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Receptor tyrosine-protein kinase ERBB3 (HER3) is expressed in most EGFR-mutated lung cancers but is not a known mechanism of resistance to EGFR inhibitors. HER3-DXd is an antibody–drug conjugate consisting of a HER3 antibody attached to a topoisomerase I inhibitor payload via a tetrapeptide-based cleavable linker. This phase I, dose escalation/expansion study included patients with locally advanced or metastatic EGFR-mutated non–small cell lung cancer (NSCLC) with prior EGFR tyrosine kinase inhibitor (TKI) therapy. Among 57 patients receiving HER3-DXd 5.6 mg/kg intravenously once every 3 weeks, the confirmed objective response rate by blinded independent central review (Response Evaluation Criteria in Solid Tumors v1.1) was 39% [95% confidence interval (CI), 26.0–52.4], and median progression-free survival was 8.2 (95% CI, 4.4–8.3) months. Responses were observed in patients with known and unknown EGFR TKI resistance mechanisms. Clinical activity was observed across a broad range of HER3 membrane expression. The most common grade ≥3 treatment-emergent adverse events were hematologic toxicities. HER3-DXd has clinical activity in EGFR TKI–resistant cancers independent of resistance mechanisms, providing an approach to treat a broad range of drug-resistant cancers. Significance: In metastatic EGFR-mutated NSCLC, after disease progression on EGFR TKI therapy, treatment approaches include genotype-directed therapy targeting a known resistance mechanism or chemotherapy. HER3-DXd demonstrated clinical activity spanning known and unknown EGFR TKI resistance mechanisms. HER3-DXd could present a future treatment option agnostic to the EGFR TKI resistance mechanism. See related commentary by Lim et al., p. 16. This article is highlighted in the In This Issue feature, p. 1

Background: I-SPY2 is a multicenter, phase 2 trial using response-adaptive randomization within biomarker subtypes to evaluate novel agents as neoadjuvant therapy for high-risk breast cancer. The primary endpoint is pathologic complete response (pCR) at surgery. The goal is to identify (graduate) regimens with ≥ 85% Bayesian predictive probability of success (i.e., demonstrating superiority to control) in a future 300-patient phase 3 1:1 randomized neoadjuvant trial with pCR endpoint within signatures defined by hormone-receptor (HR), HER2, and MammaPrint (MP) status. Regimens may leave the trial for futility (< 10% probability of success), maximum sample size accrual (with probability of success ≥ 10% and < 85%), or safety concerns as recommended by the independent DSMB. For HER2+ patients, the I-SPY2 control arm was 12 weekly cycles of paclitaxel+trastuzumab (TH, control) followed by doxorubicin/cyclophosphamide (AC) q2-3 weeks x4 and surgery. Patritumab is a fully human monoclonal antibody that inhibits HER3. In this experimental arm for HER2+ patients, patritumab was given q3w x 4 cycles (18mg/kg loading dose followed by 9mg/kg/dose) concurrent with paclitaxel and trastuzumab q1w x 12 weeks (PTH, treatment), followed by AC q2-3w. Methods: Women with tumors ≥ 2.5cm were eligible for screening. MP low/HR+ tumors were ineligible. MRI scans (baseline, 3 weeks after start of therapy, prior to AC, and prior to surgery) were used in a longitudinal statistical model to predict pCR for individual patients. Analysis was intention to treat. Patients who switched to non-protocol therapy count as non-pCR. Patients on treatment arm therapy at the time of arm closure are non-evaluable. Graduation potential was in 3 of 10 pre-defined signatures: all HER2+, HR-/HER2+, and HR+/HER2+. Results: The PTH regimen was stopped at the recommendation of the Safety Working Group and DSMB based on a safety event (bilateral sensorineural hearing loss, Gr 3) observed in one patient. At the time of arm closure, N=31 patients had received PTH treatment; 4 patients receiving PTH were changed to non-protocol therapy and removed from the analysis. The final estimated pCR report will consider 27 PTH and 31 TH as evaluable patients. Accrual was insufficient to assess graduation, however, there appears to be good signal in the HER2+HR- but not HER2+HR+ signatures. I-SPY 2 TRIAL Est. pCR at time of arm closureSignaturesPTH (Treatment)N= 31TH (Control)N = 31All (HER2+)0.40 (0.22 - 0.59), n=310.23 (0.09 - 0.37), n=31HR-/HER2+0.64 (0.36 - 0.91), n=110.30 (0.12 - 0.47), n=12HR+/HER2+0.28 (0.08 - 0.48), n=200.20 (0.06 - 0.34), n=19 HR+/HER2+0.28 (0.08 - 0.48), n=200.20 (0.06 - 0.34), n=19The patient who developed Gr3 sensorineural hearing loss 6 days after the 2nd patritumab (and 4th paclitaxel/trastuzumab) treatment, did not recover her hearing after patritumab was stopped, and also reported Gr3 vulvovaginal pain, vulvitis, and vaginal inflammation. Other gynecological symptoms in the PTH arm include: 1 pt with Gr1 vaginal hemorrhage, and 1 pt with Gr2 dyspareunia. There was a higher frequency of Gr3 hypokalaemia (12.5% vs. 3.2%). One pt in the PTH arm reported Gr3 small intestinal obstruction which resolved with conservative management. Conclusion: The I-SPY 2 study aims to assess the probability that investigational regimens will be successful in a phase 3 neoadjuvant trial; PTH was stopped due to safety concerns, although there was activity in the HER2+ HR- signature. This is the first report of Gr3 hearing loss associated with patritumab/paclitaxel/trastuzumab, and thus attribution is uncertain. Citation Format: Teresa L Helsten, Shelly S Lo, Christina Yau, Kevin Kalinsky, Anthony D Elias, Anne M Wallace, A. Jo Chien, Janice Lu, Julie E Lang, Kathy S Albain, Erica Stringer-Reasor, Amy S Clark, Judy C Boughey, Erin D Ellis, Douglas Yee, Angela DeMichele, Claudine Isaacs, Jane Perlmutter, Hope S Rugo, Richard Schwab, Nola M. Hylton, W. Fraser Symmans, Michelle E Melisko, Laura J van't Veer, Amy Wilson, Ruby Singhrao, Smita M Asare, Ashish Sanil, Donald A Berry, Laura J Esserman. Evaluation of patritumab/paclitaxel/trastuzumab over standard paclitaxel/trastuzumab in early stage, high-risk HER2 positive breast cancer: Results from the neoadjuvant I-SPY 2 trial [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P3-11-02.

Resistance of progressive cancers against chemotherapy is a serious clinical problem. In this context, human epidermal growth factor receptor 3 (HER3) can play important roles in drug resistance to HER1- and HER2- targeted therapies. Since clinical testing of anti-HER3 monoclonal antibodies (mAbs) such as patritumab could not show remarkable effect compared with existing drugs, we generated novel mAbs against anti-HER3. Novel rat mAbs reacted with HEK293 cells expressing HER3, but not with cells expressing HER1, HER2 or HER4. Specificity of mAbs was substantiated by the loss of mAb binding with knockdown by siRNA and knockout of CRISPR/Cas9-based genome-editing. Analyses of CDR sequence and germline segment have revealed that seven mAbs are classified to four groups, and the binding of patritumab was inhibited by one of seven mAbs. Seven mAbs have shown reactivity with various human epithelial cancer cells, strong internalization activity of cell-surface HER3, and inhibition of NRG1 binding, NRG1-dependent HER3 phosphorylation and cell growth. Anti-HER3 mAbs were also reactive with in vivo tumor tissues and cancer tissue-originated spheroid. Ab4 inhibited in vivo tumor growth of human colon cancer cells in nude mice. Present mAbs may be superior to existing anti-HER3 mAbs and support existing anti-cancer therapeutic mAbs.

