Introduction

Astaxanthin (AXT) (3,3′-dihydroxy-β-carotene-4,4′-dione) is a xanthophyll carotenoid with a molecular mass of 596.85 Da. It could be chemically synthesized or found naturally in plants and some species of microorganisms like bacteria, yeast, and algae. It is used as feed in aquacultures to impart the characteristic red–orange color of aquatic animals such as crabs, shrimp, salmon, and trout [1]. The US Food and Drug Administration (FDA) has approved using AXT as a food color [2]. Due to the unknown effects of synthetic AXT on human health, in the long run, natural AXT is preferred in the food, pharmaceutical, and cosmetic industries. The green alga Haematococcus pluvialis is considered the primary source of AXT for human and animal consumption [3, 4]. The use of AXT as a nutritional supplement or medicine is growing rapidly. The global market of AXT is expected to reach 398.8 million $ by 2027, as stated by Grand review research [5]. AXT has several beneficial therapeutic properties due to its high antioxidant properties and is considered a strong candidate for treating numerous disorders. AXT intake can treat or lower the risk of several illnesses, including anti-aging and anticancer, besides providing neuroprotective benefits. However, AXT’s poor bioavailability, low stability, and solubility severely limit its use as a medicinal agent in clinical practice [6]. This review provides our current understanding of AXT sources, characterization, biological activity, therapeutic use, and how bioavailability problems can be solved using different nanoparticle technologies for future therapeutic applications.

Sources of AXT

There are multiple different sources from which AXT can be produced, including microbial, plant, and animal sources (Table 1). The green microalga Haematococcus pluvialis is one of the best microbial sources of natural AXT, accumulating to 38% at the dry weight foundation [7, 8]. The first strain applied in the industrial production of AXT is Xanthophyllomyces dendrorhous. It is one of the significant AXT-producing yeast; it contains roughly 0.2–0.5 mg/g dry cell weight (DCW) carotenoids, (40–95) % of which are AXT [9].

Table 1 Sources of AXT
Full size table

AXT is found in many animals, including birds like flamingos and quails and marine creatures like salmon, shrimp, lobster, crab, and krill [10]. AXT accounts for 74% to 98% of all pigments in the crab shell. Krill oil contains significant AXT (0.1–15 mg/ml) depending on the processing technique [11]. The animals, conversely, do not produce AXT; they obtain it from natural AXT producers found in their diets. However, the animals do not generate AXT; they obtain it from the natural AXT producers found in their diets [12].

Many flowering plants in the genus Adonis also contain AXT, and their bright colors help attract pollinators. Among plants, Adonis aestivalis is the largest source of AXT; about 1% of their dry matter comprises AXT [13, 14].

Synthetic AXT differs in its makeup from natural AXT. In synthetic AXT, there are various stereoisomers, some of which do not occur naturally, have poor bioavailability, and are less technologically stable. EC Regulation No. 1925/2006 on vitamins, minerals, and other food additives prohibits synthetic AXT in food. It also is not generally considered safe (GRAS) status in the US. However, it appears that synthesized AXT might be dangerous for people or animals. It is frequently used in the feed industry, particularly aquaculture, as a pink and red color [13].

Applying genetic engineering and synthetic biology strategies makes obtaining free AXT with high purity in non-native microorganisms relatively easy and increases the production of AXT from native organisms [12]. For instance, random mutation and overexpression of AXT biosynthetic genes of Paracoccus sp have helped raise the productivity o 480 mg/L in fed-batch fermentation [15]. Also, the optimized strain of Yarrowia lipolytica generated 0.3 g/L of AXT in fed-batch cultivation with cellular content of 6 mg/g dry cell weight (DCW) [16].

AXT production challenges

AXT production by H. pluvialis, microalgae, faces several economic and laboratory challenges [17].

The bioprocess for producing AXT is complex and challenging to scale up due to the need for two-stage cultivation with a high-intensity light. Additionally, microalgae have a slow growth rate, making the production phase relatively long and more susceptible to contamination. Non-natural producers also face challenges due to the inefficiency of heterologous enzymes and pathway complexity, which can lead to the accumulation of structurally similar intermediate carotenoids [12].

Genetic engineering of carotenoid metabolic pathways has been used to increase yields in several species, but this approach presents specific challenges, including the need for detailed knowledge of carotenoid biosynthesis pathways, availability of nuclear and chloroplast genomes, and knowledge of cellular enzyme localization [18].

Another significant challenge in microalgae production is the high cost of other food commodities. The cost of producing microalgal biomass depends on the reactor’s location and the biomass’s overall quality. In Europe, the cost of producing microalgal biomass is around 10–50 €·kg−1, which is much higher than that of common food ingredients like soya, wheat, and rice [19].

