Diabetes is a significant global health challenge in the 21st century, impacting the health of individuals worldwide. Unhealthy lifestyle habits, influenced by genetic tendencies, are a significant contributor to the increasing prevalence of diabetes. Thiazolidinediones (TZDs) are type 2 diabetes medications that activate PPARs, promoting fatty acid storage in adipocytes, reducing fatty acid concentration in the bloodstream, and increasing glucose reliance. In this study, the authors implemented the bioisosterism approach to design novel analogues of Rosiglitazone (RGT), due to the presence of multiple toxicities associated with its treatment. A total of 191 pyridine bioisosteres, such as thiazolidine, phenoxy, and methoxyphenyl, were generated using the MolOpt tool. The newly designed compounds were assessed for their medicinal properties, pharmacokinetic (ADME) parameters, and toxicity profile through the ADMETLab 3.0 online tool. After screening, a total of 46 analogues found favorable drug-likeness, QED score, and Lipinski's rule. Notably, compounds RGT008, RGT029, RGT04, RGT05, RGT010, and RGT011 were identified as promising candidates based on their QED and MCE-18 scores. Additionally, compounds RGT011 and RGT021 exhibited reduced hepatotoxicity and cardiac toxicity compared to standard (RGT). The comprehensive analysis indicates that the compounds, specifically RGT05, RGT04, and RGT021, may be considered for additional assessment of the hypothesis as promising activators of PPAR-γ, serving as potential anti-diabetic agents.

Diabetes is recognized as one of the most significant health challenges of the 21st century, affecting the health of individuals globally. Approximately 830 million individuals across the globe are affected by diabetes, with a significant proportion residing in low- and middle-income nations [1]. The elevation of blood glucose levels leads to diabetes, resulting from a malfunction in insulin production. Various factors, including genetic tendencies and unhealthy lifestyle habits, can lead to this malfunction. Currently, unhealthy lifestyle choices play a major role in the rising incidence of diabetes [2]. Elevated blood glucose levels can result in vascular complications throughout the body, leading to a range of health issues. Diabetes has the potential to harm various organs, including the kidneys and eyes, and it also impacts nerve tissues and the cardiovascular system. Individuals diagnosed with diabetes face a heightened risk of experiencing heart attacks compared to those without the condition [3]. Diabetes is primarily classified into two types: insulin-dependent and non-insulin-dependent, commonly referred to as type 2 diabetes. The incidence of type 2 diabetes is frequently associated with obesity and a lack of physical activity [4]. Diabetes Mellitus is a chronic metabolic disorder marked by persistent hyperglycaemia, which occurs due to impairments in insulin secretion, insulin action, or both. This condition is linked to serious complications, including cardiovascular diseases, neuropathy, nephropathy, and retinopathy, all of which can significantly diminish the quality of life for those affected. 

The management of diabetes focuses on controlling blood glucose levels through various therapeutic agents [5]. Among these, Thiazolidinediones (TZDs), also known as glitazones, represent a class of medications utilized in the treatment of type 2 diabetes. These medications function by activating peroxisome proliferator-activated receptors (PPARs), particularly PPARγ, which are nuclear receptors. This activation promotes the increased storage of fatty acids in adipocytes (fat cells), thereby decreasing the concentration of fatty acids in the bloodstream and enhancing cellular reliance on glucose for energy [6]. There are two drugs within this class that have received approval from the FDA including Pioglitazone and Rosiglitazone. These drugs may be administered independently or in conjunction with other oral medications for type 2 diabetes, including metformin or sulfonylureas. Nonetheless, they may lead to side effects such as weight gain and an increase in peripheral fat mass. Rosiglitazone (RGT) improves insulin sensitivity and glycaemic control by activating the PPARγ. PPAR-γ activation enhances insulin sensitivity in peripheral tissues, rendering it a viable treatment option for Type 2 Diabetes Mellitus [7]. However, many glitazone medications are classified as second-line treatments and exhibit significant toxicity. For instance, while rosiglitazone was known to improve tissue sensitivity to insulin, it was banned in 2010 due to its association with heart failure. Additional serious side effects of this medication include blurred vision, numbness, bone fractures, respiratory tract infections, and strokes [8].

