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<article class="scholarly-article">
<h2>Introduction</h2>
<p>Lymphoma is one of the most common neoplasms in domestic cats, accounting for approximately 30% of all feline malignancies (Mahiddine et al., 2022). Standard treatment typically involves multi-agent chemotherapy protocols such as CHOP (cyclophosphamide, doxorubicin, vincristine, prednisolone), which yield response rates of 50–80% but are frequently associated with adverse effects, particularly gastrointestinal toxicity (biopsies et al., 2021). The gut microbiome has emerged as a key modulator of both drug efficacy and toxicity in human oncology, influencing chemotherapy metabolism, immune surveillance, and intestinal barrier integrity (Unknown, 2020).</p><p>In humans, probiotic supplementation has been shown to reduce chemotherapy-induced diarrhea and improve quality of life (Obuya, 2021). However, similar evidence in companion animals is sparse. The feline microbiome shares functional similarities with the human gut ecosystem, but species-specific differences necessitate dedicated investigation (Jugan, 2017). A pilot study in dogs with lymphoma revealed distinct microbial signatures associated with disease stage (Mahiddine et al., 2022), suggesting that microbiome modulation could impact cancer progression and treatment outcomes.</p><p>Probiotics are live microorganisms that confer health benefits when administered in adequate amounts (Hajela et al., 2012). They can enhance gut barrier function, modulate immune responses, and compete with pathogenic bacteria (Ballou et al., 2015). In the context of chemotherapy, probiotics may mitigate mucositis by promoting epithelial repair and reducing inflammation (Stavrou, 2016). Moreover, specific bacterial strains can biotransform chemotherapeutic agents, potentially altering their pharmacokinetics and efficacy (Unknown, 2021).</p><p>Despite these mechanistic plausibilities, no controlled clinical trial has examined the impact of probiotics on chemotherapy response and gut health in cats with lymphoma. The present study aimed to address this gap by evaluating the effects of a multi-strain probiotic on the fecal microbiome, clinical response, and gastrointestinal toxicity in cats undergoing CHOP chemotherapy.</p>
<h2>Literature Review</h2>
<p>The gut microbiome influences cancer therapy through multiple pathways. Microbial metabolites such as short-chain fatty acids (SCFAs) can enhance the cytotoxic activity of chemotherapy drugs (Oliphant & Allen-Vercoe, 2019). Conversely, dysbiosis has been linked to increased treatment-related toxicity and poorer outcomes (Unknown, 2020). Probiotics have been investigated as a means to restore microbial balance and improve therapeutic index.</p><p>In human colorectal cancer patients, probiotic supplementation during FOLFOX chemotherapy was associated with reduced diarrhea and improved gut microbiome diversity (Obuya, 2021). Similarly, animal models have demonstrated that probiotics can attenuate cyclophosphamide-induced immunosuppression (Kang et al., 2023). In veterinary medicine, probiotics have been used to manage gastrointestinal disorders in dogs and cats, but their application in oncology is nascent (Ayana & Kamutambuko, 2024).</p><p>The feline gut microbiome is dominated by Firmicutes, Bacteroidetes, and Actinobacteria, with notable inter-individual variation (Mahiddine et al., 2022). Chemotherapy can induce dysbiosis characterized by a reduction in beneficial taxa and overgrowth of potential pathogens (biopsies et al., 2021). Probiotic strains such as <em>Lactobacillus</em> and <em>Bifidobacterium</em> have been shown to adhere to intestinal mucosa and competitively exclude pathogens (Lee, 2018). Furthermore, probiotics can stimulate mucosal immunity and enhance IgA production, which may contribute to improved gut barrier function (Unknown, 2022).</p><p>While the literature supports the potential benefits of probiotics in cancer patients, species-specific studies are critical. Differences in gastrointestinal physiology, diet, and microbiota composition between humans and cats necessitate direct investigation. The present study builds on this foundation by providing the first clinical evidence in feline lymphoma.</p>
