Photodynamic therapy: Tumor targeting with adenoviral proteins

A brief summary of the mechanisms involved in photodynamic therapy (PDT) an d the role of delivery vehicles for photosensitizer targeting is addressed. Phthalocyanines (Pc) have been coupled to adenovirus type 2 capsid protein s including the hexon, the penton base and the fiber to enhance their targe t selectivity. Adenovirus penton base proteins contain the arginine-glycine -aspartic acid peptidic sequence (RGD) moth known to bind with great affini ty and high specificity to integrin receptors, expressed by several types o f cancer. Tetrasulfonated aluminum phthalocyanine (AIPcS(4)) was covalently coupled tee the various capsid proteins via one or two caproic acid spacer chains (A(1) of A(2)) in 7:1 up to 66:1 molar ratios, The capacity of the bioconjugates for singlet oxygen production, as measured by an L-tryptophan oxidation assay, was strongly reduced, likely reflecting scavenging by the carrier. Cell adsorption and in vitro photocytotoxicity assays were carrie d out using the A549 and HEp2 human cell lines expressing integrin receptor s, and one murine, the EMT-6 cell line, which lacks receptors for the RGD s equence, The AIPcS(4)A(2)-protein complexes induced greater cytotoxicity as compared to the analogous AIPcS(4)A(1) preparations. The penton base-AIPcS (4)A(2) derivative was the more phototoxic for all cell lines tested. Tumor response studies using Balb/c mice with EMT-6 tumor implants demonstrated that the free AIPcS(4)A(2), induced complete tumor regression at a dose of 1 mu mol/ kg and 400 J/cm(2), which is comparable to the activity of the kn own AIPcS2(adj). A mixture of adenovirus type 2 soluble proteins covalently labeled with AIPcS(4)A(2) required 0.5 mu mol/kg to induce the same respon se with the same light dose, suggesting that the high affinity RGD/receptor complex is able to target Pc for PDT.