How the Microbiome Influences the Success of Cancer Therapy

HAMBURG, Germany — The human microbiome comprises 39 to 44 billion microbes. That is ten times more than the number of cells in our body. Hendrik Poeck, MD, managing senior physician of internal medicine at the University Hospital Regensburg, illustrated this point at the annual meeting of the German Society for Hematology and Medical Oncology 2023. If the gut microbiome falls out of balance, then "intestinal dysbiosis potentially poses a risk for the pathogenesis of local and systemic diseases," explained Poeck.

Cancers and their therapies can also be influenced in this way. "Microbial diversity affects whether a tumor grows, whether it leads to inflammation, immune escape mechanisms or genomic instability, or whether therapeutic resistances develop," said Poeck.

Microbial diversity could be beneficial for cancer therapy, too. The composition of the microbiome varies significantly from host to host and can mutate. These properties make it a target for precision microbiotics, which involves using the gut microbiome as a biomarker to predict various physical reactions and to develop individualized diets.

Microbiome and Pathogenesis

The body's microbiome fulfills a barrier function, especially where the body is exposed to an external environment: at the epidermis and the internal mucous membranes, in the gastrointestinal tract, and in the lungs, chest, and urogenital system.

Association studies on humans and experimental manipulations on mouse models of cancer showed that certain microorganisms can have either protective or harmful effects on cancer development, on the progression of a malignant disease, and on the response to therapy.

A Master Regulator?

Disruptions of the microbial system in the gut, as occur during antibiotic therapy, can have significant effects on a patient's response to immunotherapy. Taking antibiotics shortly before or after starting therapy with immune checkpoint inhibitors (ICIs) significantly affected both overall survival (OS) and progression-free survival (PFS), as reported in a recent review and meta-analysis, for example.

Proton pump inhibitors also affect the gut microbiome and reduce the response to immunotherapy; this effect was demonstrated by an analysis of data from more than 2700 cancer patients that was recently presented at the European Society for Medical Oncology (ESMO) Annual Meeting 2023.

The extent to which the gut microbiome influences the efficacy of an ICI or predicts said efficacy was examined in a retrospective analysis published in Science in 2018, which Poeck presented. Resistance to ICI correlated with the relative frequency of the bacteria Akkermansia muciniphila in the gut of patients with cancer. In mouse models, the researchers restored the efficacy of the PD-1 blockade through a stool transplant.

Predicting Immunotherapy Response

If A muciniphila is present, can the composition of the microbiome act as a predictor for an effective ICI therapy?

Laurence Zitvogel, MD, PhD, and her working group at the National Institute of Health and Medical Research in Villejuif, France, performed a prospective study in 338 patients with non–small cell lung cancer and examined the prognostic significance of the fecal bacteria A muciniphila (Akk). The "Akkerman status" (low Akk vs high Akk) in a patient's stool correlated with an increased objective response rate and a longer OS, independently of PD-L1 expression, antibiotics, and performance status. The OS for low Akk was 13.4 months, vs 18.8 months for high Akk in first-line treatment.

These results are promising, said Poeck. But there is no one-size-fits-all solution. No conclusions can be drawn from one bacterium on the efficacy of therapies in humans, since "the entirety of the bacteria is decisive," said Poeck. In addition to the gut microbiome, the composition of gut metabolites influences the response to immunotherapies, as shown in a study with ICI.

Therapeutic Interventions

One possible therapeutic intervention to restore the gut microbiome is fecal microbiota transplantation (FMT). In a phase 1 study presented by Poeck, FMT was effective in the treatment of 20 patients with melanoma with ICI in an advanced and treatment-naive stage. Seven days after the patients received FMT, the first cycle with anti-PD-1 immunotherapy was initiated, with a total administration of three to four cycles. After 12 weeks, most patients were in complete or partial remission, as evidenced on imaging.

However, FMT also carries some risks. Two cases of sepsis with multiresistant Escherichia coli occurred, as well as other serious infections. Since then, there has been an FDA condition for extended screening of the donor stool, said Poeck. Nevertheless, this intervention is promising. A search of the keywords "FMT in cancer/transplant setting" reveals 46 currently clinical studies on clinicaltrials.gov.

Nutritional Interventions

Poeck advises caution about over-the-counter products. These products usually contain only a few species, such as Lactobacillus spp and Bifidobacterium spp. "Over-the-counter probiotics can even delay the reconstitution of the microbiome after antibiotics," said Poeck, according to a study. In some studies, the response rates were significantly lower after probiotic intake or led to controversial results, according to Poeck.

In contrast, Poeck said prebiotics (that is, a fiber-rich diet with indigestible carbohydrates) were promising. During digestion, prebiotics are split into short-chain fatty acids by bacterial enzymes and promote the growth of certain microbiota.

In this way, just 20 g of extremely fiber-rich food had a significant effect on PFS in 128 patients with melanoma undergoing anti-PD-1 immunotherapy. With 20 g of fiber-rich food per day, the PFS was stable over 60 months. The most significant benefit was observed in patients with a sufficient fiber intake who were not taking probiotics.

What to Recommend?

In summary, Poeck said that it is important to "budget" well, particularly with antibiotic administration, and to strive for calculated therapy with as narrow a spectrum as possible. For patients who experience complications such as cytokine release syndrome as a reaction to cell therapy, delaying the use of antibiotics is important. However, it is often difficult to differentiate this syndrome from neutropenic fever. The aim should be to avoid high-risk antibiotics, if clinically justifiable. Patients should avoid taking antibiotics for 30 days before starting immunotherapy.

Regarding nutritional interventions, Poeck referred to the recent Onkopedia recommendation for nutrition after cancer and the 10 nutritional rules of the German Nutrition Society. According to Poeck, the important aspects of these recommendations are a fiber-rich diet (> 20 g/d) from various plant products and avoiding artificial sweeteners and flavorings, as well as ultraprocessed (convenience) foods. In addition, meat should be consumed only in moderation, and as little processed meat as possible should be consumed. In addition, regular (aerobic and anaerobic) physical activity is important.

"Looking ahead into the future," said Poeck, "we need a uniform and functional understanding and we need a randomized prediction for diagnosis."

This article was translated from the Medscape German edition.

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