: A total of 375 ADC patients were identified. MIP/SOL was the subtype most commonly positive for various biomarkers. PD-L1 Negative/high TMB was associated with better outcomes in terms of OS (HR = 0.46 [0.23-0.89], P = .021) and DFS (HR = 0.52 [0.30-0.90], P = .02), relative to PD-L1 Negative/low TMB. High TMB predicted worse outcome with AC use in terms of OS (ratio of hazard ratio rHR = 2.75 [1.07-7.04], P = .035). Marked TILs had better outcome with AC for DFS (rHR = 0.22 [0.06-0.87], P = .031 and LCSS (rHR = 0.08 [0.01-0.66], P = .019) respectively. There was also a beneficial effect of AC among patients with Marked TILs/low TMB in terms of DFS (rHR = 0.06 [0.01-0.53], P = .011). CONCLUSION: High TMB has a prognostic role in resectable lung ADC. The high TMB group had a poor outcome with AC, suggesting that this group may be better served with immune checkpoint therapy.

Brief Report: Canadian Cancer Trials Group IND.227: A Phase 2 Randomized Study of Pembrolizumab in Patients With Advanced Malignant Pleural Mesothelioma (NCT02784171).

Immune checkpoint inhibitors have activity in mesothelioma. IND.227 was a phase 2 trial (120 patients planned) comparing progression-free survival of standard platinum and pemetrexed (CP) versus CP + pembrolizumab (pembro) versus pembro. Accrual to the pembro arm was discontinued on the basis of interim analysis (IA-16 wk disease control rate). CP + pembro was tolerable, with progression-free survival similar between arms and median survival and overall response rate higher than those of CP alone (19.8 mo [95% confidence interval or CI: 8.4-41.36] versus 8.9 mo [95% CI: 5.3-12.8] and 47% [95% CI: 24%-71%] versus 19% [95% CI: 5%-42%], respectively). The subsequent phase 3 trial has completed accrual; results are expected in 2023.

Meta-Analysis of the Test-Retest Repeatability of [18F]-Fluorodeoxyglucose Standardized Uptake Values: Implications for Assessment of Tumor Response.

PURPOSE: Currently, guidelines for PET with 18F-fluorodeoxyglucose (FDG-PET) interpretation for assessment of therapy response in oncology primarily involve visual evaluation of FDG-PET/CT scans. However, quantitative measurements of the metabolic activity in tumors may be even more useful in evaluating response to treatment. Guidelines based on such measurements, including the European Organization for Research and Treatment of Cancer Criteria and PET Response Criteria in Solid Tumors, have been proposed. However, more rigorous analysis of response criteria based on FDG-PET measurements is needed to adopt regular use in practice. EXPERIMENTAL DESIGN: Well-defined boundaries of repeatability and reproducibility of quantitative measurements to discriminate noise from true signal changes are a needed initial step. An extension of the meta-analysis from de Langen and colleagues (2012) of the test-retest repeatability of quantitative FDG-PET measurements, including mean, maximum, and peak standardized uptake values (SUVmax, SUVmean, and SUVpeak, respectively), was performed. Data from 11 studies in the literature were used to estimate the relationship between the variance in test-retest measurements with uptake level and various study-level, patient-level, and lesion-level characteristics. RESULTS: Test-retest repeatability of percentage fluctuations for all three types of SUV measurement (max, mean, and peak) improved with higher FDG uptake levels. Repeatability in all three SUV measurements varied for different lesion locations. Worse repeatability in SUVmean was also associated with higher tumor volumes. CONCLUSIONS: On the basis of these results, recommendations regarding SUV measurements for assessing minimal detectable changes based on repeatability and reproducibility are proposed. These should be applied to differentiate between response categories for a future set of FDG-PET-based criteria that assess clinically significant changes in tumor response.

Results of the phase I CCTG IND.231 trial of CX-5461 in patients with advanced solid tumors enriched for DNA-repair deficiencies.

CX-5461 is a G-quadruplex stabilizer that exhibits synthetic lethality in homologous recombination-deficient models. In this multicentre phase I trial in patients with solid tumors, 40 patients are treated across 10 dose levels (50-650 mg/m2) to determine the recommended phase II dose (primary outcome), and evaluate safety, tolerability, pharmacokinetics (secondary outcomes). Defective homologous recombination is explored as a predictive biomarker of response. CX-5461 is generally well tolerated, with a recommended phase II dose of 475 mg/m2 days 1, 8 and 15 every 4 weeks, and dose limiting phototoxicity. Responses are observed in 14% of patients, primarily in patients with defective homologous recombination. Reversion mutations in PALB2 and BRCA2 are detected on progression following initial response in germline carriers, confirming the underlying synthetic lethal mechanism. In vitro characterization of UV sensitization shows this toxicity is related to the CX-5461 chemotype, independent of G-quadruplex synthetic lethality. These results establish clinical proof-of-concept for this G-quadruplex stabilizer. Clinicaltrials.gov NCT02719977.

iRECIST and atypical patterns of response to immuno-oncology drugs.

