Peptide-mediated targeting of Quantum Dots in a 3D model of head and neck cancer

Oral squamous cell carcinoma (OSCC) treatment mainly relies on surgery. The status of surgical margin is a major prognostic factor for patients as positive margins are associated with lower survival. However, the anatomical particularities of this area complicate margin establishment. Fluorescence guided surgery (FGS) could be employed as an intraoperative technique to improve tumor resection and margin investigation. Quantum dots (QDs) serve as ideal contrast agents in this technique due to their brightness and stability. Since αVβ6 integrin is overexpressed in OSCC, coupling QDs with A20FMDV2 peptide (QDs-A20) targeting the αVβ6 integrin constitute a real opportunity. This study investigates the accumulation of QDs-A20 in 2D and 3D tongue cancer models, as well as QDs coupled to a scrambled version of this peptide (QDs-Scr) or without peptide (QDs-SPP), for imaging purposes.

CdSeCdS/ZnS quantum dots were coated with sulfobetaine polymers (QDs-SPP) and conjugated to A20FMDV2 peptide (QDs-A20) or its scrambled version (QDs-Scr). Two-dimensional (2D) and three-dimensional (3D) tongue cancer cells HSC-3 were employed to test the effectiveness of intracellular accumulation of all types of QDs. Targeting ability of each QDs was assessed by flow cytometry, while the depth of penetration into cancerous spheroids was assessed by fluorescence microscopy.

QDs coating with sulfobetaines polymers (QDs-SPP) completely prevented their internalization by HSC-3 cells in 2D and 3D models, making QDs stealthy and preventing their non-specific accumulation. Conversely, peptides conjugated QDs (QDs-A20 & QDs-Scr) labeled HSC-3 monolayers and managed to label spheroid periphery up to 23 µm deep. However, no difference in accumulation was found between these two QDs whereas only A20 peptide could potentially target αVβ6 integrin. It appears that peptide conjugation increased QDs zeta potential, promoting their adsorption and subsequent endocytosis by cells, independently from αVβ6 integrin.

The present study highlighted the impact of peptide conjugation on QDs internalization in 2D and 3D tongue cancer cell models. QDs-SPP were stealthy and did not accumulate in cells. Peptides conjugated QDs could be used as contrast agents, but in a passive targeting approach. Modifications to surface chemistry are required to target αVβ6 integrin through active targeting. This study also highlights the need for controls such as scrambled peptides, the absence of which can lead to misinterpretation of results.