Photochemical internalization (PCI) of HER2-targeted toxins: synergy is dependent on the treatment sequence

Photochemical internalization (PCI) is a modality for cytosolic release of drugs trapped in endocytic vesicles. The method is based upon photosensitizers localized in the membranes of endocytic vesicles which create membrane rupture upon light exposure by generating reactive oxygen species (ROS), predominantly singlet oxygen ((1)O(2)).

The human epidermal growth factor receptor 2 (HER2)-targeted immunotoxin (IT), trastuzumab-saporin, was evaluated in combination with PCI using TPCS(2a) (Amphinex®), a new photosensitizer approved for clinical use.

PCI synergistically enhanced the cytotoxicity of trastuzumab-saporin on trastuzumab-resistant HER2(+) Zr-75-1 cells. The PCI effect was only observed when the IT was administered prior to the photochemical treatment ("light after" strategy), while administration of a non-targeted drug may equally well be performed after light exposure. Mechanistic studies showed reduced ligand-induced HER2 phosphorylation and receptor-mediated endocytosis after TPCS(2a)-PDT. Photochemical disruption of the cytoplasmic domain of HER2 was found to be induced by (1)O(2) generated both by photosensitizer located in the endocytic vesicles and in the outer leaflet of the plasma membrane.

Administration of the HER2-targeted toxin prior to light exposure is a prerequisite for successful PCI-mediated delivery of HER2-targeted toxins.

PCI of HER2-targeted toxins is demonstrated as a highly effective treatment modality which may overcome trastuzumab resistance. The mechanistic studies of the lack of PCI effect of the "light first" procedure is of outermost importance when designing a clinical PCI treatment protocol for delivery of HER2-targeted therapies.