Purpose: The goal of this clinical trial is to determine whether probiotics can reduce gastrointestinal and psychosocial symptoms in post-treatment cancer survivors by impacting the gut microbiome.

Objectives: The main questions the investigators aim to answer are:

• Does taking the probiotic reduce gastrointestinal (e.g. belly pain) and psychosocial (e.g. depressive symptoms, fatigue) symptoms, and impact the gut microbiome?
• What relationships exist between gut bacteria, gastrointestinal, and psychosocial symptoms?

Methods: The study team will investigate this by giving a group of adult post-treatment cancer survivors either a probiotic capsule (intervention) or placebo capsule (control) over 12 weeks. Investigators will then analyze the bacterial diversity in participants' stool samples before and after these 12 weeks to see how the bacterial composition changed due to the treatment. Surveys will be used to ask participants questions about their physical and mental health, including specific gastrointestinal and psychosocial symptoms.

Implications: Cancer recovery is tough enough, and to minimize treatment-related long-term effects through a simple probiotic capsule would immensely improve the well-being and health of survivors.

Background: Survivors of cancer experience chronic gastrointestinal (GI) and psychosocial health symptoms, and reduced gut microbial diversity. This may compromise survivors' long-term health and overall wellbeing. No studies have investigated probiotics for managing both GI and psychosocial symptoms together, or the composition of the gut microbiota in post-treatment cancer survivors.

Aims: To investigate the effects of a probiotic vs. placebo on (i) abdominal pain and depressive symptoms (primary outcomes); (ii) (a) GI (i.e. gas/bloating, diarrhea, constipation) and psychosocial (i.e. anxiety, cognitive function, fatigue) symptoms, and general health; and (b) gut microbiota composition; (iii) relationships between bacteria, GI and psychosocial symptoms.

Methods: This double-blinded, placebo-controlled, 2-arm, randomized trial will recruit N=66 participants allocated to the probiotic or placebo group for a 12-week trial. Adult survivors diagnosed with a solid tumour or blood cancer who have completed chemotherapy within the last 5 years, and show elevated GI or psychosocial symptoms will be included. The probiotic capsule contains Lactobacillus and Bifidobacterium strains, ingested orally once daily. Stool samples will be collected at baseline and week-12 and analyzed using GA-Map dysbiosis test and 16S rRNA gene sequencing. GI and psychosocial surveys will be completed at baseline, weeks 6 and 12. Descriptive statistics, frequencies, paired samples t-tests, linear mixed models, and Spearman's correlations analyses will be used.

Implications: This study explores a novel, microbiota-based treatment for chronic GI and psychosocial symptoms in cancer survivors. These findings and commitment to patient-centred knowledge translation via creating patient materials (e.g. infographics, personal results summary) will enable patients to make informed decisions about their health.

Inclusion Criteria:

1. Male, female, and non-binary, any ethnicity
2. Aged 18 years or older
3. Diagnosed with hematological cancers (leukemia, Hodgkin's, and non-Hodgkin lymphoma), breast, osteosarcoma, Ewing's sarcoma, gynecological (cervical, endometrial), prostate, or testicular
4. Stages I - IV, including metastatic if stable and off treatment
5. Have received chemotherapy (with or without radiation, surgery, or hormone therapy)
6. Have completed primary cancer treatments
7. Within 5 years from their final cancer treatments, with emphasis placed on recruiting those within the first year post-treatment
8. Not currently pregnant or planning to become pregnant during the 12-week study
9. Evidence of clinically elevated levels (i.e. a score of 56 or higher) of GI and/or comorbid psychosocial symptoms as determined using PROMIS short forms for abdominal pain, gas/bloating, and general mental and physical health
10. Able to provide stool samples
11. Fluent in English, and have access to a computer, smartphone, or tablet with internet access to complete questionnaires
12. Provide written informed consent

Exclusion Criteria:

1. Diagnosis of central nervous system tumor, or colorectal cancer
2. Taken antibiotics and/or daily probiotic (including probiotic yogurt) within the 1 month prior to study participation
3. Currently or previously receiving immunotherapy
4. Diagnosed with irritable bowel syndrome or inflammatory bowel disease prior to being diagnosed with cancer
5. Diagnosed with a developmental/cognitive delay prior to cancer (e.g. autism spectrum disorder, downs syndrome)
6. Pregnancy

The study protocol will be published. Deidentified data will be made available to other researchers after primary data has been published and upon reasonable request to the PI.

1. The study protocol is anticipated to be published by/before winter 2023
2. Deidentified data will be made available to other researchers after primary data has been published (anticipated 2026)

Reasonable request to the PI

Participants will be randomized individually upon enrollment after completing baseline assessments into the probiotic or placebo condition in a 1:1 allocation ratio using a computer-generated stratified randomization strategy (to balance age and sex). Randomization sequence will be pre-determined by randomization software and sequential allocations prepared in sealed envelopes, only to be revealed to a research assistant after consent and baseline questionnaires are completed. Once randomized, a research assistant will be responsible for assigning the probiotic or placebo for each participant to their study package so that the study coordinator, who will have direct contact with patients, will remain blinded to participants' condition. A member of the study team who does not have contact with participants will also be provided a copy of the treatment allocation.