Biomaterial therapeutic strategies for treatment of bacterial lung infections

Abstract

Bacterial infections of the lung frequently occur as a secondary infection to many respiratory viral infections and conditions, including influenza, COVID-19, chronic obstructive pulmonary disease (COPD), and cystic fibrosis (CF). Currently, clinical standard treats bacterial infections of the lung with antibiotic drugs. However, the use of broad-spectrum antibiotics can disrupt host microbiomes, lead to patient discomfort, and current clinical settings face the constantly increasing threat of drug-resistant bacteria. Biofilms further obstruct effective treatment due to their protective matrix layer, which shields bacteria from both the host immune system and antimicrobial drugs and subsequently promotes drug resistance. Alternative antimicrobial agents, including bacteriophages and antimicrobial peptides, have been utilized to treat drug-resistant bacteria. However, these antimicrobial agents have significant limitations pertaining to their ability to arrive at infection sites without compromised function and ability to persist over an extended period to fully treat infections. Enhanced delivery strategies present great promise in addressing these issues by using micro/nanoparticle carriers that shield antimicrobial agents in transit and result in sustained release, enhancing subsequent therapeutic effect and can even be modulated to be multi-functional to further improve recovery following bacterial infection.

1. Background

Bacterial infections of the lung frequently occur as a secondary infection to respiratory viral infections such as influenza [1] and COVID-19 [2,3] and conditions such as chronic obstructive pulmonary disease (COPD) [4] and cystic fibrosis (CF) [5]. Common causal bacteria include Staphylococcus aureus (S. aureus) and Pseudomonas aeruginosa (P. aeruginosa), with additional species documented [6]. To treat such infections, the current standard of clinical care is to employ antimicrobial drugs (e.g., antibiotics). In the current clinical setting, however, drug resistance in bacteria has become an ever-increasing problem, which complicates the use of antimicrobial drugs. To address this issue, clinical care strategies recommend administering empirical antibiotic regimens, which entails the use of multiple antimicrobial drugs [6,7]. However, this strategy not only has the potential to further exacerbate the ability of bacteria to resist drugs but also leads to side effects resulting from non-selective killing of human gut and lung flora. In turn, these effects can lead to additional discomfort for patients and can lead to imbalances in the body including the promotion of opportunistic infections [8].

Under normal conditions, the lung contains its own microbiome with significant variations between the upper and lower respiratory tracts that are thought to arise from bacterial species travelling through the nose and the passages connected to it [[9], [10], [11]]. Studies also support that the lung microbiome is connected to the gut microbiome, and that these microbiomes are critical for maintaining homeostasis and directing immune responses [9,12]. When a bacterial infection occurs, the microbiome becomes disrupted and exhibits a decreased diversity in bacterial presence (i.e.: fewer different species of bacteria present) [9,11]. With these findings in mind, a sweeping antibiotic treatment may not only result in discomfort associated with gut flora changes, but also potential exacerbation of the respiratory system condition after treatment [9,13], prompting the need for treatment options with greater specificity, be it through limiting range of affected bacteria or more localized delivery of antibiotic agents.

1.1. Significance of biofilms and development of antibiotic resistance

Importantly, bacteria can form biofilms, presenting another obstacle to effective treatment by antimicrobial drugs. Formation of a biofilm begins after planktonic bacteria proliferate and attach to a surface. From there, these bacteria create microcolonies, consisting of bacteria aggregates covered in a matrix, which can then further proliferate and turn into a biofilm. At this point, the biofilm consists mostly of a highly protective matrix layer consisting of extracellular polymeric substances (referred to as EPS), such as polysaccharides and proteins [14,15]. This matrix serves to shield bacteria from threats, such as the immune system and antimicrobial drugs. The EPS can also serve as a diffusional barrier (in addition to physical), causing antibiotic drugs that are administered to arrive at bacteria at reduced, nonlethal concentrations over prolonged periods time which promotes drug tolerance [16]. This drug tolerance is the result of the reduced concentration antibiotic drugs causing selection towards bacteria with mutations that extend the period of time where bacteria grow in size but not in number, subsequently promoting the rise of phenotypes that allow bacteria to be in a more dormant or quiescent state (known as persister bacteria) and evade drugs that target actively proliferating cells [14,17]. As these persister bacteria proliferate, genetic transfer of genes for antibiotic drug resistance is greatly enabled, leading to the development of biofilms with highly drug resistant bacteria [14,18,19].

