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Mirena (Levonorgestrel Intrauterine) - Description and Clinical Pharmacology

 
 



DESCRIPTION

Mirena is intended to provide an initial release rate of 20 mcg/day of levonorgestrel

Levonorgestrel USP, (-)-13-Ethyl-17-hydroxy-18,19-dinor-17α-pregn-4-en-20-yn-3-one, the active ingredient in Mirena, has a molecular weight of 312.4, a molecular formula of C21H28O2, and the following structural formula:

Mirena

Mirena (levonorgestrel-releasing intrauterine system) consists of a T-shaped polyethylene frame (T-body) with a steroid reservoir (hormone elastomer core) around the vertical stem. The reservoir consists of a white or almost white cylinder, made of a mixture of levonorgestrel and silicone (polydimethylsiloxane), containing a total of 52 mg levonorgestrel. The reservoir is covered by a semi-opaque silicone (polydimethylsiloxane) membrane. The T-body is 32 mm in both the horizontal and vertical directions. The polyethylene of the T-body is compounded with barium sulfate, which makes it radiopaque. A monofilament brown polyethylene removal thread is attached to a loop at the end of the vertical stem of the T-body.

Schematic drawing of Mirena

Schematic drawing of Mirena

Inserter

Mirena is packaged sterile within an inserter. The inserter, which is used for insertion of Mirena into the uterine cavity, consists of a symmetric two-sided body and slider that are integrated with flange, lock, pre-bent insertion tube and plunger. Once Mirena is in place, the inserter is discarded.

Diagram of Inserter

Diagram of Inserter

CLINICAL PHARMACOLOGY

Mechanism of Action

The local mechanism by which continuously released levonorgestrel enhances contraceptive effectiveness of Mirena has not been conclusively demonstrated. Studies of Mirena prototypes have suggested several mechanisms that prevent pregnancy: thickening of cervical mucus preventing passage of sperm into the uterus, inhibition of sperm capacitation or survival, and alteration of the endometrium.

Pharmacodynamics

Mirena has mainly local progestogenic effects in the uterine cavity. The high local levels of levonorgestrel1 lead to morphological changes including stromal pseudodecidualization, glandular atrophy, a leukocytic infiltration and a decrease in glandular and stromal mitoses.

Ovulation is inhibited in some women using Mirena. In a 1-year study approximately 45% of menstrual cycles were ovulatory and in another study after 4 years 75% of cycles were ovulatory.

Pharmacokinetics

Absorption

Low doses of levonorgestrel are administered into the uterine cavity with the Mirena intrauterine delivery system. Initially, levonorgestrel is released at a rate of approximately 20 mcg/day. This rate decreases progressively to half that value after 5 years. A stable serum concentration, without peaks and troughs, of levonorgestrel of 150–200 pg/mL occurs after the first few weeks following insertion of Mirena. Levonorgestrel concentrations after long-term use of 12, 24, and 60 months were 180±66 pg/mL, 192±140 pg/mL, and 159±59 pg/mL, respectively.

Distribution

The apparent volume of distribution of levonorgestrel is reported to be approximately 1.8 L/kg. It is about 97.5 to 99% protein-bound, principally to sex hormone binding globulin (SHBG) and, to a lesser extent, serum albumin.

Metabolism

Following absorption, levonorgestrel is conjugated at the 17β-OH position to form sulfate conjugates and, to a lesser extent, glucuronide conjugates in serum. Significant amounts of conjugated and unconjugated 3α, 5β- tetrahydrolevonorgestrel are also present in serum, along with much smaller amounts of 3α, 5α-tetrahydrolevonorgestrel and 16βhydroxylevonorgestrel. Levonorgestrel and its phase I metabolites are excreted primarily as glucuronide conjugates. Metabolic clearance rates may differ among individuals by several-fold, and this may account in part for wide individual variations in levonorgestrel concentrations seen in individuals using levonorgestrel–containing contraceptive products.

Excretion

About 45% of levonorgestrel and its metabolites are excreted in the urine and about 32% are excreted in feces, mostly as glucuronide conjugates. The elimination half-life of levonorgestrel after daily oral doses is approximately 17 hours.

Specific Populations

Pediatric: Safety and efficacy of Mirena have been established in women of reproductive age. Use of this product before menarche is not indicated.

Geriatric: Mirena has not been studied in women over age 65 and is not currently approved for use in this population.

Race: No studies have evaluated the effect of race on pharmacokinetics of Mirena.

Hepatic Impairment: No studies were conducted to evaluate the effect of hepatic disease on the disposition of Mirena.

Renal Impairment: No formal studies were conducted to evaluate the effect of renal disease on the disposition of Mirena.

