In situ forming hydrogels for colorectal cancer therapy

Abstract

In situ forming hydrogels (ISFGs) have emerged as a promising platform for localized drug delivery in colorectal cancer (CRC) therapy, addressing limitations of systemic treatments such as off-target toxicity and poor drug accumulation at tumor sites. These hydrogels undergo sol–gel transitions in response to stimuli (e.g., temperature, pH, redox) to form drug-laden depots within the tumor microenvironment or at the surgical bed. This review outlines recent advances in ISFG materials—including thermosensitive poloxamers, ion-responsive alginates, Schiff-base dynamic networks, and ROS/pH-sensitive polymers—highlighting their injectability, adhesiveness, and biocompatibility. Therapeutically, ISFGs have been successfully engineered to deliver cytotoxic agents, immunomodulators, photosensitizers, and metabolic inhibitors, enabling synergistic chemo-immunotherapy, postoperative cancer vaccines, and tumor microenvironment remodeling. Preclinical studies have demonstrated enhanced antitumor efficacy, reduced recurrence, immune activation, and even systemic abscopal effects, with minimal systemic toxicity. Clinical translation is advancing, with some ISFG systems entering early-phase trials. This review emphasizes ISFGs’ unique capacity to combine spatially confined treatment with systemic immune priming, making them ideal for neoadjuvant, adjuvant, or interventional CRC therapy. Future directions include personalized hydrogel-vaccine platforms, multimodal integration (e.g., ablation + immunotherapy), and smart hydrogels tailored to CRC biology. ISFGs thus represent a versatile and translationally promising strategy for improving local tumor control and long-term outcomes in colorectal cancer.

Graphical abstract

1. Introduction

Colorectal cancer (CRC) remains among the most prevalent malignancies and a leading cause of cancer deaths worldwide [1]. Despite advances in surgery, chemotherapy, and radiotherapy, clinical outcomes are often limited by systemic toxicity, incomplete tumor eradication, and high rates of local recurrence or metastasis [2]. Beyond these general limitations, CRC exhibits marked inter- and intratumoral heterogeneity (e.g., microsatellite status and driver mutations such as KRAS, BRAF, and PIK3CA) and therapy-resistance mechanisms (epithelial-mesenchymal transition (EMT), hypoxia-driven quiescence, immune evasion) that further compromise durable control [[3], [4], [5]]. These challenges highlight the urgent need for locoregional therapeutic strategies that can enhance local tumor control while limiting systemic adverse effects [6].

Systemic nanoparticle strategies have been extensively explored to improve tumor-site accumulation by passive or active targeting [7,8]. While this strategy can enhance biodistribution profiles to some extent, the improvement in local drug concentration within colorectal tumors remains modest due to physiological barriers and tumor heterogeneity, including dense/fibrotic extracellular matrix, poor perfusion, and hypoxic–acidic niches in CRC [9]. Moreover, such approaches often fail to maintain prolonged exposure at the disease site, especially in poorly perfused solid tumors.

In contrast, localized delivery—particularly intratumoral or peritoneal administration—offers more direct access to the tumor bed but faces its own limitations, such as rapid clearance and insufficient retention, exacerbated in CRC by peristalsis, luminal flow, and irregular post-resection geometries that promote backflow and non-uniform coverage. In this context, in situ forming hydrogels (ISFGs) have emerged as a promising platform for local drug delivery in CRC. These injectable formulations remain liquid during administration and undergo gelation upon exposure to physiological conditions—such as temperature, pH, or ionic strength—within the tumor microenvironment (TME) or surgical cavity. The resulting hydrogel conforms to irregular anatomical surfaces and forms a local depot for sustained, site-specific drug release [10]. Beyond their injectability and biocompatibility, ISFGs offer exceptional versatility in payload encapsulation. They can incorporate a wide range of therapeutic agents (e.g., small molecule drugs, proteins, and even nanoparticle formulations), thereby enabling combinatorial or synergistic treatment strategies [[11], [12], [13], [14], [15]]. Furthermore, the hydrogel matrix provides a protective environment that prolongs local drug retention and minimizes premature diffusion or enzymatic degradation, addressing key limitations of both systemic and conventional local delivery systems. In practice, these features translate into three delivery routes for ISFGs, including intratumoral/peritumoral injection, endoscopic submucosal injection, and intraperitoneal administration (via catheter/port); intraoperative application (post-resection bed/cavity filling and sealant depots); and luminal instillation (enema) for selected mucosal lesions.

Structurally, hydrogels are cross-linked polymer networks capable of absorbing large amounts of water while mimicking the mechanical properties of soft tissues [16]. They can be constructed from natural polymers (e.g., chitosan, hyaluronic acid (HA), alginate) or synthetic materials (e.g., Pluronic poloxamers, poly(ethylene glycol) (PEG)), and further engineered with stimuli-responsive or bioadhesive features to enable spatial control and improve tissue affinity [10,17]. For CRC, in particular, ISFGs are well-suited due to the surgical accessibility, the need for postoperative drug depots, and the peritoneal cavity's suitability for gel distribution and containment. Recent reports in gastrointestinal contexts, like rectal thermosensitive gels and colon-targeted polysaccharide platform, further support the feasibility of localized hydrogel delivery and inform CRC-specific ISFG deployment [[18], [19], [20]].

As illustrated in Fig. 1, this review outlines recent progress in ISFG systems for CRC therapy, including their material design, therapeutic payloads, delivery strategies, and translational prospects in the context of CRC-specific barriers (e.g., dense ECM, hypoxia, localized immunosuppression). We highlight how these hydrogels have been employed to deliver chemotherapeutics, immunotherapies, and combination regimens, summarize key preclinical studies and their outcomes, and discuss translational considerations and future directions for clinical implementation.

Fig. 1.

Schematic illustration of in situ forming hydrogels (ISFGs) for colorectal cancer (CRC) therapy. ISFGs can be administered via injection (intratumoral/peritumoral, submucosal, or intraperitoneal), enema (luminal instillation), or intraoperative application (post-resection beds or cavity filling). Upon exposure to physiological or exogenous triggers (e.g., temperature, pH, ions, enzymes, external stimuli), the precursor solution undergoes in situ sol–gel transition. Various therapeutic agents—including chemotherapeutics, immunomodulators, photosensitizers, and metabolic regulators—can be incorporated to achieve local tumor control, immune activation, and postoperative recurrence prevention. This high-level schematic is meant to orient the reader; CRC-specific design considerations are detailed in Sections 2, 3, 4.

2. Materials and design of In situ forming hydrogels

To fulfill the need for localized, sustained, and minimally invasive CRC therapy, a variety of ISFG systems have been developed. These systems are typically administered as low-viscosity liquids that rapidly undergo sol–gel transition upon exposure to physiological triggers, such as body temperature, pH, or ionic strength—thereby forming a localized drug depot at the disease site [21]. However, beyond gelation alone, ideal materials must satisfy multiple functional requirements: biocompatibility, injectability, tissue adhesion, and the ability to encapsulate and release diverse therapeutics (including advanced network design such as conductive/self-healing or structurally colored hydrogels, and nanoparticle-in-hydrogel composite designs) [14,15,22].

