Phytochemical-Chemotherapeutic Interactions in Gastrointestinal Cancers: Synergistic Mechanisms, Pharmacokinetic Barriers, and Translational Challenges

Phytochemicals have garnered substantial interest as potential adjuncts to standard chemotherapy, particularly in gastrointestinal (GI) cancers, owing to their ability to modulate cancer-associated signaling pathways and potentially improve chemosensitivity. In this review, we critically examine preclinical findings alongside early-phase clinical data addressing phyto-chemotherapeutic interactions, with emphasis on widely investigated agents such as curcumin, epigallocatechin gallate (EGCG), resveratrol, quercetin, berberine, thymoquinone, and triptolide. We review the proposed mechanisms in support of synergy, including modulation of apoptosis, oxidative stress responses, cell-cycle progression, and survival signaling pathways, while also addressing major translational obstacles that continue to limit clinical success. Major translational barriers include poor oral bioavailability, rapid metabolic degradation, uncertain dose-response relationships, interindividual pharmacogenomic variability, and medicinal chemistry concerns such as PAINS-related behavior and target promiscuity. Particular attention is given to the discrepancy between robust in vitro findings and the often modest or inconsistent outcomes reported in clinical settings to avoid overstating therapeutic efficacy. Available clinical data remains heterogeneous and largely derived from small-scale or regional studies, underscoring the need for rigorously designed clinical trials using standardized formulations with pharmacokinetic profiling and strict safety assessment. This review highlights mechanistic insights together with pharmacokinetic and drug-likeness considerations in order to clarify why many phytochemicals demonstrating promising preclinical activity ultimately fail to progress into approved anticancer therapies.