Thus, addressing these challenges is crucial for the cost-effective and efficient production of AXT and other microalgae-based products.

Characterization of AXT

Primary structure and isomers of AXT

Metabolically active AXT (3,3′-dihydroxy-β, β-carotene-4,40-dione) is a xanthophyll carotenoid; with a molecular formula C40H52O4 and a molar mass of 596.84 g/mol [1]. Like most known carotenoids, AXT comprises a C40 skeleton of linked isoprene units. The system consists of 11 conjugated double bonds, determining the characteristics red color of AXT and is responsible for its antioxidative activity. The structure comprises two β-ionone–terminal rings joined by a linear polyene chain. Both terminal rings contain a hydroxyl group (OH) localized at 3 and 3′ positions of the two asymmetric carbons and two keto groups (=O) located at carbons C4 (Fig. 1). The structure’s polar and non-polar groups enabled AXT to express its antioxidative activity in the lipid system. The hydrophobic polyene chain can fit inside the membrane’s lipid bilayer, while its polar groups are oriented near the membrane surface, constituting a unique structure among known carotenoids. Based on the configuration of the hydroxyl group of the asymmetric carbon C3, three different stereoisomers are formed: (3S,3′S), (3R,3′R), and (3R,3′S) (Fig. 1). The abovementioned isomers differ in bioavailability, physicochemical and biological properties, and their occurrence ratio in nature depends on the source where they are obtained. In addition, the hydroxyl group can react with fatty acids, such as palmitic, oleic, stearic, or linoleic acid, to form mono- or diesters, accordingly, which increases its solubility in the cell and makes it more stable to oxidation [5, 20]. AXT also exists as trans and cis (E and Z) geometrical isomers, depending on the configuration of the double bonds in the polyene chain. All-trans AXT is the dominant isomer, although at least two cis-isomers (9-cis and 13-cis) also occur in nature, depending on the host species and body part. In addition, they are found in synthetic preparations [21] (Fig. 2).

Fig. 1
figure 1

Chemical structure of astaxanthin and its three configurational isomers. AXT (PubChem CID: 5281224). Meso-AXT (3S,3'R) AXT or 3R,3'S-AXT (PubChem CID 12358422). 3R,3'R-AXT or (3R,3'R)-AXT (PubChem CID: 12358421). (3S,3'S)-AXT (PubChem CID: 5281224). AXT: Astaxanthin

Full size image
Fig. 2
figure 2

AXT trans and cis structure. All-trans (PubChem CID: 118989688), 9-cis (PubChem CID: 91827091), and 13-cis-AXT (PubChem CID: 91827159). AXT: Astaxanthin

Full size image

Although AXT possesses a high pharmacological potential, it has few applications in the clinical setting due to poor drug stability [22]. The molecular structure of carotenoids contains C=C double bonds, which render the molecule vulnerable to air oxidation and photo-oxidation. These elements collectively can transform them into a different stereoisomer (with trans–cis isomerization) [1]. Furthermore, they increase the molecule’s sensitivity to thermal degradation and worsen its water solubility [5] Therefore, creative approaches are needed to overcome the limits of this intriguing natural active compound, thus allowing its exploitation in the therapeutic field.

The safety profile of AXT

According to various studies and experiments, AXT has been demonstrated to have a favorable safety profile. Its global market value has risen significantly over the years. Safety testing of an AXT-enriched extract showed no clinically significant changes in blood pressure, hematology, or blood chemistry parameters in healthy adults at 6 or 20 mg daily for eight weeks and four weeks, respectively. Furthermore, a dose of 8 mg daily for three months did not cause gastrointestinal tract distress or other side effects [23].

Studies on oral administration of natural AXT have not reported any adverse effects, even at high doses of up to 1240 mg/kg/day in rats for 90 days. Clinical safety studies have shown that AXT is safe compared to the placebo, even at low doses of up to 12 mg and high doses of up to 100 mg per day [24].

Numerous experiments, including acute toxicity, mutagenesis, transgenicity, fetal toxicity, and reproductive toxicity, have confirmed the safety of AXT [25].

Furthermore, there has been an exponential increase in the number of studies examining the health, beauty, and safety of AXT, and a review of 87 AXT clinical trials involving humans found no serious adverse effects [26].

Overall, the safety profile of AXT appears to be excellent based on the available evidence.

Pharmacokinetics and bioavailability of AXT

Of note, the bioavailability of AXT is affected mainly by its chemical structure [27, 28] (Fig. 3). The absorption of AXT in organisms takes place through its passive diffusion alongside fatty acids into the intestinal epithelium. After ingestion, AXT is combined with bile acid in the intestine, forming micelles. Intestinal mucosal cells absorb AXT from micelles, which are then incorporated into chylomicrons and carried to the liver. Lipoproteins then transport AXT to various tissues. AXT protects the mitochondrial redox state and functions by inserting itself into lipid bilayers and maintaining membrane integrity. It is believed that carotenoids are finally transported to the tissues by circulating, where they are incorporated into very low-density lipoproteins (VLDL), low-density lipoproteins (LDL), or high-density lipoproteins (HDL) [28, 29].

