Probiotic Supplements on Oncology Patients’ Treatment-Related Side Effects: A Systematic Review of Randomized Controlled Trials

Cancer affects more than 19.3 million people and has become the second leading cause of death worldwide. Chemo- and radiotherapy, the most common procedures in these patients, often produce unpleasant treatment-related side effects that have a direct impact on the quality of life of these patients. However, innovative therapeutic strategies such as probiotics are being implemented to manage these complications. Thus, this study aimed to evaluate the efficacy of probiotics supplements as a therapeutic strategy in adult oncology treatment-related side effects. A systematic review of randomized controlled trials was conducted in PubMed, Scielo, ProQuest and OVID databases up to and including January 2021, following the PRISMA guidelines. The quality of the included studies was assessed by the Jadad Scale. Twenty clinical trials published between 1988 and 2020 were included in this review. Seventeen studies (85%) revealed predominantly positive results when using probiotics to reduce the incidence of treatment-related side effects in oncology patients, while three studies (15%) reported no impact in their findings. This study sheds some light on the significance of chemotherapy and radiotherapy in altering the composition of gut microbiota, where probiotic strains may play an important role in preventing or mitigating treatment-related side effects.


Introduction
According to the World Health Organization (WHO), cancer is the second leading cause of death, affecting more than 19.3 million people and claiming 10 million lives worldwide, and the number of new cases is expected to double by 2040 [1]. This disease is diagnosed differently in men and women, with one in every five people developing cancer at some point in their lives, resulting in the death of one in every eight men and one in every eleven women diagnosed with cancer. In this sense, breast, colorectal, lung, cervical, and thyroid cancer are the most common cancers in women, while lung and prostate cancer are the most common in men [2].
There are diverse therapeutic strategies to reduce cell proliferation and disease progression, with surgery, chemotherapy, radiotherapy, and, more recently, immunotherapy and hormone therapy being the most commonly used treatments [3,4]. These treatments have significant side effects, particularly chemotherapy and radiotherapy, which is why it is frequently necessary to use combination treatments to increase effectiveness, despite the fact that this strategy multiplies side effects [5]. As a result, cancer treatments have the greatest impact on cells with the highest rate of cell division, resulting in low cell counts in blood cells, which manifests as anemia, infections, and bleedings. Likewise, gastrointestinal cells are also altered, resulting in nausea, vomiting, diarrhea, taste disturbances, mucositis, and swallowing difficulties [6,7], which cause many patients to postpone or discontinue their treatments [8].
Chemo-and radiotherapy modify the composition of intestinal microbiota in a process known as dysbiosis, which is often associated with biochemistry and immunologic disorders in the gastrointestinal tract [9,10]. Multiple strategies are being developed to modify microbiota with the underlying idea of propelling this dysbiosis toward eubiosis or the hemostasis of the gut microbiota in order to prevent or inhibit cancer progression [11,12]. In this regard, it has been reported that paclitaxel, a mitosis inhibitor, is able to increase matrix metalloproteinase 9 (MMP9) and tumor necrosis factor-alpha (TNF-α) levels and alter bacterial diversity in female mice colon [13]. Probiotics, defined as live microorganisms that provide a health benefit to the host when administered in adequate amounts [14], have been shown to be effective in the management of diarrhea and constipation, as well as highly effective in the treatment of inflammatory bowel diseases by improving bowel function [15,16]. For example, a probiotic mixture improved altered intestinal tight junction levels in mice with dextran sodium sulfate (DSS)-induced colitis [17]. Consequently, probiotics containing one or more strains could indeed restore the composition of altered gut microbiota and improve certain parameters, leading to significant homeostasis in animal models of obesity, Parkinson's disease, and depression [15,18,19]. Similarly, immune function may improve after the administration of a probiotic combination. Treatment with Bifidobacterium longum, Lactobacillus lactis, and Enterococcus faecium significantly reduced the occurrence of radio-chemotherapy-induced oral mucositis, as well as increased CD4+, CD8+, and CD3+ T cells in oncological patients [20]. In that manner, 5-fluorouracilinduced intestinal mucositis has also shown an improvement after probiotic treatment by reducing TNF-α, IL-6, and IFN-γ levels in mice [21].
In this context, corticosteroids and antiemetics are key elements in oncology to be used prior to the administration of chemotherapy to avoid side effects [5]. However, relatively little is understood about including probiotics in this preventive regimen due to beneficial results in intestinal disorders and altered immunity, which could be of great interest in reducing certain oncology treatment-related side effects, such as diarrhea, mucositis, or constipation. Therefore, this review aims to evaluate the efficacy of probiotic supplements to ameliorate chemo-and radiotherapy-related side effects in adults.

