Minibody-Indocyanine Green Based Activatable Optical Imaging Probes: The Role of Short Polyethylene Glycol Linkers

Abstract

Minibodies show rapider blood clearance than IgGs due to smaller size that improves target-to-background ratio (TBR) in in vivo imaging. Additionally, the ability to activate an optical probe after binding to the target greatly improves the TBR. An optical imaging probe based on a minibody against prostate-specific membrane antigen (PSMA-MB) and conjugated with an activatable fluorophore, indocyanine green (ICG), was designed to fluoresce only after binding to cell-surface PSMA. To further reduce background signal, short polyethylene glycol (PEG) linkers were employed to improve the covalent bonding ratio of ICG. New PSMA-MBs conjugated with bifunctional ICG derivatives specifically visualized PSMA-positive tumor xenografts in mice bearing both PSMA-positive and -negative tumors within 6 h postinjection. The addition of short PEG linkers significantly improved TBRs; however, it did not significantly alter the biodistribution. Thus, minibody-ICG conjugates could be a good alternative to IgG-ICG in the optical cancer imaging for further clinical applications.

PSMA-targeted contrast agents for intraoperative imaging of prostate cancer,

“…7–9 However, surgery is performed without intraoperative image guidance because there are no clinically available optical contrast agents to help the surgeon ensure clean tumor margins and find small occult metastases in the surgical field. 9–11 …”

“…Overall, PEG4 spacer (–CH 2 –CH 2 –O–; C: sp 3 hybridization) provided sufficient flexibility for KUE to overcome spatial limitations in order to effectively interact with the PSMA on the cell membrane. 10,11 …”

“…3b, in vitro PSMA targeting efficiency revealed that PEG2 (10 atoms, ≈11 Å) and PEG4 (16 atoms, ≈18 Å) provided an optimum length for keeping the potent PSMA specificity of KUE. 10,11 Interestingly, LNCaP cells treated with KUE-ZW800+3C showed negligible signals on the cell membrane because the S1 pocket of PSMA is a tunnel-like region, about 20 Å towards the surface of the enzyme. 5,27 On the other hand, PEG12 (40 atoms, ≈47 Å) and PEG24 (76 atoms, ≈89 Å) resulted in diminished NIR signals on the membrane of LNCaP cells due to the steric hindrance of long, flexible linkers, where the KUE moiety buried with limited exposure to PSMA.…”

Monoclonal antibody-based optical molecular imaging probes; considerations and caveats in chemistry, biology and pharmacology

“…[30,31] Thus, Ab-ICG conjugates are capable of becoming activatable with conjugation ratios as low as 1:1 to 1.5:1, however, during purification processes, complete removal of non-covalently associated ICG-derivative to antibody was rather difficult. [32–34] An important caveat is that idiosyncratic biodistributions of mAb-conjugates can defeat attempts to create an activatable probe. For instance when a diabody (Db) was activatably labeled with ICG it was not a successful imaging agent.…”

Current Trends and Emerging Future of Indocyanine Green Usage in Surgery and Oncology: An Update

“…Preclinical trials of ICG-loaded nanoparticles for tumorspecific diagnosis and therapy are underway. [1][2][3][4][5][6][7][8][9][10][11][12][13] In 2011, Polom et al presented a thoughtful review of the trends and future directions of ICG in surgical oncology, focusing on its role as an NIR fluorophore that facilitates sentinel lymph node (SLN) mapping. 14 Since then, many series have been published.…”