Hierarchical Macro-Mesoporous Silica Monolithic Tablets as a Novel Dose-Structure-Dependent Delivery System for the Release of Confined Dexketoprofen

This study reports the application of hierarchical porous monoliths as carriers for controlled and dose-adjustable release of model pharmaceutical (dexketoprofen, DEX). The synthesis and detailed characterization of the hierarchical porous scaffolds are provided before and after the adsorption of three doses of DEX─a widely used nonsteroidal anti-inflammatory drug. The drug incorporated in the mesopores of silica was stabilized in an amorphous state, while the presence of macropores provided sufficient space for drug crystallization as we demonstrated via a combination of powder X-ray diffraction, differential scanning calorimetry, and imaging techniques (scanning electron microscopy and EDX analysis). Drug release from silica matrices was tested, and a mechanistic model of this release based on the Fick diffusion equation was proposed. The hierarchical structure of the carrier, due to the presence of micrometric macropores and nanometric mesopores, turned out to be critical for the control of the drug phase and drug release from the monoliths. It was found that at low drug content, the presence of an amorphous component in the pores promoted the rapid release of the drug, while at higher drug contents, the presence of macropores favored the crystallization of DEX, which naturally slowed down its release. Both the hierarchical porous structure and the control of the drug phase (amorphous and/or crystalline) were proven important for adjustable (fast or prolonged) release kinetics, desirable for effective pharmacotherapy and patient compliance. Therefore, the developed materials may serve as a versatile formulation platform for the smart manipulation of drug release kinetics.

Controllable Enzyme Immobilization via Simple and Quantitative Adsorption of Dendronized Polymer-Enzyme Conjugates Inside a Silica Monolith for Enzymatic Flow-Through Reactor Applications

Although many different methods are known for the immobilization of enzymes on solid supports for use in flow-through applications as enzyme reactors, the reproducible immobilization of predetermined amounts of catalytically active enzyme molecules remains challenging. This challenge was tackled using a macro- and mesoporous silica monolith as a support and dendronized polymer-enzyme conjugates. The conjugates were first prepared in an aqueous solution by covalently linking enzyme molecules and either horseradish peroxidase (HRP) or bovine carbonic anhydrase (BCA) along the chains of a water-soluble second-generation dendronized polymer using an established procedure. The obtained conjugates are stable biohybrid structures in which the linking unit between the dendronized polymer and each enzyme molecule is a bisaryl hydrazone (BAH) bond. Quantitative and reproducible enzyme immobilization inside the monolith is possible by simply adding a defined volume of a conjugate solution of a defined enzyme concentration to a dry monolith piece of the desired size. In that way, (i) the entire volume of the conjugate solution is taken up by the monolith piece due to capillary forces and (ii) all conjugates of the added conjugate solution remain stably adsorbed (immobilized) noncovalently without detectable leakage from the monolith piece. The observed flow-through activity of the resulting enzyme reactors was directly proportional to the amount of conjugate used for the reactor preparation. With conjugate solutions consisting of defined amounts of both types of conjugates, the controlled coimmobilization of the two enzymes, namely, BCA and HRP, was shown to be possible in a simple way. Different stability tests of the enzyme reactors were carried out. Finally, the enzyme reactors were applied to the catalysis of a two-enzyme cascade reaction in two types of enzymatic flow-through reactor systems with either coimmobilized or sequentially immobilized BCA and HRP. Depending on the composition of the substrate solution that was pumped through the two types of enzyme reactor systems, the coimmobilized enzymes performed significantly better than the sequentially immobilized ones. This difference, however, is not due to a molecular proximity effect with regard to the enzymes but rather originates from the kinetic features of the cascade reaction used. Overall, the method developed for the controllable and reproducible immobilization of enzymes in the macro- and mesoporous silica monolith offers many possibilities for systematic investigations of immobilized enzymes in enzymatic flow-through reactors, potentially for any type of enzyme.