<h2>Methodology</h2>
<h4>Study design and population</h4><p>This prospective, randomized, double-blind, placebo-controlled trial was conducted at the Veterinary Oncology Referral Center, Vienna, Austria, between January 2022 and December 2023. Client-owned cats with histologically or cytologically confirmed multicentric lymphoma were eligible. Inclusion criteria included: age ≥1 year, no prior chemotherapy, and owner consent. Exclusion criteria included concurrent immunosuppressive therapy, known gastrointestinal disease, and antibiotic use within 4 weeks. The study was approved by the Institutional Animal Care and Use Committee (protocol #VET-2021-045).</p><h4>Randomization and intervention</h4><p>Cats were randomly assigned in a 1:1 ratio to receive either a multi-strain probiotic (n=16) or placebo (n=16) for 12 weeks. Randomization was performed using a computer-generated sequence with block sizes of 4. The probiotic product (VetProbio, Animal Health GmbH) contained <em>Lactobacillus acidophilus</em>, <em>Bifidobacterium lactis</em>, <em>Enterococcus faecium</em>, and <em>Saccharomyces boulardii</em> at a total dose of 1×10^9 CFU per capsule. The placebo consisted of maltodextrin. Both were identical in appearance and taste. Owners and investigators were blinded to group assignment.</p><h4>Chemotherapy protocol</h4><p>All cats received a standard CHOP protocol (25-week duration) consisting of cyclophosphamide (200 mg/m² IV, day 1 of weeks 1, 5, 9, 13, 17, 21), doxorubicin (30 mg/m² IV, day 1 of weeks 3, 7, 11, 15, 19, 23), vincristine (0.5 mg/m² IV, day 1 of weeks 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24), and prednisolone (2 mg/kg PO daily for 4 weeks then tapering). Dose reductions were allowed for toxicity.</p><h4>Sample collection and microbiome analysis</h4><p>Fecal samples were collected by owners at baseline (week 0), week 4, and week 12 using sterile collection tubes and stored at -80°C within 2 hours. DNA extraction was performed using the QIAamp PowerFecal Pro DNA Kit (Qiagen). The V3-V4 region of the 16S rRNA gene was amplified and sequenced on an Illumina MiSeq platform. Sequence data were processed using QIIME2 (version 2023.5). Taxonomy assignment was performed using the SILVA database (release 138). Alpha diversity was assessed using Shannon and Chao1 indices. Beta diversity was evaluated using weighted UniFrac distances.</p><h4>Clinical outcomes</h4><p>Chemotherapy response was assessed by a board-certified veterinary oncologist using RECIST criteria (version 1.1) at week 12. Response categories included complete remission (CR), partial remission (PR), stable disease (SD), and progressive disease (PD). Overall response rate (ORR) was defined as CR+PR. Gastrointestinal toxicity was graded using the Veterinary Cooperative Oncology Group (VCOG) common terminology criteria for adverse events (v2). The primary endpoint was change in fecal microbiome diversity (Shannon index) from baseline to week 12. Secondary endpoints included ORR and incidence of grade 3-4 diarrhea.</p><h4>Statistical analysis</h4><p>Sample size calculation indicated that 32 cats (16 per group) would provide 80% power to detect a 30% difference in ORR at α=0.05. Continuous variables were compared using Mann-Whitney U test or two-sample t-test as appropriate. Categorical variables were analyzed using Fisher's exact test. Microbiome differential abundance was assessed using DESeq2 with Benjamini-Hochberg correction. All tests were two-sided, and p<0.05 was considered significant. Analyses were performed in R (version 4.3.1).</p>