2.1. ISFG types for CRC therapy

Recent CRC-focused studies have demonstrated that ISFGs can be constructed using various material types and gelation mechanisms. In this part, we summarize the most representative categories reported in CRC models, broadly classified according to their gelation triggers and functional design principles (Fig. 2).

Fig. 2.

Representative design strategies of in situ forming hydrogels (ISFGs) for colorectal cancer (CRC). Shown are thermosensitive polymers (e.g., poloxamers, PEG–PCL, PNIPAm), pH/ion-sensitive systems (e.g., alginate/Ca²⁺, chitosan, PAA), bioadhesive hydrogels (e.g., HA-, gelatin-, PEG-based catechol conjugates), and dynamic covalent/self-healing networks (e.g., Schiff base, boronate ester), with their gelation mechanisms and CRC-relevant features. CRC-specific challenges addressed include anti-washout retention on resection beds (bioadhesives), release in hypoxic–acidic niches (pH/ion-responsive), conformal filling of irregular cavities (self-healing/dynamic covalent), and peri-operative handling (thermosensitive). The classification is tailored to CRC–relevant ISFG systems, and may not cover the full spectrum of hydrogel designs explored in other tumor types or non-oncologic indications. Abbreviations: PEG–PCL, poly(ethylene glycol)-block-poly(ε-caprolactone); PNIPAm, poly(N-isopropylacrylamide); HA, hyaluronic acid; PAA, poly(acrylic acid).

One widely adopted design involves thermosensitive polymers (e.g., poloxamers, PEG–PCL, PNIPAm-based copolymers), which remain liquid at room temperature and undergo sol–gel transition at body temperature. For example, poloxamer copolymers (Pluronic F127/P407) form dynamic networks where micellar aggregation leads to a sharp sol–gel transition at body temperature [23]. Al Sabbagh et al. developed a Poloxamer 407/188-based hydrogel with alginate that remained injectable at ambient temperature and solidified around ∼26 °C, forming a stable depot at 37 °C [24]. This formulation (20 % P407, 2 % P188, 1 % alginate) demonstrated shear-thinning behavior and solid-like elasticity at the colon tumor site. Similarly, Dinh et al. prepared a thermosensitive hydrogel by combining Poloxamer 407 with HA and κ-carrageenan, which effectively gelled in the peritoneal cavity and served as an anti-adhesion barrier post-surgery [25]. With a tan δ < 1 at 37 °C, this formulation demonstrated strong elasticity and sustained drug release capacity. These thermosensitive hydrogels allow convenient injection into the colon or abdominal cavity, followed by rapid in situ gelation to immobilize the formulation, supporting postoperative cavity filling and depot persistence to reduce local recurrence risk.

Alternatively, pH- and ion-sensitive hydrogels utilize the gastrointestinal pH gradient or ionic microenvironment to trigger gelation in the colon. Poly(acrylic acid) (PAA), a pH-responsive polymer, swells and forms hydrogen-bonded networks at higher pH levels [26]. Lo et al. reported a dual-responsive hydrogel based on Pluronic F127 and PAA, which remained fluid in the acidic stomach but gelled in the neutral colon, achieving colon-specific epirubicin delivery [27]. This oral ISFG showed a sponge-like structure during gel state, with sustained release characteristics for 96 h, and could be retained at the end of the ileum near the colon of SD rats. Ionically crosslinked alginate is another classic system: its gelation is triggered by crosslinking of alginate chains with calcium ions. Lyu et al. used CaCO₃ nanoparticles as an internal calcium source that released Ca²⁺ in acidic tumor microenvironments, thereby inducing alginate gelation in situ and forming a localized depot with enhanced payload retention [28]. Such ion-sensitive strategies are particularly appealing for colon-targeted delivery, exploiting local calcium levels and colonic pH. Chen et al. also reported calcium-triggered alginate gelation for energy-responsive CRC treatment [29]. These pH/ion responsive strategies utilized the hypoxic–acidic niches and localized Ca²⁺ availability in CRC to trigger in situ-gelation and extend drug exposure at the disease site.

In other designs, bioadhesive hydrogels are introduced to enhance mucosal retention and minimize displacement after topical or intraluminal administration. Hydrogel-tissue interface dynamics strongly influence efficacy in the colorectum [30]. Bioadhesiveness governs anti-washout retention on inflamed mucosa and resection beds [31], whereas approximate modulus matching helps maintain conformal contact under peristaltic shear and reduces backflow/leakage. Catechol- or dopamine-functionalized polymers, inspired by mussel adhesive proteins, exhibit strong adhesion to mucosal surfaces [32]. For example, Lin et al. developed a polyphenol-adhesive hydrogel by crosslinking catechol-bearing polymers, which firmly bound to the gut mucosa and delivered growth factor (KGF) in a colitis model [33]. ISFGs that 0.1 % polyphenol (epigallocatechin gallate, EGCG) exhibited a strong adhesive force toward colon tissue at 37 °C, with 780 ± 20 mN, which was 2.5-fold higher than that of the non-EGCG group. After rectal administration, the fluorescence intensity of Cy7.5 labeled bioadhesive ISFGs in ex vivo colon at 6 h was about 2-fold higher than that of non-EGCG group. The strong interaction of tissue components with EGCG helped prolong the hydrogel retention in the colorectum [34]. In the oncologic context, Qiu et al. used a dopamine-crosslinked HA hydrogel to seal colorectal surgical wounds, achieve rapid hemostasis, and deliver immunomodulatory agents as an “in situ vaccine” [35]. The adhesive hydrogel quickly stopped bleeding and adhered in the cavity, avoiding gel displacement by peristalsis, and then gradually released co-loaded drugs to treat residual disease. Chitosan-based systems, due to their cationic mucoadhesion, are also widely studied [36]; Piotrowska and Orzechowska reviewed multiple smart chitosan hydrogels that adhere to the colon wall and undergo sol–gel transition in situ, highlighting their promise for colon-targeted therapy [37]. By resisting peristaltic shear and luminal flow, bioadhesive properties reduce backflow/leakage and can enhance local immune priming when immunomodulators are co-delivered. Beyond macroscopic adhesion, cell-material interactions could also modulate early host responses (e.g., immune cell infiltration, macrophage polarization), and thus influence overall efficacy. These interfacial factors are particularly relevant in the irregular post-resection geometries typical of CRC care; accordingly, material choice, surface chemistry, and crosslink dynamics should balance retention with biocompatibility in ISFG design.

Finally, dynamic covalent and self-healing hydrogels offer reversible crosslinking, allowing for injectability and conformal gelation within complex tissue architectures. Schiff base chemistry is frequently employed in this category [38]. For instance, Dalei et al. formulated a biodegradable nanocomposite hydrogel via Schiff-base reaction between chitosan and dialdehyde-functionalized pectin, incorporating ZnO nanoparticles synthesized from marigold extract [39]. The imine bonds endowed the hydrogel with self-healing capability and injectability, while the embedded ZnO conferred antioxidant and anti-inflammatory benefits [39]. Notably, this biopolymer hydrogel was biodegradable and allowed controlled 5-fluorouracil (5-FU) release in the colorectum, leading to improved cytotoxic effects on colon cancer cells. Wu et al. reported another dynamic hydrogel system using oxidized dextran and polyethylenimine, which co-delivered 5-FU and metformin, with synergistic anticancer effects in vivo [40]. These dynamic networks can reform after shear, making them ideal for injection through narrow endoscopic needles and for conforming to irregular tumor cavities, and improving depot retention at CRC surgical sites.

