ATRA has been found to be effective in the management of multiple dermatologic diseases and has been developed into various topical formulations. However, several limitations of ATRA require further research to alleviate the adverse effects and improve its stability while maintaining or enhancing the activity towards the target. ATRA has proven to be effective and potent, but its lack of solubility in water imparts a challenge for its delivery. Furthermore, ATRA is unstable in the presence of air, light, and heat [47]. When considering the topical dosage form, retinoic acid is also limited by the skin reactions that it causes, which include erythema, irritation, and peeling [48,49]. Hence, encapsulation of ATRA in a colloidal carrier system is the best strategy to overcome these challenges. Some of the drug delivery systems used for ATRA include solid lipid nanoparticles (SLNs), nanostructured lipid carriers (NLCs), liposomes, ethosomes, and niosomes, as illustrated in Figure 7. The different types of formulations and its method of preparation are also summarised in Table 2. 4.1. Veisuclar Drug Delivery SystemsMany different drug delivery systems have been extensively explored in the past few decades to determine the best delivery systems for different drugs to target different diseases. Considerable attention has been given to vesicular drug delivery systems. A vesicular drug delivery system is a highly ordered assembly of concentric bilayers formed in the presence of water by self-assembling amphiphilic building blocks [59]. Liposomes are a type of vesicular carrier that have been well studied because of their various forms. Liposomes are spherical carriers with an internal aqueous core surrounded by drug molecules [47]. Liposomes are able to increase the efficacy and stability of drugs while reducing the toxicity and, most importantly, enabling targeted delivery [60]. A study on ATRA encapsulated in liposomes showed that the presence of cholesterol or a negative charge inducer had opposite effects on ATRA release. Cholesterol significantly lowered the percentage of ATRA released from the liposomes inducing a sustained release effect, whereas the negative charge inducer significantly increased ATRA release. Clinically, ATRA in liposomes has shown superior efficacy against non-inflammatory lesions and less irritation potential for erythema compared with free ATRA and a marketed product [61]. However, issues of low solubility, leaking, and fusing of the encapsulated drug as well as the high production cost have stimulated further research [59,62].As a result, niosomes, ethosomes, transferosomes, and many other delivery systems have been developed and applied to the delivery of ATRA. Niosomes are vesicular carriers with non-ionic surfactants; ethosomes contain a high percentage of ethanol and transfersomes, or deformable liposomes, and use a surfactant as an edge activator to destabilize the lipid bilayers [60,63,64]. Liposomes, hexosomes, glycerosomes, and ethosomes could all produce vesicles with similar zeta potentials and polydispersity indices; however, the entrapment efficiency and particle sizes were significantly better in hexosomes than in the other formulations. In in vitro and in vivo studies, the highest skin retention in the stratum corneum, epidermis, and dermis was observed with hexosomes and the least retention with liposomes. Liposomes also showed some permeation through the skin, unlike the other formulations, indicating that the other vesicular formulations were able to provide a targeted effect in the skin layers while reducing systemic side effects because there was less skin permeation. Hexosomes also showed superior anti-rosacea activity, whereas liposomes were unable to suppress inflammatory cell recruitment [51].When liposomes were compared to niosomes in penetration enhancer containing vesicles (PEVs), niosomes were found to have similar properties and performance to liposomes in terms of drug incorporation and ATRA skin delivery when the penetration enhancer Labrasol® was added. Liposome, liposome-PEV, noisome, and niosome-PEV formulations were prepared using Labrasol® as the penetration enhancer in the PEV formulation. Both liposome formulations had a smaller particle size, a more negative zeta potential, and higher entrapment efficiency compared with the niosome formulations. The niosome characteristics were significantly improved with the addition of Labrasol®. None of the formulations permeated through the skin, instead ATRA accumulated in the skin with the liposomes showing the highest skin drug deposition. Niosomes had the lowest drug deposition, but this was significantly improved by the addition of Labrasol® [52].ATRA encapsulated in deformable liposomes was also studied through a full factorial design with nine formulations using different ratios of soy phosphatidylcholine (SPC) and transcutol. Higher levels of SPC showed better encapsulation efficiency and slower drug release to a certain extent, whereas a high amount of transcutol had the opposite effect. A drug release study showed an initial burst and then reduced release of ATRA, except for formulation seven (SPC/transcutol = 25:5). Optimized formulations were prepared with ratios of SPC/transcutol of 15.5:14.5, 24:7, and 25:5. The levels of penetration of the optimized formulations were all higher than that of tretinoin cream, with the 15.5:14.5 formulation having the highest penetration as the optimized amount of transcutol acted as a solubilizer. The PEVs had low irritation compared with tretinoin cream in an in vivo skin irritation study [47]. Formulation in liposomes also improved the characteristics of ATRA, and good permeation through the skin was observed because of the nature of the liposomes. In an advanced version of the formulation, the properties were improved with better permeation as observed with hexosomes. Liposomes have the flexibility to be adjusted and modified to suit different goals in disease management. 