With the advent of immunotherapy as one of the keystones of the treatment of our patients with cancer, a number of atypical patterns of response to these agents has been identified. These include pseudoprogression, where the tumor initially shows objective growth before decreasing in size, and hyperprogression, hypothesized to be a drug-induced acceleration of the tumor burden. Despite it being >10 years since the first immune-oncology drug was approved, neither the biology behind these paradoxical responses has been well understood, nor their incidence, identification criteria, predictive biomarkers, or clinical impact have been fully described. Immune-based Response Evaluation Criteria in Solid Tumors (iRECIST) guidelines have been published as a revision to the RECIST V.1.1 criteria for use in trials of immunotherapeutics, and the iRECIST subcommittee (of the RECIST Working Group) is working on elucidating these aspects, with data sharing a current major challenge to move forward with this unmet need in immuno-oncology.

Complementary Medicine Use Amongst Patients with Metastatic Cancer Enrolled in Phase III Clinical Trials.

BACKGROUND: Complementary medicines (CM) are frequently used by patients with cancer. Controversy exists over the effectiveness and risk that CM may add to conventional cancer therapy. The incidence of CM use among patients enrolled in phase III clinical trials is unknown. METHODS: Medication lists from 6 international phase III clinical trials were retrospectively reviewed to identify patients using CM. Patients had metastatic breast, colorectal, or lung cancers. Quality of life, adverse events, overall survival, and progression-free survival were compared between CM users and non-users. Baseline differences between groups were adjusted with propensity score matching groups. RESULTS: Seven hundred and six of 3446 patients (20.5%) used at least one CM. CM use was highest among patients with breast cancer (35.6%). CM users had more favorable baseline prognostic factors (ECOG 0-1, non-smoking status, younger age, and fewer metastases). CM use was associated with lower rates of adverse events (50% vs. 62%, P = .002) and quality of life was similar between both groups. After adjustment with propensity score matching, CM use was also associated with longer overall survival in patients with lung cancer (adjusted hazard ratio 0.80, 95%CI, 0.68-0.94, P =.0054). However, several key control variables like EGFR status were not available. CONCLUSION: One in 5 patients in phase III clinical trials report using CM. CM was not associated with worse cancer-specific outcomes. However, CM users had more favorable baseline prognostic factors, and likely other confounders that may have contributed to improved outcomes observed in the lung cohort. Physicians should monitor for CM use and potential interactions with clinical trial drugs.

Prognostic and predictive effect of KRAS gene copy number and mutation status in early stage non-small cell lung cancer patients.

BACKGROUND: In the current analysis, we characterize the prognostic significance of KRAS mutations with concomitant copy number aberrations (CNA) in early stage non-small cell lung cancer (NSCLC), and evaluate the ability to predict survival benefit from adjuvant chemotherapy. METHODS: Clinical and genomic data from the LACE (Lung Adjuvant Cisplatin Evaluation)-Bio consortium was utilized. CNAs were categorized as Gain (CN ≥2) or Neutral (Neut)/Loss; KRAS status was defined as wild type (WT) or mutant (MUT). The following groups were compared in all patients and the adenocarcinoma subgroup, and were correlated to survival endpoints using a Cox proportional hazards model: WT + Neut/Loss (reference), WT + Gain, MUT + Gain and MUT + Neut/Loss. A treatment-by-variable interaction was added to evaluate predictive effect. RESULTS: Of the 946 (399 adenocarcinoma) NSCLC patients, 41 [30] had MUT + Gain, 145 [99] MUT + Neut/Loss, 125 [16] WT + Gain, and 635 [254] WT + Neut/Loss. A non-significant trend towards worse lung cancer-specific survival (LCSS; HR =1.34; 95% CI, 0.83-2.17, P=0.232), DFS (HR =1.34; 95% CI, 0.86-2.09, P=0.202) and OS (HR =1.59; 95% CI, 0.99-2.54, P=0.055) was seen in KRAS MUT + Gain patients relative to KRAS WT + Neut/Loss patients. A negative prognostic effect of KRAS MUT + Neut/Loss was observed for LCSS (HR =1.32; 95% CI, 1.01-1.71, P=0.038) relative to KRAS WT + Neut/Loss on univariable analysis, but to a lesser extent after adjusting for covariates (HR =1.28; 95% CI, 0.97-1.68, P=0.078). KRAS MUT + Gain was associated with a greater beneficial effect of chemotherapy on DFS compared to KRAS WT + Neut/Loss patients (rHR =0.33; 95% CI, 0.11-0.99, P=0.048), with a non-significant trend also seen for LCSS (rHR =0.41; 95% CI, 0.13-1.33, P=0.138) and OS (rHR =0.40; 95% CI, 0.13-1.26, P=0.116) in the adenocarcinoma subgroup. CONCLUSIONS: A small prognostic effect of KRAS mutation was identified for LCSS, and a trend towards worse LCSS, DFS and OS was noted for KRAS MUT + Gain. A potential predictive effect of concomitant KRAS mutation and copy number gain was observed for DFS in adenocarcinoma patients. These results could be driven by the small number of patients and require validation.

