Dexibuprofen, a member of the nonsteroidal anti-inflammatory drug (NSAID) class, is well-known for its anti-inflammatory, antipyretic, and analgesic effects achieved by inhibiting cyclooxygenase 1 and 2 (COX-1, COX-2) enzymes responsible for prostaglandin synthesis.^1-3) The active S-(+) enantiomer of ibuprofen, known as dexibuprofen, has been shown to be approximately 160 times more effective in prostaglandin inhibition than the inactive R-(-) form.^4,5) Specifically, dexibuprofen offers advantages over the racemic mixture of ibuprofen by effectively inhibiting COX-1 and COX-2 at lower doses, resulting in reduced gastrointestinal toxicity, improved clinical efficacy, and decreased variability in therapeutic effects.⁶⁾

Derived from ibuprofen, dexibuprofen presents promising prospects in various therapeutic applications, offering pharmacokinetic improvements and decreased gastrointestinal side effects.⁷⁾ In the field of pediatric healthcare, Maxibupen^® Syrup (dexibuprofen 12 mg/mL, Hanmi Pharmaceutical Co., Ltd.) has firmly established itself as an over-the-counter drug.^8,9) Specifically formulated for infants and toddlers, this syrup effectively masks the bitterness of dexibuprofen, enhancing medication compliance. However, the inherent characteristics of dexibuprofen, with a pharmacological half-life (t_1/2) ranging from 1.8 to 3.5 hours, result in irregular bioavailability and absorption variability.¹⁰⁾ Administering appropriate doses at 4 to 6-hour intervals for infants and toddlers aged six months and above poses practical difficulties, especially in cases of unconsciousness or symptoms like nausea, vomiting, and seizures that make oral administration unfeasible.^11-13) In such circumstances, rectal administration proves to be a viable alternative, well-established for drug delivery.¹⁴⁾ Therefore, this study aims to develop a sustained-release thermosensitive hydrogel suppository for rectal administration, allowing for prolonged and controlled drug release over 12 hours or more. By addressing the limitations of conventional formulations, this sustained-release approach provides a practical and effective method for maintaining consistent therapeutic effects in infant and toddler patients.¹⁴⁾

Traditional solid suppositories have been the most widely used delivery system for rectal drug administration, constituting over 98% of all rectal delivery forms.¹⁵⁾ However, patient aversion due to discomfort and a sense of foreignness is a common issue associated with conventional solid suppositories.¹⁶⁾ Moreover, Dexibuprofen (DXI) is classified as a drug belonging to the Biopharmaceutical Classification System (BCS) class II, characterized by low solubility and high permeability.^4,17) Drug solubility is a crucial variable in designing safe and effective formulations. Both the absorption of the active ingredient and biological activity depend significantly on aqueous solubility. Low solubility and dissolution rate pose significant challenges for drugs classified under BCS class II (low solubility and high permeability).^10,18,19)

To comprehensively address these challenges, the development of a thermosensitive liquid suppository appears promising. This formulation facilitates rectal administration, provides mucosal adhesion without mucosal damage or leakage, and reduces discomfort and aversion commonly associated with conventional solid suppositories.²⁰⁾ Additionally, rectal drug delivery offers a bypass of the liver through drainage into the inferior and middle rectal veins, minimizing first-pass metabolism before systemic effects occur.^15,21) The rectal area, rich in lymphatic circulation, further contributes to increased systemic absorption of specific highly lipophilic drugs.²²⁾ Thermosensitive systems allow for effective control of drug release by varying the types and concentrations of components. This versatility in formulation can positively impact drug therapeutic effects.^23,24) Polymers exhibiting thermosensitivity and mucoadhesiveness include Poloxamer, Poly-N-isopropylacrylamide, Cellulose, PLGA (poly(lactic-co-glycolic acid), and PEG (polyethylene glycol).^25-27) Poloxamers, in particular, are tri-block copolymers consisting of hydrophilic polyethylene oxide (PEO) and hydrophobic polypropylene oxide (PPO) blocks, exhibiting a sol-gel phase transition due to their thermosensitive behavior.²⁸⁾

Therefore, this study aims to overcome the shortcomings of existing commercial products and to develop and evaluate a sustained-release rectal thermosensitive hydrogel suppository containing dexibuprofen. The evaluations include appearance, droplet size, gelation temperature, gelation time, viscosity, mechanical properties (gel strength and adhesiveness), and drug release characteristics. The ultimate goal is to develop an optimal formulation that addresses the challenges associated with pediatric drug delivery, ensuring both efficacy and patient compliance. Through this research, we aim to make a significant contribution to the advancement of drug delivery systems, particularly in the field of pediatric medicine.

