Aquasome

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Aquasomes are self-assembling nanoparticle drug carrier systems composed of three layers: a ceramic core, an oligomer coat, and a biochemically active molecule. Aquasomes are utilized for targeted drug delivery to achieve specific therapeutic effects, and are biocompatible, biodegradable, and stable. Due to their structure, aquasomes are capable of delivering several types of substrates, and can be used for applications such as delivery of antigen, insulin, and hemoglobin.

Aquasomes were first explored by Kossovsky et al. in 1996 in experiments proposing their use in antigen delivery, drug delivery, and hemoglobin delivery in vitro and in animal models.^[1] The aim of creating these aquasomes was to address the molecular denaturation of polypeptide pharmaceuticals by forming a molecular carrier using the novel surface modification process.^[1] Kossovsky et al. suggested that this system would be able to combat physical and chemical degradative agents affecting bioactive molecules while preserving the molecular structure of the drug.^[1]

Since this initial exploration, the researched applications of aquasomes and the scientific knowledge of their composition have increased. Upon synthesis of each layer of the aquasome, aquasomes self-assemble into triple-layered particles. The tri-layer structure enables aquasomes to deliver and release poorly soluble drugs in a controlled manner. Delivery of these poorly soluble drugs within aquasomes increases their solubility, bioavailability, and stability. These drugs are adsorbed on the surface of the aquasome, forming its third layer, which confers bioactive properties to the aquasome.

Structure

Aquasomes form a three-layered structure, made of a polyhydroxy oligomer coated core upon which the drug is loaded. The biochemically active molecules are able to interact with the coated core through different Van der Waal forces, entropic forces, and ionic and non-covalent bonds.^[2] The structure of aquasomes enables them to carry a variety of substrates, facilitating applications such as protein and peptide delivery and protection, and the delivery of nucleic acids for gene therapy applications.^[3] The tri-layer formulation is attributed to several drug delivery properties of the aquasome.

Aquasomes’ solid core, made of ceramic or polymeric material, is attributed to the structural stability of the nanoparticle itself, and can result in improved solubility and biocompatibility of the drug.^[3] Different core designs have also been shown to affect the controlled release properties of the drug molecule. A commonly used core material is the ceramic calcium phosphate, which naturally occurs in the body.^[3] Hydroxyapatite, which is found in bone, is another commonly used core material. Hydroxyapatite cores have been shown to contribute to targeted delivery of encapsulated hepatitis B antigens intracellularly.^[3] However, the ability to target aquasomes to specific cells or tissues has not been explored beyond intracellular delivery of aquasome payloads.

The second layer of aquasomes is the carbohydrate coat, onto which the drug is adsorbed. Due to carbohydrate’s action as a dehydroprotectant, it has been shown to function as a natural stabilizer to preserve the conformation and shape of soft drugs.^[3] The dehydroprotectant property of the carbohydrate coat also enables protection of the biochemically active molecule from dehydration and protein degradation.^[3]

The size of aquasomes ranges from 60 to 300 nanometers, hence their characterization as a nanoparticle drug carrier.^[1] The nanoscale of aquasomes gives them a high surface area to volume ratio. The smaller the core, the higher the surface area to volume ratio, which increases the drug loading capacity of the aquasome.^[2] Aquasomes possess water-like properties due to the presence of the carbohydrate coating, enabling them to protect and preserve fragile biological molecules. The size of aquasome particles increases as a function of the ratio between the concentration of the core to the coat due to the availability of free surface core particles for the coating material.^[3]

The self-assembly process of aquasomes into their tri-layer structure is achieved by non-covalent and ionic bonds, along with physicochemical properties of their components. Calcium phosphate nanoparticles are formed before the carbohydrate coat is adsorbed onto the surface of the core through electrostatic interactions. Layers are then added to the structure to achieve desired size, while crosslinked polymers aid in further stabilization. The sonication process during the reaction of disodium hydrogen phosphate and calcium chloride to prepare calcium phosphate impacts the self-assembly process of aquasomes by increasing surface free energy of the core prepared. This assembly process allows the design of aquasomes for specific drug delivery applications.^[2]

The structure of aquasomes can contribute to controlled drug release, drug stability, and intracellular targeting of the drug. Other commonly used nanoparticle drug delivery systems include niosomes, liposomes, and vesosomes, the compositions of which contribute to different properties of the resulting nanoparticle compared to aquasomes. Niosomes are composed of non-ionic surfactants and bilayer structures, allowing them to encapsulate hydrophilic and hydrophobic drugs. Liposomes are composed of phospholipids and a similar bilayer structure to niosomes, and can deliver toxic or poorly soluble drugs. Vesosomes have a core-shell structure similar to aquasomes, but contain a lipid bilayer core and a polymer shell, while aquasomes consist of a ceramic or polymeric core and a carbohydrate coat. Vesosomes are used for encapsulating imaging agents and aiding in imaging techniques such as MRI. ^[4][5][6]