Purpose: HER3 is a compelling target for cancer treatment; however, no HER3-targeted therapy is currently clinically available. Here, we produced U3-1402, an anti-HER3 antibody–drug conjugate with a topoisomerase I inhibitor exatecan derivative (DXd), and systematically investigated its targeted drug delivery potential and antitumor activity in preclinical models. Experimental Design: In vitro pharmacologic activities and the mechanisms of action of U3-1402 were assessed in several human cancer cell lines. Antitumor activity of U3-1402 was evaluated in xenograft mouse models, including patient-derived xenograft (PDX) models. Safety assessments were also conducted in rats and monkeys. Results: U3-1402 showed HER3-specific binding followed by highly efficient cancer cell internalization. Subsequently, U3-1402 was translocated to the lysosome and released its payload DXd. While U3-1402 was able to inhibit HER3-activated signaling similar to its naked antibody patritumab, the cytotoxic activity of U3-1402 in HER3-expressing cells was predominantly mediated by released DXd through DNA damage and apoptosis induction. In xenograft mouse models, U3-1402 exhibited dose-dependent and HER3-dependent antitumor activity. Furthermore, U3-1402 exerted potent antitumor activity against PDX tumors with HER3 expression. Acceptable toxicity was noted in both rats and monkeys. Conclusions: U3-1402 demonstrated promising antitumor activity against HER3-expressing tumors with tolerable safety profiles. The activity of U3-1402 was driven by HER3-mediated payload delivery via high internalization into tumor cells.

HER3 is overexpressed in several cancers, including colorectal cancer. Although therapies with anti-HER3 antibodies have been investigated, significant clinical benefits have not been reported. U3-1402 is a novel HER3-antibody–drug conjugate (ADC) composed of the HER3 antibody patritumab and a novel topoisomerase I inhibitor, DX-8951 derivative (DXd). The sensitivity of DXd was evaluated by a growth inhibition assay. The antitumor activity of U3-1402 was evaluated in a murine xenograft model in which its effects on cells, with a range of HER3 expression levels, were compared with those of patritumab alone, irinotecan, control-ADC, and saline. In the growth inhibition assay, all colorectal cancer cell lines were sensitive to DXd. In the tumor xenograft model, significant tumor regression with U3-1402 was observed both in the DiFi cell line (high HER3 expression; KRAS wild type) and in SW620 (high HER3 expression; KRAS mutation), but no treatment effect was observed in Colo320DM (low HER3 expression). Notably, SW620 tumor growth was significantly suppressed with U3-1402 compared with the saline-treated group (P < 0.001) and showed greater activity compared with the irinotecan group. By contrast, patritumab alone, control-ADC, and saline did not significantly differ in tumor growth inhibition. The antitumor activity of U3-1402 was dependent on HER3 expression level, but not on KRAS mutation status. These results support further investigation of development strategies for U3-1402 in patients with HER3-expressing colorectal cancer.

HER2‐targeted therapy, especially the anti‐HER2 antibody trastuzumab, is standard for HER2‐positive breast cancer; however, its efficacy is limited in a subpopulation of patients. HER3 ligand (heregulin)‐dependent HER2‐HER3 interactions play a critical role in the evasion of apoptosis and are therefore a target for oncotherapy to treat HER2‐positive breast cancer. The anti‐HER2 antibody pertuzumab and anti‐HER3 antibody patritumab both target this heregulin–HER3‐HER2 complex in different ways. This study examined the anticancer efficacy of dual HER2 and HER3 blockade in trastuzumab‐resistant HER2‐positive breast cancer. HER2‐positive SKBR3 or BT474 cells overexpressing heregulin (SKBR3‐HRG, BT474‐HRG) were used to evaluate the efficacy of trastuzumab, pertuzumab, and patritumab in vitro by performing cell viability, immunoblotting, and clonogenic assays. The effects of these agents were then evaluated in vivo using BT474‐HRG and an intrinsic heregulin‐expressing and HER2‐positive JIMT‐1 xenograft models. SKBR3‐HRG and BT474‐HRG cells lost sensitivity to trastuzumab, which was accompanied by Akt activation. Unexpectedly, trastuzumab in combination with pertuzumab or patritumab also showed limited efficacy toward these cells. In contrast, trastuzumab/pertuzumab/patritumab triple treatment demonstrated potent anticancer efficacy, concomitant with strong repression of Akt. Finally, in heregulin‐expressing BT474‐HRG and JIMT‐1 xenograft models, the addition of pertuzumab and patritumab to trastuzumab also enhanced antitumor efficacy leading to tumor regression. The current study found that triple blockade of HER2 and HER3 using trastuzumab, pertuzumab, and patritumab could overcome resistance to trastuzumab therapy in heregulin‐expressing and HER2‐positive breast cancer, which could be exploited clinically.

Càncer de mama; Puntuació CelTIL; Patritumab deruxtecan Cáncer de mama; Puntuación CelTIL; Patritumab deruxtecan Breast Cancer; CelTIL Score; Patritumab deruxtecan Background: Preclinical data support a key role for the human epidermal growth factor receptor 3 (HER3) pathway in hormone receptor (HR)–positive breast cancer. Recently, new HER3 directed antibody drug conjugates have shown activity in breast cancer. Given the need to better understand the molecular biology, tumor microenvironment, and mechanisms of drug resistance in breast cancer, we designed this window-of-opportunity study with the HER3 directed antibody drug conjugate patritumab deruxtecan (HER3-DXd; U3-1402). Trial Design: Based on these data, a prospective, multicenter, single-arm, window-of-opportunity study was designed to evaluate the biological effect of patritumab deruxtecan in the treatment of naïve patients with HR-positive/HER2-negative early breast cancer whose primary tumors are ≥1 cm by ultrasound evaluation. Patients will be enrolled in four cohorts according to the mRNA-based ERBB3 expression by central assessment. The primary endpoint is a CelTIL score after one single dose. A translational research plan is also included to provide biological information and to evaluate secondary and exploratory objectives of the study. We thank Daiichi Sankyo for their provision of patritumab deruxtecan and their financial contribution to this clinical study. Pas a Pas and Save the Mama to AP. Fundación SEOM (SEOM 2018 Grant: Fellowship for Training in Research in Reference Centers) to TP.

Background: Pathological complete response (pCR) is accepted by FDA as a surrogate endpoint for accelerated approval of targeted agents in combination with chemotherapy based on better long-term outcomes compared to residual disease (Cortazar 2014). Methods: The multi-center, adaptively-randomized I-SPY2 platform trial uses pCR as the primary endpoint to identify investigational agents that will improve outcomes in women with stage 2/3 breast cancer with high risk of early recurrence, across all signatures, based on hormone receptor (HR), HER2, and 70-gene (MammaPrint) status. For patients with HR+ HER2- tumors, only 70-gene (Mammaprint) high-risk patients are enrolled. To date, 1200+ patients have been randomized to one of 14 arms: control (paclitaxel followed by AC); veliparib/carboplatin; neratinib; MK2206; trebananib; trastuzumab/pertuzumab; ado-trastuzumab emtansine/pertuzumab; pembrolizumabx4; ganitumab/metformin; ganetespib; PLX-3397. 7 agents graduated in at least one signature (> 85% probability of success in a 300-patient phase III confirmatory trial); 2 did not graduate; 1 stopped for toxicity, and 3 are enrolling (patritumab/trastuzumab, talazoparib/irinotecan, pembrolizumabx8). Local pathologists were centrally trained using the Residual Cancer Burden (RCB) assessment to ensure uniform evaluation and response classification; RCB 0 = pCR. Results: We evaluated the relationship between pCR and event free (EFS) and distant disease free survival (DDFS) in the first 522 pts (median follow-up:2.5 years). 180 pts achieved pCR (36%) while 338 did not (RCB=1-3). There were 82 EFS and 65 DRFS events. Over the entire group (including all arms), pCR was highly associated with 3-year EFS (p 3-year survival (pCR group)Hazard Ratio OverallOverallHR+/HER2-HER2+TNBCEFS97%0.08 (0.03-0.23)0.14 (0.02-1.04)0 (NA)0.11 (0.03-0.37)DDFS98%0.08 (0.03-0.26)0.17 (0.01-1.23)0 (NA)0.09 (0.02-0.40) Conclusions: The first long-term efficacy results from the I-SPY2 TRIAL demonstrate that achieving pCR is a very strong surrogate endpoint for improved EFS and DDFS in a high-risk population, across all treatment arms, regardless of subtype. I-SPY2 shows substantially lower estimated EFS hazards for patients achieving pCR, compared to the 5 yr EFS hazard ratio for pCR vs not in Cortazar (hazard ratio 0.49), demonstrating important differences between a metaanalysis compared to a platform trial with uniform high-risk eligibility, standardized pathology assessment, and multiple targeted therapies. Our data support the use of pCR as a primary endpoint for accelerated approval of new drugs if EFS is evaluated in the same population. Based on these findings, the I-SPY2 TRIAL will test whether therapy can be deescalated or escalated for individual patients with the goal of achieving pCR for all. Citation Format: Yee D, DeMichele A, Isaacs C, Symmans F, Yau C, Albain KS, Hylton NM, Forero-Torres A, van't Veer LJ, Perlmutter J, Rugo HS, Melisko M, Chen Y-Y, Balassanian R, Krings G, Datnow B, Hasteh F, Tipps A, Weidner N, Zhang H, Tickman R, Thornton S, Ritter J, Amin K, Klein M, Chen B, Keeney G, Ocal T, Feldman M, Klipfel N, Sattar H, Mueller J, Gwin K, Baker G, Kallakury B, Zeck J, Duan X, Ersahin C, Gamez R, Troxell M, Mansoor A, Grasso LeBeau L, Sams S, Wisell J, Wei S, Harada S, Vinh T, Stamatakos MD, Tawfik O, Fan F, Adams A, Rendi M, Minton S, Magliocco A, Sahoo S, Fang Y, Hirst G, Singhrao R, Asare SM, Wallace AM, Chien AJ, Ellis ED, Han HS, Clark AS, Boughey JC, Elias AD, Nanda R, Korde L, Murthy R, Lang J, Northfelt D, Khan Q, Edmiston KK, Viscusi R, Haley B, Kemmer K, Zelnak A, Berry DA, Esserman LJ. Pathological complete response predicts event-free and distant disease-free survival in the I-SPY2 TRIAL [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr GS3-08.

