Hydrafinil (9-Fluorenol)

Hydrafinil (9-Fluorenol, CRL-40,941)

Hydrafinil (also known as 9-hydroxyfluorene, 9-fluorenol, or by the developmental code CRL-40,941¹) is a white-cream colored solid at room temperature, with the hydroxy group positioned on the bridging carbon between the two benzene rings. It can be readily oxidized to fluorenone.² Chemically, it is a hydroxylated derivative and metabolite of fluorene, a combustion-derived polycyclic aromatic hydrocarbon (PAH) classified as a possible human carcinogen.² The compound was first patented in 1940 for use as an insecticidal agent due to its ability to disrupt the growth and survival of agricultural pests while exhibiting low toxicity to warm-blooded animals.^7,9 Owing to its metabolic and structural relation to fluorene derivatives, hydrafinil has also been investigated as a potential urinary biomarker for estimating fluorene exposure in environmental and occupational health studies.^2,12

Later research identified hydrafinil as a secondary alcohol derivative of fluorene with structural similarity to modafinil and other benzhydryl-sulfinyl compounds investigated for wakefulness-promoting activity.^2,14 Although limited data exist in humans, preclinical pharmacological evaluation suggests mild eugeroic (wake-promoting) properties consistent with weak dopamine reuptake inhibition.^14,15

Chemistry

Hydrafinil (9-hydroxyfluorene; C₁₃H₁₀O) is a secondary alcohol positioned at the bridge carbon (C-9) of the fluorene nucleus, producing a planar tricyclic aromatic structure with no stereocenters and a molecular weight of 182.22 g/mol.¹ It occurs as a cream-white solid at room temperature and is practically insoluble in water while readily soluble in organic solvents such as acetone and mineral oils.² The compound is combustible but only weakly flammable and oxidizes readily to fluorenone upon exposure to air or light.³

Spectral characterization has confirmed the secondary-alcohol functionality and the aromatic C–H stretching vibrations typical of fluorene derivatives. Infrared and NMR spectra show diagnostic absorptions at 3400 cm⁻¹ (O–H) and 3050–750 cm⁻¹ (aromatic C–H), while mass spectrometry gives a molecular ion at m/z 182 and base fragment at m/z 181.^1,7 In analytical toxicology, hydrafinil and its phase II conjugates—namely hydrafinil sulfate and hydrafinil glucuronide—have been detected by HPLC-HRMS and GC-MS/MS in human urine reference materials, displaying reproducible diagnostic ions at m/z 277 and 357 respectively.^7,8

Pharmacology

Pharmacodynamics

Mechanism of Action

The mechanism of action of fluorenol (hydrafinil) is unknown¹³. The lipophilicity of fluorenol (LogP ≈ 2.4) is higher than that of drugs like modafinil (LogP ≈ 1.7) and amphetamine (LogP ≈ 1.8), suggesting that it may penetrate the blood–brain barrier more readily^1,11,12. Although its precise molecular targets have not been confirmed, in vitro data indicate weak inhibition of dopamine reuptake at the dopamine transporter (DAT), producing measurable wake-promoting effects in preclinical studies¹⁴. This limited dopaminergic activity, combined with greater lipophilicity, may influence its pharmacological profile—producing central stimulation without strong sympathomimetic activity. However, the relative contribution of other neurotransmitter systems remains undetermined^13,14.

Eugeroics

A study conducted by Cephalon in the course of developing next-generation wake-promoting agents found that a fluorenol-based analog exhibited 39 % greater wake-promotion in mice over a four-hour period compared to modafinil¹⁴. Subsequent pharmacokinetic analysis determined that the observed eugeroic effect was primarily attributable to fluorenol itself, acting as the active metabolite of the tested analog¹⁴. In vitro assays characterize fluorenol as a weak dopamine reuptake inhibitor with an IC₅₀ of approximately 9 µM—around 59 % weaker than modafinil (IC₅₀ = 3.7 µM)—which may account for its lower stimulant liability despite similar wake-promoting potential^11,15.