Materials

Dexibuprofen used in this study was provided by Hanmi Pharmaceuticals (Hwasung, South Korea), and poloxamer (Kolliphor P407, P188) was purchased from BASF (Ludwigshafen, Germany).

Various analytical instruments were employed in this research, including a UV-vis spectrophotometer (UV-1800, Shimadzu, Japan), high-performance liquid chromatography (HPLC, 1260 Infinity II, Agilent Technologies, USA), nanoparticle analyzer (Zetasizer Nano ZS, Malvern, UK), viscometer (DV2TRVTJ0, Ametek Brookfield, USA), texture analyzer (CT3-1000, Ametek Brookfield, USA), and dissolution tester (RC-8D S, Minhua Pharmaceutical Machinery, China).

HPLC analysis

For HPLC analysis, a VDSpher PUR 100 C18-M-SE column (150×4.6 mm, 5 μm particle size, VDS optilab, Germany) was utilized. The mobile phase consisted of a mixture of phosphate buffer (pH 3.5) and acetonitrile in a volumetric ratio of 40:60 (v/v). The flow rate was set at 1.0 mL/min, column temperature at 25°C, detection wavelength at 220 nm, and the injection volume at 20 μL.²⁹⁾

Using this HPLC system, linearity, limit of detection (LOD), and limit of quantification (LOQ) were determined. All experiments were conducted in triplicate for accuracy and reliability.

Preparation of thermosensitive hydrogel suppositories containing dexibuprofen

Dexibuprofen was added to room temperature purified water at 2% (w/w) of the total formulation. The mixture was vortexed and then heated at 70°C in a constant temperature water bath for 1 minute. Pre-prepared 23% (w/w) P407 and 29% (w/w) P188 solutions were added to the dexibuprofen solution in various ratios and mixed. The entire mixture was then melted by heating at 90°C in a constant temperature water bath for 30 minutes. After mixing, the liquid was stored in a 4°C refrigerator for 24 hours to obtain a liquid-state dexibuprofen hydrogel suppository (Fig. 1).

Fig. 1. Manufacturing method of a thermosensitive hydrogel suppository containing dexibuprofen.
RT: room temperature.

Characterization of thermosensitive hydrogel suppositories

Appearance

Dexibuprofen-containing thermosensitive hydrogel suppositories, prepared at various ratios of P407 and P188, were visually observed for their appearance after 1 hour of storage at 25 and 37°C (as shown in Table 1).

Table 1 Formulation of a thermosensitive hydrogel suppository containing dexibuprofen with various concentrations of P407 and P188. Properties and gelation temperatures of thermosensitive hydrogel suppository containing dexibuprofen. Each value of gelation temperature represents the mean±SD (n=3)

g \ No.	F1	F2	F3	F4	F5	F6	F7	F8	F9	F10	F11	F12	F13	F14	F15	F16	F17
Dexibuprofen	0.4	0.4	0.4	0.4	0.4	0.4	0.4	0.4	0.4	0.4	0.4	0.4	0.4	0.4	0.4	0.4	0.4
Kolliphor P407	1.8	1.8	1.8	1.8	1.8	1.8	1.8	2.4	2.4	2.4	2.4	2.4	2.4	2.8	2.8	2.8	2.8
Kolliphor P188	1.4	1.6	1.8	2	2.2	2.6	3	1.4	1.6	1.8	2	2.2	2.6	1.4	1.6	1.8	2
D.W	16.4	16.2	16	15.8	15.6	15.2	14.8	15.8	15.6	15.4	15.2	15	14.6	15.4	15.2	15	14.8
Total	20	20	20	20	20	20	20	20	20	20	20	20	20	20	20	20	20
25°C	sol	sol	sol	sol	sol	sol	sol	sol	sol	sol	sol	sol	sol	sol	sol	sol	gel
37°C	sol	sol	sol	gel	gel	gel	gel	sol	gel	gel	gel	gel	gel	gel	gel	gel	gel
Gelation temp. (°C)	-	-	-	37.2±0.6	35.0±0.8	29.9±1.2	28.2±1.7	38.8±0.7	34.8±0.7	33.5±0.6	32.4±0.4	27.7±0.1	26.8±0.6	30.2±0.5	29.7±1.3	27.9±0.1	25.6±0.7
Gelation time (min)	-	-	-	-	6.0±0.4	3.8±0.1	2.7±0.5	-	-	6.8±0.5	4.1±0.3	3.5±0.2	2.9±0.3	4.2±0.3	4.0±0.2	3.4±0.4	1.8±0.3

