Structural Modifications of Curcumin toward Clinical Applications

2.1. Reduced Analogues of Curcumin

The analogues of curcumin, such as tetrahydrocurcumin and hexahydrocurcumin, have been the focus of several studies due to their improved chemical stability at physiological pH. Unlike curcumin, these analogues lack the unsaturated β-diketo chain, which contributes to curcumin's instability. Holder et al [13] identified glucuronides of tetrahydrocurcumin and hexahydrocurcumin. These compounds have been evaluated for various pharmacological properties, including anti-inflammatory, antioxidant, anticancer, hepatoprotective, antidiabetic, and photoprotective activities. Despite a broad bioactive profile, the low aqueous solubility of tetrahydrocurcumin limits its bioavailability [14, 15].

Hexahydrocurcumin, another analogue of curcumin, has shown better bioavailability in Caco2 permeability assays [16], and its anticancer, antioxidant, anti-inflammatory, and vasorelaxation activities have been identified [17]. Studies have shown that reduced curcumin analogues exhibit higher kinetic solubility than curcumin. However, tetrahydrocurcumin has lower metabolic stability compared to both hexahydrocurcumin and octahydrocurcumin in in vitro assays [18].

Dihydrocurcumin, another compound obtained from the reduction of curcumin, has demonstrated activity against human and rat 17β-HSD1 enzymes. It was found to inhibit oestradiol secretion in human BeWo cells at concentrations of ≥5–10 μM [19]. However, like other reduced analogues, dihydrocurcumin’s clinical utility is limited by poor bioavailability.

The number of publications evidences the interest in these reduced analogues. Over 400 articles in SCOPUS-indexed journals on tetrahydrocurcumin and more than 80 on hexahydrocurcumin have been published, highlighting their therapeutic potential and the need for improved formulations to address their poor pharmacokinetic profiles.

2.2. Monocarbonyl Analogues of Curcumin

To address the instability associated with curcumin’s β-diketo chain, researchers have explored monocarbonyl analogues, which replace the β-diketo group with a more stable heterocyclic moiety. These compounds have been discussed here as piperidone analogues (Fig. 1, series 1a-1h), straight chain analogues (Fig. 2, series 2a-2j), cyclopentanone analogues (Fig. 3, series 3a-3h), and cyclohexanone analogues (Fig. 4, Series 5a – 5e)).

Youssef et al. [20] synthesised analogues in which the β-diketo group was replaced with various rings, including piperidone, cycloheptanone, cyclopentane, and cyclohexane. These modifications were aimed at increasing physiological stability while enhancing antioxidant and anticancer activities. However, removing or replacing methoxy groups with ethoxy groups diminished antioxidant potency. Additionally, the presence of a p-hydroxylic group and an electron-releasing group ortho to the phenolic hydroxyl was found to be crucial for maintaining antioxidant activity. Chen et al. [21] identified the brominated analogue NL04 (Fig. 1a), which exhibited pain-relieving properties in bone cancer models via NLRP3 and IL-1β inhibition.

Adams et al. [22] synthesised a series of monocarbonyl analogues, which feature a shortened five-carbon chain between the aromatic rings instead of curcumin’s seven-carbon chain. Among monocarbonyl curcumin analogues, EF-24 (Fig. 1b), a piperidone analogue, was found to have higher potency than curcumin with respect to anticancer and anti-angiogenic properties. Krasinski et al. [23] reported that EF-24 affects the NF-κB pathway, while Reid et al [24] identified its high oral bioavailability and rapid clearance in rats. Despite its promising activity, formulation is a challenge. Agashe et al. [25] reported a liposome-encapsulated EF-24 formulation with a short half-life. EF-31 (Fig. 1c), a more potent analogue of EF-24, features phenyl rings replaced with piperidine rings, further enhancing its biological activity [26].