From a CRC perspective, priority design criteria include injectability through endoscopic or laparoscopic devices, robust anti-washout retention on mucosa and resection beds, degradation kinetics aligned with adjuvant treatment windows to prevent recurrence, and matrices or payloads capable of priming local immunity. Thermosensitive hydrogels allow convenient injection into the colon or peritoneal cavity, followed by rapid in situ gelation that immobilizes the depot, thereby supporting postoperative cavity filling and sustained drug release. However, they may soften under dilution or shear. By contrast, pH- and ion-responsive systems exploit the hypoxic–acidic tumor niches or local calcium levels to trigger gelation in situ, yielding depots with improved retention. Yet, Ca²⁺ availability and control remain a translational concern. Bioadhesive matrices (e.g., catechol- or cationic-based) enhance mucosal retention under peristalsis but require careful evaluation of hemocompatibility. Dynamic covalent and self-healing networks provide conformability to irregular resection cavities and resilience against luminal flow, although bond stability in acidic CRC microenvironments remains a limiting factor. Thus, material choice should align with both clinical use scenarios (endoscopic submucosal injection versus postoperative cavity sealing) and payload class (small molecules, proteins, nanoparticles), balancing injectability, retention, and biocompatibility.

2.2. Formulation characterization and evaluation framework

To enable meaningful cross-study comparison, CRC-specific ISFGs should be evaluated with a standardized set of physicochemical and functional parameters. Traditional reports often emphasize sol–gel transition alone, but this metric is insufficient to predict in vivo performance [11]. Instead, a broader evaluation framework is needed that covers rheological properties, injectability, tissue adhesion, degradation behavior, structural organization, drug–matrix interactions, and responsiveness to physiological stimuli [41]. These parameters collectively determine depot formation, release dynamics, and tissue compatibility, all essential for safe and effective translation.

Rheological profiling (e.g., storage/loss modulus vs. temperature or time, gel point, and recovery after shear) links directly to injectability and mechanical stability at the disease site. Adhesion tests (e.g., syringeability force, lap-shear/peel assays on colon or peritoneal tissue) provide insight into handling feasibility and mucosal retention under peristaltic stress. Swelling and degradation kinetics measured in relevant media help estimate depot longevity and the balance between burst and sustained release. Network structure, approximated by mesh size calculations or pore imaging, and drug–matrix interaction proxies (partition coefficient, binding affinity) influence effective diffusivity and encapsulation stability. Finally, responsiveness to stimuli such as pH, ions, redox, or temperature allows for microenvironment- or trigger-controlled release.

By reporting these attributes alongside pharmacological outcomes (tumor inhibition, survival, immune activation, and safety), ISFG studies can provide a stronger mechanistic link between material design and therapeutic efficacy. Such harmonization would not only enhance reproducibility across CRC studies but also support dose translation and regulatory evaluation. Table 1 summarizes recommended reporting items as a practical checklist for ISFG characterization.

Table 1.

Recommended reporting items for CRC-specific ISFG characterization and release.

Aspects	Recommended readouts	Relevance to CRC ISFG
Rheology & Gelation	G′/G″ vs. T/time; gel point; recovery after shear	Predicts injectability, mechanical match, and in situ stability
Injectability & Adhesion	Syringeability force; lap-shear/peel tests on colon tissue	Determines handling feasibility; retention under peristalsis
Swelling/Degradation	Mass loss or erosion t50 in relevant media	Governs hydrogel durability and payload diffusivity
Network Structure	Mesh size estimates; pore imaging	Controls permeability (molecules vs. nanoparticles)
Drug–Matrix Interaction	Partitioning/binding proxies	Shapes burst fraction and sustained release
Stimuli Responsiveness	Trigger thresholds (pH, ion, redox, light)	Enables on-demand or programmable release

2.3. Release mechanisms and modeling

Drug release from ISFGs is a multi-phase process shaped by both material properties and local physiological conditions [42]. Release typical shows (i) an initial burst from the ungelled or near-surface fraction before gelation is complete, followed by (ii) a post-gel diffusion phase through the hydrated network [43]. For small molecules, this phase is often described by Higuchi or Korsmeyer–Peppas models (with n ≲ 0.5 for Fickian diffusion) [44,45]. In contrast, macromolecules may be constrained by mesh size and matrix relaxation, while nanoparticle payloads encounter additional convective and interfacial barriers. When swelling or polymer erosion contributes, profiles shift to erosion-controlled or zero-order kinetics; in degradable networks, erosion often dominates at later stages [45]. Stimuli-responsive designs (pH/ion, redox, light/ultrasound, temperature) can superimpose trigger-dependent changes, enabling on-demand release [46]. Key determinants include the test medium (pH, ionic strength, enzyme/protein content), temperature, drug–matrix interactions, and depot geometry. For cross-study comparison, it is important to report burst fraction, gelation time, and cumulative release with model fits (Higuchi, zero/first-order, Korsmeyer–Peppas with n and goodness-of-fit), along with full test conditions (medium composition, temperature, and sink criteria). Consistent and standardized reporting is essential, as many CRC ISFG studies still lack harmonized release methods. Overall, rigorous modeling of release kinetics not only informs formulation design but also strengthens the translational relevance of ISFGs by linking material behavior with therapeutic exposure.

In summary, researchers have developed a modular toolkit of gelation strategies—from thermoresponsive block copolymers and ion-triggered polysaccharides to mucoadhesive and dynamic covalent systems—to enable precise and sustained drug release in CRC therapy. The choice of material and crosslinking chemistry should align with the clinical scenario (e.g. endoscopic injection vs. surgical application) and drug properties (e.g., small molecule, biologic, nanoparticle). In the next section, we explore how such hydrogel systems have been leveraged to improve the delivery and efficacy of various therapeutic agents in colorectal cancer models.

3. Localized chemotherapy delivery

Systemic chemotherapy remains a cornerstone treatment for CRC, with first-line regimens commonly including 5-FU, oxaliplatin, and irinotecan [1]. However, their clinical utility is often constrained by dose-limiting toxicities—particularly hematologic, gastrointestinal, and cardiotoxic effects—resulting from non-specific distribution and systemic exposure [47]. ISFGs offer an alternative strategy by concentrating therapeutic agents directly at the tumor site or surgical bed, thereby enhancing local efficacy while reducing systemic toxicity [48]. Their ability to provide sustained release and site-specific retention makes them especially attractive in adjuvant or intraoperative settings. Several studies have demonstrated the utility of ISFGs to deliver chemotherapeutics in CRC models, as summarized below.