Droplet size and polydispersity index (PDI)

A sample diluted 50-fold with purified water was injected into a DTS1070 polystyrene/polyurethane cuvette. The cuvette was then mounted on a dynamic light scattering instrument (ZEN3600, Malvern, UK) with the temperature set to 37°C. After 120 seconds of temperature equilibration, the average droplet size and polydispersity index (PDI) of the dexibuprofen-containing hydrogel suppository were measured at 4 mW, 633 nm, and a scattering angle of 173°. All measurements were conducted in triplicate to ensure accuracy.

Gelation temperature

First, 20 g of thermosensitive hydrogel in a sol phase at 10°C was injected into a 30 mL scintillation vial with a magnetic bar. The vial was secured in a beaker containing 150 mL of water maintained at 55°C. This assembly was then mounted on a hot plate, and a contact-type digital thermometer was carefully inserted into the sample without touching the magnetic bar. The hot plate was set to 55°C, and stirring speed was maintained at 100 rpm. The gelation temperature of the dexibuprofen-containing hydrogel suppository was defined as the temperature at which the magnetic bar, in motion during stirring, came to a stop as the sample gradually underwent gelation.³⁰⁾ All measurements were conducted in triplicate to ensure accuracy.

Measurement of gelation time through viscosity changes

The measurement of gelation time and viscosity was conducted using a viscometer (DV2TRVTJ0, Ametek Brookfield, USA). A 13 mL sample of the sol-state thermosensitive hydrogel at 10°C was injected into the small sample adapter of the viscometer, which was set to 37°C. The SC4-29 spindle was employed, and measurements were conducted at 50 rpm for 10 minutes. Gelation time was defined as the moment when the viscosity reached 12,000 cps.³¹⁾ Considering the compliance of the target population, infants, as a crucial factor in enhancing the effectiveness of drug therapy, the target gelation time was set to be within 5 minutes. The experiments were conducted in triplicate, and the results were described with representative images.

Gel strength and adhesiveness

The measurement of gel strength and adhesiveness was conducted using a texture analyzer (CT3-1000, Ametek Brookfield, USA). A 17 g sample of the thermosensitive hydrogel at 37°C, contained in a scintillation vial, was affixed to the TA-RT-KIT support base of the texture analyzer. To mitigate the influence of air inflow, a barrier was installed around the sample. The TA10 (θ=12.7 mm) probe, moving at a speed of 3.00 mm/s, touched the sample surface and descended 15 mm with a dwell time of 10 seconds. The test type was set to compression, and the target type was distance. Gel strength was defined as the overall resistance value when the gel was compressed by the probe and adhesiveness was defined as the work required to overcome the cohesive forces between the sample and the probe.^32,33) All measurements were conducted in triplicate to ensure accuracy.

In-vitro Dissolution test

A selected 2 mL sample of the dexibuprofen-containing thermosensitive hydrogel formulation was injected into preprepared membrane tubing (Spectra/Por^® 2 dialysis membrane, MWCO 12~14kD, Repligen, USA), and both ends were secured with clamps. The dissolution test was conducted according to the second method (paddle method) of the Korean Pharmacopoeia Twelfth Edition at 100 rpm, using 600 mL of phosphate buffer solution (pH 6.8) as the dissolution medium, at 37±0.5°C. Samples of 2 mL were withdrawn at predetermined time points (0.5, 1, 3, 5, 6, 8, 10, 12, 14, and 16 hours), filtered through a 0.45 μm nylon syringe membrane filter, and analyzed using HPLC under the same conditions.³⁴⁾ The release rate (%) of dexibuprofen for each time point was calculated using the constructed calibration curve. All measurements were conducted in triplicate to ensure accuracy.

