Improvement in the efficacy of dexketoprofen by its prodrug in lipid emulsion

Authors


Abstract

A lipid emulsion delivery system was developed to improve the anti-inflammatory and anti-nociceptive activities of dexketoprofen. The dexketoprofen isopropyl ester (DE) lipid emulsion (DEE) with a particle size of 208.1 ± 33.1 nm and a charge of −34.81 mV was administered to rats and then compared with dexketoprofen injection solution (DS). There was no statistical significance in their pharmacokinetic parameters. The anti-inflammatory effect of DEE was evaluated by experiments involving egg-albumin-induced paw edema in rats and xylene-induced ear swelling in mice. In the paw edema test, the swelling of the DEE group recovered quickly from 1 to 3 h (p<0.05), compared with the DS group. In the ear swelling test, the inhibition rate produced by DEE and DS was 57.79 and 28.57%, respectively. Acetic acid-induced abdominal constriction and hot-plate experiments were used to evaluate the peripheral and central anti-nociceptive actions of DEE. In the acetic acid-induced abdominal constriction test, DEE significantly restrained the writhing responses with a pain inhibition rate of 66.38% compared with 30.06% for the DS group. In the hot-plate test, both preparations had a similar pain threshold increasing percentage. This study shows that the anti-inflammatory and anti-nociceptive activities of dexketoprofen are markedly improved after incorporation into a lipid emulsion.

Practical applications: Dexketoprofen is an important drug for the management of inflammation and treatment of pain. However, its commercial preparations have usually been applied to treat diseases which are less serious because of their limited efficiencies. In this study, the anti-inflammatory and anti-nociceptive activities of dexketoprofen are markedly improved by incorporating its prodrug into the lipid emulsion. Therefore, the lipid emulsion preparation can be used to treat more kinds of inflammation and pain in clinic.

Abbreviations:

DE, dexketoprofen isopropyl ester; DEE, dexketoprofen isopropyl ester lipid emulsion; DS, dexketoprofen injection solution; MCT, medium-chain triglyceride; LCT, long-chain triglyceride; PG, prostaglandin; UPLC, ultra performance liquid chromatography

Introduction

Ketoprofen(2-[3-benzoylphenyl] propionic acid), a nonsteroidal anti-inflammatory drug (NSAID), was initially launched by Sanofi (formerly known as sanofi-aventis) in 1975 for the treatment of pain associated with arthritis, dysmenorrhea, gout, osteoarthritis, rheumatoid arthritis, and ankylosing spondylitis, and it inhibits arachidonic acid (AA) metabolism by an inhibitory action on cyclooxygenase and lipooxygenase 1, 2. The drug is an aryl carboxylic acid derivative with a chiral center 3, which exists in two enantiomeric forms, S-(+)-ketoprofen (dexketoprofen) and R-(−)-ketoprofen (levoketoprofen).

It has been reported that dexketoprofen is the active optical isomer (eutomer) of ketoprofen, which produces effective analgesic, anti-inflammatory, and antipyretic effects at half the dosage of ketoprofen 4, 5. The eutomer has been separated to provide half the dosage required and maximize its relative effectiveness as an analgesic. The inactive isomer (distomer) was discarded in the hope of eliminating or reducing the potential side effects and avoiding any toxic side effects 6. Dexketoprofen oral dosage forms have been launched in Europe, USA, and Japan. To avoid the gastrointestinal bleeding induced by acid stimulation, a dexketoprofen injection was developed. Both the oral and parenteral preparations are very important agents for the management of inflammation and treatment of pain. However, these preparations have usually been applied to treat diseases which are less serious because of their limited efficiencies. To solve this problem, a dexketoprofen emulsion delivery system was developed in this paper based on the pathological characteristics of the inflamed tissues.