EF-31 (Fig. 1c), a more potent analogue of EF-24, has been found to induce anticancer effects through DNA methyltransferase downregulation in pancreatic cancer cells and increases PON1 activity in the U87MG glioblastoma cell line [27]. Further studies on EF-24 analogues, such as UBS109, demonstrated additional anticancer activity, including migration inhibition in HeLa and SiHa cervical cancer cells by influencing matrix metalloproteinases and the phosphorylation of the p38 signalling pathway [28-31].

In addition to EF-24, other monocarbonyl analogues have been studied for their potential therapeutic applications. Malave et al. [32] synthesised N-Boc-protected analogues of piperidone chalcones, identifying four with potent selectivity for colorectal cancer cells. Ghosh et al. [33] explored structural modifications of EF-24 and observed that N-acylation of EF-24 did not enhance its potency. However, when fluorine was introduced at the third, or both the third and fourth positions, of the benzylidene rings, and a trifluoromethyl group was added at the fourth position, the resulting acrolyl analogues exhibited a significantly improved activity profile compared to EF-24. These modifications suggest that substituting with electron-withdrawing groups, such as fluorine and trifluoromethyl, may enhance the pharmacological potential of curcumin analogues.

Zhuang et al. [34] reported ACE inhibitory activity for the curcumin analogue – UBS109 (Fig. 1d), with better bioavailability than curcumin. Further evaluations by Razali et al. followed. [35], who tested the anticancer effects of two piperidone analogues, FLDP-5 and FLDP-8, in glioblastoma cells. D’Arcy et al. [36] identified b-AP15 (Fig. 1e) as a proteasome inhibitor that targets deubiquitinating enzymes in the 19S subunit of the proteasome.

Nishimura et al. [37] evaluated NC2603 (Fig. 1f), another monocarbonyl analogue of curcumin, for its selective cytotoxicity activity against MCF-7 breast cancer cells (IC₅₀ = 5.6 μM). Transcriptomic analysis of cells treated with NC2603 showed upregulation of CDKN1A, potentially via ESR1 or GADD45A. NC2603 was also shown to inhibit cell migration in BT-20 breast cancer cells (IC₅₀=3.5 µΜ), possibly through EGR3 regulation [38].

Selvendiran et al. [39] reported HO-3867 (Fig. 1h), a difluoro monocarbonyl curcumin analogue substituted with N-hydroxy proline. The compound showed improved cell permeability as compared to curcumin [40]. The anticancer potential of HO-3867 has been evaluated against various cancers, including osteosarcoma [41] and hepatocellular carcinoma [42]. It has been shown to inhibit arterial smooth muscle proliferation and arterial stenosis [43]. The compound's effect on the JNK signalling pathway has also been explored, highlighting its effect on multiple pathways.

Liu et al. [44] synthesised a dibenzylidene piperidone analogue of curcumin, with trifluoromethyl groups at the third position of the benzylidene rings. This compound showed potent and selective anticancer activity against NCI-H460 lung cancer cells by inducing reactive oxygen species (ROS) generation. However, other structural modifications, including replacing the piperidone ring with a cyclohexane ring or substituting the piperidone with N-methyl-piperidone, yielded less potent compounds than the lead molecule identified in this study, underscoring the importance of specific structural features for anticancer activity.

Various research groups have continued to explore piperidone analogues of curcumin, testing them for a wide range of pharmacological activities. These analogues have been found to exhibit α-glucosidase inhibition [45], aldose reductase inhibition [46], anticancer properties [47-49] and anti-inflammatory activity [50], indicating their broad therapeutic potential. The structures of the most potent piperidone analogues of curcumin showing anticancer activity (Fig. 1g and i) [51, 52], antifungal activity (Fig. 1m and j) [53, 54], anti-inflammatory activity (1k) [55] and antitubercular activity [56] have been represented in Fig. (1).

Wang et al. [57] further expanded on this work by identifying a monocarbonyl curcumin analogue that induced apoptosis in H460 human lung cancer cells by upregulating C/EBP-homologous protein expression, a key regulator of cell stress response pathways. In related studies, Xiao et al. [58] discovered B63 (Fig. 2a), a monocarbonyl curcumin analogue where the bismethoxyphenyl groups were connected by a penta-1,4-dien-3-one chain. B63 demonstrated apoptotic activity against human non-small cell lung cancer, highlighting the importance of chain length and substitution patterns in improving the efficacy of curcumin analogues.