Methods of Preparation

The three major units of aquasomes are fabricated together according to self-assembly, a thermodynamically driven process that organizes subunits of a system in a manner that results in the lowest Gibbs Free Energy available, known as ΔG. Self-assembly as a mixing process offers high accuracy and control over sizes on the nanometer scale, which is especially relevant for aquasomes, which exist on this size scale. The three layers of aquasomes can be synthesized differently using a variety of techniques depending on the intended functions or desired therapeutic effects.^[3] The general scheme of aquasome fabrication involves a sequential synthesis of a nanocrystalline core, followed by a polyhydroxy coating, and finished with integration of bioactive molecules. Throughout this process, several intermittent steps are included that involve selective filtering and purification to remove byproducts while isolating the desired products for further processing.^[3]

Layer 1: Core

The core of an aquasome can be made from either ceramic or polymeric materials. Examples of such polymers include acrylates and gelatin. However, because ceramic materials are more ordered due to their naturally occurring crystalline structure, they are more often preferred as the material type for the core.^[3] Some of the most common ceramic materials used in the formation of an aquasome core include tin oxide, calcium phosphate, and even diamond. Another characteristic that ceramic materials provide is enhanced binding of the carbohydrate layer due to the high surface energy present on the orderly surface. The binding affinity of the carbohydrate layer also reduces surface tension for its bond to the ceramic core.^[1] The first aquasomes fabricated with a nanocrystalline core using ceramic material are detailed in Kossovsky et al. in 1996.^[1] Calcium phosphate ceramic nanoparticles (brushite) were first prepared via the method of solution precipitation and sonication.^[1] Precipitation methods are the most common techniques employed when synthesizing the core of an aquasome as they offer control over the homogeneity and purity of the precipitated products, which are important design features in the core structure.^[3] Once the cores are prepared, they are separated by centrifugation and then washed to remove any salt byproducts from the solution precipitation process. Finally, the washed cores are passed through a Millipore filter to selectively isolate core particles of a certain size.^[1]

Layer 2: Carbohydrate

After synthesizing and purifying the core, the carbohydrate layer is added to its surface. Common coating materials are typically polyhydroxy oligomers such as cellobiose, citrate, lactose, and sucrose.^[1] This layer seems to be important for the properties of aquasomes, as it influences several drug characteristics including adsorption, molecular stability, and conformation, and acts as a dehydroprotectant.^[3] The addition of the carbohydrate layer to the surface of the nanocrystalline core is commonly carried out by passive adsorption through incubation and sonication. Similar to the processing of the core, the carbohydrate layer is subjected to centrifugation, washing, and further sonification followed by heated air drying.^[3]

Layer 3: Bioactive Molecule

Finally, the bioactive molecule of interest is loaded into the carbohydrate layer. This process typically occurs through either lyophilization or passive adsorption, and the fully functionalized aquasome is then characterized.^[3]

Characterization/Mechanism of Action

Aquasomes can be characterized by a variety of techniques that analyze the properties of their three functional units: the ceramic core, carbohydrate coating, and bioactive drug. Characterization of aquasomes after synthesis is done to gain a better understanding of each of the facets that provide or contribute to their functionality. Analyzing characteristics of aquasomes such as size distribution, the carbohydrate coat-to-core ratio, and electrical potential between the nanoparticles can be important in understanding an aquasome’s function.

Solution precipitation as a core synthesis technique produces homogenous-sized nanoparticles, which can be advantageous in controlling specific physical properties such as surface tension and packing density of the atoms in a crystalline lattice structure.^[3] The most common methods of characterizing nanoparticle size distribution and morphology of the core in aquasomes include scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In a study by Kommimeni et al. in 2012, researchers employed TEM to verify that the ceramic particles were spherical and also in the acceptable nano-range for aquasomes.^[7] The carbohydrate coating size can also be characterized using SEM and TEM, but a Fourier Transform Infrared Spectroscopy (FTIR) is commonly utilized to check for the presence of the coat. In a study by Kommimeni et al. in 2020, FTIR was used to confirm the presence of the coating by analyzing the IR spectra bands that correspond to the functional groups of either the core or the sugar coat.^[7]