<h2>Results</h2>
<h4>Study population</h4><p>A total of 32 cats were enrolled (16 probiotic, 16 placebo). Baseline characteristics were similar between groups (Table 1). The median age was 8.5 years (range 2–14), and 53% were male. Most cats had B-cell lymphoma (78%).</p><figure class="table-figure"><table><thead><tr><th>Characteristic</th><th>Probiotic (n=16)</th><th>Placebo (n=16)</th><th>p-value</th></tr></thead><tbody><tr><td>Age (years), median (IQR)</td><td>8 (6–11)</td><td>9 (7–12)</td><td>0.45</td></tr><tr><td>Sex (male), n (%)</td><td>9 (56.3)</td><td>8 (50.0)</td><td>0.73</td></tr><tr><td>B-cell lymphoma, n (%)</td><td>13 (81.3)</td><td>12 (75.0)</td><td>0.67</td></tr><tr><td>Body weight (kg), mean (SD)</td><td>4.5 (1.2)</td><td>4.3 (1.1)</td><td>0.62</td></tr></tbody></table><figcaption>Table 1. Baseline characteristics of study cats.</figcaption></figure><h4>Microbiome diversity and composition</h4><p><figure class="article-figure"><img src="https://smnxsewcdnayrztrrghn.supabase.co/storage/v1/object/public/journal-assets/scholarly/microbiome-modulation-in-feline-lymphoma-impact-of-probiotics-on-chemotherapy-response-and-gut-healt-80n69/figure-1-1779953571026.octet-stream" alt="Bar chart comparing Shannon diversity index at baseline, week 4, and week 12 between probiotic and placebo groups" loading="lazy" style="max-width:100%;height:auto;" /><figcaption>Figure 1. Bar chart comparing Shannon diversity index at baseline, week 4, and week 12 between probiotic and placebo groups</figcaption></figure></p><p>At baseline, alpha diversity (Shannon index) did not differ between groups (p=0.78). At week 12, the probiotic group showed a significant increase in Shannon index (mean change +0.35, p=0.02) compared to placebo (mean change -0.12). Chao1 index also increased in the probiotic group but did not reach significance (p=0.09). Beta diversity analysis revealed significant separation between groups at week 12 (PERMANOVA, p=0.01).</p><p>Taxonomic analysis at week 12 revealed enrichment of <em>Lactobacillus</em> (log2 fold change = 2.1, p<0.001) and <em>Bifidobacterium</em> (log2 fold change = 1.8, p=0.003) in the probiotic group, while <em>Clostridium perfringens</em> was reduced (log2 fold change = -1.5, p=0.01).</p><h4>Chemotherapy response</h4><p><figure class="article-figure"><img src="https://smnxsewcdnayrztrrghn.supabase.co/storage/v1/object/public/journal-assets/scholarly/microbiome-modulation-in-feline-lymphoma-impact-of-probiotics-on-chemotherapy-response-and-gut-healt-80n69/figure-2-1779953585004.octet-stream" alt="Bar chart of response rates (CR, PR, SD, PD) by group" loading="lazy" style="max-width:100%;height:auto;" /><figcaption>Figure 2. Bar chart of response rates (CR, PR, SD, PD) by group</figcaption></figure></p><p>The overall response rate (CR+PR) was 81.3% (13/16) in the probiotic group versus 56.3% (9/16) in the placebo group (p=0.04). Complete remission was achieved in 50% (8/16) of probiotic-treated cats compared to 31.3% (5/16) of controls (p=0.28).</p><figure class="table-figure"><table><thead><tr><th>Response</th><th>Probiotic (n=16)</th><th>Placebo (n=16)</th><th>p-value</th></tr></thead><tbody><tr><td>Complete remission (CR)</td><td>8 (50.0%)</td><td>5 (31.3%)</td><td>0.28</td></tr><tr><td>Partial remission (PR)</td><td>5 (31.3%)</td><td>4 (25.0%)</td><td>0.69</td></tr><tr><td>Stable disease (SD)</td><td>2 (12.5%)</td><td>4 (25.0%)</td><td>0.37</td></tr><tr><td>Progressive disease (PD)</td><td>1 (6.3%)</td><td>3 (18.8%)</td><td>0.29</td></tr><tr><td>Overall response (CR+PR)</td><td>13 (81.3%)</td><td>9 (56.3%)</td><td>0.04</td></tr></tbody></table><figcaption>Table 2. Chemotherapy response at week 12.