Bioisosteres represent a significant concept that has been widely utilized in the development of drugs and continues to be an essential strategic element in contemporary medicinal chemistry practices [9]. This concept involves the deliberate substitution of specific atoms or functional groups within a molecule with alternatives that possess comparable properties. This concept plays a crucial role in medicinal chemistry, enabling researchers to improve drug candidates by altering their pharmacological properties [10]. The objective may include enhancing metabolic stability or solubility while maintaining biological efficacy. Structural modifications classified as bioisosteres do not alter the biological function of the compound. Substitutions that involve changes to a compound's core are often termed scaffold hops, while those that modify the substituents of a core are typically referred to as functional group replacements [11,12]. Bioisosteres can be divided into classical and non-classical categories. classical bioisosteres involve the replacement of an atom or group with another that has similar dimensions, shape, and charge distribution. For example, substituting a hydrogen atom with a halogen atom (such as fluorine or chlorine) or an oxygen atom with a sulfur atom. Nonclassical bioisosteres involve more complex substitutions that may modify the electronic or steric properties of the molecule such as substituting a benzene ring with a thiophene ring [13,14]. The use of bioisosterism is particularly advantageous in addressing drug resistance, minimizing side effects, improving pharmacokinetic profile and enhancing drug-receptor interactions [15,16].The practice of substituting one part of a molecule with another that possesses comparable physical or chemical characteristics is a well-recognized approach in medicinal chemistry, aimed at enhancing drug features such as efficacy, selectivity, and safety [17-19].

This study concentrated on examining novel analogues of RGT to enhance the pharmacokinetic (ADME) profile and diminish toxicity, aiming to produce safer and more efficacious PPAR-γ activators for diabetes therapy.

2.1 Designing of Rosiglitazone Analogues

Authors have selected the pyridine group in the RGT molecule for the design of novel RGT analogues. A variety of bioisosteres of the pyridine group were generated by the MolOpt tool. This online web server facilitates in silico drug design by employing bioisosteric transformation rule. It assists medicinal chemists by automatically producing lists of analogues via bioisosteric replacement, which can subsequently be evaluated for potential synthesis. Additionally, it serves as a benchmark for appraising advancements in molecular optimization algorithms, with an emphasis on sample efficiency. The tool accommodates 25 molecular design algorithms across 23 tasks, rendering it a valuable resource for researchers and scientists [20,21].

2.2 Pharmacokinetic and Toxicological (ADMET) Profile Calculation

The produced analogues of RGT were subsequently utilized to calculate the ADMET profile employing the ADMETlab 3.0 online tool. It is an advanced tool within the field and a comprehensive software platform developed to prediction the absorption, distribution, metabolism, excretion, and toxicity characteristics of newly designed compounds [22,23]. This platform is a comprehensive online source containing eighty-four quantitative and four qualitative regression models that provide reliable and extensive predictions of ADMET properties for new ligands that mimic mammalian ADMET characteristics. It combines various computational models and databases to forecast the ADMET profiles of chemical compounds. This tool is important during the initial phases of drug discovery, as it aids in the identification of potential drug candidates with favourable ADMET properties [24,25]. By predicting the ADMET profiles of bioisosteric replacements, authors can choose the most promising candidates for subsequent screening studies.

2.3Drug Likeness (DL) and Drug Score (DS) Prediction

Drug likeness (DL) and drug score (DS) assessments play a crucial role in the initial stage of the drug development process. These evaluations aid researchers in identifying and prioritizing compounds, enabling them to focus on candidates with a higher likelihood of success in later development and clinical trials. The Osiris properties explorer (PEO) was utilized to compute DL and DS [26,27].

3.1 Designing of Analogues of Rosiglitazone

RGT serves as a second-line oral hypoglycaemic agent for managing diabetes mellitus. However, numerous patients have reported experiencing various toxicities associated with this medication. Therefore, it is essential to modify the structure of RGT to develop safer and less toxic analogues. MolOpt generated a total of 189 substitutable groups for the pyridine group within the RGT molecule (Figure 1). All 189 analogues were chosen for subsequent evaluations, including ADMET, DL, and DS predictions. After the screening, 46 selected analogues of RGT, along with their 2D structures are being shown in Table 1.MolOpt generates the variety of the bioisosteres for the pyridine group including pyrazole-3-amine, 2-(tert butyl)-4-methylthiazole, thiazole, n-cyclopropylformamide, cyclopropane-1-one, imidazolidine-2-one, 3-chorophenol, 6-methylpyridin-3-ol (Figure 2).

                                                              Figure 1. Structure of Rosiglitazone and its Modified Pyridine Group using Bioisosteric Approach.

                                Table 1. Structure of the Rosiglitazone Analogues and their Physicochemical Properties

MW (Molecular weight), nHA (Number of hydrogen bond acceptor), nHD (Number of hydrogen bond donor), nRot (Number of rotatable bonds), TPSA (Topological polar surface area), logP (The logarithm of aqueous solubility value).

