Aerobic glycolysis in tumor cells serves as the foundation for producing IMP synthesis precursors. Most cancer cells adhere to the Warburg effect, which prioritizes glycolysis to convert glucose into lactate even in the presence of abundant oxygen, whereas normal cells usually generate energy by mitochondrial oxidative phosphorylation. According to the theory by early Warburg, tumor cells prefer aerobic glycolysis, an inefficient ATP production method that yields only 2 ATP molecules, whereas oxidative phosphorylation produces 36 ATP. Warburg explained that this was a passive adaptation due to mitochondrial dysfunction caused by hypoxia, lactate accumulation and other factors, which results in irreversible respiratory impairment (15). However, subsequent studies revealed that most cancer cells have normal mitochondrial function and are still capable of oxidative phosphorylation; their metabolic activity has changed to glycolysis (16-18). It has been found that cancer cells preferentially use aerobic glycolysis to meet the biosynthetic demands of proliferation by allocating glucose-derived carbon toward biosynthesis rather than complete oxidative phosphorylation. The pentose phosphate pathway (PPP) converts ~10% of glucose into 5-phosphoribose to supply nucleotide synthesis; another portion is metabolized into acetyl-CoA for lipid synthesis; and only a small fraction is completely oxidized to carbon dioxide (CO₂) to meet basic ATP requirements. Cancer cells coordinate the PPP with glutamine metabolism in an integrated process, in addition to allocating carbon through aerobic glycolysis (19). A total of three vital enzymes are critical to the aerobic glycolysis of tumor cells: Hexokinase 2 (HK2), pyruvate kinase M2 (PKM2) and lactate dehydrogenase A (LDH-A) (20). HK2 is highly expressed in many tumors and localizes to the outer mitochondrial membrane to bind with the voltage-dependent anion channel (VDAC) along with hexokinase 1 and hexokinase domain component 1. This arrangement allows direct use of VDAC-derived ATP for glucose phosphorylation, thereby improving hexokinase activity, accelerating glucose-6-phosphate (G-6-P) synthesis, modulating glycolytic flux and inhibiting G-6-P accumulation (21). By contrast, LDH-A inhibition significantly suppresses cell proliferation. LDH-A-specific inhibitors (such as oxaloacetate) block lactate production in tumor cells, causing intracellular carbon accumulation, impaired diversion of glycolytic intermediates toward nucleotide and lipid synthesis, decreased NAD⁺ regeneration, reduced glycolytic flux and diminished NADPH production, because glutamine metabolism depends on glycolysis-coupled NAD⁺ regeneration, ultimately leading to growth arrest or apoptosis. Therefore, LDH-A is crucial for preserving the precursors needed for IMP synthesis (22). Furthermore, proliferative signals inhibit the activity of PKM2, a tumor-specific isoform. Glycolytic intermediates, such as 3-phosphoglycerate (3-PG) and phosphoenolpyruvate (PEP), in their low-activity dimeric forms, are inefficiently converted to pyruvate and are instead redirected into anabolic pathways. Particularly, 3-PG and fructose-6-phosphate enter the non-oxidative branch of the PPP to produce ribose-5-phosphate, whereas PEP facilitates the biosynthesis of alanine and serine (23,24).

