Effect of FB301 on Vaginal Bacterial Profile in Women With Asymptomatic Vaginal Dysbiosis Undergoing a Mock Frozen Embryo Transfer Cycle

Vaginal dysbiosis characterized by reduced relative abundance of Lactobacillus species has been associated with reproductive outcomes in women undergoing frozen embryo transfer (FET). This study evaluates the safety, tolerability, and molecular effects of FB301 in women undergoing FET who have no known underlying clinical gynecological conditions but are identified as having molecular dysbiosis by metagenomic sequencing of a vaginal swab collected at screening.

Participants will be randomized to one of three arms: (1) FB301 following a single initial vaginal cleansing with chlorhexidine (an antiseptic solution), (2) FB301 following a single sham cleansing with saline, or (3) placebo following sham cleansing. Cleansing occurs once prior to initiation of study treatment. Participants will then receive 15 days of study treatment during a mock frozen embryo transfer (FET) cycle in which participants receive hormonal preparation of the endometrium but no embryo is transferred. Following completion of the mock cycle, participants may proceed with their planned frozen embryo transfer during a subsequent menstrual cycle, in accordance with standard clinical practice. Participants who become pregnant will be followed until birth.

The primary objective is to evaluate whether treatment increases the proportion of participants achieving a predefined Lactobacillus-dominant vaginal bacterial profile following therapy. Secondary objectives include evaluation of biochemical pregnancy (confirmed by serum beta-hCG testing) and clinical pregnancy (confirmed by transvaginal ultrasound) following the subsequent planned FET cycle.

• FB-301 (Drug): FB301 capsule
• Placebo (Drug): FB301 Placebo capsule
• Chlorhexidine (0.5%) Vaginal Cleanse (Other): Vaginal cleanse with 0.5% Chlorhexidine solution prior to first IP administration
• Sham vaginal cleanse (saline) (Other): Vaginal cleanse with saline solution prior to first IP administration.

Primary outcomes

• Proportion of participants with ≥90% combined relative abundance of Lactobacillus crispatus, L. jensenii, L. mulieris, L. gasseri, and L. paragasseri as measured by metagenomic sequencing. [Baseline to Day 16 (Mock FET cycle)]: Metagenomic sequencing is performed on cervical vaginal swab samples. Relative abundance is expressed as a percentage of bacterial sequencing reads that are assigned to a specific bacterial species.

Secondary outcomes

• Percentage change from baseline in relative abundance of Lactobacillus crispatus, L. jensenii, L. mulieris, L. gasseri, and L. paragasseri as measured by metagenomic sequencing. [Baseline to Day 16 (Mock FET cycle)]: Metagenomic sequencing is performed on cervical vaginal swab samples. Relative abundance is expressed as a percentage of bacterial sequencing reads that are assigned to a specific bacterial species.
• Biochemical pregnancy rate following frozen embryo transfer (FET), defined as a positive serum beta-human chorionic gonadotropin (hCG) test. [12-14 days after frozen embryo transfer (FET)]
• Clinical pregnancy rate following frozen embryo transfer (FET), defined as the presence of a fetal heartbeat confirmed by transvaginal ultrasound. [5-6 weeks after frozen embryo transfer (FET)]
• Frequency and intensity of all treatment-emergent adverse events (TEAEs), adverse events (AEs), serious adverse events (SAEs), and adverse events of special interest) AESI [Baseline to End of Study (up to neonatal follow-up assessment (Visit 11), approximately 10 months after FET)]
• The incidence of early pregnancy loss, defined as a positive beta-hCG test (biochemical pregnancy) at 12-14 days after FET but no transvaginal ultrasound-confirmed fetal heartbeat at 5-6 weeks after FET (Visit 7) [5-6 weeks after FET]

Sex: Female, ages: 18 Years to 45 Years

Inclusion Criteria:

1. Able to understand the study and sign a consent form.
2. Is a woman aged 18 to 45 years.
3. Still has regular menstrual periods (pre-menopausal).
4. Has had at least one previous frozen embryo transfer (FET) that did not result in pregnancy within the past 3 years, and this was not due to either male infertility or blocked /damaged fallopian tubes.
5. Willing to take part in mock frozen embryo transfer (FET) cycle and is considered medically suitable to subsequently have a frozen embryo transfer (FET) using one good-quality embryo (grade 3BB or higher).
6. Willing to have only one embryo transferred during the study cycle.
7. A vaginal swab taken at the first study visit shows a specific vaginal microbiome imbalance required for this study (vaginal molecular dysbiosis).
8. An ultrasound scan shows no problems with uterus or pelvis that would prevent pregnancy or greatly reduce the chance of pregnancy.
9. Recent fertility blood tests show FSH less than 12 IU/L (within the last 6 months)
10. Recent fertility blood tests show AMH greater than 1 ng/mL (at the time of oocyte retrieval).
11. Body mass index (BMI) is between 18 and 42 kg/m².
12. From the first study treatment until the mock FET cycle, agrees to:

    * Avoid vaginal intercourse or use condoms (without spermicide or lubricant),
    * Avoid swimming or hot tubs,
    * Avoid using tampons, menstrual cups, sex toys, vaginal cleansers, lubricants, or other vaginal products unless approved by the study team.
13. Willing to answer questions about sexual and reproductive activity during the study.
14. Willing to provide vaginal swab samples at the clinic.
15. Willing to receive the first study treatment in the clinic and use the study treatment at home as instructed.
16. Screening blood tests are negative for hepatitis B, hepatitis C, HIV, and syphilis.
17. Willing to attend all study visits and follow-up assessments and provide information about pregnancy and newborn health outcomes.

Exclusion Criteria:

1. History of five or more failed frozen embryo transfers (FET) in the past 5 years.
2. Have a condition that weakens the immune system (such as uncontrolled diabetes, active cancer, or an organ transplant).
3. Currently taking medications that suppress the immune system (such as systemic steroids, biologics, chemotherapy, or transplant medications).
4. Is already pregnant before completing the mock FET cycle.
5. Test positive for a sexually transmitted infection (such as chlamydia, gonorrhea, Mycoplasma genitalium, or Trichomonas vaginalis).
6. Current vaginal yeast infection that requires treatment.
7. Cannot safely take the hormone medications required for a frozen embryo transfer (such as estradiol or progesterone).
8. Use of intravaginal progesterone gels or solutions.
9. Needs regular antibiotics or are likely to require antibiotics during the study.
10. Has taken oral, injectable, or vaginal antibiotics within 30 days before screening.
11. History of gynecological cancer or other significant gynecological condition that would make participation unsafe.
12. Has a medical condition that would make participation unsafe or difficult.
13. Has an unstable medical or psychiatric condition that could make it difficult to follow study requirements.
14. Has any other condition that could interfere with participation.