Bupivacaine in Microcapsules Prolongs Analgesia After Subcutaneous Infiltration in Humans: A Dose-Finding Study

Abstract

In this study, we examined the onset and duration of local analgesic effects of bupivacaine incorporated into biodegradable microcapsules (extended-duration local anesthetic; EDLA) administered as subcutaneous infiltrations in different doses in humans. In 18 volunteers, the skin on the medial calf was infiltrated with 10 mL of EDLA, and the opposite calf was infiltrated with 10 mL of aqueous bupivacaine (5.0 mg/mL) in a double-blinded, randomized manner. Three different concentrations of EDLA were tested (6.25, 12.5, and 25 mg/mL), with 6 subjects in each group. Pain responses to mechanical and heat stimuli and sensory thresholds (touch, warm, and cold detection thresholds) were examined by von Frey hairs and contact thermodes. Assessments were made before and 2, 4, 6, 8, 24, 48, 72, 96, and 168 h after the injections. Safety evaluations were performed daily for the first week and at 2 wk, 6 wk, and 6 mo after the injections. The time to maximum effects was significantly shorter for aqueous bupivacaine (2–6 h) than for EDLA (4–24 h), but there were no significant differences between the maximum effects of EDLA and aqueous bupivacaine. From 24 to 96 h after the injections, EDLA was significantly more efficient than aqueous bupivacaine for all variables, and significant effects of EDLA were demonstrated for at least 96 h for all variables. In general, a dose-response gradient was seen in the EDLA group for 5 of 7 variables when the curves expressing effect over time for the different concentrations were evaluated. No serious side effects were observed for up to 6 mo after administration. In conclusion, bupivacaine incorporated in microcapsules provided analgesia for 96 h after subcutaneous infiltration.

In Brief

IMPLICATIONS: Incorporation of bupivacaine and dexamethasone in micro-capsules compared with aqueous bupivacaine significantly prolonged analgesia after subcutaneous infiltration in volunteers. Analgesia was seen 96 h after infiltration.

Incisional administration of local anesthetics is routine clinical practice in many smaller operations, but the duration of analgesia is limited to approximately 4–8 h, even with long-acting local anesthetics (¹). Various attempts have been made to prolong the analgesic effects of local anesthetics by loading these into liposomes (^2,3) or a biodegradable polymer matrix formulated as microcapsules (^4–6), which serve as a slow-release local anesthetic delivery system. Microdialysis in humans has demonstrated that the bupivacaine concentration in subcutaneous tissue increased for 24–34 h after subcutaneous injection of microcapsules loaded with bupivacaine (⁷).

Although these systems are attractive, clinical data have been limited, despite findings in preliminary studies that demonstrate a prolonged duration of local analgesia to approximately 4–6 days with subcutaneous administration (^7,8) or intercostal blocks (⁹). We have therefore performed a dose-finding study of bupivacaine microcapsules (slow-release system) compared with aqueous bupivacaine in a double-blinded randomized study in humans.

Methods

We studied 18 healthy male volunteers aged 22 to 36 yr. Informed consent was obtained, and the study was approved by the local ethics committee, the Danish Medicines Agency, and the Danish National Health Board. Volunteers were interviewed about their health history and underwent a physical examination by a physician. The examination included a prestudy blood sample to measure hemoglobin, hematocrit, platelet count, white blood cell count with differential count, fasting blood glucose, urea nitrogen, creatinine, potassium, sodium, chloride, uric acid, albumin, calcium, ^CO2^, triglycerides, inorganic phosphate, alkaline phosphatase, serum glutamine/oxalacetic transaminase, serum glutamine/pyruvate transaminase, lactate dehydrogenase, total bilirubin, and cholesterol. A urinalysis was also performed (pH, albumin, glucose, ketones, and blood). Only volunteers with normal test results were enrolled in the study, and none was taking any medication except vitamins. They were familiar with the heat and the von Frey hair stimuli, and they trained in ratings with verbal ranking scale (VRS) before the study.