Fig. 3
figure 3

Factors affecting AXT bioavailability. AXT: Astaxanthin

Full size image

In their study, Odeberg et al. administered 40 mg of AXT to healthy male volunteers to determine AXT’s pharmacokinetic parameters. Depending on the source, this preparation was either a commercial dietary supplement or a lipid-based formula. The bioavailability of lipid-based formulations was significantly higher than reference formulations (p < 0.05) [30]. More effective methods, such as nanoformulations and targeted therapies, are needed to improve AXT’s bioavailability. The pharmacokinetic parameters of AXT-containing formulations were studied in animal models, but few human studies have been reported. In a study by Singh et al., they administered macro, nano, and oil solutions to rats. They investigated the correlation between the formulation’s size and AXT’s tissue distribution. Their study reported that nano-sized emulsions improved the distribution of AXT in tissues and enhanced its bioavailability [31].

According to a study of spleen contents, spleens had the highest AXT concentrations (1673.28 ± 99.86 μg/g wet weight). These results may illuminate why AXT has a prominent effect on the immune system. The kidney is the primary excretory organ for AXT, following the spleen. In descending order, the spleen, kidney, heart, lung, and liver had the highest total concentrations [32, 33].

It is worth noting that there is a vast difference in carotenoids’ bioavailability, which varies from 10 to 50% in humans. AXT can be absorbed through intestinal epithelial cells and released into lymph as chylomicrons due to its low solubility and dispersibility in gastrointestinal fluids [29]. Evidence suggests that various parameters influence carotenoids’ absorption. These parameters can be categorized into chemical properties and diet- and non-diet-related parameters (such as age, gender, fat type, and fat content). Carotenoid bioavailability is determined by three types of chemical complexity: chemical structure, optical stereochemistry, and geochemistry of the isomeric form. In addition, food consumption (especially fatty food) increases AXT absorption, while smoking significantly reduces the half-life of AXT’s elimination [27, 33].

AXT’s poor solubility and stability limit its use in functional food, health products, and medicines [34]. One approach for improving AXT’s in vitro release is to apply novel formulations with functional ingredients. Although these formulations included molecular complexes, microemulsions, liposomes, solid nanoparticles, biopolymer particles, and microgels, pharmacokinetic studies did not show enough AXT to reach a high blood concentration. Hence, Nanopreparations can be an appropriate drug delivery form, increasing insoluble drugs’ bioaccessibility [35].

Improving AXT bioavailability

AXT is a carotenoid with potent antioxidant and anti-inflammatory activities that result from its highly unsaturated molecular structures. It could be a good candidate for treating many diseases, possesses many valuable therapeutic functions such as anti-aging, anticancer, and has a neuroprotective effect [7]. However, AXT’s limited stability and solubility lead to poor absorption, significantly limiting its therapeutic use in clinical practice.

Notably, AXT delivery systems, such as lipid-based nanocarriers, macro capsulation, nanoparticles, and cyclodextrin inclusion systems, improve AXT bioavailability and antioxidant effect [36, 37] (Fig. 4).

Fig. 4
figure 4

Role of AXT delivery systems in improving the bioavailability and antioxidant effect. AXT: Astaxanthin

Full size image

Recently, nanomedicine, particularly solid lipid nanoparticles (SLNs), has been attracting considerable attention due to its potential advantages for enhancing pharmacokinetics and the pharmacodynamic activity of naturally derived constituents. SLNs are colloidal carriers made from solid biodegradable lipids stabilized by surfactants, with a mean diameter ranging from 50 to 1000 nm [38] presenting several advantages over other colloidal carriers (liposomes and polymeric nanoparticles) such as higher drugs, absence of toxicity, easy large-scale production, and the possibility of lyophilization [39].

These nanoparticles’ composition and characteristics make them ideal for carrying/delivering sensitive bioactive compounds, protecting them against chemical degradation and facilitating their application in various administration routes [40].

The encapsulation of drugs into these nanocarriers increases their solubility, stability, cell uptake, specificity, tolerability, and therapeutic index [41, 42].

Unfortunately, the use of SLNs through systemic administration is limited by the opsonization process; an important mechanism used by the immune system to fight pathogens, which is responsible for their short half-life (3–5 min) after intravenous administration [43, 44].