The bioactive drug loaded onto the aquasome can be characterized in a variety of ways depending on the molecular classification of the drug. In Kossovsky et al. in 1996, which studied the effect of insulin as the bioactive drug of interest, immunogold labeling was employed. Through this technique, the different binding efficiencies of carbohydrate coatings for insulin were able to be observed.^[1]

To characterize the loading of the bioactive drug, it is common to employ mathematical equations that use theoretical estimations for drug concentrations entrapped in the aquasome. One such parameter is entrapment efficiency, which is the amount of drug added that was adsorbed onto aquasomes. Another parameter of interest is drug loading, which is the percentage of the aquasome’s weight that is composed of the drug. In a study done by Kutlehria et al. in 2018, the following equations were used to quantify the entrapment efficiency and drug loading::^[8]

Finally, measuring the mechanism of action of the formulated aquasomes is important for understanding their potential therapeutic effect. Several studies have carried out in vitro drug release studies that place the aquasomes in a dissolution media and measure the subsequent release of drug into the solution. By obtaining these values, a release profile of the aquasome can be postulated and further optimizations can be determined for aquasomes to meet specific therapeutic indices or pharmacological outcomes.^[10]

Applications

Until recently, clinical uses for aquasomes were primarily for targeted drug delivery of general treatment drugs. Additional applications have been since explored, including delivery of antigen, insulin, hemoglobin, and vaccines.

Drug Delivery

In a 2022 study by Chaudhary et al., aquasomes were explored for delivery of the drug dithranol, which is a treatment for psoriasis. A limitation of the practical application of free dithranol in the treatment of psoriasis is its degradation when encountering oxygen, light, alkaline pH, and heavy metallic elements, leading to the exploration of aquasomes as a delivery system for dithranol to overcome these limitations.^[11] Aquasomes displayed a 72% drug entrapment efficiency for dithranol, and drug release studies showed 55% release within 12 hours in vitro and good deposition of the drug ex vivo, indicating a strong controlled release profile for dithranol-loaded aquasomes.^[12] This study indicates support for aquasomes as a drug delivery system due to their ability to stabilize easily degradable drugs such as dithranol, while also providing controlled drug release profiles.

In a 2019 study by Kutlehria et al., aquasomes were explored for the oral delivery of the drug bromelain, which inhibits platelet aggregation and modulates anti-inflammatory cytokines, and has shown anti-tumor activity.^[13] A challenge in the administration of bromelain has been its limited ability to reach the site of therapeutic action before it degrades. The water-absorbent nature of aquasomes allows for the aqueous transfer of bromelain. The Kutlehria et al. study demonstrated desirable drug-carrying properties for bromelain-loaded aquasomes, such as a drug entrapment efficiency of 72% to 79% and sustained release in vitro, showing promise as an oral delivery mechanism to increase the bioavailability of bromelain.^[14] Such applications may be useful for the transport and targeting of poorly soluble drugs, enabled by the structure and polysaccharide coating of aquasomes.

Dual drug delivery is another application of aquasomes enabled by their structure. Dual drug delivery systems can deliver two drugs simultaneously, and aim to enhance the therapeutic efficiency and reduce the side effects of the drugs delivered. Such systems can be useful in treating patients suffering from multiple diseases. Challenges in dual drug delivery include independently controlling release rates of each of the drugs loaded in the system. In a 2019 study by Damera et al., aquasomes were used to deliver bovine serum albumin (BSA) in combination with one of three therapeutic drugs (C153, WAR, and IBU), allowing release of a bioactive molecule and a hydrophobic drug simultaneously.^[15] Damera et al. suggested that dual drug delivery was enabled by the bioactive molecule layer of the aquasome being BSA. This BSA layer interacted with the hydrophobic therapeutic drugs, and the strength of the binding interactions was shown to affect the release behaviors of the drugs. Dual drug delivery with aquasomes thus shows promise for treatment of patients with coexisting diseases alongside hypoalbuminemia, as the albumin from BSA can treat the hypoalbuminemia while the additional drug treats the disease.^[15]

Vaccine Delivery

Aquasomes have also shown promise as vaccine carriers. In a 2022 study by Jitendra et al., aquasomes prepared with hydroxyapatite and merozoite surface protein-119 (MSP-119) were shown to possess immunoadjuvant properties.^[16] Specifically, the small size and relatively large surface area of the nanoparticles contributed to their strong adsorption efficiency of immunogens, and showed slower antigen release and biodegradability in vitro compared to traditional adjuvants.^[17] These properties show promise for increasing the time that immunogens are present in vivo. This could increase the exposure of immunogens to antigen-presenting cells and lymphocytes and thus the immunological function of aquasome-based vaccines. Aquasomes prepared in such a manner were shown to induce significant release of antibodies of the immunoglobulin G class in addition to secretion of pro-inflammatory cytokines when compared to MSP-119 alone or MSP-119 with alum, a commonly used immunoadjuvant.^[18] These results indicate that aquasomes show promise as vaccine carriers due to their adjuvanticity and their ability to deliver vaccines to tissues.