4.2. Lipid NanoparticlesThe use of lipid nanoparticles is an advanced alternative to traditional colloidal carrier systems suitable for lipophilic drug delivery, such as emulsions. With the benefits of colloidal carriers but with fewer shortcomings, lipid nanoparticles can encapsulate both hydrophilic and lipophilic drugs equally well. Lipid nanoparticles can be categorized into four types: SLNs, NLCs, lipid drug conjugates, and polymer lipid hybrid nanoparticles [65,66]. The lipid matrix of SLNs consists of solid lipids with a melting point above 40 °C, ensuring that the matrix remains solid at normal room and body temperatures [54,67]. SLNs have the advantages of reduced toxicity, increased loading capacity, controlled release, enhanced stability, biodegradability, and ease of upscaling to an industrial scale because of the low cost and organic-solvent-free process [23,38,49]. SLNs also have a large surface area to enable better penetration through the skin and to release the active molecules in a controlled manner [55].A formulation of ATRA loaded into SLNs showed improvements in the ATRA properties and activity. The efficacy of SLNs loaded with ATRA was assessed based on the desired use. For a topical acne formulation, a rhino mouse model was used to compare a commercial formulation with SLNs loaded with ATRA. In this model, the encapsulated ATRA produced a comedolytic effect and caused epidermal proliferation similar to marketed products and the epidermal granular layer showed greater thickening compared with the commercial formulations. Furthermore, the study showed a significant reduction in skin irritation because of the gradual distribution of ATRA in the skin through the sustained delivery [23,38]. The effects of ATRA on chronic wound healing were assessed using a diabetic mouse model. Wound healing was improved via a reduction in macrophages and neutrophil infiltration. Collagen deposition was also increased, specifically type III collagen, which is associated with healing with reduced scar tissue. ATRA-SLN improved the wound healing without any detrimental effects although applied directly to the wound bed [38]. A study found that ATRA had a low loading capacity in SLNs because of a low solubilization rate in the lipid but had high encapsulation efficiency, which was attributed to the lipophilicity of ATRA. The addition of chitosan to the formulation resulted in less cytotoxicity towards keratinocytes and better antibacterial properties against C. acnes and Staphylococcus aureus, making chitosan a good candidate for ATRA delivery [53].The encapsulation efficiency of SLNs for ATRA is, however, quite low. The encapsulation efficiency could be improved by forming an ion pair between ATRA and a lipophilic amine, such as maprotiline or benethamine [38,49]. The use of the ion pair improved the encapsulation efficiency and enhanced the particle size and zeta potential of the particles [38]. To investigate further improvements in the permeation, limonene, a terpene, was added as a skin permeation enhancer, which improved the solubility of ATRA, thus also increasing the drug loading capacity. As a skin penetration enhancer, limonene improved the ATRA skin permeation by increasing the ATRA solubility within the skin and its partitioning into the skin [55]. SLNs have been shown to be beneficial for the encapsulation of ATRA, not only by improving its efficacy in managing different skin issues, but also in alleviating unwanted side effects. Some problems that occurred because of the chemical properties of ATRA were resolved by adding an ingredient that could act synergistically with ATRA.NLCs are composed of a mixture of both a solid and a liquid lipid matrix, with a lower melting point compared with SLNs, but still maintain a solid state at room and body temperature. The development of NLCs was aimed to overcome the limitations observed with the lipid matrix of SLNs. With the addition of a liquid lipid that has varied density in the fatty acid chain, a less ordered structure in the lipid matrix is produced, enabling a much greater drug loading capacity. This less ordered structure in the solid lipid matrix also means the carrier system is more thermodynamically stable [48,56,67]. A formulation of ATRA loaded in NLCs for topical application had high entrapment efficiency, especially with the addition of cholesterol. The use of a sonication method produced NLCs with a smaller particle size and polydispersity index. ATRA loaded in NLCs showed a reduction in the initial drug release and a sustained release effect in studies using Franz cells. Skin irritancy tests for ATRA in NLCs indicated there was no irritation or erythema after 7 days of application, which was in contrast to a marketed formulation. The stability was also improved as there are no changes in the particle size and no significant drug loss, after 4 weeks [56].The effects of different factors, such as the drug amount, type of lipid, and solid to liquid lipid ratio of the NLCs have been investigated and the best NLC formulation was compared with SLN and suspension formulations. All of the formulations showed an encapsulation efficiency above 95%, but oils with less solubility towards ATRA, such as soybean oil, resulted in slightly less encapsulation. All NLC formulations showed a faster and greater cumulative drug release than SLN formulations. A high amount of medium chain triglycerides (MCTs) resulted in the slowest release rate, but MCTs in general showed better release rates than oleic acid, linoleic acid, or soybean oil. Among the NLCs, the permeation was greatest with a high drug amount, a solid/liquid lipid ratio of 2:1, and with oleic acid because of its superior permeation enhancing properties. Formulations with SLNs had the best permeation, followed by NLC and suspension formulations [54].In a study of SLN, NLC, and nano-emulsion (NE) formulations, all of the formulations had acceptable characteristics, with the NE having the least desirable characteristics in terms of size, yield, and ATRA content. The permeation of ATRA was improved by the addition of a terpene, especially with limonene in comparison with 1,8-cineole, used at a high percentage. The highest permeation was obtained with SLN, followed by NLC, NE, and suspension formulations because of the better occlusion effect provided by the solid lipid [55]. For the encapsulation of retinoic acid with NLCs, the same ion pairing method used for SLNs has been used to improve the encapsulation efficiency of ATRA. A study used benethamine as the lipophilic amine to pair with ATRA, which resulted in higher encapsulation as the lipophilic properties of ATRA were enhanced by benethamine enabling better incorporation of ATRA into the lipid matrix [48]. When tested for anti-psoriatic activity, ATRA in NLCs and liposomes showed enhanced orthokeratosis, which indicated better anti-psoriatic activity compared with marketed gel products and SLN and ethosome formulations [50]. Improvements in the properties of ATRA were also obtained using NLCs; however, when compared with SLNs, NLCs were inferior in terms of the permeation and controlled drug release but could be superior in terms of efficacy.