Rational design of a receptor-targeted photodynamic molecular beacon for the multi-level control of singlet oxygen production and PDT activity in cancer cells

Photodynamic therapy (PDT) involves the combined action of light, oxygen and a photosensitizer (PS). It offers unique control in the PS's action because the key cytotoxic agent, singlet oxygen (O-1(2)), is only produced in situ upon irradiation. The O-1(2) production can be controlled in three levels. The first level involves the judicious use of fiber optics to selectively deliver light to disease tissues. The second level is to exert control over the PS's localization by selectively delivering PS to cancer cells. The third level is to exert control of the PS's ability to generate O-1(2) in responding to specific cancer biomarkers. Here, we present two PDT agents based on the latter two levels of O-1(2) control. The first PDT agent "PPF" contains a PS (Pyro) and a tumor homing molecule (folate) and a peptide linker. PPF was found to be selectively accumulated in cancer cells via folate receptor (FR) pathway. The second PDT agent "PPMMP7B" is a matrix metalloproteinase-7 (MMP7)-triggered photodynamic molecular beacon (PMB) containing a PS (Pyro), a O-1(2) quencher (BHQ3) and a MMP7-cleavable peptide linker. Thus, the O-1(2) production of PPMMP7B is highly sequence-specific and its photodynamic cytotoxicity is NMP7-dependent. Since these agents are designed to share functional modules (PS and peptide linker) and common cancer cell model (KB cells overexpress both FR and NMP7), it forms the basis for rational design of receptor-targeted PMB for achieving a multi-level control of O-1(2) production in cancer cells, which in term, could provide a much higher level of PDT selectivity.