Design
A systematic review of randomized controlled trials was undertaken in January 2021, following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines (Supplementary File 1) [22]. This review used a structured Patient-Intervention-Outcome (PIO) question as follows [23]: "In adult oncology patients (P), what is the efficacy of probiotics supplements (I) on treatment-related side effects (O)?" The protocol for this review was not registered.

Search Strategy
The electronic databases included PubMed, Scielo, ProQuest, and OVID, using natural and structured language in the following search strategy: ( . This search strategy was adapted for use across databases (Appendix A). "Randomized clinical trial", "humans", and "adult:19+ years" search filters were applied for this search strategy.

Selection Criteria
The following inclusion criteria were used: (i) randomized clinical trials, (ii) published in English or Spanish, (iii) related to the aim of the study; the use of probiotics supplements on adult oncology-related treatments and side effects; and (iv) published until January 2021. Likewise, the exclusion criteria included (i) studies on other pathologies than cancer or symptoms related to cancer treatments, (ii) symbiotics and other treatment combinations, (iii) re-publications, and (iv) studies with animals. No articles were excluded after quality appraisal.

Data Screening
Initially, the two authors (MR, AM) performed a first screening by titles and abstracts, following the selection criteria independently and in duplicate. Once a third author (CR) double-checked the screening and discussed any discrepancy, a full-text reading was performed for their quality appraisal by authors.

Quality Appraisal
The quality of selected articles was assessed by two researchers independently (MR, AM). Any disagreements on quality ratings were discussed with a third author (CR) and a consensus was reached. The Jadad Scale of Clinical Trials was used to assess the methodological quality of experimental human studies included. This is a scale with five simple items and it has known reliability and external validity. A score below 3 points indicate low quality based on (i) the quality of randomization, (ii) double blinding, and (iii) drop-outs extracted from each study [24].

Data Abstraction and Synthesis
Consecutively, the relevant data from the included studies were extracted and tabulated according to (i) authors, (ii) country, (iii) population, (iv) probiotic strains, (v) variables, (vi) measures, and (vii) main findings.