Metabolites as Prognostic Markers for Metastatic Non-Small Cell Lung Cancer (NSCLC) Patients Treated with First-Line Platinum-Doublet Chemotherapy.

The metabolic requirements of metastatic non-small cell lung (mNSCLC) tumors from patients receiving first-line platinum-doublet chemotherapy are hypothesized to imprint a blood signature suitable for survival prediction. Pre-treatment samples prospectively collected at baseline from a randomized phase III trial were assayed using nuclear magnetic resonance (NMR) spectroscopy (n = 341) and ultra-high performance liquid chromatography - mass spectrometry (UPLC-MS) (n = 297). Distributions of time to event outcomes were estimated by Kaplan-Meier analysis, and baseline characteristics adjusted Cox regression modeling was used to correlate markers' levels to time to event outcomes. Sixteen polar metabolites were significantly correlated with overall survival (OS) by univariate analysis (p < 0.025). Formate, 2-hydroxybutyrate, glycine and myo-inositol were selected for a multivariate model. The median OS was 6.6 months in the high-risk group compared to 11.4 months in the low risk group HR (Hazard Ratio) = 1.99, 95% C.I. (Confidence Interval) 1.45-2.68; p < 0.0001). Modeling of lipids by class (sphingolipids, acylcarnitines and lysophosphatidylcholines) revealed a median OS = 5.7 months vs. 11. 9 months for the high vs. low risk group. (HR: 2.23, 95% C.I. 1.55-3.20; p < 0.0001). These results demonstrate that metabolic profiles from pre-treatment samples may be useful to stratify clinical outcomes for mNSCLC patients receiving chemotherapy. Genomic and longitudinal measurements pre- and post-treatment may yield addition information to personalize treatment decisions further.

A phase IB study of durvalumab with or without tremelimumab and platinum-doublet chemotherapy in advanced solid tumours: Canadian Cancer Trials Group Study IND226.

This multicenter phase Ib study aimed to establish a recommended phase II dose for durvalumab (Du) ± tremelimumab (Tr) in combination with standard platinum-doublet chemotherapy. Eligible patients were enrolled into one of six dose levels (DL) of Du ± Tr which included concomitant treatment with standard platinum-doublet regimens; (pemetrexed, gemcitabine, etoposide, (each with cisplatin or carboplatin) or nab-paclitaxel (with carboplatin)). Dose escalation was according to a Rolling Six type design. Both weight-based and fixed dosing of Du and Tr were explored. Du was continued until progression. Tr dosing was finite (up to 6 doses) with increasing dose and/or frequency by DL. 136 patients were enrolled. The majority of drug-related adverse events (AEs) were ≤ grade 2 and attributable to chemotherapy. AEs considered related to immunotherapy were mainly ≤ grade 2; the most frequent (occurring ≥10 %) were colitis/diarrhea, skin, and thyroid dysfunction. Seven patients had DLTs including pneumonitis, myocarditis, diarrhea, encephalitis, motor neuropathy, and enterocolitis. There were 2 treatment-related deaths. Tr and Du exposures did not appear affected by chemotherapy. Among the 73 non-small cell lung cancer (NSCLC) patients treated, the objective response rate was 51 % (95 %CI = 38.7-62.6 %) with a median progression-free and overall survival of 6.5 months (95 % CI = 5.5-9.4 months) and 19.8 months (95 % CI = 14.8 months - not yet reached) respectively. Anti-tumour activity was observed across PD-L1 subtypes. Du 1500 mg q3w and Tr 75 mg q3wx5 can be safely combined with platinum-doublet chemotherapy. Efficacy among NSCLC patients appears comparable to results from other immunotherapy and chemotherapy combination trials. NCT02537418.