One of the most used strategies to prolong these carriers’ systemic residence is modifying the nanoparticle surface to achieve ‘stealth’ systems to overcome the defense line represented by macrophages through a biomimetic mechanism. A strategy used to realize ‘stealth’ systems; is to coat the nanoparticle surface with surfactants, such as poloxamers. This hydrophilic coating modifies the SLN biodistribution, improving blood circulation times and deposition in non-RES (reticuloendothelial system) organs [45, 46].

Among the different surfactants used to modify particle distribution, polysorbate 80 (p80) is one of the most effective surfactants for enhancing concentration in the central nervous system (CNS).

The remarkable scavenging activity of AXT is due to its chemical structure which traps radicals in the polyene chain (Fig. 1). It is a highly lipophilic molecule. However, the presence of two terminal hydrophilic groups determines a slight solubility in water.

It’s dual nature and small size allow it to permeate cell membranes readily. It is a carotenoid that carries out its antioxidant and anti-inflammatory actions at the level of the eyes, brain, and CNS, as it can cross the blood–brain and blood-retinal barriers [47].

Therapeutic uses of AXT

AXT’s role as an antioxidant and anti-inflammatory

The chemical structure of AXT makes it a potent antioxidant with the capacity to scavenge free radicals, protect the mitochondria, reduce inflammation, and prevent glycation [48]. It can quench single oxygen and significantly protect against oxidative stress and age-related disorders. Also, AXT cooperates to protect against lipid peroxidation [49].

The AXT reduces the O2− level in the Lipopolysaccharide (LPS)-stimulated pro-monocytic, human myeloid leukemia cell line (U937). Treatment with AXT results in the antioxidant network’s functional recovery by reducing O2− levels. Therefore, nuclear factor erythroid 2–related factor 2 (Nrf2) does not translocate into the nucleus, and there is not any evidence of increased expression and catalytic activity of heme oxygenase 1 (HO-1) [50].

Consuming AXT in the diet improves immune function, reduces inflammation, and is a biomarker for deoxyribonucleic acid (DNA) oxidative damage in young, healthy females. However, antioxidants generally show more significant physiologic modulation in cases with excessive amounts of oxidative stress and immuno-compromised individuals [32].

In dry eye disease (DED) models, AXT may suppress inflammation by downregulating the production of high-mobility group box 1 (HMGB1) and inflammatory cytokines, tumor necrosis factor-alpha (TNF-α), interleukin-1 (IL-1). So, AXT may be a viable natural derivative for ocular surface protection in DED [51].

AXT reduced inflammation in endotoxin-induced uveitis (EIU) in a dose-dependent manner. For instance, 100 mg/kg of AXT had an anti-inflammatory impact on the ocular comparable to 10 mg/kg of prednisolone. AXT may have an anti-inflammatory impact on the ocular by directly inhibiting the activity of the nitric oxide synthase (NOS) enzyme, which could also reduce the generation of nitric oxide (NO), prostaglandin E2 (PGE2), and TNF-α [52].

According to preclinical and clinical research, AXT’s anti-inflammatory and antioxidant properties slow the development of cardiovascular diseases as AXT reduces the buildup of cholesterol in foam cells and the development of atherosclerotic plaques. These qualities of AXT make it a prospective choice for the preventive and/or adjuvant treatment of cardiovascular diseases [53].

AXT as an anticancer agent

According to in vitro research, AXT inhibits cell proliferation, arrests the cell cycle, and induces apoptosis to prevent cancer through various pathways. Additionally, AXT improved the sensitivity of tumor cells to treatment and reduced its adverse effects [54]. AXT’s antioxidative property prevents stress from impairing natural kill (NK) cell antitumor activity, which may help explain why it inhibits cancer spreading [55].

Different stereoisomeric AXT significantly inhibited human colon cancer (HCT116 and HT29 cells). As a result of the stereoisomeric AXT’s modulation of oncogenic signalling proteins, the suppression of cancer cells was linked to substantial cell cycle arrest and apoptosis. The findings contradicted the initial theory that the anticancer activity of AXT was linked to the structures mentioned earlier and suggested that hydroxyl at C3 and C3′ of the terminal ring structure may not be the primary factor that affects the anticancer activity of AXT. The structure of long conjugated chains, unsaturated ketone (C=O), and α-hydroxy ketones probably contribute to the anticancer effects of AXT stereoisomers [56].

Treatment with AXT is an efficient method for preventing breast cancer cells from proliferating and migrating. It has been consistently established that AXT can reduce several different cancers. These discoveries could lead to various medical studies that could influence how cancer is treated today [57].

AXT was discovered to increase radiosensitivity and trigger apoptosis in esophageal squamous epithelial cell carcinoma cells (LS-180 cells) by decreasing B-cell lymphoma 2 (Bcl-2), CyclinB1, cell division cycle 2 (Cdc2) and favoring Bcl-2 associated X-protein (Bax) expression. By upregulating the expression of the genes Bax and Caspase-3 and downregulating the expression of the gene Bcl-2, it has been discovered that AXT induces apoptosis in LS-180 cells and inhibits the growth and multiplication of cancer cells [58].

