Synthesis, photophysical properties, tumor uptake, and preliminary in vivo photosensitizing efficacy of a homologous series of 3-(1′-alkyloxy)-ethyl-3-devinylpurpurin-18-N-alkylimides with variable lipophilicity

Starting from methylpheophorbide-a, a homologous series of purpurinimides containing alkyl substituents at two different positions [as 3-(1¹-O-alkyl) and 13²-N-alkyl] were synthesized. These compounds with variable lipophilicity (log P 5.32-16.44) exhibit long wavelength absorption near) λ_max700 nm (ε: 45 000 in dichloromethane) with singlet oxygen (¹O₂) production in the range of 57-60%. The shifts in in vivo absorptions and tumor/skin uptake of these compounds were determined in C₃H mice bearing RIF tumors by in vivo reflectance spectroscopy. The results obtained from a set of photosensitizers with similar lipophilicity (log P 10.68-10.88) indicate that besides the overall lipophilicity, the presence and position of the alkyl groups (O-alkyl vs N-alkyl) in a molecule play an important role in tumor uptake, tumor selectivity, and in vivo PDT efficacy. At present, all purpurinimide analogues are being evaluated at various doses, and experiments are underway to establish a quantitative structure-activity relationship on a limited set of compounds. The 1D and 2D NMR and mass spectrometry analyses confirmed the structures of the desired purpurinimides and the byproducts formed during various reaction conditions. The mechanisms of the formation of the unexpected 12-formyl- and 12-(hydroxymethyl)purpurinimides under certain reaction conditions are also discussed.