During inflammation or pain secondary to inflammation, the blood vessels in the impaired tissues differ from normal ones because of evident extension and increased permeation. These differences provide an opportunity for targeted drug delivery. Lipid emulsions (oil-in-water), also known as lipid microspheres, can be used to achieve such delivery. Such emulsions are colloidal dispersion systems with a particle size of about 200 nm, and are composed of soybean oil and lecithin. The particle size in this range confers many interesting properties. Also, they can distinguish between the lesion region and the normal one due to their permeation and uptake prioperties. After administration, the emulsion drops with loaded drug can concentrate at the inflammed lesions, vascular lesions, tumor cells, and reticuloendothelial cells 7, thereby improving the treatment efficacy.

In this study, a lipid emulsion delivery system was used to improve the anti-inflammatory and anti-nociceptive effect of dexketoprfen. The pharmacokinetic and pharmacodynamic characteristics of this dexketoprfen emulsion were evaluated, and compared with those of a dexketoprfen injection solution.

Materials and methods

Materials

The dexketoprfen injection solution (DS) containing 25 mg/mL dexketoprfen, 17 mg/mL L-arginine and water for injection was provided by our laboratory. It was prepared as follows: L-arginine was dissolved by water for injection and then dexketoprofen was added. After decolorizing with 0.1% active carbon and filtrating by 0.22 µm microporous membrane, this solution was sterilized in an 121°C autoclave for 10 min. The oils used in this experiment were medium-chain triglyceride (MCT; Lipoid KG, Ludwigshafen, Germany) and long-chain triglyceride (LCT; TieLing BeiYa Pharmaceutical Co., Tieling, China). The emulsifier egg yolk lecithin PL-100M (PC >80%) was purchased from Q.P. Corporation (Tokyo, Japan). Poloxamer 188 (Pluronic F68) (BASF AG, Ludwigshafen, Germany), glycerol (Zhejiang Suichang Glycerol Plant, Zhejiang, China), sodium oleate (Nation Drug Group Chemical Agents Ltd., Co., Shanghai, China) were used for the preparation. Xylene and acetic acid were purchased from Tianjin Baishi Chemical Industry Co., Ltd. (Tianjin, China) and Tianjin Bodi Chemical Holding Co., Ltd. (Tianjin, China), respectively. Ammonium acetate was purchased from Dima Technology Inc. (Richmond Hill, USA). All other chemicals and reagents used were of analytical or chromatographic grade.

All the animals used in this study were purchased from the Experimental Animal Center (Shenyang Pharmaceutical University, Shenyang, China). The experimental protocol was evaluated and approved by the University Ethics Committee for the use of experimental animals and conformed to the Guide for the Care and Use of Laboratory Animals.

The animals were kept in plastic cages and housed under standard condition of 12 h light–dark cycles with ad libitum access to food and water during the quarantine period. Food was withdrawn 12 h prior to the experiments. The number of animals was the minimum compatible with obtaining consistent effects of the drug treatments.

Methods

Preparation of oil/water emulsion with DE

Dexketoprofen was first esterified to produce its prodrug, dexketoprofen isopropyl ester (DE) which was then incorporated into a lipid emulsion (DEE) by high-pressure homogenization 8, 9. In brief, DE was dissolved in a 10% w/v oil phase (the MCT and LCT were both 5%) at 80°C, in which the emulsifier egg yolk lecithin PL-100M (1.8%) had already been uniformly dissolved. The aqueous phase consisting of glycerol (2.25%), F68 (0.4%), and sodium oleate (0.1%) was uniformly dispersed at 80°C in a water bath and then the coarse emulsion was prepared by high shear mixing at 10 000 rpm (ULTRA TURRAX T18 basic, IKA WORKS Guangzhou, China) and rapidly adding the aqueous phase to the oil phase. The high shear mixing process was carried out for 3 min and repeated three times. Then, the volume was adjusted to 100 mL with purified water and the pH was adjusted to 7.5 with 0.1 mol/L HCl or 0.1 mol/L NaOH. The final emulsion was obtained by high-pressure homogenization (Nano homogenizer AH100D, ATS Engineering Inc., Shanghai, China) at 800 bar for eight cycles. The temperature of the whole homogenization process was kept below 40°C in an ice-water bath. Finally, the emulsion was transferred to vials after adding nitrogen gas and sterilized in a 121°C autoclave for 10 min. The concentration of DEE was 25 mg/mL (based on dexketoprofen).