                                                                           Figure 2: Variety of Pyridine Bioisosteres in the Rosiglitazone Molecule.

3.2 Calculation of Medicinal Profile

The medicinal properties of the designed analogues such as QED (Quantitative estimation of drug-likeness), SA (Synthetic accessibility), Fsp3 (The number of sp3 hybridized carbons/total carbon count), and MCE-18 (Medicinal chemistry evolution in 2018) has been carried out and their results are tabulated in Table 2. A QED, evaluates the drug-like characteristics of potential drug candidates. This assessment is grounded in desirability, which includes nine properties associated with drug likeness. The QED scores ofall designed analogues with exception of RGT012, fall within a favorable range (greater than 0.67), which was favourable as RGT (0.821). These scores indicate that all analogues represent attractive compounds as standard (RGT). The MCE-18 metric effectively evaluates the novelty of compounds based on their overall sp3 complexity. The MCE-18 scores ofall newly developed analogue found more than 45 indicatingsufficient novelty, necessitate visual examination to assess their target profiles and drug-likeness. Lipinski’s rule has been satisfied by all analogues, indicating their potential for adequate absorption or permeability. Furthermore, Pfizer’s criteria were also met by all analogues, suggesting favorable ADMET profiles. The Golden Triangle (GT) rule encompasses two parameters: molecular weight (MW) (≤200 and ≥50) and LogP (≤5 and ≥−2). All analogues complied with the acceptance criteria of the GT rule. 

                                                                                     Table 2. Medicinal Properties of Rosiglitazone Analogues.

QED (Quantitative estimation of drug-likeness), SA (Synthetic accessibility score), Fsp3 (The number of sp3 hybridized carbons/total carbon count), MCE-18 (Medicinal chemistry evolution in 2018), GSK (GlaxoSmithKline), GT (Golden Triangle)

3.3 Prediction of Pharmacokinetic (ADME) Properties

Pharmacokinetic parameters play a vital role in the development of new drugs, as they provide critical information regarding the absorption, distribution, metabolism, and excretion of a drug within the human body. The pharmacokinetic parameters, which encompass absorption (Caco-2, MDCK, and HIA), distribution (BBB, PPB, and VD), metabolism (CYP1A2), and excretion (CL and T1/2), have been calculated for the designed analogues, with their respective scores tabulated in Table 3. The Caco-2 (human colon adenocarcinoma cell line) is a commonly used in vitro model for predicting the intestinal permeability of drugs and evaluating their potential for oral absorption. Consequently, the evaluation of permeability through Caco-2 cells has become a significant factor in assessing the viability of a therapeutic compound. The results indicate that the Caco-2 scores for analogues RGT04, RGT05, RGT011, RGT012, RGT013, RGT014, RGT16, RGT020, RGT036, and RGT042 found more than -5.15, suggesting favorable in vivo drug permeability. A strong MDCK score further implies that all analogues possess the capability to permeate and traverse cellular membranes. Human intestinal absorption (HIA) scores ranging from 0 to 0.3 suggest that these analogues may exhibit good oral bioavailability. The blood-brain barrier (BBB) scores for analogues RGT01-046 are below 0.03, indicating a potential safety profile against central nervous system side effects. Furthermore, the designed analogues, including RGT01-046, demonstrate poor plasma protein binding (PPB) of less than 90%, suggesting limited distribution throughout the body. The volume of distribution (VD) scores for all analogues falls between 0.04 and 20, indicating an adequate distribution in body fluids and tissue uptake. Cytochrome P450 (CYP P450), a family of isozymes, is essential for phase-I and phase-II drug metabolism. All analogues exhibit a high substrate score and a low inhibitor score for CYP1A2.Metabolism (CYP1A2-inh and sub, CYP2C19-inh and sub, CYP2C9-inh and sub, CYP2D6-inh and sub, CYP3A4-inh and sub), clearance and half-life (T1/2) scores were calculated and their results are shown in Table4. 

                                                                                       Table 3: Absorption and Distribution Profile of the Analogues.

Caco2 (Human colon adenocarcinoma cell line), MDCK (Madin−Darby Canine Kidney cells), PAMPA (The Parallel Artificial Membrane Permeability Assay),              HIA (Human intestinal absorption), BBB (Blood Brain Barrier), PPB (Plasma protein binding), FU (Fraction Unbound).