The PPP, which is an essential part of carbohydrate metabolism, is primarily responsible for producing 5'-phosphoribulose and NADPH. The former serves as a critical precursor for IMP synthesis within the body, whereas the latter is essential for maintaining redox homeostasis (25). The PPP consists of two functionally complementary branches: Oxidative and non-oxidative. The oxidative branch in tumor cells, driven mainly by glucose-6-phosphate dehydrogenase (G6PD) and 6-phosphogluconate dehydrogenase, produces NADPH to support antioxidant defense and lipid biosynthesis, while the non-oxidative branch is adapted to hypoxic and highly proliferative conditions. This branch circumvents the limitations of oxidative phosphorylation under hypoxia by converting glycolytic intermediates into 5-phosphoribose independently of mitochondria. It is catalyzed by transketolase-like 1 (TKTL1) and transaldolase. This reversible pathway supplies >60% of the 5-phosphoribose required by tumor cells and dynamically connects glycolysis to biosynthetic pathways to distribute carbon in accordance with energetic and anabolic requirements (26,27). These two branches are interconnected rather than independent: The oxidative branch produces NADPH, which is generated by the oxidative branch, protects non-oxidative enzymes such as TKTL1 from reactive oxygen species (ROS)-mediated damage, whereas the non-oxidative branch replenishes G-6-P for the oxidative pathway by recycling intermediates into glycolysis, forming a metabolic cycle that collectively supports tumor proliferation, apoptosis resistance and metastatic potential (26). It is now recognized that tumor cells exhibit upregulation of the PPP pathway, particularly in rapidly growing tumors such as glioblastoma. Increased expression of enzymes such as G6PD indicates enhanced PPP activity, providing an abundance of ribose-5-phosphate and promoting de novo IMP synthesis. Elevated IMP, adenosine monophosphate (AMP) and guanosine monophosphate (GMP) levels were verified in five patient-derived glioblastoma models, maintaining cellular proliferation and stemness. Furthermore, concurrent NADPH synthesis also improves invasive phenotypes by maintaining tumor cell redox homeostasis. In conclusion, tumor cells modulate the PPP pathway to boost 5-phosphoribose needed for IMP synthesis and NADPH to maintain a stable tumor microenvironment (28). It has been demonstrated that combining therapy with G6PD and glycolysis inhibitors significantly improves radiosensitivity in human gliomas, suggesting that this combination may be a novel targeted treatment approach (29).

Glucose is taken up by cancer cells and metabolized into precursors such as serine and glycine, which are essential for one-carbon metabolism and provide the carbon units needed to synthesize IMP and glycine (30). The primary source of cytoplasmic serine is extracellular uptake through the alanine/serine/cysteine/threonine transporter 1 and the diversion of ~10% of the glycolytic intermediate 3-PG into de novo serine synthesis, which is catalyzed by phosphoglycerate dehydrogenase (PHGDH). Serine is subsequently produced through transamination by phosphoserine aminotransferase 1 and dephosphorylated by phosphoserine phosphatase. Cytoplasmic serine hydroxymethyltransferase 1 (SHMT1) or mitochondrial serine hydroxymethyltransferase 2 (SHMT2) then catalyze the combination with tetrahydrofolate (THF) to produce glycine and 5,10-methylenetetrahydrofolate (31-33). Most cancer cells preferentially use mitochondrial SHMT2 to boost mitochondrial folate synthesis rather than cytoplasmic SHMT1 to convert serine to glycine; SHMT2 knockout leads to increased accumulation of AICAR, an intermediate in IMP synthesis (34). It is interesting to note that SHMT2 generates S-adenosylmethionine (SAM) through one-carbon metabolism, increasing N6-methyladenosine (m⁶A) modification of PPAT mRNA. This modification enables recognition and stabilization by insulin-like growth factor (IGF) 2 mRNA-binding protein 2, which eventually increases PPAT expression to promote renal cell cancer development (35). Tumor cells synthesize required IMP through extracellular serine uptake when serine availability is limited. This finding suggests that lowering IMP synthesis may enhance the efficacy of therapies in specific tumor cells (36). The primary carbon source for purine ring formation in the cancerous tissues of non-small cell lung cancer (NSCLC) is glucose. This process involves the activation and compartmentalization of the glucose-to-serine pathway in the cytoplasm, as well as an enhanced reverse one-carbon flux that reduces the incorporation of external serine into purine synthesis (30,37). Numerous types of tumor cell have been shown to exhibit a significant increase in gene expression linked to serine metabolism. For instance, PHGDH is markedly overexpressed in melanoma, breast cancer and NSCLC, particularly in 70% of triple-negative breast cancers, which is associated with poor prognosis (37,38).