Characteristics of DEE (particle size and zeta potential)

The particle size and zeta potential were measured by a laser dynamic light-scattering particle sizer, Nicomp™ 380 particle sizing system (Santa Barbara, USA) which was based on the principle of photon correlation spectroscopy (DLS) and electrophoretic light scattering (ELS), respectively. Samples were diluted with purified water appropriately before measurement and it was verified beforehand that dilution of the samples did not alter the size distributions 10.

Pharmacokinetic study of DEE

Sample collection and disposal

Rats weighing 180–220 g were divided into two equal groups of five rats per group. These groups received either DEE or DS at 5.25 mg/kg (based on dexketoprofen) via the tail vein. Serial blood samples (0.5 mL) were collected by puncture of the retro-orbital sinus in heparinized tubes according to the following schedule: 5 min, 15 min, 30 min, 1 h, 1.5 h, 2 h, 4 h, 6 h, 8 h, 12 h, and 24 h post-dosing. The heparinized blood samples were centrifuged immediately at 6000 rpm for 10 min to obtain plasma and the plasma samples were labeled and kept frozen at −20°C until analysis.

Twenty microliters ibuprofen methanol solution (100 µg/mL), the internal standard, and 100 µL HCl solution (1 mol/L) were sequentially added to 200 µL plasma. The mixture was then vortexed for 1 min in a Liquid Fast Mixer, and then extracted with 3.5 mL ether by vortexing for 10 min. After centrifugation for 15 min at 4000 rpm, the supernatant (3 mL) was transferred to a clean tube and evaporated to dryness. The residue was reconstituted in 200 µL methanol. After vortexing for 10 min and then centrifugation for 10 min at 12 000 rpm, a 5 µL aliquot of the solution was injected into the ultra performance liquid chromatography (UPLC)-MS/MS system.

Blood sample analysis

Chromatography was performed on an ACQUITYTM UPLC system (Waters Corp., Milford, MA, USA) with a conditioned autosampler at 4°C, and a column temperature maintained at 35°C. The separation was carried out on an ACQUITY UPLCTM BEH C18 column (2.1 mm × 50 mm inside diameter, 1.7 µm; Waters Corp., Milford, MA, USA), and using a mobile phase consisting of acetonitrile and 2 mmol/L ammonium acetate solution (80:20 v/v). The gradient conditions are shown in Table 1. The injection volume was 5 µL and the partial loop mode was selected for sample injection. A Waters ACQUITYTM TQD triple-quadrupole tandem mass spectrometer (Waters Corp., Manchester, UK) was connected to the UPLC system involving the negative ion (NI) ESI mode. The capillary voltage was 1.6 kV, and the extractor and RF voltages were 3.0 and 0.1 V, respectively. The temperature of the source and desolvation were set at 100 and 400°C, respectively. Nitrogen was used as the desolvation gas (550 L/h) and cone gas (50 L/h). For collision-induced dissociation (CID), argon was used as the collision gas at a flow rate of 0.18 mL/min. The multiple reaction monitoring (MRM) mode was used for quantification. Dexketoprofen and ibuprofen (IBU) responses were optimized at the transitions of 253.17>209.11 and 205.24>161.14, respectively. All data collected in the centroid mode were acquired using Masslynx™ NT4.1 software (Waters Corp., Milford, MA, USA). Post-acquisition quantitative analyses were performed using a QuanLynx™ program (Waters Corp.).

Table 1. Gradient conditions for UPLC
InletTime (min)Flow rate (mL/min)A (%)aB (%)bCurve
  • a

    2 mmol/L ammonium acetate water.

  • b

    Acetonitrile.

1Initial0.2005050
21.00.20020806
31.80.20020806
42.20.20050506
53.00.20050506

The validation of analytical method for DE showed that the method was precise and accurate with a linear range of 0.05–80 µg/mL. The mean recovery of DE from plasma in the quality control samples (0.1, 10, and 64 µg/mL) was 71.97 ± 4.45%, 70.53 ± 2.18%, and 59.91 ± 1.41%, respectively.

