Enter the concept of tissue-selective estrogen complex (TSEC)
Some postmenopausal women are extremely intolerant of any progestogen added to estrogen therapy to confer endometrial protection in those with a uterus. According to the results of a clinical trial of postmenopausal women, bazedoxifene is the only SERM shown to decrease endometrial thickness compared with placebo.26 This is the basis for thinking that perhaps a SERM like bazedoxifene, instead of a progestogen, could be used to confer endometrial protection.
A further consideration comes out of the evaluation of data derived from the 2 arms of the Women’s Health Initiative (WHI).27 In the arm that combined conjugated estrogen with medroxyprogesterone acetate through 11.3 years, there was a 25% increase in the incidence of invasive breast cancer, which was statistically significant. Contrast that with the arm in hysterectomized women who received only conjugated estrogen (often inaccurately referred to as the “estrogen only” arm of the WHI). In that study arm, the relative risk of invasive breast cancer was reduced 23%, also statistically significant. Thus, the culprit in the breast cancer incidence difference in these 2 arms appears to be the addition of the progestogen medroxyprogesterone acetate.27
Since the progestogen was used only for endometrial protection, could such endometrial protection be provided by a SERM like bazedoxifene? Preclinical trials showed that a combination of bazedoxifene and conjugated estrogen (in various estrogen doses) resulted in uterine wet weight in an ovariectomized rat model that was no different than that with placebo.28
In terms of effects on breast, preclinical models showed that conjugated estrogen use resulted in less mammary duct elongation and end bud proliferation than estradiol by itself, and that the combination of conjugated estrogen and bazedoxifene resulted in mammary duct elongation and end bud proliferation that was similar to that in the ovariectomized animals and considerably less than a combination of estradiol with bazedoxifene.29
Five phase 3 studies known as the SMART (Selective estrogens, Menopause, And Response to Therapy) trials were then conducted. Collectively, these studies examined the frequency and severity of vasomotor symptoms (VMS), BMD, bone turnover markers, lipid profiles, sleep, quality of life, breast density, and endometrial safety with conjugated estrogen/bazedoxifene treatment.30 Based on these trials with more than 7,500 women, in 2013 the FDA approved a compound of conjugated estrogen 0.45 mg and bazedoxifene 20 mg (Duavee in the United States and Duavive outside the United States).
The incidence of endometrial hyperplasia at 12 months was consistently less than 1%, which is the FDA guidance for approval of hormone therapies. The incidence of bleeding or spotting with conjugated estrogen/bazedoxifene (FIGURE 3) in each 4-week interval over 12 months mirror-imaged that of placebo and ranged from 3.9% in the first 4-week interval to 1.7% in the last 4 weeks, compared with conjugated estrogen 0.45 mg/medroxyprogesterone acetate 1.5 mg, which had a 20.8% incidence of bleeding or spotting in the first 4-week interval and was still at an 8.8% incidence in the last 4 weeks.31 This is extremely relevant in clinical practice. There was no difference from placebo in breast cancer incidence, breast pain or tenderness, abnormal mammograms, or breast density at month 12.32
In terms of frequency of VMS, there was a 74% reduction from baseline at 12 weeks compared with placebo (P<.001), as well as a 37% reduction in the VMS severity score (P<.001).32 Statistically significant improvements occurred in lumbar spine and hip BMD (P<.01) for women who were 1 to 5 years since menopause as well as for those who were more than 5 years since menopause.33
Packaging issue puts TSEC on back order
In May 2020, Pfizer voluntarily recalled its conjugated estrogen/bazedoxifene product after identifying a “flaw in the drug’s foil laminate pouch that introduced oxygen and lowered the dissolution rate of active pharmaceutical ingredient bazedoxifene acetate.”34 The manufacturer then wrote a letter to health care professionals in September 2021 stating, “Duavee continues to be out of stock due to an unexpected and complex packaging issue, resulting in manufacturing delays. This has nothing to do with the safety or quality of the product itself but could affect product stability throughout its shelf life… Given regulatory approval timelines for any new packaging, it is unlikely that Duavee will return to stock in 2022.”35
Other TSECs?
The conjugated estrogen/bazedoxifene combination is the first FDA-approved TSEC. Other attempts have been made to achieve similar results with combined raloxifene and 17β-estradiol.36 That study was meant to be a 52-week treatment trial with either raloxifene 60 mg alone or in combination with 17β-estradiol 1 mg per day to assess effects on VMS and endometrial safety. The study was stopped early because signs of endometrial stimulation were observed in the raloxifene plus estradiol group. Thus, one cannot combine any estrogen with any SERM and assume similar results.
Clinical application
The combination of conjugated estrogen/bazedoxifene is approved for treatment of VMS of menopause as well as prevention of osteoporosis. Although it is not approved for treatment of moderate to severe VVA, in younger women who initiate treatment it should prevent the development of moderate to severe symptoms of VVA.
Finally, this drug should be protective of the breast. Conjugated estrogen has clearly shown a reduction in breast cancer incidence and mortality, and bazedoxifene is a SERM. All SERMs have, as a class effect, been shown to be antiestrogens in breast tissue, and abundant preclinical data point in that direction.
This combination of conjugated estrogen/bazedoxifene, when it is once again clinically available, may well provide a new paradigm of hormone therapy that is progestogen free and has a benefit/risk ratio that tilts toward its benefits.
Potential for wider therapeutic benefits
Newer SERMs like ospemifene, approved for treatment of VVA/GSM, and bazedoxifene/conjugated estrogen combination, approved for treatment of VMS and prevention of bone loss, have other beneficial properties that can and should result in their more widespread use. ●