Objectives Patritumab is a fully human anti-human epidermal growth factor receptor 3 (HER3) antibody that blocks activation by its ligand, heregulin (HRG). Preclinical studies have demonstrated the efficacy of patritumab in aberrantly high HRG-expressing non-small cell lung cancer (NSCLC). In the phase II randomized, placebo-controlled double-blind study HERALD ( n =212 patients with NSCLC), patritumab plus erlotinib did not improve progression-free survival (PFS) compared with placebo plus erlotinib. The current study examined whether soluble HRG (sHRG) level in serum correlated with the efficacy of patritumab plus erlotinib. Materials and methods Serum was obtained from participants prior to treatment ( n =202). sHRG level was measured using a validated quantitative immune assay, and correlations with survival were blindly assessed. Results sHRG level was various (−1346–11,772pg/mL). Participants were divided into the sHRG-high or -low subgroups at the concentration defining near the third quartile, 980pg/mL. Patritumab plus erlotinib significantly improved PFS relative to placebo in the sHRG-high subgroup ( n =46, hazard ratio 0.42 [0.19–0.96], p =0.0327). In contrast, the HRG-low subgroup ( n =148) had no improvement in PFS with patritumab. Conclusion sHRG seems to be a predictive biomarker for the efficacy of patritumab plus erlotinib in NSCLC patients.

Background This phase 1 study evaluated the safety, tolerability, pharmacokinetics and efficacy of patritumab (U3-1287) Process 2, a new formulation of fully human anti-HER3 monoclonal antibody in combination with erlotinib, an epidermal growth factor receptortyrosine kinase inhibitor (EGFR-TKI) in prior chemotherapy treated Japanese patients with advanced non-small cell lung cancer (NSCLC). Methods Patients received intravenous patritumab Process 2 formulation at 9 mg/kg every 3 weeks after initiation of 18 mg/kg loading dose combined with continuous daily dose of erlotinib (150 mg QD) until any of the withdrawal criteria are met. Adverse events (AEs) were assessed using CTCAE v4.0 and tumor response was assessed using RECIST v1.1. Full pharmacokinetic sampling and serum biomarker analyses were mainly performed during cycle 1 and 2. Results Total of six EGFR-mutant NSCLC patients including one EGFR-TKI naïve patient received patritumab Process 2 formulation combined with erlotinib. No dose-limiting toxicities were observed. The most frequent AEs were gastrointestinal or skin toxicities, which were generally mild and manageable. One patient discontinued from study due to reversible grade 3 interstitial lung disease. The mean area under the curve (AUC) value was 2640 μg/day/mL; the Cmax value was 434 μg/mL, respectively. The median progression-free survival (95% confidence interval) was 220.0 (100.0–363.0) days. HER3 ligand heregulin was detected in serum from only a patient that maintained most durable stable disease. Conclusions Patritumab Process 2 formulation in combination with erlotinib was well tolerated compatible with favorable PK profile in Japanese patients with advanced NSCLC.

Background The fully human monoclonal antibody patritumab blocks HER3 activation, a resistance mechanism to cetuximab, induced by heregulin (HRG). A phase Ib study in recurrent and/or metastatic (R/M) squamous cell carcinoma of the head and neck (SCCHN) demonstrated tolerability and tumour response of patritumab + cetuximab + platinum. Methods This was a randomised, double-blind, phase II study of patritumab + cetuximab with platinum-based therapy for first-line treatment of R/M SCCHN ( Clinicaltrials.gov identifier: NCT02633800 ). Patients aged ≥18 years received patritumab or placebo, both combined with cetuximab + cisplatin or carboplatin. Co-primary end-points were progression-free survival (PFS) in the intent-to-treat (ITT) and the high-expression HRG (HRG high) populations. Results Eighty-seven patients (n = 43 in the patritumab group; n = 44 in placebo group) enrolled. A median (range) of 6.5 (1–24) patritumab cycles were completed. Median PFS was similar between the patritumab group and placebo group in the ITT population (5.6 versus 5.5 months; hazard ratio [HR] 0.99 [95% confidence interval [CI], 0.6–1.7]; P = 0.96) and HRG-high subgroup (n = 51; 5.6 versus 5.6 months; HR 0.93 [95% CI, 0.5–1.8]; P = 0.82). Median overall survival in the ITT population was also similar (10.0 versus 12.7 months; HR 1.3 [95% CI, 0.69–2.29]; P = 0.46). All patients experienced ≥1 treatment-emergent adverse event (TEAE). Grade ≥III TEAEs were more frequent in the patritumab than the placebo group (84.1% versus 60.5%). The most common grade ≥III patritumab-related TEAE in the patritumab group (20.5% overall) was rash (6.8%). Conclusion Patritumab + cetuximab + platinum was tolerable but not superior to cetuximab + platinum.

TPS9139 Background: Few treatment options have demonstrated therapeutic benefit in epidermal growth factor receptor–mutated ( EGFRm) non–small cell lung cancer (NSCLC) that has progressed after treatment with EGFR tyrosine kinase inhibitors (TKIs) and platinum-based chemotherapy. HER3, a member of the human epidermal growth factor family, is detectable in most EGFRm NSCLC, and its expression has been linked to worse clinical outcomes. There are no approved HER3 directed therapies for the treatment of NSCLC. HER3-DXd is a novel, potentially first-in-class HER3 directed antibody drug conjugate that has demonstrated preliminary evidence of safety and antitumor activity in patients (pts) with EGFRm TKI–resistant NSCLC in an ongoing Phase 1 study, providing proof of concept of HER3-DXd. The Phase 2 study (HERTHENA-Lung01) is further evaluating HER3-DXd in pts with previously treated metastatic or locally advanced EGFRm NSCLC. Methods: This randomized, open-label Phase 2 study will enroll up to 420 pts at approximately 135 study sites in North America, Europe and the Asia-Pacific region. Eligible pts will have metastatic or locally advanced NSCLC with an activating EGFR mutation (exon 19 deletion or L858R), progression during or after systemic treatment with ≥1 EGFR TKI and ≥1 platinum-based chemotherapy regimen, and ≥1 measurable lesion confirmed by blinded independent central review (BICR) per RECIST v1.1. Pts with an EGFR T790M mutation must have received and progressed on prior osimertinib. Pts with stable brain metastases are eligible. Exclusion criteria include evidence of previous small cell or combined small cell/non–small cell histology or any history of interstitial lung disease. Tumor tissue will be assessed retrospectively for HER3 expression and molecular mechanisms of TKI resistance. HER3 expression will not be used to select pts for enrollment. Pts will be randomized 1:1 to receive 1 of 2 HER3-DXd Q3W dose regimens that will be independently evaluated: a 5.6 mg/kg fixed-dose regimen (Arm 1) or an up-titration dose regimen (Arm 2: Cycle 1, 3.2 mg/kg; Cycle 2, 4.8 mg/kg; Cycle 3 and beyond, 6.4 mg/kg). After review of data from an ongoing Phase 1 study with similar patients treated with either of these dose regimens, a decision could be made to continue enrollment into 1 or both arms. The primary objective is to evaluate the efficacy of HER3-DXd as measured by objective response rate (ORR) by BICR. Secondary objectives are to evaluate the efficacy and safety/tolerability of HER3-DXd and to assess the relationship between efficacy and HER3 expression. Secondary endpoints include duration of response, progression-free survival, ORR by investigator, disease control rate, time to response, best percentage change in the sum of diameters of measurable tumors, and overall survival. The study is enrolling and is planned to finish in 2023. Clinical trial information: NCT04619004.

