Role of Drug Adsorption onto the Silica Surface in Drug Release from Mesoporous Silica Systems

High loading of lipophilic compounds in mesoporous silica for improved solubility and dissolution performance

Loading poorly soluble active pharmaceutical ingredients (API) into mesoporous silica can enable API stabilization in non-crystalline form, which leads to improved dissolution. This is particularly beneficial for highly lipophilic APIs (log D7.4 > 8) as these drugs often exhibit limited solubility in dispersion forming carrier polymers, resulting in low drug load and reduced solid state stability. To overcome this challenge, we loaded the highly lipophilic natural products coenzyme Q10 (CoQ10) and astaxanthin (ASX), as well as the synthetic APIs probucol (PB) and lumefantrine (LU) into the mesoporous silica carriers Syloid® XDP 3050 and Silsol® 6035. All formulations were physically stable in their non-crystalline form and drug loads of up to 50 % were achieved. At increasing drug loads, a marked increase in equilibrium solubility of the active ingredients in biorelevant medium was detected, leading to improved performance during biorelevant biphasic dissolution studies (BiPHa + ). Particularly the natural products CoQ10 and ASX showed substantial benefits from being loaded into mesoporous carrier particles and clearly outperformed currently available commercial formulations. Performance differences between the model compounds could be explained by in silico calculations of the mixing enthalpy for drug and silica in combination with an experimental chromatographic method to estimate molecular interactions.

Characterization of Drug with Good Glass-Forming Ability Loaded Mesoporous Silica Nanoparticles and Its Impact Toward in vitro and in vivo Studies

Solid oral dosage forms are mostly preferred in pharmaceutical formulation development due to patient convenience, ease of product handling, high throughput, low manufacturing costs, with good physical and chemical stability. However, 70% of drug candidates have poor water solubility leading to compromised bioavailability. This phenomenon occurs because drug molecules are often absorbed after dissolving in gastrointestinal fluid. To address this limitation, delivery systems designed to improve the pharmacokinetics of drug molecules are needed to allow controlled release and target-specific delivery. Among various strategies, amorphous formulations show significantly high potential, particularly for molecules with solubility-limited dissolution rates. The ease of drug molecules to amorphized is known as their glass-forming ability (GFA). Specifically, drug molecules categorized into class III based on the Taylor classification have a low recrystallization tendency and high GFA after cooling, with substantial "glass stability" when heated. In the last decades, the application of mesoporous silica nanoparticles (MSNs) as drug delivery systems (DDS) has gained significant attention in various investigations and the pharmaceutical industry. This is attributed to the unique physicochemical properties of MSNs, including high loading capacity, recrystallization inhibition, excellent biocompatibility, and easy functionalization. Therefore, this study aimed to discuss the current state of good glass former drug loaded mesoporous silica and shows its impact on the pharmaceutical properties including dissolution and physical stability, along with in vivo study. The results show the importance of determining whether mesoporous structures are needed in amorphous formulations to improve the pharmaceutical properties of drug with a favorable GFA.

Drug-Coformer Loaded-Mesoporous Silica Nanoparticles: A Review of the Preparation, Characterization, and Mechanism of Drug Release

Drug-coformer systems, such as coamorphous and cocrystal, are gaining recognition as highly effective strategies for enhancing the stability, solubility, and dissolution of drugs. These systems depend on the interactions between drug and coformer to prevent the conversion of amorphous drugs into the crystalline form and improve the solubility. Furthermore, mesoporous silica (MPS) is also a promising carrier commonly used for stabilization, leading to solubility improvement of poorly water-soluble drugs. The surface interaction of drug-MPS and the nanoconfinement effect prevent amorphous drugs from crystallizing. A novel method has been developed recently, which entails the loading of drug-coformer into MPS to improve the solubility, dissolution, and physical stability of the amorphous drug. This method uses the synergistic effects of drug-coformer interactions and the nanoconfinement effect within MPS. Several studies have reported successful incorporation of drug-coformer into MPS, indicating the potential for significant improvement in dissolution characteristics and physical stability of the drug. Therefore, this study aimed to discuss the preparation and characterization of drug-coformer within MPS, particularly the interaction in the nanoconfinement, as well as the impact on drug release and physical stability.

The processes behind drug loading and release in porous drug delivery systems

Porous materials are ubiquitous and exhibit properties suitable for depositing therapeutic compounds. Drug loading in porous materials can protect the drug, control its release rate, and improve its solubility. However, to achieve such outcomes from porous delivery systems, effective incorporation of the drug in the internal porosity of the carrier must be guaranteed. Mechanistic knowledge of the factors influencing drug loading and release from porous carriers allows rational design of formulations by selecting a suitable carrier for each application. Much of this knowledge exists in research areas other than drug delivery. Thus, a comprehensive overview of this topic from the drug delivery aspect is warranted. This review aims to identify the loading processes and carrier characteristics influencing the drug delivery outcome with porous materials. Additionally, the kinetics of drug release from porous materials are elucidated, and the common approaches to mathematical modeling of these processes are outlined.

