Acridone: Biological Effects and Practical Uses

Acridone is a tricyclic organic compound whose core scaffold shows up in an unusually wide range of biological activities, from killing malaria parasites to blocking viral replication and slowing cancer cell growth. Found naturally in citrus-family plants, acridone and its many synthetic derivatives have become a favorite starting point for medicinal chemists looking for new drugs. Beyond medicine, the compound’s strong fluorescence has made it useful in environmental sensing and green chemistry. What makes acridone interesting is less any single blockbuster application and more the sheer breadth of things it can do when its structure is tweaked in the right ways.

A Scaffold From Citrus Plants

Acridone alkaloids occur naturally in the Rutaceae family, the botanical group that includes oranges, lemons, grapefruits, and their wild relatives. These plants produce acridone-based compounds as part of their chemical defense toolkit. The first naturally occurring azaacridone alkaloid, dubbed azacridone-A, was isolated from the roots of Marsh grapefruit in the early 1990s.¹ Since then, researchers have pulled acridone alkaloids from an assortment of Rutaceae species across continents. Two prenylated acridone alkaloids were discovered in the stems of Balsamocitrus paniculata, a Cameroonian medicinal plant, alongside eighteen other known compounds.² Other studies have focused on Swinglea glutinosa, another Rutaceae member, from which five acridone alkaloids were isolated and tested for their effects on photosynthesis, motivated by the search for natural herbicide models.³

The fact that these plants evolved acridone chemistry as a defense mechanism hints at why the scaffold is so biologically active. Plants do not invest metabolic energy in making complex alkaloids unless those molecules help them survive, whether by repelling insects, inhibiting competing plants, or fighting off microbial invaders. That ecological role is one reason medicinal chemists keep returning to acridone as a starting structure for drug design.

Fighting Malaria on Multiple Fronts

Antimalarial research is arguably where acridone derivatives have made the strongest case for clinical relevance. Malaria parasites have a complicated life cycle that moves through the human bloodstream, the liver, and the mosquito gut, and most existing drugs only target one of those stages. Acridone-based compounds are unusual in that some hit all three.

A large-scale structural optimization of novel acridones produced lead compounds with picomolar-level activity against drug-resistant blood-stage Plasmodium falciparum in cell cultures. In mice, these compounds cured blood-stage malaria after oral dosing, prevented liver-stage infection from developing, and blocked the parasite from establishing in mosquitoes.⁴ That kind of multistage activity is rare and would be valuable for programs aiming to eradicate malaria rather than just treat symptomatic infections.

More recent work has identified a lead compound called T111 that takes this multistage profile even further. T111 achieved oral single-dose cure in a rodent blood-stage malaria model, killed sexual-stage parasites that would otherwise transmit malaria to mosquitoes, showed activity against relapsing liver parasites in primate cells, and worked synergistically with tafenoquine, an existing antimalarial drug. Analysis of parasites that developed resistance to T111 pointed to inhibition of the mitochondrial electron transport chain through a mechanism distinct from other antimalarials currently in use or in clinical trials.⁵ A novel mechanism of action matters because drug resistance is the central problem in malaria control, and a compound that kills parasites in a completely new way would not be undercut by resistance to older drugs.

Acridones also show promise as chemosensitizers, compounds that restore sensitivity to drugs that resistant parasites have learned to pump out of their cells. Certain acridone derivatives reversed chloroquine resistance in multidrug-resistant P. falciparum strains while only producing additive effects in strains that were already chloroquine-sensitive. They also enhanced the effectiveness of other quinoline antimalarials, behaving similarly to verapamil, a known resistance reverser.⁶ The implication is that even if acridone derivatives never become standalone antimalarials, they could extend the useful life of existing drugs by making resistant parasites vulnerable again.

Broad Antiviral Activity

Acridone derivatives show activity against a surprisingly wide range of viruses, including both DNA and RNA types. The list spans herpes simplex virus, cytomegalovirus, adenovirus, hepatitis C, dengue, and Junin virus, among others.⁷ That breadth itself is a clue about how these compounds work. When a molecule is active against many unrelated viruses, the target is likely something inside the host cell that all viruses need, rather than a virus-specific protein. The predominant mechanism appears to involve interference with nucleic acid synthesis, and some acridone compounds can slip into double-stranded RNA or DNA, disrupting the replication process.

Dengue virus provides a concrete example. An N-allyl acridone compound inhibited infection by all four dengue serotypes in cell cultures, with the drug blocking viral RNA synthesis without interfering with the virus’s ability to enter cells.⁸ Hepatitis C virus has been another target. An acridone called Fac4 achieved over 90% inhibition of HCV replication at a low concentration with no effect on cell survival. It also inhibited viral release from infected cells by roughly 80%, though it had no effect on the virus getting into cells in the first place and did not directly shut down the HCV polymerase enzyme.⁹ One limitation noted was that Fac4 did not work against genotype 3 HCV, a reminder that antiviral activity can be genotype-specific.

