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The Pharmacokinetics and Pharmacodynamics of Bupivacaine-Loaded Microspheres on a Brachial Plexus Block Model in Sheep

Estebe, Jean-Pierre, MD; Le Corre, Pascal, PharmD, PhD; Du Plessis, Laure, PharmD; Chevanne, François, BS; Cathelineau, Guy, DD, PhD; Le Verge, Roger, PharmD, PhD; Ecoffey, Claude, MD

doi: 10.1213/00000539-200108000-00042
TECHNOLOGY, COMPUTING, AND SIMULATION: Society for Technology in Anesthesia: Research Report
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We evaluated bupivacaine-loaded microspheres (B-Ms) using a brachial plexus block model in sheep. In the first step, pharmacokinetic characterization of 75 mg bupivacaine hydrochloride (B-HCl) (IV infusion and brachial plexus block) was performed (n = 12). In the second step, a brachial plexus block dose response study of B-HCl was performed with 37.5 mg, 75 mg, 150 mg, 300 mg, and 750 mg. As a comparison, evaluations were performed using a 750-mg bupivacaine base (B). In the third step, evaluations of brachial plexus block were performed with B-Ms (750 mg of B as B-Ms) using two formulations, 60/40 and 50/50 (w/w %); drug-free microspheres were also evaluated. Toxicity evaluations were also performed after IV administration of B-HCl (750 mg and 300 mg), B-Ms (750 mg), and drug-free microspheres (30 mL over 1 min). As the B-HCl dose increased, the time of onset of block decreased and the duration of complete motor blockade increased at the expense of an increase in bupivacaine plasma concentrations. The time of maximum concentration appeared to be independent of the B-HCl dose. In brachial plexus block, a 37.5-mg dose of B-HCl did not induce motor blockade whereas a dose of 750 mg of B-HCl was clinically toxic. In the case of IV administration, doses of 300 mg of B-HCl were as toxic as 750 mg of B-HCl. Compared with the 75 mg of B-HCl administration for brachial plexus block, administration of 750 mg of B as B-Ms increased the duration of complete motor blockade without significant difference in maximum concentration. No significant clinical difference between the two formulations of B-Ms was demonstrated. The IV administration of B-Ms was safe. We conclude that the controlled release of bupivacaine from microspheres prolonged the brachial plexus block without obvious toxicity.

*Service d’Anesthésie Réanimation Chirurgicale; †Laboratoire de Pharmacie Galénique et Biopharmacie; and ‡Laboratoire des Biomatériaux en Site Osseux, Université de Rennes 1, Rennes, France

Presented, in part, at the American Society of Anesthesiology, Dallas, Texas, October 1999 and San Francisco, California, October 2000.

April 18, 2001.

Address correspondence and reprint requests to JP Estebe, MD, Service d’Anesthésie Réanimation Chirurgicale 2, Hôpital Hôtel Dieu: 2 rue de l’Hôtel Dieu, 35000, Rennes, France. Address e-mail to jean-pierre.estebe@chu-rennes.fr.

Various approaches to prolong the duration of local anesthetics for the management of both acute and chronic pain have been studied. Biodegradable microspheres are an attractive alternative to implants (1). Previous work in our laboratory has demonstrated the feasibility of prolonged spinal or epidural anesthesia with bupivacaine-loaded microspheres (B-Ms) (2,3). These drug delivery systems have been evaluated on single peripheral nerves (4,5) but never for brachial plexus blockade. The aim of this study was to evaluate the pharmacokinetics and pharmacodynamics of B-Ms after injection into the brachial plexus of sheep.

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Materials and Methods

There are extensive data concerning the pharmacology of local anesthetics after IV (6), epidural (7), intercostal (5), and brachial plexus administration (8) in sheep that are comparable in size and weight to humans.

Nonpregnant Lacaunes ewes (2.5 ± 1.0 yr, weighing 75 ± 12 kg) were included in accordance with the rules and guidelines concerning the care and use of laboratory animals and after approval of our local Animal Research Committee. Only animals in good health with no demonstrable motor deficits were included (INRA Number Fr 35 240 046).