Mesoporous Drug Delivery System: From Physical Properties of Drug in Solid State to Controlled Release

Mesoporous materials, which exhibit great potential in the control of polymorphs and delivery of poorly water-soluble drugs, have obtained considerable attention in the field of pharmaceutical science. The physical properties and release behaviors of amorphous or crystalline drugs may be affected by formulating them into mesoporous drug delivery systems. In the past few decades, an increasing amount of papers have been written about mesoporous drug delivery systems, which play a crucial role in improving the properties of drugs. Herein, mesoporous drug delivery systems are comprehensively reviewed in terms of their physicochemical characteristics, control of polymorphic forms, physical stability, in vitro performance, and in vivo performance. Moreover, the challenges and strategies of developing robust mesoporous drug delivery systems are also discussed.

Physical stability and dissolution behaviors of amorphous pharmaceutical solids: Role of surface and interface effects

Amorphous pharmaceutical solids (APS) are single- or multi-component systems in which drugs exist in high-energy states with long-range disordered molecular packing. APSs have become one of the most effective and widely used pharmaceutical delivery approaches for poorly water-soluble drugs in the last several decades. Considerable efforts have been made to investigate the physical stability and dissolution behaviors of APSs, however, the underlying mechanisms remain imperfectly understood. Recent studies reveal that surface and interface properties of APSs could strongly affect the physical stability and dissolution behaviors. This paper provides a comprehensive overview of recent studies focusing on the physical stability and dissolution behaviors of APSs from both surface and interface perspectives. We highlight the role of surface or interface properties in nucleation, crystal growth, phase separation, dissolution, and supersaturation. Meanwhile, the challenges and scope of research on surface and interface properties in the future are also briefly discussed. This review contributes to a better understanding of the surface- and interface-facilitated processes, which will provide more efficient and rational guidance for the design of APSs.

Recent Advances in Enhancement of Dissolution and Supersaturation of Poorly Water-Soluble Drug in Amorphous Pharmaceutical Solids: A Review

Amorphization is one of the most effective pharmaceutical approaches to enhance the dissolution and oral bioavailability of poorly water-soluble drugs. In recent years, amorphous formulations have been experiencing rapid development both in theoretical and practical application. Based on using different types of stabilizing agents, amorphous formulations can be mainly classified as polymer-based amorphous solid dispersion, coamorphous formulation, mesoporous silica-based amorphous formulation, etc. This paper summarizes recent advances in the dissolution and supersaturation of these amorphous formulations. Moreover, we also highlight the roles of stabilizing agents such as polymers, low molecular weight co-formers, and mesoporous silica. Maintaining supersaturation in solution is a key factor for the enhancement of dissolution profile and oral bioavailability, and thus, the strategies and challenges for maintaining supersaturation are also discussed. With an in-depth understanding of the inherent mechanisms of dissolution behaviors, the design of amorphous pharmaceutical formulations will become more scientific and reasonable, leading to vigorous development of commercial amorphous drug products.

Oral Drug Delivery Systems Based on Ordered Mesoporous Silica Nanoparticles for Modulating the Release of Aprepitant

Two different types of ordered mesoporous nanoparticles, namely MCM-41 and MCM-48, with similar pore sizes but different pore connectivity, were loaded with aprepitant via a passive diffusion method. The percentage of the loaded active agent, along with the encapsulation efficiency, was evaluated using High-performance Liquid Chromatography (HPLC) analysis complemented by Thermogravimetric Analysis (TGA). The determination of the pore properties of the mesoporous particles before and after the drug loading revealed the presence of confined aprepitant in the pore structure of the particles, while Powder X-ray Diffractometry(pXRD), Differential Scanning Calorimetry (DSC), and FTIR experiments indicated that the drug is in an amorphous state. The release profiles of the drug from the two different mesoporous materials were studied in various release media and revealed an aprepitant release up to 45% when sink conditions are applied. The cytocompatibility of the silica nanoparticles was assessed in Caco-2 cell monolayers, in the presence and absence of the active agent, suggesting that they can be used as carriers of aprepitant without presenting any toxicity in vitro.