Liu et al. [59] identified that B82, a bromo-substituted analogue of B63, which effectively inhibited H460 tumour growth in in vivo experiments, underlining the anticancer potential of monocarbonyl curcumin analogues. Further, Luo et al. [60] and Tan et al. [61] reported dibenzylidene thiopyranone derivatives of curcumin, more resistant to hydrolysis. These compounds were more potent than curcumin and triggered apoptosis in leukemic cells by regulating endoplasmic reticulum stress signalling pathways. This mechanism represents a novel therapeutic approach for targeting cancer cells resistant to conventional chemotherapeutic agents.

Hutzen et al. [62] identified GO-Y030 (Fig. 2c), a straight chain monocarbonyl analogue of curcumin, that exhibited potent cytotoxicity against both breast cancer and pancreatic cancer cell lines. GO-Y030's anticancer efficacy was linked to the regulation of cancer cell proliferation by downregulating the transcription factor STAT3. Kudo et al. [63] investigated GO-Y030 and its analogue GO-Y078 (Fig. 2f), identifying that both compounds effectively targeted multiple oncogenic pathways, including NF-κB, PI3K/AKT, JAK/STAT3, and IRF4. These pathways play crucial roles in tumour cell survival and drug resistance, making these analogues promising candidates for further development.

The ability of GO-Y030 and its analogues to inhibit the ATP-binding cassette (ABC) transporter, a protein involved in cancer cell drug resistance mechanisms, was identified by Murakami et al. [64]. However, the bioavailability of GO-Y030 was limited by its poor water solubility. To address this issue, Yamakoshi et al. [65] synthesised GO-Y199, a prodrug of GO-Y030. This prodrug, modified with propargyl-polyethene glycol and 1-glycosylazide, exhibited significantly improved aqueous solubility at concentrations of 100 µM while maintaining anticancer activity comparable to that of the parent compound. These modifications open the door to analogues with a better pharmacokinetic profile, suitable for in vivo application. Extensive research continues to explore the in vitro and in vivo anticancer activities of these curcumin analogues, reinforcing their potential as therapeutic agents.

Li et al. [66] identified two hexamethoxy-diarylpentanedienone analogues of curcumin (Fig. 2g and h), which were shown to induce G2/M phase arrest in NCI-H460 lung cancer cells, possibly through the induction of reactive oxygen species (ROS). By overwhelming the cell's redox buffering system, these analogues significantly disrupted cellular homeostasis, leading to cell cycle arrest. The structures of potent straight-chain monocarbonyl curcumin analogues with anticancer activity (Fig. 2b, d, and j) [67-69], antitubercular activity [70] have been included in Fig. (2).

Further expanding on the therapeutic potential of curcumin analogues, Liang et al. [71] synthesised dibenzylidenecyclopentanone analogues (Fig. 3f and g) and a mono-keto analogue, where the phenyl rings were replaced with thiophene (Fig. 2i). These compounds demonstrated potent anti-inflammatory properties by inhibiting the release of key inflammatory mediators, including TNF-α, IL-1β, IL-6, MCP-1, COX-2, PGES, iNOS, and p65 NF-κB mRNA. Notably, the analogue with a dipropyloxy dimethylamino group (Fig. 3d) showed enhanced water solubility when formulated as its hydrochloride salt, which improved its potential for clinical applications.

Chen et al. identified cyclopentanone curcumin analogues synthesised from cinnamaldehyde and demonstrated potential anticancer activity in vitro and in murine models. One compound (Fig. 3a) showed inhibitory activity against the SGC-7901 and AGS cell lines, with an IC50 value of less than 1 µM. Replacement of the hydroxyl group with halogens resulted in a complete lack of antiproliferative activity [72].

