Local pathology and systemic serum bupivacaine after subcutaneous delivery of slow-releasing bupivacaine microspheres

BACKGROUND: Prolonged local anesthesia, particularly desirable to minimize acute and chronic postoperative pain, has been provided by microspheres that slowly release bupivacaine (MS-Bup). In this study, we report on the systemic drug concentrations and the local dermatopathology that occur after subcutaneous injection of MS-Bup. METHODS: Rats (approximately 300 g) were injected under the dorsolumbar skin with MS-Bup containing 40 mg of bupivacaine (base) or with 0.4 mL of 0.5% bupivacaine-HCl (BupHCl; 1.78 mg bupivacaine). Blood was drawn, under sevoflurane anesthesia, at 10 minutes to 144 hours, and the serum analyzed for total bupivacaine by liquid chromatography-tandem mass spectrometry. In different animals, skin punch biopsies (4 mm) were taken at 1, 3, 7, 14, and 30 days after the same drug injections, sectioned at 5 μm, and stained with hematoxylin-eosin. Samples from skin injected with BupHCl, with MS-Bup suspended in carboxymethyl cellulose (MS-Bup.CMC), or in methyl cellulose (MS-Bup.MC) were compared with their respective drugfree controls (placebos). RESULTS: Serum bupivacaine reached a maximal average value (n = 8) of 194.9 ng/mL at 8 hours after injection of MS-Bup (95% upper prediction limit = 230.2 ng/mL), compared with the maximal average (n = 6) serum level of 374.9 ng/mL (95% prediction limit = 470.6 ng/ mL) at 30 minutes after injection of BupHCl. Serum bupivacaine decreased to undetectable levels (<3.23 ng/mL) at 8 hours after BupHCl and was detectable at approximately 20% of the maximal value at 144 hours after MS-Bup injection. BupHCl injection resulted in moderate lymphocytic infiltration of skeletal muscle at 1 and 3 days. MS-Bup.CMC and placebo-CMC caused extensive infiltration of macrophages, lymphocytes, and some neutrophils at 1 to 7 days, whereas MS-Bup.MC and placebo-MC caused only mild inflammation. CONCLUSIONS: Subcutaneous administration of microspheres releasing bupivacaine results in lower blood levels lasting for much longer times than those from bupivacaine solution.