TWIRLA is a TDS designed with an active matrix core containing LNG and EE. TWIRLA delivers medication to the systemic circulation by absorption of LNG and EE through the skin.
Absorption
Following application of TWIRLA, both LNG and EE reach a plateau by 24 to 48 hours (Figures 4 and 5). Delivery of hormones is continuous over the 7 days of TWIRLA wear. The mean pharmacokinetic parameters (Css and AUC0‑168) for LNG and EE following two consecutive cycles of TWIRLA are summarized in Table 6.
Table 6: Mean (%CVCoefficient of Variation
) Pharmacokinetic Parameters of Levonorgestrel and Ethinyl Estradiol Following Two Consecutive Cycles of TWIRLA Wear on the Buttock |
| Analyte | Parameter | Cycle 1 Week 1 (N=18) | Cycle 1 Week 3 (N=18) | Cycle 2 Week 1 (N=18) | Cycle 2 Week 3 (N=18) |
| LNG | Css (pg/mL) Average concentration within the 48-168 h time interval | 842 (41.2) | 2009 (47.2) | 1389 (46.5) | 2209 (44.5) |
AUC0-168 (ng∙h/mL)AUC0-168: area under the plasma drug concentration-time curve calculated between 0 and 168 h | 120.0 (39.1) | 339.0 (41.1) | 207.0 (44.1) | 378.0 (43.8) |
| t1/2 (h) t1/2: elimination half-life | NC | 38.2 (22.7) | NC | 40.5 (15.4) |
| EE | Css (pg/mL) | 31.9 (37.4) | 34.8 (37.4) | 38.6 (41.7) | 40.3 (38.9) |
AUC0-168 (pg∙h/mL) | 5040 (35.4) | 6210 (34.2) | 6060 (35.9) | 7120 (36.6) |
| t1/2 (h) | NC | 19.7 (18.8) | NC | 20.5 (18.2) |
In multiple dose studies, AUC0-168 for LNG and EE showed within-cycle and between cycle increases and the mean serum concentrations of EE and LNG were highest during the third Week of Cycle 2 after two consecutive cycles of wear (Figures 4 and 5). In a three-cycle study, the steady-state pharmacokinetics of EE and LNG was reached during Cycle 2. Upon removal of TWIRLA, serum levels of EE and LNG reach non‑measurable levels and low levels within 3 days, respectively.
Figure 4. Mean Serum Ethinyl Estradiol Concentrations in Healthy Female Volunteers Following Two Consecutive Cycles of TWIRLA Wear on the Buttock (Vertical arrow indicates time of TWIRLA removal)
Figure 5. Mean Serum Levonorgestrel Concentrations in Healthy Female Volunteers Following Two Consecutive Cycles of TWIRLA Wear on the Buttock (Vertical arrow indicates time of TWIRLA removal)
The absorption of LNG and EE following application of TWIRLA to the buttock, abdomen, and upper torso (excluding the breasts) was examined. While absorption from the abdomen was slightly lower than from other sites, absorption from all three anatomic sites was considered to be therapeutically equivalent.
The absorption of LNG and EE following application of TWIRLA was studied under various external conditions including sauna, whirlpool, treadmill, and in a cold-water bath. Somewhat lower drug concentration levels were reported for whirlpool and treadmill with geometric ratios within the 78-90% range for both LNG and EE and dry sauna (LNG only).
Distribution
LNG in serum is primarily bound to sex hormone-binding globulin (SHBG). EE is about 97% bound to plasma albumin. EE does not bind to SHBG but induces SHBG synthesis.
Elimination
Metabolism
Since TWIRLA is applied transdermally, first-pass metabolism (via the gastrointestinal tract and/or liver) of LNG and EE that would be expected with oral administration does not occur. Hepatic metabolism of LNG and EE occurs as described below.
Levonorgestrel: The most important metabolic pathways are reduction of the Δ4-3-oxo group and hydroxylation at positions 2α, 1β, and 16β, followed by conjugation. Most of the circulating metabolites are sulfates of 3α, 5β-tetrahydro-levonorgestrel, while excretion occurs predominantly in the form of glucuronides. Some of the parent LNG also circulates as 17β-sulfate. Metabolic clearance rates may differ among individuals by several-fold, and this may account in part for the wide variation observed in LNG concentrations among users.
Ethinyl estradiol: Cytochrome P450 enzymes (CYP3A4) in the liver are responsible for the 2-hydroxylation that is the major oxidative reaction. The 2-hydroxy metabolite is further transformed by methylation and glucuronidation prior to urinary and fecal excretion. Levels of CYP3A4 vary widely among individuals and can explain the variation in rates of EE 2-hydroxylation.
Excretion
LNG and its metabolites are excreted in the urine (40% to 68%) and in feces (16% to 48%). The mean terminal elimination half-life for LNG in TWIRLA is approximately 41 ± 6.2 hours at steady state.
EE is excreted in the urine and feces as glucuronide and sulfate conjugates and undergoes enterohepatic recirculation. The terminal elimination half-life of EE in TWIRLA is approximately 21 ± 3.7 hours at steady state.