In photodynamic therapy (PDT), localized monochromatic light is used to activate targeted photosensitizers (PS) to
induce cellular damage through the generation of cytotoxic species such as singlet oxygen. While first-generation PS
passively targeted malignancies, a variety of targeting mechanisms have since been studied, including specifically
activatable agents. Antibody internalization has previously been employed as a fluorescence activation system and could
potentially enable similar activation of PS. TAMRA, Rhodamine-B and Rhodamine-6G were conjugated to trastuzumab
(brand name Herceptin), a humanized monoclonal antibody with specificity for the human epidermal growth factor
receptor 2 (HER2), to create quenched PS (Tra-TAM, Tra-RhoB, and Tra-Rho6G).
Specific PDT with Tra-TAM and Tra-Rho6G, which formed covalently bound H-dimers, was demonstrated in HER2+
cells: Minimal cell death (<6%) was observed in all treatments of the HER2- cell line (BALB/3T3) and in treatments the
HER2+ cell line (3T3/HER2) with light or trastuzumab only. There was significant light-induced cell death in HER2
expressing cells using Tra-TAM (3% dead without light, 20% at 50 J/cm2, 46% at 100 J/cm2) and Tra-Rho6G (5% dead
without light, 22% at 50 J/cm2, 46% at 100 J/cm2). No efficacy was observed in treatment with Tra-RhoB, which was also non-specifically taken up by BALB/3T3 cells and which had weaker PS-antibody interactions (as demonstrated by
visualization of protein and fluorescence on SDS-PAGE).