Serine metabolism and glycolysis are closely related due to complex feedback regulation. Elevated serine levels activate the low-activity M2 isoform of PKM2, which catalyzes the conversion of phosphoenolpyruvate to pyruvate in the final stage of glycolysis (39). This process maintains glycolytic flux while restricting diversion of 3-PG into de novo serine synthesis. Serine depletion lowers PKM2 activity, impairs the terminal glycolytic step and results in 3-PG accumulation that is redirected into the serine biosynthetic pathway. Simultaneously, elevated 2-phosphoglycerate (2-PG) activates PHGDH, a crucial enzyme in serine and glycine synthesis, thereby increasing de novo serine production; 2-PG is converted to 3-PG by phosphoglycerate mutase, which is immediately upstream of PHGDH. As a result, rapid tumor proliferation and high IMP consumption cause serine deficiency, inhibit PKM2 activity, increase 3-PG accumulation and promote compensatory serine synthesis. This feedback circuit partially explains the limited effectiveness of the purine synthesis inhibitor methotrexate. Furthermore, ribonucleotides-purine biosynthetic intermediates-can allosterically activate PKM2 under glucose-restricted conditions, offering an additional adaptive mechanism for cancer cell survival (40). Thus, focusing on the association between serine metabolism and glycolysis may provide a viable strategy for managing cell growth and developing cancer therapies.

Glutamine is necessary for cancer cell proliferation and, together with its metabolite aspartate, serves as a crucial precursor for IMP synthesis. Rapid tumor proliferation diverts vital tricarboxylic acid cycle (TCA cycle) intermediates toward biosynthesis, disrupting the balance of the TCA cycle. Glutamine is converted to glutamate through the glutaminase pathway and then metabolized by glutamate dehydrogenase 1 into α-ketoglutarate, which enters the TCA cycle to replenish carbon sources (41,42). Furthermore, α-ketoglutarate can be further oxidized to produce succinate and fumarate. These metabolites assist the TCA cycle in cancer cells by providing ATP, NADH and FADH₂. They also act as essential tumor metabolites that promote rapid tumor cell proliferation (41,43,44). Additionally, glutamate is converted by glutamic-oxaloacetic transaminase (GOT) and glutamic-pyruvic transaminase (GPT) into alanine and aspartate, which provide tumor cells with nitrogen sources. In particular, GPT catalyzes the conversion of glutamate and pyruvate into α-ketoglutarate and alanine, with α-ketoglutarate replenishing the TCA cycle and alanine supporting protein synthesis. It has been demonstrated that GPT2 has a crucial role in breast cancer, glioblastoma and KRAS-driven colorectal cancer (45-47). GOT catalyzes the transformation of glutamate and oxaloacetate into α-ketoglutarate and aspartate. Endogenous synthesis is essential since cancer cells have a limited ability to absorb exogenous aspartate. Aspartate production exerts a central oncogenic function by promoting nucleotide and protein synthesis and contributing to the malate-aspartate shuttle, which produces NADPH to maintain tumor cell redox homeostasis (43,48). Additionally, asparagine synthetase (ASNS) utilizes glutamate as a substrate to produce asparagine. This process consumes significant amounts of aspartate and stimulates its synthesis in tumor cells (49). Research has revealed the involvement of ASNS in tumor development and metastasis, in addition to its link to poor survival rates in several cancers, including NSCLC (50). Notably, glutamate catalyzed from glutamine serves as an important precursor for glutathione (GSH). It produces GSH, an essential reducing agent that scavenges ROS and protects cancer cells from apoptosis, together with cysteine and glycine (51). Furthermore, the ability of this metabolic pathway to convert glutamine into biosynthetic precursors is particularly critical in hypoxic conditions (52).

