Addressing the PEG Mucoadhesivity Paradox to Engineer Nanoparticles that “Slip” through the Human Mucus Barrier

Abstract

Mucus linings serve as the bodys first line of defense at exposed surfaces of the eye and respiratory, gastrointestinal, and cervicovaginal tracts. The high viscoelasticity and adhesivity of mucus traps and limits exposure to foreign pathogens, [1, 2] toxins, [3] and environmental ultrafine particles, [1,4] which are all typically removed by normal mucus clearance mechanisms. Numerous studies have demonstrated that human mucus also strongly immobilizes conventional synthetic nanoparticles, [5][6][7] and therefore represents a hurdle for localized drug and gene delivery at mucosal surfaces, such as aerosol-based gene carriers for cystic fibrosis gene therapy. [8] To increase the bioavailability of cargo therapeutics, it is important that carrier particles rapidly penetrate mucus to avoid being shed.We recently discovered that nanoparticles coated with low molecular weight (MW) poly(ethylene glycol) (PEG) possess hydrophilic and near neutrally-charged surfaces that minimize mucoadhesion by reducing hydrophobic or electrostatic interactions. [5] These coatings were inspired by viruses with similar surface properties that are capable of moving rapidly through human mucus. [7,9] However, a vast amount of literature has shown that PEG can be strongly mucoadhesive, presumably by interpenetrating polymer network (IPN) effects between PEG chains and the mucus mesh [10][11][12] and/ or hydrogen bonding between ether oxygen atoms in PEG and sugars on glycosylated mucins. [13] To reconcile the para-doxical reports of PEGs interactions with mucus, we correlated the physicochemical properties of PEG-coated nanoparticles, specifically PEG MW and degree of surface coverage, to the dynamics of coated particles in fresh, undiluted human cervicovaginal mucus (CVM). The realtime transport rates of the particles were studied in mucus using multiple-particle tracking, a powerful technique that allows quantitative measurements of hundreds of individual particles. [5,14] We show that low PEG MW and high (dense) PEG surface coverage are both required for rapid mucus penetration of coated particles, and that high MW PEG can increase mucoadhesion.To determine the effect of PEG MW on the interactions of coated particles with mucus, we covalently conjugated a dense surface coverage of either 2 kDa (PS-PEG2k High ) or 10 kDa (PS-PEG10k High ) methoxy-PEG-amine onto 200 nm fluorescent carboxylated polystyrene (PS) nanoparticles. The MWs were chosen on the basis of our previous finding that coating particles with 2 kDa PEG provided mucus-penetrating transport properties [5] and literature suggesting that adhesive IPN effects are likely enhanced at higher MWs. [11] A dense surface coating was confirmed for each formulation by the nearneutral surface charge of coated particles, as well as fluorimetric assays that revealed 65-70 % of surface carboxy groups conjugated with PEG (Table 1). A fivefold reduction in PEG MW, from 10 kDa to 2 kDa, produced a 1000-fold increase in the mean-square-displacement (MSD) of coated particles in CVM at a time sca...

In vivo study of the mucus-permeating properties of PEG-coated nanoparticles following oral administration

“…All of these observations are553 in line with previous results described by Wang and co-workers 554 who described that 200 nm poly(styrene) particles coated with 555 either 5 kDa-PEG diffused in vitro as rapidly through mucus as 556 those with 2 kDa PEG, but a further increase of PEG MW to557 10 kDa resulted in 1000-fold slower transport in human cervico 558 vaginal mucus[19]. In fact the 10 kDa PEG chains may be long 559 enough to significantly entangle with mucins [37].Another important factor that appears to modulate the mucuspermeating properties of PEG-coated nanoparticles is the extent of 562 the PEG surface coverage.…”

Polymer-based nanocarriers for vaginal drug delivery

“…The use of PEG with adequate MW is essential in order to achieve sufficient surface coating while avoiding the presence of long polymer chains that could promote adhesion by interpenetration mechanisms with mucin fibers. Values of 2-5 kDa were shown ideal for enhanced diffusive behavior, while longer PEG chains (10 kDa) were able to delay the transport of 200 nm PS NPs in cervicovaginal mucus as determined using MPT [75]. Similar results were obtained by Cu and Saltzman [76] for the transport of PEG-modified PLGA NPs in cervical mucus, as assessed by fluorescent microscopy and using a mass transport mathematical model, except for 10 kDa PEG-modified particles.…”

On the synthesis of mucus permeating nanocarriers

“…Hydrogen bonding or Van der Waals interactions are other types of interactions which can also contribute to the ion-pairing process [2][3][4]. Apart from nanoparticles with a densely charged, yet net neutral surface, an uncharged nanoparticle may be also considered as mucoinert as viral capsids, provided it is sufficiently hydrophilic with a low hydrogen bonding http capability (e.g., polyethylene glycol (PEG), cyclodextrin coatings) [1,5]. Wang et al [5] covalently modified the surfaces of polystyrene nanoparticles with CH 3 O-PEG-NH 2 of various molecular weights (MW = 2, 5, 10 kDa) and showed that low PEG MW and high PEG surface coverage are both required for rapid mucus penetration of coated NPs.…”

“…Apart from nanoparticles with a densely charged, yet net neutral surface, an uncharged nanoparticle may be also considered as mucoinert as viral capsids, provided it is sufficiently hydrophilic with a low hydrogen bonding http capability (e.g., polyethylene glycol (PEG), cyclodextrin coatings) [1,5]. Wang et al [5] covalently modified the surfaces of polystyrene nanoparticles with CH 3 O-PEG-NH 2 of various molecular weights (MW = 2, 5, 10 kDa) and showed that low PEG MW and high PEG surface coverage are both required for rapid mucus penetration of coated NPs. More specifically, Wang et al [5] observed that there exists a critical MW between 5 and 10 kDa where dense PEG coatings transition from being mucoinert to mucoadhesive.…”