AICAR Research

AICAR research is unusual in one respect. Most research compounds have thin human data. AICAR has a large, modern, and clearly negative human trial record, published under the name acadesine. Its preclinical literature is extensive and often striking. The gap between the two is the most useful thing to understand about it. This article summarizes what the published evidence has investigated. It describes research findings, not outcomes anyone should expect.

What AICAR is

AICAR is a nucleoside, not a peptide. Chemically it is 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside. It is also called acadesine or AICA riboside. PubChem lists it as CID 17513 under CAS 2627-69-2, with the formula C9H14N4O5 and a molecular weight of 258.23 g/mol.

Its phosphorylated form is a separate molecule with its own record. That form is ZMP, also called AICA ribotide, listed as PubChem CID 65110, formula C9H15N4O8P, molecular weight 338.21 g/mol. The difference between the two is exactly one phosphate group.

Naming in this field is inconsistent and causes real confusion. Some papers use AICAR for the nucleoside and ZMP for the phosphate. Others reverse it. Here, AICAR means the nucleoside and ZMP the phosphate.

AICAR AMPK activation: what actually does the activating

This is the central point, and it is often stated loosely. AICAR is a precursor. It does not act on AMP-activated protein kinase itself.

The sequence has three steps. AICAR is carried into cells by nucleoside transporters rather than crossing the membrane freely.^[1] Once inside, adenosine kinase phosphorylates it to ZMP, and does so without the usual feedback restraint, so ZMP accumulates.^[2] ZMP then resembles AMP closely enough to act on AMPK in its place.^[3]

Two experiments carry most of the weight. Blocking nucleoside transport prevents AICAR from activating AMPK in cells.^[1] Blocking adenosine kinase, the enzyme that makes ZMP, also abolishes the effect.^[4] If the nucleoside were doing the work, neither block should matter. ZMP itself is poorly membrane permeable, which is why the nucleoside exists as a delivery form.^[5]

Research-grade AICAR is supplied as the nucleoside, so the conversion step sits inside every mechanistic claim made about it.

One honest gap is worth stating. No published experiment appears to have applied the unphosphorylated nucleoside directly to purified AMPK and reported it inactive. The case rests on the convergent cell-based evidence above rather than on a single decisive cell-free comparison.

Describing AICAR as an AMPK activator is therefore reasonable shorthand for what happens in a cell. It is inaccurate as a description of mechanism.

What ZMP does at the enzyme

ZMP binds the regulatory gamma subunit of AMPK, at the same nucleotide sites AMP occupies. A crystal structure of the human gamma1 subunit domain pair resolved both AMP and ZMP in place.^[6] That work came from a commercial structural biology group.

ZMP is a much weaker activator than AMP. Direct measurement on kinase purified from rat liver found it needed far higher concentrations for the same effect.^[3] The size of the gap depended on how much ATP was present. Review-level sources put it at roughly forty to fifty times weaker.

Whether ZMP reaches the same maximum as AMP is unsettled. The rat liver work reported equal maximal activation.^[3] A later study using purified recombinant human enzyme reported that maximal activity was not reached.^[7] Different enzyme sources and assay formats may explain it. It has not been resolved.

One further caution belongs here. Reviews often say ZMP protects the enzyme from being switched off by dephosphorylation. That claim traces back to work on AMP rather than ZMP, and the original AMP finding was itself later reinterpreted as an artifact of contaminated preparations.^[8] It should not be stated as established for ZMP.

AICAR metabolism beyond AMPK

AICAR is not a clean tool, and the field has said so directly. A 2021 systematic review concluded that many effects previously attributed to AMPK activation are AMPK-independent, and called for caution in interpreting AICAR-based studies.^[9]

Several specific examples are documented:

• In rat liver mitochondria, AICAR inhibited oxidative phosphorylation independently of AMPK.^[10]
• In mouse adipocytes, AICAR reduced insulin-stimulated glucose uptake through a mechanism that required conversion to ZMP but did not involve AMPK.^[11]
• In rat brain slices, AICAR competed with adenosine at the transporter, raising extracellular adenosine and activating adenosine receptors, a confound entirely separate from AMPK.^[1]

The second example matters most for interpretation. Requiring ZMP does not mean acting through AMPK.

AICAR research on energy metabolism in animals

In perfused rat hindlimb muscle, AICAR activated AMPK, inactivated acetyl-CoA carboxylase, and increased fatty acid oxidation and glucose uptake.^[12] This is an isolated limb preparation, not a whole animal.

The most quoted result is a 2008 mouse study. Sedentary mice given AICAR for four weeks ran 44% longer on a treadmill, alongside induction of metabolic genes.^[13] It is a mouse result, in sedentary animals, measured as treadmill endurance. No human trial of endurance or exercise capacity has been published.

Acadesine research in humans

Here the picture changes sharply.

In cardiac surgery, three randomized trials in the 1990s each missed their primary endpoints. A 1997 individual-patient meta-analysis pooling five trials and 4,043 patients then reported reductions in perioperative myocardial infarction and cardiac death.^[14] That pooled result is the source of nearly every positive claim about acadesine in circulation.