Although the emergence of drug-resistant bacteria is problematic, these bacteria still have weaknesses that can be exploited. Drug resistance in bacteria partially relies on the rise of randomly emerging genetic mutations, which occur at low frequency due to low error rate with replication of bacterial genomes. As a consequence of this reliance on low probability, it is unlikely for bacteria to acquire resistance to multiple antibiotic agents (including unresisted antibiotic drugs) when they are administered simultaneously [20,21]. Drug-resistant bacteria can still be affected by changes in population dynamics, which means that disrupting this balance—such as by probiotic strategies—can disrupt the ability of drug-resistant pathogenic bacteria to colonize [20,22]. In healthy conditions, the lung has a mucosal layer that protects the epithelial lining and provides a diffusion barrier. This mucosal layer is also circulated and cleaned of foreign material and pathogens by the beating of the cilia of the epithelial layer. However, under diseased conditions (e.g., COPD and CF), the mucus viscosity increases and the motion of the cilia can be inhibited, causing any bacteria in the mucosal layer to be trapped as opposed to transported for expulsion from the body. Thus, in context of the lung, it is these trapped bacteria that often proliferate and develop into biofilms [14,18]. Additionally, insertion of some medical devices (such as endotracheal tubes) leads to the device becoming coated with host proteins, which can then promote the attachment of bacteria to the device as well as the damaged tissue and introduce biofilm formation. Planktonic bacteria from the device biofilm can then spread and attach to nearby biologic surfaces, including the mucosal layer of the lung [18].

1.2. Alternative antimicrobial strategies

Because of the drug-evading and drug-resistant properties associated with biofilms, there is a great need for either alternative targets for antimicrobial agents (to eliminate quiescent bacteria in addition to active bacteria) or enhanced delivery to penetrate the biofilm matrix and promote earlier elimination of biofilm bacteria to prevent development of antibiotic resistance (Fig. 1). To deal with the drug-circumvention ability of persister bacteria, alternative antimicrobial agents have been sought out. Most notable of these alternative antimicrobial agents are bacteriophages—bacteria-targeting viruses—and antimicrobial peptides (AMPs).

Fig. 1.

Overview of biofilm environment and notable targets for improving treatment. Biofilms have several properties that make them difficult to treat. Biofilms have a protective matrix layer, which provides obstacle to effective delivery of antibiotic drugs to the bacteria within. Additionally, some of the bacteria involved (highlighted as planktonic bacteria and bacteria in biofilm in this image) can be of a quiescent or persister phenotype, which diminishes the effect of drugs targeting actively replicating bacteria and thus creates a need for antibiotic agents with alternative targets.

1.2.1. Bacteriophages

Bacteriophages are viruses that can target and kill bacteria. Since the mechanism by which bacteriophages cause bacterial death is different from antibiotic drugs, bacteriophages can be used to treat drug-resistant strains. Bacteriophages that undergo lytic cycle (as opposed to lysogenic phages) are typically used as antimicrobial agents. To proliferate, bacteriophages will attach to target host bacteria and inject their genetic material. The host bacterium then replicates the bacteriophage's genetic material and synthesizes bacteriophage components, which then self-assemble. As this process proceeds, increasing amounts of phage specific proteins are synthesized. These phage specific proteins can then cause lysis of bacteria and release more bacteriophages that can repeat this cycle [3,23,24].

