Microstructure and elution of tetracycline from block copolymer coatings

Method for assessing coating uniformity of angioplasty balloons coated with poly(lactic-co-glycolic acid) nanoparticles loaded with quercetin

Significance

Drug-coated angioplasty balloons (DCBs) are used to treat peripheral artery disease, and proper dosage depends on coating characteristics like uniformity and number of layers.

Aim

Quantify coating uniformity and correlate fluorescence intensity to drug loading for DCBs coated with 5, 10, 15, or 20 layers of poly(lactic-co-glycolic acid) nanoparticles (NPs) entrapped with quercetin.

Approach

Images of DCBs were acquired using fluorescence microscopy. Coating uniformity was quantified from histograms and horizontal line profiles, and cracks on the balloons were measured and counted. Fluorescence intensity was correlated with the drug loading of quercetin found from gravimetric analysis coupled with high-performance liquid chromatography (HPLC).

Results

Higher numbers of coating layers on DCBs may be associated with less uniform coatings. Cracks in the coating were present on all balloons, and the length of cracks was not significantly different between balloons coated with different numbers of layers or balloons coated with the same number of layers. A strong positive correlation was identified between fluorescence intensity and drug loading.

Conclusion

There may be a relationship between the number of NP layers and the uniformity of the coating, but further investigation is needed to confirm this. Fluorescence intensity appears to be a strong predictor of drug loading on DCBs coated with quercetin-entrapped NPs, demonstrating that fluorescent imaging may be a viable alternative to drug release studies.

Strategies for antimicrobial peptide coatings on medical devices: a review and regulatory science perspective

Indwelling and implanted medical devices are subject to contamination by microbial pathogens during surgery, insertion or injection, and ongoing use, often resulting in severe nosocomial infections. Antimicrobial peptides (AMPs) offer a promising alternative to conventional antibiotics to reduce the incidence of such infections, as they exhibit broad-spectrum antimicrobial activity against Gram-negative and Gram-positive bacteria, microbial biofilms, fungi, and viruses. In this review-perspective, we first provide an overview of the progress made in this field over the past decade with an emphasis on the local release of AMPs from implant surfaces and immobilization strategies for incorporating these agents into a wide range of medical device materials. We then provide a regulatory science perspective addressing the characterization and testing of AMP coatings based on the type of immobilization strategy used with a focus on the US market regulatory niche. Our goal is to help narrow the gulf between academic studies and preclinical testing, as well as to support a future literature base in order to develop the regulatory science of antimicrobial coatings.

Controlled initial surge despite high drug fraction and high solubility

Potential connections between release profiles and solvent evaporation rates alongside polymer chemistry were elucidated for the release of tetracycline hydrochloride from two different poly (d, l-lactide-co-glycolide) (PLGA) film matrices containing high drug fractions (50%, 30%, and 15%), and prepared at two distinct solvent evaporation rates. At highest tetracycline concentrations (50%), (i) the early release rates were ≤0.5 μg/min in all cases; (ii) release was linear from systems fabricated with lower lactic content and slower solvent evaporation rate and bimodal from systems fabricated with higher lactic content and faster evaporation rate; (iii) surface fractions covered by the drug were similar at both evaporation rates for 85:15 PLGA but very different for 50:50 PLGA, leading to unexpectedly reduced early release from 50:50 PLGA than from 85:15 PLGA when both the matrices were fabricated using a slower evaporation rate. These features remained unaffected in case of low drug concentration. Results suggested that during the formation of the drug-polymer microstructure, the combined effect of polymer chemistry and solvent evaporation rate sets apart the surface characteristics and the initial release profiles of systems containing high drug fraction, and an appropriate combination of these parameters may be utilized to control the early stage of drug release.

Application of quality by design (QbD) approach to ultrasonic atomization spray coating of drug-eluting stents

The drug coating process for coated drug-eluting stents (DES) has been identified as a key source of inter- and intra-batch variability in drug elution rates. Quality-by-design (QbD) principles were applied to gain an understanding of the ultrasonic spray coating process of DES. Statistically based design of experiments (DOE) were used to understand the relationship between ultrasonic atomization spray coating parameters and dependent variables such as coating mass ratio, roughness, drug solid state composite microstructure, and elution kinetics. Defect-free DES coatings composed of 70% 85:15 poly(DL-lactide-co-glycolide) and 30% everolimus were fabricated with a constant coating mass. The drug elution profile was characterized by a mathematical model describing biphasic release kinetics. Model coefficients were analyzed as a DOE response. Changes in ultrasonic coating processing conditions resulted in substantial changes in roughness and elution kinetics. Based on the outcome from the DOE study, a design space was defined in terms of the critical coating process parameters resulting in optimum coating roughness and drug elution. This QbD methodology can be useful to enhance the quality of coated DES.