Anesthesia was induced with IV thiopental (5–8 mg/kg). As described previously (7), blocks were performed using electrical stimulation through a 50-mm insulated needle (0.7-mm inner diameter; Stimuplex; B. Braun, Melsungen, Germany) for the injection of bupivacaine solutions, and via a 55-mm insulated needle (1.3-mm inner diameter; Contiplex A Set; B. Braun) for the administration of microspheres. The 30-mL solution was injected over a period of 1 min.

After recovery from brief general anesthesia, motor blockade was defined as the animal’s ability to support its own body weight on its forelimb. When an animal was lying down, the investigator was permitted to help it stand for evaluation. The motor block scale used was a modification of that suggested by Bromage et al. (9) as follows: Level 0, free movement of the animal using its forelimbs; Level 1, limited or asymmetrical movement of forelimbs to support the body and walk; Level 2, inability to support its own weight on forelimbs with detectable ability to move the forelimb; Level 3, total paralysis of forelimbs. A single investigator performed all observations. Motor activity was recorded every 5 min until Level 3 was confirmed with three successive measurements; mea-surements were then taken at 30-min intervals. The onset of motor blockade was defined as the time from the end of injection to the time to reach the Level 3. The duration of motor blockade was defined as the time from the onset of motor blockade until the animal regained the ability to support its own weight (confirmed with three successive measurements of Level 0). Because of the wide distribution of nerves, interspecies differences, and the lack of a standard methodology, sensory blockade was not evaluated in this study.

Bupivacaine was encapsulated as a base (B) obtained from a bupivacaine hydrochloride (B-HCl) form (Laboratoires Astra-Zeneca; Nanterre, France). Microspheres were prepared using a spray-drying method with polylactide-co-glycolide polymer (Resomer: RG503H 50:50; Boehringer Ingelheim, Saint-Germain-en-Laye, France). Two different weight ratios between B and polymer were prepared: 60/40 and 50/50 (w/w %), respectively. Drug content was evaluated by high-performance liquid chromatography (10). In vitro release studies were performed to mea-sure the cumulative release of bupivacaine. Mean dissolution time (Td) was derived from the fit of the percent released-time plots. For in vivo evaluation, microspheres were dispersed in 30 mL of an aqueous phase containing 5% mannitol and 0.05% Tween 20. Microspheres were freeze-dried under vacuum and maintained at 4°C until administration; they were suspended in 30 mL of sterile water immediately before injection.

The pharmacokinetic and pharmacodynamic evaluations were divided into four parts (experimental design is summarized in Table 1 for each animal).

Table 1

Table 1

The first stage consisted of the study of B-HCl after IV infusion (75 mg of B-HCl, 50 mL over 15 min) and after brachial plexus administration (75 mg of B-HCl, 30 mL over 1 min) were performed (n = 12).

In the second step, a dose-response study was performed with a range of doses of B-HCl (0.125% to 2.5%, i.e., 37.5 mg, 75 mg, 150 mg, 300 mg, and 750 mg;n = 3 in each group). As comparison, evaluations of brachial plexus block were performed with 750 mg of bupivacaine B as suspension (n = 4).

In the third step, B-Ms were evaluated after brachial plexus administration (750 mg of bupivacaine as B-Ms). Two polylactide-co-glycolide microspheres formulations were evaluated (60/40, n = 8; 50/50, n = 4). Drug-free microspheres were also evaluated as control (n = 4).

At the end of study, toxicity evaluations were performed using IV B-HCl (750 mg and 300 mg, n = 3 in each group), B-Ms (750 mg, n = 4), and drug-free microspheres (30 mL over 1 min, n = 4).

Venous blood samples were drawn from a catheter immediately before injection and then at intervals of 1, 3, 5, 8, 10, 15, 20, 30, 45, 60, 90, 120, 150, 180, 210, 240, 270, and 300 min and then each 60 min until complete recovery. The measurement of plasma bupivacaine concentration was performed by high-performance liquid chromatography (10). The limit of detection was 2 ng/mL; within-day and the between-day reproducibilities were 2.1% and 5.6%, respectively.

After IV infusion administration, a model with one and two-exponential functions and first-order elimination from the central compartment was fitted to the bupivacaine plasma concentrations. In all cases, the biexponential function provided a better fit. Total body clearance (CL), apparent volume of distribution of the central compartment (Vc), apparent volume of distribution at steady-state (Vss), distribution rate constants (K12 and K21), the elimination rate constant (K10), and apparent distribution and elimination half-lives (t1/2 α and t1/2 β) were all calculated.

