Cysteine Cathepsins in Breast Cancer: Promising Targets for Fluorescence-Guided Surgery

Abstract

The majority of breast cancer patients is treated with breast-conserving surgery (BCS) combined with adjuvant radiation therapy. Up to 40% of patients has a tumor-positive resection margin after BCS, which necessitates re-resection or additional boost radiation. Cathepsin-targeted near-infrared fluorescence imaging during BCS could be used to detect residual cancer in the surgical cavity and guide additional resection, thereby preventing tumor-positive resection margins and associated mutilating treatments. The cysteine cathepsins are a family of proteases that play a major role in normal cellular physiology and neoplastic transformation. In breast cancer, the increased enzymatic activity and aberrant localization of many of the cysteine cathepsins drive tumor progression, proliferation, invasion, and metastasis. The upregulation of cysteine cathepsins in breast cancer cells indicates their potential as a target for intraoperative fluorescence imaging. This review provides a summary of the current knowledge on the role and expression of the most important cysteine cathepsins in breast cancer to better understand their potential as a target for fluorescence-guided surgery (FGS). In addition, it gives an overview of the cathepsin-targeted fluorescent probes that have been investigated preclinically and in breast cancer patients. The current review underscores that cysteine cathepsins are highly suitable molecular targets for FGS because of favorable expression and activity patterns in virtually all breast cancer subtypes. This is confirmed by cathepsin-targeted fluorescent probes that have been shown to facilitate in vivo breast cancer visualization and tumor resection in mouse models and breast cancer patients. These findings indicate that cathepsin-targeted FGS has potential to improve treatment outcomes in breast cancer patients.

Keywords: Breast cancer; Cysteine cathepsins; Fluorescence-guided surgery; Near-infrared fluorescence imaging; Targeted molecular imaging.

Fig. 1

Cathepsin-targeted fluorescence-guided surgery. 1 A quenched, cathepsin-activatable fluorescent probe is administered intravenously prior to surgery or topically during surgery. 2 The probe is activated by cysteine cathepsins overexpressed by tumor and/or stromal cells. 3 The fluorescence signal generated by the activated probe is detected using a NIR sensitive camera system and 4 displayed on a screen in the operating theater. Q, quencher; F, fluorophore; pacman shape, cysteine cathepsin activating the probe; green dotted arrow, fluorescent signal generated by the activated probe in the tumor after illumination with NIR light from the camera system. Abbreviations: NIR, near infrared.

Fig. 2

Different types of quenched cathepsin-activatable fluorescent probes. A Quenched substrate-based probe. The probe is activated by enzymatic cleavage of the peptide linker by a target cathepsin. Upon cleavage, two fragments—one containing the quencher and the other the now unquenched fluorophore—are released. B Quenched activity-based probe. The probe covalently binds in the active site of the target cathepsin forming a permanent bond. Upon binding the active site, the quencher is released and the probe is activated. Abbreviations: qSPB, quenched substrate-based probe; qABP, quenched activity-based probe; Q, quencher; F, fluorophore.