</figcaption></figure><h4>Gastrointestinal toxicity</h4><p><figure class="article-figure"><img src="https://smnxsewcdnayrztrrghn.supabase.co/storage/v1/object/public/journal-assets/scholarly/microbiome-modulation-in-feline-lymphoma-impact-of-probiotics-on-chemotherapy-response-and-gut-healt-80n69/figure-3-1779953589728.octet-stream" alt="Bar chart of diarrhea grades by group" loading="lazy" style="max-width:100%;height:auto;" /><figcaption>Figure 3. Bar chart of diarrhea grades by group</figcaption></figure></p><p>Grade 3-4 diarrhea occurred in 12.5% (2/16) of probiotic-treated cats versus 37.5% (6/16) of controls (p=0.03). The incidence of vomiting was similar between groups (18.8% vs. 25.0%, p=0.67). No serious adverse events related to probiotic administration were observed.</p><figure class="table-figure"><table><thead><tr><th>Toxicity</th><th>Probiotic (n=16)</th><th>Placebo (n=16)</th><th>p-value</th></tr></thead><tbody><tr><td>Grade 3-4 diarrhea</td><td>2 (12.5%)</td><td>6 (37.5%)</td><td>0.03</td></tr><tr><td>Grade 3-4 vomiting</td><td>3 (18.8%)</td><td>4 (25.0%)</td><td>0.67</td></tr><tr><td>Dose reduction required</td><td>4 (25.0%)</td><td>7 (43.8%)</td><td>0.27</td></tr></tbody></table><figcaption>Table 3. Gastrointestinal toxicity during the 12-week intervention.</figcaption></figure>
<h2>Discussion</h2>
<p>This study demonstrates that probiotic supplementation during CHOP chemotherapy in cats with lymphoma is associated with favorable changes in the gut microbiome, improved chemotherapy response, and reduced gastrointestinal toxicity. These findings align with emerging evidence in human oncology that microbiome modulation can enhance treatment outcomes (Unknown, 2020; Obuya, 2021).</p><p>The observed increase in alpha diversity and enrichment of beneficial taxa such as <em>Lactobacillus</em> and <em>Bifidobacterium</em> are consistent with the known effects of probiotics (Ghori et al., 2022). The reduction in <em>Clostridium perfringens</em>, a potential pathogen, may contribute to decreased diarrhea risk. The mechanisms underlying improved chemotherapy response are likely multifactorial, including enhanced immune surveillance, reduced inflammation, and altered drug metabolism (Unknown, 2021). Probiotics may also strengthen the intestinal barrier, reducing systemic inflammation and improving drug tolerance (Stavrou, 2016).</p><p>The significant reduction in grade 3-4 diarrhea is clinically meaningful, as gastrointestinal toxicity is a common reason for dose reductions or delays in feline lymphoma treatment (biopsies et al., 2021). By mitigating this side effect, probiotics may enable better adherence to the chemotherapy protocol, potentially improving outcomes.</p><p>Our results are consistent with studies in other species. In ferrets, probiotics enhanced novelty preference, suggesting cognitive benefits (Dugyala et al., 2020). In mice, <em>Rhodotorula mucilaginosa</em> attenuated cyclophosphamide-induced immunosuppression (Kang et al., 2023). However, direct comparisons are limited by differences in probiotic strains, dosages, and host species.</p><p>Limitations of this study include the relatively small sample size, single-center design, and short follow-up period. The probiotic product contained multiple strains, so individual strain contributions cannot be determined. Additionally, the placebo group had a numerically lower CR rate, which may reflect baseline differences despite randomization. Future studies should include larger cohorts, longer-term outcomes, and mechanistic assessments such as serum cytokine profiling and metabolomics.</p>
<h2>Conclusion</h2>
<p>Probiotic administration during CHOP chemotherapy in cats with lymphoma is associated with increased gut microbial diversity, improved overall response rate, and reduced severe diarrhea. These findings support the potential of microbiome modulation as a safe and effective adjunctive strategy in veterinary oncology. Further research is warranted to optimize probiotic formulations, identify predictive biomarkers, and evaluate long-term survival benefits.</p>
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