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Journal of Clinical Endocrinology & Metabolism, doi:10.1210/jc.2007-0026
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The Journal of Clinical Endocrinology & Metabolism Vol. 92, No. 6 2074-2079
Copyright © 2007 by The Endocrine Society

Effects of Oral and Transvaginal Ethinyl Estradiol on Hemostatic Factors and Hepatic Proteins in a Randomized, Crossover Study

Régine Sitruk-Ware, Geneviève Plu-Bureau, Joël Menard, Jacqueline Conard, Sushma Kumar, Jean-Christophe Thalabard, Barbara Tokay and Philippe Bouchard

Center for Biomedical Research (R.S.-W., S.K., B.T.), Population Council, New York, New York 10021; Rockefeller University (R.S.-W.), New York, New York; Assistance Publique-Hôpitaux de Paris (G.P.-B., J.C., J.-C.T.), Université Paris V and Hôtel-Dieu, 75004 Paris, France; Institut National de la Santé et de la Recherche Médicale (INSERM) (G.P.-B.), U780, 75004 Paris, France; INSERM (J.M.), U652, 75005 Paris, France; and Hôpital Saint-Antoine (P.B.), 75012 Paris, France

Address all correspondence and requests for reprints to: Régine Sitruk-Ware, M.D., Center for Biomedical Research, Population Council, 1230 York Avenue, New York, New York 10021. E-mail: regine{at}popcbr.rockefeller.edu.


   Abstract
Top
 Abstract
Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
Context: The use of combined hormonal contraceptives with ethinyl estradiol (EE) and a progestin results in alterations in potential biomarkers of venous thromboembolism risk. Evaluation of the impact of delivery route on these changes is difficult due to an interaction between EE and the progestin component.

Objective: The aim of the study was to compare the impact of oral and vaginal administration of EE alone on hemostatic variables and estrogen-sensitive liver proteins.

Design: This was a single-center, randomized, crossover study with two treatment cycles separated by a washout cycle.

Setting: The study was conducted in an academic outpatient center.

Participants: Fourteen healthy postmenopausal women were enrolled; 13 completed the study and were included in the analyses.

Intervention: Participants were randomized to receive EE (15 µg/d) delivered by oral tablet or vaginal ring for 21 d in one of two treatment sequences.

Main Outcome Measures: Changes in plasma concentration or activity of 10 hemostatic variables and six estrogen-sensitive liver proteins between baseline and d 21 of treatment were the primany outcomes.

Results: Prothrombin fragment 1 + 2 plasma level was unaffected by treatment or delivery route. Angiotensinogen (expressed as plasma level of angiotensin I) increased similarly with oral and vaginal delivery; mean (SD) increases were 2757 (1033) and 2864 (893) ng /ml, respectively (P = 0.0002). Alterations in other study variables, except total cholesterol, were similar with oral and vaginal administration.

Conclusion: Our results provide evidence that the customary effects of combined hormonal contraceptives on hemostatic variables and estrogen-sensitive liver proteins are largely related to EE and independent of delivery route during short-term treatment.


   Introduction
Top
 Abstract
 Introduction
Subjects and Methods
 Results
 Discussion
 References
 
THE USE OF COMBINED oral contraceptives (OCs) containing ethinyl estradiol (EE) has been associated with an increase in the risk of venous thromboembolism (VTE) in numerous clinical studies (1, 2, 3, 4, 5, 6, 7, 8); the risk doubles for users of OCs with second-generation progestins and doubles again with use of OCs incorporating third-generation progestins (9). A similar relationship may exist between progestin generation and VTE risk for older women using hormone replacement therapy (HRT) (10).

VTE risk in OC users may be affected by many other factors as well, including estrogen dose (2), age (11), smoking status (11), duration of use (2, 4), and genetic mutations affecting hemostasis (12). VTE risk has been reduced to some extent by lowering the daily dose of EE in OCs to 40 µg or less. In a 5-yr national case-control study (987 cases, 4054 controls), the odds ratio for VTE was calculated after adjusting for progestin type and use duration. Odds ratios for OCs with 20 and 50 µg of EE were 0.6 [95% confidence interval (CI) 0.4–0.9] and 1.6 (95% CI, 0.9–2.8), respectively [P (trend) = 0.02], as compared with a 30–40 µg/d reference standard (2). In pooled risk analyses incorporating data from 22 studies, the risk ratios for VTE were elevated for OCs containing 30–40 µg of EE, increasing to 3.0 (95% CI, 2.6–3.4), 4.8 (95% CI, 2.5–7.7), 2.4 (95% CI, 1.6–3.5), and 1.1 (95% CI, 0.4–2.9) for case control studies, retrospective cohort studies, prospective cohort studies, and randomized controlled trials, respectively (1).

