Stable Synthetic Bacteriochlorins: Potent Light-Activated Anti-Cancer Drugs

Photodynamic therapy (PDT) uses non-toxic dyes called photosensitizers (PS) to absorb light of visible or near-infrared wavelengths at non-thermal power densities that (in the presence of oxygen) combine together to produce reactive oxygen species (ROS). The main ROS formed are singlet oxygen and hydroxyl radicals that can both have cytotoxic effects. PDT has two separate mechanisms of specificity due to specific accumulation in the tumor coupled with spatially confined illumination. The first-generation PS were porphyrins, but due to the need for long-wavelength activation to improve tissue penetration, second- generation chlorins (far red) and bacteriochlorins (BC) (near infrared [NIR]) have been investigated. This review covers quantitative structure-function relationship with charge and lipophilicity, the effect the central metal atom has on determining the photochemical mechanism, the effect that electron withdrawing di-cyano groups have on preventing photobleaching, the improvement in activity found when micellar nanocarriers are employed, and the relative superiority of mono-substitution over symmetrical di- substitution. Finally, the necessity for long-wavelength (730 nm) activation is underlined by looking at PDT of pigmented melanoma in vitro and in a mouse model of melanoma.