Pharmacodynamic study of DEE

Egg-albumin induced paw edema in rats

Rats (180–220 g) were divided into three groups (n = 6). Before treatment, the circumference of ankle joint of the right hindpaw was measured as the zero time circumference. Peripheral inflammation was induced by intraplantar injection of 10% egg-albumin solution (0.1 mL) into the middle of the plantar surface of the right hindpaw, 15 min after i.v. injection of DEE (5.25 mg/kg based on dexketoprofen), DS (5.25 mg/kg), or physiological saline (as the control group) into the tail. The circumference of the ankle joint of the right hindpaw was measured at 30 min, 1 h, 2 h, 3 h, and 4 h after injection of the 10% egg-albumin solution.

equation image

C1, circumference before administration, C2, circumference after administration.

Xylene-induced ear swelling in mice

The mice (18–22 g) were placed into three random groups (n = 6), and each animal received 50 µL xylene on the anterior and posterior surfaces of the right ear lobe 0.5 h after intravenous injection of DEE (7.58 mg/kg based on dexketoprofen), DS (7.58 mg/kg), or physiological saline (as the control group) into the tail. The left ear was considered as a control. Two hours later, the animals were sacrificed by cervical dislocation and both ears were sampled. Circular sections were taken, using a cork borer with a diameter of 9 mm, and weighed immediately. The degree of ear swelling was calculated based on the weight of the left ear without application of xylene 11.

equation image
equation image

SD1, SD of the control group; SD2, SD of the test group.

Acetic acid-induced abdominal constriction in mice

This test was carried out using a technique described previously 12. Mice (18–22 g) were placed into three groups (n = 6) and given intraperitoneal injections of 0.1 mL/10 g body weight of 0.6% acetic acid solution in saline 0.5 h after injection of DEE (7.58 mg/kg based on dexketoprofen), DS (7.58 mg/kg), or physiological saline (as the control group) via the tail vein. Writhing was characterized by a wave of contraction of the abdominal musculature followed by the extension of the hind limbs. The frequency of writhing observed was recorded 20 min after the injection of acetic acid.

equation image

Wc, writhing count of the control group; Wt, writhing count of the test group.

Hot-plate test in mice

The hot-plate test was performed using a modification of a published method 13. Female mice (18–22 g) were habituated twice to the hot plate 2 h prior to the test. For the test, female mice were placed on a hot plate maintained at 55 ± 5°C. The time that elapsed until the occurrence of either a hind paw licking or a jump off the plate surface was recorded as the hot-plate latency. Mice were placed in three groups (n = 10), and those with baseline latencies of <10 or >30 s were removed from the study. After the baseline determination of the response latencies, the hot-plate latencies were measured at 30 min, 1 h, 2 h, 3 h, and 4 h after intravenous injection of DEE (7.58 mg/kg based on dexketoprofen), DS (7.58 mg/kg), or physiological saline (as the control group) into the tail. A value of 60 s was recorded if animals made no characterized action until 1 min, and animals stayed no more than 1 min on the hot-plate avoiding dermal damage every time.

equation image

T1, action time before administration; T2, action time after administration.

Statistical analysis

Results in this study were expressed as mean ± SD. Data were analyzed by a one-way ANOVA. p<0.05 was considered to be significant.

Results and discussion

Preparation and evaluation of DEE (particle size and zeta potential)

The lipophilicity of the active pharmaceutical ingredient (API) is crucial for its solubility in oils for the preparation of lipid emulsions. However, dexketoprofen was only sparingly soluble in pharmaceutical oils due to its polar carboxy group. So it was rational to increase the lipophilicity of dexketoprofen by esterifying the carboxy group with isopropyl alcohol. In this study, the carboxy group was first converted to acyl chloride with thionyl chloride, and then the active intermediate was reacted with isopropyl alcohol to form dexketoprofen isopropyl ester (DE). The log P oil/water value of DE was 3.3, which confirmed its excellent partition into oil. Its solubility in oil (LCT:MCT = 1:1) was also greatly improved and it dissolved immediately in oil at any ratio. However, dexketoprofen could only dissolve in oil at a ratio of <1:50 with long time stirring. These results show that the physical characteristics of dexketoprofen were changed after esterification and DE was more suitable for loading into an emulsion as a prodrug.