Drug-Drug Interactions

No drug-drug interaction studies were conducted with Mirena [see Drug Interactions (7)].

NONCLINICAL TOXICOLOGY

Carcinogenesis, Mutagenesis, Impairment of Fertility

Carcinogenicity

Long-term studies in animals to assess the carcinogenic potential of levonorgestrel releasing intrauterine system have not been performed. There is no evidence of increased risk of cancer with short-term use of progestins. There was no increase in tumorigenicity following parenteral administration of levonorgestrel to rats for 2 years at approximately 5 mcg/day, or following oral administration to dogs for 7 years at up to 0.125 mg/kg/day, or to rhesus monkeys for 10 years at up to 250 mcg/kg/day. In another 7 year dog study, oral administration of levonorgestrel at 0.5 mg/kg/day did increase the number of mammary adenomas in treated dogs compared to controls. There were no malignancies. The nonclinical doses above are respectively 16, 200, 240 and 810 times the release rate of levonorgestrel by Mirena (20 mcg/day), based on body surface area [see Warnings and Precautions ].

Mutagenicity

Levonorgestrel was not found to be genotoxic in the Ames assay, in vitro mammalian culture assays utilizing mouse lymphoma cells and Chinese hamster ovary cells, and in an in vivo micronucleus assay in mice.

Impairment of Fertility

There are no irreversible effects on fertility following cessation of exposures to levonorgestrel or progestins in general.

Animal Toxicology and/or Pharmacology

Carcinogenicity

Long-term studies in animals to assess the carcinogenic potential of levonorgestrel releasing intrauterine system have not been performed. There is no evidence of increased risk of cancer with short-term use of progestins. There was no increase in tumorigenicity following parenteral administration of levonorgestrel to rats for 2 years at approximately 5 mcg/day, or following oral administration to dogs for 7 years at up to 0.125 mg/kg/day, or to rhesus monkeys for 10 years at up to 250 mcg/kg/day. In another 7 year dog study, oral administration of levonorgestrel at 0.5 mg/kg/day did increase the number of mammary adenomas in treated dogs compared to controls. There were no malignancies. The nonclinical doses above are respectively 16, 200, 240 and 810 times the release rate of levonorgestrel by Mirena (20 mcg/day), based on body surface area [see Warnings and Precautions].

Mutagenicity

Levonorgestrel was not found to be genotoxic in the Ames assay, in vitro mammalian culture assays utilizing mouse lymphoma cells and Chinese hamster ovary cells, and in an in vivo micronucleus assay in mice.

Impairment of Fertility

There are no irreversible effects on fertility following cessation of exposures to levonorgestrel or progestins in general.

CLINICAL STUDIES

Clinical Trials on Intrauterine Contraception

Mirena has been studied for safety and efficacy in two large clinical trials in Finland and Sweden. In study sites having verifiable data and informed consent, 1,169 women 18 to 35 years of age at enrollment used Mirena for up to 5 years, for a total of 45,000 women-months of exposure. Subjects had previously been pregnant, had no history of ectopic pregnancy, had no history of pelvic inflammatory disease over the preceding 12 months, were predominantly Caucasian, and over 70% of the participants had previously used IUDs (intrauterine devices). The reported 12-month pregnancy rates were less than or equal to 0.2 per 100 women (0.2%) and the cumulative 5-year pregnancy rate was approximately 0.7 per 100 women (0.7%).

About 80% of women wishing to become pregnant conceived within 12 months after removal of Mirena.

Clinical Trial on Heavy Menstrual Bleeding

The efficacy of Mirena in the treatment of heavy menstrual bleeding was studied in a randomized, open-label, active-control, parallel-group trial comparing Mirena (n=79) to an approved therapy, medroxyprogesterone acetate (MPA) (n=81), over 6 cycles. The subjects included reproductive-aged women in good health, with no contraindications to the drug products and with confirmed heavy menstrual bleeding (≥ 80 mL menstrual blood loss [MBL]) determined using the alkaline hematin method. Excluded were women with organic or systemic conditions that may cause heavy uterine bleeding (except small fibroids, with total volume not > 5 mL). Treatment with Mirena showed a statistically significantly greater reduction in MBL (see Figure 10) and a statistically significantly greater number of subjects with successful treatment (see Figure 11). Successful treatment was defined as proportion of subjects with (1) end-of-study MBL < 80 mL and (2) a ≥ 50% decrease in MBL from baseline to end-of-study.

Figure 10. Median Menstrual Blood Loss (MBL) by Time and Treatment

Figure 10. Median Menstrual Blood Loss (MBL) by Time and Treatment

Figure 11. Proportion of Subjects with Successful Treatment

Figure 11. Proportion of Subjects with Successful Treatment

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