It did not hold. RED-CABG, a phase 3 trial across 300 sites in seven countries, was stopped for futility. The primary composite outcome occurred in 5.0% of placebo patients and 5.1% on acadesine, an odds ratio of 1.01.^[15] In hematology, a phase I/II study in relapsed or refractory chronic lymphocytic leukemia produced no conventional responses.^[16]

No regulator has approved acadesine for any indication.

Two human studies measured what the compound actually did to AMPK in skeletal muscle. Both found AMPK phosphorylation and activity unchanged, even where glucose uptake rose.^[17][18] A separate infusion study confirmed that AICAR is taken up and phosphorylated to ZMP in human red blood cells, while finding no effect on glucose disposal.^[19] One human positive stands out: in type 2 diabetes, intravenous AICAR reduced hepatic glucose output.^[18]

Limitations and research directions

The most practically important human finding is pharmacokinetic. In a phase I study that gave both oral and intravenous doses, oral bioavailability was under 5%.^[20] The cardiac surgery trials and the metabolic studies described above all used intravenous infusion.

AICAR falls under section S4 of the World Anti-Doping Agency Prohibited List, covering hormone and metabolic modulators, and is prohibited at all times. Sub-numbering within that section is revised annually and is not cited here.

What the literature does not establish is substantial. There is no human evidence for endurance, fat oxidation, mitochondrial adaptation, or body composition. The two human studies that looked for AMPK activation in muscle did not find it.

Conclusion

AICAR is a nucleoside precursor. Its intracellular phosphate, ZMP, is the species that acts on AMPK. It does so more weakly than AMP, and less selectively than its reputation suggests. Preclinical work in rodents established it as the standard laboratory tool for raising AMPK activity, with striking metabolic and endurance findings. A large and well-conducted human program tested the cardiac hypothesis and failed.

The distinction between what is administered and what acts is the thing to carry into any paper on this compound. AICAR and ZMP are different molecules, and the literature does not always make clear which one it means.

References

1. 1

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   Boß M, Newbatt Y, Gupta S, Collins I, Brüne B, Namgaladze D. AMPK-independent inhibition of human macrophage ER stress response by AICAR. Sci Rep. 2016;6:32111. doi:10.1038/srep32111. PMID 27562249.
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   Hunter RW, Foretz M, Bultot L, Fullerton MD, Deak M, Ross FA, et al. Mechanism of action of compound-13: an alpha1-selective small molecule activator of AMPK. Chem Biol. 2014;21(7):866-879. PMID 25036776.
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   Day P, Sharff A, Parra L, Cleasby A, Williams M, Hörer S, et al. Structure of a CBS-domain pair from the regulatory gamma1 subunit of human AMPK in complex with AMP and ZMP. Acta Crystallogr D Biol Crystallogr. 2007;63(Pt 5):587-596. PMID 17452784.
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   Višnjić D, Lalić H, Dembitz V, Tomić B, Smoljo T. AICAr, a widely used AMPK activator with important AMPK-independent effects: a systematic review. Cells. 2021;10(5):1095. doi:10.3390/cells10051095. PMID 34064363.
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    Merrill GF, Kurth EJ, Hardie DG, Winder WW. AICA riboside increases AMP-activated protein kinase, fatty acid oxidation, and glucose uptake in rat muscle. Am J Physiol. 1997;273(6):E1107-E1112. PMID 9435525.
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    Mangano DT. Effects of acadesine on myocardial infarction, stroke, and death following surgery. A meta-analysis of the 5 international randomized trials. JAMA. 1997;277(4):325-332. PMID 9002496.
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    Newman MF, Ferguson TB, White JA, Ambrosio G, Koglin J, Nussmeier NA, et al. Effect of adenosine-regulating agent acadesine on morbidity and mortality associated with coronary artery bypass grafting: the RED-CABG randomized controlled trial. JAMA. 2012;308(2):157-164. PMID 22782417.
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    Cuthbertson DJ, Babraj JA, Mustard KJ, Towler MC, Green KA, Wackerhage H, et al. 5-aminoimidazole-4-carboxamide 1-beta-D-ribofuranoside acutely stimulates skeletal muscle 2-deoxyglucose uptake in healthy men. Diabetes. 2007;56(8):2078-2084. PMID 17513706.
18. 18

    Boon H, Bosselaar M, Praet SF, Blaak EE, Saris WH, Wagenmakers AJ, et al. Intravenous AICAR administration reduces hepatic glucose output and inhibits whole body lipolysis in type 2 diabetic patients. Diabetologia. 2008;51(10):1893-1900. PMID 18709353.
19. 19

    Bosselaar M, Smits P, van Loon LJ, Tack CJ. Intravenous AICAR during hyperinsulinemia induces systemic hemodynamic changes but has no local metabolic effect. J Clin Pharmacol. 2011;51(10):1449-1458. PMID 21148051.
20. 20

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