Among the agents used in ISFG-based delivery, 5-FU has been most extensively investigated. Al Sabbagh et al. encapsulated 5-FU into a poloxamer/alginate thermosensitive ISFGs for intratumoral injection in a mouse CRC model [24]. This ISFGs system released 5-FU with an initial burst release in the first hour, followed by sustained release with poloxamer erosion. Notably, in mice bearing subcutaneous CT26-luc tumor model, tumor sizes were comparable between the 5-FU ISFG group and the free 5-FU group (231.4 ± 67.6 mm³ vs. 275.5 ± 36.9 mm³), but body weight loss was significantly less in the ISFG group (−0.4 % ± 2.9 % vs. −3.6 % ± 3.0 %). This finding suggests that intratumor injection of 5-FU in ISFGs form reduce the chance of systemic absorption and overall increase its efficacy and safety. When applied to the tumor bed following surgical excision, the same gel effectively prevented CT26 CRC local recurrence with more steady mice body weight as compared to solution group. This study not only directly compared the formulation advantages of ISFGs, but also highlights its potential in both intratumoral injection and post-excision application. Similarly, Dinh et al. developed a HA/carrageenan/poloxamer ISFGs loaded with 5-FU, serving both an anti-adhesive barrier and a chemotherapeutic depot [25]. Both Higuchi and Korsmeyer-Peppas models indicated that the sustained release kinetics of 5-FU from this gel system involved diffusion. Moreover, the application of ISFGs significantly reduced the anti-adhesion score after 10 days, with the best formulation represented <25 % adhesion that separated to the abdominal wall using Sprague-Dawley rat models. Therein, the ISFGs could provide a high initial drug level to eliminate residual tumor cells while minimizing postoperative adhesions, acting as a “therapeutic anti-adhesive” in CRC surgery. Of particular note, Zhang et al. started an exploratory clinical study (NCT06385418) at The Second Hospital of Nanjing Medical University to study 5-FU thermosensitive gels for CRC [49]. In this study, they proposed an integrated approach utilizing colonic transendoscopic enteral tubing to cover the entire colon flexibly, coupled with ISFG system to prolong the 5-FU exposure.

Doxorubicin (Dox), though not first-line in CRC, has been used in hydrogel systems to improve local cytotoxicity while minimizing cardiotoxicity. Fiorica et al. developed an injectable hydrogel (named as HA-EDA/β-CD-VS(Dox)) composed of HA grafted with β-cyclodextrin, allowing Dox encapsulation via host–guest complexation (Fig. 3) [50]. The hydrogel was formed by a Michael-type addition (between HA-amine and a vinylsulfone-modified cyclodextrin), which could be prepared rapidly during surgery. It exhibited robust rheological properties for easy handling and gelation within minutes (∼3 min). The HA backbone targeted CD44-expressing cancer cells, while cyclodextrin modulated sustained drug release. CRC HCT-116 spheroids treated with HA-EDA/β-CD-VS(Dox) maintained their size and morphology, whereas those exposed to free Dox grew larger with irregular margins. In vivo, a single peritumoral injection significantly reduced tumor size without inducing systemic cardiotoxicity—unlike equivalent systemic Dox, which caused cardiac injury. This highlights the advantage of ISFG-mediated local delivery in enhancing efficacy while maintaining handling feasibility and minimizing off-target effects.

Fig. 3.

A hyaluronic acid/cyclodextrin-based injectable hydrogel for localized doxorubicin delivery (Dox) in colorectal cancer. (A) Schematic illustration of hydrogel (HA-EDA/β-CD-VS(Dox)) formation via Michael-type addition between amine-functionalized hyaluronic acid (HA-EDA) and vinylsulfone-modified β-cyclodextrin (β-CD-VS), with Dox complexed into the β-CD cavity. (B) Physicochemical characterization, including: (a) DSC thermograms; (b) time-dependent rheological curves (G′, G″) showed a gel point at 37 °C; (c) complex viscosity profile; and (d) cumulative Dox release kinetics. (C) MRI scans at 6, 8, and 9 weeks post-treatment; red arrows indicate hydrogel retention at tumor site. Reprinted from Ref. [50], with permission from Elsevier, 2020. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Irinotecan and oxaliplatin, the clinical backbones of systemic CRC therapy, have also been adapted into ISFGs for localized intervention. Wang et al. reported a supramolecular peptide hydrogel co-loaded with a STING agonist and camptothecin (the active metabolite of irinotecan), achieving potent tumor inhibition via extended intratumoral release and minimal systemic exposure in CRC models [51]. Lee et al. embedded oxaliplatin in a crosslinked hydrogel film designed for implantation post-tumor resection. This film not only provided sustained oxaliplatin release to eradicate residual tumor cells but also reduced postoperative adhesions—an important consideration in peritoneal metastases and carcinomatosis management [52].

Beyond conventional chemo-drugs, plant-derived compounds such as curcumin have been explored for local [53,54]. For example, curcumin loaded polymeric micelles have been developed to improve anti-colon cancer activity after intratumoral injection [55], but this approach required frequent administration to maintain effective concentrations. ISFGs loading helps overcome this limitation by providing sustained local release. Zhang et al. designed a GelMA (gelatin methacrylate) and SilMA (silk fibroin methacrylate) mixed hydrogel containing curcumin-shellac nanoparticles for CRC therapy [56]. The system enabled gradual curcumin release at the tumor site, achieving superior efficacy over free curcumin or nanoparticle formulation. Also, the sustained curcumin release was achieved through the disintegration of the nanoparticles (pH dependent and colon relevant) and the swelling and degradation of the hydrogel matrix. Similar benefits were observed with thermosensitive hydrogels incorporating curcumin-loaded micelles in CRC metastasis models [57]. Actually, curcumin is often used as a supplementary agent and co-loaded with other therapeutics in ISFGs for CRC treatment [58] These findings demonstrate that ISFGs can expand the utility of compounds otherwise limited by pharmacokinetic barriers.

Overall, locoregional chemotherapy using hydrogels has shown promising outcomes, including significant tumor growth inhibition, reduced recurrence following surgery, and markedly reduced systemic toxicity [6]. Co-loading of multiple agents is also feasible. For instance, an injectable polypeptide hydrogel was designed to co-deliver combretastatin A4 phosphate (a vascular disrupting agent) and cisplatin, achieving synergistic effects in resistant colon tumors [59]. By tuning hydrogel composition, drug release can be sustained over days to weeks. In many cases, the Higuchi diffusion model accurately fits the release profile, suggesting that diffusion dominates the release mechanism. Importantly, the hydrogel matrix protects labile drugs from degradation in the gastrointestinal environment and promotes their accumulation at the tumor site.

In summary, ISFGs offer a robust platform for localized chemotherapy, with strong preclinical evidence of improved efficacy and safety in CRC models. Building on these findings, researchers are now incorporating combination strategies—integrating chemotherapy with immunotherapies, radiosensitizers, or metabolic modulators—into hydrogel systems to better address the multifactorial nature of cancer. These integrated approaches are discussed in the following sections.

4. Combination therapies and tumor microenvironment modulation

While chemotherapy remains the cornerstone of CRC treatment, it is increasingly evident that monotherapy alone cannot address the complexity of tumor progression, metastasis, and immune evasion. With growing understanding of the TME and tumor–host interactions, there is heightened interest in combination therapies that integrate cytotoxic, immunomodulatory, and physical approaches [60,61]. ISFGs provide a versatile platform for delivering such multimodal regimens in a localized, sustained, and spatially confined manner. This section explores how ISFG systems are being utilized not only to carry traditional and immunotherapeutic agents but also to modulate physical, metabolic, and stromal features of the TME. We organize these strategies into three emerging paradigms: chemo-immunotherapy and in situ vaccination post-resection (Section 4.1), energy-activated therapies like phototherapy and radiotherapy (Section 4.2), and metabolic or microenvironment-targeted strategies (Section 4.3).