Investigations of mechanism of Ca2+ adsorption on silica and alumina based on Ca-ISE monitoring, potentiometric titration, electrokinetic measurements and surface complexation modeling

Research on Ca2+ adsorption onto the mineral surface is of significant importance with regard to geochemical processes. Sverjensky (Geochim Cosmochim Acta 70(10), 2427–2453, 2006) assumed that alkaline earths form two types of surface species on oxides: tetranuclear (> SOH)2(> SO−)2_M(OH)+ and mononuclear > SO−_M(OH)+. To look into the above assumption we investigated calcium adsorption on SiO2 and Al2O3 because they are the most widespread minerals in the environment. We have determined the proton surface charge, electrokinetic potential and metal adsorption as a function of pH. The Ca2+ uptake and concentration in the system were monitored by the calcium ion-selective electrode (Ca-ISE). The Ca-ISE measurements indicated a similar affinity of Ca2+ for both materials despite their differently charged surface, negative for silica and mainly positive for alumina. This may suggest that simple electrostatic interactions are not the primary driving force for calcium adsorption, and that solvation of calcium ions at the surface may be crucial. We have analyzed our experimental data using the 2-pK triple-layer model (2-pK TLM). Three calcium complexes on the mineral surface were reported. Two of them were the same for both oxides, i.e. the tetranuclear ($$>$$ >  SOH)2($$>$$ >  SO−)2_Ca2+ and mononuclear complexes > SO−_CaOH+. Additionally, minor contribution from >SOH…Ca2+ for silica was assumed. In the case of Al2O3 the hydrolyzed tetranuclear complexes ($$>$$ >  SOH)2($$>$$ >  SO−)2_CaOH+ at pH > 7.5 occurred based on the modeling results. Two types of surface complexes suggested by Sverjensky allowed for the correct description of proton and calcium uptake for alumina. However, the electrokinetic data excluded hydrolyzed tetranuclear surface species for this oxide.

Comparison of Drug Release and Adsorption under Supersaturating Conditions for Ordered Mesoporous Silica with Indomethacin or Indomethacin Methyl Ester

Incomplete drug release from mesoporous silica systems has been observed in several studies. This work aims to increase the understanding of this phenomenon by investigating the mechanism of drug-silica interactions and adsorption behavior from supersaturated aqueous solutions of two similar drug molecules with different hydrogen bonding capabilities. Drug-silica interactions between indomethacin or its methyl ester and SBA-15 were investigated using spectroscopic techniques (infrared, fluorescence and X-ray photoelectron) and adsorption experiments. The results demonstrate that the predominant mechanism of interaction of both drugs with silica is hydrogen bonding between drug acceptor carbonyl groups with donor groups on the silica surface. The presence of a drug hydrogen bond donor group did not enhance drug adsorption. No evidence was obtained for drug adsorption through nonspecific hydrophobic interactions. Drug adsorption onto the silica surface was investigated under supersaturating conditions through the generation of adsorption isotherms. Similar adsorption isotherms were observed for each compound when the concentration scale was normalized to the drug amorphous solubility. In other words, the equilibrium between the drug adsorbed on the silica surface and free drug in solution was related to the drug activity in solution. The high tendency of the drug to adsorb when the solution is supersaturated was, in turn, found to limit the extent of drug release during dissolution under nonsink conditions. Thus, adsorption provides an explanation for incomplete drug release.

Hansen solubility parameters obtained via molecular dynamics simulations as a route to predict siloxane surfactant adsorption

HYPOTHESIS

The Hansen Solubility Parameters (HSP) derived from Molecular Dynamics (MD) simulations can be used as a fast approach to predict surfactants adsorption on a solid surface. Experiments and simulations: We focused on the specific case of siloxane-based surfactants adsorption on silicon oxide surface (SiO₂), encountered in inkjet printing processes. A simplified atomistic model of the SiO₂ surface was designed to enable the computation of its solubility parameter using MD, and to subsequently determine the interactions of the SiO₂ surface with the siloxane-based surfactant and the various solvents employed. Surfactant adsorption was characterized experimentally using contact angle goniometry, ellipsometry, XPS and AFM.

FINDINGS

Comparison of the numerical results with experiments showed that the HSP theory allows to identify the range of solvents that are likely to prevent surfactant adsorption on the SiO₂ surface. The proposed approach indicates that polar solvents, such as acetone and triacetin, which are strongly attracted to the silicon oxide surface might form a shield that prevents siloxane-based surfactants adsorption. This simple approach, can guide the selection of adequate solvents for surfaces and surfactants with specific chemical structures, providing opportunities for controlling interfacial adsorption.