Characteristics of Selected Studies
Firstly, a total of 402 articles were retrieved through databases searching (PubMed (n = 349), Scielo (n = 6), ProQuest (n = 29) and OVID (n = 18)), from which 68 papers were discarded by duplicity. After title, abstract, and full-text screening, a total of 314 articles were excluded following the selection criteria. Twenty studies were included in this review ( Figure 1).
All trials and patients' characteristics are summarized in Table 1. On the whole, all individuals were treated using conventional cancer therapy methods, such as radiotherapy (n = 11, 55%), chemotherapy (n = 6, 30%), or both (n = 3, 15%). In some studies, sex was not specified (n = 2, 10%), and only women were included in studies dealing with specific carcinomas of the female reproductive tract (n = 4, 20%), such as endometrial, vaginal, uterine, and cervical cancers. The age range of the patients ranged from 18 to 75 years old (with a mean age of 57.41 years), enrolling a total of 2508 participants. All studies included were published between 1988 and 2020, and 15 studies (75%) were not registered in any clinical trial registry. Most of these studies were conducted in Asia (n = 9), Europe (n = 8), but also in America (n = 2) and Oceania (n = 1). As regards the use of probiotics, 10 of the selected studies (50%) used a single probiotic strain, while the remaining 10 (50%) used two or more probiotics combined. The presentation and forms of administration varied from study to study, with the most commonly used forms being capsules, gelatine, and yoghurt. The time of administration as well as the dose administered to the patients were also varied, which ranged from 1 to 24 weeks and 10 6 to 10 11 CFU/day, respectively. Finally, 17 studies (85%) revealed predominantly positive results when using probiotics to reduce the incidence of treatment-related side effects in oncology patients, while three studies (15%) reported no impact in their findings.  All trials and patients' characteristics are summarized in Table 1. On the whole, all individuals were treated using conventional cancer therapy methods, such as radiotherapy (n = 11, 55%), chemotherapy (n = 6, 30%), or both (n = 3, 15%). In some studies, sex was not specified (n = 2, 10%), and only women were included in studies dealing with specific carcinomas of the female reproductive tract (n = 4, 20%), such as endometrial, vaginal, uterine, and cervical cancers. The age range of the patients ranged from 18 to 75 years old (with a mean age of 57.41 years), enrolling a total of 2508 participants. All studies included were published between 1988 and 2020, and 15 studies (75%) were not registered in any clinical trial registry. Most of these studies were conducted in Asia (n = 9), Europe (n = 8), but also in America (n = 2) and Oceania (n = 1). As regards the use of probiotics, 10 of the selected studies (50%) used a single probiotic strain, while the remaining 10 (50%) used two or more probiotics combined. The presentation and forms of administration varied from study to study, with the most commonly used forms being capsules, gelatine, and yoghurt. The time of administration as well as the dose administered to the patients were also varied, which ranged from 1 to 24 weeks and 10 6 to 10 11 CFU/day, respectively. Finally, 17 studies (85%) revealed predominantly positive results when using probiotics to reduce the incidence of treatment-related side effects in oncology patients, while three studies (15%) reported no impact in their findings. The data synthesis revealed four categories related to the use of probiotic supplements for treatment-related side effects in clinical oncology. For that matter, these categories would study the effects of probiotic treatments for different treatment-related side effects in oncology such as gastrointestinal side effects, immune-related side effects, inflammatory side effects, and performance status-related side effects. These categories are described below.

Gastrointestinal Side Effects
Probiotics have been shown to be effective in the treatment of some common oncology treatment-related gastrointestinal adverse reactions, as demonstrated in 11 of 20 trials (55%) [25][26][27][28][29][30][31][33][34][35][36]. The main adverse effects identified and treated were mainly diarrhea, with other drawbacks being abdominal pain, nausea and vomiting, constipation, bloating, abdominal distension, and lactose intolerance caused by chemotherapy. The most commonly probiotic strains used along these studies were Lactobacillus acidophilus; L. rhamnosus GG ATCC 53103; and L. casei var. rhamnosus. Likewise, other probiotic strains used in combination were (L. acidophilus LA-5 along with Bifidobacterium animalis subsp. Conversely, 2 trials (10%) showed inconclusive results for their benefits to control stool constituency and flatulence, although their findings were promising to prevent radiotherapy-induced diarrhoea [32,37]. The probiotic strains used in these studies in-cluded: L. acidophilus NCDO1748, and (S. thermophilus, L. delbrueckii subsp. bulgaricus, and L. casei DN-114 001). The treatment for these studies ranged from 1 to 6 weeks and were observed only in women.

Immune-Related Side Effects
Despite having only one study [38], positive results of the probiotics on immunerelated side effects have also been observed. A combination of probiotic strains (Bifidobacterium, Lactobacillus and S. thermophilus) was used for 1 to 2 weeks, in which patients improved their immune and nutritional status as well as rehabilitation, showing improved cellular immune parameters and tolerance to abdominal pain, bloating and diarrhoea. These authors used glutamine along with fish oil in their treatment as it has been shown to enhance epithelial cell growth and repair of intestinal mucous membrane, prevent bacterial translocation and reduce barrier injury, among others, which may actually be able to work synergistically with probiotics to protect the intestinal mucosa barrier and reduce permeability. In this manner, radiation-induced injuries may be alleviated by these probiotic strains, while other eco-nutrients feed the intestinal membrane.