Toxicity Profile

Fluorenol exhibits measurable toxicity toward aquatic organisms, including algae, bacteria, and crustaceans⁹. It has been identified as an algaecide against Dunaliella bioculata and related green algae species¹⁰. The compound was first patented in 1939 as an insecticidal agent effective against mosquito larvae, codling moths, and corn borers, while demonstrating comparatively low acute toxicity in warm-blooded animals⁷. Environmental and occupational safety data indicate that fluorenol is classified as hazardous to aquatic life with long-lasting effects, poorly soluble in water, and combustible though only weakly flammable^2,3. In regulatory assessments, it is categorized under GHS “Aquatic Acute 1” and “Aquatic Chronic 1” hazard classes, with precautionary guidance to prevent environmental release and to collect any spillage³.

Safety and Handling

Industrial safety documentation recommends maintaining closed systems, minimizing dust formation, and using protective equipment when working with powdered fluorenol³. Direct contact requires gloves composed of butyl or fluoro-rubber, and respiratory protection should be used if airborne particulates are present³. Work areas should provide adequate ventilation, and spills must be collected without generating dust to prevent environmental contamination. No human toxicity data are available, and mammalian toxicity appears limited to general irritant effects. Collectively, current data suggest that fluorenol presents a moderate environmental hazard but relatively low systemic toxicity in mammals under standard laboratory conditions^2,3,7,9,10.

Pharmacokinetics

Metabolism and Biotransformation

Fluorenol undergoes oxidation in biological and environmental systems to yield fluorenone and related carboxylated metabolites⁸. In mammalian metabolism, it is detected in urine as a hydroxylated derivative of fluorene, supporting its role as a secondary metabolite produced during the biotransformation of polycyclic aromatic hydrocarbons (PAHs)². Oxidative interconversion between fluorenol and fluorenone is readily reversible, reflecting metabolic cycling between alcohol and ketone forms under aerobic conditions^8,9.

Microbial degradation studies with Pasteurella sp. IFA demonstrated partial oxidation of fluoranthene to fluorenone, 9-hydroxyfluorene, and several aromatic acids, confirming that fluorenol can appear as both an intermediate and terminal metabolite within PAH biodegradation pathways⁸. Environmental monitoring confirms that fluorenol (hydrafinil) appears in human urine following fluorene exposure, though its concentration does not correlate with carcinogenic outcomes². These findings indicate that fluorenol participates in reversible redox transformations linking parent fluorene structures to their oxygenated metabolites, functioning primarily as a transient intermediate rather than a persistent bioaccumulative compound^2,8,9.

Excretion Profile

Fluorenol and its oxidative counterpart fluorenone are primarily eliminated through urinary excretion as hydroxylated fluorene metabolites². Analytical toxicology studies have identified 9-hydroxyfluorene in urine following exposure to fluorene and combustion-derived polycyclic aromatic hydrocarbons, confirming renal clearance as the dominant elimination route^2,12. Detection relies on liquid chromatography–mass spectrometry methods capable of distinguishing fluorenol from other PAH-OH metabolites, including 1- and 3-fluorenols².

Human biomonitoring data indicate that urinary concentrations of fluorenol rise transiently after occupational or dietary exposure but return to baseline rapidly, suggesting limited tissue retention and efficient metabolic turnover^2,12. Environmental and microbial studies further support that fluorenol acts as an intermediate in oxidation–reduction cycles rather than a persistent bioaccumulative product⁸.

Research

Animal Studies

A study conducted by Cephalon evaluated a series of fluorene-based analogs of modafinil for wake-promoting activity in rats. One compound in the series produced a 39 % greater increase in wakefulness over a four-hour period compared with modafinil. Pharmacokinetic testing identified fluorenol as the active metabolite responsible for this effect¹⁴.

Earlier toxicological testing described in the 1939 insecticidal patent reported that fluorenol and related fluorene derivatives were lethal to mosquito larvae and corn borers while exhibiting relatively low acute toxicity in warm-blooded animals⁷. Later environmental bioassays found that 9-hydroxyfluorene showed moderate toxicity toward algae, bacteria, and crustaceans, but was significantly less toxic than the parent polycyclic hydrocarbon fluoranthene^9,10.