Prediction and validation of diffusion coefficients in a model drug delivery system using microsecond atomistic molecular dynamics simulation and vapour sorption analysis

Diffusion of small to medium sized molecules in polymeric medical device materials underlies a broad range of public health concerns related to unintended leaching from or uptake into implantable medical devices. However, obtaining accurate diffusion coefficients for such systems at physiological temperature represents a formidable challenge, both experimentally and computationally. While molecular dynamics simulation has been used to accurately predict the diffusion coefficients, D, of a handful of gases in various polymers, this success has not been extended to molecules larger than gases, e.g., condensable vapours, liquids, and drugs. We present atomistic molecular dynamics simulation predictions of diffusion in a model drug eluting system that represent a dramatic improvement in accuracy compared to previous simulation predictions for comparable systems. We find that, for simulations of insufficient duration, sub-diffusive dynamics can lead to dramatic over-prediction of D. We present useful metrics for monitoring the extent of sub-diffusive dynamics and explore how these metrics correlate to error in D. We also identify a relationship between diffusion and fast dynamics in our system, which may serve as a means to more rapidly predict diffusion in slowly diffusing systems. Our work provides important precedent and essential insights for utilizing atomistic molecular dynamics simulations to predict diffusion coefficients of small to medium sized molecules in condensed soft matter systems.

Impact of copolymer ratio on drug distribution in styrene-isobutylene-styrene block copolymers

Drug-polymer composite coatings, composed of styrene-isobutylene-styrene (SIBS) tri-block copolymers, are frequently used in controlled drug release biomedical device applications. In this work, we used atomic force microscopy to characterize the effects of different drug loadings and polymer chemistries (i.e., block copolymer ratio) on the variation of surface structures and compositions of SIBS-tetracycline (SIBS-TC) cast composites including tetracycline (TC) drug amount, drug phase size distribution, and drug and polymer phase morphologies. We tested the structural variations by fabricating and characterizing two types of composite specimens, that is, SIBS15 and SIBS30, composed of 15 and 30 Wt % of polystyrene (PS), respectively. The differences in the distribution of TC drug, PS, and polyisobutylene (PIB) polymer phase structures observed in SIBS15 and SIBS30 resulted in more drug at the surface of SIBS30 compared to SIBS15. To support the experimental findings, we have determined the Hildebrand solubility parameter of TC using molecular dynamics (MD) computation and compared it to the polymer components, PS and PIB. The MD results show that the solubility parameter of TC is much closer to that of PS than PIB, which demonstrates a higher thermodynamic stability of TC-PS mixtures.

Molecular dynamics study of the role of the free surface on block copolymer thin film morphology and alignment

Next-generation applications of block copolymer thin films will require a better understanding of the driving forces unique to thin film coatings, specifically those arising from the polymer-air interface. Previous modeling studies of film morphology have treated rigidly confined films, neglecting free surface considerations altogether. We report in this article the first systematic molecular dynamics investigation of block copolymer thin film ordering for unconfined films. We investigate the molecular basis of the formation of a number of experimentally relevant coating features, including surface islands and vertical lamellae. Surface islands are found to form in response to film incommensurability, whereas commensurability considerations are insufficient to explain vertical lamellar formation. Dynamics of lamellar formation presented herein demonstrate that vertical lamellar orientation is initiated in the surface regions of the film, most strikingly at the free surface. We conclude that the free surface plays a pivotal role in the free energy balance determining overall film morphology, and that confinement models provide an incomplete explanation of the physical basis of morphology selection in block copolymer coatings.

Predicting microstructure development during casting of drug-eluting coatings

We have devised a novel diffuse interface formulation to model the development of chemical and physical inhomogeneities, i.e. microstructure, during the process of casting drug-eluting coatings. These inhomogeneities, which depend on the coating constituents and manufacturing conditions, can have a profound affect on the rate and extent of drug release, and therefore the ability of coated medical devices to function successfully. By deriving the model equations in a time-dependent reference frame, we find that it is computationally viable to probe a wide, physically relevant range of material and process quantities. To illustrate the application of the model, we have evaluated the impact of manufacturing solvent, coating thickness and evaporation rate on microstructure development. Our results suggest that modifying these process conditions can have a strong and nearly discontinuous effect on coating microstructure, and therefore on drug release. Further, we demonstrate that the model can be applied to processes that involve the incremental application of the coating in layers or passes. This new model formulation, which can also be used to predict the kinetics of drug release, provides a tool to elucidate and quantify the relationships between process variables, microstructure and performance. Establishing these relationships can reduce empiricism in materials selection and process design, providing a facile and efficient means to tailor the underlying microstructure and achieve a desired drug-release behavior.