**3.2 Designing antimicrobial-coating for endotracheal tube to prevent ventilator-associated respiratory tract infections**

Mechanical ventilation (endotracheal intubation) is an effective intervention performed for breathing support in patients admitted in the intensive care unit, but it is also identified as one of the highest risk factors for developing ventilator-associated pneumonia (VAP) [29]. VAP is a type of nosocomial infection that results in a higher mortality (increase from 20–75%) and morbidity rate, prolonged lengths of hospitalization, and also increased hospitalization costs (\$10,000 to \$25,000) [30–32]. Furthermore, each year, approximately 50 million patients in the intensive care unit are intubated with an endotracheal tube (ETT) for breathing support worldwide [33].

Most cases of VAP are caused by the aspiration of infected (bacteria and/or virus) secretions from the oropharynx, although a small number of cases can result from direct bloodstream infection [34]. Moreover, there is a growing concern associated with the ETT as the primary target related to VAP by biofilm formation on its surface [35]. Biofilms are characterized by its resistance to commercial antibiotics that favor resistant microorganisms' proliferation and make them inaccessible to antimicrobials [36].

Regarding VAP occur by ETT, aspiration occurs when there is distal migration of microorganisms present in the secretions accumulated above the ETT cuff. Moreover, biofilm is formed and attached in the lumen of ETT facilitating the transfer to the sterile bronchial tree [37], as presented in **Figure 1**.

Currently, there are methods used to prevent VAP based on its pathogenesis such as prevent aspiration of secretions and bacterial colonization of aerodigestive tract. Lastly, strategies include measures to minimize the risk of contaminated equipment but these methods show some practical limitations. In this regard, the development of strategies and new medical devices to avoid VAP is urgently need.

New medical devices based on the development of antimicrobial coated for ETT surface should be considered if they have been able to prevent VAP in well-designed clinical studies and be cost-effective [38]. Along the years, different strategies and antimicrobial coated for ETT surface (e.g. metal/antiseptics, metal/zeolites/d- tyrosine, nanorough/fructose, antimicrobial peptides, antibiotics/antiseptics, photo-based therapy, micropatterned surfaces, nanorough surfaces, and hydrophobic/hydrophilic) have been evaluated aiming to prevent the biofilm formation and VAP [38] (**Figure 2**).

These antimicrobial coated are functionalized on ETT surface via covalently or ionic bonding or creating a matrix on a polymer (e.g. polyvinyl chloride (PVC)) depending on the molecular structure of both antimicrobial and type of polymerbased ETT and the presence of additives on ETT constitution [39].

As a selected example, in 2020, the Optics and Photonics Research Center from University of São Paulo developed a photo-based antimicrobial coating for ETT *via* functionalization of a natural product (curcumin) photosensitizer on PVC-based ETT surface [40] (**Figure 3**).

This therapeutic approach is based on the photoactivation of curcuminfunctionalized endotracheal tube using an optical fiber followed by the production of reactive oxygen species and 1 O2 able to destroy biofilm and preventing its formation in the lumen of ETT. In this regard, the authors observed a photoelimination of bacteria biofilm such as *E. coli* (72%), *S. aureus* (95%), and *P. aeruginosa* (73%) previously formed on the ETT surface using a light dose of 50 J/cm<sup>2</sup> . Moreover,

**Figure 1.** *Pathogenesis of ventilator-associated pneumonia (VAP). Copyright (2020) National Academy of Sciences.*

*Antimicrobial Photodynamic Therapy of the Respiratory Tract: From the Proof of Principles… DOI: http://dx.doi.org/10.5772/intechopen.95602*

**Figure 2.** *Antimicrobial coatings for ETT.*

#### **Figure 3.** *Curcumin-functionalized endotracheal tube. Copyright (2020) National Academy of Sciences.*

a prevention on formation of *S. aureus* bacteria biofilm in the lumen of curcuminfunctionalized endotracheal tube was observed when it was under illumination (at 450 nm, 35 mW/cm<sup>2</sup> ) [39]. Furthermore, no degradation and leaching for curcumin-functionalized endotracheal tube under different pH values (2.0, 4.5, 7.0, 8.0, and 10.0) were observed. These results pave the way for developing of photosensitizers-functionalized ETT and photodynamic action to combat hospitalacquired infections like VAP [40].

Overall, the development and application of antimicrobials coatings for ETT have shown great promise and continue to progress. Significant results are being obtained with a wide family of the antimicrobial coating, including photosensitizers. From perspective, these *in vitro* methodologies developed so far could be applied in *ex vivo* and *in vivo* tests to evaluate and optimize these antimicrobial medical devices to be applied in clinical trials. In sum, this approach possesses excellent potential to reduce the number of deaths worldwide and decrease healthcare costs.

### **3.3 Lower respiratory tract infections and current treatment challenges**

Lower respiratory infections are the fourth-largest cause of death worldwide and the main cause of death in low-income countries [14]. The most frequent lower respiratory infections are acute bronchitis and bronchiolitis, influenza, and pneumonia [41]. In Brazil, pneumonia is the number one cause of hospitalization [42]. It is also the main worldwide cause of death of children younger than 5 years old [43]. Although the number of hospitalizations has decreased over the past decades, the in-hospital mortality increased, mainly explained by the aging of the population and the occurrence of pneumonia cases that are more difficult to treat [42].