Zhao et al. [73] identified that diarylpenta-1,4-diene-3-one analogues of curcumin exhibited superior anti-inflammatory activity compared to both dibenzylidene cyclohexanone and dibenzylidene cyclopentanone analogues in LPS-stimulated RAW 264.7 macrophages. One particular analogue, C66 (Fig. 3b), a trifluoromethyl-substituted dibenzylidene cyclopentanone, was found to prevent renal injury in mice by inhibiting high glucose-induced inflammation and reducing macrophage infiltration [74].

Taking these findings further, Feng et al. [75] explored 34 asymmetric analogues of C66 (Fig. 3c and e) and demonstrated their potent in vivo anti-inflammatory effects in LPS-induced acute lung injury in rats. These analogues inhibited the expression of inflammatory mediators at the mRNA level and demonstrated cytoprotective effects, as evidenced by histopathological examination of the treated rats.

Hu et al. [76] identified that C66 reduced the disease activity index in a dextran sodium sulphate (DSS) induced IBD (inflammatory bowel disease) model in mice, showing promise for treating IBD. Mladenov et al. [77] reported the cardioprotective effect of C66 against oxidative damage in diabetic cardiac tissue, possibly through regulation of Nrf2 and JNK2 pathways. C66's improved bioavailability and metabolic stability over curcumin make it a valuable candidate for further clinical studies.

Ai et al. [78] synthesised monocarbonyl hybrids of curcumin and ligustrazine with selectivity against both drug-resistant and drug-sensitive lung cancer cells. These findings suggest that hybrid curcumin analogues may offer a novel approach to overcoming cancer drug resistance.

Tang et al. [79] identified a diaryl cyclopenta-1,4-diene-3-one analogue of curcumin (Fig. 3h) where the aryl rings were substituted with a fluorine atom and a trifluoromethyl group. This compound effectively inhibited extracellular matrix accumulation in models of chronic kidney disease, suggesting its potential to prevent kidney fibrosis and related pathologies.

Cyclohexanone analogues of curcumin (Fig. 4, 5a) with antiparasitic activity against Trichomonas vaginalis have been identified by Carapina de Silva et al. It reduced in vitro parasite viability by 70% as compared to the control group [80]. Liu B et al. identified the antiangiogenic activity of a cyclohexanone analogue of curcumin (Fig. 4, 5b) in in vitro, ex vivo, and in vivo studies, through modulation of NADH/NADPH oxidase-derived ROS [81]. This discovery could have implications for the control of tumour growth in cancer therapy.

A cyclohexanone analogue of curcumin with an IC₅₀ of 6.27 µM against lung cancer cells - H460 and H1650, with a potential for the development of treatment options for lung cancer, was identified. Lee et al. [82] synthesised asymmetrically substituted dibenzylidene cyclohexanone and diarylpentan-1,4-dien-3-one curcumin analogues, discovering significant anti-inflammatory activity. Among them, the analogue BDMC33 (Fig. 4, 5d) showed superior activity, interfering with the NF-κB and MAPK signalling pathways in IFN-γ/LPS-stimulated macrophages. Additionally, BDMC33 disrupted LPS signalling by reducing the expression of CD-14 accessory molecules, thus inhibiting NO and TNF-α release in microglial cells. These properties position BDMC33 as a promising therapeutic agent for the treatment of chronic inflammatory conditions. In line with this, Mostafa et al. [83] identified BDMC33's anti-inflammatory potential in a model of bone morphogenetic protein-mediated inflammation and arthritis in zebrafish larvae.

Revalde et al. [84] identified A13 (Fig. 4, 5e), another monocarbonyl curcumin analogue, which showed significant potential in inhibiting the ATP-binding cassette (ABC) transporter. This activity is one of the crucial pathways in overcoming drug resistance in cancer cells. This positions A13 as a promising agent in combination therapies to combat multidrug-resistant cancers.

