Figure 6.

A schematic representation of the three archetypal targeted PDT probes that either selectively deliver PSs to tumor cells (A), are selectively activated within tumor tissue (B) or are activated within tumor cells following targeted delivery (C). A) Targeted probes conjugated to PS molecules can be delivered selectively to tumors cells through the blood or through direct contact on surfaces, whereby the probes actively bind to their targets and deliver their payload in a disease-specific manner. B) Microenvironmental characteristics of tumors, such as tumor-specific proteases that are upregulated during disease progression can be targeted with substrate-mimetic quenched PDT probes that become selectively activated within the tumor upon cleavage. C) Some activatable targeted probes combine the mechanisms of probe systems described in (B) and (C) to selectively deliver the PS payload to tumor cells, yet remain optically inactive until cancer-cell specific internalization, proteolytic degradation and PS dequenching.