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.

Near infrared spectroscopy of magnesium stearate hydrates and multivariate calibration of pseudopolymorph composition.

Samples of magnesium stearate monohydrate and dihydrate were used to prepare standard mixtures of known pseudopolymorphic composition. Near infrared spectra (NIR) of the standard mixtures were measured to develop multivariate calibration models for the pseudopolymorphic composition of magnesium stearate by partial least squares (PLS) regression. Magnesium stearate hydrate compositions of the standard mixtures were compared against the hydrate composition based on thermogravimetric analysis (TGA). The mixture compositions determined from TGA mass loss on drying (LOD) measurements were found to be inaccurate. PLS regression was applied to the TGA thermograms of the standard mixtures to generate more accurate reference values, and this model was then applied to a set of validation samples. Application of the NIR PLS model to the validation sample set resulted in precise estimates of sample pseudopolymorphic composition when compared to the TGA PLS reference values. The NIR PLS model was found to be more sensitive than TGA LOD to small quantities of hydrates, and the TGA PLS model was also found to be more sensitive that TGA LOD. The results demonstrate the challenges and opportunities that arise when rapid, nondestructive spectroscopic methods depend on insensitive or inaccurate reference methods for development of multivariate calibration models.