The study was performed as a double-blinded, randomized trial, with each subject acting as his own control. The randomization codes were generated by the Biostatistics and Clinical Data Management Department of Purdue Pharma LP (Stamford, CT). The volunteers were infiltrated on the medial part of both calves with the extended-duration local anesthetic (EDLA) and aqueous bupivacaine in a randomized manner. The skin areas were infiltrated via injections performed in the superficial part of subcutis from 2 corners of marked rectangular areas (35 ×60 mm) in a fanlike fashion, thus covering the area evenly. The areas were infiltrated with 10 mL of the assigned treatment by using a 0.8 × 50 mm (21-gauge) needle. Injections and preparation of EDLA and bupivacaine were performed by a person who was not involved in the sensory testing.

The study medication was polylactic-co-glycolic acid polymer microcapsules loaded with bupivacaine and dexamethasone. The microcapsules contained 72% bupivacaine and 0.04% dexamethasone; the microcapsule size was 25–125 μm, and the molecular weight was 40 kd. The microcapsules were manufactured by Purdue Pharma LP. We tested 3 microcapsule concentrations: 1) 6.25 mg/mL (4.5 mg/mL bupivacaine; 2.5 μg/mL dexamethasone; n = 6), 2) 12.5 mg/mL (9.0 mg/mL bupivacaine; 5.0 μg/mL dexamethasone; n = 6), and 3) 25.0 mg/mL (18.0 mg/mL bupivacaine; 10.0 μg/mL dexamethasone; n = 6). The EDLA was compared with aqueous bupivacaine (5.0 mg/mL; Purdue Pharma LP) in 18 volunteers. The EDLA was stored frozen at less than −5°C until use and diluted immediately before the injections. The pain on injection—not needle insertion—was rated with a VRS (0–10). The VRS was anchored by the descriptors “no pain” (0) and “worst pain imaginable” (¹⁰).

Assessments of pain and sensory thresholds were made before the injections (baseline) and 2, 4, 6, 8, 24, 48, 72, 96, and 168 h after the injections. All sensory testing was performed at the same time of the day in a quiet room with a temperature of 22°C–24°C. The subjects were resting in a relaxed position with their eyes closed during all assessments.

The mechanical pain threshold (MPT) and the touch detection threshold (MTDT) within the area of injection were determined by mechanical stimuli with von Frey hairs (Senselab; Somedic AB, Stockholm, Sweden). Twelve different von Frey hairs were used that covered the range from 3 to 402 mN. The MPT was defined as the least force of mechanical stimulation that produced a sensation of pain or discomfort, and the subjects were instructed to report the first sensation of pain. The MTDT was defined as the least force that produced a sensation of touch or pressure. Eight stimuli covering the infiltrated area were made with each hair from the thinnest until at least one-half of the stimulations with one hair caused the specified sensation. The 8 stimuli were applied with a rate of approximately 0.5 Hz. The threshold assessment was repeated 3 times at each measurement point, and the median was reported as the threshold. If Hair 17 (402 mN) did not produce any sensation, we assigned the value 18 to that observation (the threshold was not assessable). Pain responses (0–10 VRS) to mechanical stimuli (PRMS) were assessed by five stimuli of 402 mN. The VRS was anchored by the descriptors “no pain” (0) and the “worst pain imaginable” (¹⁰).

Thermal thresholds were determined by using a computerized contact thermode (Somedic AB). All thresholds were assessed with a 2.5 × 5.0 cm thermode, whereas heat pain responses were evaluated with a 1.5 × 2.5 cm thermode. The heat pain threshold (HPT) was the lowest temperature perceived as painful, and the volunteers were instructed to react to the first sensation of pain. Cold and warm detection threshold (CDT and WDT) was defined as the smallest change from baseline that the volunteer could perceive, and the volunteer pressed a button as soon as the specified sensation was perceived. All thermal thresholds were determined as the average of 3 assessments performed at 9-s intervals, from a baseline temperature of 32°C, and with a rate of change of 1°C/s. The upper cutoff limit was 52°C, and the lower limit was 25°C. When the volunteer did not perceive the specified sensation by 52°C, the value of 53°C was recorded. When the volunteer did not perceive cooling by 7°C (from 32°C to 25°C), the value of 8°C was recorded. Pain responses (0–10 VRS) to heat (HPS) were evaluated by a heat stimulus (3.75 cm²) of 45°C lasting 5 s, which was preceded by a temperature increase from 40°C to 45°C in 5 s.

