Hyper-viscoelastic CRSM secretions not only play a key role in the pathogenesis of CRS disease, but also serve as a critical diffusional barrier that blocks particles from reaching the underlying epithelium and/or distal sinus cavities. Here, we show that synthetic nanoparticles at least as large as ~200 nm in diameter, including biodegradable nanoparticles composed of commonly used biomaterials, can be engineered to rapidly penetrate a majority of CRSM samples. CRSM-penetrating particles must have surfaces that minimize adhesion to CRSM constituents, which can be achieved via a dense covalent coating of low MW polyethylene glycol (PEG), or via a non-covalent coating with Pluronic F127. Without coatings that can minimize adhesive interactions between particles and CRSM constituents, nanoparticles are extensively trapped in CRSM. Importantly, the average speed of the biodegradable PLGA/F127 particles was only ~20-fold reduced compared to their theoretical speed in water, with the fastest 20% of the particles only slowed 6-fold compared to their average speeds in water. The ability to engineer nanoparticle-based carriers, using likely safe materials, that rapidly penetrate the formidable CRSM barrier and slowly release encapsulated therapeutics over time may lead to new generations of nanomedicines with tailored pharmacokinetics for local treatment of CRS. This includes CRSM-penetrating particles that, upon nasal instillation, can slowly release drugs like corticosteroids or antibiotics locally in the sinuses, thereby potentially mitigating much of the adverse side effects associated with systemic exposure of both drugs.
The PLGA/F127 particle platform reported here likely represents a safe delivery vehicle to the sinuses. PLGA, one of the most widely used polymers in drug delivery, has a well established biocompatibility and biodegradability profile [26
]. PLGA is FDA-approved for use in various biomedical devices, such as resorbable sutures and drug delivery devices, including the Lupron Depot® (intramuscular injection) and Atridox® (periodontal gel) [31
]. Likewise, Pluronic has a long history of safety for use in oral, intravenous and ophthalmic applications [29
], including toothpastes and mouthwashes, laxatives, and pharmaceutical products such as Oraqix® (periodontal gel), Differin® (a topical acne cream or gel) and Lariam® (oral tablet) [33
]. Reflecting their biocompatibility when applied to mucosal tissues and systemically, both PLGA and Pluronics are classified as Generally Regarded As Safe (GRAS) materials by the FDA. It is important to note that the concept of GRAS does not automatically imply biocompatibility, which must be considered in the context of a specific application since biocompatibility is not a property of the material. Nevertheless, the ability to engineer mucus-penetrating particles composed entirely of GRAS materials, without generating any new chemical entity (NCE), may facilitate quicker and more cost-effective clinical development of this drug delivery platform.
In addition to their safety profile, we anticipate that the PLGA/F127 particles will also provide sustained delivery of a wide array of therapeutics, since many therapeutic molecules can be encapsulated and released from PLGA particles [37
]. Importantly, our current method involves only incubation of pre-formulated PLGA particles with Pluronic, and does not require altering the formulation process of drug-loaded PLGA particles; the simplicity of the process should help ensure ease of manufacturing and production scalability.
The rapid diffusion of coated particles in CRSM suggests that CRSM, like CVM and CFS, possesses a mesh structure with a low viscosity, water-like, interstitial fluid between the structural elements through which properly engineered particles can penetrate [11
]. Nevertheless, there appear to be distinct differences in the barrier properties of CRSM against nanoparticles compared to other human mucus secretions. CRSM slowed the transport of all PS-PEG particles to a greater extent than CVM, especially to 100 nm and 500 nm PS-PEG than 200 nm PS-PEG [13
]. Since 200 nm PS-PEG exhibited faster transport than 100 nm PS-PEG in the same CRSM samples, the slower transport of 100 nm PS-PEG particles cannot be attributed to steric obstruction. Instead, 100 nm PS-PEG must be slowed by adhesive interactions with CRSM, perhaps a consequence of less effective PEG coating on smaller particles due to the higher degree of curvature. Nevertheless, the same 100 nm PS-PEG particles rapidly penetrated CVM [38
], suggesting that CRSM most likely possess greater adhesivity than CVM. The elevated adhesivity may also be reflected by the slower speeds of 200 nm PS-PEG in CRSM compared to CVM, which was slowed on average only 4-fold in CVM compared to their theoretical speeds in water, compared to 20-fold in CRSM [13
]. The hindered transport of 500 nm PS-PEG in CRSM vs. CVM likely reflects a combination of increased adhesivity and elevated steric obstruction from a denser mesh, similar to what we have previously observed with CFS [16
]. Interesting, the same trend for the speeds of 100 – 500 nm PS-PEG particles in CRSM (200 nm > 100 nm > 500 nm) was also observed with CFS, although the absolute speeds of PS-PEG particles in CRSM are faster than those in CFS, suggesting that CRSM may pose an overall less tenacious diffusional barrier than CFS [16
]. The similar barrier properties of CRSM and CFS may be attributed to the similar pathogenesis of the two diseases, in which the formation of an increasingly viscoelastic mucus gel is exacerbated by pathogenic infections and chronic inflammation [4
]. Indeed, CRS occurs extremely frequently in CF patients, and carriers of a single CF mutation have a higher prevalence of CRS than the general population, suggesting the potential genetic linkage between the two diseases may have led to similarities in their nanoscale barrier properties [40
Although we were able to engineer densely PEGylated particles to rapidly penetrate all CRSM samples from patients without nasal polyps, the particles were not able to penetrate a subset of CRSM samples from patients with nasal polyps. While the actual biochemical differences between the two groups of CRSM samples remain unclear, the disparity is likely related to polyp-induced alteration of the mucus barrier. Since polyp formation likely reflects greater prior exposure to inflammatory stress, the greater adhesivity and viscoelasticity in these CRSM samples may be attributed to increased DNA and actin content from degenerating inflammatory neutrophils and elevated mucin content from globet cell hyperplasia. This motivates the use of adjuvant therapies, such as mucolytics, to reduce the barrier properties of CRSM and improve particle penetration across these samples [42