Mitochondria, as the primary metabolic organelles in cells, coordinate cellular energy production and provide the raw materials needed for cell proliferation, given the marked demand of tumor cells for TCA cycle precursors and the energy produced by de novo IMP synthesis. Their critical role in the IMP metabolic pathway is undeniable. Mitochondria replenish TCA cycle intermediate metabolites by oxidizing glutamine-derived α-ketoglutarate or acetyl-CoA and maintain TCA cycle activity (53). Furthermore, mitochondrial respiration produces aspartate, which increases IMP synthesis in low-oxygen environments, promoting tumor growth and the progression of cancer (54,55). The blocking of mitochondrial respiration reduces intracellular aspartate levels, which in turn restricts the production of asparagine, which is derived from aspartate (56). Interestingly, fibroblasts associated with cancer serve as an additional source of aspartate, which promotes the proliferation of tumor cells (54). Furthermore, SHMT2 converts serine into glycine and one-carbon units in mitochondria, whereas mitochondria produce ATP and CO₂ by oxidative phosphorylation (57). Therefore, mitochondria serve as factories for the synthesis of de novo IMP precursors and are crucial for IMP metabolism. Additionally, purinosomes involved in IMP synthesis are intrinsically linked to mitochondria. Targeting mitochondrial function may be a viable strategy for inhibiting purine biosynthesis due to this mitochondrial dependence (8) (Fig. 2).

Purinosomes, initially identified in 2008, are dynamic intracellular structures made up of six enzymes involved in de novo purine synthesis. The core scaffold comprises PPAT, GART and FGAMS, while PAICS, ADSL and ATIC are associated with peripheral structures. This spatial organization facilitates metabolic channeling in order to produce IMP and downstream purines to meet cellular demands effectively. Notably, purinosomes are functionally coupled to mitochondria, suggesting a link between de novo purine synthesis and cellular energy metabolism. The primary function of purinosomes in growth-related pathways makes them attractive therapeutic targets, despite the fact that their significance in cancer is still unclear (8,73-75). Its formation is driven by reduced cellular purine levels and increased metabolic demand. Enzymes of the de novo synthesis pathway assemble into purinosomes when cells require high purine levels, such as during G₁/S-phase proliferation or when salvage pathways are compromised. This organization improves metabolic efficiency by channeling substrates, limiting diffusion or degradation of intermediates, and increasing de novo synthesis flux by ~50%, thereby increasing IMP production threefold as compared to normal growth conditions. Pyrimidine body production rises by 25% in cells with impaired salvage pathways, such as HGPRT knockout cells, thereby compensating to maintain purine supply. Pathway flux may be markedly more noticeable during the G₁ phase of cell proliferation because these investigations are asynchronous (8,76,77). Functional loss of any purinosomes component disrupts its stability and structure (78,79), and purine supplementation causes complete or significant depletion of purinosomes (8). According to time-lapse fluorescence microscopy analysis, the proportion of purinosome-positive cells peaked during the G₁ phase, providing essential purine nucleotides for cell growth, and progressively decreases throughout the course of the remaining cell cycle. Interestingly, expression levels of de novo purine synthesis enzymes did not change across the cell cycle, suggesting that mechanisms other than purinosome assembly and disassembly may also regulate this process (77). Available evidence suggests that a significant portion of purinosomes in purinosome-positive cells localize to mitochondria rather than being randomly distributed throughout the cytoplasm, indicating a close mitochondrial association due to the high energetic needs of purine synthesis. Microtubules provide a structural foundation for purinosome function, as evidenced by the spatial regulation of purinosome assembly being dependent on microtubules. Purinosomes travel along the microtubule network, and microtubule depolymerization disrupts their mitochondrial colocalization, leading to a roughly 50% reduction in the flux of de novo purine synthesis. Super-resolution fluorescence microscopy further revealed colocalization of purinosomes with mitochondria, with at least nine purinosome components located at the microtubule-mitochondria interface. This organization allows purinosomes to directly access ATP, one-carbon units (formate), aspartate and glutamine, thereby maximizing the efficiency of purine synthesis. Notably, purinosome formation is increased by defects in mitochondrial functions, including electron transport or oxidative phosphorylation, whereas excessive purinosome accumulation can impair mitochondrial function (5,80,81). Although the role of purine synthase in cancer has not been extensively studied, it is a promising target for possible treatments due to its crucial involvement in notable cellular growth pathways. Furthermore, its dependence on mitochondria suggests that targeting mitochondria can be a strategy to inhibit purine biosynthesis (8). Additionally, molecular chaperones heat shock protein (HSP70)/HSP90 actively contribute to purinosomes assembly, stability maintenance and functional execution by direct binding, folding assistance and degradation regulation. They serve as essential regulators of the biophysical properties and structural integrity of purinosomes, and they also provide a potential therapeutic strategy targeting purinosomes by interfering with chaperone function (82-84). Tumor cells use mechanisms to regulate the assembly and disassembly of purinosomes or their interactions with other organelles to meet their high IMP demand. Tumor cells use microtubules to assemble purinosomes when IMP is deficient, a crucial adaptive mechanism that enables cancer cells to respond to purine scarcity. The process can be inhibited by docetaxel, a microtubule-stabilizing chemotherapeutic agent (85) (Fig. 3).

