Most patients who experience moderate to severe pain, e.g., postoperative pain, posttraumatic pain, and burn pain, often require analgesics in the first 3 days after injury (1–6). An analgesic with a long-acting effect of approximately 2–3 days may be particularly valuable for these patients (1–3). Ketorolac is a potent nonsteroidal antiinflammatory drug (NSAID), which has an analgesic activity similar to opioids. It is an attractive alternative to opioids for the control of pain (4–10) and has been used frequently in the postoperative setting in both hospital inpatients and outpatients and in patients with various pain states (4–10). An injection of ketorolac 10–30 mg IM has similar analgesic efficacy as that of morphine 6–12 mg and pethidine 50–100 mg IM (4–10). Although ketorolac has a potent analgesic effect, its duration of action is relatively short (4–6 h after 10–30 mg IM). Prolonging its duration of action would make ketorolac more clinically valuable for treating pain.
Depot formulation with prodrug design is one of the methods used to increase the duration of a short-acting drug (11–13). It is produced by esterifying a drug to form a bioconvertable prodrug-type ester and then formulating it in an injectable oily formulation (13,14), which forms a drug reservoir at the site of injection. After IM injection, a long-duration of action will occur (11–14). In our laboratory, several novel depots of ketorolac esters (Fig. 1), such as ketorolac propyl ester, pentyl ester, heptyl ester, and decyl ester, were synthesized and formulated. The aim of the study was to evaluate the antinociceptive and antiinflammatory effects and duration of action of these depots after IM injection in rats and to observe whether they had a long-acting effect.
Male Sprague-Dawley rats, purchased from the National Laboratory Animal Center, Taiwan, weighing between 200 and 250 g, were used. They were housed in groups of 3 at least 1 wk in a climate-controlled room maintained at 21°C with approximately 50% relative humidity. Lighting was on a 12-h light/dark cycle (lights on at 6:00 am), with food and water available ad libitum except during the time of testing (10 min on average). All tests were performed in accordance with the recommendations and policies of the International Association for the Study of Pain, and the protocol was approved by the animal investigation committee of Chi-Mei Medical Center.
Ketorolac tromethamine was purchased from Sigma (Saint Louis, MO). Ketorolac base was obtained from its tromethamine salt by using a method of precipitation. After adding 1 N of HCl drop-by-drop into a ketorolac tromethamine solution, ketorolac base was precipitated. This precipitate was then extracted with ethyl acetate and evaporated to dryness. The purity of the ketorolac base was assured (>99%) by checking the melting point and the gas chromatography.
Four ketorolac esters (Fig. 1)—ketorolac propyl ester, pentyl ester, heptyl ester, and decyl ester—were synthesized by using the method previously reported (15). In brief, the ketorolac base was reacted with the respective alcohol, i.e., propyl alcohol (Mallinckrodt Baker, Phillipsburg, NJ), pentyl alcohol (Kanto Chemical, Tokyo, Japan), heptyl alcohol (Fluka, Buchs, Switzerland), and decyl alcohol (Mallinckrodt Baker) in the presence of 4-dimethylaminopyridine (Sigma). Purities (>99%) were assured by elemental analysis, nuclear magnetic resonance spectroscopy, and gas chromatography with mass detector. Ketorolac tromethamine was prepared in either saline or in injectable sesame oil (Sigma). Ketorolac base and esters were prepared in sesame oil.
Four studies were performed. In Study 1, we evaluated the antinociceptive and antiinflammatory effects of the traditional dosage form of ketorolac tromethamine (in saline) with doses of 8, 24, 80, and 240 μmol/kg. The vehicle (saline) was used as control. In Study 2, we evaluated the antinociceptive and antiinflammatory effects of ketorolac tromethamine and its base form in sesame oil with a dose of 240 μmol/kg. The vehicle (sesame oil) was used as control. In Study 3, we evaluated the antinociceptive and antiinflammatory effects of ketorolac propyl ester in oil with doses of 80, 160, and 240 μmol/kg. In Study 4, we evaluated the antinociceptive and antiinflammatory effects of the four other ketorolac esters in oil with a dose of 240 μmol/kg. All the above medications were injected IM into the right hind legs (biceps femoris and semitendinosus) of rats. Each rat received only one injection. The injection volume was 0.1 mL. For each different treatment, six rats were included into the test.
In Study 1, we performed a 10-h study. One minute after IM injection of ketorolac tromethamine or its vehicle (saline) into the right hind leg, rats received intraplantar injection of 100 μL of 1% λ carrageenin into the left hindpaw (16–18). The injection was made with a Hamilton syringe and a 30-gauge hypodermic needle. The needle was inserted subcutaneously into the central part of the paw and advanced 6–8 mm proximally toward the tarsal region (16–18). After carrageenin injection, the antinociceptive and antiinflammatory effects were determined.