Bacteriophages target bacteria with high specificity and often target only one or a few species [25,26]. This high specificity indicates that bacteriophages will not affect human cells or gut flora and thus have low off-targeting effects. Bacteriophages can also be quickly isolated and produced against newly emerging drug-resistant strains, meaning that they are antimicrobial agents that can evolve alongside their target bacterial strains. Furthermore, bacteriophages have seen use in patients both historically (in the former Soviet Union) and currently (in Poland, Russia, and Georgia) without notable side effects directly related to bacteriophages [23,27]. Because of these properties, bacteriophages represent an attractive source of treatment for drug resistant bacteria.

However, despite their promising properties, bacteriophages also have some significant drawbacks. Due to the high degree of specificity, bacteriophages need to be implemented in cocktails of different bacteriophage strains. In addition, the required bacteriophage cocktail can vary according to infection site even when the target bacteria species is the same, which can limit where in the body a particular strain of bacteriophages can be used. As a consequence of the need for multiple strains of bacteriophages in a treatment, time needed to create these cocktails and associated financial costs can increase significantly [23,28]. Furthermore, the host immune system can inactivate bacteriophages and bacteria are able to develop resistance to bacteriophages, which limits how often bacteriophages can be used to treat their target drug-resistant strains [29,30].

1.2.2. Antimicrobial peptides (AMPs)

AMPs are peptides that have potent antimicrobial activity. AMPs have different mechanisms of antimicrobial action, which enable AMPs to induce bacterial death and thus are able to treat drug-resistant bacteria [31,32]. The mechanism of antimicrobial activity varies across different kinds of AMPs, with direct action typically occurring through causing disruption of bacterial cell membrane or disruption of intracellular function. Important to the direct activity of AMPs is the presence of a positive charge, which directs AMPs selectively towards anionic bacterial membranes [31,32], and the presence of a hydrophobic component, which is needed to interact with the hydrophobic interior of bacterial membranes. After reaching a certain concentration, AMPs can then induce pore formation and enter bacteria, consequently disrupting organelle membranes [[33], [34], [35]]. Some AMPs are reported to inhibit bacterial protein and nucleic acid synthesis, block vital bacterial enzyme activity, or induce apoptosis-like death through generating reactive oxidative species [[36], [37], [38]]. AMPs also have indirect antimicrobial effect by directing the immune host response through the promotion of inflammation—improving nucleic acid recognition—and activation of immune cells [39,40].

AMPs have been reported to affect quiescent bacterial phenotypes and interfere with EPS production as well, which makes AMPs an appealing agent for biofilm prevention [[41], [42], [43]]. Additionally, bacterial resistance to AMPs does not develop quickly, which means that AMPs can remain an effective antimicrobial agent for greater number of uses than antibiotic drugs [31,32,44]. AMPs are associated with additional beneficial properties that can help with recovery from infection, such as wound-healing, which can help repair tissue damage associated with infection, and anti-inflammatory properties, which can help reduce patient pain and discomfort [31,32,45,46]. Examples of AMPs that have been investigated for treatment of respiratory infections include: amphibian AMP Esculentin (1–21) which was reported to significantly affect planktonic and biofilm-forming P. aeruginosa when administered intra-tracheal in a mouse model of pulmonary infection at 5 mg/kg animal [31,47], and Tachyplesin III which was reported to prolong survival of mice infected with co-infection of multi-drug resistant P. aeruginosa and Acinetobacter baumannii and reported to significantly reduce bacterial presence in bronchoalveolar lavage fluid when administered intravenously at 10 mg/kg intravenously [32,48].