The possibility of a compound hitting multiple viral targets at once is attractive because it could make drug resistance harder for viruses to develop. A virus would need to simultaneously evolve around multiple disrupted pathways, which is a much taller order than evading a drug that targets a single viral protein.⁷

Antibacterial and Antifungal Potential

The anti-infective utility of acridone derivatives extends beyond parasites and viruses. Recent work on a series of synthetic acridone compounds tested their activity against common bacterial pathogens. One compound produced inhibitory zones against Pseudomonas aeruginosa that were larger than those of gentamicin, a standard antibiotic, and matched or exceeded gentamicin against Escherichia coli as well.¹⁰ These are early-stage findings and a long way from clinical use, but outperforming a workhorse antibiotic in a lab assay is enough to justify continued development. Pseudomonas aeruginosa in particular is notoriously resistant to many drugs, and new chemical classes with activity against it are always in demand.

Cancer Research and Drug Resistance

Acridone derivatives have been studied as anticancer agents since at least the early 1990s, when synthetic versions of acridone alkaloids like glyfoline were tested against human leukemia cells. Those early structure-activity studies showed that small changes to the acridone ring system, such as switching between a hydroxyl group and a methoxy group at one position, could mean the difference between strong toxicity to cancer cells and none at all.¹¹

More recent work has focused on making acridone derivatives that inhibit topoisomerase II, an enzyme cancer cells rely on to untangle their DNA during division. One optimized compound functioned as a strong inhibitor of both subtypes of topoisomerase II, caused DNA damage in cancer cells, and triggered programmed cell death by disrupting the energy-producing membranes inside mitochondria.¹² Additional studies have mapped how the three-dimensional shape and orientation of substituents on the acridone ring affect anticancer potency, identifying specific structural features that guide the design of next-generation candidates.¹³

A separate and somewhat distinct angle is using acridones to overcome multidrug resistance, the ability of some cancer cells to pump chemotherapy drugs back out before they can work. Computational modeling of 2,4-dimethylacridones identified their potential to modulate P-glycoprotein, the cellular pump most often responsible for multidrug resistance.¹⁴ Pharmacophore modeling of chemosensitizing acridones active against doxorubicin-resistant leukemia cells identified the structural recipe needed for this resistance-reversing activity.¹⁵ As with the antimalarial chemosensitizing work, the idea is that an acridone compound could be paired with an existing cancer drug to make resistant tumors respond again.

Anti-Inflammatory Effects and Liver Protection

Chronic inflammation drives many diseases, and several acridone derivatives have shown the ability to dial down key inflammatory pathways. Isatinimino acridinedione compounds, tested in animal models, suppressed the inflammatory signaling proteins NF-κB, COX-2, and iNOS, along with downstream mediators like prostaglandin E2, nitric oxide, TNF-alpha, and several interleukins.¹⁶ Shutting down multiple nodes in the inflammatory cascade simultaneously could be an advantage over drugs that target only one mediator, though it also raises questions about off-target effects that would need to be sorted out in safety studies.

Liver disease is another area where acridone chemistry has surfaced. An acridone derivative designated A22 was studied for its effects on nonalcoholic fatty liver disease, a condition affecting hundreds of millions of people worldwide. A22 treatment reduced liver cell death by boosting expression of BCL-2, a survival protein, while lowering levels of the enzyme that executes cell death. It also reduced inflammatory markers in liver tissue.¹⁷ The mechanism here was unusual: A22 appeared to work by stabilizing a specific DNA structure in the promoter region of the BCL-2 gene, essentially turning up the gene’s activity by interacting directly with the DNA architecture rather than with a protein target.

Skin Disease Applications

Psoriasis, a chronic skin condition driven by excessive keratinocyte growth, has also been a target for acridone research. A series of hydroxy-acridone derivatives were tested for their ability to slow down the runaway proliferation of human keratinocytes. The most potent compound in the series, bearing an N-methyl group and a specific dihydroxy arrangement, inhibited keratinocyte growth at concentrations comparable to anthralin, a standard topical psoriasis treatment.¹⁸ Anthralin works but irritates the skin and stains clothing, so a compound with similar potency but a cleaner side-effect profile would be a genuine improvement. Whether any acridone derivative can deliver on that promise remains to be seen.

