**7. Challenges for pulmonary delivery of phage and future perspective**

Phage therapy is evolving as a promising alternative or an adjuvant to antibiotics for the battle against MDR bacteria. Although a few randomized, double-blind and placebo-controlled clinical trials have been conducted to assess tolerance and/ or efficacy of phage therapy in the past few years, none of the completed trials have yielded data supporting the promising observations noted in the experimental phage therapy conducted in animals and humans. Górski et al. highlighted the importance of the quality and titer of the phage preparations and their delivery efficiency to the target sites to ensure a sufficient high phage to bacteria concentration in the vicinity of infected tissues [93]. For lung infection, directly delivering phage preparation to the airways enhance the incidence of phage getting access to its host bacteria, avoiding the rapid clearance in systemic circulation. Advancements have been made in the past decade to improve the formulations for pulmonary delivery of phage. Here we highlight some hurdles remained to be tackled to bring inhaled phage therapy to clinical settings beyond compassionate use and a few prospective research directions for the commercial application of aerosol formulations.

As a sufficient amount of phage at the site of infection is the prerequisite for successful therapy, nebulizers and DPI are better choice for pulmonary delivery of phage compared with pMDI and SMI due to their capacity of high dose delivery. The detrimental effect of the various type of nebulizers to phage was found to be phagespecific, likely attributing to the tail morphology of phage [21] and compositions of the phage formulations [18]. Systematic studies to confirm their impacts on phage nebulization will provide important information in developing new phage cocktail formulations. Although liquid formulations are commonly used for phage therapy, solid phage formulations are more desirable for long-term storage and transportation. While stable phage powder formulations have been successfully achieved with storage at ambient temperature, they are usually required to be handled and stored at low humidity conditions (RH < 20%) [48–50]. These would be easily achievable in a manufacturing setting and with pharmaceutical packaging designs. As excessive environmental moisture could also be relevant in patients' homes or in healthcare settings, the impacts of humidity on powdered phage administration should be evaluated to ensure the phage product could be used successfully in different geographic regions over the world. In preparing phage-powder formulation, trehalose, lactose, and leucine are commonly employed to stabilize phage. However, these excipients have not been approved for inhalation except lactose was approved as a carrier which is not expected to be delivered to the lower respiratory tract. Further *in vivo* studies are required to evaluate the safety profile of these excipients for both short term and long term usage.

Currently, *in vivo* data of phage therapy for lung infections mostly focused on acute infections that phage preparation was given at within a few hours post-infection. However, in clinical settings, the phages are unlikely given immediately after the onset of infection, the postponed treatment may lead to significant bacterial growth and biofilm formation, more research is needed to evaluate the therapeutic efficacy of phage therapy against chronic lung infection in animal models. Moreover, more extensive *in vivo* PKPD evaluations are needed to investigate the optimal administration dose and time for pulmonary phage therapy.

The role of the immune system on phage therapy is largely unexplored in animal studies and human trials [33, 88]. Depending on administration route, phage type and phage dose, and duration of phage therapy can lead to the generation of neutralizing antibodies [94]. Together with increasing evidences showing the interactions between phage and mammalian cells [95–97], it would be worthwhile to explore the interaction between phage formulations with lung leukocytes and epithelial cells lining the alveolar surface and the conducting airways.

Current phage formulation research is largely empirical based. To speed up the research progress for phage therapy, *in silico* models and database would be required to predict phage-excipient interaction, phage-antibiotic combination and pharmacokinetic/pharmacodynamics (PKPD) profiles.

### **8. Conclusion**

In the past decade, highly acceptable formulations have been achieved with minimal phage loss and desirable stability for pulmonary delivery using both nebulizers and dry powder inhalers. The synergistic effect of the phage-antibiotic combination provides an efficient way to prevent the emergence of bacterial resistance and reduce the toxicity of antibiotic use. However, systematic PKPD profile of phage after administration by inhalation, and the modern tools to accurately predict the result of combination therapy are still pending. With the advent of phage research, the sound manufacturing and regulatory guidelines towards successful clinical trials to bring phage therapy to clinical settings will be beneficial to the patients suffering from bacterial infections.

### **Acknowledgements**

The authors gratefully acknowledge the provision of graduate studentship from CUHK to W. Yan and S. Mukhopadhyay is supported by the HKPFS. The funding support from University Grants Committee Hong Kong (ref. 24300619) for our phage research is greatly acknowledged.

*Potential of Inhaled Bacteriophage Therapy for Bacterial Lung Infection DOI: http://dx.doi.org/10.5772/intechopen.96660*