4.1. Chemo-immunotherapy and cancer vaccines

A major advantages of localized ISFG-based therapy lies in its capacity to stimulate antitumor immunity in parallel with tumor cell eradication. Tumor cell death in situ can function a vaccine by releasing tumor antigens, particularly when combined with immunostimulatory signals. Hydrogels have been engineered to co-deliver chemotherapeutics and immune modulators to transform “cold” tumors into “hot” ones [17,62]. One illustrative approach involves combining chemotherapy with cytokine-based immunotherapy. Jiang et al. developed a thermosensitive ISFG (termed OXL/IL-15 TG) embedding oxaliplatin (OXL) and IL-15 for triple-interlocked chemo-immunotherapy against CRC (Fig. 4A–C) [63]. OXL was encapsulated in the internal phase of a liposomal structure and IL-15 was dispersed in the matrix. Upon injection, the gel formed in situ, releasing payloads in a staggered, slower manner. Oxaliplatin not only directly killed tumor cells but also induced immunogenic cell death (ICD), providing tumor antigens and DAMPs, while IL-15 potently activated natural killer (NK) cells and T cells. In CT-26 tumor-bearing C57BL/6 mice, OXL/IL-15 TG significantly suppressed primary tumor growth and metastases, and prolonged survival. Immune profiling revealed increased CD8⁺ T and NK cell infiltration and decreased immunosuppressive myeloid cells, indicating effective reshaping of the TME toward an immunostimulatory phenotype. OXL/IL-15 integrates multiple functions, including synergistic OXL + IL-15 pharmacology, a liposome-in-gel architecture that enhances OXL exposure, and a long-acting ISFG depot. This study exemplifies the promise of ISFG-based chemo-immunotherapy [64].

Fig. 4.

Representative in situ forming hydrogel systems for chemo-immunotherapy and in situ vaccination in colorectal cancer. (A–C) A thermosensitive hydrogel co-loaded with oxaliplatin (OXL) and IL-15 (OXL/IL-15 TG) induces tumor killing, immunogenic cell death (ICD) induction, and NK/T cell activation. (A) Schematic of hydrogel preparation and immune mechanisms. (B) Tumor volume progression (a) and representative images (b) of collected tumors in CT26-bearing mice. (C) CD8⁺ T cell infiltration (a) by flow cytometry; IFN-γ secretion (b); NK cell phenotype (c) within the collected tumors. Reprinted from Ref. [63], with permission from Elsevier, 2024. (D–F) An immunomodulatory hydrogel (Gel-GM-CSF-BCG) combined with radiofrequency ablation (RFA) achieves complete eradication of both local and distant CRC tumors. (D) Hydrogel composition. (E) RFA plus hydrogel enhances the antitumor effect in an adjuvant treatment and maintain long-term immunity against tumor re-challenge. (a) Bioluminescence images of distant tumors; (b) distant tumor growth curves; (c) survival analysis. (F) Immunological effects of RFA and hydrogel combination therapy ± anti-PD-1. (a) Tumor growth under RFA ± anti-PD-1; (b) survival analysis. Reprinted from Ref. [65], with permission.

Beyond surgical settings, thermal ablation methods such as radiofrequency ablation (RFA) have been combined with immunostimulatory ISFGs to augment immune responses. An illustrative case of ISFG facilitated in situ vaccination comes from Lemdani et al. (Fig. 4D–F) [65]. Although RFA releases tumor antigens, it often fails to elicit sufficient immune activation to prevent recurrence. Lemdani's team injected a mucoadhesive immunogel (Gel-GM-CSF-BCG) composed of granulocyte-macrophage colony stimulating factor (GM-CSF) and Bacillus Calmette–Guérin (BCG) into the ablation zone (Fig. 4D–F) [65]. The in situ formed gel remained in place at the tumor site, gradually releasing GM-CSF to recruit dendritic cells and live-attenuated BCG as a potent immune stimulator. In human CRC liver metastasis, the authors first quantified tumor infiltrating lymphocytes (TILs) and found that RFA alone did not increase TIL counts in distant lesions, supporting the need for combination. In the CT26 mouse model, Gel-GM-CSF-BCG alone had a short-lived local effect whereas RFA induced significant regression of the treated tumor. Bioluminescence imaging showed that RFA + Gel-GM-CSF-BCG achieved complete tumor regression by day 15, while RFA + empty gel or RFA + Solution--GM-CSF-BCG produced only transient, limited inhibition. Depletion of CD8⁺ T cells abrogated tumor control, confirming CD8 dependence. In a separate study, systemic anti-PD1 further enhanced the RFA + Gel-GM-CSF-BCG effect on distant tumors and led to durable regression in majority of mice, with high survival rate (≈33 % alive at 4 month). Further analyses showed a 2- to 20-fold increase of CD45⁺ immune cells in distant tumors with anti-PD-1 + RFA + Gel-GM-CSF-BCG versus controls. This comprehensive in vivo work provides a strong rationale for clinical evaluation of RFA plus immunogel with PD-1/PD-L1 blockade in metastatic CRC.

A similar RFA + immune ISFG approach was reported by Seguin et al. [66]. They used purified protein derivatives (PPD) from Mycobacterium tuberculosis as pathogen-associated molecular patterns (PAMP) and a TLR agonist, co-loaded with GM-CSF into poloxamer P407/xanthan gum ISFG (IMT-GM-CSF-PPD). In situ gel formation and retention post-injection were confirmed by MRI and bioluminescence imaging. In a CRC liver-metastasis mouse model, all metastases disappeared by day 16 in the RFA + IMT-GM-CSF-PPD group, whereas RFA-alone animals had progressive metastases (≈350 ± 99.3 mm³). Treated mice showed >10-fold improvement in long-term survival. Safety was evaluated in a pig liver model; intralesional injection into the RFA zone was feasible after volume adjustments, without clinical, hematologic, or biochemical abnormalities. This program has advanced toward a clinical trial combining RFA with ISFG-based intratumoral immunotherapy in CRC patients [67].

Hydrogels have also been used to improve local antibody delivery. Rather than repeated systemic injections, checkpoint inhibitors can be released locally from a hydrogel matrix. For example, Zeting et al. designed an adhesive ICG-loaded granular hydrogel that, when applied after tumor resection and illuminated with near-infrared light, performed photothermal therapy (PTT) to kill residual cells and also served as a depot for anti-PD-L1 antibody [68]. The ICG was “locked” in the granular gel, preventing its diffusion and photobleaching, thereby generating sufficient heat upon NIR exposure to induce immunogenic cell death. In combination with the checkpoint inhibitor released from the gel, this approach eradicated postoperative residual and metastatic tumors in an aggressive CRC model and prevented long-term recurrence. The tissue-adhesive nature of the granular hydrogel was crucial for maintaining contact with the tumor bed during the treatment. Such localized checkpoint blockade, when paired with a local cytotoxic modality, can greatly enhance immunotherapy efficacy in immunologically “cold” tumors like microsatellite-stable CRC.

