Alginate gels are an outstanding biomaterial widely applicable in tissue engineering, medicine, and pharmacy for cell transplantation, wound healing and efficient bioactive agent delivery, respectively. This contribution provides new and comprehensive insight into the atomic-resolution structure and dynamics of polyvalent ion-cross-linked alginate gels in microbead formulations. By applying various advanced solid-state NMR (ssNMR) spectroscopy techniques, we verified the homogeneous distribution of the cross-linking ions in the alginate gels and the high degree of ion exchange. We also established that the two-component character of the alginate gels arises from the concentration fluctuations of residual water molecules that are preferentially localized along polymer chains containing abundant mannuronic acid (M) residues. These hydrated M-rich blocks tend to self-aggregate into subnanometer domains. The resulting coexistence of two types of alginate chains differing in segmental dynamics was revealed by H-C dipolar profile analysis, which indicated that the average fluctuation angles of the stiff and mobile alginate segments were about 5-9° or 30°, respectively. Next, the C CP/MAS NMR spectra indicated that the alginate polymer microstructure was strongly dependent on the type of cross-linking ion. The polymer chain regularity was determined to systematically decrease as the cross-linking ion radius decreased. Consistent with theH-H correlation spectra, regular structures were found for the gels cross-linked by relatively large alkaline earth cations (Ba, Sr, or Ca), whereas the alginate chains cross-linked by bivalent transition metal ions (Zn) and trivalent metal cations (Al) exhibited significant irregularities. Notably, however, the observed disordering of the alginate chains was exclusively attributed to the M residues, whereas the structurally well-defined gels all contained guluronic acid (G) residues. Therefore, a key role of the units in M-rich blocks as mediators promoting the self-assembly of alginate chains was experimentally confirmed. Finally, combining 2D Al 3Q/MAS NMR spectroscopy with density functional theory (DFT) calculations provided previously unreported insight into the structure of the Al cross-linking centers. Notably, even with a low residual amount of water, these cross-linking units adopt exclusively 6-fold octahedral coordination and exhibit significant motion, which considerably reduces quadrupolar coupling constants. Thus, the experimental strategy presented in this study provides a new perspective on cross-linked alginate structure and dynamics for which high-quality diffraction data at the atomic resolution level are inherently unavailable.

No abstract

A drug dissolution profile is one of the most critical dosage form characteristics with immediate and controlled drug release. Comparing the dissolution profiles of different pharmaceutical products plays a key role before starting the bioequivalence or stability studies. General recommendations for dissolution profile comparison are mentioned by the EMA and FDA guidelines. However, neither the EMA nor the FDA provides unambiguous instructions for comparing the dissolution curves, except for calculating the similarity factor f2. In agreement with the EMA and FDA strategy for comparing the dissolution profiles, this manuscript provides an overview of suitable statistical methods (CI derivation for f2 based on bootstrap, CI derivation for the difference between reference and test samples, Mahalanobis distance, model-dependent approach and maximum deviation method), their procedures and limitations. However, usage of statistical approaches for the above-described methods can be met with difficulties, especially when combined with the requirement of practice for robust and straightforward techniques for data evaluation. Therefore, the bootstrap to derive the CI for f2 or CI derivation for the difference between reference and test samples was selected as the method of choice.

The remarkably diverse affinity of alginate (ALG) macromolecules for polyvalent metal ions makes cross-linked alginate gels an outstanding biomaterial. Surprisingly, however, very little is known about their interactions and structural transformations in physiological environments. To bridge this gap, we prepared a set of ALG gels cross-linked by various ions and monitored their structural changes at different media simulating gastric and intestinal fluids and cellular environments. For these studies, we used multinuclear solid-state NMR (ss-NMR) spectroscopy, which revealed a range of competitive ion-exchange and interconversion reactions, the rate of which strongly depended on the nature of the cross-linking metal ions. Depending on the environment, ALG chains adopted different forms, such as acidic (hydro)gels stabilized by strong hydrogen bonds, and/or weakly cross-linked Na/H-gels. Simultaneously, the exchanged polyvalent ions extensively interacted with the environment even forming in some cases insoluble phosphate microdomains directly deposited in the ALG bead matrix. The extent of the transformations and incorporation of secondary phases into the alginate beads followed the size and electronegativity of the cross-linking ions. Overall, the applied combination of various macroscopic and biological tests with multinuclear ss-NMR revealed a complex pathway of alginate beads transformations in physiological environments.

Burst drug release is often considered a negative phenomenon resulting in unexpected toxicity or tissue irritation. Optimal release of a highly soluble active pharmaceutical ingredient (API) from hypromellose (HPMC) matrices is technologically impossible; therefore, a combination of polymers is required for burst effect reduction. Promising variant could be seen in combination of HPMC and insoluble Eudragits as water dispersions. These can be applied only on API/insoluble filler mixture as over-wetting prevention. The main hurdle is a limited water absorption capacity (WAC) of filler. Therefore, the object of this study was to investigate the dissolution behavior of levetiracetam from HPMC/EudragitNE matrices using magnesium aluminometasilicate (Neusilin US2) as filler with excellent WAC. Part of this study was also to assess influence of thermal treatment on quality parameters of matrices. The use of Neusilin allowed the application of Eudragit dispersion to API/Neusilin mixture in one step during high-shear wet granulation. HPMC was added extragranularly. Obtained matrices were investigated for qualitative characteristics, NMR solid-state spectroscopy (ssNMR), gel layer dynamic parameters, SEM, and principal component analysis (PCA). Decrease in burst effect (max. of 33.6%) and dissolution rate, increase in fitting to zero-order kinetics, and paradoxical reduction in gel layer thickness were observed with rising Eudragit NE concentration. The explanation was done by ssNMR, which clearly showed a significant reduction of the API particle size (150-500 nm) in granules as effect of surfactant present in dispersion in dependence on EudragitNE amount. This change in API particle size resulted in a significantly larger interface between these two entities. Based on ANOVA and PCA, thermal treatment was not revealed as a useful procedure for this system.