title: The pharmacokinetics of methimazole in a novel lipophilic formulation administered transdermally to healthy cats.
abstract: OBJECTIVE
  To determine the pharmacokinetics of a novel lipophilic formulation of transdermal methimazole compared to oral carbimazole.
  
  
  METHODS
  Healthy cats received 5 mg carbimazole orally every 12 hours for 13 treatments (n=6), then received transdermal methimazole (n=5) at a dose of 5 mg, then 10 mg, once daily on the pinna for 7 days, with 21 days between treatments. Concentrations of methimazole in serum over 24 hours and at 148 hours were determined by high performance liquid chromatography.
  
  
  RESULTS
  Concentrations of methimazole in serum for the first 24 hours were not reliably detected in all cats treated with 5 mg methimazole transdermally, while for those receiving 5 mg carbimazole orally and 10 mg methimazole transdermally all cats had detectable concentrations of methimazole in serum. The maximum concentration and area under the curve were lower in cats receiving 10 mg methimazole transdermally (108 (SD 25) ng/mL and 2544 (SD 216) mg-hour/mL, respectively) than those receiving 5 mg oral carbimazole (355 (SD 113) ng/mL and 31,866 (SD 439) ng-hour/mL, respectively) (p<0.05). The time at maximal concentration and elimination half-life were longer for 10 mg transdermal methimazole (5.2 (SD 1.1) hours and 13 (SD 3) hours, respectively) compared to 5 mg oral carbimazole (2.1 (SD 1.6) hours and 5.1 (SD 1.2) hours, respectively). At 148 hours, mean concentrations of methimazole in serum were higher in cats receiving 10 mg methimazole transdermally (506 (SD 165) ng/mL) than for 5 mg oral carbimazole (255 (SD 28) ng/mL) or 5 mg transdermally (204 (SD 76) ng/mL). The mean relative bioavailability of 10 mg transdermal methimazole compared to oral carbimazole was 48 (min 43, max 55)%.
  
  
  CONCLUSIONS
  Transdermal methimazole at a dose of 10 mg administered to the pinnae of healthy cats once daily in a novel lipophilic formulation has half the relative bioavailablity compared to 5 mg oral carbimazole.
  
  
  CONCLUSIONS
  Transdermal methimazole can be absorbed from the skin of healthy cats.
authors.affiliation: a * Institute of Veterinary, Animal and Biomedical Sciences , Massey University , Palmerston North , New Zealand.
authors.forename: K E
authors.initials: KE
authors.lastname: Hill
authors.forename: M A
authors.initials: MA
authors.lastname: Gieseg
authors.forename: J
authors.initials: J
authors.lastname: Bridges
authors.forename: J P
authors.initials: JP
authors.lastname: Chambers
mesh_terms: D000279:Administration, Cutaneous; D000818:Animals; D013956:Antithyroid Agents / Q000097:blood / Q000378:metabolism / Q000493:pharmacokinetics*; D019540:Area Under Curve; D001682:Biological Availability; D002231:Carbimazole / Q000008:administration & dosage / Q000493:pharmacokinetics; D002415:Cats / Q000097:blood / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D008297:Male; D008713:Methimazole / Q000097:blood / Q000378:metabolism / Q000493:pharmacokinetics*
chemical_list: D013956:Antithyroid Agents; D008713:Methimazole; D002231:Carbimazole
keywords: hyperthyroidism; methimazole; once daily; transdermal