Structure-activity relationship among purpurinimides and bacteriopurpurinimides: Trifluoromethyl substituent enhanced the photosensitizing efficacy

At similar lipophilicity, compared to the nonfluorinated purpurinimide 11, the corresponding fluorinated analog 8 with a trifluoromethyl substituent at the lower half (position-13²) of the molecule showed enhanced photosensitizing efficacy. The structural parameters established in purpurinimides (λ_max: 700 nm) were successfully translated to the bacteriopurpurin imide system 19 (λ_max: 792 nm) and within both series, a monotonic relationship between the lipophilicity and the in vivo PDT activity was observed. For preparing water-soluble compounds, the photosensitizers 8 and 19 were converted into the corresponding aminobenzyldiethylenetriamine pentaacetate conjugates 23 and 26. Acid treatment of purpurinimide 23 produced the corresponding water-soluble analog 24. Bacteriochlorin 26 under acidic or basic conditions mainly gave the decomposition products. At similar in vivo treatment conditions (C3H mice with RIF tumors and BALB-C mice with colon-26 tumors) the water-soluble purpurinimide 24 was found to be more effective than the methyl ester analog 8. These results suggest that besides overall lipophilicity the inherent charge of the photosensitizer also influences the PDT efficacy.