The antinociceptive effects of medications were evaluated by using the TSE Analgesia System (TSE Technical & Scientific Equipment GmbH, Bad Homburg, Germany). This system is designed to perform a rapid and accurate screening of analgesic drugs on the normal and inflamed paw of small laboratory animals, according to the Randall-Selitto method (16–19). During testing, the rat's left hindpaw was placed on a plinth, and an increasing pressure was applied to it by the sensor tip. This force was measured. The sensor was made from smooth plastic to prevent paw injury during testing. The baseline pressure of paw withdrawal was approximately 140–190 g. To prevent tissue injury, a cutoff pressure of 350 g was set.
The antiinflammatory effect of medication was assessed by measuring the changes in thickness (cm) of the central part of the left hindpaw after intraplantar injection of carrageenin (17). The paw thickness was measured by using a vernier caliper (No. 530-104, Mitutoyo, Kanagawa, Japan). To avoid the interference between the antinociceptive and antiinflammatory tests, 2 groups (n = 6 for each group) of rats were used after one of the medications: one group for the antinociceptive test, and another for the antiinflammatory test.
In Studies 2–4, we performed a series of 4-day studies. After intraplantar injection of carrageenin, paw edema and pain occurred gradually with a maximum intensity at 6 h and were then reduced gradually (16–18). To keep a similar condition of carrageenin-induced paw edema and pain at each testing day, Studies 2–4 were accomplished by conducting a series of 4 1-day studies consecutively (from Day 1 to Day 4). All rats received only one medication (one dose of ketorolac tromethamine, its derivatives, or the vehicle) IM at the start of study (Day 1) and then received one intraplantar injection of carrageenin at Day 1, 2, 3, or 4. Each rat received only one injection of carrageenin. After carrageenin injection, rats were observed for 8 h to determine either the antinociceptive or the antiinflammatory effect of the medications. The methods for determining the antinociceptive and the antiinflammatory effects were the same as those described in Study 1. Six rats were used for each day of each medication.
Values are expressed as mean ± sem. A two-way analysis of variance (ANOVA) with one-way repeated method was used in Study 1, whereas a three-way ANOVA with one-way repeated method was used in Studies 2–4. The Bonferroni test was used for post hoc analysis to evaluate the differences among groups, whereas the Dunnett test was used to evaluate the differences between the medication groups and the vehicle group at each time point. Bonferroni correction was used when appropriate. A P value <0.05 was considered significant.
After IM injection, ketorolac tromethamine (in saline) produced dose-related antinociceptive and antiinflammatory effects (Fig. 2 and Table 1). Ketorolac tromethamine 24–240 μmol/kg IM produced a significant antinociceptive effect (P < 0.05–0.01) with an onset time of 2 h and duration of 6–8 h (Fig. 2A and Table 1) and a significant antiinflammatory effect (P < 0.05–0.01) with an onset time of 2 h and duration of 8 h (Fig. 2B and Table 1). The antinociceptive effects among ketorolac tromethamine 24, 80, and 240 μmol/kg were significantly different (240 > 80 > 24 μmol/kg; P < 0.05 for each comparison; Fig. 2A) as were the antiinflammatory effects between ketorolac tromethamine 24 and 80 μmol/kg (80>24 μmol/kg; P < 0.05; Fig. 2B).
After IM injection, ketorolac tromethamine and its base form when prepared in oil did not demonstrate any significant antinociceptive and antiinflammatory effects when compared with the vehicle (sesame oil) group (Fig. 3).
After IM injection, ketorolac propyl ester 80, 160, and 240 μmol/kg (in oil) produced long-acting and dose-related antinociceptive and antiinflammatory effects (Fig. 4 and Table 1). Ketorolac propyl ester 160 and 240 μmol/kg produced significant antinociceptive and antiinflammatory effects with an onset time of 2 h and a duration of approximately 26–52 h Fig. 4 and Table 1). The antinociceptive and antiinflammatory effects among ketorolac propyl ester 80, 160, and 240 μmol/kg were significantly different (240 > 160 > 80 μmol/kg; P < 0.05 for each comparison; Fig. 4).
After IM injection of 240 μmol/kg (in oil), ketorolac esters such as pentyl ester, heptyl ester, and decyl ester also produced significant long-acting antinociceptive and antiinflammatory effects with an onset time of approximately 2–4 h and a duration of approximately 52–76 h (Fig. 5 and Table 1).