Numerous vital concerns about the structure and function of purinosomes remain unresolved despite initial observations. The present review summarized the main knowledge gaps and emerging hypotheses in purine vesicle biology to highlight their potential for cancer research.

Purinosomes play a central role in cellular metabolism, but little is known about their regulatory mechanisms and functional properties in the tumor context. First, whether various tumor types exhibit distinct patterns of purinosome assembly remains unknown. For instance, although myelocytomatosis oncogene (MYC)-driven tumors frequently exhibit increased de novo purine synthesis flux (86), systematic studies of assembly principles unique to tumor subtypes are limited. Second, purinosomes are markedly enriched in the vicinity of mitochondria; nevertheless, the functional mechanism responsible for this spatial coupling remains undefined. Additionally, purinosomes may receive essential substrates from mitochondria, such as ATP, formate and aspartate (8). Can purinosomes specifically inhibit tumor cells by disrupting mitochondrial colocalization, potentially developing a new therapeutic avenue?

Furthermore, the assembly of purinosomes depends on molecular chaperones such as HSP90/HSP70, indicating that they constitute functional targets for chaperone inhibitors (83). However, the extent of their dependence on the chaperone system and their specificity in tumor cells remains to be confirmed. Finally, the morphological, dynamic and functional traits of the purinosome closely resemble those of liquid-liquid phase separation (LLPS)-formed aggregates (82,87,88). Although LLPS is the most favored proposed mechanism to explain its development and characteristics, there is still limited evidence. There are likely other/further mechanisms.

Overall, these unresolved issues constitute important areas of research in purine metabolism. An in-depth investigation of their mechanisms may help clarify patterns of nucleotide metabolism in tumors and provide crucial information for developing novel metabolic-targeted treatment approaches.