Overall, these examples highlight the ability of ISFGs to deliver immunotherapy safely and effectively alongside tumor killing interventions—whether through chemotherapy, ablation, phototherapy, or targeted agents. ISFGs create a localized, pro-inflammatory microenvironment enriched with tumor antigens and immunoadjuvants, thereby enhancing robust T cell responses and even abscopal effects. Importantly, because immunostimulatory agents largely remain confined within the in situ gel, systemic immune-related adverse effects (e.g., cytokine storm or autoimmunity) are minimized—for instance, no detectable systemic GM-CSF was found in mice treated with RFA + immunogel [66]. This approach represents a paradigm shift—treating cancer not only as a local disease but as one with systemic immunological implications, by harnessing the host's own immune system to eliminate residual tumor cells.

4.2. Photo- and radiotherapy Enhancement

ISFGs have increasingly been integrated into energy-activated cancer treatments, such as PTT, photodynamic therapy (PDT), sonodynamic therapy (SDT), and radiotherapy. These strategies rely on converting external energy sources (e.g., light, ultrasound, or ionizing radiation) into localized cytotoxic effects. However, their efficacy is often limited by insufficient sensitizer accumulation, rapid systemic clearance, or off-target toxicity [69]. Hydrogels, with their tunable matrix properties and depot-forming capability, provide an effective solution by anchoring sensitizers or adjuvants within the TME, allowing for sustained retention, controlled release, and spatiotemporal confinement of therapeutic activity [70].

A representative example is SDT, where ultrasound activates a sensitizer to generate reactive oxygen species (ROS) for tumor cell killing. To enhance SDT efficacy in CRC, Chen et al. developed an injectable alginate-based hydrogel (termed as A-BMP-BSO) that incorporated PpIX-conjugated MnO₂ nanoparticles (serving as a sonosensitizer and oxygen generator) and L-buthionine sulfoximine (BSO), a glutathione-depleting agent (Fig. 5A–C) [29]. The alginate hydrogel gelated within 3 s upon exposure to 1.8 mM Ca²⁺, simulating extracellular conditions. Within the acidic TME, MnO₂ generated oxygen to alleviate hypoxia, thereby amplifying ROS production under ultrasound, while BSO blocked intracellular glutathione synthesis, preventing ROS scavenging. In CT26 BALB/c mouse model, the A-BMP-BSO + US group showed the smallest tumor volume of all the groups, with tumor weights were ∼8-fold lower than those in the US alone group. Essentially, the hydrogel created a more ROS-permissive microenvironment, thereby potentiating SDT via enhanced oxidative stress. This study underscores that stimuli-responsive hydrogels (here responding to both low pH and ultrasound) can coordinate multiple mechanisms: sustained sensitizer release, on-demand oxygen generation, and microenvironment conditioning for maximal therapy [70,71].

Fig. 5.

In situ forming hydrogels enhances energy-based therapies in colorectal cancer. (A–C) A composite hydrogel (named as A-BMP-BSO) modulating the tumor redox microenvironment to enhance ultrasound-triggered sonodynamic therapy. (A) Schematic of preparation process and redox modulation strategy. (B) Hydrogel characterization, including (a) Gel appearance post-injection in the presence of Ca²⁺; (b) L-buthionine sulfoximine (BSO) cumulative release kinetics; (c) O₂ generation upon ultrasound. (C) In vivo antitumor efficacy: (a) Treatment timeline; (b) tumor growth curves; (c) dissected tumor images; (d) tumor weights; (e) tumor GSH levels. Reprinted from Ref. [29], with permission from Royal Society of Chemistry, 2022. (D–G) An alginate-based hydrogel doped with AIEgen and CaCO₃ nanoparticles to remodel the tumor microenvironment and boost FLASH immunoradiotherapy. (D) Schematic of gelation. (E) Physicochemical characterization: (a) TEM images; (b) pH-dependent Ca²⁺ release; (c) gel photos; (d) rheological properties of FA hydrogel. (F) In vivo efficacy and immune response in CT26 tumor-bearing mice: (a) Treatment timeline; (b) retention of Cy5-labeled FCC; (c) Western blot and (d) immunofluorescence staining of HMGB1 and CRT. (G) Evaluation of systemic (abscopal) effects: (a) Schematic of dual-tumor model; (b) primary and (c) distant tumor growth; (d) survival curves; (e) flow cytometry analysis of mature DCs in lymph nodes and CD8⁺ T cells in distant tumors; (f, g) intratumoral levels of TNF-α and IFN-γ. Reprinted from Ref. [28], with permission from Elsevier, 2025.

Similarly, PTT benefits from ISFG-based localization and concentration of thermal agents. Ouyang et al. reported a hydrogel-based NIR-II-triggered system combining a photothermal “ink” with an azo-based radical initiator (AIPH) [72]. Upon NIR-II irradiation, the photothermal agent elevated local temperatures to <45 °C, sufficient to thermally cleave AIPH and release cytotoxic alkyl radicals. The combination of mild hyperthermia and radical-induced damage induced more pronounced apoptosis than conventional high-temperature PTT, while minimizing collateral thermal injury. Furthermore, the mild heat stress upregulated tumor PD-L1 expression, suggesting synergy with checkpoint immunotherapy. The in situ formed hydrogel matrix confined both the photothermal agent and AIPH within the tumor, preventing systemic diffusion of radicals. Indocyanine green (ICG), a clinically approved photothermal dye, has also been used in an injectable adhesive hydrogel for CRC treatment. Zeting et al. demonstrated that ICG-loaded granular hydrogels produced localized hyperthermia under NIR light, eradicating residual tumor cells post-surgery [68]. The hydrogel matrix immobilized ICG at the tumor bed, enabling repeated laser treatments. When combined with systemic checkpoint blockade, the treatment prevented postoperative recurrence in an aggressive CRC model.

Beyond monotherapies, ISFGs have also been engineered to combine PDT with radiotherapy, especially in the context of emerging FLASH radiotherapy. Lyu et al. developed an AIEgen-alginate hydrogel to boost the cutting-edge modality of FLASH radiotherapy (ultra-high dose rate radiation) (Fig. 5D–F) [28]. The hydrogel contains flower-like CaCO₃ nanoparticles doped with an aggregation-induced emission luminogen (CQu). Upon injection, the acid in the tumor triggers alginate gelation (via Ca²⁺ release) and traps the CQu within the tumor. Under a low-power laser, CQu performs PDT, generating ROS that cause mitochondrial damage and excess Ca²⁺ influx into cancer cells. This calcium overload and ROS not only directly kill cells but also make them more susceptible to radiation. Subsequent FLASH radiotherapy (which delivers >40 Gy/s) then induces immunogenic cell death in this primed environment. The combination produced durable tumor regression and an abscopal effect in mice (immune-mediated clearance of tumors outside the radiation field). The sustained ROS production and Ca²⁺ released by the hydrogel effectively remodeled the TME, overcoming the incomplete tumor kill that FLASH alone might leave. A similar approach using pH-sensitive CaCO₃-triggered calcium overload to sensitize CRC cells to NIR irradiation has also been reported [73], with ISFGs enhancing this effect through prolonged intratumoral localization.