Brain protection and neuroprotective role

AXT is gaining considerable attention as a pharmacological agent for treating various neurological diseases such as Alzheimer’s disease (AD), Parkinson’s disease (PD), neuropathic pain (NP), depression, autism, aging, and injuries to the brain and spinal cord [59].

Experimental data from animal studies have demonstrated that AXT has neuroprotective effects. In Vascular dementia, AXT improves memory and protects against stroke and hypertension. Also, AXT provides brain protection because it can cross the blood–brain barrier. Earlier findings also reported reduced blood pressure in individuals with a daily dose of 6–8 mg [49]. Fassett et al. reported that a dose of 50 mg/kg of AXT oil lowered systolic and diastolic blood pressure in rats with metabolic syndrome suffering from spontaneous hypertension, suggesting that AXT improves cognitive functions and protects against aging in rats [49, 60].

Alzheimer’s is regarded as one of the most common chronic and severe neurodegenerative disorders. It is reported that the antioxidant AXT can fight Alzheimer’s disease. Previous results reported that supplementing Alzheimer’s disease Wistar rats’ model with AXT powder isolated from shrimp shells has greatly ameliorated cognitive functions. Also, it is reported that the synthetic formula of AXT’ docosahexaenoic-acid-acylated AXT diesters (AXT-DHA) ameliorated cognitive disorders in mice with amyloid protein precursor/presenilin-1 (APP/PSEN1) by reducing chronic neuroinflammation and oxidative stress [76, 77]. For example, zinc oxide nanoparticles have been synthesized from naturally available eggplant leaf extract or Solanum melongena leaf extract [78]. Previous studies have indicated that an extracellular mechanism for synthesizing silver nanoparticles using bacteria is widely employed. This mechanism involves the reduction of silver ions (Ag+) by several bacterial species, such as Streptomyces sp. LK3 and Bacillus licheniformis via nicotinamide adenine dinucleotide hydrogen (NADH)-dependent nitrate reductase-mediated reduction [79]. One study synthesized silver nanoparticles using aqueous extracts of fresh leaves from Impatiens balsamina and Lantana camara plants. These nanoparticles were comparable to ciprofloxacin in inhibiting bacterial growth [80].

Additionally, research has shown that the aqueous extract of Gundlia tournefortii leaves is an effective bioreductant in synthesizing gold nanoparticles to treat bacterial, fungal, and skin diseases. This further demonstrates the potential of using biological systems for the green synthesis of nanoparticles and their applications in medicine [81].

The nanoparticles produced are synthesized using various qualitative and quantitative techniques. Qualitative techniques comprise Fourier Transform Infrared Spectroscopy (FT-IR), UV–Vis spectrophotometry, Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), and Atomic Force Microscopy (AFM). On the other hand, the quantitative techniques consist of Transmission Electron Microscopy (TEM), Annular Dark-Field Imaging (ADF), and Intracranial Pressure (ICP) [82].

AXT in the nanotechnology era

Innovative AXT-loaded stealth lipid nanoparticles (AXT-SSLNs) were developed to overcome AXT's high stability in clinical practice and to improve AXT bioavailability in the brain. Solvent-diffusion technique was used to prepare AXT-SSLNs suitable for parenteral administration [83].

The stability of AXT was evaluated in a range of carriers and storage settings. It has been studied utilizing microencapsulation with chitosan, polymeric nanospheres, emulsions, and beta-cyclodextrin [5].

Enhancement of stability and solubility of AXT

To increase AXT's bioavailability, Zhu et al. applied polyethene glycol-grafted chitosan (PEG-g-CS) to AXT using the solvent evaporation method. In an in vitro release assay, AXT that was well-encapsulated by PEG-g-CS (Table 2) was released immediately within 15 min. This formulation increases the oral bioavailability of AXT in vivo and in vitro [84].

Table 2 Formulations that increase the stability and solubility of AXT
Full size table

AXT nanodispersions were created using the solvent-diffusion method with the addition of ascorbic acid and α-tocopherol, which can both retard the degradation of AXT. The addition of ascorbic acid (ascorbic acid/AXT w/w) and α-tocopherol (α-tocopherol/AXT w/w) at proportions of 0.4 and 0.6, respectively, would offer AXT the greatest chemical stability, which was measured by using a response surface methodology (RSM) response optimizer. The results showed the superiority of using α-tocopherol to stabilize AXT over ascorbic acid [85].