PRPS is an important enzyme involved in the early phases of the de novo purine synthesis pathway. PRPS1 is an essential isoform in normal cells, whereas PRPS2 is a crucial dependence subtype in tumor cells (particularly those that overexpress MYC) (89). It catalyzes the synthesis of PRPP, an essential precursor of IMP that provides the phosphoribose backbone for IMP. The reaction proceeds as follows: 5'-ribose-5'-phosphate + ATP → PRPP + AMP. Mutations that cause hyperactivation or overexpression of this enzyme have been identified in several cancers. PRPS2-dependent nucleotide synthesis is a crucial factor in determining cancer cell survival in hepatocellular carcinoma, diffuse large B-cell lymphoma, lung squamous cell carcinoma, lung adenocarcinoma and colorectal adenocarcinoma. Consequently, increased PRPS activity results in elevated PRPP production, thereby accelerating nucleotide synthesis to meet the high nucleic acid demands of proliferating tumor cells (60,90,91). Additionally, PRPS2 evades classical ADP/GDP-mediated allosteric feedback inhibition by using four non-conservative key residues, which leads to sustained ATP production that promotes tumor cell proliferation and survival. Studies further demonstrated that PRPS2 has unique carcinogenic roles in regulating RNA methylation in tumor cells. PRPS2, by sustaining ATP production, promotes SAM synthesis and improves methionine adenosyltransferase 2A stability through direct interaction, thereby increasing ATP use and SAM availability. This subsequently promotes RNA m⁶A methylation through the methyltransferase-like 3 (METTL3)-METTL14-Wilms tumor 1-associated protein complex, thereby aiding the growth and metastasis of lung tumors. These results highlight that PRPS2 is an important regulator of tumor progression and a potential therapeutic target (92). Notably, increased glycolysis, PPP and PRPS activities were observed in tumor cells overexpressing cellular myelocytomatosis oncogene protein (c-MYC) (86). Additional research has documented a feedback loop involving phosphoribosyl PRPS1 and its substrate 5-phosphoribose (R-5P) in regulating the PPP. These results indicate that elevated PRPS activity in tumor cells consumes significant amounts of R-5P from the PPP, increasing PPP flux and supplying biosynthetic substrates to promote tumor proliferation (93). Simultaneously, PRPS2 facilitates the synergistic regulation of 'protein synthesis-nucleotide metabolism' by MYC, which maintains tumor metabolic homeostasis (89). From a metabolic perspective, improving the PPP and coordinating protein synthesis probably extends beyond PRPS; these two traits may be shared by enzymes regulating de novo IMP synthesis pathways. Beyond this, several aspects of PRPS mechanisms within tumor cells require further investigation, including their coordinated interactions with other enzymes. Comprehensive studies may offer a scientific basis for targeted combination treatments.

PPAT is the first key/rate-limiting enzyme involved in the de novo synthesis of purine nucleotides, which catalyzes the following initial step of this process: PRPP + Gln → 5-phosphoribosylamine + Glu + PPi. This reaction provides a critical precursor for the subsequent synthesis of IMP, AMP and GMP, directly determining the overall flux of purine nucleotide synthesis (94,95). It has been demonstrated that PPAT has a high expression level in several tumors, including nasopharyngeal carcinoma, renal cell carcinoma, small cell lung cancer, thyroid cancer, gastric cancer and colorectal cancer. Its elevated expression promotes de novo synthesis of purine nucleotides, which supports tumor cell growth and invasion. Silencing PPAT significantly reduces cell proliferation and clonogenic potential, arresting the cell cycle in the G0/G1 phase and reducing migration and invasion. This further underscores the crucial role of PPAT in maintaining the proliferation and malignant behavior of tumor cells (65). Furthermore, PPAT connects glutamine metabolism and purine synthesis pathways. Elevated PPAT expression in tumor cells diverts glutamine from the TCA cycle to purine synthesis, thereby reducing its contribution to the TCA cycle. This metabolic shift meets the high nucleotide requirements of tumor cells and also affects cellular energy metabolism and other biosynthetic processes, thereby supporting the growth and survival of tumor cells (96). Other studies demonstrated that various factors in the tumor microenvironment, such as hypoxia and fluctuations in nutrient concentrations, may also impact the activity and expression of PPAT. Tumor cells may regulate PPAT under hypoxic conditions through specific signaling pathways and may synergize with other metabolic regulators to increase its activity. This regulation maintains purine synthesis, which provides vital nucleotides and energy to support tumor cell survival and progression in harsh conditions (97-99).

The human genome encodes two isoforms of IMPDH: IMPDH1, located on chromosome 7, and IMPDH2, located on chromosome 3. These isoforms are essential enzymes that catalyze the conversion of IMP to xanthosine monophosphate, regulating the influx of guanine nucleotides. Interestingly, they are found in nearly all organisms (100). The assembly of intracellular IMPDH filaments forms rod-like and ring-like structures. This process enables proliferating cells to maintain high levels of guanine nucleotides, which are necessary for rapid cell division. Furthermore, IMPDH is frequently overexpressed in numerous types of tumor cells (101).

