Development of combined schemes for the treatment of oncological diseases is a promising strategy to improve the effectiveness of antitumor therapy. This paper shows the fundamental possibility of multiplying the antitumor effect by combining targeted and photodynamic therapy. It was demonstrated that sequential treatment of HER-2 positive breast cancer cells with the targeted toxin DARPin-LoPE and the photoactive compound photodithazine leads to a synergistic enhancement of their effect. In the future, this approach is intended to achieve the maximum therapeutic effect while minimizing the risks of negative side effects.