Daniel S. Costa

Inhibition by local bupivacaine-releasing microspheres of acute postoperative pain from hairy skin incision.

BACKGROUND:Acute postoperative pain causes physiological deficits and slows recovery. Reduction of such pain by local anesthetics that are delivered for several days postoperatively is a desirable clinical objective, which is approached by a new formulation and applied in animal studies reported here. METHODS:We subcutaneously injected a new formulation of poly-lactic-co-glycolic acid polymer microspheres, which provides steady drug release for 96+ hours into rats at the dorsal region 2 hours before surgery. A single 1.2-cm-long skin incision was followed by blunt dissection of skin away from the underlying fascia, and closed by 2 sutures, followed by 14 days of testing. Microspheres containing 5, 10, 20, and 40 mg bupivacaine were injected locally 2 hours before surgery; bupivacaine-free microspheres were the vehicle control, and bupivacaine HCl solution (0.5%), the positive control. Mechanical sensitivity was determined by the frequency of local muscle contractions to repeated pokes with nylon monofilaments (von Frey hairs) exerting 4 and 15 g forces, testing, respectively, allodynia and hyperalgesia, and by pinprick. RESULTS:Injection of bupivacaine microspheres (40 mg drug) into intact skin reduced responses to 15 g von Frey hairs for 6 hours and to pinprick for 36 hours. Respective reductions from bupivacaine HCl lasted for 3 and 2 hours. Skin incision and dissection alone caused mechanical allodynia and hyperalgesia for 14 days. Microspheres containing 20 or 40 mg bupivacaine suppressed postoperative hypersensitivity for up to 3 days, reduced integrated allodynia (area under curve of response versus time) over postoperative days 1 to 5 by 51% ± 20% (mean ± SE) and 78% ± 12%, and reduced integrated hyperalgesia by 55% ± 13% and 64% ± 11%, for the respective doses. Five and ten milligrams bupivacaine in microspheres and the 0.5% bupivacaine solution were ineffective in reducing postoperative hypersensitivity, as were 40 mg bupivacaine microspheres injected contralateral to the incision. CONCLUSIONS:Significant suppression of postoperative pain by the slow-release bupivacaine preparation outlasts its anesthetic action on intact skin. These findings demonstrate preventive analgesia and indicate the importance of acute processes in the development of chronic postoperative pain.

Prolonged nerve block by microencapsulated bupivacaine prevents acute postoperative pain in rats.

Background and Objectives 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.

Prolonged amelioration of experimental postoperative pain by bupivacaine released from microsphere-coated hernia mesh.

Background and Objectives Postoperative pain alters physiological functions and delays discharge. Perioperative local anesthetics are effective analgesics in the immediate 1- to 2-day postoperative period, but acute pain often lasts longer. The goal of this work was to develop a local anesthetic formulation adhering to an intraoperative implanted device that reduces pain for at least 3 days after surgery. Methods Six groups, each with 8 rats, were studied. In a control group (group I), one 1.2-cm-long incision of the skin was followed by blunt dissection to separate the skin away from the underlying tissues and closing with 2 sutures. In 3 of the treatment groups, the same surgical procedure was used, with the subcutaneous space formed by the blunt dissection lined with a 1-cm square patch of hernia mesh coated with poly lactide co-glycolic acid microspheres containing approximately 17 mg of bupivacaine (group II), no drug (placebo; group III), or bupivacaine free-base powder (group IV). Uncoated mesh implants (group V) served as a secondary control. A standard bupivacaine solution (0.4 mL, 0.5%; 2-mg dose) was infiltrated subcutaneously 30 minutes before the surgery and served as a standard control (group VI). Mechanosensitivity of the skin was tested by the local subcutaneous muscle responses to cutaneous tactile stimulation by von Frey hairs with forces of 4 g (for allodynia) and 15 g (for hyperalgesia) preoperatively and for 7 postoperative days. Results Control rats (group I) showed mechanohypersensitivity, indicative of postoperative allodynia and hyperalgesia, for all 7 postoperative days. Mechanohyperalgesia in rats that received mesh coated with bupivacaine-releasing microspheres (group II) was reduced during this period to 13% of control postoperative values (P < 0.001); mesh coated with bupivacaine base (group IV) reduced it by 50% (P = 0.034). The placebo mesh (group III) and uncoated mesh (group V) caused no significant reduction of mechanohypersensitivity, and bupivacaine solution infiltrated before the incision (group VI) reduced hypersensitivity for only approximately 2 hours, an overall insignificant effect. Conclusions Bupivacaine slowly released for 72 hours from microspheres adsorbed to the hernia mesh significantly suppresses evoked postoperative hypersensitivity for at least 1 week and is more effective than a suspension of these microspheres or preoperative single-shot infiltration of bupivacaine.