The purine catabolism cascade: ATP to uric acid
Purines are the molecular currency of cellular energy, but they are also signalling molecules whose fate after release from cells shapes inflammation, immune suppression, and oxidative stress. The cascade begins with adenosine triphosphate (ATP), the universal intracellular energy carrier. In dying, stressed, or activated cells, ATP is released into the extracellular space through pannexin channels, P2X7 receptor gating, or simple membrane rupture. Once extracellular, ATP undergoes stepwise dephosphorylation: ectonucleoside triphosphate diphosphohydrolases (NTPDases) remove the gamma and beta phosphates to yield ADP and then AMP; 5’-ectonucleotidases — principally CD73, the protein encoded by NT5E — then remove the final phosphate to generate adenosine; and adenosine deaminase (ADA) converts adenosine to inosine, a largely inert end-product. Inosine can be further catabolised through hypoxanthine and xanthine to uric acid, with each xanthine oxidase step generating superoxide.
This pathway matters because the different metabolites have profoundly different biological effects. ATP in the extracellular space is a danger signal — it activates P2X and P2Y purinergic receptors on immune cells, triggering NLRP3 inflammasome assembly, cytokine release, and cell death signalling. Adenosine, by contrast, has the opposite immunological valence: it signals through four G-protein-coupled receptors (A1R, A2AR, A2BR, A3R) and, in immunological contexts, is predominantly immunosuppressive. The transition from inflammatory ATP to immunosuppressive adenosine is therefore a molecularly encoded immunological off-switch — one that tumours and chronic infections learn to exploit.
The CD39/CD73 axis: ectonucleotidases as immune regulators
The enzymes that execute the critical ATP-to-adenosine conversion are co-expressed on the surface of regulatory T cells (Tregs), endothelial cells, and many tumour cells. CD39 (official name ENTPD1, ectonucleoside triphosphate diphosphohydrolase 1) converts ATP to ADP and then to AMP. CD73 (NT5E, 5’-ectonucleotidase) converts AMP to adenosine. Together, CD39 and CD73 form a membrane-bound enzymatic relay that transforms pro-inflammatory ATP into immunosuppressive adenosine at the cell surface and in the immediate extracellular environment.
On Tregs, co-expression of CD39 and CD73 is a key effector mechanism for peripheral tolerance. When activated Tregs encounter ATP — released by damaged tissue or activated immune cells — they convert it to adenosine locally, suppressing nearby effector T cells and NK cells. This is not a pathological process in itself; it is how the immune system terminates acute inflammation and prevents autoimmunity. The pathology arises in chronic contexts — particularly in tumours — where the CD39/CD73/adenosine axis becomes constitutively active, creating a persistently immunosuppressive microenvironment that shields tumour cells from immune clearance.
In the tumour microenvironment (TME), multiple CD73 sources converge: tumour cells themselves frequently upregulate CD73 expression (driven by HIF-1α under hypoxia, and by TGF-β from Tregs), and tumour-associated macrophages (TAMs) and cancer-associated fibroblasts (CAFs) contribute additional ectonucleotidase activity. The result is sustained adenosine generation from the ATP continuously released by dying tumour cells and infiltrating immune cells.
A2A receptor signalling and T cell paralysis
The immunosuppressive consequences of adenosine in the TME are mediated primarily through the A2A adenosine receptor (ADORA2A), expressed at high levels on CD8+ cytotoxic T cells and NK cells. A2AR is a Gαs-coupled receptor that increases intracellular cAMP upon adenosine binding. Elevated cAMP activates protein kinase A (PKA), which phosphorylates and inhibits multiple downstream signalling nodes critical for T cell effector function: PKA phosphorylates Csk (C-terminal Src kinase), which in turn phosphorylates and inactivates Lck, the tyrosine kinase that initiates T cell receptor (TCR) signalling. PKA also phosphorylates the inhibitory regulator ICER (inducible cAMP early repressor) and suppresses NFAT nuclear translocation — the transcription factor required for IL-2, IFN-γ, and perforin expression.
The practical consequence is that adenosine-rich environments render tumour-infiltrating lymphocytes functionally exhausted: they express high levels of inhibitory receptors (PD-1, TIM-3, LAG-3), cannot proliferate effectively, and fail to produce cytotoxic granules. This is distinct from — but synergistic with — the PD-1/PD-L1 axis targeted by checkpoint inhibitor therapies. This distinction is clinically important because it explains why checkpoint inhibitors targeting PD-1 alone achieve durable responses in only a minority of solid tumours: adenosine-mediated immunosuppression operates through a completely independent pathway that checkpoint inhibitors do not address.