Poly-lactic-co-glycolic acid (PLGA) nanoparticles coated with chitosan oligosaccharides (COS) (Table 2) were used to encapsulate AXT by the anti-solvent precipitation method. Dynamic light scattering, transmission electron microscopy, and scanning electron microscopy were used to analyze the loaded nanoparticles' characteristics. The results showed the loading capacity (> 15%) and encapsulation efficiency (> 85%) of the comparatively high nanoparticles. The nanoparticles displayed high cytocompatibility, good aqueous solution dispersibility, and stability [22].

Zanoni et al. create a technique to stabilize AXT and enhance its nutritive and functional qualities. They used the emulsification-solvent evaporation approach, where whey protein concentrate (WPC) was a stabilizer, and the encapsulation efficiency was 96%. Whey protein-based nanoparticles that contain AXT had a maximum bio-accessibility of 76%, with 75% of the AXT being converted to the more bioavailable free form [86].

By Whitaker et al., oil-in-water nanoemulsions of AXT were prepared by specific energy methods. The effect of nano-emulsification of AXT on its bioabsorption, degradation, and antioxidant activity was evaluated in different models. It was found that nano-emulsified AXT increased carotenoid plasma concentration up to 7.5-fold compared with the control solution [87].

High-pressure homogenization was used to create oil-in-water nanoemulsions of AXT. The AXT-containing dispersed particles had an average diameter of 160 and 190 nm. The nanoemulsion did not significantly change following one month of storage under light and temperature conditions, showing that the nanoemulsion had a stable colloid with a zeta potential of less than − 41 mV [88].

The freeze-dried AXT-encapsulated with poly (ethylene oxide)-4-methoxycinnamoylphthaloyl-chitosan (PCPLC) nanospheres (Table 2) by using the solvent displacement method showed better dispersibility in water, yielding stable aqueous suspensions of 300–320 nm nanoparticles. Nuclear magnetic resonance (NMR) analysis indicated that after a two-hour heating at 70 ◦C in an aqueous environment, PCPLC nano encapsulated AXT showed minimal heat degradation of AXT [89].

Recently, a study evaluated the physicochemical stability and in vitro digestibility of AXT-loaded Pickering emulsions stabilized by two different nanoparticles, zein and Adzuki bean seed coat polyphenol (ABSCP). Using these nanoparticles was an effective delivery system for AXT [90].

Enhancement of antioxidant activity of AXT

Formulation of AXT-loaded nanostructure lipid carriers using green chemistry and sunflower oil can stabilize and preserve the AXT antioxidant capacity [91]. The AXT-loaded DNA/chitosan (ADC) nanoparticles showed a more potent cytoprotective impact against H2O2-induced oxidative cell damage and increased cell viability. ADC nanoparticles had a ROS scavenging effectiveness of up to 54.3%, two times greater than free AXT. The intestinal epithelial cells could endocytose the encapsulated AXT and absorb it rapidly [92]. The Solid lipid-polymer hybrid nanoparticles (SLPN) were used to encapsulate AXT. After encapsulation in SLPN, the antioxidant activity of AXT was significantly increased in aqueous conditions, and a sustained release was achieved in simulated gastrointestinal (GI) fluids [93].

AXT-nanoparticle enhancing anticancer activity

Blocking inflammatory feed-forward loops

Breast cancer development and metastasis depend on inflammatory feed-forward loops, including the “inflammatory cell recruitment processes.” A doxorubicin micellar low-molecular-weight heparin-AXT nanoparticle (LMWH-AXT/DOX, LA/DOX NP) was created. The nuclear factor-κB (NF-κB) and signal transducer and activator of transcription 3 (STAT3) signalling pathways may be inhibited by the hydrophobic AXT. Therefore LA/DOX NPs can stop these loops by preventing the development of neutrophil extracellular traps (NETs), which prevent breast cancer and limit liver and lung metastasis and reduce the inflammatory and immunosuppressive milieu in tumors (Table 3) [94].

Table 3 Formulations of AXT that manage cancer and different other diseases
Full size table

Apoptosis induction in metastatic lung melanoma

Natural edible peanut oil and D-α-tocopheryl polyethene glycol succinate (TPGS) were combined by Haung et al. to create a stable oil-in-water (O/W) nanoemulsion D-α-tocopheryl that was loaded with AXT (denoted as TAP-nanoemulsion). Otherwise, TAP-nanoemulsion effectively activated the apoptosis pathway in vivo, as the study demonstrated that oral treatment significantly affectselanoma with lung metastases due to the formulation's antioxidant efficacy [95].

Treatment of skin cancer

The anti-photodamage effect in immortalized human keratinocytes (HaCaT cells) was examined by Hu et al., who synthesized and improved PLGA nanoparticles loaded with AXT utilizing the emulsion solvent evaporation approach. In vitro research on HaCaT cells showed that the optimized nanoparticles had high cell viability and cellular uptake. The findings indicate that creating PLGA-coated AXT nanoparticles was highly practical and might be used to treat skin conditions or create cosmetic products by reducing ROS levels and restoring mitochondrial membrane potential (Table 3) [96].