9007 Background: Patients (pts) with advanced EGFRm NSCLC have limited treatment options after failure of EGFR TKI and platinum-based chemotherapy (PBC). HER3-DXd is an antibody drug conjugate consisting of a fully human monoclonal antibody to HER3 attached to a topoisomerase I inhibitor payload via a tetrapeptide-based cleavable linker. We previously presented efficacy/safety data (median follow-up, 5.4 mo) from an ongoing study of HER3-DXd in EGFRm NSCLC after failure of EGFR TKI therapy. We now present extended follow-up of pts receiving the recommended dose for expansion (5.6 mg/kg IV Q3W). Methods: This Ph 1 dose-escalation/expansion study included pts with locally advanced or metastatic EGFRm NSCLC with prior EGFR TKI therapy (NCT03260491). Pts with stable brain metastases (BM) were allowed. The primary endpoint was confirmed ORR by blinded independent central review (BICR) per RECIST v1.1; secondary endpoints included DOR, PFS and safety. Results: At data cutoff (Sept 24, 2020), 57 pts were treated with HER3-DXd 5.6 mg/kg IV Q3W; median follow-up, 10.2 mo (range, 5.2-19.9 mo). Median number of prior anticancer regimens was 4 (range, 1-10). 100% had prior EGFR TKI (86% prior osimertinib [OSI]) and 91% had prior PBC. 47% had a history of BM. Median treatment duration was 5.5 mo (range, 0.7-18.6 mo); treatment was ongoing in 18 pts (32%). Confirmed ORR by BICR was 39% (22/57; 95% CI, 26.0%-52.4%; 1 CR, 21 PR, 19 SD) with 14/22 responses occurring within 3 mo of starting HER3-DXd. DCR was 72% (95% CI, 58.5%-83.0%). Median DOR was 6.9 mo (95% CI, 3.1 mo-NE), and median PFS was 8.2 mo (95% CI, 4.4-8.3 mo). Antitumor activity was observed across diverse mechanisms of EGFR TKI resistance, including those not directly related to HER3 ( EGFR C797S, MET or HER2 amp, and BRAF fusion). Among pts with prior PBC, ORR was 37% (19/52; 95% CI, 23.6%-51.0%); in pts with prior OSI and PBC, ORR was 39% (17/44; 95% CI, 24.4%-54.5%). Among 43 pts evaluable for HER3 expression, nearly all expressed HER3; median membrane H-score by IHC was 180 (range, 2-280). Median H-score (range; N) was 195 (92-268; 15) in pts with CR/PR, 180 (4-280; 15) with SD, 126.5 (2-251; 6) with PD, and 180 (36-180; 7) in pts unevaluable for best overall response. The most common grade ≥3 adverse events (AEs) were thrombocytopenia (30%), neutropenia (19%), and fatigue (14%). Drug-related interstitial lung disease by central adjudication occurred in 4 pts (7%; 1 grade ≥3 [2%]; no grade 5). 6/57 pts (11%) had AEs associated with treatment discontinuation (none were due to thrombocytopenia). Conclusions: HER3-DXd 5.6 mg/kg IV Q3W demonstrated antitumor activity across various EGFR TKI resistance mechanisms in heavily pretreated metastatic/locally advanced EGFRm NSCLC. The safety profile was consistent with previous reports. A Ph 2 study of HER3-DXd in pts with EGFRm NSCLC after failure of EGFR TKI and PBC has been initiated (NCT04619004). Clinical trial information: NCT03260491.

TPS157 Background: Patritumab deruxtecan (HER3-DXd; U3-1402) is a novel, investigational antibody drug conjugate comprising an anti-HER3 monoclonal antibody, a tetrapeptide-based linker, and a topoisomerase I inhibitor payload. Ongoing clinical trials of HER3-DXd in pts with metastatic breast cancer or non-small cell lung cancer have shown promising clinical activity and acceptable safety. HER3 (human epidermal growth receptor 3), a member of the tyrosine kinase receptor family, is overexpressed in most CRC tumors and associated with an adverse prognosis. Significant tumor regression with HER3-DXd has been observed in CRC murine xenograft models, regardless of KRAS mutation status. Here we introduce the design of a phase 2 study (U31402-A-U202) that is evaluating HER3-DXd in previously treated pts with advanced/metastatic CRC. Methods: U31402-A-U202 (NCT04479436) is an open-label, multicenter phase 2 study that will enroll 80 pts in the USA, Europe and Asia. Pts are enrolled who are aged ≥ 18 years with advanced/metastatic colorectal adenocarcinoma that is resistant/refractory/intolerant to ≥ 2 prior lines of therapy including a fluoropyrimidine, irinotecan, a platinum agent, an anti-EGFR agent (if clinically indicated), an anti-VEGF agent (unless contraindicated [CI]), and an immune checkpoint inhibitor (unless CI) for microsatellite instability-high CRC. Pts with current/previous interstitial lung disease or clinically severe pulmonary compromise are excluded. Archival tumor biopsy and pre-treatment tumor biopsy are collected from all pts at screening, with HER3 protein expression measured by immunohistochemistry (IHC). In part 1, results of the HER3 IHC assay from the pre-treatment tumor biopsy are used to assign pts into 1 of 2 cohorts (C). C1: HER3 high (IHC 3+, 2+), n = 24; C2: HER3 low/negative (IHC 1+, 0), n = 12. Pts receive 5.6 mg/kg HER3-DXd IV every 3 weeks. An interim futility analysis will be conducted separately for C1 and C2 and will determine enrollment in part 2, with 2 potential scenarios: enrollment continues irrespective of HER3 IHC status, or enrollment continues in HER3 high pts only. The primary objective is the evaluation of the antitumor activity of HER3-DXd as measured by objective response rate (ORR) (assessed by BICR according to RECIST v1.1). ORR will be summarized with the 2-sided 95% confidence interval. Secondary objectives include the evaluation of efficacy as measured by ORR (assessed by investigator according to RECIST v1.1), duration of response, time to tumor response, disease control rate, progression-free survival (assessed by investigator and BICR according to RECIST v1.1), overall survival, safety and tolerability, HER3 protein expression in tumor tissue and relationship with efficacy, and pharmacokinetic properties. Clinical trial information: NCT04479436.

The mutation or expression of HER family members serves as a therapeutic target for tyrosine kinase inhibitors or monoclonal antibodies in diverse cancers, such as non-small cell lung cancer, breast cancer, gastric cancer, head and neck cancer, colorectal cancer, pancreatic cancer and glioblastoma. HER3, which heterodimerizes with HER1 and HER2, has received much attention as a potential target for anti-EGFR treatment. Patritumab is a novel, fully human monoclonal antibody directed against HER3. Areas covered: In this review article, an overview of the market, chemistry, pharmacodynamics, pharmacokinetics, efficacy, and safety of patritumab is provided based on data from phase I studies, a combination phase I trial, and a randomized phase II trial comparing two doses of patritumab. Expert opinion: The combination of patritumab plus erlotinib has shown a promising efficacy and safety in early-phase clinical trials. In a randomized phase II trial, higher mRNA expression of heregulin (a ligand of HER3) was associated with better progression-free survival and a tendency toward improved overall survival. In the era of precise treatment based on an appropriate target with a predictive biomarker, further studies with patritumab are needed to realize its potential in cancer treatment.