Fundamental evaluation and optimization of porous spherical silica for developing functional fine particles via fluidized bed coating

Particle coating, a taste-masking technique for drugs, is limited by its long manufacturing time, which is caused by the decrease in the spray rate required to prevent particle agglomeration. Mesoporous silica particles, which have a high surface area and pore sizes in the range of 2-50 nm, possess high surface free energy; they have attracted significant interest for numerous applications in adsorption, separation, and catalysis and drug delivery. A form of mesoporous silica, microbead silicate, can prevent particle aggregation because of its good water absorbency and drying properties. Hence, it has been suggested to be applicable for particle coating. This study evaluated the physical properties and drug release capability of microbead silicate with different pore sizes. Although microbead silicate with small pores displayed a rapid drug release profile, drug release was incomplete. Contrastingly, microbead silicate with large pores achieved complete drug release even with high drug loading. Furthermore, in the case of 100% layering, the porosity of microbead silicate was maintained, thus sufficiently preventing aggregation due to the prevention of formation of liquid bridging of the coating solution. These results suggest that using microbead silicate with large pores for particle coating enables complete drug release while improving manufacturability.

Antioxidant properties of probucol released from mesoporous silica

Antioxidants play a vital role in scavenging reactive oxygen species (ROS) produced by the reduction of molecular oxygen from various cellular mechanisms. Under oxidative stress, an increase in the levels of ROS overwhelms the antioxidant response, causing oxidative damage to biological molecules, and leading to the development of various diseases. Drug compounds with potent antioxidant properties are typically poorly water soluble and highly hydrophobic. An extreme case is Probucol (PB), a potent antioxidant with reported water solubility of 5 ng/ml, and oral bioavailiability of <10%. In this study, PB was loaded in mesoporous silica at various drug loadings to understand the changes to the physical properties of the loaded drug, and it's in vitro drug release. Further in vitro studies were conducted in endothelial and microglia cell models to compare the free radical scavening efficiency of ascorbic acid, PB, and PB release from mesoporous silica particles. Out of the three different mesostructured particles studied, the maximum loading of PB was achieved for large pore mesoporous particles (SBA-15) at 50 wt% drug loading, before complete pore filling was observed. For all materials, loadings above complete pore filling resulted in the recrystallization of PB on the external surface. In vitro drug release measurements showed a rapid dissolution rate at low drug loadings compared to a bimodal release profile of amorphous and crystalline drug at higher drug loadings. PB loaded in mesoporous particle was shown to enhance the antioxidant response to extracellular ROS in the endothelial cell line model, and to intracellular ROS in the microglia cell model. Our results indicate that the antioxidant properties of PB can be significantly improved by using mesoporous silica as a delivery vehicle.

Enhancing oral bioavailability of poorly soluble drugs with mesoporous silica based systems: opportunities and challenges

Porous silica-based drug delivery systems have shown considerable promise for improving the oral delivery of poorly water-soluble drugs. More specifically, micro- and meso-porous silica carriers have high surface areas with associated ability to physically adsorb high-drug loads in a molecular or amorphous form; this allows molecular state drug release in aqueous gastrointestinal environments, potential for supersaturation, and hence facilitates enhanced absorption and increased bioavailability. This review focuses primarily on the ability of porous silica materials to modulate in vitro drug release and enhance in vivo biopharmaceutical performance. The key considerations identified and addressed are the physicochemical properties of the porous silica materials (e.g. the particle and pore size, shape, and surface chemistry), drug specific properties (e.g. pKa, solubility, and nature of interactions with the silica carrier), potential for both immediate and controlled release, drug release mechanisms, potential for surface functionalization and inclusion of precipitation inhibitors, and importance of utilizing relevant and effective in vitro dissolution methods with discriminating dissolution media that provides guidance for in vivo outcomes (i.e. IVIVC).

Supersaturation Potential of Ordered Mesoporous Silica Delivery Systems. Part 1: Dissolution Performance and Drug Membrane Transport Rates

Ordered mesoporous silica materials have shown great potential as oral drug delivery systems for poorly soluble drugs. However, the ability of these delivery systems to generate drug supersaturation has not been widely investigated, and the recently noted phenomenon of incomplete drug release is not well understood. Therefore, the aim of this study was to comprehensively evaluate the release of hydrophobic drug molecules into solution from ordered mesoporous silica, focusing on the extent and duration of drug supersaturation. The dissolution and supersaturation behavior of ritonavir, following loading into mesoporous SBA-15 silica particles, was investigated by undertaking simple in vitro dissolution studies in phosphate buffer pH 6.8 and fasted state simulated intestinal fluid, as well as membrane flux studies using a side-by-side diffusion cell apparatus. It was found that supersaturated ritonavir solutions were generated from ritonavir-loaded mesoporous SBA-15 particles; however, drug release was always incomplete, even under sink conditions. In addition, the percentage drug release was observed to decrease significantly as the theoretical supersaturation ratio and dose of ritonavir-loaded SBA-15 formulation increased. The data obtained suggest an equilibrium exists between drug adsorbed to the SBA-15 silica surface and free drug present in solution. The findings described herein are highly significant in aiding our understanding of ordered mesoporous silica as a supersaturating drug delivery system for bioavailability enhancement.