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`Annu. Rev. Pharmacol. Toxicol. 2001. 41:443–70
`Copyright c(cid:176) 2001 by Annual Reviews. All rights reserved
`
`METABOLISM OF FLUORINE-CONTAINING DRUGS
`
`B Kevin Park, Neil R Kitteringham, and Paul M O’Neill
`Department of Pharmacology and Therapeutics, University of Liverpool,
`New Medical Building, Liverpool, United Kingdom; e-mail: bkpark@liverpool.ac.uk,
`neilk@liverpool.ac.uk, p.m.oneill01@liv.ac.uk
`
`Key Words drug metabolism, drug toxicity, drug design, drug safety,
`defluorination, metabolic stability
`n Abstract This article reviews current knowledge of the metabolism of drugs that
`contain fluorine. The strategic value of fluorine substitution in drug design is discussed
`in terms of chemical structure and basic concepts in drug metabolism and drug toxicity.
`
`INTRODUCTION
`
`Fluorine substitution can alter the chemical properties, disposition, and biological
`activity of drugs (1). Many fluorinated compounds are currently widely used in
`the treatment of disease. These include antidepressants, antiinflammatory agents,
`antimalarial drugs, antipsychotics, antiviral agents, steroids, and general anaesthet-
`ics (2). The chemistry and medicinal chemistry of fluoro-organic compounds and
`drugs have been reviewed (1, 3–5). The development of new fluorinating agents
`has vastly increased the potential for synthesis of novel fluorinated drugs. In ad-
`dition, the development of sophisticated noninvasive analytical techniques based
`on fluorine nuclear magnetic resonance (NMR) and positron emission topography
`has transformed the study of fluorinated drugs in man and animals (6–8).
`The inclusion of a fluorine atom in a drug molecule can influence both the
`disposition of the drug and the interaction of the drug with its pharmacological
`target (Figure 1). For example, the effects of fluorine substitution on the inter-
`and intramolecular forces that affect binding of ligands, and thus introduce re-
`ceptor subtype selectivity, at cholinergic and adrenergic receptors are now well
`understood (9–11). Fluorine substitution can also have a profound effect on drug
`disposition, in terms of distribution, drug clearance, route(s), and extent of drug
`metabolism (12). Such changes can be used constructively by medicinal chemists
`to improve both the safety and the efficacy of a drug. Therefore, the purpose of
`this review is twofold. First, to outline the chemical basis of changes in drug dis-
`position that can be achieved by the introduction of fluorine. Second, to consider
`the pharmacological and toxicological implications of such changes with respect
`to drug response.
`
`0362-1642/01/0421-0443$14.00
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`Figure 1 Flow diagram illustrating the
`effect of fluorine substitution on drug re-
`sponse.
`
`PHYSICOCHEMICAL PROPERTIES
`OF FLUORINATED DRUGS
`
`The replacement of a hydrogen atom or hydroxyl group by a fluorine atom is a
`strategy widely used in drug development to alter biological function. Although it
`is generally thought that fluorine for hydrogen substitution causes minimal steric
`effects at receptor sites, the actual van der Waals radius of fluorine (1.47 ˚A) lies
`between that of oxygen (1.57 ˚A) and hydrogen (1.2 ˚A) (Table 1). Despite the fact
`that fluorine has a greater size than hydrogen, several studies have demonstrated
`that it is a reasonable hydrogen mimic and exerts only a minor steric demand at
`receptor sites, at least for monofunctional analogues (3).
`In contrast to their slight differences in size, hydrogen and fluorine have quite
`different electronic properties. Fluorine is the most electronegative element in the
`periodic table (Table 1). The resulting change in the electron distribution in a
`molecule, following the replacement of a hydrogen atom for fluorine, can alter
`the pKa, the dipole moments, and even the chemical reactivity and stability of
`neighboring functional groups. The magnitude of the change in these electronic
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`TABLE 1 Physiochemical properties of the carbon-fluorine bond
`
`Element
`
`H
`F
`O(OH)
`
`Electro-
`negativity
`
`Bond length Van der Waals Bond energy
`radius ( ˚A)
`(CH2X, ˚A)
`(kcal/mol)
`
`2.1
`4.0
`3.5
`
`1.09
`1.39
`1.43
`
`1.20
`1.35
`1.40
`
`99
`116
`85
`
`properties is often determined by the bonding between the fluorine atom and the
`functional group. Thus, the presence of a fluorine atom ortho to a phenolic group
`is associated with a reduced pKa of 1.2, whereas meta and para fluoro substitutions
`have much less effect. The incorporation of two fluorine atoms at the 2- and 6-
`positions of phenol leads to a reduction of pKa of 2.7 U. Based on this effect,
`the 2,6-difluorophenol group has been used as an isostere of a carboxylic acid
`in a series of GABA aminotransferase inhibitors (13). These compounds were
`shown to inhibit the aminotransferase enzyme demonstrating the potential of this
`bioisosteric replacement.
