Shiga-like Toxin Subunit B (SLTB)-Enhanced Delivery of Chlorin e6 (Ce6) Improves Cell Killing

We used Shiga-like toxin B subunit (SLTB) to deliver the photosensitizer, chlorin e6 (Ce6), to Vero cells expressing the Gb3 receptor. Our aim was to provide an example of carrier-enhanced photodynamic cell killing with which to start a systematic consideration of photosensitizer delivery at the subcellular level. SLTB, in contrast to many other potential protein carriers, is delivered intracellularly to the Golgi apparatus and endoplasmic reticulum (ER). Ce6 was chosen both for its phototoxic properties and its potential for covalent conjugation with SLTB. Ce6-SLTB after cleanup contained ≤10% noncovalently bound Ce6. The noncovalent binding of porphyrins and chlorins to protein conjugates has been well documented, and hence the effective cleanup procedure is a significant accomplishment. We demonstrate that Ce6-SLTB enhances delivery of Ce6 to target cells as compared to free Ce6. In Vero cells, Ce6-SLTB was over an order of magnitude more photodynamically toxic than free Ce6. Moreover, we show that in the case of Ce6-SLTB, photosensitizer accumulation is in a combination of subcellular sites including mitochondria, Golgi apparatus, ER and plasma membrane. The occurrence in nature of diverse B subunit binding sites and the possibilities of varied intracellular delivery make optimized use of B subunit carriers attractive.