Efficacy of 3 Buprenorphine Formulations for the Attenuation of Hypersensitivity after Plantar Incision in Immunodeficient NSG Mice

Abstract

Buprenorphine is perhaps the most prescribed analgesic for management of postoperative pain in mice. Although various buprenorphine formulations are effective in commonly used immunocompetent mouse strains, a knowledge gap exists regarding its efficacy in immunodeficient mice. Here we used a plantar incision to evaluate the efficacy of 3 buprenorphine formulations for attenuating postoperative mechanical and thermal hypersensitivity in the immunodeficient NSG mouse strain. We also characterized the pharmacokinetics of these formulations over a 72-h period. We hypothesized that all 3 buprenorphine formulations evaluated—the standard preparation and 2 extended-release products (Bup-HCl, Bup-ER, and Bup-XR, respectively)—would attenuate postoperative mechanical and thermal hypersensitivity resulting from a plantar incision in NSG mice. Male and female NSG mice (n = 48) were allocated to 4 treatment groups: saline (0.9% NaCl, 5 mL/kg SC once); Bup-HCl (0.1 mg/kg SC, BID for 2 d); Bup-ER (1.0 mg/kg SC once); and Bup-XR (3.25 mg/kg SC once). Mechanical and thermal hypersensitivity assessments were conducted 24 h before surgery and at 4, 8, 24, 48, and 72 h afterward. All groups of mice showed mechanical and thermal hypersensitivity within the first 24 h after surgery. Behavioral pain indicators (guarding, toe-touching [intermittent partial weight bearing], licking the incision, vocalizations) were observed in some mice from each group at every postoperative time point. Plasma buprenorphine was measured in a separate group of mice and concentrations surpassed the suggested therapeutic level (1.0 ng/mL) for less than 4 h for Bup-HCl, for at least 24 h for Bup-ER, and for 72 h for Bup-XR. Our results indicate that at the dosages studied, these buprenorphine formulations do not adequately attenuate postoperative mechanical and thermal hypersensitivity in the plantar incisional model in NSG mice. These findings support the need for strain-specific analgesic protocols for mice used in research.

Comparative Pharmacokinetics and Injection Site Histopathology in Nude Mice Treated with Long-acting Buprenorphine Formulations

“…The duration of buprenorphine plasma levels greater than 1 ng/mL achieved by XR in the current study is similar to what has been previously reported, 14 although some previous studies report shorter 4 and longer 2,20,21 durations in mice. One study reported the duration of plasma levels greater than 1 ng/mL achieved by ER to be similar to our findings, 6 although several other studies report longer durations in mice.…”

“…Because we found no difference between the drug groups and the saline group, we concluded that stress caused by daily anesthesia and blood collection was the likely cause of weight loss. Other studies have similarly found no difference in weight loss between XR and control 2,15,21 or XR and ER. 2,20 This finding suggests that potential drug-induced weight loss is not a critical factor in selecting buprenorphine formulations for mouse studies.…”

“…At times when NSG mice achieve the 1 ng/mL buprenorphine plasma level, mice of this strain still display mechanical hypersensitivity. 2 Nude mice given morphine have greater sensitivity to thermal and chemical pain than their heterozygous controls. 16 Future studies that combine pain assays and plasma drug levels are necessary to identify a true therapeutic buprenorphine plasma level in nude mice.…”

“…One study reported the duration of plasma levels greater than 1 ng/mL achieved by ER to be similar to our findings, 6 although several other studies report longer durations in mice. 2,6,12,14,20 Sample collection techniques, study design differences, or the stock/strain of the mice could account for these differences. The labeled dose of XR used in this study (3.25 mg/kg) is over 3 times higher than the manufacturer-recommended dose of ER (1 mg/kg); thus, a higher injected dose of ER may have sustained higher plasma concentrations of buprenorphine over a longer duration.…”

“…7 Similar to its ER counterpart, XR has been shown to maintain purported therapeutic plasma concentrations for up to 72 h in mice. 2,20,21 Subcutaneous injection of ER and XR are both associated with injection site lesions in rodents. 1,6,8,13,21 The manufacturer of ER warns against the using the product in nude rodents based on alleged anecdotal local reactions to the novel polymer carrier.…”