Maria Pallozzi

Tryptophan (Trp) metabolism lies at the intersection of nutrition, gut microbiota, mucosal immunology, and systemic inflammation—processes that play key roles in many gastrointestinal and extraintestinal diseases. Trp is an essential amino acid obtained through dietary intake. Beyond its role in protein synthesis, it is metabolized through three principal pathways: the kynurenine pathway, the serotonin/melatonin pathway, and microbial metabolism in the gut leading to indole and related derivatives. The kynurenine pathway represents the primary route of Trp degradation and is strongly linked to inflammatory signaling. The serotonin pathway is particularly important for gastrointestinal physiology, influencing motility, secretion, and visceral sensitivity. In parallel, microbial Trp metabolism produces metabolites that regulate epithelial barrier integrity, modulate mucosal immune responses, and contribute to communication along the gut–organ axes. Across different disease states, several recurring patterns emerge. Inflammatory conditions frequently shift Trp metabolism toward the kynurenine pathway through increased IDO1 or TDO activity, resulting in changes in kynurenine metabolites and in the kynurenine-to-tryptophan (Kyn/Trp) ratio. At the same time, reduced microbial production of indole derivatives may impair aryl hydrocarbon receptor signaling and weaken barrier-protective and immunoregulatory mechanisms. Alterations in the serotonin pathway are also associated with disturbances in gastrointestinal motility and gut–brain communication. Together, these observations highlight Trp metabolism as an important framework for understanding interactions between diet, microbiota, and host responses in health and disease. However, it is important to clarify that much of the currently available evidence remains associative or is derived primarily from preclinical models. This narrative review, based on literature retrieved from major biomedical databases (e.g., PubMed/MEDLINE, Scopus, and Web of Science), aims to provide an updated synthesis of current knowledge on Trp metabolism in disease pathophysiology. Furthermore, while we highlight potential translational applications—such as proposed biomarker development and targeted therapeutic strategies—these perspectives have been moderated to acknowledge the limited level of clinical validation established to date.

Immune checkpoint inhibitors (ICIs) have revolutionized the therapeutic landscape for hepatocellular carcinoma (HCC); however, a considerable proportion of patients do not achieve durable clinical benefits. This highlights the need for reliable predictive biomarkers, which are currently lacking. The accumulated evidence supports a relevant role of the gut–liver axis in modulating immunotherapy outcomes, and several studies have identified distinct microbial features associated with either responders or non-responders. Responders to immunotherapy frequently present with higher microbial diversity and enrichment of beneficial taxa, whereas the expansion of pro-inflammatory and pathogenic bacteria has been associated with primary resistance and increased treatment-related toxicity in non-responders. However, the available findings remain heterogeneous across cohorts, likely owing to differences in geography, diet, liver disease etiology, treatment regimens, and microbiome analytical methods. Machine-learning models integrating metagenomic and metabolomic data have shown encouraging results in defining microbial signatures associated with treatment outcomes, although variability among cohorts currently limits their clinical applicability and generalizability. Beyond microbial taxonomic composition, microbiota-derived metabolites—such as short-chain fatty acids, bile acids, inosine, and tryptophan catabolites—appear to play a crucial role in shaping the tumor microenvironment and host immune responses, thus representing additional candidate biomarkers, also due to the relative ease of their measurement. Finally, microbiota-targeted interventions are emerging as potential strategies to enhance immunotherapy efficacy. Overall, the gut microbiome and its metabolic activity represent promising tools, albeit still under investigation, for patient stratification and personalized management in HCC treated with ICIs. Therefore, this review aims to summarize and critically discuss the current evidence on gut microbiota-derived biomarkers of response and resistance to ICIs in HCC, with particular focus on microbial composition, microbiota-related metabolites, and emerging microbiome-based therapeutic strategies. This narrative review provides an updated overview of the role of gut microbiota as both a biomarker and a therapeutic target in patients with hepatocellular carcinoma (HCC) receiving immune checkpoint inhibitor (ICI) therapy.

Hepatocellular carcinoma (HCC) is the sixth most diagnosed cancer worldwide and represents the second cause of cancer-related death worldwide, according to the estimates by Global Cancer Observatory in 2020. Its prognosis is strictly associated with neoplastic stage and liver function. Several treatments are involved in the management of HCC, according to its stage and the patients’ performance status. The abscopal effect is a particular phenomenon taking place in locally advanced or in metastatic cancer, as a positive off-target result of ionizing radiation at a certain distance from the irradiated zone determining a systemic effect due to the increase in tumor immune response, and resulting in the shrinkage of neoplastic lesions with a subsequent increase in tumor prognosis. If future studies will allow one to master its etiological mechanisms and deliberately trigger them, it could represent a notable weapon in the treatment of hepatocellular carcinoma, particularly at advanced stages, in which sometimes it is able to positively modify the patients’ prognosis. The aim of this review is to evaluate the literature available on this intriguing topic and the characteristics of the mechanisms that originate the abscopal effect, the treatments that can elicit this phenomenon, and the possible relevant therapeutic implications.

Hepatocellular carcinoma (HCC) is characterized by a complex disruption of hemostatic balance, increasing the risk of both thrombotic and hemorrhagic events. Thrombotic complications, most notably portal vein thrombosis (PVT) and venous thromboembolism (VTE), have a significant impact on clinical outcomes and therapeutic strategies. Cirrhosis contributes to the precarious equilibrium between pro- and anticoagulant forces through impaired synthesis of coagulation factors, endothelial dysfunction, and systemic inflammation. In the presence of HCC tumor-driven mechanisms, such as tissue factor expression, extracellular vesicle release, platelet activation, and suppression of fibrinolysis exacerbate this prothrombotic state. In this scenario, advanced diagnostic tools such as thrombin generation assay (TGA) and rotational thromboelastometry (ROTEM) offer a more accurate assessment of coagulation dynamics than conventional tests, enabling better risk stratification especially for therapeutic purposes. Anticoagulant therapy has demonstrated clinical benefit in selected cases of non-malignant PVT and VTE, particularly when liver function is preserved. While prophylactic strategies are still under investigation, data suggest they may be safely implemented in selected surgical patients. In the setting of immunotherapy, especially regimens involving anti-VEGF agents, anticoagulation may be considered with careful management of bleeding risk due to portal hypertension. An individualized approach to anticoagulation, supported by functional coagulation testing, is gaining acceptance as a means to safely reduce thrombotic burden and potentially improve outcomes in patients with HCC.