The volunteers were asked about the presence of local reactions and other adverse events, intercurrent illness, and concomitant medications at each visit to the laboratory. Two weeks after the injections, the physical examination and blood tests were repeated. Six weeks after the injections, the volunteers returned to the investigator, who evaluated the injection sites for persistent or delayed reactions, and they were questioned regarding any abnormalities. Six months after the injections, the volunteers were contacted by telephone and questioned about problems or incidents that they felt might be related to the study medication. The volunteers were requested to notify the investigator at any time in the case of any potential serious adverse event.

Localized reactions at the injection sites (erythema, discoloration, hematoma, induration, swelling, and blisters) and the presence of other sensations, such as tingling, itching, burning, pain, and hyperesthesia, were recorded. When adverse effects were present, they were rated as mild, moderate, or severe.

Normality of data and differences between groups were evaluated by using the Shapiro-Wilk test (¹⁰). Data are presented as means or medians, depending on the distribution (normal or skewed). The analysis of EDLA versus aqueous bupivacaine effects was based on the combination of the 3 different EDLA concentrations with 6 subjects in each group (in total, n = 18; paired design). Comparisons were made by using Student’s t-test for differences in a paired design when the differences between EDLA and aqueous bupivacaine showed normal distribution (¹⁰); differences showing nonnormal distribution were analyzed with Wilcoxon’s test for paired observations. Comparisons were based on summary measures such as maximum effect, time to maximum effect, and area under the curve (AUC) for the period 0 to 24 h after injection (the period with a pronounced effect of aqueous bupivacaine) and the period 24 to 96 h after injection (pronounced effect of EDLA). The dose-response relationships were evaluated by using a 3 (concentrations [between group]) × 10 (time [within subject]) analysis of variance (ANOVA) design. The dose-response data are presented without confidence intervals to improve the clarity of the curves.

The duration of neural blockade was defined as the period with significantly reduced sensory sensitivity compared with the baseline value. Because of multiple comparisons, the Bonferroni correction was used to prevent mass significance. After the Bonferroni correction, only P values less than 0.0056 (0.05/9) were considered statistically significant. In all other comparisons, P values less than 0.05 were considered statistically significant.

Results

The analysis of EDLA versus aqueous bupivacaine effects is based on the combination of the 3 different EDLA concentrations with 6 subjects in each group (in total, n = 18; paired design). The baseline sensory responses, pain thresholds, and pain responses were not significantly different between treatment groups except for CDTs (P = 0.02; Wilcoxon test) (Fig. 1). For this reason, comparisons of this variable between treatment groups were corrected through comparing changes from baseline. Thus, AUCs were calculated by using two new curves based on changes from baseline (recorded value at a specific time minus baseline value). Comparison of maximum values was corrected the same way. Time to maximum values was not changed by the corrections. Injections of EDLA were slightly less painful (mean, 4.0; SD, 1.7) than the injections of aqueous bupivacaine (mean, 4.8; SD 1.9) (P = 0.03; Wilcoxon’s test).

Figure 1.: Warm (means), cold (medians), and mechanical (medians) detection thresholds. ○(with solid lines) = bupivacaine and dexamethasone incorporated into biodegradable microcapsules (extended-duration local anesthetic, all three concentrations combined; n = 18); ▴(with dotted lines) = aqueous bupivacaine 0.5% (n = 18).