Design and Conduct of Early Clinical Studies of Immunotherapy: Recommendations from the Task Force on Methodology for the Development of Innovative Cancer Therapies 2019 (MDICT).

PURPOSE: To review key aspects of the design and conduct of early clinical trials (ECT) of immunotherapy agents. EXPERIMENTAL DESIGN: The Methodology for the Development of Innovative Cancer Therapies Task Force 2019 included experts from academia, nonprofit organizations, industry, and regulatory agencies. The review focus was on methodology for ECTs testing immune-oncology therapies (IO) used in combination with other IO or chemotherapy. RESULTS: Although early successes have been seen, the landscape continues to be very dynamic, and there are ongoing concerns regarding the capacity to test all new drugs and combinations in clinical trials. CONCLUSIONS: Optimization of drug development methodology is required, taking into account early, late, and lower grade intolerable toxicities, novel response patterns, as well as pharmacodynamic data.

A Canadian cancer trials group phase IB study of durvalumab (anti-PD-L1) plus tremelimumab (anti-CTLA-4) given concurrently or sequentially in patients with advanced, incurable solid malignancies.

Background The IND.226 study was a phase Ib study to determine the recommended phase II dose of durvalumab + tremelimumab in combination with standard platinum-doublet chemotherapy. Sequential administration of multiple agents increases total chair time adding costs overall and inconvenience for patients. This cohort of the IND.226 study evaluated the safety and tolerability of durvalumab + tremelimumab given either sequentially (SEQ) or concurrently (CON). Methods Patients with advanced solid tumours were enrolled and randomised to either SEQ tremelimumab 75 mg IV over 1 h followed by durvalumab 1500 mg IV over 1 h q4wks on the same day, or CON administration over 1 h. The serum pharmacokinetic profile of SEQ versus CON of durvalumab and tremelimumab administration was also evaluated. Results 14 patients either received SEQ (n = 7pts) or CON (n = 7 pts). There were no infusion related reactions. Drug related adverse events (AEs) were mainly low grade and manageable, and comparable in frequency between SEQ/CON- fatigue (43%/57%), rash (43%/43%), pruritus (43%/29%) and nausea (14%/29%). One patient in each cohort discontinued treatment due to toxicity. The PK profiles of durvalumab and tremelimumab were similar between CON and SEQ, and to historical reference data. Conclusions Concurrent administration of durvalumab and tremelimumab over 1 h is safe with a comparable PK profile to sequential administration.

A phase Ib study of a PI3Kinase inhibitor BKM120 in combination with panitumumab in patients with KRAS wild-type advanced colorectal cancer.

Background Resistance to Epidermal Growth Factor inhibition (EGFRi) in patients with KRAS wild-type (wt) Colorectal Cancer (CRC) may occur as a result of PI3K/AKT/mTOR signaling. We conducted a study to establish the recommended phase II dose (RP2D) and response rate of panitumumab, an EGFRi, plus BKM120, a PI3K inhibitor, in advanced CRC. Methods Patients with chemotherapy refractory KRAS wt CRC, who were EGFRi naive were enrolled. A 3 + 3 dose escalation design was utilized. The starting dose of panitumumab was 6 mg/kg iv every 2 weeks with BKM120 at 60 mg oral daily. Results Nineteen patients were treated and 17 were evaluable for response. The starting dose was not tolerable (mucositis, fatigue). At dose level (DL) 1, three of six patients discontinued treatment due to toxicity, DL - 1 had no significant toxicity. Panitumumab 6 mg/kg iv q 2 weeks with BKM120 60 mg given 5 out of 7 days per week was declared the RP2D. One patient (5.9%) who was PTEN and PIK3CA negative by IHC had a partial response, seven had stable disease, and nine had disease progression. Conclusion Panitumumab (6 mg/kg iv q 2 weeks) with BKM120 60 mg given 5 out of 7 days per week was declared the RP2D. Toxicities including fatigue, rash and mucositis. There was little evidence of activity in this biomarker unselected cohort.

A phase I study of vistusertib (dual mTORC1/2 inhibitor) in patients with previously treated glioblastoma multiforme: a CCTG study.

