Pharmacokinetics and Adverse Effects of a Long-Acting Transdermal Buprenorphine Formulation in Rats (Rattus norvegicus)
Rats regularly undergo surgical procedures that may result in pain. Alleviation of unnecessary pain is an ethical and regulatory responsibility. Buprenorphine is an opioid analgesic commonly used in rats and requires dosing every 6 to 8 h to be effective. Frequent administration is time consuming and may increase stress, post-surgical pain, and dehiscence in rats, making the use of long-acting formulations an attractive alternative. A transdermal buprenorphine solution (TBS), FDA approved for use in felines, is commercially available and effective for up to 96 h. We hypothesize that a single dose of TBS in rats will result in clinically relevant plasma buprenorphine concentrations (greater than 1 ng/mL) for up to 96 h. To test this, 39 rats were randomly assigned to the following treatment groups: low dose (LD; 5 mg/kg; n = 6 females, 6 males), high dose (HD; 10 mg/kg; n = 6 females, 6 males), and vehicle control (CON; n = 7 females, 8 males). TBS or anhydrous ethanol (CON) were topically applied. Blood was collected at 4, 24, 72, 96, and 168 h postadministration, and buprenorphine concentrations were determined via HPLC-MS. To quantitatively assess adverse effects, daily fecal output, food intake, and body weight were measured, and observations of hematuria and skin lesions were documented. Plasma buprenorphine concentrations exceeded 1 ng/mL in all TBS rats at 4, 24, 48, and 72 h. No rats experienced serious adverse effects or developed gross lesions at the application site. The HD group had decreased fecal output compared with CON. Both TBS groups had reduced weight gain compared with CON. These results suggest that TBS dosed at 5 to 10 mg/kg could provide analgesia for up to 3 d in rats, and administering a lower dose mitigates some adverse effects.
Visual representation of experiment timeline and data collected for each time point for the main study. Rats were acclimated to housing changes D-5, D-4, and D-3 before initiation of baseline data collection. Control groups did not undergo blood collection.
Change from baseline body weight averaged across all time points (A and B) and at each time point (C). Data are presented as group mean ± SEM. (A) The TBS groups had reduced overall average weight gain compared with CON (§, P ≤ 0.0001). (B) Similarly, female and male TBS groups had lower average weight gain compared with sex-matched CON groups (†, P ≤ 0.01; §, P ≤ 0.0001). The HD female and LD male groups had greater weight gain than the LD F and HD M groups respectively (*, P ≤ 0.05). (C) When analyzed by time point, D2 to D7 CON had greater weight gain than the TBS groups (*, P ≤ 0.05; §, P ≤ 0.0001). In addition to the significant differences shown, CON had reduced weight gain compared with the LD group on D1 (P = 0.0326).
Change from baseline food intake averaged across all time points (A and B) and at each time point (C). Data are presented as group mean ± SEM. (A) TBS groups had reduced overall average food intake compared with CON (†, P ≤ 0.01). (B) When considering sex effects, both female TBS groups had lower average food intake relative to female CON (†, P ≤ 0.01) while only the HD males had reduced average intake compared with CON males (‡, P ≤ 0.001). (C) For comparisons at each time point, CON had increased food intake relative to both TBS groups on D1 to D2 and D4 to D5 (*, P ≤ 0.05; §, P ≤ 0.0001). On D3, only HD had reduced intake (*, P ≤ 0.05).
Change from baseline fecal output averaged across all time points (A and B) and at each time point (C). Data are presented as group mean ± SEM. (A) HD had decreased overall average fecal output compared with CON (*, P ≤ 0.05). (B) When analyzed by sex, the HD males had reduced output compared with LD and CON males (*, P ≤ 0.05). There were no differences in fecal output between female groups. (C) Analysis of each time point showed reduced output for the HD group compared with all other groups on D1 (§, P ≤ 0.0001). In addition, CON had increased fecal output compared with LD on D2 (P = 0.0409) and HD on D4 to D5 (P = 0.0492, P = 0.0265).
Mean plasma buprenorphine concentration after administration of TBS at doses of 5 and 10 mg/kg. This data is from the main study and is presented as group mean ± SEM, with (A) and without (B) consideration for sex. The dashed line at 1 ng/mL indicates the assumed threshold for analgesic efficacy in rats. Both dose groups had a mean plasma buprenorphine concentration above the threshold at all time points through 72 h, regardless of sex. (A) There was no difference between dose groups at any time point. (B) LD females had higher plasma buprenorphine levels than HD females at 24 h (P = 0.0180). There were no differences between groups at any other time point.
Appearance of the TBS application site (A) seconds after administration of a 5-g/kg dose and (B) 10 min later, demonstrating that the product visibly dries within 10 min.
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