AICAR is not a peptide but a small molecule — a nucleoside, chemically related to the building blocks of DNA. It became famous in 2008, when sedentary mice given it for four weeks ran 44% longer on a treadmill than animals without the drug. The press called it “exercise in a pill”, and the World Anti-Doping Agency added it to the Prohibited List as early as 2009. Far less is said about the fact that AICAR — under the drug name acadesine — has been tested in more than seven thousand people, albeit in cardiac surgery, and that those trials produced contradictory and ultimately null results. This review covers both halves of the story.
What AICAR is
Its full name is 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside; the medical literature also calls it AICA-riboside, and it was investigated as a drug under the name acadesine. The most important thing to grasp: this is a molecule the human body makes for itself. AICAR is a natural intermediate in the pathway by which cells build purines, components of genetic material. That is why it can be measured in the urine of any healthy person: in a study of 499 athletes the mean concentration was about 2,186 nanograms per millilitre. This natural presence is now the central problem for anti-doping control, because detecting the compound proves nothing on its own.
How it works — AMPK as a fuel gauge
Inside the cell, AICAR is converted into a molecule called ZMP. Its shape resembles AMP, the signal a cell sends when it is running out of energy. Thanks to that resemblance, ZMP fools AMPK, the protein “fuel gauge”. AMPK normally switches on during exercise or fasting and flips the cell from storing reserves to burning them: more glucose uptake, more fat oxidation, less synthesis. AICAR is meant to press that switch without the exercise, and that is where all the hope invested in the compound comes from.
The catch is that ZMP is not AMP. It mimics only part of its action and needs high concentrations inside the cell to do anything at all. That limitation runs through every human study.
What was studied — animals and the exercise-mimetic fame
The 2008 paper in Cell tested two compounds: an agonist of the PPAR-delta receptor and AICAR. The first worked only in combination with training. The second worked on its own: four weeks of AICAR in sedentary mice switched on genes of oxidative metabolism and extended treadmill running by 44%. The authors wrote plainly that the AMPK-PPAR-delta pathway could be targeted with a drug to increase endurance without exercise. The paper appeared a few weeks before the Beijing Olympics, and it set off everything that followed, both scientific and anti-doping.
Human data — muscle
This is where the trouble starts. In a 2007 study of 29 healthy men, an AICAR infusion did increase the uptake of a glucose tracer by muscle — 2.1-fold. For comparison, cycling increased it 4.7-fold, more than twice as strongly. More importantly, the activity of AMPK itself in muscle did not rise either after 20 minutes or after 3 hours of infusion, whereas after cycling it rose clearly. Whole-body glucose disposal improved by only 7%. In other words: the compound was doing something in muscle, but apparently not through the very mechanism it was given for.
Four years later another group repeated the measurement under raised insulin in ten healthy people and found no effect at all — neither on forearm glucose uptake nor on the whole body. What it saw instead was a systemic action: heart rate rose on average from 58 to 70 beats per minute (against 60 to 63 on placebo) and blood pressure fell. The studies therefore contradict each other and cannot be averaged: one shows a moderate muscle effect without AMPK activation, the other no effect and a clear circulatory response.
Human data — liver
The one reproducible metabolic effect in humans concerns not muscle but liver. In a study of ten men with type 2 diabetes, a continuous intravenous infusion of AICAR reduced glucose output from the liver, so blood sugar fell faster than in the control condition. It also reduced the release of free fatty acids. And here the same observation returns: AMPK phosphorylation in skeletal muscle did not rise, although one of its downstream markers did. It appears, then, that intravenous AICAR acts mainly on the organ the bloodstream reaches most easily, rather than on the muscle that interests athletes.
Large clinical trials — two opposite results
The largest body of human data on AICAR comes from cardiac surgery, where acadesine was given in the hope of protecting the heart from oxygen shortage during bypass grafting. In 1997 a meta-analysis of five trials in 4,043 patients found that the drug cut the risk of perioperative heart attack by 27%, of cardiac death within the first four days by 50%, and of the combined outcome by 26%. It looked like a breakthrough.
The check came 15 years later. The RED-CABG trial — randomised, double-blind, across 300 sites in 7 countries — was stopped for futility after 3,080 of the planned 7,500 patients had been randomised. The primary outcome occurred in 5.0% of people on placebo and 5.1% on acadesine (odds ratio 1.01). There was no difference in any key secondary endpoint. This is precisely the case in which a large, well-designed trial erases an earlier signal from a meta-analysis — and after which development of the drug in that indication faded out.
Absorption and fate in the body
A 1991 pharmacokinetic study in healthy men answers two practical questions. First, oral bioavailability was below 5%: taken by mouth, the compound barely reaches the bloodstream at all. Second, after intravenous dosing it disappears very quickly (half-life 1.4 hours), and it is taken up almost instantly by red blood cells and converted there into ZMP. The same uptake by red cells was confirmed in the 2011 study. That means a substantial share of any dose is stranded in blood cells before it reaches muscle, which is the simplest explanation for why AMPK activity in human muscle failed to rise.
Doping and legal status
After the 2008 paper, the World Anti-Doping Agency added AICAR (together with GW501516) to the Prohibited List in 2009. The reasons were both the assumed performance-enhancing properties and seizures of illicitly distributed preparations containing AICAR. Testing is difficult precisely because the compound arises naturally in the body: laboratories had to develop methods that distinguish a person's own AICAR from an administered one, among them measurement of carbon isotope ratios. For an athlete subject to anti-doping control this is a concrete disqualification risk.
Safety and the limits of the evidence
The safety profile of acadesine under short, controlled hospital administration is better documented than for most compounds in this category, covering thousands of patients. In the 1997 meta-analysis the rate of adverse events was similar to placebo, with one exception: a transient rise in serum uric acid. That is expected, since AICAR feeds into the purine pathway whose end product is uric acid. The 2011 study also recorded a faster heart rate and a fall in blood pressure. There are, however, no data on long-term dosing, on giving it to healthy people outside a trial, or on giving it to improve performance. AICAR is not an approved medicine for any indication. We deliberately give no methods of use and no doses.
The wider context — does “exercise in a pill” exist
The history of AICAR is a textbook case of a rodent result falling apart on contact with humans. It worked in mice injected for four weeks; in people it failed to raise muscle AMPK in any of the three studies that measured it. The same question returns with other “exercise mimetics” — MOTS-c, which also acts through AMPK, or 5-Amino-1MQ, whose data likewise come only from rodents. The shared lesson: demonstrating a mechanism in a cell is the start of the road, not the end of it.
Summary
AICAR is a naturally occurring small molecule that, inside the cell, imitates the signal of energy shortage and thereby stimulates the AMPK enzyme. In mice this produced a spectacular rise in endurance without training. In humans the picture differs: it is barely absorbed by mouth, it is captured by red blood cells after intravenous dosing, it does not raise AMPK activity in muscle, and the largest clinical trial in cardiac surgery was stopped for lack of efficacy. It remains an interesting laboratory tool and a substance banned in sport. Evidence that it improves human performance does not exist.
Sources
- Newman MF, Ferguson TB, White JA, et al.; RED-CABG Steering Committee and Investigators. 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. DOI: 10.1001/jama.2012.7633. pubmed.ncbi.nlm.nih.gov/22782417
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For in-vitro laboratory research only. It is not a human medicine and is not for treatment.