Adenosine also has pro-angiogenic effects through A2AR on endothelial cells, where receptor activation increases VEGF transcription via cAMP-dependent mechanisms, further supporting the tumour vasculature that feeds tumour growth.
CD73 as a drug target: clinical trials with oleclumab
The recognition of adenosine as a parallel immunosuppressive axis to PD-1/PD-L1 has generated substantial pharmaceutical interest in CD73 inhibition. MEDI9447 (oleclumab, AstraZeneca) is a monoclonal antibody that binds CD73 and blocks its 5’-nucleotidase activity, reducing adenosine generation in the TME. In preclinical models, oleclumab combined with anti-PD-L1 therapy (durvalumab) showed synergistic anti-tumour activity greater than either agent alone. This has been translated into clinical trials: the COAST trial (NCT03822351) investigated oleclumab plus durvalumab in unresectable stage III non-small cell lung cancer, and multiple other trials have explored oleclumab combinations in breast, pancreatic, and colorectal cancers. Other CD73 inhibitors in clinical development include AB680 (Arcus Biosciences) and small molecule approaches targeting the enzyme active site.
A2AR antagonists offer an alternative approach. Caffeine is, of course, the most widely consumed A2AR antagonist in human history — though its adenosine receptor antagonism is non-selective and at physiological concentrations affects A1R more than A2AR. More selective A2AR antagonists including CPI-006 and AZD4635 are in clinical development. The principle is to block adenosine signalling at the receptor level rather than preventing adenosine generation, with the goal of restoring T cell and NK cell function in the TME.
The beneficial side of adenosine: cardioprotection and A1/A3 receptors
Not all adenosine signalling is harmful. The complexity of purinergic signalling lies partly in the divergent effects of different receptor subtypes in different tissue contexts. In the heart, adenosine acting through A1 and A3 receptors is cardioprotective: ischaemic preconditioning — the phenomenon where brief periods of ischaemia protect against subsequent prolonged ischaemic injury — is substantially mediated through adenosine receptor activation, particularly A1R. A1R activation in cardiomyocytes activates PI3K/Akt and ERK survival pathways, reduces mitochondrial permeability transition pore opening, and protects against reperfusion injury. This is why intravenous adenosine is used clinically in the acute management of paroxysmal supraventricular tachycardia — a relatively dramatic demonstration that adenosine is a physiological signalling molecule with potent cardiac effects.
A3 receptor activation similarly has cytoprotective effects in ischaemia and has been investigated as a cancer-protective mechanism in some contexts (A3R activation can promote apoptosis in cancer cells while being protective in normal cells). The therapeutic complexity of adenosine is thus considerable: the same molecule whose accumulation in the TME paralyses anti-tumour immunity may simultaneously protect the heart during ischaemia. This is why approaches like CD73 inhibition — which reduce adenosine generation locally at the tumour — are more nuanced than simply blocking all adenosine signalling.
Xanthine oxidase, uric acid, and gout
Downstream of adenosine, the final steps of purine catabolism involve xanthine oxidase converting hypoxanthine to xanthine, and xanthine to uric acid, with generation of superoxide radical at each step. Uric acid is the terminal purine metabolite in humans (unlike most mammals, humans lack uricase and cannot further catabolise uric acid to allantoin). This creates the clinical condition of hyperuricaemia and gout: when uric acid exceeds its solubility threshold in plasma (∼6.8 mg/dL), it crystallises as monosodium urate in joints and periarticular tissue, triggering a violent NLRP3 inflammasome-driven inflammatory response — the acute gouty flare.
Beyond gout, hyperuricaemia is associated with hypertension, kidney disease, and cardiovascular disease, partly through direct effects of uric acid on endothelial function and partly through the superoxide generated during its synthesis by xanthine oxidase. The xanthine oxidase inhibitor allopurinol (and its active metabolite oxypurinol) is the first-line pharmacological treatment for hyperuricaemia, reducing uric acid by blocking the enzyme that generates it.
Where spirulina connects: xanthine oxidase inhibition
Phycocyanin, spirulina’s principal bioactive pigment, has been shown in multiple in vitro studies to inhibit xanthine oxidase activity. The mechanism appears to involve direct interaction of phycocyanobilin (the tetrapyrrole chromophore of phycocyanin) with the enzyme’s active site, though the structural basis has not been resolved to crystallographic detail. IC50 values in cell-free assays range widely depending on the phycocyanin preparation and assay conditions, but inhibition at physiologically plausible concentrations has been demonstrated in several independent laboratories.