                                                                                  Table 4: Metabolism and Excretion profile Prediction of the Analogues

Human cytochrome P450 (five isozymes––1A2, 3A4, 2C9, 2C19 and 2D6), + (indicates the compound act as substrate against the isozymes), - (indicates the compound act as inhibitor against the isozymes), CL (The clearance of a drug), T1/2 (The half-life of a drug).

3.4 Prediction of the toxicity properties

The assessment of the toxicological characteristics of the analogues was conducted, focusing on various parameters such as drug-induced liver injury (DILI), mutagenicity as determined by the Ames test, acute oral toxicity in rats (ROA), the interaction of the molecule with the ligand-binding domain (LBD) of the androgen receptor (NR-AR-LBD), and carcinogenic potential. The results of these evaluations are presented in Table 3. Notably, all designed analogues exhibited identical human hepatotoxicity (H-HT) scores to RGT01-046, suggesting a potential for mild toxicity, with scores ranging from 0.3 to 0.7. In contrast, RGT04, RGT018, and RGT022 were predicted to have a safer DILI score, falling between 0 and 0.3, indicating lower toxicity levels. However, RGT01, RGT28, RGT30, and RGT043 were anticipated to exhibit higher toxicity in the Ames test. The toxicity profile of the analogues with their score is shown in Table 5. 

3.5 Prediction of DS and DL score

The definitions of Druglikeness (DL) and Drug Score (DS) have been formulated based on specific physicochemical characteristics of established drug compounds and their impact on molecular behavior in vitro. The DL and DS metrics are predictive of various properties, including solubility, permeability, metabolic stability, and transporter interactions, relevant to drug candidates. The DL score of these compounds may offer insights into their safety and effectiveness. Conversely, the DS serves as a holistic measure that integrates various attributes such as druglikeness, cLogP, logS, molecular weight (MW), and toxicity issues into a singular value. This metric can be utilized to evaluate the likelihood of an unknown compound fulfilling the necessary criteria to qualify as a drug [28,29]. Among all analogues, RGT46 exhibits the highest DL score, followed by others. The DL and DS scores of the analogues with their score is shown in Table 5. 

                                                              Table 5. Toxicity, Drug Likness and Drug Score Prediction of Design Analogues

H-HT (The human hepatotoxicity), DILI (Drug-induced liver injury), hERG (Human ether-a-go-go related gene or cardiotoxicity), Ames (test for mutagenicity), ROA (Rat Oral Acute Toxicity, NR-AR (Nuclear receptor-androgen receptor), DL (drug likeness), DS (Drug score).

Diabetes is a major global health concern, primarily arising from insulin dysfunction, genetic factors, and unhealthy lifestyle choices, leading to elevated blood glucose levels and an increasing incidence of the disease. Rosiglitazone, a PPAR-γ agonist used in the treatment of Type 2 Diabetes Mellitus, was removed from the market in 2010 due to its association with heart failure. Additionally, this medication is linked to serious side effects, such as blurred vision, numbness, bone fractures, respiratory infections, and strokes. In medicinal chemistry, the principle of bioisosterism involves substituting a part of a molecule with another that has comparable physical or chemical characteristics, thereby enhancing the drug's efficacy, selectivity, and safety. This research aims to explore new analogues of Rosiglitazone to improve the pharmacokinetic profile (ADME) and minimize toxicity, with the objective of developing safer and more effective PPAR-γ activators for diabetes management. A total of 191 pyridine bioisosteres, including thiazolidine, phenoxy, and methoxyphenyl, were generated using the MolOpt tool. The newly synthesized compounds were assessed for their medicinal properties, pharmacokinetic (ADME) parameters, and toxicity profiles through the ADMETLab 3.0 online platform. The distribution of lipophilicity (DL) and solubility (DS) was evaluated using PEO calculations. After the screening process, 46 analogues exhibited favorable drug-likeness, QED scores, and adherence to Lipinski's rule. Notable candidates identified include compounds RGT008, RGT029, RGT04, RGT05, RGT010, and RGT011, which were distinguished based on their QED and MCE-18 scores. Moreover, compounds RGT011 and RGT021 demonstrated reduced hepatotoxicity and cardiac toxicity compared to the standard (RGT). The comprehensive analysis indicates that compounds RGT05, RGT04, and RGT021 merit further exploration as potential PPAR-γ activators, suggesting their potential as anti-diabetic agents.

The authors have no conflicts of interest regarding this investigation. 

The authors wish to thank the Head, Department of Pharmacy, Guru Ghasidas Vishwavidyalaya, Bilaspur (CG) for providing necessary facilities to carry out the research work.