Human epidermal growth factor receptor 3 (HER3) expression in lung and breast cancers has a negative impact on survival. Patritumab, a human anti-HER3 mAb, has shown anticancer activity in preclinical models. This study examined the safety and pharmacokinetics of patritumab in combination with trastuzumab and paclitaxel in patients with HER2-overexpressing metastatic breast cancer. In this open-label, multicenter, dose-escalation, phase Ib study, patients received patritumab 9 or 18 mg/kg plus trastuzumab and paclitaxel at known tolerated doses. Safety and tolerability were assessed based on dose-limiting toxicities and other non-life threatening adverse events. The pharmacokinetic profile for patritumab was determined based on the target trough level. Clinical efficacy was evaluated based on the overall response rate and progression-free survival. Six patients received patritumab 9 mg/kg and 12 received 18 mg/kg. The most common adverse events were diarrhea, alopecia, leukopenia, neutropenia, and maculopapular rash. No dose-limiting toxicities were observed. The target trough serum concentration was achieved in all patients at a dose of 18 mg/kg. Overall response rate was 38.9% and median progression-free survival was 274 days. In conclusion, patritumab plus trastuzumab and paclitaxel was tolerable and efficacious at both doses. We recommend the dose level of 18 mg/kg for future phase II studies. (Clinical trial registration: JapicCTI-121772.).

Epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKI) have changed the treatment landscape for EGFR-mutant non-small cell lung cancers (NSCLC); however, most patients develop resistance over time. HER3 is a unique pseudokinase member of the ERBB family which functions through dimerization with other ERBB family members (EGFR and HER2) and has been difficult to target with conventional kinase inhibitor strategies. HER3 is frequently over expressed in EGFR-mutant NSCLC. U3-1402 is an antibody-drug conjugate (ADC) comprised of HER3-targeting antibody (patritumab) linked to a topoisomerase I inhibitor (DX-8951 derivative, or DXd). U3-1402 is currently in clinical development as a single agent in metastatic EGFR-mutant NSCLC (NCT03260491). We aimed to develop a preclinical strategy to enhance the efficacy of U3-1402. Pre-treatment with EGFR TKIs (gefitinib or osimertinib) increased HER3 membrane levels in six different EGFR-mutant cell lines by increasing both the amount of HER3 positive cells and the intensity of HER3 expression. Furthermore, using a proximity-ligation assay (PLA), we noted an increase in membrane EGFR:HER3 interaction following EGFR inhibitor treatment. HER3 protein levels and EGFR:HER3 interaction were similarly increased in four EGFR-mutant patient-derived xenograft (PDX) models following osimertinib treatment. HER3 expression peaked between 24 and 48 hours, and the increase in HER3 was regulated by both increased gene expression and in the level of protein stability. We further evaluated the biological consequences of increased HER3 expression on U3-1402 intake and activity. We used pH-sensitive pHrodo Red conjugate to label U3-1402 to monitor the intake of the ADC using Incucyte live cell imaging. In three EGFR-mutant cell lines, there was a 3- to 5-fold higher intake of U3-1402 over time in cell lines pre-treated with osimertinib, compared to cell lines without pre-treatment. Osimertinib pre-treatment also increased the uptake of U3-1402 in HER3-low expressing EGFR-mutant models. In contrast, osimertinib pre-treatment did not increase the uptake of a control IgG-ADC. In long-term in vitro cell growth assays, the combination of osimertinib and U3-1402 was superior to single agent U3-1402 in four EGFR-mutant cell lines. Preliminary in vivo studies demonstrated no significant toxicities as measured by mouse body weights and complete blood counts. In vivo experiments evaluating the antitumor efficacy of the combination of osimertinib and U3-1402 are currently underway. Our studies reveal that EGFR inhibitor treatment increased membrane expression of HER3 which was associated with enhanced internalization of U3-1402 in EGFR-mutant NSCLC. The combination of osimertinib and U3-1402 may be an effective treatment approach and should be evaluated in future clinical trials in patients with EGFR-mutant NSCLC. Citation Format: Heidi M. Haikala, Jens Köhler, Timothy Lopez, Pinar Eser, Man Xu, Channing Yu, Yoshinobu Shiose, Yang Qiu, Prafulla Gokhale, Pasi A. Jänne. EGFR inhibition enhances the cellular uptake and antitumor activity of the novel HER3 antibody drug conjugate U3-1402 [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5192.

Purpose: Patritumab plus cetuximab with platinum as first-line therapy for patients with recurrent and/or metastatic (R/M) squamous cell carcinoma of the head and neck (SCCHN) was evaluated for safety and to determine the recommended phase II combination dose. Patients and Methods: Patients aged ≥18 years with confirmed R/M SCCHN received intravenous patritumab (18 mg/kg loading dose; 9 mg/kg maintenance dose every 3 weeks) + cetuximab (400 mg/m2 loading dose; 250 mg/m2 maintenance dose weekly) + cisplatin (100 mg/m2 every 3 weeks) or carboplatin (AUC of 5) for six cycles or until toxicity, disease progression, or withdrawal. Primary endpoints were dose-limiting toxicities [DLT; grade ≥3 (21-day observation period)] and treatment-emergent adverse events (TEAE). Pharmacokinetics, human antihuman antibodies (HAHA), tumor response, progression-free survival (PFS), and overall survival (OS) were assessed. Results: Fifteen patients completed a median (range) of 8.7 (2.0–20.7) patritumab cycles. No DLTs were reported. Serious adverse events were reported in 9 patients (patritumab-related n = 4). TEAEs (N = 15 patients) led to patritumab interruption in 7 patients. Patritumab-related dose reductions were reported in 1 patient. Patritumab (18 mg/kg) pharmacokinetics (N = 15) showed mean (SD) AUC0–21d of 2,619 (560) μg/day/mL and maximum concentration of 499.9 (90.4) μg/mL. All patients were HAHA-negative at study end (single, transient low titer in 1 patient). Tumor response rate (complete plus partial response; N = 15) was 47%. Median (95% confidence interval) PFS and OS (N = 15) were 7.9 (3.7–9.7) and 13.5 (6.6–17.5) months, respectively. Conclusions: Patritumab (18 mg/kg loading dose, 9 mg/kg maintenance dose) plus cetuximab/platinum was tolerable, active in SCCHN, and selected as the phase II dose regimen.

The purpose of this study was to evaluate the safety, dosimetry, and apparent receptor occupancy (RO) of [(64)Cu]DOTA-patritumab, a radiolabeled monoclonal antibody directed against HER3/ERBB3 in subjects with advanced solid tumors.Dosimetry subjects (n = 5) received [(64)Cu]DOTA-patritumab and underwent positron emission tomography (PET)/X-ray computed tomography (CT) at 3, 24, and 48 h. Evaluable RO subjects (n = 3 out of 6) received [(64)Cu]DOTA-patritumab at day 1 and day 8 (after 9.0 mg/kg patritumab) followed by PET/CT at 24 h post-injection. Endpoints included safety, tumor uptake, and efficacy.The tumor SUVmax (± SD) was 5.6 ± 4.5, 3.3 ± 1.7, and 3.0 ± 1.1 at 3, 24, and 48 h in dosimetry subjects. The effective dose and critical organ dose (liver) averaged 0.044 ± 0.008 mSv/MBq and 0.46 ± 0.086 mGy/MBq, respectively. In RO subjects, tumor-to-blood ratio decreased from 1.00 ± 0.32 at baseline to 0.57 ± 0.17 after stable patritumab, corresponding to a RO of 42.1 ± 3.[(64)Cu]DOTA-patritumab was safe. These limited results suggest that this PET-based method can be used to determine tumor-apparent RO.

Objectives Human epidermal growth factor receptor 3 (HER3) is a key dimerization partner for HER family members and is associated with resistance to other HER family receptor-targeted therapeutics. This study evaluated the safety, tolerability, pharmacokinetics and efficacy of patritumab (U3-1287), a fully human anti-HER3 monoclonal antibody, in combination with erlotinib, an epidermal growth factor receptor-tyrosine kinase inhibitor in patients with previously treated advanced non-small cell lung cancer (NSCLC). Patients and methods This study enrolled patients with stage IIIB/IV NSCLC with Eastern Cooperative Oncology Group performance status 0–1, life expectancy >3 months and who had progressed after at least one prior course of chemotherapy (excluding erlotinib). This open-label study included two parts: dose escalation (Part 1) and dose expansion (Part 2). In Part 1, patients received intravenous patritumab 9 or 18 mg/kg every 3 weeks in addition to per-oral erlotinib 150 mg/day daily. In Part 2, patients received the recommended dose of patritumab as determined in Part 1. Adverse event rates, pharmacokinetics and tumor responses were determined. Results Twenty-four Japanese patients received patritumab at 9 mg/kg (n = 3) or 18 mg/kg (n = 21), and erlotinib. No dose-limiting toxicities were reported, indicating the maximum-tolerated dose was not reached. The most frequent adverse events were gastrointestinal or skin toxicities, which were generally mild and manageable. Patritumab pharmacokinetics were similar to those reported in previous studies. The median progression-free survival (95% confidence interval) was 44.0 (22.0–133.0) days for the EGFR wild-type group (n = 9) and 107.0 (74.0–224.0) days for the EGFR-activating mutation group (n = 13). Evaluation of biomarkers by immunohistochemical analysis did not indicate a relationship between efficacy and HER3 expression in tumor tissues. Conclusion Patritumab in combination with erlotinib was well tolerated and the efficacy of the combination was encouraging, especially in patients where prior gefitinib treatment failed.

