Transferrin-conjugated liposome targeting of photosensitizer AlPcS4 to rat bladder carcinoma cells

Background: The efficacy and safety of photodynamic therapy for superficial bladder cancer depend on tumor-selective accumulation of the photosensitizer. Bladder transitional-cell carcinoma cells overexpress the transferrin receptor on their surface. We examined whether transferrin-mediated liposomal targeting of the photosensitizer aluminum phthalocyanine tetrasulfonate (AIPcS(4)) is an effective strategy to attain tumor-selective accumulation of this compound when applied intravesically. Methods: AlPcS4 was stably encapsulated in unconjugated liposomes (Lip-AlPcS4) or transferrin-conjugated liposomes (Tf-Lip-AlPcS4). The accumulation of free AIPcS4, Lip-AlPcS4, and Tf-Lip-AlPcS4 in human AY-27 transitional-cell carcinoma cells and in an orthotopic rat bladder tumor model was visualized by fluorescence microscopy. In vitro AIPcS4 accumulation was quantified by fluorescence measurements following drug extraction, and the photodynamic efficacy of AlPcS4 was measured in a clonogenic assay. All statistical tests were two-sided. Results: AY-27 cells incubated with Tf-Lip-AlPcS4 had much higher intracellular AlPcS4 levels than AY-27 cells incubated with Lip-AlPcS4 (384.1 versus 3.7 muM; difference = 380.4 muM, 95% CI = 219.4 to 541.3; P =.0095). Among rats bearing AY-27 cell-derived bladder tumors, intravesical instillation with Tf-Lip-AlPcS4 resulted in mean AlPCS4 fluorescence in tumoral tissue, normal urothelium, and submucosa/muscle of 77.9 fluorescence units (fu) (95% CI = 69.1 to 86.8 fu), 4.3 fu (95 % CI = 4.0 to 4.5 fu), and 1.0 (95 % CI = 0.1 to 1.9 fu), respectively, whereas instillation of free AlPCS4 resulted in nonselective accumulation throughout the whole bladder wall, and Lip-AlPcS4 instillation resulted in no tissue accumulation. Photodynamic therapy of AY-27 cells incubated with Lip-AIPcS(4) resulted in cell viabilities greater than 90% for all concentrations and incubation times tested; photodynamic therapy of cells incubated with 1 muM Tf-Lip-AlPcS4 or AlPCS4 resulted in cell viabilities of 0.19% (95% CI = 0.02% to 0.36%) and 1.32% (95% CI = 0.46% to 2.19%), respectively. Higher concentrations of either AlPcS4 or TfLip-AlPcS4 resulted in cell kills of more than 3 logs. Conclusions: Transferrin-mediated liposomal targeting of photosensitizing drugs is a promising potential tool for photodynamic therapy of superficial bladder tumors.