Hydrogels may also augment thermal ablation (e.g., microwave or radiofrequency) by acting as conductive or immunogenic additives. One example used an injectable metal-alginate hydrogel that released calcium and immunostimulatory ions during microwave ablation of tumors, resulting in greater tumor destruction and anti-tumor immunity (including HSP release and dendritic cell activation) [74]. This concept is being explored for CRC liver metastases, where an alginate hydrogel loaded with calcium or other nanoparticles could potentiate thermal ablation and concurrently activate immune cells in situ.

In essence, ISFGs are emerging as effective vehicles for sensitizers (sonosensitizers, photosensitizers, radiosensitizers) and for combination regimens involving physical therapies. By retaining these agents within the tumor, hydrogels ensure that applied energy is concentrated at sensitizer-rich sites, maximizing therapeutic response. Additionally, hydrogels can carry additional components like oxygen generators [29], radical precursors [72], or calcium modulators [28] to counteract tumor protective mechanisms (hypoxia, antioxidant defenses, etc.) during therapy. As a result, even modalities traditionally limited by tumor environment factors (like PDT's dependence on oxygen or radiotherapy's immune suppression) can achieve deeper tumor kill and engage the immune system when coupled with a smart hydrogel. These systems have demonstrated substantial tumor regression and immune activation in CRC models, underscoring the potential of hydrogel-assisted phototherapy and radiotherapy for improving locoregional control and minimizing recurrence.

4.3. Targeting metabolic and the tumor microenvironment

CRC progression is supported not only by malignant cells but also by a reprogrammed TME, characterized by metabolic rewiring, immunosuppression, and dense stromal barriers [75]. Emerging hydrogel-based therapies aim to locally intervene in these non-cellular tumor supports by delivering agents that disrupt nutrient supply, oxidative balance, immune evasion, and fibrotic shielding [76]. For example, recent reports describe nanoenzyme loaded hydrogels (e.g., MnO₂ or Fe₃O₄ systems) that modulate the TME via in situ oxygen generation or catalytic ROS production, metabolic interventions that reshape nutrient/redox balance for cancer treatment [70,77]. Compared to systemic administration, ISFGs offer the distinct advantage of sustained and localized delivery of metabolic modulators or stromal reprogrammers, thereby avoiding dose-limiting toxicity and enhancing tumor-specific effects.

Among metabolic interventions, amino acid deprivation has emerged as an effective approach to induce tumor stress and immunogenicity [78,79]. Methionine dependency is a hallmark of many tumors, including CRC, due to their elevated demand for methyl donors and epigenetic regulators. However, systemic methionine depletion is limited by severe side effects. To overcome this, Ma et al. designed a “3M” multiplex methionine-modulating hydrogel to locally starve tumors of methionine via a triple blockade strategy (Fig. 6) [80]. Two small-molecule inhibitors—PF-9366 (targeting methionine adenosyltransferase) and adenosine dialdehyde (blocking downstream metabolism)—were loaded into tumor-targeted nanoparticles, which were co-embedded with the macromolecular Met transporter inhibitor JPH203 in a ROS-responsive injectable hydrogel. Following intratumoral injection in CRC and other tumor models, JPH203 was released extracellularly to prevent methionine uptake, while the nanoparticles were internalized to inhibit intracellular metabolism. The localized Met starvation drastically reduced S-adenosylmethionine levels and attenuated histone methylation in tumors. This in turn led to epigenetic reprogramming and potent immune activation: the nutrient-stressed tumor cells underwent immunogenic cell death, attracting innate immune cells and T cells. In CRC mouse models, the 3M hydrogel suppressed tumor growth and sensitized tumors to anti-PD-1 therapy. This work suggests a promising avenue of “metabolic therapy” delivered by hydrogel, wherein amino acid starvation in situ can ignite anti-tumor immunity and sensitize immune-refractory tumors to immunotherapy. The hydrogel confines the metabolic drugs to the tumor, avoiding systemic toxicity (e.g. neurological issues from depriving normal tissues of methionine). Other studies have also contributed to this domain of amino acid metabolism and enhancing anticancer efficacy [81]. While these studies did not employ hydrogels, their principles—transporter-guided delivery and metabolic blockade—parallel those of hydrogel-based local therapies. Such strategies could be extended by embedding transporter-targeted nanoagents into hydrogel matrices for even more precise locoregional control.

Fig. 6.

A multiplex methionine modulating (3M) hydrogel induces metabolic reprogramming and antitumor immunity. (A) Schematic illustration of in situ metabolic modulation by the 3M Gel, (a) Preparation process and (b) ROS-triggered metabolic mechanisms, induce immunogenic cell death (ICD), enhance dendritic cell (DC) maturation, and stimulate cytotoxic T cell-mediated immune responses. (B) In vivo Cy5.5 imaging and retention over time in 4T1 tumor-bearing mice. (C) (a) Tumor growth curves; (b) Calreticulin (CRT) expression (hallmark of ICD). (D) Therapeutic performance of 3M Gel in a CT26 colorectal cancer model. (a) Tumor images and volume growth; (b) CRT staining; (c) CD8⁺ T cell infiltration. Reprinted from Ref. [80], with permission from Wiley, 2025.

Another promising axis involves redox and NAD⁺ metabolism. CRC cells depend on NAD⁺ synthesis and glutathione peroxidase (GPX4) activity to survive oxidative stress, making them susceptible to ferroptosis inducers. Ye et al. created a ROS-responsive hydrogel encapsulating FK866 (a NAMPT inhibitor) and a STAT3 inhibitor to simultaneously disrupt NAD⁺ synthesis and redox signaling [82]. In CRC models, sustained FK866 release depleted NAD⁺ and suppressed STAT3-mediated transcription of antioxidant and immune-evasion genes. This led to ferroptotic cancer cell death and an inflammatory microenvironment favorable for immune activation. The combination of NAMPT inhibition and STAT3 blockade showed synergistic tumor suppression, significantly greater than either alone. Importantly, the ROS-responsiveness of the hydrogel ensured drug release was accelerated in the oxidative tumor milieu, aligning drug activity with times of high oxidative stress. Tumor-infiltrating CD8⁺ T cells increased, while immunosuppressive M2 macrophages decreased, indicating effective immune remodeling. This strategy highlights how hydrogel-mediated metabolic disruption can synergize with immunomodulation to improve therapeutic outcomes.

In addition to metabolic interventions, hydrogels have been used to overcome physical and immunologic barriers in the TME. The dense fibrotic stroma and TGF-β signaling characteristic of CRC hinder drug penetration and facilitate immune exclusion. Li et al. addressed this by developing a composite hydrogel system comprising thermosensitive polypeptide gels loaded with regorafenib (a multikinase inhibitor) and ROS-responsive nanogels encapsulating the TGF-β inhibitor LY364947 [83]. Upon intratumoral injection into orthotopic CRC tumors, regorafenib was released first to inhibit angiogenesis and induce ROS generation, which in turn triggered the release of LY364947. This sequential delivery suppressed both tumor growth and stromal-driven immune evasion. In vivo, the strategy reduced both primary and metastatic CRC lesions and enhanced CD8⁺ T cell infiltration while decreasing MDSCs and M2 macrophages. Essentially, the hydrogel composite adapted to the tumor's microenvironment: it delivered the right drug at the right time and location (an initial wave to damage tumor cells and a second wave to block stromal protection and immune suppression). This precise spatiotemporal control is difficult to achieve with systemic dosing, and exemplifies the therapeutic precision achievable with smart hydrogels.