`The presence of a single fluorine, adjacent to a carboxylic acid function in
`aliphatic systems, can also have pronounced effect on the pKa. This fact was
`used to rationalize the decrease in toxicity of new monofluorinated analogues
`of methotrexate. The incorporation of a fluorine atom adjacent to the glutamic
`carboxyl acid function results in an increase in the acidity. As a result, these
`new analogues are less toxic than methotrexate because they do not form polyg-
`lutamates, metabolites associated with undesirable prolonged cellular retention
`(14).
`Fluorine forms a strong bond with carbon (bond energy C-F D 116 kcal/mol),
`which has an increased oxidative and thermal stability compared with the carbon-
`hydrogen bond (C-H D 99 kcal/mol). The carbon-fluorine bond is one of the
`strongest known in organic chemistry. In addition to the formation of covalent
`bonds, a fluorine atom present in a molecule can also form reversible, electrostatic
`bonds with certain functional groups.
`The isosteric replacement of the hydroxyl group is a commonly used strategy in
`medicinal chemistry. This substitution is usually based on the premise that the fluo-
`rine can hydrogen bond accept in a manner similar to the oxygen of a hydroxyl func-
`tion. However, the higher electronegativity and lower polarizability of fluorine over
`oxygen has a major influence on the ability of fluorine to mimic a hydroxyl group
`(15, 16). Recent calculations have measured the strength of an optimum F...H bond
`¡1 in an adduct between fluoromethane and water. There-
`(1.9 ˚A) to be 2.38 kcal mol
`fore, the F...H bond is clearly much weaker than the corresponding O...H (conserva-
`¡1). The carbon-fluorine bond also has a strong
`tively estimated to be ca 5 kcal mol
`dipole, and this may interact, either positively or negatively, with other dipoles. For
`example, it is thought that in fluorinated derivatives of noradrenaline, interactions
`between a ring carbon-fluorine bond and the hydroxyl group on the beta-carbon
`in the side chain determine the conformation of the molecule, and hence, the
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`position of the fluorine atom in the aromatic ring can determine receptor selectivity
`(17). Fluorine also has a number of specific stereoelectronic effects, such as
`the fluorine anomeric effect in carbohydrates, the Anh-Eisenstein stabilization
`effect in lipase-mediated kinetic resolutions, and the cis effect in difluorinated
`alkanes (3).
`In contrast to the single replacement of a hydrogen for fluorine, replacement
`of a methylene function with a difluoromethylene function (CF2 for CH2) can
`have a significant effect on both conformation and physical properties (3). The
`difluoromethylene moiety has in fact been used as an electronic mimic of labile
`2¡
`2¡
`). This functional
`oxygen atoms in phosphate esters (R-CF2-PO3
`vs R-OPO3
`group has found extensive use in the design of inhibitors of enzymes that hydrolyze
`or bind phosphate esters (18). The CF2 has been proposed as a reasonable isosteric
`and isopolar replacement for the hydroxyl group because of their size, electron
`distribution, and ability to act as a hydrogen bond acceptor (19–21). The CF2H
`group is particularly favored because of its ability to act as a hydrogen donor (22),
`potentially allowing interaction with solvent and biological molecules. Further
`introduction of fluorine causes even greater steric restrictions. The frequently used
`trifluoromethyl group (-CF3) is closer in size to an isopropyl group (23). Indeed,
`several workers have suggested that the CF3 group can exert an effect comparable
`to a phenyl ring or even a tert-butyl function (24).
