Injectable suspensions are heterogeneous systems comprising a solid phase dispersed in a liquid phase, typically aqueous or nonaqueous. For pharmaceutical acceptability, they should be sterile, stable, resuspendable, syringeable, injectable, isotonic, and non-irritating 1. Recent scientific advances in parenteral drug delivery have spurred the development of systems that allow sustained or controlled drug release 2. Parenteral depot formulations have gained prominence owing to technological advancements 3, especially for drugs with limited bioavailability or narrow therapeutic indices in the gastrointestinal tract 4, 5.

Glucocorticoids (GCs) are pivotal in medicine and exhibit anti-inflammatory, immunosuppressive, anti-allergic, and antitumour properties 6, 7, 8, 9. They mitigate conditions such as rheumatoid arthritis, inflammatory bowel disease, and COVID-19 by inhibiting inflammatory mediators 10. GCs effectively suppress the immune response in autoimmune diseases and in transplant settings. They also alleviate allergic reactions such as rhinitis and urticaria, and enhance chemotherapy outcomes by reducing side effects and boosting efficacy 11, 12, 13.

A significant application of GCs is the management of asthma, a prevalent global inflammatory disease 14. Inhaled glucocorticoids have revolutionised asthma treatment and become a cornerstone therapy for chronic disease management. Advances in the understanding of how GCs suppress inflammation offer the potential for developing improved therapies 15, 16. This study aimed to optimise the ingredients and process of drug formulation for long-acting depot injections. It involved formulating the injectable dosage form, evaluating the final formulation, conducting accelerated stability studies, and comprehensively documenting all resulting data.

The goal of the present study was to develop an injectable suspension of the potent anti-inflammatory glucocorticoid drug betamethasone acetate, evaluate its physicochemical properties to investigate its release pattern and stability, and optimise its final formulation which is effective in the treatment of arthritis. Any formulation work must be preceded by pre-formulation studies, including analytical investigations and choice of the analytical method, and there is a need for the selection of preservatives and surfactants which are compatible with drugs and among themselves, as well as physiologically safe and biocompatible. Preliminary ideas about the behaviour of the dosage form were formulated using the prepared and selected ingredients, and their singular and collective effects on the physicochemical and pharmaceutical properties of the dosage form also need to be generated during this phase. Similarly, betamethasone acetate and betamethasone sodium phosphate were first subjected to a purity study using their melting points and characteristic IR spectra. The drug conformed to pharmacopoeia standards with respect to melting point and characteristic IR peaks. Furthermore, there was no discernible shift/disappearance/appearance of peaks in the drug-excipient combined spectra, indicating good drug-excipient compatibility.

Another objective of this study was to increase the duration of action and bioavailability of betamethasone acetate, which is widely used for the management of arthritis. As a primary step, the drug was subjected to pre-formulation studies according to the USP specification. The drug was found to be a white crystalline powder, which complies with the standard. Solubility was determined, and it was found that the drug was practically insoluble in water, soluble in alcohol and dichloromethane, and freely soluble in acetone. The drug was identified by the IR spectrum, which complied with the standard drug spectrum. The assay was carried out by HPLC, which was found to be 99.4%, and each single impurity and total impurity were 0.10% and 0.18%, respectively. The betamethasone acetate drug exhibits a 2.1% loss on drying and a melting point of 233⁰ C. The specific rotation was +1230. These results are within the USP limits. Drug excipient compatibility studies were carried out with different excipients selected in a 1:2 ratio for one month under accelerated stability conditions. The IR spectra confirmed that all selected excipients were compatible with the drug. Injectable suspension was adopted to formulate parenteral dosage form of betamethasone acetate based on the physicochemical properties of the drug and other excipients, the excipient selected were polysorbate 80 as a surfactant, disodium edetate as complexing agent, dibasic sodium phosphate and monobasic sodium phosphate as buffering agent, benzalkonium chloride as a preservative and water for injection as a vehicle (Table 2 ).

The formulation was carried out in three different batches by varying the surfactant and maintaining constant quantities of all the active pharmaceutical ingredients. In batch I, the formulation contained no surfactant and showed poor syringeability, poor sustained release, and brisk shaking to resuspend. To improve the sedimentation properties, syringeability, and resuspendability of polysorbate 80, PEG3350 was used in batches II and III. The polysorbate 80 formulation exhibited good syringeability, resuspendability, high zeta potential, and sustained release for 24 h. Therefore, batch II was used in further studies.

Evaluation of the injectable suspension involved several key parameters across the three batches. The injectable suspension in all three batches was observed to be a white-coloured suspension, as shown in Table 4. The syringeability of batches I, II, and III was 26, 16, and 21 seconds, respectively, indicating that batch II exhibited the best syringeability. Resuspendability for all three batches was found to be within 4–8 seconds, with batches II and III showing rapid resuspendability within just 4 seconds. The pH of the suspension in all batches fell within the USP limits of 6.8 to 7.8, indicating the absence of stability issues, such as hydrolysis and esterification. The sedimentation volumes of batches I, II, and III after one day were 0.35, 0.48, and 0.55, respectively, with batches II and III displaying better stability owing to higher sedimentation volumes. The zeta potential measurements for batches I, II, and III were 2.92, 13.6, and 12.1, respectively, with batch II showing the highest stability owing to the highest zeta potential. The assay of the injectable suspension, conducted using the HPLC method, showed results within the range of 98–103%, which is within the USP specification limits of 90–120% (Fig. 1, Fig 2 & Fig. 3). Analysis of related substances, also performed using the HPLC method, indicated that all batches were within the USP limits. Dissolution studies were conducted in phosphate buffer (pH 7.4) using a Sotax type 4 dissolution apparatus, revealing that batch I achieved complete release within 14 hours, whereas batches II and III exhibited sustained release up to 24 hours (Table 3). Scanning electron microscopy (SEM) of batch II demonstrated that the particles in the suspension were uniform in size, forming a flocculated suspension that led to a lower resuspending time. Accelerated stability studies were carried out according to ICH guidelines at 40°C ± 2°C / 75% RH ± 5% RH over a three-month period. During this period, the optimized injectable suspension formulations were analyzed for all evaluation parameters, including syringeability, resuspendability, pH, assay, and related substances.

In the 3-month stability analysis, the colour of the product was the same as that of the initial product, and no colour change was observed. The syringeability of the product was determined to be 17 seconds. The entire suspension can be resuspended within 6 seconds. The pH of all the solutions was found to be 7.24 and not much difference was observed compared to the initial pH. The assay was analyzed by HPLC and the difference was found to be within the USP limits.

From the above results, it was found that all the evaluated parameters were within the USP limits, even after three months of stability studies. Finally, it was concluded that formulation number two was the best formulation.

This study aimed to create a novel injectable dosage form of betamethasone acetate that is insoluble in water. The project aimed to overcome these drawbacks and create a stable formulation as a depot injectable suspension. Surfactants such as polysorbate 80 and PEG 3350 were selected to improve syringeability, resuspendability, and sedimentation characteristics. Monobasic and dibasic sodium phosphates were used as buffers, disodium edetate was used as a complexing agent, and benzalkonium chloride was used as a preservative. The batch-II formulation was selected because of its good appearance, syringeability, resuspendability, drainage, zeta potential, assay, pH, and related impurities. The drug release from the suspension was prolonged for 24 hours. Stability studies were conducted over three months, revealing the stability of the formulation.