Human Studies

The first verified human administration study of hydrafinil was conducted in 2021 by Knoop et al., in which three healthy male volunteers each received a single 50 mg oral dose of hydrafinil.² Urine samples collected up to 72 hours after administration confirmed the presence of hydrafinil and its metabolites using high-performance liquid chromatography and gas-chromatography high-resolution mass spectrometry.² Detectable urinary concentrations appeared within 1.5–2 hours post-dose, reached peak levels between 2–4 hours (approximately 15–80 µg/mL), and persisted up to 72 hours.² No adverse reactions were reported among participants.² Hydrafinil and its metabolites were later detected in two out-of-competition doping-control samples, demonstrating that standard analytical protocols are capable of identifying the compound under routine screening conditions.² The study established the first characterized human metabolic profile of hydrafinil, confirming its detectability, elimination kinetics, and relevance to anti-doping monitoring.²

Human Biomonitoring

In human biomonitoring and environmental toxicology, hydrafinil (9-hydroxyfluorene) serves mainly as a urinary biomarker of fluorene exposure rather than a pharmacologically active compound. Its measurement in population and occupational studies provides insight into polycyclic aromatic hydrocarbon metabolism and exposure-related effects.^2,3,15 The following studies highlight how 9-hydroxyfluorene has been applied across diverse human cohorts to examine correlations between urinary levels, physiological outcomes, and exposure sources.^3–5,15,16

Yang et al. (2025) conducted a population-based analysis using NHANES data to examine the relationship between urinary polycyclic aromatic hydrocarbon metabolites and sleep-related disorders in adults.³ In this study, 9-hydroxyfluorene, the primary metabolite of hydrafinil, was among nine urinary biomarkers quantified by gas chromatography–tandem mass spectrometry.³ Elevated urinary 9-hydroxyfluorene concentrations were significantly associated with delayed sleep onset, obstructive sleep apnea, and daytime sleepiness, with odds ratios of 1.61, 1.83, and 1.72 respectively.³ The analysis further indicated that systemic inflammation, reflected by increased white blood cell counts, partially mediated these relationships, suggesting that inflammatory processes contribute to the neurobehavioral effects associated with 9-hydroxyfluorene exposure.³

Nsonwu-Anyanwu et al. (2025) measured urinary 9-hydroxyfluorene in seventy adults in Calabar, Nigeria, to assess the influence of dietary exposure and hydration on polycyclic aromatic hydrocarbon metabolite levels.⁴ Mean urinary concentrations of 9-hydroxyfluorene were 1.46 ± 1.82 µg/g creatinine, with higher excretion observed in individuals consuming more than one liter of water daily compared to those with lower intake (p = 0.031).⁴ The findings suggested that hydration status influences the urinary elimination of 9-hydroxyfluorene, indicating that excretion kinetics may be affected by fluid intake rather than solely by environmental exposure.⁴

Zhen et al. (2025) performed a case–control study among 642 pregnant women in Hefei, China, to examine associations between urinary hydroxylated polycyclic aromatic hydrocarbons and prenatal anxiety symptoms.⁵ In this cohort, 9-hydroxyfluorene exhibited the strongest positive association with prenatal anxiety, with women in the highest tertile of exposure showing an adjusted odds ratio of 2.084 (95 % CI: 1.358–3.199).⁵ Bayesian kernel mixture regression confirmed that 9-hydroxyfluorene contributed most strongly to overall mixture effects, identifying it as a key exposure biomarker linked to gestational anxiety risk.⁵

Louro et al. (2022) reviewed occupational biomonitoring across Europe and identified 9-hydroxyfluorene as a validated biomarker of fluorene metabolism in workers exposed to polycyclic aromatic hydrocarbons.¹⁵ The review emphasized its consistent detection in urine across industries, supporting its use in exposure surveillance and confirming its analytical reliability as a human biomonitoring marker.¹⁵

Liu et al. (2025) examined differential metabolic profiles among male Tucson firefighters following fireground exposure and quantified urinary 2-, 3-, and 9-fluorenol to assess post-exposure changes.¹⁶ Although no significant elevation in 9-hydroxyfluorene levels was observed after exposure, the study reaffirmed its role as a biomarker for fluorene metabolism and exposure verification rather than as a pharmacologically active compound.¹⁶

