Efficacy of a leptin receptor antagonist peptide in a mouse model of triple-negative breast cancer

Abstract

Triple-negative breast cancers, which represent 10–20% of all mammary tumours, are characterised by the aggressive phenotype, are often found in younger women and have been associated with poor prognosis. Obesity increases the risk for triple-negative breast cancer development. Because triple-negative breast cancer patients are unresponsive to current targeted therapies and other treatment options are only partially effective, new pharmacological modalities are urgently needed. Here we examined if the leptin (obesity hormone) receptor is a viable target for the treatment of this cancer subtype. In human triple-negative breast cancer tissues, the leptin receptor was expressed in 92% (64/69) and leptin in 86% (59/69) of cases. In a model triple-negative breast cancer cell line MDA-MB-231, the leptin receptor antagonist peptide Allo-aca inhibited leptin-induced proliferation at 50 pM concentration. In an MDA-MB-231 orthotopic mouse xenograft model, Allo-aca administered subcutaneously significantly extended the average survival time from 15.4 days (untreated controls) to 24 and 28.1 days at 0.1 and 1 mg/kg/day doses, respectively. In parallel, conventional treatment with 1 mg/kg/day intraperitoneal cisplatin prolonged the average survival time to 18.6 days, while administration of 20 mg/kg/day oral Tamoxifen (negative control) had no significant survival effects relative to controls. In normal CD-1 mice, Allo-aca produced no systemic toxicity up to the highest studied subcutaneous bolus dose of 50 mg/kg, while, as expected, it induced a modest 6–10% body weight increase. Our results indicate that leptin receptor antagonists could become attractive options for triple-negative breast cancer treatment, especially in the obese patient population.

Introduction

Approximately 10–20% of breast cancers are considered triple-negative (TNBC) as they lack oestrogen and progesterone receptors (ER, PgR) and do not express or express low levels of the oncogenic receptor HER2.¹ TNBC are associated with shorter relapse-free survival and increased lymphovascular invasion.² The disease affects more frequently younger patients and it is more prevalent in African Americans and Hispanics.³ Currently no targeted therapies exist for TNBC, while conventional chemotherapy is only partially effective.⁴ Until recently, high dose combination chemotherapy was the preferred treatment option for TNBC.⁵ Current clinical trials with cisplatin show promising results,⁶ although the response rates with a combined surgery, cisplatin and radiotherapy regimen remain at 50–60%.⁷ Consequently, the development of novel targeted therapies for TNBC is of paramount importance.

The risk of TNBC is increased by obesity, especially upper-body obesity.8, 9, 10 Among obesity-related factors that are known to impact breast cancer development and progression, the prominent place is occupied by the adipokine leptin.¹¹ Leptin (obesity hormone) is produced mainly by the fat tissue, but it can also be synthesised by breast cancer cells in response to obesity-related stimuli.12, 13, 14, 15 Leptin induces breast cancer cell growth, transformation, and survival and reduces the efficacy of breast cancer treatments.11, 16, 17 Both leptin and its receptor (ObR) are overexpressed in breast cancer, especially in higher grade tumours, but are absent in normal epithelial breast tissues.11, 12, 18 The expression of the leptin system is found in 70–80% of breast cancer cases, including hormone receptor-and HER2-positive and negative cancers.12, 18 Consequently, in recent years, the leptin/ObR system has emerged as a new and promising therapeutic target for breast cancer therapy.¹⁹ The data suggest that ObR is an independent biomarker, not correlating with ER/PgR or HER2, thus ObR antagonists could be useful for management of different breast cancer subtypes.17, 20

In the course of development of specific, selective and clinically relevant ObR antagonists for cancer treatment,²¹ we generated Allo-aca (H-alloThr-Glu-Nva-Val-Ala-Leu-Ser-Arg-Aca-NH₂), a 9-residue peptidomimetic.²² The Allo-aca sequence is based on the C-terminal ObR-binding leptin site III.²³ Allo-aca inhibits leptin-induced proliferation and oncogenic signalling of MCF-7 cells and various glioblastoma cell lines at high pM concentrations. Furthermore, in an orthotopic mouse model of human hormone responsive breast cancer, the peptide added intraperitoneally (ip) or subcutaneously (sc) at 0.1 mg/kg/day doses reduces the growth of MCF-7 cell xenografts by approximately 50%. In the current report, we investigated the expression levels of ObR in tissues of human TNBC and assessed the potential of targeting ObR in in vitro and in vivo models of TNBC.