Background During early clinical development, prospective identification of a predictive biomarker and validation of an assay method may not always be feasible. Dichotomizing a continuous biomarker measure to classify responders also leads to challenges. We present a case study of a prospective–retrospective approach for a continuous biomarker identified after patient enrollment but defined prospectively before the unblinding of data. An analysis of the strengths and weaknesses of this approach and the challenges encountered in its practical application are also provided. Methods HERALD (NCT02134015) was a double-blind, phase 2 study in patients with non-small cell lung cancer (NSCLC) randomized to erlotinib with placebo or with high or low doses of patritumab, a monoclonal antibody targeted against human epidermal growth factor receptor 3 (HER3). While the primary objective was to assess safety and progression-free survival (PFS), a secondary objective was to determine a single predictive biomarker hypothesis to identify subjects most likely to benefit from the addition of patritumab. Although not identified as the primary biomarker in the study protocol, on the basis of preclinical results from 2 independent laboratories, expression levels of the HER3 ligand heregulin (HRG) were prospectively declared the predictive biomarker before data unblinding but after subject enrollment. An assay to measure HRG mRNA was developed and validated. Other biomarkers, such as epidermal growth factor receptor (EGFR) mutation status, were also evaluated in an exploratory fashion. The cutoff value for high vs. low HRG mRNA levels was set at the median delta threshold cycle. A maximum likelihood analysis was performed to evaluate the provisional cutoff. The relationship of HRG values to PFS hazard ratios (HRs) was assessed as a measure of internal validation. Additional NSCLC samples were analyzed to characterize HRG mRNA distribution. Results The subgroup of patients with high HRG mRNA levels (“HRG-high”) demonstrated clinical benefit from patritumab treatment with HRs of 0.37 (P = 0.0283) and 0.29 (P = 0.0027) in the high- and low-dose patritumab arms, respectively. However, only 102 of the 215 randomized patients (47.4%) had sufficient tumor samples for HRG mRNA measurement. Maximum likelihood analysis showed that the provisional cutoff was within the optimal range. In the placebo arm, the HRG-high subgroup demonstrated worse prognosis compared with HRG-low. A continuous relationship was observed between increased HRG mRNA levels and lower HR. Additional NSCLC samples (N = 300) demonstrated a similar unimodal distribution to that observed in this study, suggesting that the defined cutoff may be applicable to future NSCLC studies. Conclusions The prospective–retrospective approach was successful in clinically validating a probable predictive biomarker. Post hoc in vitro studies and statistical analyses permitted further testing of the underlying assumptions. However, limitations of this analysis include the incomplete collection of adequate tumor tissue and a lack of stratification. In a phase 3 study, findings are being confirmed, and the HRG cutoff value is being further refined. ClinicalTrials.gov Number NCT02134015., Highlights • High heregulin levels predict benefit from patritumab treatment in patients with NSCLC. • A prospective–retrospective approach provisionally validated a predictive biomarker. • Post hoc analyses can be used to test underlying assumptions in biomarker validation. • The median may be a reasonable initial cutoff for a unimodal continuous biomarker.

Background Integrating molecularly targeted agents with cytotoxic drugs used in curative treatment of pediatric cancers is complex. An evaluation was undertaken with the ERBB3/Her3-specific antibody patritumab (P) either alone or with the ERBB1/epidermal growth factor receptor inhibitor erlotinib (E) in combination with standard cytotoxic agents, cisplatin, vincristine, and cyclophosphamide, in pediatric sarcoma xenograft models that express receptors and ligands targeted by these agents. Procedures Tumor models were selected based upon ERBB3 expression and phosphorylation, and ligand (heregulin) expression. Patritumab, E, or these agents combined was evaluated without or with concomitant cytotoxic agents using procedures developed by the Pediatric Preclinical Testing Program. Results Full doses of cytotoxic agents were tolerated when combined with P, whereas dose reductions of 25% (vincristine, cisplatin) or 50% (cyclophosphamide) were required when combined with P + E. Patritumab, E alone, or in combination did not significantly inhibit growth of any tumor model, except for Rh18 xenografts (E alone). Patritumab had no single-agent activity and marginally enhanced the activity of vincristine and cisplatin only in Ewing sarcoma ES-4. P + E did not increase the antitumor activity of vincristine or cisplatin, whereas dose-reduced cyclophosphamide was significantly less active than cyclophosphamide administered at its maximum tolerated dose when combined with P + E. Conclusions P had no single-agent activity, although it marginally potentiated the activity of vincristine and cisplatin in one of three models studied. However, the addition of E necessitated dose reduction of each cytotoxic agent, abrogating the enhancement observed with P alone.

// Hisato Kawakami 1 , Isamu Okamoto 1, 2 , Kimio Yonesaka 1 , Kunio Okamoto 1 , Kiyoko Shibata 1 , Yume Shinkai 1 , Haruka Sakamoto 1 , Michiko Kitano 1 , Takao Tamura 1 , Kazuto Nishio 3 , Kazuhiko Nakagawa 1 1 Department of Medical Oncology, Kinki University Faculty of Medicine, Osaka-sayama, Osaka 589-8511, Japan 2 Center for Clinical and Translational Research, Kyushu University Hospital, Higashiku, Fukuoka 812–8582, Japan 3 Department of Genome Biology, Kinki University Faculty of Medicine, Osaka-Sayama, Osaka 589–8511, Japan Correspondence to: Isamu Okamoto, e-mail: okamotoi@kokyu.med.kyushu-u.ac.jp Keywords: colorectal cancer, heregulin, resistance, cetuximab, patritumab Received: September 21, 2014 Accepted: October 26, 2014 Published: December 19, 2014 ABSTRACT We previously showed that tumor-derived heregulin, a ligand for HER3, is associated with both de novo and acquired resistance to cetuximab. We have now examined whether patritumab, a novel neutralizing monoclonal antibody to HER3, is able to overcome such resistance. Human colorectal cancer (DiFi) cells that are highly sensitive to cetuximab were engineered to stably express heregulin by retroviral infection, and the effects of cetuximab and patritumab on the resulting DiFi-HRG cells were examined. DiFi-HRG cells released substantial amounts of heregulin and showed resistance to cetuximab. Cetuximab alone inhibited EGFR and ERK phosphorylation in DiFi-HRG cells, but it had no effect on the phosphorylation of HER2, HER3, or AKT, suggesting that sustained AKT activation by HER2 and HER3 underlies cetuximab resistance in these cells. In contrast, patritumab in combination with cetuximab markedly inhibited the phosphorylation of EGFR, HER2, HER3, ERK, and AKT. The combination therapy also inhibited the growth of DiFi-HRG tumor xenografts in nude mice to a greater extent than did treatment with either drug alone. Activation of HER2-HER3 signaling associated with the operation of a heregulin autocrine loop confers resistance to cetuximab, and patritumab is able to restore cetuximab sensitivity through inhibition of heregulin-induced HER3 activation.

Purpose Patritumab (U3-1287) is a human epidermal growth factor receptor-3 (HER3)-targeted antibody that blocks ligand-associated activation of HER3. This open-label, phase 1 and dose-finding study (ClinicalTrials.jp Identifier: JapicCTI-101262) aimed to assess the safety, pharmacokinetics, incidence of anti-patritumab antibody, recommended dose for subsequent clinical studies, preliminary efficacy, and patritumab-related biomarkers in Japanese patients with advanced solid tumors. Methods Patients received patritumab 9 or 18 mg/kg intravenously every 3 weeks until disease progression or intolerable toxicity occurred. Adverse events (AEs) were assessed according to the National Cancer Institute Common Terminology Criteria for Adverse Events (CTCAE version 4.0). Dose-limiting toxicities (DLTs) were evaluated from the initial dose to Cycle 1 Day 21. Tumor response was assessed with Response Evaluation Criteria in Solid Tumors (RECIST version 1.1). Results Nine patients received patritumab 9 mg/kg (n = 3) or 18 mg/kg (n = 6). Five patients were male, all patients had Eastern Cooperative Oncology Group performance status (PS) ≤ 1, and median (range) age of 67 (50–69) years. No DLTs were reported. Patritumab-related AEs reported in ≥2 patients were ALT increase (three patients), thrombocytopenia, diarrhea, stomatitis, cheilitis, rash maculo-papular and AST increase (two each). Pharmacokinetics profile was similar to the preceding US phase 1 study. Soluble HER3 concentration in serum unexpectedly increased in all patients. These changes did not correlate with clinical response. Four patients had a best response of stable disease. All patients tested had negative for anti-patritumab antibody formation. Conclusions Patritumab was well tolerated up to 18 mg/kg without DLTs in Japanese patients with advanced solid tumors. Soluble HER3 increased in all patients.