Collectively, these studies demonstrate that hydrafinil’s principal metabolite, 9-hydroxyfluorene, occurs measurably in human urine, can be influenced by environmental, physiological, and occupational factors, and is epidemiologically linked to inflammatory and neurobehavioral endpoints in exposed populations.^3–5,15,16

History

Hydrafinil, chemically known as 9-fluorenol or 9-hydroxyfluorene, was first described and patented in the late 1930s for non-pharmaceutical purposes.⁷ Hydrafinil was first described in the late 1930s for non-pharmaceutical purposes. This detailed the use of fluorene derivatives—including 9-fluorenol—as insecticidal agents intended to replace toxic arsenical pesticides.⁷

Following its initial development as a pesticide, 9-fluorenol remained primarily a chemical intermediate and research substance for several decades.⁹ By the early 2000s, 9-fluorenol and related metabolites were recognized as stable environmental residues and began to appear in human biological monitoring data as indicators of exposure to polycyclic aromatic hydrocarbons.^15,16 These applications established its secondary role as a biomarker of environmental and occupational exposure.^15,16

Pharmacological interest in hydrafinil reemerged in the early 2010s when researchers at Cephalon explored fluorenol analogues in their search for next-generation eugeroic agents related to modafinil.¹⁴ Preclinical animal studies demonstrated that fluorenol derivatives could produce wake-promoting effects exceeding those of modafinil, suggesting hydrafinil as the likely active metabolite responsible for the observed activity.¹⁴ However, the drug candidate was not further developed, and preclinical and clinical trials were not pursued.²

In the years that followed, hydrafinil resurfaced in online markets as an unregulated research chemical promoted for cognitive enhancement.^2,19

Legal Status

Controlled Substance Scheduling

Hydrafinil (9-hydroxyfluorene) is not listed in the Controlled Substances Act of the United States, the Controlled Drugs and Substances Act of Canada, the United Nations Convention on Psychotropic Substances applied in the European Union, or the Australian Poisons Standard.^17,20–23 None of these jurisdictions classify hydrafinil as a narcotic, psychotropic, or scheduled substance.^17,20–23

Approval for Human or Medical Use

Hydrafinil has not been approved for human consumption or therapeutic use by the U.S. Food and Drug Administration, the European Medicines Agency, the Therapeutic Goods Administration, or Health Canada.^24–27 The compound does not appear in any national databases of authorized medicines or active pharmaceutical ingredients maintained by these agencies.^24–27

Society and Culture

Regulation and Sports Doping

Hydrafinil has not been approved as a therapeutic agent in any country, but it has attracted attention within anti-doping and analytical chemistry contexts.^2,28 In 2021, its urinary metabolites were characterized to enable routine detection under World Anti-Doping Agency protocols, demonstrating that hydrafinil can be identified in athlete samples using existing screening methods.² Although hydrafinil itself is not named on the WADA Prohibited List, the agency includes it implicitly under the “stimulants not otherwise specified” category.²⁸ Its detection capability and pharmacological similarity to modafinil position hydrafinil within the broader class of non-approved eugeroic stimulants monitored in sports testing.^2,28

Commercial Availability

Hydrafinil is marketed online as an unscheduled “research chemical” and sold by a limited number of nootropic vendors without medical authorization or regulatory oversight.^1,19 Vendors typically advertise the compound for cognitive or wakefulness-promoting purposes, despite the absence of clinical safety or efficacy data.^1,19 Its availability varies across jurisdictions but remains accessible through international e-commerce sources labeling it as a chemical reagent rather than a dietary supplement.^1,19

Non-Medical and Personal Use

Survey data on cognitive-enhancement drug use indicate limited awareness and minimal reported use of hydrafinil among consumers.¹⁸ In a multinational survey of modafinil users, only about two percent of respondents reported having heard of or tried hydrafinil, underscoring its marginal role relative to approved eugeroics.¹⁸ Informal reports describe subjective wake-promoting or concentration-enhancing effects, but such accounts remain anecdotal and unverified by clinical research.^1,18,19