Inflammatory-Related Side Effects
Impact on inflammatory-related side effects such as oral mucositis was also reported in three trials (15%). Among these studies, two trials [20,40] showed positive and effective results in reducing the severity of oral mucositis when using different probiotic strains: (B. longum, L. lactis, and E. faecium) and L. brevis CD2. The treatment period for these studies was from 1 to 7 weeks. However, De Sanctis et al. (2019) [39] did not notice any significant changes in the severity of oral mucositis with L. brevis CD2, although their treatment lasted only 1 week due to premature closure of patient accrual. While it is true that radiochemotherapy-induced mucositis is a complex process and further prospective studies are needed to explore oral microbiota modulation in reducing its incidence, the findings of Jiang and collaborators (2019) [20] and Sharma and collaborators (2012) [40] strongly underpinned the probiotics used as a plausible strategy to manage mucositis-associated pain and reduce its incidence.

Performance Status-Related Side Effects
Concerning to the impact of probiotics in patients' general well-being and activities of daily life, three trials (15%) evaluated their effects over a 4-week treatment period [41][42][43]. Two of these studies used a single probiotic strain, S. salivarius M18 and L. casei LC9018 respectively, and the remaining study used a combination of L. acidophilus, L. rhamnosus, B. longum, and Saccharomycesboulardii. In line with the findings of Shao and collaborators (2014) [38], not only did Doppalapudi and collaborators (2020) [42] and Vesty and collaborators (2020) [41] observe clinical improvements driven by probiotic-induced changes in the oral microbiota but also a potential mechanism to improve these performance status-related side effects throughout other modulating host immune response and microbial interactions. Having said that, only one study [43] assessed the effect of probiotics in malignant pleural effusion, which is one of the most common complications in lung cancer. This complication can have a severe impact on patient performance and shortened survival, but interestingly, L. casei LC9018 has been shown to be a useful adjuvant in the treatment of this type of cancer and to prevent this complication.

Quality Assessment
On the Jadad Scale, the average quality of the analyzed studies was 3.75 ( Figure 2). Its reporting quality varied from 2 (in four studies), 3 (in two studies), 4 (in nine studies), and 5 (in five studies), with none of them having an inappropriate reporting quality (lower than 1). At last, four of the studies reviewed received support from different manufacturers, indicating the possibility of a sponsorship bias [26,31,36,40].

Discussion
This review was aimed to evaluate the efficacy of probiotics supplements as a therapeutic strategy for treatment-related side effects in adult oncology patients. After analyzing 20 randomized controlled trials, our findings showed the beneficial effects that probiotic may have in a range of common treatment-related side effects, which have a direct impact of the oncology patients' quality of life. In this manner, 11 of 20 studies (55%) observed positive outcomes among gastrointestinal adverse effects management such as diarrhea, abdominal pain, nausea, and vomiting among others. Similarly, another six studies (30%) reported promising results in the control of immune and inflammatory re-

