title: Prolonged nerve block by microencapsulated bupivacaine prevents acute postoperative pain in rats.
abstract: OBJECTIVE
  To minimize acute postoperative pain, a new formulation of slowly released bupivacaine was developed.
  
  
  METHODS
  Bupivacaine was microencapsulated at 60% (wt/wt) in poly-lactide-co-glycolide polymers and characterized for physicochemical properties and bupivacaine release kinetics. This formulation was injected around the rat sciatic nerve to produce an antinociceptive effect to toe pinch. Mechanical hyperalgesia following lateral plantar paw incision in rats was assessed for 7 to 14 days when the bupivacaine slow-release formulation was placed at the ipsilateral sciatic nerve and compared with the hyperalgesia that developed with various controls.
  
  
  RESULTS
  Bupivacaine was released in vitro at a relatively constant rate over a period of ≈ 72 to 96 hours. Complete antinociception, shown as no response to toe pinch, lasted for 23 ± 7 hours, with a half-recovery time of 42 ± 8 hours after sciatic nerve injection of 0.4 mL of the microspheres delivering 34 mg of bupivacaine. Solutions of 0.5% (wt/vol) bupivacaine-HCl (0.1 mL) produced complete antinociception for less than 2 hours and recovery half-times of 2 hours. Postincisional mechanical hyperalgesia, shown by increased withdrawal responses to von Frey filaments, was absent for 24 hours and was lower than control for 96 hours, when the sciatic nerve was blocked by bupivacaine microspheres, whereas the 0.5% bupivacaine solution reduced postincisional pain for only 4 hours.
  
  
  CONCLUSIONS
  Corresponding to its far greater functional blocking time, the microsphere-bupivacaine formulation was able to significantly reduce postoperative pain below control levels for up to 4 days. These findings of several days of postoperative pain relief, for an injectable formulation containing a single active agent, present an improved and potentially promising therapy to prevent acute pain after surgery.
authors.affiliation: COVIDIEN Surgical Solutions, Bedford, MA, USA.
authors.forename: Rachit
authors.initials: R
authors.lastname: Ohri
authors.forename: Phillip
authors.initials: P
authors.lastname: Blaskovich
authors.forename: Jeffrey Chi-Fei
authors.initials: JC
authors.lastname: Wang
authors.forename: Lan
authors.initials: L
authors.lastname: Pham
authors.forename: Gary
authors.initials: G
authors.lastname: Nichols
authors.forename: William
authors.initials: W
authors.lastname: Hildebrand
authors.forename: Daniel
authors.initials: D
authors.lastname: Costa
authors.forename: Nelson
authors.initials: N
authors.lastname: Scarborough
authors.forename: Clifford
authors.initials: C
authors.lastname: Herman
authors.forename: Gary
authors.initials: G
authors.lastname: Strichartz
mesh_terms: D000779:Anesthetics, Local / Q000008:administration & dosage* / Q000493:pharmacokinetics; D000818:Animals; D001522:Behavior, Animal; D002045:Bupivacaine / Q000008:administration & dosage* / Q000493:pharmacokinetics; D003692:Delayed-Action Preparations; D006930:Hyperalgesia / Q000517:prevention & control; D007267:Injections; D008855:Microscopy, Electron, Scanning; D008863:Microspheres; D009407:Nerve Block / Q000379:methods*; D010149:Postoperative Pain / Q000517:prevention & control*; D051381:Rats; D012584:Sciatic Nerve
chemical_list: D000779:Anesthetics, Local; D003692:Delayed-Action Preparations; D002045:Bupivacaine