This xanthine oxidase inhibition has two relevant consequences. First, it reduces uric acid production — potentially relevant to managing hyperuricaemia, though the clinical evidence in humans with gout is very limited and consists mainly of small observational studies and case reports. Second, xanthine oxidase is a significant source of superoxide during oxidative stress — particularly during ischaemia-reperfusion events where hypoxanthine accumulates and is then rapidly oxidised upon reoxygenation. Phycocyanin’s xanthine oxidase inhibition could reduce this oxidative burst, complementing its direct radical-scavenging properties (phycocyanobilin is itself a potent peroxynitrite and hydroxyl radical scavenger).
It is important to be honest about the distance between in vitro xanthine oxidase inhibition and clinical benefit in gout. Allopurinol achieves plasma concentrations of its active metabolite oxypurinol in the 5–20 μM range, which translates to near-complete xanthine oxidase inhibition. The phycocyanin concentrations required to achieve comparable inhibition in vitro are unlikely to be reached in plasma at typical spirulina supplementation doses (1–5 g/day). Spirulina is not a replacement for established urate-lowering therapy, but may provide additive antioxidant benefit relevant to the oxidative component of gout pathology.
Spirulina, AMPK, and the AMP/ATP ratio
AMPK activation by spirulina components (covered in detail in the AMPK pathway articles) has an interesting relationship with purine metabolism. AMPK senses cellular energy status through the AMP/ATP ratio — specifically, AMPK is allosterically activated by AMP binding to its regulatory γ subunit. Spirulina’s activation of AMPK through phycocyanin’s direct effects on upstream kinases (LKB1, CaMKKβ) and on mitochondrial complex I modulation does not primarily work through altering the AMP/ATP ratio; rather, it engages AMPK through upstream kinase activation that is AMP-independent.
This distinction matters because a genuine increase in the intracellular AMP/ATP ratio (as occurs in ischaemia or metabolic stress) would increase flux through the purine degradation pathway toward adenosine and eventually uric acid. AMPK activation by spirulina’s phycocyanin through LKB1-mediated mechanisms does not drive this catabolic flux, so concerns about spirulina supplementation increasing uric acid production via AMPK activation are not mechanistically founded. The two phenomena — AMPK activation and purine catabolism — are related by the adenosine nucleotide pool but are not coupled in the way a simple energetic model might suggest.
Tumour microenvironment adenosine and spirulina’s anti-tumour context
Spirulina has documented anti-proliferative effects in numerous cancer cell line studies, involving multiple mechanisms including cell cycle arrest, apoptosis induction, and NF-κB suppression. Whether the CD39/CD73/adenosine axis is relevant to these effects requires nuanced consideration.
Phycocyanin has been reported in some cell-based studies to modulate A2AR signalling and to reduce CD73 expression on certain cancer cell lines. If confirmed and extended to in vivo tumour models, this would suggest that phycocyanin could partially counteract the immunosuppressive adenosine environment in tumours by reducing the CD73-mediated adenosine generation at the tumour surface — making the TME less tolerogenic and theoretically more permissive to immune-mediated tumour clearance. This is a mechanistically coherent hypothesis, but the evidence base is thin: most relevant data comes from cell line studies where tumour-infiltrating lymphocytes, stromal cells, and the full TME complexity are absent. Extrapolating from cancer cell A2AR studies to immune-tumour interactions in vivo requires caution.
The honest position is that spirulina’s relevance to TME adenosine biology is an interesting mechanistic lead that warrants investigation in better experimental models, but is not an established anti-cancer mechanism that patients or clinicians should make treatment decisions on the basis of.
Summary
Purine catabolism generates a cascade of metabolites with distinct and often opposing biological functions: inflammatory ATP, immunosuppressive adenosine, and the pro-oxidant xanthine oxidase substrate that yields uric acid. The CD39/CD73 ectonucleotidase axis on Tregs and in the tumour microenvironment is a validated, clinically targetable immunosuppressive mechanism, with CD73 inhibitors in active clinical trials for multiple solid tumours. Spirulina touches this system at two points: phycocyanin’s xanthine oxidase inhibition reduces the terminal oxidative step of purine catabolism, with relevance to hyperuricaemia and oxidative stress; and emerging evidence suggests phycocyanin may modulate CD73 expression and A2AR signalling in cancer cells, though the in vivo significance of this remains to be established. Both connections are mechanistically grounded but require considerably more human evidence before clinical implications can be drawn.