Discussion
This review was aimed to evaluate the efficacy of probiotics supplements as a therapeutic strategy for treatment-related side effects in adult oncology patients. After analyzing 20 randomized controlled trials, our findings showed the beneficial effects that probiotic may have in a range of common treatment-related side effects, which have a direct impact of the oncology patients' quality of life. In this manner, 11 of 20 studies (55%) observed positive outcomes among gastrointestinal adverse effects management such as diarrhea, abdominal pain, nausea, and vomiting among others. Similarly, another six studies (30%) reported promising results in the control of immune and inflammatory responses, as well as other side effects related to their overall well-being and daily life activities. These findings further support the idea of previous reviews [44,45], suggesting that microbiota plays a key role in the pathogenesis of some treatment-related side effects, although further evidence is needed to determine their safety and accuracy [46,47].
The studies included in this review were heterogenous in the use of probiotic strains, where Lactobacillus acidophilus (LA-5, BMC12130, LAC-361, and NCDO1748) was the most widely used strain among other 15 different strains, both in single strain [29,37] and multiple strain trials [25][26][27][28]31,34,42]. This heterogeneity added to the number of cancers included may explain some of the between-studies variability of the results [47]. Another possible explanation may be the interindividual diversity of the microbiota composition, where personalized medicine might well contribute to predicting the most suitable probiotic strain for the individual [48]. In this vein, strong evidence suggests that the efficacy of probiotics is strain-specific as well as disease-specific, and therefore, these factors should be considered when recommending the best probiotic for the patient [49]. Furthermore, the duration of treatment may also have to be considered to demonstrate probiotic clinical position in the oncology of treatment-related side effects, 4 weeks being the most common duration of treatment among the studies included. These results are consistent with the findings of De Sanctis and collaborators (2019) [39], who stated that a probiotic treatment period of less than 4 weeks may not be sufficient to observe and confirm their beneficial effects. However, to date, there are not standardized procedures available on the minimum treatment duration for the selected probiotic strain in order to observe positive outcomes, as it requires time to promote gut microbiota re-shaping and, as a result, the beneficial effect [50].
In reference to the treatment-related side effects, authors such as Delia and collaborators (2007) [34], Golkhalkhali and collaborators (2018) [26], as well as Osterlund and collaborators (2007) [33] among others, concur that the use of probiotics and microbial cell preparations improves the intestinal immune barrier, particularly intestinal IgA responses. In line with the results of other studies, these probiotic strains are able to stabilize the intestinal microbial environment and improve the permeability of the intestinal barrier, leading to a reduction in inflammatory response and promoting changes in the intestinal flora [51,52]. This promotes an ideal environment for the growth of non-pathogenic bacteria, helping to protect epithelial cells, the process of apoptosis, and some cytoprotective processes [53]. Interestingly, similar results were found using probiotic strains such as Lactobacillus, Bifidobacterium, or Streptococcus along with other eco-nutrients such as glutamine and fish oil [38]. These results match those observed in recent preclinical studies [54,55], where the colonization of this bacteria genera enhanced the immune and anti-inflammatory response to radiation, forming an enteric-intestinal barrier that increased the thickness of the intestinal flora. Moreover, the optimization of the medium promotes the life of living microorganisms, which can restore the balance of a radiation-damaged microecosystem by repairing the intestinal membrane, inhibiting the growth of intestinal pathogens, and reducing endotoxin production [56]. These probiotic strains are antioxidant agents that act by eliminating free radicals produced by ionization and preventing lipid oxidation, thereby prioritizing the repair and regeneration of the cell membrane, DNA, and proteins, resulting in their high efficacy in reducing abdominal pain, flatulence, and diarrhea, as these authors highlight in their findings [38,56].
In accordance with these findings, Holma and collaborators (2013) [30] underline the importance of fecal pH and methane production in this type of patient, where intestinal microbiota plays a central role in the incidence of unpleasant side effects such as diarrhea and constipation, bloating, or abdominal inflammation. These results confirm the association between the higher production of elements such as methane and microbiota, where a higher production of methane is associated with a lower incidence of diarrhea and a methane deficiency is associated with a higher incidence of abdominal discomfort [57]. In this context, the results showed that the L. rhamnosus GG ATCC 53,103 strain did not alter the production of pH or methane, as opposed to studies such as Salminen and collaborators (1988) [37], in which L. acidophilus NCDO1748 was administered and increased flatulence was observed, pointing directly to the lactulose content as a non-absorbable substrate, a mechanism favoring the production of methane and probiotic absorption. In this sense, Osterlund and collaborators (2004) [35] provide information on lactose intolerance caused by low intestinal villus height in relation to its depth of treatment, resulting in malabsorption syndrome and therefore hindering the production of diarrhea, flatulence, and abdominal pain [58]. In line with the overall evidence, it is worth noting how adverse effects can be managed by modifying gut microbiota and methane production mechanisms [30,59]. Replacing lactulose with another non-absorbable substrate would not cause diarrhea and would, in turn, allow the amount of methane to be controlled to achieve balance in intestinal transit, vary the amount of substrate administered, and greatly improve or even reduce the number of treatment doses administered to patients [35,57].
On the other hand, oral mucositis and oral health stand as one of the most treated side effects as they significantly reduce the patients' quality of life [60]. In that matter, probiotics such as B. longum (BCMC02120, BB-536), L. lactis BCMC12451, E. faecium, and L. brevis CD2 have shown to reduce the incidence of severe oral mucositis by promoting the growth and protection of the bacterial flora and, as a result, decreasing the number of adverse effects, severity, and incidence of mucositis [20,[40][41][42]. These findings are in agreement with those of Vesty and collaborators (2020) [41], who identified that using S. salivarius M18 improved patients' quality of life by reducing the number of oral infections (candidiasis) and adverse effects (mucositis, diarrhea) that these patients experienced after their treatments. However, recent research found that the effects of L. brevis CD2 were unable to confirm its beneficial impact for severe oral mucositis, though one possible explanation for these findings could be the premature closure of patient accrual [39]. Lastly, it is also interesting to note the effect of probiotics on other side effects of these patients such as pleural effusion, which can severely affect their performance status and even shorten their life expectancy. Only Masuno and collaborators (1991) [43] evaluated the use of the L. casei LC9018 strain against this complication, demonstrating promising results in controlling pleural effusions by reducing the number of malignant cells at the pleural level, which are supported by preclinical models [61,62].