Subarachnoid hemorrhage therapy treatment

A promising way of transferrin conjugated to poly (ethylene glycol) (PEG)-encapsulated AXT nanoparticles (AXT-NPs) on targeted delivery was investigated. The transferrin-containing AXT-NPs exhibited enhanced cellular uptake efficiency than AXT-NPs without transferrin conjugated in primary cortical neurons. Transferrin-containing AXT-NPs with lower AXT concentration showed powerful neuroprotective effects on OxyHb-induced neuronal damage compared to free AXT. These in vitro findings provide insights into the advancement of subarachnoid hemorrhage therapy [97].

A design of a mitochondria-targeting nanocarrier for enhancing AXT delivery

A novel triphenylphosphonium bromide (TPP)—modified whey protein isolates (WPI)—dextran (DX) conjugates were prepared and characterized for AXT delivery with mitochondria-targeting ability. The TPP–WPI–DX–ASX nanoparticles with spherical shape and nano-size had good stability in stimulated blood. In vitro experiments indicated that the TPP–WPI–DX nanocarriers could effectively realize lysosomal escape and accumulate in the cellular mitochondria. The TPP–WPI–DX–ASX could significantly protect cells against oxidative damage, maintain normal levels of the MMP, and dramatically promote the viability of RAW 264.7 cells [98].

AXT conjugated polypyrrole nanoparticles for photothermal therapy

Stabilization of AXT by its conjugation with a bovine serum albumin polymer (PPy@BSA-AXT). This novel structure produces many reactive radicals to cause cell death. An experiment using fluorescent microscopic technology showed how photodynamic therapy causes cell death and intracellular organ degradation. As a result, the manufactured PPy@BSA-AXT nanoparticles can be employed as photoacoustic imaging-based prognostic agents for photothermal or photodynamic therapy [99].

AXT-loaded polymer-lipid hybrid nanoparticles (AXT-LPN) against ototoxicity

One of platinum-based chemotherapy's most common adverse effects, particularly cisplatin therapy, is ototoxicity. There are currently no FDA-approved treatments or preventative measures for this ototoxicity. However, it is commonly accepted that excessive production of ROS in the inner ear causes ototoxicity. AXT is a promising agent for preventing and treating oxidative stress-related diseases [100].

By conjugating AXT and lipid-polymer hybrid nanoparticles (LPN), the solubility of AXT increased; therefore, AXT can pretend round window membranes (RWM) and strongly affect the cisplatin-induced formation of ROS. The results showed that AXT-LPN successfully prevented cells from entering the early stages of apoptosis and restored the decrease in mitochondrial membrane potential caused by cisplatin in vitro [100].

Protective effects of AXT liposomal on UV-induced skin damage

A practical scavenging ability against singlet oxygen generation in the water phase was demonstrated using chemiluminescence-dependent liposomes carrying AXT. UV-induced skin thickening could be avoided by applying AXT-liposome to the skin before exposure to the sun. Interestingly, preadministration of Asx-lipo also reduced collagen loss brought on by UV exposure. Additionally, topical application of AXT-liposome cationic lipid suppressed melanin synthesis in UV-exposed skin (Table 3) [101].

AXT-loaded nanoparticles to enhance the neuroprotective effect

In primary cortical neurons, AXT nanoparticles with transferrin conjugated on them (AXT-NPs) demonstrated improved cellular absorption efficiency compared to AXT-NPs without transferrin conjugated. Furthermore, transferrin-containing AXT-NPs with lower AXT concentrations exhibited potent neuroprotective effects on OxyHb-induced neuronal injury compared to free AXT. The increased neuroprotective effects and improved bioavailability made AXT-NPs attractive candidates for AXT's targeted delivery and absorption (Table 3) [102].

Chitosan-caseinate-dextran ternary complex nanoparticles for potential oral delivery of AXT within liver fibrosis prevention

Stearic acid-chitosan conjugate (SA-CS) and sodium caseinate (NaCas) created biopolymer-based nanoparticles through ionic gelation. Its nanostructure was cross-linked using oxidized dextran (Odex) via Schiff base reaction. The optimized nanoparticles had uniform size distribution and a diameter of 120 nm. High encapsulation efficiency and a good AXT encapsulation capacity with up to a 6% loading ratio were achieved. The radical scavenging ability of the encapsulated AXT was dramatically boosted, which supported the considerable improvement in aqueous dispersibility. The encapsulated AXT displayed drastically increased cellular bioactivity in comparison to the anti-fibrogenic action of free AXT in the human hepatic stellate cell line (LX-2 cells) (Table 3) [103].

