Development of bacteriochlorophyll a-based near-infrared photosensitizers conjugated to gold nanoparticles for photodynamic therapy of cancer

We report the synthesis and characterization of a new sulfur-containing derivative of bacteriochlorophyll a. The latter was isolated from biomass of the nonsulfur purple bacterium Rhodobacter capsulatus strain B10. The developed photosensitizer is N-aminobacteriopurpurinimide with an exocyclic amino group acylated with a lipoic acid moiety, which is a biogenic substance that acts as a cofactor of the pyruvate dehydrogenase and α-ketoglutarate dehydrogenase complexes in the body. The disulfide moiety of lipoic acid confers the compound aurophilicity, thus allowing its conjugation with gold nanoparticles (NP-Au) via S—Au bonds. The shape and the size of the resulting nanoconjugate with immobilized photosensitizer (PS—Au) were assessed by dynamic light scattering and transmission electron microscopy. The conjugated nanoparticles are spherical with hydrodynamic diameter of 100–110 nm. The PS—Au conjugate absorbs light at 824 nm and emits strong fluorescence at 830 nm, which allowed in vivo study of its dynamic biodistribution in rats bearing sarcoma M-1. Compared to the free photosensitizer, PS loaded on the gold nanoparticles (PS—Au) showed extended circulation time in the blood and enhanced tumor uptake due to nonspecific passive targeting when the drug accumulates in tumor sites through the leaky tumor neovasculature and does not return to the circulation.