From nutrient deprivation to cytokine modulation and stromal remodeling, hydrogels provide a versatile platform for delivering unconventional agents that target tumor-supportive elements. By localizing such treatments, hydrogels reduce systemic toxicity—such as neurotoxicity from methionine deprivation or immunopathology from widespread TGF-β inhibition—while maximizing intratumoral efficacy. The case studies above demonstrate significant therapeutic gains: e.g. complete responses in previously resistant tumors, sensitization of “cold” tumors to checkpoint therapy, and inhibition of metastasis formation. This underscores a paradigm wherein cancer is treated not just by killing tumor cells, but by drugging the TME– and doing so locally for maximal effect.

5. Translational progress and future directions

ISFGs have demonstrated substantial therapeutic efficacy in preclinical models of CRC. Numerous studies have reported durable tumor regression, long-term tumor-free survival, and even abscopal effects in immunoresistant CRC models [25,41,50]. Localized delivery of chemotherapeutics via hydrogel depots has been shown to reduce recurrence after surgical resection, while immunomodulatory hydrogels have been able to initiate systemic antitumor immune responses [28,65,68,84]. These findings support the potential of ISFGs to extend the therapeutic impact of locoregional delivery strategies, with the capacity to elicit systemic effects that are typically difficult to achieve using conventional modalities.

Building on strong preclinical results, several ISFG strategies have advanced toward clinical translation. For example, a representative phase I trial (NCT04062721) is evaluating the combination of RFA with a thermogel co-loaded with GM-CSF and mycobacterial cell wall derivative (PPD) in patients with colorectal liver metastases [67]. Another recent clinical study (NCT06385418) has employed colonoscopic submucosal injection of a 5-FU-loaded ISFG in rectal cancer patients, demonstrating improved tumor regression with limited systemic toxicity [49]. Though these two studies are still in the initial stage (monocentric or exploratory), the clinical outlook is further strengthened by the favorable safety profile of ISFG components. Many hydrogel materials (e.g., hyaluronic acid, chitosan, alginate, poloxamer, and fibrin) are natural or clinically validated polymers with known biocompatibility profiles [85,86]. Their degradation products are generally well-tolerated (e.g., alginate dissolves gradually; poloxamer is eliminated renally). In large animal models, injectable formulations such as mucoadhesive immunogels have shown no detectable organ toxicity or off-target inflammation [66,87]. In addition, many ISFGs are amenable to minimally invasive administration. Formulations designed for injection through fine needles or catheters can be deployed via endoscopy or laparoscopy, facilitating local application to rectal or peritoneal lesions. For clinical usability, injectability parameters (e.g., syringeability through fine needles/catheters and rapid gelation within procedure compatible windows) are also considered in formulation design. Their stimuli-responsive properties—such as temperature, pH, or ion-triggered gelation—enable rapid in situ solidification post-injection [12,38]. Imaging studies in CRC models demonstrate hydrogel retention at the tumor site for several days to weeks [66,88], indicating prolonged and spatially confined exposure to the therapeutic payloads. Pharmacokinetic analyses further show increased intratumoral exposure and decreased systemic exposure after ISFG dosing, reflected by higher tumor/plasma ratios and lower plasma area under the curve (AUC) compared with systemic administration [89]. Some hydrogels have also been reported to reduce peritoneal adhesions after surgery, providing both mechanical and therapeutic functions [25].

Despite promising progress, several translational challenges remain. In particular, sterility assurance for injectables (e.g., terminal sterilization by gamma/e-beam/X-ray versus aseptic processing/filtration and their impact on rheology and release) requires early, explicit planning. Similarly, regulatory classification (device versus drug–device combination, depending on primary mode of action) also needs to be considered. Key concerns include polymer batch consistency, safety in large models, and predictable degradation kinetics. Some immunomodulatory hydrogels incorporate bacterial components (e.g., PPD) or virus-like nanoparticles (e.g., Cowpea Mosaic Virus), which may raise concerns about immunogenicity and require cautious dose escalation in early-phase trials. Another practical limitation is the lack of harmonized efficacy endpoints across preclinical CRC ISFG studies, which hinders cross-study comparison and dose translation; where feasible, we encourage reporting a core set (tumor growth inhibition, survival (Kaplan–Meier with log-rank), local recurrence/metastasis burden, quantitative immune readouts, and safety) together with tissue distribution imaging and pharmacokinetic parameters.

Looking ahead, next-generation ISFGs are expected to integrate more advanced functional designs. For example, personalized hydrogel vaccines incorporating patient-derived tumor lysates post-resection have shown efficacy in reducing recurrence in preclinical CRC models [90]. Combinatorial systems capable of co-delivering chemotherapeutics, checkpoint inhibitors, and physical sensitizers (e.g., photosensitizers) are under active development to achieve multi-modal synergy. Technologies such as 3D bioprinting may enable the fabrication of anatomically tailored hydrogel depots, while organ-on-chip systems embedded with hydrogel matrices facilitate in vitro modeling of drug diffusion, immune cell infiltration, and tumor heterogeneity [91,92]. Another evolving area is functional integration—ISFGs that provide both drug delivery and structural roles, such as tissue sealing, hemostasis, or adhesion prevention. For instance, dopamine-crosslinked HA gels have demonstrated simultaneous hemostatic and immunoadjuvant activity in tumor surgical sites [35]. Such multi-functional devices may streamline surgical workflows while reducing recurrence. Together, these strategies suggest that future ISFG development will not only optimize local control but also redefine systemic CRC management.

In conclusion, ISFGs represent a promising platform for improving CRC therapy. By enabling localized, sustained, and biologically responsive drug delivery, ISFGs address two major unmet needs in CRC: improving intratumoral drug accumulation and minimizing systemic toxicity. Preclinical studies have validated their therapeutic promise, and clinical trials are beginning to test their feasibility in surgical and interventional settings. Future development will likely focus on multi-functional, context-adaptive ISFGs capable of targeting the dynamic TME, with the goal of achieving both local control and systemic therapeutic benefit. With ongoing innovation in materials science, delivery engineering, and translational oncology, these hydrogels are poised to redefine how we treat CRC: not only at the tumor site, but beyond.

CRediT authorship contribution statement

Qing Yao: Writing – original draft, Methodology, Funding acquisition, Conceptualization. Yannan Shi: Writing – original draft, Validation, Methodology. Yinsha Yao: Writing – original draft, Validation. Yingyi Zhao: Writing – review & editing, Validation. Baiqun Duan: Writing – original draft, Validation, Conceptualization. Longfa Kou: Writing – review & editing, Validation, Supervision, Project administration, Funding acquisition, Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

No data was used for the research described in the article.