Bacteriophage (hereafter, phage) therapy is attracting attention as a treatment option for patients who do not respond to existing antimicrobial drug therapies. Phage therapy is broadly divided into two types: personalized phage therapy, which uses a personalized phage cocktail for each patient's causative bacteria, and fixed phage therapy, which uses a phage cocktail based on the target bacterial species or infections [1].

Personalized phage therapy is provided through frameworks such as the compassionate use of new investigational phage-based drugs pathway in the United States, the Therapeutic Goods Administration's Special Access Scheme in Australia, and the magistral preparation pathway in Belgium [1]. Increasing evidence indicates that the use of phage therapy in combination with antibacterial drugs is emerging as a promising strategy in the treatment of infections caused by drug-resistant bacteria and intractable infections involving biofilms related to prosthetic materials, where effective treatment is difficult to achieve through antibacterial drugs alone [2].

In Japan, as of December 2024, pilot clinical applications of phages have commenced in the field of veterinary medicine [3], and clinical trials of bacteriophage therapy using fixed bacteriophages for patients with common acne have begun [4]. However, personalized phage therapy in humans for the treatment of intractable infections caused by drug-resistant bacteria has not been implemented.

Since the implementation of the antimicrobial resistance (AMR) Action Plan in Japan, achievements have been reported including a reduction in the use of oral antibacterial agents and maintenance of a low detection frequency of carbapenem-resistant bacteria. Still, infections due to drug-resistant bacteria, such as methicillin-resistant Staphylococcus aureus (MRSA) and fluoroquinolone-resistant Escherichia coli, are estimated to significantly contribute to mortality [5]. Furthermore, post-COVID-19, there has been a resurgence in the number of international travel [6], and the risk of cross-border spread of highly drug-resistant bacteria has increased [7].

The aim of this pilot study was to obtain data that informs future clinical trials on the use of personalized phage therapy in the treatment of intractable infections caused by drug-resistant bacteria.