Combined hormonal contraceptives also affect a variety of hemostatic variables and estrogen-sensitive liver proteins, and these effects are also modulated by progestin generation (13, 14, 15, 16, 17, 18, 19, 20). The net impact on hemostatic variables may be a disturbance in the balance between profibrinolytic and procoagulant effects (21), but the relationship between such changes and VTE risk has not yet been established, and there is currently no agreed biomarker for VTE risk (22).

It has been suggested that non-oral delivery of EE might mitigate its effects on the liver, as has been found with 17ß-estradiol (E2). Transdermal administration of E2, with or without a progestin, eliminates the first-pass effect on liver metabolism observed with oral administration (23, 24), which may explain the reduction in the risk of deep VT with non-oral E2 (25). Comparisons of oral and non-oral combined hormonal contraceptives have suggested that EE-related changes in liver metabolism and hemostatic variables are not affected by delivery route, but the design of these studies made it difficult to distinguish the effect of delivery route from those related to progestin type or EE dose (18, 19, 20, 26).

The present crossover study was designed to compare the effects of oral and vaginal administration of the same dose of EE, given alone for 21 d, on hemostatic variables and estrogen-sensitive liver proteins.


   Subjects and Methods
Top
 Abstract
 Introduction
 Subjects and Methods
Results
 Discussion
 References
 
Ethical aspects

The study was conducted in accordance with the principles of the Declaration of Helsinki and its amendments, and current Good Clinical Practice guidelines. The Investigational Review Board of the Population Council (New York, NY) and the Medical Ethics Committee of the Hôpital Saint-Antoine (Paris, France) approved the study. All subjects provided written informed consent.

Study design and conduct

This was a randomized, single-center, crossover study. Postmenopausal women were selected as the study population to eliminate a potential effect of endogenous E2 on the outcomes. At enrollment, participants were randomized to the order of treatment (oral/vaginal or vaginal/oral); the two 21-d treatment periods were separated by a 4- to 6-wk washout period. A 15-µg tablet, given once daily for 21 d, delivered oral EE. Vaginal EE was administered via a vaginal ring (VR) delivering 15 µg of EE daily; the VR was inserted and used continuously for 21 d. Both treatments were initiated at the clinic on d 1 of the treatment period after the pretreatment blood samples were drawn. Women recorded adverse effects, including mastalgia and bleeding, in a daily diary. The selection of the 15-µg/d dose of EE was based on the results of a dose-finding study with a contraceptive vaginal ring (CVR) in which two doses of EE were tested in combination with two doses of Nestorone (NES; Gédéon Richter, Budapest, Hungary), a novel, nonandrogenic progestin (27); the 15-µg dose is also used in a commercially available CVR (NuvaRing; Organon, Oss, The Netherlands). The study variables were hemostatic variables and estrogen-sensitive liver proteins. The hemostatic variables included procoagulant factors (factor VIII, fibrinogen, factor II), markers of fibrin turnover (prothrombin fragment 1 + 2 and D-dimer), coagulation inhibitors [antithrombin activity, free protein S antigen, acquired activated protein C (APC) resistance], and fibrinolytic (plasminogen) and antifibrinolytic [plasminogen activator inhibitor (PAI)-1] factors. Estrogen-sensitive liver proteins included angiotensinogen, SHBG, high-density lipoprotein cholesterol (HDL-C), low-density lipoprotein cholesterol (LDL-C), triglycerides (TG), and total cholesterol (TC). The primary endpoints were changes in the plasma concentrations of angiotensinogen and prothrombin fragment 1 + 2 between d 1 and 21 of treatment. The secondary endpoints were changes between these time points in plasma concentrations or activities of the other study variables.

Subjects

Inclusion criteria. The participants were healthy, postmenopausal women, aged 49–58 yr, with or without ovaries who had been amenorrheic for 1 yr or had a FSH serum concentration more than 40 UI/ml and an E2 serum concentration less than 10 pg/ml.

