The role of intestinal microbiota in breast cancer treatment with hormone therapy: a pathway to new therapeutic options

Gut microbiota and host determinants evolve in symbiotic and dependent
relationships resulting in a personal ecosystem. In case of dysbiosis,
microbiota can instigate cancer development and even change response to
systemic cancer treatment.
High circulating estrogen levels are recognized as a causal factor for estrogen
receptor positive breast cancer development. Microbiota related estrogen
sources are the estrobolome (the aggregate of bacterial genes capable of
metabolizing estrogens) andbacterial ß-glucuronidase activity that increases
the availability of intestinal estrogen for reabsorption into the bloodstream.
Correlations between microbiota related estrogens and systemic estrogen levels
are already proven. However, there*s no knowledge on the influence of
microbiota composition in breast cancer treatment outcomes.
We hypothesize that microbiota and it's related ß-glucuronidase activity
influences intestinal metabolism of tamoxifen*s and it's related metabolite,
endoxifen.

The main goal is to show in postmenopausal estrogen receptor positive breast
cancer patients the influence of:
Microbiota composition and ß-glucuronidase activity on systemic endoxifen
levels during tamoxifen therapy.

An explorative prospective observational multicenter cohort study will be
conducted in Maastricht University Medical Center, Maxima Medical Center,
Elkerliek Hospital, Catharina Hospital, and Admiraal de Ruyter Hospital.

After informed consent, patient and tumor characteristics will be gathered.
Before and during hormone therapy, microbiota composition will be analyzed by
mass spectrometry and 16S rRNA Next Generation Sequencing, and absolute
abundance will be assessed through qPCR. Bacterial functional activity of ß-
glucuronidase will be measured to determine its influence on tamoxifen
metabolism and intestinal estrogen reabsorption. Blood estrogens and endoxifen
metabolites will be quantified by ultra-high
performance-liquid-chromatography-mass-spectrometry. Questionnaires on
patient*s compliance will be provided.

Postmenopausal breast cancer patients will be informed and asked to participate
in this study during their outpatient clinic visit by their physician (or their
representative; nurse practitioner / specialised nurse). After the physician
has provided initial information on the study, the researcher is asked to
explain the study in more detail, if the patient requests. Next, the patient is
asked whether the researcher can contact the patient after a minimum of 2 days,
by phone or face to face, to further explain the study. This followed by the
request to sign the informed consent (in duplicate) if the patient is willing
to participate. The informed consent will be co-signed by one of the
researchers. After informed consent, patients will undergo standard workup and
diagnostic procedures and treatments, according to the Dutch guideline.
Additional to standard treatment, fecal samples and questionnaires on
patients* (baseline) characteristics and compliance will be collected before
and during (after approximately 2 months) hormone therapy. Blood samples will
only be collected during tamoxifen therapy. Patients will have the ability to
collect their fecal samples and fill in the questionnaire up to 2 days before,
during or after their hospital visits. It will take 5 minutes to fill in the
questionnaire. All other procedures can take place during hospital visits.
Taken all together, blood collection introduces a minimal burden to the
patients.