DEE was prepared by the high-pressure homogenization method widely described in the literature 14, 15. The formulation and preparation parameters were examined. Finally, egg yolk lecithin PL-100M was selected as an emulsifier at a concentration of 1.8%, the oil phase was 5% LCT, and 5% MCT, and the operation pressure was 800 bar and eight cycles were used.

It is known that the particle size and zeta potential are two very important parameters for emulsions. Different particle sizes result in different distributions in vivo while a suitable zeta potential is a prerequisite for long-term storage and to allow the emulsion drops to remain stable in blood.

The particle size was 208.1 ± 33.1 nm and emulsions of this size distribute to organs rich in reticuloendothelial cells, such as the liver and spleen. Moreover, drops of this size are able to be retained by injured vessels. The zeta potential was determined to be −34.81 mV.

Pharmacokinetic study of DEE

A pharmacokinetic study was carried out to compare the in vivo difference between DEE and DS. The plasma concentration–time curves are shown in Fig. 1. It can be seen from this figure that the two dosage forms exhibited similar profiles. During the first 4 h, the DEE group exhibited a slightly lower plasma concentration than the DS group. This may be due to the particle size of DEE and the plasma protein binding of dexketoprofen. As a colloidal dispersing system, DEE could concentrate in liver and spleen which is rich in reticuloendothelial cells and be metabolized rapidly. This led to a reduced concentration of dexketoprofen in the blood. As far as DS is concerned, more dexketoprofen tended to distribute in the blood because of its high degree of blood protein binding (99%). Based on this result, it can be presumed that DEE and DS may have different fates after injection, which possibly leads to their different distribution in inflammed tissues. As a matter of fact, this hypothesis has been proved by some literature reports and was validated by the following pharmacodynamic experiment 16, 17.

Figure 1.

Mean plasma concentration–time profiles after i.v. administration of DEE and DS to rats (n = 5). ▪, DEE; ♦, DS.

The pharmacokinetic data were analyzed using drug and statistics (DAS) version 2.0 software (Mathematical Pharmacology Professional Committee of China, Shanghai, China). The pharmacokinetic parameters of DEE and DS after intravenous (i.v.) injection are listed in Table 2 and no significant differences were found (p>0.05). The AUC0–t values of DEE and DS were 57266.24 ± 10034.05 and 69716.68 ± 9005.70 ng h/mL, respectively. The results calculated by DAS 2.0 showed that DEE and DS were bioequivalent.

Table 2. Pharmacokinetic parameters of DEE and DS in rats after i.v. administration (n = 5)
ParametersDEEDSStatistical analysis between parameters
AUC0–t (ng h/mL)57266.24 ± 10034.0569716.68 ± 9005.70p>0.05
Ke (1/h)0.13 ± 0.050.14 ± 0.07p>0.05
T1/2 (h)5.89 ± 1.715.58 ± 1.93p>0.05
CL (L/h/kg)0.09 ± 0.020.07 ± 0.01p>0.05
V (L/kg)0.76 ± 0.280.60 ± 0.26p>0.05

Pharmacodynamic study of DEE

Anti-inflammatory study

The experiment involving egg-albumin induced paw edema in rats was used to compare the anti-inflammatory performances of DEE and DS. The hind paw edema–time curve is shown in Fig. 2. After stimulation by the short-acting inflammatory agent, egg-albumin, the hind paw exhibited marked swelling at 0.5 h, which then decreased gradually over the next few hours. Compared with the control group, rats in the DEE group exhibited a low initial degree of swelling (p<0.01) and their paws recovered quickly (p<0.01 at 0.5, 1, and 3 h, p<0.05 at 2 and 4 h). Although the rats in the DS group also exhibited a lower degree of paw edema at 0.5 and 1 h, the subsequent restoration was relatively slower and was not significantly different from the control group. As far as comparison of the DEE and DS groups was concerned, the former exhibited less edema from 1 to 3 h (p<0.05), indicating that DEE was more effective in the treatment of inflammation induced by egg-albumin.