Exclusion criteria. Carriers of the factor V Leiden or prothrombin 20210A mutations, linked to the most prevalent thrombophilias in France, were excluded. Women were also excluded if they had a previous VTE or a family history of VTE (first-degree relative < 55 yr of age); a family history of breast cancer (first-degree relative); systolic/diastolic blood pressure (BP) ≥ 135/85 mm Hg; LDL-C more than 1.6 mmol/liter; TG more than 1.5 mmol/liter; body mass index more than 30 kg/m2; or if they smoked more than 15 cigarettes daily. Women using HRT underwent a 2-month washout period before enrollment.

Blood sampling and handling

Blood samples were obtained by venipuncture after 15 min of rest on d 1 and 21 of each treatment period; participants were asked to fast for ≥12 h and to avoid the use of nonsteroidal antiinflammatory drugs for 14 d. Samples were centrifuged for 20 min at 2000 x g and 4 C within 30 min of collection, and were then separated into aliquots for each study variable, snap frozen, and stored at –80 C. SHBG was measured in EDTA plasma and all other variables in citrate plasma.

Assays

Angiotensinogen enzymatic assay. Angiotensinogen and SHBG assays were performed at INSERM 652, Paris, France. Plasma angiotensinogen was determined by measuring the generation of angiotensin I (Ang I) after addition of an excess human renin to obtain complete cleavage to Ang I as described previously (28). Plasma (0.25 ml) was incubated at 37 C for 1 h with 0.062 pmol/liter of pure human recombinant renin (0.025 Goldblatt Units; final concentration of 0.125 nmol/liter) in 0.5 ml of 0.15-mol/liter citrate/phosphate buffer (pH 5.7) containing 50 mmol/liter EDTA [Na2], 1.4 mmol/liter of phenylmethylsulfonyl fluoride, and 0.2% BSA. The reaction was stopped by placing the tubes in an iced-water bath at 4 C. The amount of Ang I generated was determined using a polyclonal antibody as previously reported (29). The standard Ang I used in these experiments was purchased from the National Biological Standards Board (Holly Hill, Hampstead, London, UK). For each sample, the mean concentration of duplicate Ang I assays was calculated using Multicalc software and the CliniGamma 1272 counter (LKB, Turku, Finland). The limit of detection for Ang I was 40 pg/ml of diluted sample. Angiotensinogen concentration was expressed as the concentration of Ang I in plasma (ng/ml). To check intraassay precision, three samples were tested eight times in the same assay, yielding the following coefficients of variation (CVs) for Ang I: 5.5% for 619 ng/ml; 2.2% for 1053 ng/ml; and 5.5% for 5600 ng/ml. For interassay precision, the same three samples were measured in 12 assays, giving CVs of 14, 7, and 7%, respectively.

Assays for hemostatic variables. Hemostatic variables were assayed at the Hemostasis Laboratory, Hôtel-Dieu, Paris, France. Fibrinogen was measured using the STA Fibrinogen method (STAGO, Asnières, France). Free protein S antigen was determined with the Liatest STA Protein S assay (STAGO). Plasminogen activity was measured with the Stachrom Plasminogen assay (STAGO). Factor VIII:C and factor II-C were determined using factor VIII or factor II deficient plasma (STAGO). BioMérieux APTT reagent and STA Neoplastin reagent were used for the determination of factor VIII and II levels. Antithrombin was measured by the Coamatic Antithrombin assay (Chromogenix, Milan, Italy), and global APTT-based APC resistance was measured by the Coatest APC Resistance assay without addition of factor-V–depleted plasma (Chromogenix). Prothrombin fragment F 1 + 2 was assayed with the Enzygnost F 1 + 2 ELISA kit (Dade Behring, Marburg, Germany). D-dimer was measured by ELFA method on Mini-Vidas (BioMérieux, Marcy-l’Etoile, France). PAI-1 was measured using Tint Elize PAI-1 reagent (DiaMed, Cressier, Switzerland).

Other assays. Plasma SHBG concentration (at 1:100 or 1:500 dilutions) was determined by immunoradiometric assay using a commercially available kit (DSL-7400 Active SHBG; DSL France, Cergy, France). TC, HDL-C, LDL-C, and TG were measured by enzymatic standard techniques using automated procedures at Hospital Necker, Paris, France.