Aluwi et al. [85] substituted one of the phenyl rings of the asymmetric dibenzylidene cyclohexanone analogues with a furan ring, resulting in a compound with potent prostaglandin E2 inhibitory activity, extending the pharmacological relevance of curcumin analogues.

Qian et al. [86] identified a monoketo analogue of curcumin with a wide range of protective effects in a murine model of cardiac myopathy induced by a high-fat diet (HFD). The analogue exhibited anti-inflammatory, antioxidant, antifibrotic, and antiapoptotic effects, highlighting its therapeutic potential for cardiovascular diseases.

The growing body of research on monocarbonyl curcumin analogues, particularly GO-Y030 and EF-24, highlights their significant potential for cancer therapy.

2.3. Heterocyclic Analogues of Curcumin

Heterocyclic analogues of curcumin with potential as anti-inflammatory and anticancer therapies have been identified. Liu et al. [87] reported the anti-inflammatory activity of NSC 757929 (Fig. 5, 4b), a pyrazole analogue of curcumin substituted with a phenyl ring on the nitrogen atom of the pyrazole. This compound showed promising effects in murine models of Parkinson’s disease, underscoring the potential of curcumin analogues for treating neuroinflammatory conditions. Sahu et al. [88] synthesised pyrazole carboxamides (NSC 777947, Fig. 5, 4a), with anticancer activity against human lung cancer cells (IC₅₀ ≈100 µM). Ahsan et al. [89] identified NSC 782199 (Fig. 5, 4f) as a potent anticancer agent across multiple human cancer cell lines. They synthesised various pyrazole analogues of curcumin, introducing modifications such as a carboxamide moiety on the pyrazole ring (Fig. 5, 4c), phenylmethanone groups (Fig. 5, 4d), and dihydropyrimidine derivatives (Fig. 6, 7a). The analogue with a phenylmethanone substitution on the pyrazole ring exhibited the highest anticancer potency, with a growth percentage of -28.71. The work was further expanded with the identification of three pyrazole analogues of curcumin - NSC 793120, NSC 782199, and NSC 782200 (Fig. 5, 4e, f, and h), which showed significant anticancer activity against multiple cancer cell lines. Substitution of the phenoxymethanone moiety with a phenoxyethanone moiety further enhanced potency, with over 80% growth inhibition.

Mohamed et al. [90] evaluated three pyrazole analogues of curcumin against cancer cells and confirmed their therapeutic potential for targeting epidermal growth factor receptor (EGFR). The analogue featuring 1-[[4-chlorophenyl] amino] propane-2-one on the pyrazole ring (Fig. 5, 4g) emerged as a potential candidate for further development. However, Ahsan et al. [91] identified the non-specific activity of these analogues, suggesting the need for additional modifications to improve selectivity against cancer cells.

Mari et al. [92] continued the structural optimisation of curcumin analogues with the synthesis of aminopyrimidine analogues (Fig. 7, 9a and b) with over 95% stability under physiological conditions. One compound, PY1 (Fig. 9a), was found to be equipotent to curcumin against prostate cancer cells. Another analogue, in which methyl groups replaced the methoxy groups of curcumin, showed better anticancer activity against HT-29 colorectal cancer cells, outperforming both curcumin and dimethoxy curcumin.

Feriotto et al. [93] focused on oxime, beta-enamine ketones, and isoxazole analogues of curcumin, targeting K562 leukaemia cells resistant to imatinib. These analogues were more stable compared to curcumin at physiological pH. The compounds containing bulky aryl rings on the enamine moiety showed greater stability than those substituted with smaller alkyl groups. Two isoxazole analogues, compounds 22 and 2 (Fig. 7, 9a and b), exhibited superior anticancer activity, with efficacy retained against imatinib-resistant cells. The absence of the methoxy group did not affect the potency of compound 22.

Caprioglio et al. [94] identified curcumin analogues with potent anticancer activity where the diketo group of curcumin was replaced with a triazole ring. The position of the methoxy and phenyl groups on curcumin's phenyl rings was varied to explore the structure-activity relationship. The analogue with the same substitution pattern as curcumin was most potent, suggesting that retaining certain structural elements may be crucial for maintaining and enhancing biological activity.

