Longitudinal evaluation of fentanyl concentrations in equine plasma and synovial fluid following application of transdermal fentanyl patches over one carpal joint.

Abstract

OBJECTIVE: To determine if transdermally delivered fentanyl can achieve greater concentrations of fentanyl in synovial fluid when applied over a synovial structure. STUDY DESIGN: Randomized, experimental study. ANIMALS: Six healthy adult horses. METHODS: Each horse had two 100&#x2009;&#x3bc;g/h fentanyl matrix patches applied on the dorsal aspect of one, randomly assigned, carpometacarpal joint (CMCJ) for 48&#x2009;h. Whole blood and bilateral synovial samples from the intercarpal joint were obtained at 0, 2, 6, 12, 24, 36 and 48&#x2009;h. Fentanyl concentrations were measured with liquid chromatography-mass spectrometry. RESULTS: All subjects achieved detectable concentrations of fentanyl in both plasma and synovial fluid. Time to peak synovial and plasma concentration was 12&#x2009;h. At 6&#x2009;h, the synovial concentration in the untreated carpus (0.104&#x2009;ng/mL&#x2009;&#xb1;&#x2009;0.106) was lower than plasma fentanyl concentrations 0.31&#x2009;&#xb1;&#x2009;0.27 (p&#x2009;=&#x2009;.036). At 12&#x2009;h, both treated (0.55&#x2009;ng/mL&#x2009;&#xb1;&#x2009;0.3) and untreated (0.53&#x2009;ng/mL&#x2009;&#xb1;&#x2009;0.28) synovial fluid fentanyl concentrations were lower than plasma (0.87&#x2009;ng/mL&#x2009;&#xb1;&#x2009;0.48) concentrations (p&#x2009;<&#x2009;.001 and p&#x2009;=&#x2009;.001, respectively). Synovial concentrations of fentanyl did not differ&#xa0;between treated and untreated joints (p > 0.608 for all time points). CONCLUSION: Application of fentanyl matrix patches directly over the CMCJ did not result in increased fentanyl concentrations in the synovial fluid of the treated intercarpal joint in normal horses. CLINICAL SIGNIFICANCE: There is likely no analgesic advantage to placing fentanyl patches directly over the affected joint, as it did not result in increased synovial concentrations at the tested site.