Pharmacokinetics of intramuscular L-carvone in sheep

To measure and model concentrations of the analgesic L-carvone, a natural component of spearmint, over time when administered IM to sheep and to characterize L-carvone's effects on CBCs and clinical biochemistry panels.

L-carvone formulated as a 50% solution (v/v) in ethanol and propylene glycol was administered at 71.6 mg/kg IM, split between each semitendinosus muscle in 6 sheep. Venous blood was sampled over 24 hours, and plasma was separated by centrifugation. Additional blood was collected for CBC and serum biochemical analysis, and tissues were sampled after euthanasia. L-carvone concentrations in plasma and tissue homogenates were measured using HPLC-MS-MS. Plasma pharmacokinetic data were described using a nonlinear mixed effects model. Complete blood count and biochemistry data were compared to baseline values using repeated-measures ANOVA and Holm-Šidák tests (P < .05).

Maximum plasma concentrations ranged from 0.28 to 1.93 µg/mL and occurred within 9 to 15 minutes after injection. Pharmacokinetics were best described using 2 compartments. Elimination half-life was 33.7 minutes and 390.2 minutes in the central and peripheral compartments, respectively. Mild increases in neutrophil count and significant increases in creatinine kinase and aspartate aminotransferase were associated with injection site myonecrosis. No physical examination, behavioral, or other clinically significant laboratory changes were noted.

Intramuscular L-carvone exhibits rapid time to peak concentration, relatively slow plasma elimination, and low tissue concentrations after 24 hours.

L-carvone exhibits a favorable pharmacokinetic profile for an analgesic drug. A new L-carvone formulation or administration route is needed to reduce inflammation and necrosis at the injection site.