2.4. Curcumin Analogues with a hepta-1,6-diene-3,5-dione

Yue et al. [95] identified AC17 (Fig. 8, 6c), a 4-arylcurcumin analogue with tenfold more potency than curcumin in inhibiting the growth of multiple cancer cell lines, including nasopharyngeal carcinoma [CNE2] and lung cancer [A549] cells. Ma et al. [96] found that formulating AC17 into transmucosal delivery microneedles coated with hyaluronic acid increased its bioavailability in mice. This formulation successfully blocked the growth of oral squamous carcinoma cells by regulating the FOXO3 signalling pathway.

Hussain et al. [97] identified methyl, methoxy, and chloro-substituted curcumin analogues that exhibited potent antioxidant and antidepressant activities, highlighting the diverse therapeutic potential of curcumin derivatives. Liu et al. [98] elucidated the mechanism by which a curcumin analogue, T63, induces apoptosis and cell cycle arrest in nasopharyngeal cancer cells (CNE2 and CNE2R). The structure of T63 is characterised by an arylidene group at the methylene carbon, which contributes to its efficacy in overcoming cancer cell resistance [99].

Yang et al. [100] studied the potential of ASC-J9 (Fig. 8, 6a), a dimethoxy analogue of curcumin, in treating spinal and bulbar muscular atrophy [SBMA]. The compound regulated the toxicity of the androgen receptor in in vitro, Drosophila, and murine models. Bott et al. [101] identified ASC-JM17 (JM17) (Fig. 8, 6b), an orally bioavailable curcumin analogue with improved efficacy in both in vitro and in vivo models of SBMA. Structural modifications, including reduction of the diketo group to a monoketogroup and substitution of the methylene carbon with a cyclobutane ring, improved the pharmacokinetic profile of this molecule. JM17 also exhibited potent antioxidant activity by regulating Nrf2. Wu et al. [102] compared the efficacy of JM17 (1 µM) to the known Nrf2 activator omaveloxolone (0.3 µM). JM17 was more effective at reducing mitochondrial ROS and lowering glutathione levels.

Seghetti et al. [103] evaluated novel 1,4,6-heptatrien-3-one analogues, incorporating a substituted triazole ring on the active methylene group to pan-assay interference (PAINS) properties characteristic of curcumin. Triazole-substituted analogues on the phenolic hydroxyl group showed less potency than those with triazole substitution on the methylene group. The analogue substituted with a para-fluorobenzyl moiety (Fig. 8, 6d) was the most potent. This compound outperformed curcumin in inducing apoptosis in leukaemia T cell lines, validating the influence of chemical structure on pharmacological activity.

Aloor et al. [104] adopted a computational approach for designing chlorinated curcumin analogues with antiproliferative activity. Docking and molecular modelling tools were used to identify analogues with favourable drug-like properties and stronger binding affinity than curcumin. One analogue showed superior binding interactions with the BRCA1-BRCT-c domain and demonstrated significant anticancer activity against breast cancer cell lines, emphasising its therapeutic potential in breast cancer treatment.

2.5. Dibenzylidenecyclohexaneoxime Analogues of Curcumin

Wang et al. [105] identified JM-2, an oxime analogue of dibenzylidenecyclohexane (Fig. 9, 5a), that demonstrated promising therapeutic effects in murine models of diabetic cardiomyopathy. Its efficacy was linked to NF-κB-mediated inhibition of inflammation, highlighting its potential for treating inflammation-related complications in diabetes.

In another study, Lou et al. [106] discovered SJ-12 (Fig. 9, 5b), a curcumin analogue that may be beneficial in the treatment of cardiomyopathy. The probable mechanism of action is through the regulation of the calcium signalling pathway. In a murine model of streptozocin-induced diabetic cardiomyopathy, SJ-12 did not influence blood glucose levels, suggesting its potential use in cardiac diseases without affecting glucose metabolism.