HPLC analysis

The analysis of dexibuprofen standard solutions (7.5~187.5 μg/ mL) revealed a retention time of 5.5 minutes (Fig. 2A). The determination coefficient (R²) of the calibration curve was excellent at 0.9999, indicating outstanding linearity (Fig. 2B). The calculated limit of detection (LOD) was 1.178 μg/mL, while the limit of quantification (LOQ) was determined to be 3.570 μg/mL.

Fig. 2. (A) HPLC analysis chart of dexibuprofen and (B) Linearity curve of dexibuprofen by HPLC.

Appearance

Fig. 3 illustrates the sol-gel phase transition phenomenon of the dexibuprofen-containing thermosensitive hydrogel suppository. The hydrogel, which appears as a light white color from a colorless state, is in a liquid-state sol at room temperature and transitions to a nearly immobile gel state at 37°C.

Fig. 3. Photographs of the phase transition of the thermosensitive hydrogel containing dexibuprofen F11 as a function of ambient temperature.
(A) at 25°C and (B) at 37°C.

The transition phenomenon mediated by poloxamer occurs as the critical dissolution temperature (CDT) increases. As the temperature rises, the hydrophobic portion of poloxamer, the PPO chains, undergoes polymer self-assembly and aggregation within the aqueous solution. This process leads to the formation of a denser network structure, resulting in the gelation of the thermosensitive hydrogel and exhibits high viscosity (Fig. 4).³⁵⁾

Fig. 4. Schematic representation of micellization and gel formation of aqueous solutions using poloxamers.

Droplet size and PDI

To improve the bioavailability of the administered drug compound absorbed by the rectal mucosa leading to systemic circulation, the particle size should be very small to better penetrate the rectal mucosa. Particularly, when the particle size is between 10-100 nm, the system is recognized as a stable isotropic liquid system with a more uniform size and favorable physicochemical properties.³⁶⁾ Additionally, droplet size and PDI are crucial factors influencing the physicochemical stability of the formulation. When PDI is less than 0.3, the particle size and distribution are considered uniform.³⁷⁾

The droplet sizes of formulations F1~F17 (except F13) were confirmed to be less than 40.0 nm, and the droplet size of formulation F13 was confirmed to be 58.0±11.2 nm. The particle size of all formulations was observed to be less than 100 nm and the PDI was less than 0.3 (Fig. 5). These findings suggest that all formulations (F1 to F17) have appropriate droplet size and uniformity to support their potential for rectal drug delivery. The formulations were subjected to further evaluation, and the results were systematically assessed to select the optimal formulation.

Fig. 5. Droplet size and PDI of the thermosensitive hydrogel suppository containing dexibuprofen.
Each value represents the mean±SD (n=3).

Gelation temperature

For rectal administration of the thermosensitive hydrogel suppository, it is essential that the formulation exists in a sol state at room temperature for ease and convenience. After administration, it should undergo a sol-gel phase transition at an appropriate temperature within the rectal mucosa for optimal attachment and absorption of the active pharmaceutical compound. Therefore, formulations with gelation temperatures within the range of 30~36°C were selected.^38,39)

When the ratio of P407 was fixed, increasing the ratio of P188 resulted in a lower gelation temperature. Similarly, when the ratio of P188 was fixed, a higher ratio of P407 led to a lower gelation temperature. The results from the evaluation indicate that, with P407 fixed at 9% (w/w), formulations F4, F5, and F6 with increased P188 ratios showed gelation temperatures of 37.2, 35.0, and 29.9°C, respectively. Similarly, with P188 fixed at 10% (w/w), formulations F4, F11, and F17 exhibited gelation temperatures of 37.2, 32.4, and 25.6°C, respectively. Among the formulations tested formulations F1, F2, and F3 did not undergo gelation even beyond the set temperature of 50°C for the hotplate stirrer. In contrast, formulations F5, F9, F10, F11, and F14 showed gelation within the target range of 30~36°C. These formulations are considered suitable candidates for further study, as they are expected to transition to a gel state at body temperature upon rectal administration (Table 1, Fig. 6).