Mesenchymal stem cells vs AXT encapsulated polymerically

Polymeric micelles were created using the synthetic methoxy polyethene glycol-polycaprolactone (mPEG-PCL) copolymer to encapsulate AXT. After that, human mesenchymal stem cells (MSCs) were used to study the impacts on proliferation and differentiation using AXT-loaded polymeric micelles. Results of MSC differentiation revealed that 20 ng/mL polymeric micelles encapsulating AXT increased MSCs' adipogenesis, chondrogenesis, and osteogenesis by 52%, 106%, and 182%, respectively [104].

Role of AXT to reverse idiopathic pulmonary fibrosis

AXT and trametinib-loaded surface-engineered nanoparticles (PER NPs) are attached to monocyte-derived multipotent cells (MOMC) to create programmed therapies. PER NPs retarget wounded alveolar epithelial cells type II (AEC II) after reacting to matrix metalloproteinase-2 (MMP-2) in Idiopathic pulmonary fibrosis (IPF) tissues. Then, released AXT can boost trametinib's synergistic impact to block myofibroblast activation, and MOMC can also repair damaged AEC II to encourage the regeneration of damaged lung tissue (Table 3) [105].

AXT natural antioxidant nanosystem for diabetic nephropathy therapy

A novel AXT-based natural antioxidant nanosystem called ligand-modified AXT liposomes (AXT-GLU-LIP) was created and used to treat diabetic nephropathy in rats. It enhanced renal absorption and bioavailability of AXT. Thus AXT showed better performance as an antioxidant agent. As a treatment for diabetic nephropathy, AXT-GLU-LIP may significantly enhance the renal pathological morphology to save the kidney (Table 3) [106].

AXT-alpha tocopherol nanoemulsion as an anticancer, wound healing, and antibacterial agent

The AXT-alpha tocopherol nanoemulsion (ATNE) was found to be more toxic at higher concentrations than at lower concentrations, according to cytotoxicity assays on three different cancer cells exposed to it for 24 and 48 h. A scratch assay demonstrated the nanoemulsion's ability to heal wounds faster. Significant antibacterial ability to compromise the integrity of the bacterial cell membrane was demonstrated by the minimal inhibitory concentration (MIC) and minimum bactericidal concentrations (MBC) techniques [107].

The characteristics of AXT and alpha-tocopherol for the control of wound healing in people with diabetes have garnered much interest. In comparison to STZ-induced diabetic mice, the AXT and alpha-tocopherol with κ-carrageenan nanoemulsion (AXT-TP@KCNE) demonstrated better management of hyperglycemia and more excellently corrected the problems associated with diabetes mellitus (Table 3) [108].

Pitfalls and caveats in nanoencapsulation of AXT

The encapsulation of AXT within nanoparticles is associated with several challenges that must be addressed to ensure successful delivery. One of these challenges is the low solubility and instability of AXT. This challenge can be overcome by selecting an appropriate and stable nanocarrier that possesses desirable physicochemical properties and high encapsulation efficiency. Additionally, the use of stabilization agents can help to maintain AXT stability. Another challenge is the variability in AXT bioavailability, which can be addressed by optimizing nanocarrier attributes such as size, shape, and surface charge. The consideration of physiological barriers such as the blood–brain barrier is also crucial. The limited understanding of AXT metabolism necessitates the characterization of AXT metabolites and their effects on bioavailability. Furthermore, interindividual differences in metabolism must be considered. The lack of standardized methods for evaluating AXT bioavailability is another challenge that must be addressed. This requires the development of validated methods for measuring AXT absorption, distribution, metabolism, and excretion. Standardized protocols for preclinical and clinical studies must also be established. The potential toxicity of nanocarriers is another concern that must be addressed. Biocompatible and biodegradable nanocarriers must be selected, and preclinical studies must be conducted to evaluate potential toxicity. Finally, regulatory compliance with nanomaterial guidelines and consideration of patent protection for novel encapsulation methods are essential to address regulatory requirements and intellectual property issues (Table 4) [5, 14, 20, 34,35,36,37, 109,110,111,112,113].

Table 4 AXT encapsulation challenges and potential solution
Full size table

Conclusions

Due to poor solubility and stability, AXT has limited applications in clinical settings. The double bonds make the molecule liable to oxidation and can transform into different stereoisomers (with trans–cis isomerization). In addition, AXT can be easily degraded by heat and it doesn't dissolve well in water. Therefore, innovative strategies are required to overcome this intriguing natural active compound's limitations and enable its application in the therapeutic sector. It has been found that AXT solubility and stability improved in combination with nanoparticles. Accordingly, several studies have directed their immediate attention to the therapeutic impact of the AXT nanoparticles combination. This article attempts to understand what is new about AXT sources, structures, therapeutic effects, and their applications in the nanotechnology era.