The opportunity of delivering drugs by penetration into or across the skin has enormous implications for the local treatment of dermatological diseases or systemic transportation of orally non-absorbed or poorly absorbed drugs (Vogt et al., 2016). However, the permeation efficiency of the skin is the main milestone to be crossed to achieve the expected outcome, because the majority of drugs are difficult to pass through the stratum corneum, the outermost layer of the skin (Kakadia and Conway, 2015, Iyer et al., 2021). In the past decades, the advent of nanotechnology has revolutionized the arena of skin drug delivery, and a plethora of nano-scaled drug carriers like liposomes, nanoemulsions, carbon/silica-based nanomaterials, and polymeric micelles/nanoparticles have been extensively explored aiming at the dermal, follicular or transdermal penetration enhancement (Kakadia and Conway, 2015, Chen et al., 2019). Nevertheless, successful translations of these nano-based drug delivery systems have been limited, as shown by a tiny number of advanced clinical studies (Goyal et al., 2016). Crucial factors that have close relevance to this problem include drug loading, safety, stability, large-scale and low-cost production, and reproducibility (Lai et al., 2013, Wang et al., 2022).

Nanosuspensions can break through the above limitations of other types of nano-based drug delivery systems. By definition, they are colloidal dispersions of drug particles in the nano-size range stabilized only by small amounts of surfactants, or/and polymers, therefore possessing obviously higher drug loading capacity (Biswas et al., 2017, Lai et al., 2013). Owing to the well-developed production technologies, almost forty commercial nanosuspensions have been available in the market mostly for oral administration (Ahire et al., 2018). There is now a paradigm shift in using them from conventional delivery routes to dermal applications, and the commercialization of rutin nanosuspensions (Juvedical®) approved for cosmetic use intrigues the interest in research and development of nanosuspensions in the dermal and transdermal drug delivery field (Shi et al., 2020).

The relationship between the properties of nanoparticles and their skin penetration is a significant research topic for the improvement of skin drug delivery performance. However, the size dependence of particle penetration is debated, because it may vary greatly among different types of nanoparticles (Vogt et al., 2016, Chen and Feng, 2022). For example, the optimal size for follicular particle penetration can be quite different for different processed materials, such as 640 nm for PLGA (Patzelt et al., 2011), 80 nm for nanoemulsions (Su et al., 2017), and 40–250 nm for polystyrene nanoparticles (Friedman et al., 2021). However, related studies about nanosuspensions have been rarely reported. It was recently observed that particle size might be a determinant for the skin drug delivery of curcumin nanosuspensions (Shi et al., 2020). Since nanosuspensions are usually prepared in the absence of carrier chemicals, the size effect of nanosuspensions on skin penetration may be differential for specific drugs, and the related investigation would offer more insight into the ambiguity of size-dependence transport.

Andrographolide (AG, as shown in Fig. 1) is a diterpenoid lactone presented in Andrographis paniculate, possessing a wide range of biological activities, such as anti-cancer, anti-inflammatory, anti-bacterial, anti-viral, and hepatoprotective effects (Kandanur et al., 2019, Kumar et al., 2020). These functions make andrographolide useful in the prevention and treatment of dermatological diseases, such as skin cancers (Zhong et al., 2022), psoriasis (Shao et al., 2016), and wounds (Sanad and Abdel-Bar, 2017). However, investigations attempting to develop its topical formulations are still lacking, probably because of its intractable physico-chemical properties. In addition, the high dose of AG makes conventional nano-based drug carriers powerless to accommodate the required amount of drug with a minimum dose volume. Therefore, formulation into nanosuspensions would be a good option for the topical application of AG. In our previous study, we successfully prepared the AG nanosuspensions with three different particle sizes, i.e., AG-NS250 (249.8 ± 1.3 nm), AG-NS450 (485.2 ± 3.7 nm), and AG-NS1000 (1015.0 ± 36.1 nm), and revealed their size effect of on the IVIVC performance in oral absorption (Yao et al., 2022). To the best of our knowledge, there have been no investigations focusing on the transdermal or dermal delivery of AG.

Herein, we compared the skin penetration of AG nanosuspensions with three different particle sizes using Franz cells. The drug release profiles, disposition in different skin layers, and penetration across the skin were all determined to investigate the effect of particle size on the skin penetration of AG from their nanosuspension formulations. For elucidating the penetration mechanisms, laser scanning confocal microscopy (LSCM) was employed to visualize the delivery pathway of these three delivery systems, and a histopathological study was carried out to probe the structural alteration of the skin induced by the formulations.