Figure 2.

Anti-inflammatory effects of DEE and DS on egg-albumin induced rat hind paw edema (n = 6). ♦, DEE group; ▴, DS group; ▪, control group. Δp<0.05 compared with the control group, ΔΔp<0.01 compared with the control group, *p<0.05 compared with the DS group.

To evaluate the anti-inflammatory effect of DEE in other experiment animals, mice were selected and the xylene-induced ear swelling test was carried out. Two hours after smearing with xylene, the degree of swelling in the control group, DS group and DEE group was 15.4 ± 3.7, 11.0 ± 3.0, and 6.5 ± 2.9 mg, respectively. As can be seen from Table 3, the ear edema in DEE group was significantly lower compared with the DS group (p<0.05). The inhibition rates for the DEE and DS group were 57.79% and 28.57%, respectively, which showed that DEE had twice the anti-inflammatory effect compared with DS.

Table 3. Anti-inflammatory effects of DEE and DS on xylene-induced ear swelling in mice (n = 6)
GroupSwell degree (mg)Inhibition rate (%)
  • Δ

    p<0.05 compared with the control group.

  • ΔΔ

    p<0.01 compared with the control group.

  • *

    p<0.05 compared with the DS group.

Control15.4 ± 3.7
DS10.0 ± 3.0Δ28.57
DEE6.5 ± 2.9ΔΔ,*57.79

The pharmacodynamic differences between DEE and DS in these two experiments may be the result of the pathological features of the inflammation and distribution differences between the two preparations. Vascular reactions always occur in the early stage of the inflammation process. In this stage, the inflamed tissue produces many kinds of inflammatory mediators, such as prostaglandins (PGs), bradykinin, and histamine. These substances act on the endothelial cells of the blood vessels, resulting in the shrinkage of the endothelial cells and the formation of endothelial cell gaps. In addition, other mechanisms, such as leukocyte-mediated endothelial cell injury, also lead to enhanced local vasopermeability. After i.v. administration of DEE, the emulsion drops pass into the blood circulation. Once they reach the injured vessels, they are entrapped, and accumulate there (see Fig. 3). Then, some of the drops pass from the basement membrane to the out layer of the vessels and accumulate in the inflamed tissues, as has been proved in the literature 16, 17. The DE loaded in the DEE is degraded to dexketoprofen by esterases, and then the activity of the cyclooxygenase in inflamed tissue is inhibited and, thereafter, the amount of PGs and the swelling of tissues were gradually reduced. For the DS, its distribution was relatively regular and no targeted effect was seen.

Figure 3.

Illustration of the accumulation process for DEE in inflamed tissue.

Anti-nociceptive study

The acetic acid-induced abdominal constriction experiment was used to evaluate the anti-nociceptive effect of DEE, in comparison with DS. As shown in Table 4, the writhing count of the control group, DS group and DEE group was 18.83 ± 3.54, 13.17 ± 2.64, and 6.33 ± 1.21, respectively. The DS group had a limited anti-nociceptive effect with a pain-inhibition rate of 30.06%. However, DEE significantly restrained the writhing responses with a pain inhibition rate of 66.38%.

Table 4. Anti-nociceptive effects of DEE and DS on acid-induced abdominal constriction in mice (n = 6)
GroupWrithing countPain-inhibition rate (%)
  • Δ

    p<0.05 compared with the control group.

  • ΔΔ

    p<0.01 compared with the control group.

  • **

    p<0.01 compared with the DS group.