Although AMPs are a promising approach to treat drug-resistant strains of bacteria while bestowing relevant recovery enhancing properties—such as wound-healing and anti-inflammatory properties—there are still notable limitations. First, AMPs isolated from natural sources are associated with challenges balancing antimicrobial activity and mammalian cell toxicity. To address this issue, investigators have modulated or synthetically created AMPs to enhance antimicrobial ability [31,49]. However, this workaround demonstrates that additional steps may be required for effective treatment by AMPs. Furthermore, some AMPs are reported to not be stable in biological fluids, which can impede activity on the way to the infection site [31,32]. Additionally, high concentrations, often to the point of mammalian cell cytotoxicity, are needed for situations where multiple bacteria species are involved, which is common for clinical cases of bacterial biofilms, and development of AMP therapies can entail high financial costs [32]. However, efforts have been made in the field to address these issues including the cyclization of AMP molecules to reduce protease and peptidase interaction, modulation of physical and chemical properties (including size, sequence, structure/shape, charge, and hydrophobicity) to reduce associated toxicity and hemolytic properties, and recombinant biosynthesis in bacteria or fungi (as opposed to isolation from animals) [31,32,50].

1.2.3. Limitations in delivery of current alternative antimicrobial strategies

Although these alternative antimicrobial agents show great promise in treating drug-resistant bacteria, further constraints remain when seeking to treat infections in the lung. Multiple routes of delivery have been utilized for treatment of lung infections including oral, systemic/intravenous, and parenteral routes. However, these methods of delivery generally result in inaccurate concentration of antimicrobial agents at the ultimate site of delivery [51,52]. For this reason, pulmonary delivery has been considered as an alternative for treating infection of the lung. The pulmonary route entails inhalation of treatment agents, aiming to directly deliver treatments to the epithelial layer of the lung, which reduces how much treatment agents get metabolized compared to oral and other routes of administration. This route also touts being a needle-free technique, enabling greater patient comfort, and having high surface exchange area—thanks to the alveoli and bronchioles—enabling quicker and more effective drug absorption [53,54]. This delivery target is appropriate for systemic drugs and has been reported to enhance the therapeutic effect in the context of treating lung infections [53,55]. In summary, pulmonary delivery affords efficient delivery of anti-microbial agents at the site of infection. As such, this localized delivery addresses limitations of systemic (i.e., intravascular, oral) delivery by (i) reducing the overall dose and systemic distribution as well as associated off-target effects and (ii) increasing local concentrations and persistence time of the therapeutic. However, the lung is designed to expel and/or clear any foreign material present, which presents further challenges to the delivery and persistence of treatment agents [53,56].

Because of the issues presented here, there is a need for either improving the ability of antimicrobial agents to reach bacteria or improving the ability of antimicrobial agents to persist long enough to achieve significant antimicrobial effect. In other words, there is a great need for improved delivery of antimicrobial agents overall. Biomaterials provide promising strategies to address these issues through a variety of means. In this review, we discuss applications of biomaterial-based strategies to improve delivery of treatments against drug-resistant bacteria and biofilms in the lung. In particular, micro/nanoparticle-based strategies for enhanced delivery by pulmonary route are highlighted (Fig. 2).

Fig. 2.

Overview of frequently used biomaterial strategies for treatment of lung infections. This review is focused on biomaterial strategies for enhanced delivery (Highlighted as the overlaps in the biomaterial types presented in the image).

1.3. Enhanced delivery by micro/nanoparticles

Biomaterial strategies focus on the use of carriers to enable enhanced delivery through obstacles, such as biofilms, or sustained release of antimicrobial agents at the site of infection. Enhanced delivery biomaterial strategies can also protect or shield antimicrobial agents from degradation in the in vivo environment, which can preserve the activity of antimicrobial agents during transit to infection site and thus enhance subsequent therapeutic effects [51,[57], [58], [59]]. Additionally, these strategies can be used to address other issues associated with antimicrobial agents and drugs, such as in vivo solubility [51,60] (Table 1).

Table 1.

Summary of pros and cons of currently used antimicrobial agents.

