Pharmacokinetics of Sublingual Versus Oral Estradiol in Transgender Women

ObjectiveTo investigate the pharmacokinetics of 17β-estradiol (E2) administered orally versus those of 17β-E2 administered sublingually in transgender women.MethodsSingle doses of 17β-E2 were administered orally (1 mg) to 10 transgender women and then sublingually (1 mg) after a 1-week washout period. Blood samples were collected at baseline (0 hour) and at 1, 2, 3, 4, 6, and 8 hours after dosing. The samples were frozen and analyzed using liquid chromatography mass spectrometry (LC-MS/MS) and immunoassay.ResultsThe results demonstrated that sublingual E2 had a significantly higher peak serum E2 concentration of 144 pg/mL, measured using LC-MS/MS, compared with an oral E2 concentration of 35 pg/mL, measured using LC-MS/MS (P = .003). Sublingual E2 peaked at 1 hour and oral E2 peaked at 8 hours, as measured using LC-MS/MS. The area under the curve (AUC) (0-8 hours) for sublingual E2, measured using LC-MS/MS, was 1.8-fold higher than the AUC (0-8 hours) for oral E2, measured using LC-MS/MS. Additionally, sublingual E2 was found to have an increased E2-to-estrone ratio at all time points (1.1 ± 1.0 vs 0.7 ± 0.4, P ≤ .0001), the clinical significance of which is unclear.ConclusionOral E2 administered sublingually has a different pharmacokinetic profile, with higher serum E2 levels and AUC (0-8 hours) than traditionally administered oral E2. Multidaily dosing may be necessary to suppress testosterone levels with sublingual E2. The appropriate dosing, efficacy, and safety of sublingual E2, compared with those of other E2 preparations, are unknown.