Fluorescent Sensors for Metal Detection

Outside of medicine, acridone’s strong fluorescence makes it a workhorse in analytical chemistry. The acridone core emits light efficiently, and attaching different chemical groups to it can make the fluorescence switch on or off in the presence of specific metal ions, turning the molecule into a sensor. An acridone-based N-acylhydrazone compound was developed as a selective fluorescent sensor for copper ions in water, with the copper quenching the compound’s fluorescence in a way that could be measured quantitatively.¹⁹ A related compound, 9-acridone-4-carboxylic acid, was established as an efficient chromium(III) sensor, enabling trace-level detection and speciation of chromium in mixed water samples.²⁰

These sensors address real problems. Copper contamination in water is an environmental concern near mining sites and in aging plumbing systems. Chromium speciation matters because chromium(III) is relatively benign while chromium(VI) is carcinogenic, and distinguishing the two is essential for water safety assessments. The acridone scaffold’s tunability, the ability to modify its structure so it recognizes one metal over another, makes it a versatile platform for developing cheap, field-deployable detection tools. Beyond metals, acridone derivatives are being explored for broader optical applications including organic light-emitting diodes, leveraging their high fluorescence quantum yield and tunable electronic structures.²¹

Green Photocatalysis

Synthetic chemistry is increasingly looking for ways to drive reactions with light instead of heat or harsh reagents, and acridone has found a role here too. N-substituted acridones work as visible-light photosensitizers for organic reactions, meaning they absorb light and transfer that energy to kick-start chemical transformations that would otherwise require expensive or toxic catalysts. These acridone photosensitizers proved highly efficient for a class of carbon-carbon bond-forming reactions, are stable, cheap to make, and dissolve in water, giving them appeal for greener industrial chemistry.²²

Electron-deficient acridones take this further, acting as powerful photooxidants through an unexpected mechanism. Unlike related compounds that work through radical cation intermediates, these electron-poor acridones operate in a closed-shell, neutral state, following a diffusion-controlled pathway that was not predicted beforehand.²³ This discovery broadens the toolkit available for photoelectrochemistry and suggests that acridone-based catalysts might access reaction pathways that other photocatalysts cannot.

Acridone as an Environmental Contaminant

For all its useful properties, acridone has a less welcome presence in the environment. Carbamazepine, a widely prescribed antiepileptic drug, breaks down under UV light in wastewater and natural water bodies. One of its major degradation products is acridone. Studies have confirmed that as carbamazepine degrades during UV water treatment, acridone concentrations rise, and the mixture of transformation products is more toxic to aquatic organisms than carbamazepine itself. Even after extended UV treatment, ecotoxicity remained high, with more than 60% inhibition of test populations relative to controls.²⁴

Acridone does not simply stay in the water column. Monitoring studies using outdoor aquatic mesocosms have detected acridone in fish tissue, demonstrating that the compound can bioaccumulate through the food web.²⁵ This is a notable gap in current environmental risk assessments, which tend to evaluate the toxicity of pharmaceutical parent compounds while ignoring the transformation products that actually persist in treated water. Acridone’s case illustrates why those assessments need updating.

Metabolism and the Challenge of Turning a Scaffold Into a Drug

Having biological activity in a test tube and becoming a useful drug are very different things. One of the persistent challenges with acridone-based drug candidates is understanding how the body processes them. Research on unsymmetrical bisacridines, a class of anticancer acridone compounds, has revealed that metabolism can go in strikingly different directions depending on the specific compound. One candidate, C-2028, was conjugated with glutathione inside cells and retained there, meaning its metabolite stayed put and may have contributed to killing the cancer cell. Another candidate, C-2045, was processed through glucuronidation and exported from the cell, following a conventional detoxification and elimination path.²⁶ Those compound-specific differences show that small structural changes in the acridone scaffold can profoundly alter not just potency but also how long the drug lingers in the right place and whether it causes toxicity.

Computational screening of tetracyclic acridone derivatives has offered some encouragement on the safety front. Predicted ADMET properties for one series suggested good oral absorption, strong blood-brain barrier penetration potential, and robust plasma protein binding. Toxicity predictions for the series came back negative, indicating the compounds would likely be tolerable.²⁷ These are computational predictions, not clinical data, so they carry all the usual caveats about in silico models. Still, the blood-brain barrier penetration finding is worth noting: it raises the possibility that some acridone derivatives could be developed for central nervous system diseases, a direction that review literature on Alzheimer’s disease has started to explore using acridone as one of several promising nitrogen-containing scaffolds.²⁸

The synthesis side has also matured. Where early acridone chemistry relied on harsh conditions, newer methods use metal-catalyzed strategies to build acridone structures efficiently and with more control over where substituents land on the ring system.²⁹ The practical result is that medicinal chemists can now make a wider variety of acridone derivatives more easily, testing more structural ideas faster. That synthetic accessibility, combined with the scaffold’s broad biological activity, is why acridone continues to show up across so many different areas of drug discovery and chemical technology.