According to available data, IMPDH is highly expressed in a variety of tumor types, with IMPDH2 expression being particularly high. Increased IMPDH activity enhances the GMP biosynthetic pathway, leading to markedly elevated GMP and GTP production. This elevated nucleotide output meets tumor cell demands for purines, energy and signaling necessary for DNA and RNA synthesis, thereby promoting proliferation (102). For instance, increased expression of IMPDH in colorectal cancer encourages cell proliferation, invasion and migration. Similar results were observed in acute myeloid leukemia, glioblastoma and small-cell lung cancer (67,70,103,104). There is evidence that the conversion of IMP to GMP consumes NAD⁺ and produces NADH. This process is accelerated by high IMPDH expression in tumor cells, which increases NADH synthesis and significantly lowers the cytoplasmic NAD⁺/NADH ratio. This change impairs GAPDH activity and disrupts glycolytic flux because the glycolytic enzyme GAPDH needs NAD⁺ as a cofactor. As a result, glucose-derived intermediates are redirected toward the PPP, thereby increasing 5-phosphoribose synthesis and accelerating de novo nucleotide synthesis in tumor cells (105,106). Furthermore, IMPDH is essential in the tumor microenvironment. De novo purine synthesis is inhibited when important substrates such as 5'-ribose-5-phosphate and glucose are limited; under these conditions, increased IMPDH expression compensates by using salvage pathway-derived IMP to maintain GMP and GTP production (1). According to recent research, cellular hypoxia also increases IMPDH expression, which encourages GMP production to sustain basal proliferation in hypoxic conditions (107). However, further research on this mechanism is still needed.

The dependence of different tumor types on de novo IMP synthesis varies significantly; this heterogeneity is controlled by factors such as tissue origin, driver mutations, metabolic status and the tumor microenvironment (108). Cellular dependency is mainly determined by the differential expression and functional divergence of the IMPDH1 and IMPDH2 isoforms. This section summarizes IMPDH reliance in various tumor types and its potential treatment implications.

Malignant tumors of the hematopoietic system exhibit a high degree of dependence on IMPDH2. This dependency arises from their high rates of proliferation, reduced ability to recycle purines and a MYC-driven increase in purine synthesis metabolism. For instance, acute myeloid leukemia (AML) with mixed lineage leukemia (MLL) rearrangements exhibits high sensitivity to IMPDH inhibition, highlighting the crucial role of IMPDH2 in the initiation and maintenance of leukemia (72). Multiple myeloma and certain acute lymphoblastic leukemia subtypes also depend on IMPDH2 to maintain GTP supply, thereby sustaining their proliferative capacity (109,110). These tumors frequently struggle to maintain nucleotide homeostasis through metabolic compensation pathways when IMPDH is inhibited, making IMPDH inhibition a unique metabolic vulnerability in such tumors.

By contrast, solid tumors exhibit subtype-specific dependence on IMPDH. IMPDH2 is consistently increased in glioblastoma and is involved in ribosome biosynthesis and DNA damage repair. Inhibiting IMPDH2 improves the effectiveness of radiotherapy and Temozolomide chemotherapy, indicating a distinct metabolic therapeutic window (70). IMPDH2 is also markedly elevated in highly proliferative solid tumors such as ovarian cancer, and is associated with disease progression and poor prognosis. Its suppression influences purine synthesis and may also regulate microenvironmental processes, including inflammation and angiogenesis (111,112). Despite having higher IMPDH2 levels, colorectal cancer is more sensitive to local pharmacokinetic features (104). Meanwhile, IMPDH1 expression can be increased in certain solid tumors under conditions such as hypoxia or nutritional deprivation to maintain oxidative metabolism and NADPH homeostasis, thereby reducing sensitivity to pure D-type IMPDH inhibitors. This implies that combination strategies must be considered (70).

Furthermore, IMPDH2 upregulation and elevated nucleolar activity are observed in certain virus-associated or immune-coupled tumors (such as Epstein-Barr virus-associated lymphoproliferative disorders), a dependency resulting from GTP metabolic requirements and aberrant immune regulation (108,113). IMPDH inhibition may have both antiproliferative and immunomodulatory effects in these diseases, providing unique therapeutic potential.