MTDT and MPT were not significantly different in comparing aqueous bupivacaine in the period 0 to 24 h with EDLA (MTDT [AUC 0–24 h]:P = 0.34, Student’s t-test; MPT [AUC 0–24 h]:P = 0.68, Wilcoxon’s test;Figs. 1 and 2), whereas EDLA increased both thresholds compared with aqueous bupivacaine in the period 24 to 96 h (MTDT, P = 0.00004; MPT, P = 0.001; Student’s t-test). PRMS (0–10 VRS), assessed by 5 pinpricks of 402 mN, showed that aqueous bupivacaine reduced pain in the period 0 to 24 h more efficiently than EDLA (P = 0.02; Wilcoxon test;Fig. 2), whereas pain reduction was significantly better by EDLA in the period 24 to 96 h (P = 0.007; Student’s t-test).

Figure 2.: Heat pain thresholds (means), heat pain responses to 45°C (medians), mechanical pain thresholds (medians), and mechanically induced pain (medians). ˆ(with solid lines) = bupivacaine and dexamethasone incorporated into biodegradable microcapsules (extended-duration local anesthetic, all three concentrations combined; n = 18); = ▴(with dotted lines) aqueous bupivacaine 0.5% (n = 18).

There were no significant differences between the maximum effects regarding MPT, MTDT, or PRMS. The median time to maximum effect was 2–3 h for aqueous bupivacaine and 4–24 h for EDLA (MPT, 4 h; MTDT, 24 h; PRMS, 6 h), and the maximum effects developed significantly later in the EDLA group (P < 0.01; Wilcoxon’s tests).

The duration of the neural blockade may be evaluated on the basis of different criteria. One definition of block duration may be the period in which the sensitivity of the sensory response was significantly reduced compared with the baseline value. According to this criterion, the block duration in the EDLA group was longer than 96 h for MTDT, MPT, and PRMS. Block duration in the aqueous bupivacaine group was approximately 24 h for MTDT, MPT, and PRMS.

A significant dose-response relationship was detected only for MTDT (P = 0.003; two-way ANOVA for repeated measurements). However, the analysis of concentration effects was influenced by the small number of volunteers in each group (microcapsule concentrations: 6.25 mg/mL [n = 6], 12.5 mg/mL [n = 6], and 25.0 mg/mL [n = 6]), although a clear dose-response gradient was seen in the EDLA group for all mechanical stimuli when the curves expressing effect over time were evaluated for the different concentrations (Figs. 3 and 4).

Figure 3.: Warm (means), cold (medians), and mechanical (medians) detection thresholds for different doses of microcapsules (extended-duration local anesthetic). ○ = microcapsule concentration of 6.25 mg/mL; ▵ = 12.5 mg/mL; □ = 25.0 mg/mL (also see text). There were six subjects in each group.

Figure 4.: Heat pain thresholds (means), heat pain responses to 45°C (medians), mechanical pain thresholds (medians), and mechanically induced pain (medians). ○ = microcapsule concentration of 6.25 mg/mL; ▵ = 12.5 mg/mL; □ = 25.0 mg/mL (also see text). There were six subjects in each group.

CDTs were not significantly different between aqueous bupivacaine and EDLA in the first 24 h after injections (CDT:P ≤0.12; Student’s t-test;Fig. 1), whereas EDLA increased CDT significantly compared with aqueous bupivacaine in the period 24 to 96 h after injections (P = 0.0007; Student’s t-test). In contrast, WDTs were increased the most by EDLA both 0–24 h (P = 0.048; Student’s t-test;Fig. 1) and 24–96 h (P = 0.0008; Student’s t-test) after the injections. For HPT and HPS (0–10 VRS), there was no significant difference between the effects of aqueous bupivacaine and EDLA in the period 0 to 24 h (Fig. 2), whereas the pain was significantly reduced by EDLA compared with aqueous bupivacaine in the period 24 to 96 h (HPT:P = 0.0005, Student’s t-test; HPS:P = 0.007, Wilcoxon’s test).

