Curcumin is a hydrophobic polyphenol found in Curcuma longa rhizome extract. This substance has a wide range of proven biological activities (Xu et al., 2018; Salem et al., 2014; Tsuda, 2018). In particular, curcumin has been shown to have an anti-inflammatory effect by reducing the expression of inflammatory mediators (Kang et al., 2016; Kurd et al., 2008; Antiga et al., 2015). The biological activity of curcumin is determined by the peculiarities of its chemical structure, which includes two aromatic rings functionalized by hydroxy and methoxy groups interconnected by a spacer of two α,β-unsaturated carbonyl groups. This structure allows the existence of beta-diketone and keto-enol tautomeric forms of curcumin (Payton et al., 2007). It has been shown that due to the peculiarities of the chemical structure, curcumin is able to interact and bind with various biomolecules, affecting their activity and modulating signaling pathways in the cell (Shehzad et al., 2013; Curcumin, 2023). At the same time, curcumin is characterized by extremely low solubility in hydrophilic media, low stability and bioavailability, and is rapidly metabolized and excreted from the body (Mittal et al., 2023; Khurana and Ho, 1988; Gao et al., 2011; Vareed et al., 2008). However, there are approaches that make it possible to overcome the above limitations. One of these approaches is the development of co-crystals with improved physico-chemical properties and increased stability (Lu and Rohani, 2010). The previously developed curcumin co-crystals demonstrated a reduced melting point and increased solubility (Goud et al., 2012; Sanphui et al., 2011; Pantwalawalkar et al., 2021; Paulazzi et al., 2022). There are also approaches for the delivery and stabilization of curcumin using various nanoformulations and nanoconjugates, based on liposomes (Li et al., 2005), exosomes (Sun et al., 2010), micelles (Mohanty et al., 2010; Podaralla et al., 2012; Gao et al., 2013), nanoparticles (Sindhu et al., 2013; Jambhrunkar et al., 2013) and dendrimers (Shi et al., 2007). Thus, curcumin is a biologically active substance with significant potential for biomedical applications, which can be effectively used as a functional ligand together with its stabilizing nanoformulation.

Cerium-based nanoparticles (nanoceria) can act as an effective platform for binding curcumin. These include cerium dioxide (CeO2, hereinafter NDC) nanoparticles capable of mimicking the activity of natural enzymes such as superoxide dismutase, oxidase, catalase, and peroxidase (Wu et al., 2019; Wei and Wang, 2013), photolyase (Tian et al., 2019), phospholipase (Khulbe et al., 2020) and nuclease (Xu et al., 2019). The enzyme–like activity of NDC is due to the structural features of the crystal lattice of nanoparticles, the presence of Ce3+ and Ce4+ ions and defects in its composition, called oxygen vacancies, and manifests itself depending on the configuration of nanoparticles and microenvironment conditions (Shcherbakov et al., 2016; Celardo et al., 2011; Tan et al., 2020). This, in turn, determines their bioactivity. In particular, NDC can exhibit an anti-inflammatory activity by reducing the expression of pro-inflammatory factors (Sangomla et al., 2018; Domala et al., 2020). Cerium fluoride (CeF3) nanoparticles are also nanoceria. They have redox activity (Chukavin et al., 2024a), due to which they are able to selectively modulate the cellular response to X-ray irradiation (Chukavin et al., 2024b), and can also be modified for luminescent imaging (Popov et al., 2022a). At the same time, CeO2 and CeF3 nanoparticles exhibit radioprotective properties in relation to Schmidtea mediterranea under X-ray irradiation conditions (Filippova et al., 2023). In addition, both types of cerium nanoparticles demonstrate a synergistic antimicrobial effect with cold atmospheric plasma (Ermakov et al., 2023). Thus, nanoceria represents a promising platform for biomedical applications.

Modification of the cerium dioxide nanoparticles with functional elements makes it possible to obtain nanoformulations with improved or new properties. The conjugate of NDC and calcein has a unique feature of detecting the level of intracellular reactive oxygen species (ROS) along with their neutralization (Chukavin et al., 2023; Chukavin et al., 2022), whereas the NDC-mildronate composite exhibits mitochondrially targeted radioprotective activity (Popov et al., 2022b). Nanoformulations of NDC and doxorubicin (Krysanov et al., 2019), TNF-α (Shydlovska et al., 2018), ssDNA (Bülbül et al., 2016), inhibitor of Hsp90, doxorubicin and folate (Sulthana et al., 2017), labeled dsDNA (Yang et al., 2016), chitosan, ZM241385 and pilocarpine (Luo et al., 2020), clindamycin (Saha et al., 2023), morin (Thakur et al., 2022), FITC (Kolmanovich et al., 2024a) and miRNA-146a (Zgheib et al., 2019; Stager et al., 2022; Dewberry et al., 2022), as well as NDC doped with gadolinium (Kolmanovich et al., 2024b; Kolmanovich et al., 2023), are also known.

Approaches to creating nanoformulations of curcumin based on nanoceria are being actively developed. The binding of these components is achieved through the formation of hydrogen bonds between them and the formation of chelate complexes. (Wang et al., 2023). Among the various methods of curcumin binding using NDC, those based on polysaccharide stabilization (Kalashnikova et al., 2017), incorporation into biopolymers (Naveen et al., 2024), hydrogels (Zhao et al., 2024) and extracellular matrix (Singh et al., 2022), binding by phospholipids (Huang et al., 2025), encapsulation in copolymers (Liu et al., 2022), coordination by polypeptides (Jiang et al., 2024) and formation of nanoconjugates with curcumin in colloidal form (Zholobak et al., 2020). These nanoformulations show a wide range of bioactivities, such as anti-inflammatory, antioxidant, and wound healing effects, as well as selective antitumor activity. This opens up prospects for their use in the treatment of oncological and chronic inflammatory diseases. However, the potential cytotoxicity of such nanoformulations remains poorly understood. Despite the existence of mechanistic studies of curcumin cytotoxicity, showing the ability of curcumin to cause cell death by generating ROS and inducing apoptosis (Singh and Singh, 2009; Fujisawa et al., 2004), there is a gap in the study of its possible cytotoxicity in nanoformulations with nanoparticles. In this paper, we investigated the effect of excess curcumin on the cytotoxicity of its nanoconjugate with nanoceria (Scheme 1).