The European Respiratory Society defines pneumonia as an acute illness of the lower respiratory tract that includes cough and at least one other symptom: new focal chest signs, new lung shadowing shown by radiography, otherwise unexplained fever for more than 4 days, or otherwise unexplained tachypnea/ dyspnea [41]. Community Acquired Pneumonia (CAP) is contracted from contact with the infection in day-to-day life [41]. It is predominantly bacterial in origin, being *Streptococcus pneumoniae* its most prevalent pathogen [44]. Other important agents are *Haemophilus influenza*, *Pseudomonas aeruginosa* [44, 45]*.* Also, about 30% of cases are coinfections with viruses [46]. However, in the vast majority of CAP cases, there is no investigation of the etiological agent [42]. In such situations, the treatment is based on the most prevalent microorganisms of that locality [42].

Hospital Acquired Pneumonia (HAP), also called nosocomial pneumonia, is the one that develops after at least 48 hours after the patients' admission [47]. Its reported mortality rate ranges from 20 to 50%, the highest among nosocomial infections [47]. As mentioned above, ventilator-associated pneumonia (VAP) is the one contracted at least 48–72 hours after endotracheal intubation [41]. The most relevant HAP and VAP agents are also bacteria, like *Staphylococcus aureus, Pseudomonas aeruginosa*, *Escherichia coli*, and *Klebsiella*, *Acinetobacter*, and *Enterobacter* species [48]. Knowledge of the etiological agents is essential in treating these infections since patients who receive the wrong initial therapy have a high risk of mortality and morbidity [47]. However, the delay in starting the treatment also leads to poor prognostic [47]. A significant concern in HAP and VAP cases is the present of methicillin-resistant *Staphylococcus aureus* (MRSA), which is associated with elevated mortality rates and treatment costs [49]. Traditionally, the firstchoice drug for MRSA infections is vancomycin, which due to its low penetration in the lungs and high renal toxicity, leads to a failure rate the can reach 70% [49].

Even with new drugs like linezolid, tigecycline and ceftaroline, persists the difficulty in increasing the success rate of treatments and the worry with the development of resistance [49, 50]. Linezolid, for example, was approved for clinical use in 2000, and cases of resistance in patients were reported as early as 2002 [51]. In a study from 2014, the occurrence of non-susceptibility to this antibiotic remained relatively low, but several different resistance mechanisms had already been observed by then [51].

Another approach to hinder the burden of pneumonia is vaccination. Two types of vaccines are currently available for *S. pneumoniae*, the main agent in CAP: the pneumococcal polysaccharide vaccine (PPV) has been recommended for adults since the mid-1980's, but it lacks efficacy in neonates and infants [52]; the pneumococcal conjugate vaccines (PCVs), designed to overcome that, were first approved in 2000 [53]. However, pneumococcal vaccination faces two main challenges: first, each vaccine is only effective against the serotypes contained in it; second, the reduction of the said serotypes increases the colonization of other serotypes that are not covered by the vaccines, and of other pathogen species like *S. aureus* and *H. influenza* [52]. Thus, new vaccines need to be developed continuously, similarly to what happens to antibiotics [52].

*Antimicrobial Photodynamic Therapy of the Respiratory Tract: From the Proof of Principles… DOI: http://dx.doi.org/10.5772/intechopen.95602*

In face of so many challenges, PDT using indocyanine green (ICG) and infrared light has been studied in the treatment of bacterial pneumonia. ICG is a water-soluble dye that emits fluorescence when exposed to infrared light [54]. Its absorption peak in human plasma is 805 nm [55]. It is desirable to have the light excitation at this range because it penetrates deeper into biological tissue, since it is less absorbed by water, melanin and hemoglobin [56].

In an *in vitro* study by Leite *et al*., the *in vitro* inactivation of *S. pneumoniae* was effective using concentrations of ICG as low as 5 μM when combined with a 780 nm laser device or 10 μM when using an 850 nm LED. In these conditions, the treatment was safe for RAW 264.7 macrophages, and seemed to enhance their ability to fight the bacteria [57]. Other studies have also investigated similar protocols for other relevant pneumonia pathogens. Topaloglu *et al.* found an effective *in vitro* killing of *S. aureus* using 84 J/cm2 of light (809 nm) with 6 μg/mL of ICG, and of *P. aeruginosa* using 125 μg/mL ICG and 252 J/cm2 [58]. Kassab *et al.* had similar results for *S. aureus*, and showed that the same protocol, with up to 200 J/cm2 and 10 μM of ICG, was harmless to multiple mammalian cell lines [59].

The first *in vivo* investigation of the proposed protocol, performed by Geralde *et al.*, found a reduction in the bacterial burden and an increase in the survival rate of SKH-1 hairless mice infected with *S. pneumoniae* after a single PDI session using ICG 100 μM and 120 J/cm2 of light at 780 nm, with a waiting interval of 3 minutes [60]. In this study, the light exposure did not seem to be harmful to the animals. Additionally, the ICG alone was no different form the control, suggesting that the activation with light was essential to the observed effects. It was then demonstrated that nebulization would be a viable delivery method for ICG to reach the lungs. ICG is compatible with air-jet nebulization in multiple concentrations, and it reaches and distributes in the lungs similarly as intranasal instillation [59, 61]. Additionally, mice exposed to pulmonary PDT using ICG and 216 J/cm2 of light at 808 nm showed no clinical signs of toxicity or histological damage to the lungs, liver or stomach 7 days after the treatment [59]. Replicating such results in larger models and patients might be challenging due to the layers of biological tissue the light needs to go through to reach the target. Nonetheless, aPDT using ICG and infrared light shows good efficacy and safety in pre-clinical studies, and has great potential to become a treatment for lower respiratory infections.
