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Photodynamic therapy (PDT) is a minimally invasive therapeutic modality that has been clinically approved for various cancer treatment [1]. To extend the potential of PDT to various types of cancers, it is important to develop new photosensitizers or drug delivery systems that can accumulate selectively within target tumors. Up to now, tremendous efforts have been devoted to the development of photosensitizers, some of which are already clinically used [2]. In addition to these efforts, recent studies have reported photosensitizers and drug delivery systems targeting specific molecules and environment in malignant tumors [3,4]. For example, we developed functional polymers possessing multiple cyclic RGD peptides in the side chains for photosensitizer delivery to target α v β 3 integrins, and this polymer-photosensitizer conjugate selectively accumulate within subcutaneous tumors in mice upon intravenous injection and accomplished appreciable PDT effects [4]. Such studies indicate that active targeting of tumor-associated molecules or environment should be a promising approach to achieve efficient PDT effects without giving unfavorable photochemical damage to normal tissues. In this context, we recently developed functional polymers targeting tumor-associated amino acid transporters [5]. The polymer had multiple amino acid moieties in the side chains and provided multivalent interaction with target amino acid transporters, exerting efficient tumor accumulation after systemic administration. In this study, based on this strategy, we fine-tuned the chemical structure of the polymers targeting tumor-associated amino acid transporters for PDT. In in vitro study, the fine-tuned polymer-photosensitizer conjugate exhibited strong phototoxicity without substantial dark toxicity to cultured cancer cells in an amino acid transporter-selective manner. Even in in vivo study, the polymer-photosensitizer conjugate demonstrated significantly enhanced PDT effect. Our polymers may be promising drug delivery systems for PDT. [ABSTRACT FROM AUTHOR]