Several long-acting opioid analgesics have been developed for the treatment of moderate to severe pain (1–3). However, there have been no long-acting NSAID developed for this purpose. In our study, the antinociceptive and antiinflammatory effects of several novel depots of ketorolac esters were evaluated. We found that these depots produced significant antinociceptive and antiinflammatory effects with duration of action of approximately 52–76 hours, which were 6.5- to 9.5-fold longer than the traditional dosage form of ketorolac tromethamine (eight hours).
For the management of moderate to severe pain, e.g., postoperative pain, posttraumatic pain, and burn pain, ketorolac has been used for more than 10 years (4–10). Preoperative and intraoperative administration of ketorolac reduces pain and analgesic requirements in the immediate postoperative period (20–23). Postoperative administration of single or multiple doses of ketorolac 10–30 mg IV or IM have similar analgesic efficacy to IM morphine 6–12 mg, pethidine 50–100 mg, pentazocine 30 mg, or IV morphine 2–4 mg and greater efficacy than IM diclofenac 75 mg (4–10). Although ketorolac has a potent analgesic effect, its duration of action is relatively short. Prolonging the duration of action would make ketorolac more clinically valuable for treating pain.
Depot formulation with prodrug design is one method used to increase the duration of drugs (11–13). For example, several prodrugs, such as haloperidol decanoate, fluphenazine enanthate, estradiol cypionate, and testosterone undecanoate, are synthesized from their active drugs, namely haloperidol, fluphenazine, estradiol, and testosterone, by esterification (13,14,24). After IM injection, the depot formulations of these prodrug-type esters demonstrated a long duration of action (13,14,24). Esterification of drugs with various fatty acids results in an increase in their lipophilicity (11–13). When these prodrug-type esters are dissolved in injectable oils and given IM, long durations of actions are obtained because of their slow release from the oily vehicles (13). Once released from the vehicles within the muscle, esters will be hydrolyzed by esterases and become their active drugs (13,14,24). There are esterases in many tissues and organs, such as blood, brain, liver, lung, etc. Several prodrug-type esters of ketorolac had been previously synthesized, but none of them was prepared as a depot formulation for a long-acting effect (15). In our study, several ketorolac prodrug-type esters were synthesized and prepared as depot formulations. Our study further demonstrated that these depot formulations had long-acting effects.
Ketorolac tromethamine and its base, when prepared as depot formulations, did not demonstrate any significant antinociceptive and antiinflammatory effect. This was because both the ketorolac tromethamine and its base were almost insoluble in sesame oil. It was therefore, in these oil dosage forms, that most of the drug particles were suspended, and only a very small part of drug particles was dissolved. Pharmaceutically, a drug must be in dissolved form to be absorbed into the systemic circulation (11). However, the dissolved form of ketorolac in these oil preparations was quite small; therefore, the pharmacologic effects of these formulations were not significant (11).
The use of injectable vegetable oils (e.g., peanut oil, cotton seed oil, sesame oil, etc.) as vehicles for IM injection is allowed and well documented in the literature (13,14). Several clinically used long-acting drugs are formulated in these oils and injected IM (13,14), e.g., estradiol valerate, fluphenazine decanoate, fluphenazine enanthate, testosterone cypionate, and testosterone enanthate. Among the clinically available long-acting preparations, injectable sesame oil is one of the most frequently used vehicles (13,14,24). In our study, we followed this method, and the vehicle we used was injectable sesame oil.
Ketorolac is an attractive alternative to opioids for the control of acute pain (4–10). It lacks opioid-related adverse effects such as sedation, emesis, pruritus, or respiratory depression (4–10). However, as with other NSAIDs, ketorolac inhibits platelet function, and this feature limits its usefulness in patients who are prone to bleeding, e.g., postoperative bleeding (4–6). For this reason, the depots of ketorolac esters are designed to be used in patients who are not at risk of bleeding. They may be used in conditions similar to the use of ketorolac for acute pain management (4–10).
In clinical practice, IM ketorolac tromethamine 30 mg (80 μmol) given to an adult provides a six-hour duration of analgesic action (4–7). In our study, IM injection of 80 μmol/kg ketorolac tromethamine in rats had a six-hour duration of antinociceptive action. According to the ratio of 1 obtained from humans and rats (six hours/six hours), we estimate that IM injection of proper doses of ketorolac esters, such as propyl ester, pentyl ester, heptyl ester, and decyl ester, in humans may have a 2.2- to 3.2-day (52- to 76-h) duration of action. Because patients who experience moderate to severe pain, such as postoperative pain, posttraumatic pain, and burn pain, may need analgesics in the first three days after trauma, the novel depots of ketorolac esters may be a suitable alternative to traditional NSAIDs for these patients.
In conclusion, IM injection of novel depots of ketorolac esters in rats produced long-lasting effects on nociception and inflammation that were 6.5- to 9.5-fold longer than that of the traditional dosage form.
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