Background Signal transduction upon HER3/EGFR dimerization to PI3K/AKT/mTOR pathways are thought to be involved in cancer survival, proliferation and up-regulation of PD-L1 expression. It is therefore hypothesized that inhibition of HER3 signal could enhance anti-tumor immunity by regulating the expression of PD-L1. Methods Two patient-derived xenograft (PDX) models of breast cancer were evaluated with patritumab (anti-HER3 antibody), polyclonal activated autologous T cells (PATCs) or a combination of patritumab and PATCs (P-PATCs). Tumor size was measured for anti-tumor effects of each treatment. To test the immunological modulation by patritumab, tumors and liver tissue were immunohistochemically stained with anti-CD3, CD4, CD8, CD137 and PD-L1 antibody. Furthermore, quantitative real-time PCR and RNA-sequencing was performed on the tumor samples from the PDX models. Results Although treatment of PATCs or patritumab alone showed limited anti-tumor effects, P-PATCs treatment demonstrated a significantly greater anti-tumor response in both 2 PDX models. Interestingly, the proportion of CD137 expressing T cell infiltration was much higher in the P-PATCs group compared to the patritumab or PATCs group while the proportion of CD4+ T cell infiltration was also higher in P-PATCs group. There was an equal proportion of T cells without CD137 expression present in liver tissue in both PATCs and P-PATCs groups. Conclusions These data suggest that patritumab could help augmentation of antigen-specific T cell activation leading to additional anti-tumor effects on patritumab treatment alone by means of immunological mechanisms. Thus, patritumab treatment might become a novel treatment strategy for HER3/neuregulin (ligand for HER3)-expressing breast cancer patients in the future.

Background: HER3 (human epidermal growth factor receptor 3) is a member of the HER family and overexpressed in a broad range of cancer types including colorectal cancer. HER3 targeted therapies such as anti-HER3 antibodies have been investigated in clinical trials; however, no HER3-targeting therapy has been approved to date. U3-1402 is a novel antibody drug conjugate (ADC) composed of a fully human anti-HER3 antibody (patritumab) and a novel topoisomerase I inhibitor (DXd) which is conjugated via an enzymatically cleavable peptide-linker with high drug antibody ratio (DAR: 7-8 to 1). Purpose: To investigate the antitumor efficacy of U3-1402 in HER3-expressing colorectal cancer xenograft models. Material and methods: The following cell lines were used in this study; WiDr, Colo320DM, and DiFi with KRAS wild type, DLD-1, HCT15, HCT116, LoVo, SW480, and SW620 harboring KRAS mutation. The sensitivities of 9 colorectal cancer cell lines to DXd were evaluated by in vitro cell killing assay. Cells were treated with DXd at different concentrations (0.5pM to 500nM) over 144 hours, and the 50% inhibitory concentration (IC50) values were determined. Cell surface HER3 antigen was quantified by flow cytometry. In vivo screening assay and tumor growth inhibition assay were performed in tumor xenograft models exhibiting a range of HER3 expression levels. In addition, the efficacy of U3-1402 was compared to that of parental antibody patritumab, CPT-11, control-ADC, and saline in SW620 tumor xenograft model harboring KARS mutation. All results were expressed as mean ± standard deviation (SD) and statistical significance was analyzed using ANOVA with Dunnet multiple comparison. Results: In in vitro study, the range of IC50 value of DXd for each cell line was 2.16 ± 0.11 nM to 16.04 ± 0.10 nM. Most of the cell lines were sensitive to DXd. In in vivo screening assay, the maximum regression from U3-1402 was observed in the tumor xenograft model of both DiFi (high HER3 expression; KRAS wild type) and SW620 (high HER3 expression; KRAS mutation). In in vivo tumor growth inhibition assay, the antitumor efficacy was dependent on the HER3 expression level. Notably, SW620 tumor growth was significantly suppressed with U3-1402 compared with that of saline group (p < 0.001) and had improved potency compared with CPT-11. Of note, no significant difference in tumor growth inhibition was observed amongst the treatments with the same dose of patritumab or control-ADC and saline. Tumor regression was only observed in U3-1402 treated group, and some of which achieved complete regression. Conclusions: The antitumor efficacy of U3-1402 in xenograft models was dependent on HER3 expression and did not show a clear relationship to KRAS mutation status. These results support further investigation of development strategies for U3-1402 in patients with HER3-expressing colorectal cancer. Citation Format: Shigehiro Koganemaru, Yasutoshi Kuboki, Yoshikatsu Koga, Takashi Kojima, Mayumi Yamauchi, Naoyuki Maeda, Takashi Kagari, Kenji Hirotani, Masahiro Yasunaga, Yasuhiro Matsumura, Toshihiko Doi. U3-1402, a novel HER3-targeting ADC, inhibits the tumor growth of colorectal cancer xenografts [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 220.

Background: Human epidermal growth factor receptor 3 (HER3), a member of the ErbB receptor tyrosine kinase family, is overexpressed in a variety of human cancers and plays an important role in cell proliferation and survival. U3-1402 is a novel HER3-targeting antibody-drug conjugate (ADC) consisting of a fully human anti-HER3 antibody (patritumab), a tetrapeptide-based linker, and a topoisomerase I inhibitor payload. It is currently being investigated in clinical trials for HER3-positive breast cancer (phase 1/2) and NSCLC (phase 1). In principle, effective action of ADCs depends on multiple dynamic factors including antigen-specific binding, internalization, trafficking to lysosomes, and payload release, as well as other inherent characteristics such as payload potency and linker stability. Methods: To assess the targeted delivery potential of U3-1402, the HER3-mediated molecular dynamics of U3-1402 (cell surface binding, internalization, trafficking, and payload release) were investigated in 8 cancer cell lines with various HER3 expression levels, along with cell growth inhibition activity. In vitro molecular dynamics were compared between U3-1402 and trastuzumab (anti-HER2 antibody) using the MDA-MB-453 cell line, which expresses both HER2 and HER3. The activity of patritumab (naked antibody of U3-1402) was tested. In vivo efficacy of U3-1402 was also evaluated and immunostaining of HER3 and γH2AX (marker of DNA double-stranded breaks) as well as drug concentration assessment were performed. Results: U3-1402 bound to the cell surface of HER3-positive cell lines in HER3 expression level-dependent manner, and was then internalized into cells. Interestingly, the internalization rate of U3-1402 in MDA-MB-453 cells at 1 h was 64%, which was higher than that observed with trastuzumab (2%). Subsequently, U3-1402 was translocated to the lysosome, followed by payload release, leading to an improved cell growth inhibition compared to that noted with patritumab. In vivo antitumor efficacy was also demonstrated in a HER3-dependent manner in MDA-MB-453 xenograft model. In the study, internalization of HER3 from cell membrane into the intracellular compartment by U3-1402 treatment was observed, followed by increased free payload and γH2AX levels in tumor tissue. Conclusion: U3-1402 has a high internalization property for effective payload delivery to HER3-expressing cancer cells, resulting in a favorable ADC-driven efficacy. Citation Format: Kumiko Koyama, Yuuri Hashimoto, Yasuki Kamai, Yoshinobu Shiose, Manabu Abe, Yuki Kaneda, Naoyuki Maeda, Kenji Hirotani, Yusuke Ogitani, Tsuyoshi Karibe, Takashi Kagari, Kenichi Wakita, Suguru Ueno, Toshinori Agatsuma, Masato Murakami. U3-1402, a novel HER3-targeting antibody-drug conjugate, exhibits its antitumor activity through increased payload intracellular delivery via highly efficient drug internalization [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr LB-275.

