Step 1: Synthesis of pyridinium halide

A conical flask containing 100 mmol of pyridine and 60 ml of ethyl acetate was stirred with 100 mmol of chloroacetic acid at 90°C for 2 hours. The resulting yellowish solution was then refrigerated for 3 hours, during which time a solid formed. This solid was subsequently filtered, air-dried, and recrystallized from hot methanol and the yield of the process was 60% to 98%.

Step 2 : Synthesis of indolizine-1-carboxylate

A suspension comprising pyridinium halide (10 mmol), methyl acrylate (50 mmol), triethylamine (1.5 ml), and manganese dioxide (80 mmol) in 80 ml of toluene was stirred at 90°C for 2 hours. Following cooling to room temperature, the resulting brown oil was collected through distillation at 130°C. The oil was then washed with water and dried over calcium chloride. The yield of this process ranged from 57% to 92% 9.

Step 3: Synthesis of Pyridinium Halide

In a conical flask, 100 mmol of pyridine and 60 ml of ethyl acetate were combined, and then 100 mmol of chloroacetic acid was added to the mixture. The resulting solution was stirred at 90°C for 2 hours, during which it turned yellowish. After stirring, the mixture was refrigerated for 3 hours, allowing a solid to form. This solid was subsequently filtered, air-dried, and recrystallized from hot methanol. The yield of this process ranged from 60% to 98%.

Step 4 : Synthesis of indolizine-1-carboxylate

A suspension consisting of pyridinium halide (10 mmol), methyl acrylate (50 mmol), triethylamine (1.5 ml), and manganese dioxide (80 mmol) in 80 ml of toluene was stirred at 90°C for 2 hours. Following this, the mixture was cooled to room temperature, and the resulting brown oil was collected through distillation at 130°C. The oil was then washed with water and dried over calcium chloride. The yield of this process ranged from 57% to 92% 9.

Step 5: Synthesis of various indolizinyl derivatives of various NSAIDs

The synthesis of various indolizinyl derivatives of NSAIDs commenced with the preparation of the indolizinyl derivative of sodium salicylate. In this process, indolizine-1-carboxylate (0.25 mol) and sodium salicylate (2.5 mol) were refluxed with concentrated sulfuric acid (2.7 ml) at 90°C for 6 hours. The reaction mixture was then cooled to 4-5°C for 24 hours, allowing the formation of solid crystals. These crystals were subsequently filtered, washed with cold water, dried, and recrystallized from ethanol 10. The same procedure was employed for the synthesis of indolizinyl derivatives of aspirin, wherein aspirin (2.5 mol) was used in place of sodium salicylate. This involved refluxing indolizine-1-carboxylate (0.25 mol) and aspirin (2.5 mol) with concentrated sulfuric acid (2.7 ml) at 90°C for 6 hours. After the reflux, the reaction mixture was cooled to 4-5°C for 24 hours, leading to the formation of solid crystals. These crystals were then filtered, washed with cold water, dried, and recrystallized from ethanol 10. For the synthesis of indolizinyl derivatives of mefenamic acid, indolizine-1-carboxylate (1.5 g) and mefenamic acid (2.9 g) were refluxed with methanol (20 ml) and concentrated sulfuric acid (2.7 ml), and the mixture was boiled at 80°C for 8 hours. The reaction mixture was then cooled to 4-5°C for 24 hours, resulting in the formation of crystals. These crystals were subsequently filtered, washed, dried, and recrystallized from ethanol 11. Similarly, the indolizinyl derivatives of flufenamic acid were synthesized by refluxing indolizine-1-carboxylate (1.5 g) and flufenamic acid (3.95 g) with methanol (20 ml) and concentrated sulfuric acid (2.7 ml). The mixture was boiled at 80°C for 8 hours, then cooled, filtered, and recrystallized from ethanol to obtain the flufenamic acid derivatives 11.

The synthesized compounds were identified and characterized using various analytical techniques. The melting point determination, performed using the Thiel’s tube method, served as a crucial criterion for assessing the purity of the compounds. Thin layer chromatography (TLC) was employed to monitor the progress of reactions and evaluate the purity of the end products, utilizing pre-coated silica plates with a mobile phase of Hexane: Ethyl acetate (4:1 or 1:1) and detecting spots under UV light.