Limitations
That being said, there are some limitations to bear in mind when interpreting the findings of this study. Fifteen of the analyzed studies were not registered, and therefore there may be a risk of reporting bias, whereas these studies are consistent trials on the Jadad Scale. On the other hand, heterogeneity in strains, length of treatment, and population could be confounding factors, and hence, generalizations should be made with caution. As a result of this heterogeneity in strains, interventions, and data collection methods, neither meta-analysis nor meta-regression were considered in this review. Given the small number of studies included, further work is still needed on the clinical position of probiotic supplements in adult oncology treatment-related side effects, in particular to determine the efficacy of individual probiotic strains, which could help to compare strains and lead more closely to preventive approaches.
As a whole, this review contributes to the existing literature, providing evidence of the current clinical position of probiotics supplements for some common treatmentrelated side effects in adult oncology patients. Despite the main findings of these studies concluded in terms of the safety and efficacy of probiotics supplements for the treatment or prevention of these side effects, further research with larger groups, specific strains, and duration of treatment is needed to conclude the beneficial effects for each of these side effects. More broadly, research is needed to determine the effects of individual and combined probiotic strains in order to draw confident conclusions about their benefits for both general oncology treatment-related side effects and specific cancers. Future research will be particularly interesting in determining how the use of probiotics and prebiotics may enhance the beneficial effect of the first to improve therapeutic responses in patients with cancer.

Conclusions
This study has shown that some probiotic strains (L. acidophilus, L. casei, B. longum, or L. rhamnosus among others) are a valid therapeutic strategy in some common treatmentrelated side effects in adult oncology patients, using both single or multiple strain combinations for at least 4 weeks of treatment. The beneficial variation between the different strains in the selected studies has been similar, which is why all of them represent a possible strategy for complications such as gastrointestinal side effects, immune or inflammatory side effects, and performance status-related side effects. Furthermore, despite its exploratory nature, this study provides some insight into the importance of chemotherapy and radiotherapy, inducing major changes in the composition of microbiota, where these probiotic strains may play an important role to prevent or treat such complications.

Implications for Clinical Practice
Common treatment-related side effects such as diarrhea, vomiting, mucositis, or abdominal pain are unpleasant for patients who have to undergo chemo-or radiotherapy treatments. Although more research is clearly needed, it has been shown that the gut microbiota plays a key role in immunity, and therefore, probiotics could be considered as a potential therapeutic strategy for treating and preventing these complications in immunocompromised cancer patients. Certain probiotic strains (e.g., Lactobacillus or Bifidobacterium) have shown to be safe and effective for some of these effects secondary to chemo-and radiotherapy, but also to significantly enhance immune response in these patients. Rather than concluding on this topic, this review provides a common ground to explore more in detail the use of certain probiotic strains for common side effects such as pleural effusions, which have a profound impact on the quality of life and life expectancy of these patients.

Data Availability Statement:
The dataset used and/or analyzed in this study is available from the corresponding author on reasonable request.