In conversation with my oncologist I was made aware that there are clinical studies which show that a probiotic could enhance the performance of immune checkpoint inhibitors - which was of particular interest as I was now taking pembrolizumab (Keytruda) alone.
The probiotic used in the studies is Clostridium butyricum MIYAIRA 588 - a specific strain of anaerobic, butyric acid forming, Gram-positive bacterium isolated from a soil sample in Japan.
Its primary use as a supplement is to regulate gut health (see the label above from the product I ordered from a supplier that imports from Japan).
As documented earlier in my blog - 7 years ago, I began a low carb/high fat diet which, along with green exercise and intermittent and periodic prolonged fasting, was key to losing 100 lbs and reversing type 2 diabetes.  I have maintained my weight and blood glucose by continuing to follow this regimen.  CBM 588 is believed to modulate some potential gastroinstestinal side effects of a ketogenic diet, increase insulin sensitivity and optimize lipid metabolism.  While more work needs to be done, in addition to bowel regulation, CBM 588 appears to be a good addition to my diet.
What is most impressive for my current health journey are the results being reported when CBM 588 is used with immune checkpoint inibitors - specifically for renal and non-small cell lung cancer.  The gut biome is known to play a key role in regulating the immune system.  Imbalances, known as dysbiosis can lead to immune disfunction. 
In a phase 1b trial for renal cancer - the objective response rate (ORR) was 58% for those receiving immunotherapy and CBM 588, compared to 20% for those receiving immunotherapy alone.  The immunotherapy was a combination of nivolumab (Opdivo) and ipilimumab (Yervoy).  Nivolumab is a PD-1 checkpoint inhibitor like pembrolizumab (Keytruda).   Ipilimumab targets the CTLA-4 receptor on T-cells.  The improvements are consistent with the hypothesis that supplementation with CBM 588 works to reduce dysbiosis, support the immune system and improve efficacy of immune checkpoint inhibitors.  A phase 1b trial examines safety - e.g. dosage and pharmokinetics.  A phase 2 trial is planned to look at efficacy (as well as further examination of safety and optimal dosing).
In the lung cancer study cited - 75% of the patients had a combination of chemotherapy and pembrolizumab.  The addition of CBM 588 was associated with a longer overall survival and progression free survival.
Less relevant for me, but interesting nonetheless - CBM 588 also been shown to be as effective against (non-muscle invasive) bladder cancer cells and potentially safer than the first immunotherapy  BCG in in-vitro and in-vivo laboratory studies.
I found a supplier of Strong Miyarisan, a supplement supplying CBM 588 on eBay (see label above).  Each tablet contains 30 mg CBM 588.  The recommended adult dose is 3 tablets 3 times per day or 270 mg.  The dose administered in the renal cancer trial was 80 mg twice per day or 160 mg.
I would welcome (and will lobby as I can for) a clinical trial of patients beginning a first line treatment of Padcev + Keytruda for locally advanced or metastatic muscle invasive bladder cancer  - one arm supplemented with CBM 588 - the other without.  This is the pathway that would allow doctors to prescribe CBM 588 as part of treatment.
In the meantime, I am proceeding with the assumption that CBM 588 may boost efficacy of pembrolizumab. I am going to start with 2 tablets 3 times per day or 180 mg.​  As this supplement is considered safe, I believe that at a minimum I will have good intestinal health, likely benefits for my ketogenic diet (including insulin sensitivity and improved bowel health) and potentially improved efficacy of Keytruda fighting any cancer that remains.  I will report any adverse effects.  I currently have no evidence of disease by imaging or circulating tumor DNA and I am just one patient so it will be difficult to know if the addition of CBM 588 has been effective (or harmful) - I will continue to monitor any relevant trials. 
As with any decision affecting treatment - patients should discuss with their care team.

I received my third negative circulating tumor DNA (Signatera) test today.
I discovered my disease - which turned out to be locally advanced muscle invasive bladder cancer (MIBC) - 390 days ago.
I had a transurethral resection of the bladder tumor (TURBT) 15 days later and began treatment with enfortumab vedotin + pembrolizumab (EV/pembro or Padcev/Keytruda) 89 days after discovering the disease.  The tumor sample was used by Natera to design my Signatera test to find evidence of residual disease by detecting tumor DNA shed into the blood from anywhere in the body. 
3 months after my TURBT, 1 month after beginning treatment, I underwent an endoscopic procedure to remove and possibly replace a stent in my right ureter placed during the TURBT due to the tumor obstructing flow from my right kidney.  The kidney flow was now brisk and there was no evidence of my primary tumor in the bladder.
84 days after beginning treatment (4 cycles) PET/CT now showed no significant uptake in my previously active pelvic lymph nodes or evidence of new metastatic disease.  Two follow-up CT scans with contrast would also show no new significant findings.
Given the good results, due to low grade adverse effects, we suspended the Padcev component after 7 cycles and have continued with Keytruda alone (currently 7 more cycles).  Sensation in my fingertips and toes, sense of taste and hair have since returned.
29% of the participants in the EV 302 trial (that led to the accelerated approval of EV/pembro as a first line treatment for patients with locally advanced or metastatic muscle invasive bladder cancer) had a complete pathological response.  I have no plans to pursue removal of or chemoradiation to the bladder.  I am working to see that more high responders are followed to see how durable the response is.  In the meantiume I hope to encourage more patients try EV/pembro as their first line treatment before pursuing radical cystectomy or bladder sparing with trimodality therapy.  The vision was articulated by a leader in the field - Dr. Shilpa Gupta - at the recent EMSO 2024 meeting