2.6. β-cyclocitral Analogue of Curcumin

Han et al. [107] synthesised β-cyclocitral-derived monocarbonyl curcumin analogues. A19 (Fig. 10) exhibited strong cytotoxicity against hepatocellular carcinoma (HCC) cell lines HepG2 and Huh-7. It reduced cell viability, suppressed colony formation, and induced G2/M phase arrest while promoting apoptosis in cancer cells, with high selectivity. A19 induced DNA damage by inhibiting the ERK/JNK/p38 MAPK signalling pathway, which plays a pivotal role in cell survival and stress response.

Combining A19 with sorafenib, a standard chemotherapeutic for HCC, enhanced A19's cytotoxicity, suggesting A19's potential as a combination therapy for liver cancer. A detailed structure-activity relationship [SAR] study further investigated the impact of various substituents on the cyclohexanone ring of A19 analogues. Analogues without substituents exhibited weak activity similar to β-cyclocitral. However, the introduction of halogenophenyl groups, particularly meta-bromophenyl and dihalogenophenyl substituents, improved anticancer activity. Compounds like A8, with para-[tert-butyl] phenyl [IC₅₀ = 35.69 μM], exhibited notable efficacy. Analogue A9 [substituted with 2,5-ditri-fluoromethylphenyl] showed reduced activity. The compound A10, which had an ortho-ethoxy group, exhibited lower potency. These results highlight the effect of electron-withdrawing substituents on activity.

2.7. Azepane Derivatives of Curcumin

Wang et al. [108] synthesised VLX1570 (Fig. 11), an analogue of b-AP15 with increased potency and hydrophilicity. However, the compound faced limitations in clinical trials due to pulmonary toxicity [109]. This was attributed to the presence of the 1,4-diene-3-one moiety, which forms glutathione complexes in vitro [110, 111].

2.8. Conjugation Strategies

Various conjugation approaches have been explored to enhance the stability, potency, and solubility of curcumin analogues.

2.9. Synthetic Routes for Curcumin Analogues

3.1. Methodology

LigPrep was used to prepare the compounds and ensure proper geometry and protonation states. Neutral forms of molecules were used for analysis to align with typical physiological conditions. QikProp was used to predict the properties in batch mode.

QikProp calculates various molecular descriptors, including molecular weight, hydrogen bond acceptor/donor counts, octanol/water partition coefficient [QPlogPo/w], aqueous solubility [QPlogS], Caco-2 cell permeability [QPPCaco], blood-brain barrier penetration [QPlogBB], human oral absorption [PHOA], and Lipinski's Rule of Five violations, among others. The output data was exported into Excel for further analysis of ADME properties [117].

Molecular docking of curcumin analogues was performed using the Maestro suite (118). Protein preparation of the DYRK2 protein [PDB ID: 6HDR] was done at a simulated pH of 7.4. Protein preparation involved filling missing side chains, optimising hydrogen bond assignments, and minimising/deleting water molecules. The Receptor Grid Generation module in GLIDE was used to generate a grid around the ligand-binding site.

The docking protocol was validated by redocking the co-crystallised ligand into the prepared grid using both Standard Precision [SP] and Extra Precision [XP] docking modes. The docked poses were superimposed onto the original structure of the co-crystallised ligand to identify the most suitable docking mode. The XP docking pose showed the minimum root mean square deviation [RMSD] value of 0.363 Å compared to the crystallographic pose (Fig. 16), indicating a valid docking setup.

The data from in silico analysis is available as a supplementary file.

The curcumin analogues, prepared with LigPrep, were docked flexibly using XP docking into the DYRK2 receptor. Only the top-scoring pose per ligand was retained. Per-residue interaction scores for amino acid residues within 4 Å of the grid centre were recorded and analysed to understand key ligand-protein interactions.

The SMILES representations of the curcumin analogues were input into the SWISS ADME web tool to predict their cytochrome P450 [CYP] enzyme interactions and to identify potential substrates of P-glycoprotein [P-gp] [119].