Hemoglobin Delivery

Aquasomes have been explored as carriers for hemoglobin throughout the body. In a 2002 study by Khopade, Khopade, and Jain, aquasomes were used to act as red blood cell substitutes with hemoglobin attached to the oligomer surface. Aquasomes in this application demonstrated minimal toxicity while obtaining a hemoglobin content of 80%, supplying blood and oxygen in a manner similar to regular red blood cells.^[19] Hemoglobin aquasomes with spherical hydroxyapatite cores have been shown to retain oxygen-affinity and cooperativity for 30 days in rats in vivo, causing no red blood cell hemolysis or blood coagulation, demonstrating potential capability as effective oxygen transporters. Additionally, aquasomes protected hemoglobin from degradation while maintaining hemoglobin function. Future exploration of aquasomes as hemoglobin carriers may explore controlled release of the aquasomes themselves to mimic typical oxygen release properties to aid in biomedical applications that require specific targeting and delivery of hemoglobin.^[19]

Insulin Delivery

Aquasomes with calcium phosphate ceramic cores may be useful for the pharmaceutical administration of substrates such as insulin where drug action is conformationally specific. In a 2000 study by Cherian et al., disaccharides such as trehalose were used to coat the core before insulin was loaded onto the coated cores via adsorption.^[20] Albino rats were used as test subjects to test these aquasome insulin formulations, and the efficiency of different carbohydrate coat molecules on the aquasome was explored. Pyridoxal-5-phosphate-coated particles were shown to lower blood glucose levels more efficiently when compared to trehalose- or cellobiose-coated particles, which may be due to their differences in structural stability.^[20] The use of these nanoparticles for the delivery of insulin in vivo in rabbits demonstrated that insulin-bearing aquasomes showed slower release and prolonged activity compared to standard insulin solution.^[20] Similar to their role in carrying hemoglobin, the carbohydrate layer of aquasomes may be responsible for the ability to protect insulin from degradation when injected subcutaneously as in the albino rats tested. Aquasomes were also shown to release insulin in controlled manners, mimicking the typical release of insulin from the pancreas.^[20] This application shows the promise of aquasomes in aiding and improving the efficacy of insulin therapy, which may be used for diabetes treatment upon further investigation of aquasomes’ in vivo behavior.

Limitations

A potential challenge of aquasome-based drug delivery could be toxicity due to burst release of drugs if poorly absorbed on the carbohydrate coat.^[3] Aquasomes can also be expensive to formulate, particularly due to their step-by-step synthesis. Careful attention is needed during aquasome production to tune the thickness of each layer, and leaching and aggregation may occur during prolonged storage of aquasomes. A physiological challenge aquasomes present is that upon their entry into the bloodstream, they may be taken up nonspecifically, leading to opsonization and phagocytic clearance by the immune system. To prevent this, aquasome surfaces can be coated with polyethylene glycol (PEG) to block opsonin binding through steric hindrance; however, the effect of PEGylation on aquasome drug release has not been sufficiently explored to enable clinical applications.^[3] Polymer degradation in different physiological environments can change the stability and drug loading of aquasomes over time, as their surface properties directly impact drug release. Aquasomes may also be challenging to scale up and prepare as it is difficult to ensure consistent formulation quality. More research is needed to demonstrate both the efficiency and safety of aquasomes in clinical use.^[3]

Prospect

Further advances in aquasome research require additional investigation of their in vivo drug release and targeting. Applications such as delivery of dithranol for the treatment of psoriasis and oral delivery of bromelain for the treatment of inflammatory diseases such as cancer show promising results in vitro and ex vivo. However, such applications have been unexplored in vivo, limiting their clinical use. Applications using aquasomes as carriers of hemoglobin, vaccines, and insulin have been tested in vivo in small animal models such as rats, mice, and rabbits, but current literature lacks in vivo testing in more advanced animal models, preventing their use as treatments for human conditions. Aquasomes are promising drug delivery mechanisms due to their ability to stabilize and transport a variety of substrates while allowing for controlled drug release. Prior to expanding the clinical applications of aquasomes, the gap existing in current literature will need to be filled by further investigating immune clearance of aquasomes, exploring additional surface modifications such as PEGylation, and expanding in vivo drug testing.^[3]

References