Light-controlled endosomal escape of the novel CD133-targeting immunotoxin AC133–saporin by photochemical internalization — A minimally invasive cancer stem cell-targeting strategy

Abstract

The cancer stem cell (CSC) marker CD133 is an attractive target to improve antitumor therapy. We have used photochemical internalization (PCI) for the endosomal escape of the novel CD133-targeting immunotoxin AC133–saporin (PCIAC133–saporin). PCI employs an endocytic vesicle-localizing photosensitizer, which generates reactive oxygen species upon light-activation causing a rupture of the vesicle membranes and endosomal escape of entrapped drugs. Here we show that AC133–saporin co-localizes with the PCI-photosensitizer TPCS2a, which upon light exposure induces cytosolic release of AC133–saporin. PCI of picomolar levels of AC133–saporin in colorectal adenocarcinoma WiDr cells blocked cell proliferation and induced 100% inhibition of cell viability and colony forming ability at the highest light doses, whereas no cytotoxicity was obtained in the absence of light. Efficient PCI-based CD133-targeting was in addition demonstrated in the stem-cell-like, triple negative breast cancer cell line MDA-MB-231 and in the aggressive malignant melanoma cell line FEMX-1, whereas no enhanced targeting was obtained in the CD133-negative breast cancer cell line MCF-7. PCIAC133–saporin induced mainly necrosis and a minimal apoptotic response based on assessing cleavage of caspase-3 and PARP, and the TUNEL assay. PCIAC133–saporin resulted in S phase arrest and reduced LC3-II conversion compared to control treatments. Notably, co-treatment with Bafilomycin A1 and PCIAC133–saporin blocked LC3-II conversion, indicating a termination of the autophagic flux in WiDr cells. For the first time, we demonstrate laser-controlled targeting of CD133 in vivo. After only one systemic injection of AC133–saporin and TPCS2a, a strong anti-tumor response was observed after PCIAC133–saporin. The present PCI-based endosomal escape technology represents a minimally invasive strategy for spatio-temporal, light-controlled targeting of CD133+ cells in localized primary tumors or metastasis.