The individual absorption kinetics of bupivacaine after brachial plexus administration of 75 mg B-HCl were evaluated. The values of the distribution rate constants (K12 and K21) and elimination rate constant (K10) were derived from those obtained after the IV infusion administration (postinfusion curve was used with a mathematical derivation to determine the rate constant identical to IV bolus injection curve) (11). To study the absorption kinetics from the brachial site (one or two phases), the amount of bupivacaine remaining to be absorbed (%) was adapted according to a one and biexponential function. The overall absolute bioavailability (F) of bupivacaine after brachial dosing was calculated from the ratio of the area under the plasma curve (AUC) after brachial and after infusion dosing. The rapid (T1/2 abs-1) and slow (T1/2 abs-2) absorption half-lives as well as the extent of absorption corresponding to rapid (F1) and slow (F2) phases were calculated. After the brachial plexus bupivacaine administration, the peak plasma concentration (Cmax) and corresponding time to peak concentration (Tmax) were derived from raw data. For each set of individual plasma concentration data, obtained after administration of bupivacaine, the apparent elimination half-life (t1/2 β) was determined. The AUC was calculated (zero to the last sampling point = AUCt-last and zero to infinity = AUC-inf).

Results are presented as mean ± sd. Data were analyzed using the analysis of variance followed by the paired Student’s t-tests with Bonferroni correction for pharmacokinetic analysis and Wilcoxon’s test for pharmocodynamic analysis as needed when the same animals were used. The Mann-Whitney U-test and unpaired Student’s t-tests were performed to make a comparison between two groups. Statistical significance was defined as P < 0.05.

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Results

The brachial plexus response to the nerve stimulation was reached in <5 min and within 5 cm depth in the neck. No inadvertent vascular injection occurred and no hematomas were recorded during the 56 stimulations performed by a single operator. One to two minutes were usually sufficient to recover from anesthesia and to allow motor evaluation.

The time course of median motor block response with 37.5, 75, 150, and 300 mg of B-HCl is depicted in Figure 1A and Table 2. Despite the small number of animals in the Dose Response group (n = 3, in each group), significant differences in the duration and complete recovery time of motor blockade were observed between different doses. B-HCl at a dose of 37.5 mg did not induce motor blockade. Within this range of doses (37.5 to 300 mg of B-HCl) no apparent signs of cardiac electrocardiogram (ECG) or central nervous system toxicity were recorded. With the injection of 750 mg of B-HCl into the brachial plexus (n = 4) hypotonia and drowsiness were extensive, precluding clinical evaluation. With 750 mg of B, the preparation of a homogenous and stable suspension was impossible to obtain (some material remained in the flask after withdrawing for injection). The mean onset time and duration of complete motor blockade are summarized in Table 2.

Figure 1

Figure 1

Table 2

Table 2

After brachial plexus injections of drug free microspheres (n = 4) motor blockade was not observed. With 750 mg of B-Ms, no significant difference between the two formulations (50/50 vs 60/40) was demonstrated (Table 2). The onset time to complete motor blockade was 10 ± 2 min. The duration of complete motor blockade was 23 ± 1 h and complete recovery was seen at 31 ± 1 h (Fig. 2A). No apparent signs of cardiac or central nervous system toxicity were recorded after B-Ms injection.

Figure 2

Figure 2

The IV infusion of B-HCl 75 mg (over 15 min) was uneventful. When the largest doses of B-HCl (750 and 300 mg;n = 3 in each group) were administered as a bolus, the death of the test sheep occurred before the injection could be finished. A seizure occurred in one case after an IV administration of 300 mg B-HCl. No apparent sign of cardiac or central nervous system toxicity was recorded in the case of IV drug-free microspheres or after IV injection of 750 mg of B-Ms.

The experimental drug-content values in microspheres were close to the theoretical drug-content values. In vitro release kinetic profiles of B-Ms showed significant differences (Fig. 3) related to the drug/polymer ratio (mean AUC0–12h: 1300 and 820 for 50/50 and 60/40 w/w ratio, respectively). Both formulations displayed a biphasic pattern with a burst effect (in the first 2 h) followed by a slow release phase.

