Transferrin-Conjugated Liposome Targeting of Photosensitizer AlPcS ₄ to Rat Bladder Carcinoma Cells

Abstract

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 (AlPcS ₄ ) is an effective strategy to attain tumor-selective accumulation of this compound when applied intravesically. Methods: AlPcS ₄ was stably encapsulated in unconjugated liposomes (Lip-AlPcS ₄ ) or transferrin-conjugated liposomes (Tf-Lip–AlPcS ₄ ). The accumulation of free AlPcS4, Lip-AlPcS ₄ , and Tf-Lip–AlPcS ₄ in human AY-27 transitional-cell carcinoma cells and in an orthotopic rat bladder tumor model was visualized by fluorescence microscopy. In vitro AlPcS ₄ accumulation was quantified by fluorescence measurements following drug extraction, and the photodynamic efficacy of AlPcS ₄ was measured in a clonogenic assay. All statistical tests were two-sided. Results: AY-27 cells incubated with Tf-Lip–AlPcS ₄ had much higher intracellular AlPcS ₄ levels than AY-27 cells incubated with Lip-AlPcS ₄ (384.1 versus 3.7 μM; difference = 380.4 μM, 95% CI = 219.4 to 541.3; P = .0095). Among rats bearing AY-27 cell–derived bladder tumors, intravesical instillation with Tf-Lip–AlPcS ₄ resulted in mean AlPcS ₄ 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 AlPcS ₄ resulted in nonselective accumulation throughout the whole bladder wall, and Lip-AlPcS ₄ instillation resulted in no tissue accumulation. Photodynamic therapy of AY-27 cells incubated with Lip-AlPcS ₄ resulted in cell viabilities greater than 90% for all concentrations and incubation times tested; photodynamic therapy of cells incubated with 1 μM Tf-Lip–AlPcS ₄ or AlPcS ₄ 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 AlPcS ₄ or Tf-Lip–AlPcS ₄ 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.