Statistical methods

Study calibration. The initial sample size was calculated from angiotensinogen data abstracted from a dose-ranging study with NES/EE CVRs delivering 15 or 20 µg/d of EE in which EE dose did not affect angiotensinogen change (Population Council, data on file, 2006), as well as from published studies reporting plasma angiotensinogen change during OC use (13) and prothrombin fragment 1 + 2 alteration during HRT (24). A sample size of 16 was considered adequate to detect an effect of treatment (defined as a change > 1000 ng/ml) on plasma angiotensinogen at the 0.05 level of significance with 80% power. Subsequently, the results of a crossover study comparing effects of a NES/EE CVR (15 µg of EE) and a levonorgestrel/EE OC (30 µg of EE) on angiotensinogen indicated that reducing the angiotensinogen change by 1–rho ({rho}; lower limit: 0.67) would not affect the statistical significance level or study power (Population Council, data on file, 2006), thus reducing the required sample size to 12. For prothrombin fragment 1 + 2, published data from an HRT study (24) were used to calculate the sample size required to detect a 1-SD change in plasma concentration at the 0.05 level of significance with 80% power; assuming a {rho} of 0.4–0.5 (SD 0.3), a sample size of 11 was considered sufficient (30).

Statistical analysis. Descriptive statistics were used to summarize demographic characteristics of the subjects at baseline. Baseline and end-of-treatment period values for each study variable during the oral and vaginal administration treatment periods were compared using paired t and Wilcoxon tests. A general linear (GL) model using log-transformed values was used to evaluate the effects of treatment and route of administration on the study variables (30). All statistical analyses were performed with the statistical package R (version 2.1) and SAS statistical software (version 8.2; SAS Institute Inc., Cary, NC). An alternate analysis was performed on the lipid variables in which data from oral and vaginal treatment periods were combined, using an extended generalized equation (GEE) model with period before/after treatment and type of treatment as controlled, dichotomic factors, and considering the intrasubject dependence of the data.


   Results
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
Discussion
 References
 
The disposition of women screened or enrolled in the study is summarized in Fig. 1Go. The initial recruitment efforts produced a pool of 159 volunteers who agreed to participate and were screened by telephone interview; of these, 55 women met initial personal and family history criteria and were invited to a clinic screening visit. A total of 25 women were screened at the clinic, and 14 met eligibility criteria and agreed to participate. Reasons for exclusion after the clinic screening included previously undetected factor V Leiden mutation (1), breast microcalcifications (3), elevated BP (3), and hypercholesterolemia (3). One participant developed a superficial VT on d 8 of the first treatment period while using oral EE. Treatment was stopped immediately; this participant was included in the safety, but not the efficacy, analyses. Baseline demographic characteristics of the 13 volunteers who completed the study are summarized in Table 1Go.


Figure 1
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  		FIG. 1. Disposition of women screened and enrolled in the study. SAE, Serious adverse event.

 

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  		TABLE 1. Baseline characteristics of 13 women completing the study

 
The mean changes in plasma level or activity of each study variable for each route of administration, and the results of the treatment and delivery route analyses are presented in Table 2Go. Small, but statistically nonsignificant, increases were noted for factor VIII and fibrinogen with both oral and vaginal administration. Both delivery routes were associated with statistically significant (P ≤ 0.02) decreases in antithrombin, protein S, and acquired APC resistance, and with statistically significant (P ≤ 0.004) increases in D-dimer and plasminogen. Plasma concentrations of angiotensinogen and SHBG, liver proteins highly sensitive to estrogen, increased in a similar, highly statistically significant fashion (P < 0.0002) with both routes of administration.


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  		TABLE 2. Effect of 21 d of oral and vaginal administration of EE (15 µg/d) on hemostatic variables and estrogen-sensitive liver proteins

 
Treatment effects were also expressed as a percent change between d 1 and 21. The ratios of percent changes with oral and vaginal administration were close to one for all study variables (data not shown). The route of administration did not affect the percent changes of prothrombin fragment 1 + 2, a marker for coagulation activation, or those of plasminogen, PAI-antigen, or D-dimer, measures of fibrinolysis. For factor VIII, a procoagulation factor, vaginal administration resulted in a larger, but nonsignificant, increase than did oral delivery. Delivery route did not affect percent changes of measures of coagulation inhibition (antithrombin, protein S, or APTT-based APC resistance).

