Pten loss promotes MAPK pathway dependency in HER2/neu breast carcinomas.

Loss of the tumor suppressor gene PTEN is implicated in breast cancer progression and resistance to targeted therapies, and is thought to promote tumorigenesis by activating PI3K signaling. In a transgenic model of breast cancer, Pten suppression using a tetracycline-regulatable short hairpin (sh)RNA cooperates with human epidermal growth factor receptor 2 (HER2/neu), leading to aggressive and metastatic disease with elevated signaling through PI3K and, surprisingly, the mitogen-activated protein kinase (MAPK) pathway. Restoring Pten function is sufficient to down-regulate both PI3K and MAPK signaling and triggers dramatic tumor regression. Pharmacologic inhibition of MAPK signaling produces similar effects to Pten restoration, suggesting that the MAPK pathway contributes to the maintenance of advanced breast cancers harboring Pten loss.

p32 is a novel mammalian Lgl binding protein that enhances the activity of protein kinase Czeta and regulates cell polarity.

Lgl (lethal giant larvae) plays an important role in cell polarity. Atypical protein kinase C (aPKC) binds to and phosphorylates Lgl, and the phosphorylation negatively regulates Lgl activity. In this study, we identify p32 as a novel Lgl binding protein that directly binds to a domain on mammalian Lgl2 (mLgl2), which contains the aPKC phosphorylation site. p32 also binds to PKCzeta, and the three proteins form a transient ternary complex. When p32 is bound, PKCzeta is stimulated to phosphorylate mLgl2 more efficiently. p32 overexpression in Madin-Darby canine kidney cells cultured in a 3D matrix induces an expansion of the actin-enriched apical membrane domain and disrupts cell polarity. Addition of PKCzeta inhibitor blocks apical actin accumulation, which is rescued by p32 overexpression. p32 knockdown by short hairpin RNA also induces cell polarity defects. Collectively, our data indicate that p32 is a novel regulator of cell polarity that forms a complex with mLgl2 and aPKC and enhances aPKC activity.

A Phase 1 Study of a CDH6-Targeting Antibody-Drug Conjugate in Patients with Advanced Solid Tumors with Evaluation of Inflammatory and Neurological Adverse Events.

PURPOSE: This first-in-human study (NCT02947152) evaluated the safety, tolerability, pharmacokinetics, and preliminary efficacy of HKT288, a first-in-class CDH6-targeting antibody-drug conjugate (ADC). EXPERIMENTAL DESIGN: HKT288 was administered intravenously (IV) every 3 weeks until patients experienced unacceptable toxicity or progressive disease (PD). The starting dose of 0.3 mg/kg was determined based on the highest nonseverely toxic dose in monkeys, which was 2 mg/kg IV weekly. Based on preclinical toxicology, skin, eyes, bone marrow, and liver were expected targets of toxicity. RESULTS: Nine patients were enrolled: 5 with renal cell carcinoma and 4 with epithelial ovarian cancer. The best overall response on the 0.3 mg/kg cohort in patients with measurable disease was RECIST v1.1 stable disease in 3 patients and PD in 2 patients. The most frequent adverse events (AEs) regardless of causality were pyrexia (44.4%), constipation (44.4%), fatigue (33.3%), and vomiting (33.3%). Three suspected-related neurologic AEs (Grade 2) were reported on the 0.75 mg/kg cohort: seizure in 1 patient and another patient with aphasia and encephalopathy. Further studies were unable to identify the underlying mechanism of the neurologic AEs, and the study was terminated early. CONCLUSIONS: Preclinical toxicology did not predict the neurotoxicity observed with HKT288, and a comprehensive assessment performed post hoc did not identify the mechanism of toxicity. The development of further CDH6-targeting ADCs should be pursued with caution.

Discovery and Optimization of HKT288, a Cadherin-6-Targeting ADC for the Treatment of Ovarian and Renal Cancers.

Despite an improving therapeutic landscape, significant challenges remain in treating the majority of patients with advanced ovarian or renal cancer. We identified the cell-cell adhesion molecule cadherin-6 (CDH6) as a lineage gene having significant differential expression in ovarian and kidney cancers. HKT288 is an optimized CDH6-targeting DM4-based antibody-drug conjugate (ADC) developed for the treatment of these diseases. Our study provides mechanistic evidence supporting the importance of linker choice for optimal antitumor activity and highlights CDH6 as an antigen for biotherapeutic development. To more robustly predict patient benefit of targeting CDH6, we incorporate a population-based patient-derived xenograft (PDX) clinical trial (PCT) to capture the heterogeneity of response across an unselected cohort of 30 models-a novel preclinical approach in ADC development. HKT288 induces durable tumor regressions of ovarian and renal cancer models in vivo, including 40% of models on the PCT, and features a preclinical safety profile supportive of progression toward clinical evaluation.Significance: We identify CDH6 as a target for biotherapeutics development and demonstrate how an integrated pharmacology strategy that incorporates mechanistic pharmacodynamics and toxicology studies provides a rich dataset for optimizing the therapeutic format. We highlight how a population-based PDX clinical trial and retrospective biomarker analysis can provide correlates of activity and response to guide initial patient selection for first-in-human trials of HKT288. Cancer Discov; 7(9); 1030-45. ©2017 AACR.This article is highlighted in the In This Issue feature, p. 920.

An antibody that locks HER3 in the inactive conformation inhibits tumor growth driven by HER2 or neuregulin.

HER2/HER3 dimerization resulting from overexpression of HER2 or neuregulin (NRG1) in cancer leads to HER3-mediated oncogenic activation of phosphoinositide 3-kinase (PI3K) signaling. Although ligand-blocking HER3 antibodies inhibit NRG1-driven tumor growth, they are ineffective against HER2-driven tumor growth because HER2 activates HER3 in a ligand-independent manner. In this study, we describe a novel HER3 monoclonal antibody (LJM716) that can neutralize multiple modes of HER3 activation, making it a superior candidate for clinical translation as a therapeutic candidate. LJM716 was a potent inhibitor of HER3/AKT phosphorylation and proliferation in HER2-amplified and NRG1-expressing cancer cells, and it displayed single-agent efficacy in tumor xenograft models. Combining LJM716 with agents that target HER2 or EGFR produced synergistic antitumor activity in vitro and in vivo. In particular, combining LJM716 with trastuzumab produced a more potent inhibition of signaling and cell proliferation than trastuzumab/pertuzumab combinations with similar activity in vivo. To elucidate its mechanism of action, we solved the structure of LJM716 bound to HER3, finding that LJM716 bound to an epitope, within domains 2 and 4, that traps HER3 in an inactive conformation. Taken together, our findings establish that LJM716 possesses a novel mechanism of action that, in combination with HER2- or EGFR-targeted agents, may leverage their clinical efficacy in ErbB-driven cancers.