(8b) Optimization of Mucus Penetrating Particles and Vehicle Composition for Improved Mucosal Surface Drug Delivery

Introduction

Local mucosal drug delivery has many applications, including
treatment of various cancers, inflammatory conditions such as inflammatory
bowel disease and chronic obstructive pulmonary disease, and prevention of
sexually transmitted infections. However, achieving effective drug delivery to mucosal
epithelia can be challenging. Epithelial surfaces of the female reproductive
tract, gastrointestinal tract, and lung have numerous folds and/or topography,
making significant portions of the epithelial surface difficult to access.
Additionally, mucosal epithelia are highly absorptive and permeable, which can
limit the duration of delivery of soluble drug systems. Nanoparticles can be
used as controlled-release delivery systems to slow the absorption of drugs,
but the viscoelastic, adhesive mucus layers lining mucosal epithelia trap
conventional nanoparticles (CP), limiting distribution and facilitating rapid
clearance. We recently discovered that densely coating the surface of
nanoparticles with low molecular weight polyethylene glycol (PEG) effectively
shields the nanoparticle core from adhesive interactions with human
cervicovaginal mucus (CVM) and human respiratory mucus (1,2). These so-called
mucus penetrating particles (MPP) rapidly diffuse through the fluid-filled
pores in the mucus mesh, whereas CP are adhesively immobilized. However,
diffusion alone may not be rapid enough for MPP to distribute throughout
mucosal surfaces, including penetrating the mucus blanket. Thus, we have found
that using osmotic gradients to induce fluid absorption and subsequent
advective delivery to the epithelial surface is very effective; MPP delivered
in a hypotonic vehicle rapidly distributed throughout the vaginal tract of
mice, including reaching into the deep folds within 10 minutes (3). By reaching
the more slowly cleared mucus layers in the rugae, MPP were well-retained in
the vagina, whereas mucoadhesive CP were rapidly cleared with the luminal mucus
layers (3). We anticipate that similar improvements in distribution and
retention with hypotonically-administered MPP can be achieved at various
mucosal surfaces, potentially improving drug delivery for a wide range of
conditions. Here, we optimize nanoparticle surface PEG density and vehicle
tonicity for enhanced distribution, facilitating effective drug delivery.

Materials and Methods

Non-biodegradable probe MPP for testing various vehicles,
capable of diffusing through human CVM, were prepared as previously reported (4).
Biodegradable nanoparticles composed of poly(lactic-co-glycolic acid) (PLGA)
and PEG copolymer (PLGA-PEG) were synthesized with various PEG surface densities
using a self-assembly emulsification method and varying ratios of
PLGA:PLGA-PEG. PEG surface density was quantified using two NMR methods: the
first method involved dissolving the nanoparticles and determining total PEG
content, while the second method measured surface PEG content of intact
particles. The PEG density (Γ)
was calculated as the number of PEG molecules on the nanoparticle surface per
100 nm². The number of unconstrained molecules per 100 nm²
(Γ*) was
calculated by assuming PEG behaves as a flexible polymer in a random-walk
conformation. The ratio Γ/Γ* represents the
relative steric constraint on the surface PEG; as Γ/Γ*
increases, the PEG conformation progresses from the mushroom to the brush
regime (5). Transport rates of MPP and CP were quantified using multiple
particle tracking (6). Vehicles of varying osmolality were prepared with
various amounts of water and saline. Vaginal distribution was assessed by
qualitatively (cross-sectional images) and quantitatively (surface area covered
on flattened tissue).

Results

We found that increasing PEG content more effectively
shielded the nanoparticle core, reducing mucoadhesion and increasing transport
rates in human mucus. Increasing PEG content reflected an increase in the
relative ratio of PEG chains per surface area compared to the number of
unconstrained PEG chains that would occupy a given surface area, indicating a
more constrained surface conformation and close packing in the ?brush? regime.
Improvement in mucus penetration correlated well with improved vaginal
distribution and surface coverage in mice. Administering MPP in hypotonic
vehicles led to rapid fluid absorption and distribution throughout the vaginal
tract. We found that even minimally hypotonic vehicles (220 mOsm/kg)
facilitated favorable distribution over the entire epithelial surface,
including the deep folds. In contrast, MPP administered in an isotonic vehicle
were found diffusely spread throughout the vaginal lumen, rather than forming
the uniform coating on the vaginal epithelium seen with
hypotonically-administered MPP.

Discussion and Conclusion

Many conventional vehicles for vaginal and rectal delivery
are hypertonic, which causes fluid secretion that facilitates clearance (7).
Hypertonicity has also been demonstrated to cause inflammation that increases
susceptibility to sexually transmitted infections (8,9). Although purely
hypotonic vehicles appear to be safe after daily vaginal administration for 1 week
(3), we anticipate that minimally hypotonic vehicles are less likely to cause
any toxicity or epithelial distress, particularly with repetitive
administration. As such, we demonstrated here that improved epithelial
distribution can be achieved with a minimally hypotonic vehicle. Although
osmotically-induced fluid absorption appears to be advantageous for
nanoparticle delivery at mucosal surfaces, it is vital that the nanoparticle be
sufficiently mucoinert to penetrate through the mucus barrier. Although we
previously demonstrated that nanoparticles sufficiently coated with PEG can
rapidly penetrate human CVM, here we more thoroughly characterize what is a
?sufficient? coating. We found that increasing PEG content, such that the
conformation was in the ?brush? regime, correlated with increased diffusion
rates in human CVM and improved distribution in the mouse vagina. We anticipate
that the PEG surface density can be similarly optimized for penetration in
mucus at other mucosal surfaces, including the GI tract and the lungs.
Additionally, mucosal surfaces such as the GI tract and the lung are also
absorptive, such that hypotonic vehicles will also be beneficial for
distribution and retention. Combining MPP with and hypotonic vehicles has
promise for more efficacious drug and gene delivery to various mucosal
epithelia, potentially improving prevention and treatment of a wide array of
mucosal diseases and conditions.

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