Control18.83 ± 3.54
DS13.17 ± 2.64Δ30.06
DEE6.33 ± 1.21ΔΔ,**66.38

In fact, the experiment involved two analgesic stages. During the first stage, pain resulted from the direct stimulation of acetic acid. Then, the low pH injured the viscus and parietal peritoneum, and caused inflammatory pain in deep tissue. Because DEE has a stronger anti-inflammatory effect, it could relieve the pain secondary to inflammation more quickly. This speculation was supported by the observation in the experiment that the writhing counts were similar in the first 5 min between the DEE and DS groups, but different in the period that followed.

It is generally assumed that the analgesic effects of nonsteroidal anti-inflammatory drugs (NSAIDs) are due to inhibition of PG synthesis in the peripheral inflamed tissue 18, as shown in the earlier test. Nevertheless, some researchers have argued that dexketoprofen is also effective in normal non-inflamed animals by reducing the wind-up of nociceptive reflexes in the spinal cord 19, 20. Therefore, the hot-plate experiment was selected subsequently because no inflammation would occur in this experiment and only nociceptive reflexes were involved in the pain process.

The results of the hot-plate test are shown in Fig. 4. Because of the physiological adaptation, the pain threshold value of the control group increased approximately 10% after 0.5 h. From 0.5 to 3 h, the pain threshold values were significantly increased in the DEE and DS groups (p<0.01). Compared with the DS group, the degree of increase in the DEE group was slightly higher, although the difference was not statistically significant. These results indicate that both the dexketoprofen preparations had similar analgesic effects in animals without inflammation. In this type of pain, the analgesic mechanism may involve the following: dexketoprofen could reduce the activity of cyclooxygenase in the spinal cord and then reduce the wind-up of nociceptive reflexes, thereby reducing the nociceptive behavior in animals. The supraspinal effect may also be involved in this process 21, 22. Therefore, it appears that if the amount of dexketoprofen crossing the blood-brain barrier was increased, the analgesic effect would also be increased. Several researchers have reported that the penetration of drugs across the blood-brain barrier could be increased by loading them into lipid colloidal delivery systems 23, 24. Unfortunately, this phenomenon was not observed in our study. One possible reason may be that dexketoprofen itself could pass through the blood-brain barrier easily 25, 26, which masked the latent enhancement in the penetration ability produced by the emulsion.

Figure 4.

Anti-nociceptive effects of DEE and DS in the hot-plate experiment in mice (n = 10). ♦, DEE group; ▪, DS group; ▴, control group. ΔΔp<0.01 compared with the control group.

In summary, DEE has a strong analgesic effect on pain secondary to inflammation compared with DS, but it has a similar analgesic activity with regard to central pain.

Conclusions

To improve the solubility in oil, dexketoprofen was esterified with isopropyl alcohol and then incorporated into a lipid emulsion using high pressure homogenization technology. After administration by i.v., the AUC0–t value of DEE and DS was 57266.24 ± 10034.05 ng h/mL and 69716.68 ± 9005.70 ng h/mL, respectively. The DEE group exhibited a slightly lower plasma concentration than the DS group, however the difference found in this study was not statistically significant. We carried out anti-inflammatory and anti-nociceptive studies to evaluate the pharmacodynamic superiority of DEE. Compared with DS, DEE exhibited a higher anti-inflammatory activity in the experiments involving egg-albumin-induced paw edema in rats and xylene-induced ear swelling in mice, possibly resulting from its targeted delivery to the inflamed tissues. The results of the anti-nociceptive experiments were complicated. DEE displayed a higher pain-inhibition rate in the acetic acid-induced abdominal constriction experiment while this was not the case in the subsequent hot-plate experiment. It can be deduced that DEE has a marked analgesic effect in secondary pain due to inflammation while it has a similar analgesic activity in central pain, compared with DS. In conclusion, the anti-inflammatory and anti-nociceptive activities of dexketoprofen were markedly improved by its incorporation into a lipid emulsion delivery system.

Acknowledgements

Support from the Program for Liaoning Excellent Talents in University (LNET) is acknowledged. Dr. David B. Jack is gratefully thanked for correcting English of the manuscript.

The authors have declared no conflict of interest.

Ancillary