In times where only a few novel antibiotics are to be expected, antimicrobial resistance remains an expanding global health threat. In case of chronic infections caused by therapy-resistant pathogens, physicians have limited therapeutic options, which are often associated with detrimental consequences for the patient. This has resulted in a renewed interest in alternative strategies, such as bacteriophage (phage) therapy. However, there are still important hurdles that currently impede the more widespread implementation of phage therapy in clinical practice. First, the limited number of good-quality case series and clinical trials have failed to show the optimal application protocol in terms of route of administration, frequency of administration, treatment duration and phage titer. Second, there is limited information on the systemic effects of phage therapy. Finally, in the past, phage therapy has been applied intuitively in terms of the selection of phages and their combination as parts of phage cocktails. This has led to an enormous heterogeneity in previously published studies, resulting in a lack of reliable safety and efficacy data for phage therapy. We hereby present a study protocol that addresses these scientific hurdles using a multidisciplinary approach, bringing together the experience of clinical, pharmaceutical and molecular microbiology experts.

Bacteriophages have (re)gained popularity as an adjunct antibacterial therapy. On top of the self-amplifying character of phages, their titer can be influenced by several factors which could lead to reduced titers after preparation and storage. The stability of phage preparations, used for the treatment of chronic rhinosinusitis (CRS), sepsis, pulmonary, and musculoskeletal infections, was investigated. Staphylococcus aureus phage ISP and Pseudomonas aeruginosa phages PNM, 14-1 and PT07 were evaluated. These phages were magistrally prepared by diluting active pharmaceutical products and storing them in different polypropylene/polycarbonate syringes at 4°C. Titers were monitored for 7–21 days using the double-agar overlay method. For 7 days, titers remained stable. Afterwards, the phage titer reduced for all phages with an average reduction of 0.78 log ( P < 0.0001) plaque-forming units (PFUs)/mL after 14 days and 1 log PFU/mL after 21 days ( P < 0.0001). PT07 titers dropped below the predefined threshold after 2 days in polycarbonate syringes. In addition, the stability after irrigation using a nasal douche (CRS) and infusion through a catheter (sepsis) was assessed. No changes were observed after using these devices with a mean change in titer of −0.07 log PFU/mL ( P = 0.9062) and −0.03 log PFU/mL ( P = 0.7350), respectively. In conclusion, a decreased titer after storage of a phage solution can lead to administration of subtherapeutic concentrations. In our experiments, we demonstrated that liquid phage solutions can be stored in syringes at 4°C for 5–7 days. PT07 appeared to be less stable, especially in polycarbonate syringes. Irrigation using a nasal douche or infusion through a catheter does not impact phage titer. IMPORTANCE: As antimicrobial resistance becomes more prevalent, the application of (bacterio)phage therapy as an alternative treatment for difficult-to-treat infections is (re)gaining popularity. Over the past decade, numerous promising case reports and series have been published demonstrating the therapeutic potential of phage therapy. However, important questions remain regarding the optimal treatment protocol and, unlike for medicinal products, there are currently no predefined quality standards for the stability of phage preparations. Phage titers can be influenced by several factors which could lead to reduced titers after preparation and storage and, ultimately, subtherapeutic applications. Determining the stability of different phages in different recipients according to the route of administration is therefore one of the first important steps in establishing a standardized protocol for phage therapy.

Chronic rhinosinusitis is a common condition affecting 5–12% of the general population worldwide. In a limited number of cases, the disease is recalcitrant to medical and surgical interventions, causing a major impact on physical, social and emotional well-being and increasing pressure on healthcare systems. Biofilm formation and dysbiosis caused by Staphylococcus aureus and Pseudomonas aeruginosa play a role in the pathogenesis of recalcitrant chronic rhinosinusitis. In these cases, a promising treatment alternative is the application of bacteriophages, which are viruses that infect and lyse bacteria. In this review, we appraise the evidence for the use of bacteriophages in the treatment of recalcitrant chronic rhinosinusitis. Additionally, (dis)advantages of bacteriophages and considerations for implementation of phage therapy in otorhinolaryngology practice will be discussed.