Antimicrobial Agent	Description/Examples	Pros	Cons
Antibiotic Drugs	Commercially available antibiotic drugs. Multiple drugs are typically administered simultaneously to treat infection by drug-resistant strains of bacteria	+Clinical standard	-Development and approval of new drugs is slow compared to emergence of drug resistant bacteria
	Example Administrations:	+Readily available	-Low water solubility in some cases → does not absorb efficiently into body
	Vancomycin, 15 mg/kg IV		-Delivery through oral and systemic methods not optimized for effective concentration at lung infection site
	Ciprofloxacin, 400 mg IV
	Tobramycin, 5–7 mg/kg IV
Bacteriophages	Bacteria-targeting viruses	+High selectivity towards target bacteria	-Entails use of cocktail of multiple strains → increases time and financial cost of development
		+Low off-target effects	-Bacteriophage strain dependent on infection site even when targeting same bacteria strain
		+Can evolve alongside emerging strains of drug resistant bacteria	-Bacteria can become resistant to bacteriophages
		+Reported effective use in humans → potential for lower regulatory barrier	-Host immune responses can inactivate phages
Antimicrobial Peptides (AMPs)	Peptides with the ability to kill bacteria	+Wide range of mechanisms for antimicrobial effect	-High financial cost for development
	Examples:	+Can interfere with biofilm protective matrix	-Issues associated with balancing antimicrobial activity with toxicity
	Esculentin (1–21), 5 mg/kg intra-tracheal	+Associated with wound healing and anti-inflammatory properties → helps with recovery outcomes	-Modulation needed to better optimize AMP properties including antimicrobial effect, toxicity, and persistence ability in biological fluids
	Tachyplesin III, 10 mg/kg IV	+Bacteria do not rapidly develop resistance to AMPs

1.3.1. Commonly used materials for micro/nanoparticle carriers

Micro/nanoparticles for enhanced delivery have been made of many different materials. Frequently reported is the use of polymeric particles, both of natural and synthetic origin. Polymeric particles are popular in the field as they allow easy encapsulation methodologies, can slow the absorption of drugs, and enable sustained release. Natural polymers frequently used are chitosan and alginate, although other materials such as dextran have been reported [61]. Natural polymers have been favored owing to greater cytocompatibility and biodegradability. Furthermore, both chitosan and alginate have been reported to result in improved aerosolization [51,[62], [63], [64], [65], [66]], enabling easier preparation for pulmonary delivery. Alginate is also attractive because of its relatively lower costs. However, these natural polymers have limitations. Safety issues in the lung have been reported with the use of chitosan and chitosan derivatives [51,67] while drug delivery with alginate involves rapid release, which is not ideal in treatment of persistent infections [51,68]. Furthermore, the ability to tune the physicochemical properties of these materials, which impacts drug release and clearance rates, is limited. As an alternative to naturally-sourced materials, synthetic polymers provide greater control can be exercised over delivery profile and release kinetics. Because of this aspect, synthetic polymers can therefore have more effective drug release over time for sustained delivery [51,69]. As a result, a variety of synthetic polymeric particles have been used to deliver antibiotic agents including poly(2-ethyl-2-oxazoline) (PEtOX), poly(lactic-co-glycolic) acid (PLGA), poly(lactic) acid (PLA), polyvinyl alcohol (PVA) and other biodegradable polymers [51,57,59,70]. Mesoporous-silica based nanoparticles have been used to deliver the antibiotic drug rifampin to the lung as well [71,72]. Despite the great benefit of significant control over particle properties, there are notable downsides. Many synthetic processes involve harsh reagents and/or solvents, which makes toxic residue a significant concern, as well as the possibility of reducing the bioactivity of the therapeutic agent [51,73]. Furthermore, high working temperatures for methods such as spray-drying can lead to thermal degradation of particular materials, which can affect the efficacy of encapsulation [51,74].