Overall, the dependencies of various tumors on IMPDH vary widely, ranging from high-proliferation-dependent types centered on IMPDH2 to adaptive dependency types regulated by the microenvironment and pharmacokinetics, and finally to metabolism-immune coupling types. These differences imply that IMPDH-targeted therapies should be customized through specialized investigation of tumor type, subtype characteristics, IMPDH1/2 expression patterns and metabolic compensatory capacity to improve treatment selectivity and clinical translation potential.

IMP metabolism is a critical node in purine metabolism. The downstream conversion of its metabolites, GMP/GTP, directly or indirectly impacts T-cell activation, myeloid cell polarization and the transmission of adenosine signaling pathways, thereby exerting significant effects on the human immune system.

First, T cells exhibit a marked rise in GTP demand during activation and proliferation. GTP possesses dual roles as both an essential metabolic substrate and signaling molecule. It serves as a crucial substrate for RNA synthesis, supporting T-cell clonal expansion, while also functioning as a key molecule in receptor signaling mediated by small GTPases. GTP is involved in vital activities, including immunological synapse formation, T-cell receptor signal amplification, cytoskeletal remodeling and integrin activation (148-151). Furthermore, it is the only precursor needed to synthesize tetrahydrobiopterin (BH4). BH4 is essential for T-cell mitochondrial electron transport, ATP supply maintenance and cell cycle progression. It affects nucleotide synthesis, ultimately resulting in decreased T-cell proliferation (152). Thus, a substantial GTP pool is a metabolic requirement for T cells to progress from initial activation to effector differentiation. T cells demonstrate functional suppression when IMPDH is inhibited because intracellular GTP levels significantly decrease, and T cells are unable to maintain normal clonal expansion and effector differentiation. These mechanisms explain why IMPDH inhibitors cause immunosuppression in clinical settings (138) and suggest that the metabolic imbalance of IMPs in the tumor microenvironment may weaken antitumor immunity by limiting GTP availability, providing an important theoretical foundation for targeting IMP metabolism to boost immune responses (153).

Second, IMP metabolism is closely associated with macrophage polarization. An adequate supply of purine nucleotides significantly increases the pro-inflammatory gene expression, migration, phagocytosis and anti-pathogen capabilities of classically activated macrophages under inflammatory conditions, suggesting that IMP/GMP/GTP levels are essential for M1 macrophage function (154). Increased purine degradation and uric acid production in the tumor microenvironment cause macrophages to adopt an immunosuppressive tumor-associated macrophage phenotype with high programmed cell death ligand 1 (PD-L1) expression, which can be re-polarized by inhibiting IMP metabolism (155). Furthermore, impaired purine degradation in monocyte-derived macrophages raises isocitrate dehydrogenase 3 activity and causes increased production of α33sdz5 d-ketoglutarate, which promotes M2-like polarization (156).

Furthermore, the body maintains equilibrium by an intriguing cross-regulation mechanism in which high levels of AMP and GMP block their own synthesis, while GTP stimulates AMP synthesis. By contrast, ATP promotes the synthesis of GMP. Accordingly, increased IMP/GMP/GTP metabolism in tumor cells leads to enhanced IMP/AMP/ATP metabolism. Subsequently, AMP/ATP can catalyze the production of additional adenosine using ectonucleoside triphosphate diphosphohydrolase 1/ecto-5'-nucleotidase (157,158). The role of adenosine in immunology is unmatched. It is an essential class of immunosuppressive signaling molecules in immune regulation that maintains immune homeostasis, limits inflammatory responses and modulates the tumor immune microenvironment (156,159,160). Consequently, the balance of IMP/GMP/GTP metabolism affects the operation of the adenosine signaling pathway, eventually affecting the immune microenvironment, including tumor cells.

In conclusion, immune system function depends on the equilibrium of IMP/GMP/GTP metabolism. It exhibits significant associations with T cells, macrophages, adenosine and other components and exerts an unparalleled impact on the immune microenvironment. Scientific investigation into the immunological implications of this extraordinarily complex process has never stopped.