`The presence of a fluorine atom can influence the lipophilicity of a molecule
`and hence affect the partitioning of the drug into membranes, and also facilitate
`hydrophobic interactions of the drug molecule with specific binding sites, on either
`receptors or enzymes. The replacement of a single aromatic hydrogen atom usually
`results only in a modest increase in lipophilicity, whereas the CF3 group is among
`the most lipophilic of all substituents.
`The fluoride ion is a good leaving group, being the conjugate base of a strong
`acid. Therefore, the fluoride ion can be lost, in both displacement and elimination
`reactions, and this aspect of fluorine chemistry can be utilized in the design of drugs
`or chemical agents that form stable covalent bonds with target receptors or enzymes
`as part of their pharmacological response. This is the basis of the “lethal synthesis”
`concept (25). More recently, fluorinated inhibitors of GABA aminotransferase have
`been synthesized as potential mechanism-based inhibitors (13).
`The presence of fluorine can alter the oxidation potential of an aromatic system,
`and thus alter the rate of autoxidation and formation of quinones and quinoneimines.
`Sequential introduction of fluorine atoms into the nucleus of paracetamol produced
`an increase in the oxidation potential of the molecule, as measured by cyclic
`voltammetry (26).
`Before embarking on fluorination as a synthetic strategy to alter drug dispo-
`sition, it is imperative to determine whether the changes in the physicochemical
`properties will diminish the inherent pharmacological activity of the drug. Molecu-
`lar modeling techniques can, in theory, be used to examine (a) the importance of the
`group to be replaced in the drug-receptor interaction and (b) whether the resulting
`C-F bond can provide the same chemical interaction with the receptor. However,
`it must be stressed that modeling of the C-F bond in drug-receptor interactions
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`
`is not a trivial task, even for very simple molecules in an aqueous environment
`(27).
`
`STRATEGIES FOR THE USE OF FLUORINE
`SUBSTITUTION TO ALTER DRUG DISPOSITION
`
`The introduction of fluorine into a molecule can be used to alter the rate, route, and
`extent of drug metabolism. Fluorine substitution can also be used to dissociate the
`pharmacological and toxicological properties of a drug when a toxic metabolite
`has been identified.
`Such alterations are most commonly achieved by fluorine substitution at the site
`of metabolic attack, based on the premise that the carbon-fluorine bond is much
`more resistant to direct chemical attack by cytochrome P450, in comparison to the
`carbon-hydrogen bond. In addition, substitution at sites adjacent to and, in some
`instances, distal to the site of metabolic attack can also affect drug metabolism,
`by either inductive/resonance (through bond) effects or conformational and elec-
`trostatic (through space) effects. The presence of a fluorine atom adjacent to a
`site of metabolic attack could, in theory, either increase or decrease the rate of
`biotransformation, depending on (a) whether the metabolic attack is nucleophilic
`or electrophilic in nature and (b) inductive or resonance effects of the fluorine
`atom predominate in the reaction. For example, in a simple saturated system, the
`inductive effect of fluorine should reduce the susceptibility of adjacent groups to
`attack by P450 enzymes. In contrast, it might be anticipated that the presence of
`fluorine ortho to a phenolic group might increase its reactivity as a nucleophile in
`methylation and glucuronidation reactions, and there is some evidence to support
`this hypothesis (28, 29).
`Fluorine substitution can therefore have complex effects on drug metabolism.
`The framework outlined in Figure 1 is used to consider the importance of the role of
`fluorinated subgroups on various aspects of drug disposition, and the consequent
`impact on drug efficacy and drug toxicity.
`
`THE EFFECT OF FLUORINE SUBSTITUTION
`ON DRUG DISTRIBUTION
`
`The inclusion of fluorine in a molecule has two benefits with respect to drug distri-
`bution. First, certain fluorine-containing functional groups enhance lipophilicity
`and therefore passive diffusion of drug across membranes. Second, noninvasive
`techniques can be used to assess penetration of the drug to the site of action,
`whether brain, tumor, or site of infection (30–32).
`Centrally acting drugs must pass through the blood brain barrier in sufficient
`concentration to elicit their pharmacological effect. For example, there are three
`categories of neuroleptics that act by blocking dopamine receptors in the cen-
`tral nervous system (CNS): tricyclics, butyrophenones, and diarylbutylamines.
