Photodynamic therapy (
PDT) is a
cancer treatment modality where
photosensitizer (PS) is activated by visible and near IR light to produce
singlet oxygen ((1)O2). However, (1)O2 has a short lifetime (<40 ns) and cannot diffuse (<20 nm) beyond the cell diameter (e.g., ∼ 1800 nm). Thus, (1)O2 damage is both spatially and temporally limited and does not produce bystander effect. In a heterogeneous
tumor, cells escaping (1)O2 damage can regrow after
PDT treatment. To overcome these limitations, we developed a
prodrug concept (PS-L-D) composed of a
photosensitizer (PS), an anti-
cancer drug (D), and an (1)O2-cleavable linker (L). Upon illumination of the
prodrug, (1)O2 is generated, which damages the
tumor and also releases anticancer
drug. The locally released
drug could cause spatially broader and temporally sustained damage, killing the surviving
cancer cells after the
PDT damage. In our previous report, we presented the superior activity of our
prodrug of CA4 (combretastatin A-4), Pc-(L-CA4)2, compared to its non-cleavable analog, Pc-(NCL-CA4)2, that produced only
PDT effects. Here, we provide clear evidence demonstrating that the released anticancer
drug, CA4, indeed damages the surviving
cancer cells over and beyond the spatial and temporal limits of (1)O2. In the limited light illumination experiment, cells in the entire well were killed due to the effect of released anti-
cancer drug, whereas only a partial damage was observed in the pseudo-
prodrug treated wells. A time-dependent cell survival study showed more cell death in the
prodrug-treated cells due to the sustained damage by the released CA4. Cell cycle analysis and microscopic imaging data demonstrated the typical damage patterns by CA4 in the
prodrug treated cells. A time-dependent histological study showed that
prodrug-treated
tumors lacked mitotic bodies, and the
prodrug caused broader and sustained
tumor size reduction compared to those seen in the
tumors treated with the pseudo-
prodrug. This data consistently support that the released CA4 overcomes the spatiotemporal limitations of (1)O2, providing far superior antitumor effect.