Phase I pharmacokinetic (PK) study assessed circulating estrogens in breast cancer (BC) patients on a non-steroidal aromatase inhibitor (NSAI) with vaginal atrophy using vaginal ultra-low-dose 0.03 mg estriol (E3) and Lactobacillus combination vaginal tablets (Gynoflor®). 16 women on NSAI with severe vaginal atrophy applied a daily vaginal tablet of Gynoflor® for 28 days followed by a maintenance therapy of 3 tablets weekly for 8 weeks. Primary outcomes were serum concentrations and PK of E3, estradiol (E2), and estrone (E1) using highly sensitive gas chromatography–mass spectrometry. Secondary outcomes were clinical measures for efficacy and side effects; microscopic changes in vaginal epithelium and microflora; and changes in serum FSH, LH, and sex hormone-binding globulin. Compared with baseline, serum E1 and E2 did not increase in any of the women at any time following vaginal application. Serum E3 transiently increased after the first application in 15 of 16 women, with a maximum of 168 pg/ml 2–3 h post-insertion. After 4 weeks, serum E3 was slightly increased in 8 women with a maximum of 44 pg/ml. The vaginal atrophy resolved or improved in all women. The product was well tolerated, and discontinuation of therapy was not observed. The low-dose 0.03 mg E3 and Lactobacillus acidophilus vaginal tablets application in postmenopausal BC patients during AI treatment suffering from vaginal atrophy lead to small and transient increases in serum E3, but not E1 or E2, and therefore can be considered as safe and efficacious for treatment of atrophic vaginitis in BC patients taking NSAIs.

Vaginal administration of estradiol is a well known and safe alternative to systemic estrogen therapy, but studies demonstrated significant increases in plasma concentrations of estradiol. Such observations have also been reported in postmenopausal breast cancer patients treated with AIs. Further studies are needed to explore risk of breast cancer recurrence after vaginal estrogen application for patients on adjuvant endocrine therapy with AIs.

On account of the good prognosis for patients with breast cancer, improving or maintaining the quality of life in the aftercare period is becoming more and more important. In particular, the increasing usage of aromatase inhibitors in the past few years has led to an increased incidence of vaginal atrophy with symptoms such as vaginal dryness, petechial bleeding, dyspareunia and recurrent cystitis. And just these symptoms have a detrimental impact on the quality of life of breast cancer patients. Application of a topical estrogen therapy represents the most effective means to treat vaginal atrophy. The use of a systemic or, respectively, topical hormone therapy is, however, contraindicated for breast cancer patients. Further clinical trials are needed in order to assess the safety of vaginal estrogen therapy.

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