Infrared spectroscopy (IR), conducted with a SHIMADZU FTIR 8400S spectrometer, provided essential information on functional groups and molecular structures through the analysis of molecular vibrations, particularly in the fingerprint region (1300-650 cm⁻¹). Nuclear magnetic resonance (NMR) spectroscopy, carried out using a Bruker spectrospin-400 NMR spectrophotometer with chloroform and DMSO as solvents, allowed for the observation of energy absorption related to the magnetic dipolar nature of spinning nuclei, aiding in structure determination. Mass spectroscopy, using an electron spray mass spectrometer, offered insights into atomic and molecular weights, structural information, and reaction kinetics by analyzing positively charged ions produced via electron bombardment.

The synthesized compounds were characterized as follows:

INS (C₁₆H₉NNaO₄, 303.23 g/mol): This compound is a dark brown solid, soluble in chloroform and ethanol, with a melting point of 204–210°C and a yield of 66%. Key IR peaks include 1621.8 cm⁻¹ (C=C) and 1732 cm⁻¹ (C=O), while the NMR shifts show δ 7.44-7.59 (aromatic H). The molecular ion peak is at m/z 303.

INA (C₁₄H₁₁NO₄, 281.26 g/mol): This compound is a dark brown crystalline solid, soluble in chloroform, with a melting point of 138–140°C and a yield of 70%. IR spectra display peaks at 3045.95 cm⁻¹ (C-H) and 1733.69 cm⁻¹ (C=O), with NMR shifts at δ 7.28-7.82 (aromatic H) and a molecular ion peak at m/z 281.

INM (C₂₅H₂₃N₂O₃, 384.42 g/mol): This compound is a blackish-brown solid, soluble in chloroform and methanol, with a melting point of 232–240°C and a yield of 78%. IR peaks include 3072.5 cm⁻¹ (C-H) and 1654.62 cm⁻¹ (C=O), with NMR shifts at δ 7.09-7.74 (aromatic H) and a molecular ion peak at m/z 384.

INF (C₂₃H₁₅F₃N₂O₃, 424.37 g/mol): This compound is a brown crystalline solid, soluble in chloroform, with a melting point of 135–140°C and a yield of 80%. Its IR spectra show peaks at 3324.68 cm⁻¹ (NH) and 1245.79 cm⁻¹ (C-O), with NMR shifts at δ 2.20-2.64 (aliphatic H) and a molecular ion peak at m/z 424.

Inflammation: The inflammatory process, triggered by stimuli such as infections, ischemia, antigen-antibody interactions, or physical injury, progressed through three distinct phases. The acute phase was characterized by local vasodilation and increased capillary permeability, resulting in redness and swelling. This was followed by the subacute phase, which involved the infiltration of leukocytes and phagocytic cells to clear pathogens and damaged cells. Finally, chronic inflammation led to tissue degeneration and fibrosis, causing scarring and loss of function. Each phase was mediated by different mechanisms and was crucial for the body's response to injury or infection 12.

Nonselective COX Inhibitors

Anti-inflammatory drugs were classified based on their chemical structure into several categories. Salicylic acid derivatives, such as aspirin, sodium salicylate, choline magnesium trisalicylate, diflunisal, salicylsalicylic acid, sulfosalazine, and oxsalazine, formed one category. Another category included Para-aminophenol derivatives, exemplified by acetaminophen. Indole and indene acetic acids, including indomethacin, sulindac, and etodolac, constituted a separate group. Heteroaryl acetic acids, such as tolmetin, diclofenac, and ketorolac, were also identified as a distinct category. Arylpropionic acids, comprising ibuprofen, naproxen, flurbiprofen, ketoprofen, fenoprofen, and oxaprozin, were another classification. Anthranilic acids, specifically the fenamates (mefenamic acid and flufenamic acid), formed another group. Enolic acids, which included oxicams like piroxicam and tenoxicam, as well as pyrazolidinediones such as phenylbutazone and oxyphenbutazone, were classified separately. Finally, Alkanones, represented by nambumetone, constituted the last category. These classifications were based on the distinct chemical structures of these anti-inflammatory drugs.

Selective COX-2 Inhibitors

Selective COX-2 inhibitors, such as rofecoxib (a diaryl-substituted furanone) and celecoxib (a diaryl-substituted pyrazole), targeted the COX-2 enzyme to reduce inflammation and pain while minimizing gastrointestinal side effects compared to traditional NSAIDs. Etodolac, an indole acetic acid, also acted as a selective COX-2 inhibitor. The synthesized derivatives were screened for their analgesic and anti-inflammatory activities. The LD50 (lethal dose 50) estimation was performed using mice, with doses ranging from 150 to 200 µg. The LD50 was determined to be 9 mg/kg body weight, and the effective dose was found to be 2 mg/kg.