Fig. 6. Gelation temperature of the thermosensitive hydrogel suppository containing dexibuprofen.
Each value represents the mean±SD (n=3).

Measurement of gelation time through viscosity changes

For rectal drug delivery, thermosensitive hydrogel suppositories require appropriate gelation time and viscosity to prevent drug loss due to leakage or excessive spread from the rectal end, ensuring absorption through the rectal mucosa. Additionally, considering the mobility of infant and toddler patients after administration, rapid gelation within 5 minutes is necessary to minimize concerns about leakage during daily activities.⁴⁰⁾

In formulations F1~F17, when the ratio of P407 was fixed, an increase in the ratio of P188 tended to result in higher viscosity for each sample at specific time points. The results from the evaluation indicate that with a fixed P407 ratio, increasing the P188 ratio led to a decrease in gelation time. When P407 was fixed at 9% (w/w), formulations F5, F6, and F7, with increased P188 ratio, showed gelation times of 6.0, 3.8 and 2.7 minutes, respectively. Similarly, with P188 fixed at 10% (w/w), formulations F11, and F17 exhibited gelation times of 4.1 and 1.8 minutes, respectively. These values indicate that a higher poloxamer content generally promotes faster gelation. The rapid increase in viscosity signifies the transition from the sol state to the gel state, with initial viscosities around 2-3 cps in the solution state, rising to 10,000-12,000 cps after gelation. These findings are similar to those in previous studies by Park, J.H. et al. and Bonacucina, G. et al..³¹⁾, ⁴¹⁾ Additionally, the time required to reach the target viscosity of 12,000 cps was shortened as the Poloxamer ratio increased. For F14 and F15, the viscosity of F14 was higher than that of F15 up to 3.5 minutes after the start of measurement. However, after 3.6 minutes, the viscosity of F15 was recorded to be higher. The gelation time, derived from viscosity changes as shown in Fig. 7, aligns with the data presented in Table 1. This transition is driven by micellar aggregation, which accelerates as the temperature increases to physiological levels (37°C). The onset of gelation is marked by the formation of a 3D network, as poloxamer micelles come together, trapping water molecules and transitioning into a gel. The optimized concentrations of P407 and P188 in formulations F6, F7, F11, F12, F13, F14, F15, F16, and F17 led to shorter gelation times, likely due to a faster dehydration of the hydrophobic PPO block, facilitating micelle aggregation. This behavior is consistent with previous research, which highlights that both the PEO and PPO block lengths, as well as their relative proportions, play critical roles in determining gelation kinetics.⁴¹⁾ Shorter gelation times, as seen in formulations with higher poloxamer ratios, can be attributed to the specific balance between hydrophilic and hydrophobic interactions within the poloxamer structure. Therefore, formulations that met the target gelation time and viscosity were identified as F6, F7, F11, F12, F13, F14, F15, F16, and F17. These formulations are considered suitable candidates for further evaluation, as they demonstrate appropriate gelation times.

Fig. 7. Viscosity of the thermosensitive hydrogel suppository containing dexibuprofen.
Results as a function of concentration of P407. (A) 9, (B) 12, and (C) 14 (%, w/w).

Gel strength and adhesiveness

The evaluation of gel strength and adhesiveness of the gelled formulation is crucial in manufacturing an appropriate thermosensitive hydrogel suppository that does not leak after administration and reaches the rectal end without difficulty. If the gel strength is too strong, it may make administration challenging and cause discomfort due to the gelation effect post-administration. Conversely, if the gel strength is too low, there is a risk of leakage, necessitating a balance in the mechanical properties of the gel. Additionally, a higher adhesiveness value is a preferred property because it provides stronger adhesion to the tissue surface, which may increase retention time and improve clinical efficacy.^32,42)

The results from the evaluation indicate that, with a fixed P407 ratio, an increase in the P188 ratio results in higher values of gel strength and adhesiveness, as shown in Fig. 8. With P407 fixed at 9% (w/w), formulations F4, F6, and F7 with increased P188 ratios exhibited gel strengths of 0.48, 1.63, and 2.15 N, respectively. Similarly, with P188 fixed at 10% (w/w), formulations F4, F11, and F17 showed gel strengths of 0.48, 1.26, and 2.01 N, respectively. For equal ratios, variations in P188 had a more pronounced effect on the intensity values than changes in P407. This trend was also observed in adhesiveness.