Figure 3

Figure 3

The plasma concentration time-course of bupivacaine after IV infusion is illustrated in Figure 4 (CL = 1.89 ± 0.44 L/min; Vss = 164 ± 36 L). The time course displayed a biphasic pattern characterized by a rapid distribution phase (T1/2 α = 9.6 ± 4 min) and an apparent elimination half-life of 121 ± 26 min. Pharmacokinetic data for brachial plexus administration are depicted in Figure 4. The mean t1/2 β was 299 ± 157 min. The mean overall bioavailability after brachial administration of plain bupivacaine was low (54.5 ± 30.4%). Absorption kinetic variables indicated that approximately 35% (F1) of bupivacaine was absorbed during the rapid phase of absorption (T1/2 abs 1 = 26.5 ± 9.9 min) and 22% (F2) was absorbed during the slow phase (T1/2 abs 2 = 192.8 ± 155 min).

Figure 4

Figure 4

After B-HCl administration in the brachial plexus, Cmax increased with the dose (95 ± 31, 112 ± 52, 584 ± 268, and 1278 ± 1096 ng/mL for 37.5 [n = 3], 75 [n = 15], 150 [n = 3], 300 mg [n = 3], respectively). Tmax appeared to be independent in the case of the B-HCl dose (22 ± 20, 30 ± 16, 38 ± 25, and 35 ± 30 min for 37.5, 75, 150, and 300 mg, respectively)(Fig. 1B). With a brachial dose of 750 mg of B-HCl, Cmax was 1105 ± 764 ng/mL whenever sheep were completely hypotonic.

The instability of the B 750 mg suspension made it impossible to administer the whole dose (n = 4). Consequently, the actual dose administered was in average 450 ± 165 mg. The mean Cmax was 195 ± 105 ng/mL and the Tmax was 470 ± 503 min.

After brachial plexus administration of the 60/40 B-polymer formulation, the Cmax obtained (71 ± 6 ng/mL) was lower than that obtained with the 50/50 B-polymer formulation (125 ± 54 ng/mL) in accordance with the in vitro differences in the release rates. This tendency was noticeable in the first few hours of in vitro release (Td = 849 ± 874 and 686 ± 579 min for 50/50 and 60/40 formulations, respectively). After administration for both microspheres formulations, the plasma profiles displayed an apparent plateau lasting approximately 25 h, suggesting a constant input rate of B (Fig. 2B).

At the end of IV injection when the death of animals occurred (5/6), Cmax were 25,484 ± 12,731 and 4891 ± 361 ng/mL after 750 and 300 mg of B-HCl. When 750 mg of B-Ms were IV administered, Cmax was 2113 ± 1489 ng/mL and Tmax was 161 ± 181 min (Fig. 5).

Figure 5

Figure 5

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Discussion

The pharmacokinetic IV variables were in agreement with data previously reported (12) and were close to those demonstrated for human subjects (CL = 0.58 L/min; Vss = 73 L) (13). We evaluate the pharmacokinetics of bupivacaine as B-HCl after IV and brachial plexus administration at the same dose in the same animal (n = 12). After brachial plexus administration of 75 mg B-HCl, the absorption data from these animals were best characterized by biphasic absorption similar to that reported in monkeys after epidural administration (14) or after axillary plexus block in humans (15). The t1/2 β of B-HCl was longer than after IV administration, an observation that had been reported for human (16–18) and animal models (7). This longer t1/2 β might indicate that the elimination rate of bupivacaine is absorption-limited after brachial plexus administration. Such a “flip-flop” phenomenon has been shown for ropivacaine and bupivacaine after administration into brachial plexus (15). Despite the small number of animals in the Dose-Ranging group, we were able to approach the effect of B-HCl dose response. As in humans, a 0.125% (37.5 mg) concentration of B-HCl is unsuitable. The increasing B-HCl dose induced a decrease in the onset and an increase in the duration of motor blockade at the expense of an increase in bupivacaine plasma concentrations. However, although Cmax increased with the dose, Tmax appeared to be independent, thereby suggesting that the rate of absorption was not dose-dependent. The onset of motor block was more rapid than has been seen in human studies. When a 750-mg B-HCl dose was used for brachial plexus administration, clear reabsorption toxicity occurred. Bupivacaine toxicity is well known and is similar in humans (19) and in sheep (6,20–21). At the lower rate of administration (brief infusion and prolonged short infusion), the bupivacaine dose required to produce seizures in sheep was 5 to 8 mg/kg, corresponding to maximum serum concentration 6.0 to 7.5 μg/mL (6,19–20). The total dose seemed to be the predominant factor of block duration. A 300-mg B-HCl brachial dose led to a large peak plasma concentration that could be fatal in the event of inadvertent direct IV B-HCl injection. As is true for humans, a 1% B-HCl concentration (300 mg or 4 mg/kg) must be avoided because the risk of vascular uptake could never be absolutely avoided.