In the per-protocol analysis, TC and LDL-C decreased, and HDL-C and TG increased with both oral and vaginal administration of EE. The reduction in TC was greater with vaginal than with oral administration, and the difference was statistically significant (P = 0.022). For LDL-C, the difference in change related to route of administration approached statistical significance (P = 0.07). In the alternate analysis, HDL-C and TG increased from baseline to end of treatment, while TC and LDL-C declined (P ≤ 0.0007; Table 3Go); delivery route did not influence these outcomes. Delivery route affected TC before (P = 0.04), but not after (P = 0.32), the application of the Bonferroni correction for number of analyses.


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  		TABLE 3. Alternate analysis of lipid variables using a GEE model

 
Mean weight increased slightly during both oral and vaginal administration of EE [0.4 (SD = 0.9) and 0.5 (0.7) kg, respectively]; the change was statistically significant with vaginal administration (P = 0.04). Both delivery routes were associated with similar small, statistically nonsignificant BP declines [systolic BP: –1.0 (7.6) and –1.0 (7.2); diastolic BP: –0.7 (6.7) vs. –1.1 (5.3) mm Hg, oral and vaginal, respectively]; route of administration had no effect.


   Discussion
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
References
 
In this crossover study, both vaginal delivery and oral administration of EE (15 µg/d for 21 d) were associated with similar effects on hemostatic variables and estrogen-sensitive liver proteins. These findings differ from those found in studies with E2, in which transdermal delivery abolished the hepatic effects induced by oral administration (31, 32, 33).

The differences in potencies of EE and E2 may explain the differences in the impact of delivery route. Estrogens are more extensively metabolized in the liver than in other organs (34), and their relative potencies have been linked to their rates of liver metabolism. Studies in hepatocyte culture have suggested that E2 is metabolized more rapidly and extensively to inactive metabolites than EE (35, 36), a difference attributed to the 7{alpha}-ethinyl group of the EE molecule (37). Because E2 is so rapidly inactivated, the E2 concentration required to promote nuclear translocation of the estrogen receptor is 100-fold greater than the concentration of EE (35, 36). Similarly, a clinical study demonstrated that EE was more potent than E2 on a weight basis for all estrogen-related activities but particularly for hepatic effects, including increases in corticosteroid-binding globulin (CBG) and SHBG (38).

With oral administration, E2 enters the liver via the portal vein and undergoes substantial metabolism; non-oral delivery eliminates the initial hepatic impact (39). No matter what the delivery route, EE undergoes limited hepatic metabolism and remains in the liver for a longer period, thus limiting the benefit of avoiding first-pass metabolism (39). Earlier comparisons of vaginal and oral delivery of EE have yielded conflicting results (37, 40). In one study, an oral EE formulation was approximately four to five times more potent than vaginally delivered EE (5 vs. 20 µg) with respect to stimulation of CBG or SHBG, but oral and vaginal delivery produced similar effects with equipotent doses; difficulties with the oral formulation might have contributed to the results (37). In a second study, a 20- to 30-µg EE tablet was twice as potent when given vaginally rather than orally with respect to ovulation suppression and CBG stimulation (40). In the current study, oral and vaginal delivery of 15 µg of EE was equipotent.

Angiotensinogen is exquisitely sensitive to estrogen stimulation, which was demonstrated in the present study by the large increases in plasma concentration following 21 d of oral or vaginal EE administration (Table 2Go). As has been shown previously, progestins, given alone or with estrogen, do not affect angiotensinogen plasma levels (41, 42), thus excluding a contributory effect by the progestin in a combined hormonal contraceptive. In the current study, EE produced large mean increases in SHBG when given orally or vaginally. It is well known, however, that the progestin in a hormonal contraceptive may modulate the impact of EE on SHBG or HDL-C, which are sensitive to androgen as well as estrogen (43, 44, 45). Thus, angiotensinogen may prove to be a better marker of the hepatic effects of EE during use of combined hormonal contraceptives than SHBG or HDL-C.