1.3.2. Micro/nanoparticle-based enhanced delivery strategies for antibiotic drugs

The current clinical standard for the treatment of lung infections is to administer an empirical regimen of antibiotic drugs. However, in addition to contributing to the development of drug-resistant strains of bacteria, many antibiotic drugs are primarily delivered by oral and systemic administration can significantly change the final concentration in the lung. Furthermore, some antibiotic drugs are associated with lack of water-solubility, making it difficult to absorb into the body. To address these issues, micro/nanoparticles have been used as delivery vehicles. Antibiotic drugs that have been encapsulated using micro/nanoparticles include ciprofloxacin, rifampin, isoniazid, and ofloxacin [53,57,71,72].

Biomaterial-based delivery strategies for antibiotic drugs enable improved solubility and enables sustained release. For example, Sabuj et al. created PEtOX nanoparticles loaded with ciprofloxacin to address its issue of poor in vivo solubility and poor delivery when administered orally and intravenously. These ciprofloxacin-loaded particles exhibited sustained release over the course of 14 days, significantly extending the persistence ability of ciprofloxacin [57]. By increasing the persistence of antibiotic drugs at effective concentrations, enhanced delivery enables improved antimicrobial activity against more persistent bacteria, such as those involved in biofilms. However, certain negative micro/nanoparticulate properties, particularly potential toxicity (be it associated with fabrication process or otherwise), need to be kept in consideration.

1.3.3. Micro/nanoparticle-based enhanced delivery strategies for alternative antimicrobial agents

Biomaterials have also been exploited for enhanced delivery of alternative antimicrobial agents such as bacteriophages and AMPs. For example, Agarwal et al. used inhalable porous PLGA microparticles to enhance bacteriophage delivery in terms of tissue distribution and persistence for the treatment of infections caused by P. aeruginosa and reported reduced bacterial presence in healthy and CF mouse in vivo models of infection. Notably, it was also reported that the antimicrobial effect of the phage-loaded microparticles did not diminish even when mice experienced previous exposure to the phage-loaded microparticles, indicating that these antimicrobial particles did not generate neutralizing immunity and suggesting the potential for multiple dosing interventions [70]. Building off these findings, Kalelkar et al. tested these PLGA microparticles for treating infections caused by drug-resistant S. aureus and co-infection of S. aureus and P. aeruginosa and showed reduced bacterial presence with in vivo microparticle delivery of bacteriophages. Additionally, mice treated with bacteriophage-loaded particles had higher concentrations of bacteriophages recovered after takedown, indicating longer sustained release of bacteriophages and ability of the phages to infect and replicate in their host bacteria (compared to free bacteriophages) [59]. These studies demonstrate that biomaterial-based delivery of bacteriophages enables sustained release and ensures that bacteriophages remain at a sufficient concentration to achieve effective antimicrobial effects. For AMPs, delivery using biomaterials maintains prolonged presence at concentrations high enough for significant antimicrobial effects while also shielding AMPs from degradation. Additionally, the shielding of AMPs means that non-target tissues and cells experience less exposure to AMPs, thus reducing the associated toxicity. For example, Falciani et al. has developed dextran-based nanoparticles that were loaded with AMPs and reported significant killing of Pseudomonas aeruginosa with prolonged in vivo presence and tolerable levels of toxicity [61].

Overall, biomaterial-based delivery strategies for alternative antimicrobial agents enable sustained release of these agents and, in the case of AMPs, reduced toxicity. However, for encapsulation of AMPs, there is a general trend of needing to compromise efficacy with toxicity. Additionally, while these strategies do improve some of the limitations of alternative antimicrobial agents, they do not address issues associated with the time-intensive and high-cost production methods for these alternative antimicrobial agents.

