Development of combined schemes for the treatment of oncological diseases is a promising strategy to improve the effectiveness of antitumor therapy. As a result of combination, an additive or synergistic effect at a reduced therapeutic dose of each individual component can be observed. The high potential was recently shown for combination of targeted therapy and photodynamic treatment (PDT). In this work, we studied the therapeutic potential of the combined action of the anticancer targeted toxin and PDT against HER2-positive breast cancer in vitro.

The human breast adenocarcinoma SK-BR-3 cells were sequential treated with recombinant HER2-targeted toxin DARPin-LoPE and then Photodithazine followed by light irradiation and the effect was compared with monotherapeutic options. Photodynamic treatment led to photoinduced cell death with IC₅₀ 0.64 µM and after incubation with the toxin for 48 h IC₅₀ was 2.8 pM. When using two therapeutic agents at IC₅₀ doses, it led to significant increase in the effectiveness; the viability of the combination-treated cell culture did not exceed 10%. To analyze the mode of the drug interaction, we compared the dose-response curves for DARPin-LoPE and Photodithazine at monotherapeutic or combined treatment. The ratio between agents’ concentrations was maintained equal (1:250), which corresponds to the IC₅₀ ratio. The calculated combination index was 0.07 indicating a significant synergistic effect of the agents. The hakf-maximal decrease in the viability of the SK-BR-3 culture was observed at concentrations of the toxin and photosensitizer that are virtually non-toxic to cells when the agents are used separately. We hypothesize that the protein synthesis arrest by DARPin-LoPE toxin is the main cause for sensitization of the cells to the subsequent PDT treatment.

This work was supported by the Russian Science Foundation projects nos. 19-14-00112 (obtaining the recombinant toxin) and 19-74-20168 (studying the efficiency of combination therapy).