In this short-term study, large increases in angiotensinogen were observed after 21 d of daily administration of EE (15 µg/d), but BP remained stable. Although a very small increase in BP (3–4 mm Hg) is observed in most women using OCs, the estrogen-related increase in angiotensinogen secretion is generally accompanied by a decrease in renin secretion, which limits the risk of developing hypertension. The risk of treatment-emergent hypertension increases only among women with both high levels of Ang I and impaired renal function. In fact, the development of hypertension during OC use is a rare event, as confirmed by the results of a large epidemiological study enrolling more than 68,000 nurses in which the risk of developing hypertension was approximately four per 1000 woman-years during the use of OCs (46); EE doses in this study were 30–50 µg/d, higher than doses used today. The results of this study led to the suggestion that the feedback loop between angiotensinogen and renin was absent in a few women.

The circulating levels or activities of the hemostatic variables evaluated in the current study are modified by combined hormonal contraceptives, with differences noted between contraceptives incorporating second- or third-generation progestins (13, 14, 15, 16, 17, 18, 19, 20). In the absence of agreed biomarkers for VTE risk, the European Medicines Agency currently recommends evaluation of a variety of hemostatic variables and cardiovascular risk factors during the development of a new hormonal contraceptive (22); most of the recommended evaluations of hemostasis were included in this study.

In this study, both vaginal and oral delivery of EE were associated with decreased activity of the coagulation inhibitory system, as measured by APC resistance, free protein S, and antithrombin activity (Table 2Go). Given the well-known variability in the CVs for most hemostatic factors (14), we did not perform power calculations for these variables. Instead, we determined the sample size and power for angiotensinogen and prothrombin fragment 1 + 2, the primary endpoints. We note that the study was adequately powered to detect the effects of treatment and route of administration for these variables as indicated by observed variances within the ranges used in the original power calculations. The effects of EE on the hemostatic variables appeared to be similar with both routes of administration.

When caused by inherited disorders, similar modifications in these hemostatic variables are well-known risk factors for VTE (47, 48). In the same way, when levels of prothrombin fragment 1 + 2 and D-dimer, markers of coagulation activation, are elevated, a state of hypercoagulation may be anticipated. It should be noted that VTE has a multifactorial etiology, and nongenetic factors (surgery, immobilization, or steroid hormone use) may also be important, especially when a genetic susceptibility is present.

In summary, daily vaginal and oral administration of 15 µg of EE for 21 d led to similar and expected alterations in hemostatic variables and estrogen-sensitive liver proteins. These results provide evidence, for the first time, that the customary effects of combined hormonal contraceptives on clotting factors and markers of coagulation and fibrinolysis are largely due to the EE component, and are independent of the route of administration. Similarly, EE had the expected effects on lipids. HDL-C and TG increased, and LDL-C and TC decreased; the changes did not appear to be affected by delivery route. Our findings contrast with observations from studies of combined hormonal contraceptives in which the progestin component may cancel or attenuate the effects of EE (13, 14, 15, 16, 17, 18, 19, 20).

Our study has several limitations. First, we studied only one dose of EE. Second, we looked only at the effects of short-term treatment. Our data suggest, however, that 15 µg of EE given daily for 21 d has similar effects on estrogen-sensitive hemostatic variables and liver proteins whether the steroid is delivered via a VR or an oral tablet.


   Acknowledgments
 
We thank Hoffmann-La Roche, Inc., for its generous gift of pure human recombinant renin.


   Footnotes
 
This work was supported by a grant from the U.S. Agency for International Development Cooperative Agreement (HRN-A-00-99-00010).

Disclosure Statement: R.S.-W., G.P.-B., J.C., S.K., J.-C.T., and B.T. have nothing to disclose. J.M. has previously consulted for Novartis, currently consults for and has given a lecture for Sanofi-Adventis, and formerly had an equity interest in Actelion. P.B. has previously consulted for Wyeth and Organon.

First Published Online March 20, 2007

Abbreviations: Ang I, Angiotensin I; APC, activated protein C; BP, blood pressure; CBG, corticosteroid-binding globulin; CI, confidence interval; CVR, contraceptive VR; CV, coefficient of variation; E2, 17ß-estradiol; EE, ethinyl estradiol; GEE, extended generalized equation; GL, general linear; HDL-C, high-density lipoprotein cholesterol; HRT, hormone replacement therapy; LDL-C, low-density lipoprotein cholesterol; NES, Nestorone; OC, oral contraceptive; PAI, plasminogen activator inhibitor; TC, total cholesterol; TG, triglycerides; VR, vaginal ring; VTE, venous thromboembolism.

Received January 5, 2007.

Accepted March 9, 2007.


   References
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 

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