The PI3K/AKT/mTOR pathway activation plays a central role in glioblastoma multiforme (GBM) development and progression, and in resistance to anti-cancer therapies. Inhibition of the PI3K pathway has been shown to sensitize cultured glioma cells and tumor xenografts to the effects of temozolomide (TMZ) and radiation. Vistusertib is an oral inhibitor of mTORC1/2 complexes. The primary objective of this Canadian Cancer Trials Group phase I study was to determine the recommended phase II dose (RP2D) of vistusertib in patients with GBM receiving TMZ at first progression following primary treatment. Vistusertib was administered at a starting dose of 100 mg bid 2 days on/5 days off weekly with TMZ 150 mg/m2 daily for 5 days/28-days cycle. Dose escalation was according to a 3 + 3 design. Secondary objectives included assessment of vistusertib safety and toxicity profile, and preliminary efficacy. 15 patients were enrolled in the study (median age 66 (range 51-77), females 8). Vistusertib 125 mg BID in combination with TMZ 150 mg/m2 daily for 5 days was well tolerated. Vistusertib treatment-related adverse events were generally grade 1-2, with the most frequently reported being fatigue, gastrointestinal symptoms, and rash. Of 13 response evaluable patients, 1 patient (8%) had a partial response ongoing at 7.6 months of follow-up, and 5 patients had stable disease (38%) as best response (median duration 9.6 months, range 3.7-not yet reached). Six-month progression-free survival (PFS) rate was 26.6%. Combination of vistusertib with TMZ in GBM patients at first recurrence demonstrated a favorable safety profile at the tested dose levels.

A Phase Ib Trial of Durvalumab in Combination with Trastuzumab in HER2-Positive Metastatic Breast Cancer (CCTG IND.229).

BACKGROUND: Immune checkpoint inhibitors are active in a broad range of cancers, including programmed death ligand 1 (PD-L1)-positive, triple-negative, metastatic breast cancer (MBC). Antibody-dependent cell-mediated cytotoxicity is a mechanism of action of trastuzumab. We performed a phase Ib trial of durvalumab and trastuzumab in HER2-positive MBC previously treated with chemotherapy and anti-HER2 antibodies to assess safety, efficacy, and correlative endpoints. PATIENTS AND METHODS: Patients with HER2-positive MBC were enrolled on a standard 3 + 3 design. Dose level 1 was durvalumab (1,125 mg intravenously day 1) and trastuzumab (8 mg/kg intravenously loading, then 6 mg/kg day 1) on a q3 weekly cycle. An expansion cohort at the recommended phase II dose (RP2D) performed tumor biopsies at baseline and after cycle 1. The primary endpoint was to establish the RP2D. RESULTS: Fifteen patients were accrued from April to December 2016, of which 14 were evaluable for response. Median age was 54 years (range 40-86); the majority had visceral disease (87%) and at least three prior (adjuvant and/or metastatic) lines of chemotherapy (73%), including trastuzumab (93%), pertuzumab (60%), and trastuzumab-emtansine (93%) for MBC. No dose-limiting toxicities were observed at dose level 1 (n = 6) or dose expansion (n = 9) during cycle 1. One patient developed a grade ≥3 immune-related adverse event (grade 4 diabetes mellitus). No responses by RECIST were seen, with 4 of 14 patients (29%) demonstrating stable disease as best response at week 6 (median duration, 2.7 months). All patients had <1% PD-L1 expression on either archival tissue (7/15) or prestudy biopsy (8/15). In the dose expansion cohort, evaluable pretreatment and on-treatment tumor biopsies (n = 5) showed minimal CD8 cell infiltration. CONCLUSION: The RP2D of durvalumab and trastuzumab is standard full doses of both agents. No significant clinical activity was observed in patients with heavily pretreated HER2-positive PD-L1-negative MBC. IMPLICATIONS FOR PRACTICE: This phase Ib trial with associated correlative endpoints provides insights into the lack of activity of the combination of durvalumab and trastuzumab in heavily pretreated HER2-positive metastatic breast cancer (MBC). No significant clinical activity was observed in patients with heavily pretreated HER2-positive programmed death ligand 1 (PD-L1)-negative MBC with evidence of cytotoxic T-cell exhaustion. Furthermore, all patients had no expression of PD-L1 in the tumor cells. These data support the importance of PD-L1 as an important selection biomarker and the need to assess the tumor microenvironment for immune regulatory cells. Further work is needed to understand how to activate the "cold" tumors to be able to combine current immune-oncology agents.