Fig. 8. Gel strength and adhesiveness of the thermosensitive hydrogel suppository containing dexibuprofen. Results as a function of concentration of P407.
(A) 9, (B) 12, and (C) 14 (%, w/w). Each value represents the mean±SD (n=3).

The study is similar to previous research by Choi, H. G. et al., which investigated acetaminophen-containing liquid suppositories and showed that higher poloxamer ratios led to increased gel strength and adhesiveness values.^24,38) Therefore, the formulation with the highest mechanical property values, including gel strength, was prioritized in this study.

Dissolution test

Dissolution tests were conducted on five formulations (F5, F9, F10, F11, and F14), selected based on their gelation temperature range (30~36°C), using membrane tubing with a molecular weight cutoff (MWCO) of 12-14 kD in a pH 6.8 phosphate buffer solution. All samples achieved a dissolution rate of 10.4±0.6% within 30 minutes and 18.0±1.3% within 1 hour from the start of dissolution. F14 showed a dissolution rate of 57.8±6.5% within 3 hours, whereas F5, F9, F10, and F11 did not achieve a dissolution rate of 50% within the first 3 hours. By 5 hours, both F5 and F14 surpassed 80.0% dissolution, while F9, F10, and F11 exceeded 80.0% by 6 hours. The five formulations tested (F5, F9, F10, F11, and F14) achieved a sustained dissolution rate of over 100.0% within 10 hours, which was maintained consistently throughout the duration of the experiment (16 hours). As the micelle packing arrangement of the poloxamer rapidly dissociates in the presence of an excess aqueous medium, the breakdown of the gel matrix results in the release of the drug, allowing it to diffuse out of the gel.⁴³⁾ Notably, F11 demonstrated a relatively high dissolution rate from 10 hours onward, suggesting its potential for extended drug release. This result, combined with F11’s favorable gelation characteristics observed in previous evaluations, supports its selection as the final formulation for rectal delivery. The dissolution profiles of these formulations are consistent with the initial goal of sustained release over 12 hours or more, demonstrating a gradual drug release that can support prolonged therapeutic effects. This sustained-release profile ensures a steady release that can allow for longer dosing intervals and minimize fluctuations in drug levels (Fig. 10).⁴⁴⁾

Fig. 9. In-vitro release profile of the thermosensitive hydrogel suppository containing dexibuprofen.
Each value represents the mean±SD (n=3).

This study focused on the development and evaluation of sustained-release thermosensitive hydrogel suppositories containing dexibuprofen for rectal administration. The formulations F5, F9, F10, F11, and F14 demonstrated appropriate gelation temperatures within the range of 30~36°C. Analysis of droplet size and polydispersity index (PDI) indicated uniform particle size and distribution, crucial indicators for maintaining physical and chemical stability. All formulations, except F13, showed acceptable results in these parameters. The gelation time and viscosity measurements demonstrated that formulations F6, F7, F11, F12, F13, F14, F15, F16 and F17 met the desired gelation time of 5 minutes or less, which is crucial for ensuring compliance among infant and toddler patients. Additionally, evaluations of mechanical properties such as gel strength and adhesiveness showed that formulations with higher P188 content exhibited increased mechanical strength. Dissolution tests indicated that formulations F5, F9, F10, F11 and F14 rapidly formed gels after administration, continuously releasing the drug. Taken together, the results of this evaluation suggest that among the developed thermosensitive hydrogel suppositories, formulation F11 stands out as the most promising candidate for rectal drug delivery. In other words, comprising 20 mg/g dexibuprofen and a ratio of 12:10 (%, w/w) of P407 and P188, it offers properties conducive to efficient drug absorption, mechanical stability, and patient compliance. Overall, the findings of this study suggest that the newly developed sustained-release thermosensitive hydrogel suppository containing dexibuprofen based on poloxamers holds potential as a convenient and effective dosage form for managing fever, inflammation, and pain in infant and toddler patients. Further preclinical and clinical studies are necessary to validate its safety and efficacy specifically for the infant and toddler population.

All authors declare no conflicts of interest.

Seo Wan Yun : Graduate student

Tae Han Yun : Graduate student

Kyeong Soo Kim : Professor