Analgesic screening involved methods to evaluate pain relief without inducing loss of consciousness. Pain, a subjective experience, can be categorized as superficial, deep non-visceral, visceral, referred, or psychogenic. The synthesized compounds were tested for their analgesic efficacy using several methods. The tail-flick method measured the reaction time to radiant heat applied to a mouse’s tail, with tail withdrawal indicating pain; an increased reaction time post-drug administration indicated an analgesic effect. Acetic acid-induced writhing in mice was used to assess chemical-induced pain, where a reduction in the writhing response suggested analgesic activity. Eddy's Hot Plate method involved placing mice on a hot plate (55°C), observing paw-licking or jumping responses to pain, and noting increased reaction times after drug administration as indicative of analgesia. The tail immersion method involved immersing a rat’s tail in 55°C water and recording the time taken to withdraw the tail; longer reaction times after drug administration indicated analgesia, with a cut-off time of 15 seconds to prevent harm. These methods collectively provided a comprehensive assessment of the analgesic properties of the synthesized derivatives.

In vitro assays for screening anti-inflammatory compounds have evolved from non-specific earlier methods to more targeted biochemical approaches. Initially, methods such as the inhibition of protein denaturation, fibrinolysis, and platelet aggregation were used, but these showed poor correlation with in vivo activity. More recent assays focus on regulating the arachidonic acid cascade, including the inhibition of phospholipase, cyclooxygenase, thromboxane synthetase, and lipoxygenase. Additionally, these assays target the neutralization of leukocyte functions such as phagocytosis and chemotaxis, as well as the inhibition of tissue degeneration by suppressing proteases and the release of lysosomal hydrolases. Experimental studies often utilized rats, with chemical stimulants injected into the hind paw to induce an inflammatory response, allowing for the evaluation of non-narcotic analgesic compounds.

Various methods were employed to screen for anti-inflammatory activity in rats. Brewer's yeast was used to induce edema and inflammation in the hind paw, with volume changes measured using techniques such as the Randall-Selitto method. Freund's adjuvant, a mixture of Mycobacteria and mineral oil, caused lesions 10-15 days post-injection, and edema was measured using a micrometer. The silver nitrate method involved injecting a 1% solution into the rat's ankle joint to induce inflammation, with pain relief from joint flexing serving as the criterion for efficacy. Kaolin suspension and bovine testicular extract were also used to provoke inflammatory responses in the paw, while granulomas were formed by injecting air or implanting cotton pellets subcutaneously. Both steroidal and non-steroidal compounds were found to reduce inflammation in these models. Formaldehyde was used to induce inflammation in the rat’s paw, with drug efficacy assessed by measuring edema reduction. Lastly, the carrageenan method utilized plethysmography to measure paw volume changes after inflammation was induced by carrageenan injection, and percent edema inhibition was calculated for each treatment group. These diverse methods collectively provided a comprehensive assessment of anti-inflammatory activity in the rat models.

Statistical analysis was performed using one-way ANOVA followed by Tukey’s multiple comparisons test. The data were presented as mean ± SEM, with n=6. Significance levels were indicated as *P ≥ 0.05, **P ≤ 0.01, ***P ≤ 0.001 for comparisons between the Normal control and INS, INA, INM, INF; •P ≥ 0.05, ••P ≤ 0.01, •••P ≤ 0.001 for comparisons between the Toxic control and INS, INA, INM, INF; ^aP ≥ 0.05, ^aaP ≤ 0.01, ^aaaP ≤ 0.001 for comparisons between SS and INS; ^bP ≥ 0.05, ^bbP ≤ 0.01, ^bbbP ≤ 0.001 for comparisons between MA and INM; and ^cP ≥ 0.05, ^ccP ≤ 0.01, ^cccP ≤ 0.001 for comparisons between FA and INF. The drug release profiles of all synthesized derivatives were evaluated through in vitro hydrolysis kinetics at pH 1.2, 4.5, and 6.8, with half-lives determined by hydrolysis and absorption at pH 7.4. The study revealed that the NSAID derivatives INS, INA, INM, and INF exhibited significant analgesic and anti-inflammatory activity compared to SS, ASP, MA, and FA, with INM showing the most notable activity among all the derivatives.