1.3.4. Hybrid micro/nanoparticle strategies for enhanced delivery

Notably, micro/nanoparticles can also be enabled with additional functionalities to improve other infection recovery outcomes (such as reduced inflammation) or additional control for antimicrobial agent release. As an example, Zhao et al. created polymeric nanoparticles with bacterial targeting motifs that were loaded with drugs and photosensitive agents to enable phototherapy for suppression of inflammatory response caused by bacteria. These nanoparticles, when exposed to near infrared light, disrupted biofilms in vitro and significantly reduced bacterial presence and inflammatory response in a mouse model of pneumonia [75]. In a similar manner, Gao et al. created size and charge adaptive nanoparticles made of poly(amidoamine) dendrimer and 2,3-dimethyl maleic anhydride-modified poly(ethylene glycol)-block-polylysine (PEG-b-Plys) and coupled them to an antibiotic drug. Due to the charge adaptive aspect, the nanoparticles were able to directly attach to bacteria and increase membrane permeability in addition to having controlled release, resulting in long circulation time with low toxicity towards mammalian cells [76]. Notably, Yang et al. created multi-functional gold/silver composite nanocages with AMPs and hyaluronic acid (HA). The nanometal component enables phototherapy, AMPs provide enhanced targeting and bactericidal effects, and HA suppresses immune responses and provides nontoxic coating of nanometal particles. As a result of the combination of these components, following exposure to near-infrared light, the nanocages significantly reduced bacterial presence at 3 days after treatment in a mouse model of pneumonia while demonstrating low mammalian cell toxicity low blood toxicity and decreased inflammatory responses [77].

Overall, these hybridized delivery strategies allow for the existence of alternative treatments that not only can be used to enhance antimicrobial effects, but also can be used to improve other related outcomes, such as inflammatory response, as well. Additionally, the ability to combine multiple therapeutic strategies indicates a potential for high degree of modulation, meaning that significant control can be exerted over multiple functions. However, the increased complexity of these strategies can increase the complication of fabrication as well as associated time and financial costs as well as regulatory burden.

1.4. Outlook

Biomaterials have provided powerful and versatile platforms to address current issues in the treatment of lung infections. Biomaterial-based strategies for enhanced delivery have been crucial to not only enhancing therapeutic effects of current antibiotic drugs, but also to addressing the shortcomings of alternative antimicrobial agents as well, including bacteriophages and AMPs. In particular, micro/nanoparticles can shield antimicrobial agents from degradation en route to infection sites and allow controllable and sustained release of antimicrobial agents. As a result of these properties, enhanced delivery strategies enable greater therapeutic effects against drug resistant infections and biofilms and even enables additional administration methods for some antimicrobial agents.

Whereas enhanced delivery biomaterials present promising applications towards addressing the problem of respiratory infections caused by drug-resistant bacteria, limitations still need to be addressed (Table 2). Although enhanced delivery biomaterials can improve delivery to the lung, there are still physiological constraints on the effectiveness of these biomaterials. Due to the lung's tendency to exhale/clear foreign material [53], it is important that particle-based delivery vehicles are in a particular size range to reduce clearance. In addition to mucus expulsion, host immune cells can phagocytize the micro/nanoparticles [78]. In other words, nano/microparticles need to be small enough to be inhaled and deposit deeply into the lung while also being large enough to stem degradation by host defense mechanisms and ensure that enough of the delivered antimicrobial agent is present for intended therapeutic effect [51,78]. However, a method of dealing with this constraint has been developed via the use of porous microparticles. By adding pores to the microparticles, microparticle density is reduced and thus enables larger sized microparticles to be able to deposit into the lung while also resisting degradation by host defense mechanisms [70,79].

Table 2.

Summary of micro/nanoparticle-based strategies for enhanced delivery of antimicrobial agents to the lung.

