Anticancer drug released from near IR-activated prodrug overcomes spatiotemporal limits of singlet oxygen

Abstract

Photodynamic therapy (PDT) is a cancer treatment modality where photosensitizer (PS) is activated by visible and near IR light to produce singlet oxygen (1O2). However, 1O2 has a short lifetime (<40ns) and cannot diffuse (<20nm) beyond the cell diameter (e.g., ∼1800nm). Thus, 1O2 damage is both spatially and temporally limited and does not produce bystander effect. In a heterogeneous tumor, cells escaping 1O2 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 1O2-cleavable linker (L). Upon illumination of the prodrug, 1O2 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 1O2. 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 1O2, providing far superior antitumor effect.