In accordance with previous data in sheep (2–4), B-Ms could prolong the block in the brachial plexus administration. Both poly (lactide-co-glycolide) formulations displayed a biphasic pattern with burst effect followed by a slow release phase as described in a previous study (22). The small mean diameter of microspheres allowed us to perform nerve blocks using the nerve stimulator technique. The motor blockade was prolonged for more than one day. To evaluate the pharmacodynamic effect, pharmacodynamic AUC (time by level of motor blockade) was calculated and found to be greatly increased, with 750 mg of B-Ms more than 6 times than after 75 mg of B-HCl administration (clinical AUC 770 ± 231, 4599 ± 326, and 5422 ± 1461, for 75 mg B-HCl, 50/50, and 60/40 B-Ms, respectively). This effect could not be attributed to microspheres polymer because the drug-free microspheres had no clinical effect. Cmax obtained after brachial nerve injection of 75 mg B-HCl or 750 mg of B-Ms was not significantly different (approximately 100 ng/mL). Such data indicated the in vivo controlled release of the bupivacaine from the microspheres. In case of inadvertent direct IV B-Ms injection, no acute adverse effects were recorded, in contrast to the toxic large doses of B-HCl (300 and 750 mg). After the IV administration of B-Ms, the bupivacaine plasma level did not cause toxicity. This concentration takes into account the amount of bupivacaine released and of bupivacaine loaded into circulating microspheres in the plasma; we were unable to single out this phenomenon. However, bupivacaine present in circulating microspheres may significantly contribute to these levels as a result of a slow release rate, thereby explaining the lack of toxicity (Fig. 5).

The IV administration of drug-free microspheres was also safe. Although there were differences in the in vitro release rates between both microsphere formulations, such differences disappeared in vivo. Such discrepancies may be explained by the fact that the in vitro differences were observed only in the first hours of release. The in vivo differences can only be evidenced in the first hours after administration. The lower resulting Cmax observed with the slowest formulation is an illustration of this phenomenon. The prolonged zero-order absorption of bupivacaine resulting from the release of the drug from the microspheres is of particular interest to obtain a constant and prolonged pharmacological effect at the site of action.

Our work is noteworthy inasmuch as it is not a sensory evaluation of brachial plexus block. Indeed, there was no relevant and discriminant model available to evaluate the sensory block of forelimbs in sheep (i.e., somatosensory evoked potentials). However, despite the lack of information about the sensory block, we would predict that sensory block duration was at least as long as motor block.

Our results are of particular value in surgical situations where the clinical motor and sensory blockade must be prolonged for one or two days; for example for the block from an interscalene brachial plexus catheter (23). Our results are very close to the doses recommended for continuous blockade of peripheral nerves (range, 5–30 mg/h or 100–600 mg per day of bupivacaine) (24). The release of bupivacaine around the nerve roots can avoid indwelling catheter insertions and associated problems (displacement, ineffectiveness resulting from the loss-of-volume effect, or misplacement).

Because pharmacodynamics and pharmacokinetics of bupivacaine used on experimental sheep were similar to those used on humans, brachial plexus block in the sheep provides a worthwhile model to evaluate local anesthetics. To prolong motor blockade, the increasing dosage of B-HCl tends to lead to an increase of bupivacaine plasma concentration in this model. This tendency could be unsafe because the attendant reabsorption phenomenon or in the case of inadvertent vascular injection. The slow controlled release rate of B from polylactide-co-glycolide polymer and the small size of this microsphere were two factors that allowed it to be used in regional anesthesia while still providing a broad safety margin. We obtained injectable B-Ms formulations allowing 1–2 days of motor blockade in the brachial plexus. Further studies are necessary to determine the best microspheres formulation and to define the optimal dose of bupivacaine suited to be evaluated in human clinical trials for the management of postsurgical acute pain.

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