Strategy type	Description	Pros	Cons
Enhanced delivery of antibiotic drugs	Micro/nanoparticle carriers encapsulating/loaded with antibiotic drugs	+Sustained release allows antibiotic drug to be present at effective concentration at infection site	-Potential toxicity associated with particle fabrication methods
		+Delivery by MP/NP circumvents poor solubility by directly delivering drug to lung epithelial layer for direct absorption	-Inflammatory responses to biomaterial carrier
Enhanced delivery of alternative antimicrobial agents	Micro/nanoparticle carriers encapsulating or loaded with bacteriophages or AMPs	+Sustained release allows greater persistence of effective concentration at infection site	-Potential toxicity associated with particle fabrication methods
		+Helps reduce toxicity for AMPs	-Inflammatory responses to biomaterial carrier
			-AMPs need to compromise effectiveness for toxicity
			-Does not address fabrication issues associated with AMPs and bacteriophages
Hybridized strategies of enhanced delivery	Hybrid micro/nanoparticle based strategies that add extra control mechanism for antimicrobial agent release or additional functionalities not related to bacterial killing	+Multi-functional	-Increased complexity for fabrication
		+Enables combination of multiple treatment options to address multiple aspects related to infection (e.g.: phototherapy + drug delivery)	-Increased time, costs, and regulatory burden

While promising delivery vehicles for anti-microbial agents, micro/nanoparticles are but a component to a device. To administer these enhanced delivery materials, aerosolization or nebulization is relied upon, resulting in administration through devices such as inhalers. Enhanced delivery strategies have also seen several clinical trials and commercialization efforts, particularly in strategies based around drug encapsulation. Pulmosphere is a commercialized inhalable microparticle treatment that has been loaded with antibiotic drugs to treat lung infections, distributed in the form of an inhaler [54,80]. It has seen an approved clinical trial for delivery of tobramycin with clinically significant reduction of sputum bacterial presence at 112 mg of inhalable treatment, twice daily [81] and completed clinical trials to assess safety in healthy people [82] and for delivery of ciprofloxacin in COPD patients, including pediatric patients [83].

Other enhanced delivery therapeutics that have been commercialized include liposomal nanoparticles encasing amikacin for inhalation and nebulized tobramycin [84]. Arikayce is a commercialized version of liposomal nanoparticles encasing amikacin that has been evaluated in clinical trials, treating infections of P. aeruginosa in COPD patients [84]. In one clinical trial, a 560 mg dose resulted in improved lung function and clinically significant reduction in sputum P. aeruginosa [84,85]. These nanoparticles have also been tested in patients in treating Mycobacterium abscessus with 590 mg dose and reported to have no presence of M. abscessus in sputum with stabilized/improved pulmonary function [86].

Other micro/nanoparticle encapsulated drugs have been developed. Pulmaquin (Linhaliq) is a dual release formulation of nanoliposomal ciprofloxacin that includes both free ciprofloxacin and encapsulated ciprofloxacin that is administered as an aerosol. The product has seen a phase 2 clinical trial that resulted in clinically significant reduction (log 4.2 reduction) of P. aeruginosa after 28 days of treatment in non-CF patients with bronchiectasis [87]. Two parallel phase 3 clinical trials were also conducted and demonstrated reduced exacerbation frequency and P. aeruginosa sputum density with the treatment reported as safe and well tolerated [88].

In conclusion, biomaterial-based strategies for enhanced delivery of antimicrobials demonstrate great potential for not only enhancing existing therapeutic strategies but also for expanding the range of antimicrobial agents available for treatment of lung infections. However, in order to reach full potential, there are still a number of obstacles that need to be overcome including challenges related to the delivery site's physiology as well as process associated issues such as concerns about toxicity from residual organic solvents used in the manufacturing process for polymeric micro/nanoparticles. Overall, this area in the field of biomaterials is full of promise for addressing the challenges of treating the lung in an environment rife with drug resistant bacteria but still requires further development and optimization. However, some of these enhanced delivery strategies have seen commercialization and successful clinical trials, demonstrating the potent potential these strategies have in the fight against drug resistant infection of the lung.

CRediT authorship contribution statement

Eunice Chee: Writing – original draft, Conceptualization, Visualization. Andrés J. García: Writing – review & editing, Funding acquisition, Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

No data was used for the research described in the article.