There were no significant differences between the maximum effects regarding WDT, CDT, HPT, or HPS. The maximum effects occurred after median of 6 h (CDT), 24 h (WDT), 24 h (HPT), and 7 h (HPS) in the EDLA group and after a median of 2 h (CDT), 5 h (WDT), 6 h (HPT), and 2 h (HPS) in the aqueous bupivacaine group. Thus, again, the maximum effects occurred significantly earlier in the aqueous bupivacaine group (P ≤0.02; Wilcoxon’s test and Student’s t-test). The block duration in the EDLA group was longer than 96 h for WDT, CDT, HPT, and HPS. The block duration in the aqueous bupivacaine group was 24 h for WDT, CDT, and HPT and was 8 h for HPS.

A significant dose-response relationship was detected only for CDT (P = 0.04; two-way ANOVA for repeated measurements). However, a clear dose-response gradient was seen for WDT, CDT, and HPS when the curves expressing effect over time were evaluated for the different concentrations (Figs. 3 and 4).

Two subjects developed mild skin indurations in the area infiltrated with EDLA (one subject was infiltrated with 6.25 mg/mL and one with 25 mg/mL) that lasted approximately 1–2 mo. One of the indurations was described as only a possible induration. This was the only adverse effect unique for EDLA. Five subjects had hematomas in the injection sites (two on the EDLA site and three on the bupivacaine site), and three subjects experienced transient discoloration of the injection site (three on the EDLA site and one on the bupivacaine site). However, the hematomas were related to the administration technique and not to the medication. None of the subjects reported pruritus. However, they were not asked specifically for this. No serious adverse effects were observed.

Discussion

We found a significant prolongation of analgesia after subcutaneous administration of microcapsules loaded with bupivacaine compared with aqueous bupivacaine. In this model, the duration of analgesia with bupivacaine in microcapsules was approximately 96 hours, compared with approximately 24 hours with aqueous bupivacaine. The adverse effects were minimal, with only mild indurations at the injection site in 2 of 18 subjects. The indurations were expected from the normal foreign body reaction induced by the polysaccharide microcapsules (^4–6,11). Mild pruritus (56% of injections) and mild indurations (42% of injections) were seen after subcutaneous infiltration in a recent study (⁷). However, more microcapsules were injected than in this study. Obviously, further safety data are necessary, especially with peripheral nerve blocks, for which the combined effect of the inflammatory reaction to the microcapsules and the prolonged local anesthetic exposure should be investigated.

Limited clinical data are available with bupivacaine microcapsules (^7–9). In both studies with subcutaneous infiltration in volunteers (^7,8) and in the study of intercostal nerve blocks in volunteers (⁹), the loading of bupivacaine and dexamethasone in microcapsules prolonged analgesia to approximately four days. These results may have significant clinical implications, because the analgesia after incisional local anesthetic administration is usually only four to eight hours (¹). The pharmacokinetic variables for bupivacaine release from microcapsules in subcutaneous tissue (⁷) corresponded with the pharmacodynamic effects observed in this study.

The largest concentration (25 mg/mL) was the most efficient. This is supported by the study by Kopacz et al. (⁷), but this concentration seems to produce more adverse effects (mild pruritus) (⁷). The clinical implications of our study and other studies with slow-release bupivacaine microcapsules (^7–9) should, obviously, be further explored. The most promising clinical use may be incisional administration in small-size operations (e.g., herniorrhaphy, port-site infiltration in laparoscopic procedures, or varicectomy). The administration of long-acting bupivacaine preparations may be less suitable for peripheral blocks, in which motor function may be affected. However, in distal orthopedic procedures with limited motor function (hand and foot surgery) or thoracic procedures, such as thoracotomy, the long-acting preparations may also have important implications.

In summary, bupivacaine and dexamethasone loaded in microcapsules produced prolonged analgesia (four days) after subcutaneous infiltration in volunteers. The largest concentration of microcapsules (25 mg/mL) was the most efficient, and no serious side effects were observed. These findings may have major implications for the future treatment of acute pain.