Background: Patritumab is a fully human anti-HER3 monoclonal antibody that has shown potent antitumor activity in vivo. HER3 is a key dimerization partner for other HER family members, and studies suggest that the HER3:HER2 heterodimer is the most potent signaling pair. Therefore, combined inhibition of HER3 and HER2 may synergistically inhibit breast cancer tumor growth. In the CLEOPATRA trial, pertuzumab combined with trastuzumab and docetaxel prolonged progression-free survival compared to trastuzumab and docetaxel alone, demonstrating the benefit of comprehensive blockade of HER2 dimer signaling. Trastuzumab is approved in combination with paclitaxel for first-line treatment of HER2+ MBC. This phase 1b/2 study is investigating patritumab in combination with trastuzumab and paclitaxel in patients (pts) with newly-diagnosed MBC. Results of the phase 1b portion are reported here. Methods: Eligible pts had HER2+ newly-diagnosed MBC. In the open-label, phase 1b portion of this trial, pts received intravenous (IV) patritumab 18 mg/kg in combination with trastuzumab (8 mg/kg IV loading dose; 6 mg/kg IV maintenance dose) and paclitaxel (175 mg/m2 IV) every 3 weeks (Q3W). In the event that 18 mg/kg was not tolerated based on dose-limiting toxicity (DLT) assessment, sequential cohorts were to receive de-escalating doses of patritumab. Phase 1b study end points included adverse event (AE) incidence, human antihuman antibody (HAHA) formation, pharmacokinetics (PK), and tumor response. Results: Six pts were enrolled in the phase 1b portion of the trial, with a median age of 61 years (range, 51-78). There were no reported DLTs. Grade ≥3 treatment-related AEs occurred in 3 pts: 1 pt had a serious AE of grade 3 pneumonia; 1 pt had grade 3 worsening of arm pain; 1 pt had grade 3 oral mucositis, prolonged QTc, flu-like syndrome, and increased transaminases. In this limited pt population, most ECG changes were within or slightly above normal limits of the QTc interval. Only 2 pts had increased values close to 50 msec compared with baseline. It should be noted that baseline QTc values for 3 pts (including the pt that experienced grade 3 prolonged QTc) were close to the upper limit of normal. There were no grade 4 AEs, and no other serious AEs. All other treatment-related AEs were grades 1 or 2. All 6 pts tested negative for HAHA formation after drug administration. PK data are consistent with previous studies with patritumab. Two pts had complete response (CR) as their best overall response, 2 pts had partial response (PR), 1 pt had stable disease, and 1 pt was not evaluated for tumor response. All 6 pts have discontinued treatment; 2 due to progressive disease, 3 due to pt decision (2 with CR and 1 with PR), and 1 at the investigator's discretion. Conclusions: Results to date indicate that the combination of patritumab with trastuzumab and paclitaxel is generally well tolerated, with a promising response rate. As no DLTs were reported, the recommended phase 2 dose is patritumab 18 mg/kg with trastuzumab (8 mg/kg loading; 6 mg/kg maintenance) and paclitaxel 175 mg/m2 Q3W. Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P4-16-09.

Purpose: HER3 is a key dimerization partner for other HER family members, and its expression is associated with poor prognosis. This first-in-human study of U3-1287 (NCT00730470), a fully human anti-HER3 monoclonal antibody, evaluated its safety, tolerability, and pharmacokinetics in patients with advanced solid tumor. Experimental Design: The study was conducted in 2 parts: part 1—sequential cohorts received escalating doses (0.3–20 mg/kg) of U3-1287 every 2 weeks, starting 3 weeks after the first dose; part 2—additional patients received 9, 14, or 20 mg/kg U3-1287 every 2 weeks, based on observed tolerability and pharmacokinetics from part 1. Recommended phase II dose, adverse event rates, pharmacokinetics, and tumor response were determined. Results: Fifty-seven patients (part 1: 26; part 2: 31) received U3-1287. As no dose-limiting toxicities were reported, the maximum-tolerated dose was not reached. The maximum-administered dose was 20 mg/kg every 2 weeks. The most frequent adverse events related to U3-1287 were fatigue (21.1%), diarrhea (12.3%), nausea (10.5%), decreased appetite (7.0%), and dysgeusia (5.3%). No patient developed anti-U3-1287 antibodies. In these heavily pretreated patients, stable disease was maintained 9 weeks or more in 19.2% in part 1 and 10 weeks or more in 25.8% in part 2. Conclusion: U3-1287 treatment was well tolerated, and some evidence of disease stabilization was observed. Pharmacokinetic data support U3-1287 dosing of 9 to 20 mg/kg every 2 to 3 weeks. Combination studies of U3-1287 are ongoing. Clin Cancer Res; 19(11); 3078–87. ©2013 AACR.

UNLABELLED Emerging evidence suggests that human epidermal growth factor receptor 3 plays a critical role in cell-survival and drug-resistance in cancer cells. Several kinds of agents targeting this receptor are currently progressing through preclinical or clinical investigations. These agents are usually monoclonal antibodies with unique characteristics, and some have shown efficacy and been welltolerated in clinical trials. For example, patritumab and seribantumab are thought to compete with ligand binding and have proven efficacy for some malignancies in Phase II clinical trials. LJM716 locks the human epidermal growth factor receptor 3 in the inactive conformation in both ligand-dependent and - independent cancers. Lumretuzumab is a glycoengineered antibody, which enhances antibody-dependent cell-mediated cytotoxicity. Duligotumab is an antibody that targets both the human epidermal growth factor receptors 1 and 3. Heregulin is a human epidermal growth factor receptor 3 ligand that represents an encouraging candidate biomarker for the prediction of the efficacy of agents targeting this receptor. CONCLUSION A number of antibodies that interact with human epidermal growth factor receptors have been evaluated for clinical use. Ongoing clinical trials will address the remaining issues related to optimization of drug combination therapy and improving the targeting of each agent to the most appropriate individuals.

// Giulia Bon 1, * , Rossella Loria 1, * , Carla Azzurra Amoreo 2 , Alessandra Verdina 1 , Isabella Sperduti 2 , Arianna Mastrofrancesco 3 , Silvia Soddu 1 , Maria Grazia Diodoro 2 , Marcella Mottolese 2 , Matilde Todaro 4 , Giorgio Stassi 4 , Michele Milella 5 , Ruggero De Maria 6 , Rita Falcioni 1 1 Department of Research, Advanced Diagnostic, and Technological Innovation, IRCCS Regina Elena National Cancer Institute, Rome, Italy 2 Department of Research, Advanced Diagnostic, and Technological Innovation, IRCCS Regina Elena National Cancer Institute, Rome, Italy 3 Physiopathology Laboratory of Skin, IRCCS San Gallicano Dermatological Institute, Rome, Italy 4 Surgical and Oncological Sciences, University of Palermo, Palermo, Italy 5 Department of Experimental Clinical Oncology, IRCCS Regina Elena National Cancer Institute, Rome, Italy 6 General Pathology, Catholic University of Rome, Rome, Italy * These authors contributed equally to this work Correspondence to: Rita Falcioni, email: rita.falcioni@ifo.gov.it Ruggero De Maria, email: demaria@iss.it Keywords: colon cancers, HER3, PI3K, MAPK, drug resistance Received: April 21, 2016 Accepted: July 10, 2016 Published: August 19, 2016 ABSTRACT Although the medical treatment of colorectal cancer has evolved greatly in the last years, a significant portion of early-stage patients develops recurrence after therapies. The current clinical trials are directed to evaluate new drug combinations and treatment schedules. By the use of patient-derived or established colon cancer cell lines, we found that the tyrosine kinase receptor HER3 is involved in the mechanisms of resistance to therapies. In agreement, the immunohistochemical analysis of total and phospho-HER3 expression in 185 colorectal cancer specimens revealed a significant correlation with lower disease-free survival. Targeting HER3 by the use of the monoclonal antibody patritumab we found induction of growth arrest in all cell lines. Despite the high efficiency of patritumab in abrogating the HER3-dependent activation of PI3K pathway, the HER2 and EGFR-dependent MAPK pathway is activated as a compensatory mechanism. Interestingly, we found that the MEK-inhibitor trametinib inhibits, as expected, the MAPK pathway but induces the HER3-dependent activation of PI3K pathway. The combined treatment results in the abrogation of both PI3K and MAPK pathways and in a significant reduction of cell proliferation and survival. These data suggest a new strategy of therapy for HER3-overexpressing colon cancers.