`Many of these drugs contain either a CF3 group or a fluoro-phenyl group, which
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`contribute to the overall pharmacological activity of the compounds by enhanc-
`ing CNS penetration and retarding metabolic degradation. Several clinically use-
`ful phenothiazines containing fluorine have been introduced (4, 5). Trifluoropro-
`mazine, trifluperazine, and fluphenazine are all more active than chlorpromazine
`(5-, 50-, and 100-fold, respectively).
`The most widely used butyrophenone is haloperidol. It has been established
`that the para-fluorophenyl group is optimal for neuroleptic activity of the bu-
`tyrophenones. In the search for more potent neuroleptics, it was found that the
`diarylbutylamine, pimozide, which contains two fluorophenyl groups, was longer
`acting than haloperidol (12).
`Another important class of centrally acting drugs containing fluorine that have
`gained clinical prominence over the past few years is the selective serotonin up-
`take inhibitors. Of the most widely used agents in this class (fluoxetine, fluvoxa-
`mine, paroxetine sertraline, and citalopram), only sertraline does not contain a
`fluorine atom in its structure. Sertraline has the shortest half-life of the drugs men-
`tioned (26 h), is the most slowly absorbed, and undergoes almost total metabolic
`conversion, principally through presystemic elimination: all properties that may
`be associated with the lack of fluorine (2).
`Quinolone antibacterial agents act by inhibition of bacterial DNA synthesis.
`Fluoroquinolones were first introduced in the early 1980s (norfloxacin) and include
`ofloxacin, enoxacin, ciprofloxacin, tosufloxacin, sparfloxacin, grepafloxacin, lev-
`ofloxacin, and, most recently, trovafloxacin. This class of drugs is active against a
`wide range of gram-positive and gram-negative pathogens, possesses improved
`oral absorption and systemic distribution, and therefore has extended clinical
`applications that include urinary tract infections, respiratory tract infections, skin
`infections, and soft tissue infections. The structural features, which determine tis-
`sue distribution and cellular uptake, are complicated by specific efflux mechanisms
`(33). Nevertheless, it has been proposed that the 5-fluoro group is important for
`both cell penetration and gyrase affinity (34).
`
`THE EFFECT OF FLUORINE SUBSTITUTION
`ON THE RATE OF DRUG METABOLISM
`
`Fluorine substitution has been used to extend the biological half-life of endogenous
`compounds and synthetic compounds. Fluorinated analogues are not only useful
`tools for physiological investigations (Figure 2), they may also be potential thera-
`peutic agents. For example, the prostanoids, prostacyclin and thromboxane, play
`an essential physiological role in the regulation of the cardiovascular system and
`platelet function. Both compounds have short (<5 min) half-lives in vivo. Conse-
`quently, there has been a concerted effort to synthesize analogues with enhanced
`stability and, thus, extended activity.
`The potent platelet aggregating agent thromboxane A2 has a half-life of only
`32 s under physiological conditions. Incorporation of fluorine into the oxetane ring
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`
`Figure 2 Examples of how fluorination
`of endogenous compounds can enhance
`metabolic stability.
`
`alpha to the acetalic linkage reduces the rate of carbonium ion formation and acid
`hydrolysis. Thus, model compounds related to 7,7-difluoro-TxA2 have a rate of
`hydrolysis that is 108-fold slower than that of TXA2 (35), whereas 10,10-difluoro-
`TxA2 is also a stable analogue and retains thromboxane-like activity (36).
`Prostacyclin, which is an inhibitor of platelet aggregation, contains an acid-
`labile enol-ether group, which is responsible for its short biological half-life.
`Electrophilic attack of a hydroxonium ion at the enol-ether double bond is the
`rate-limiting step in the metabolic degradation of the compound. Fluorination
`alpha to the labile group reduces the electron density on the enol-ether group
`and, thus, improves the stability of the molecule toward acid hydrolysis. Thus
`10,10-difluoro-13-dehydro-prostacyclin exhibits a half-life 150 times greater than
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`that of PGI2 and has equal potency to the natural compound (37). Similarly, the int-
`roduction of a 7-fluoro group into PGI2 stabilized the enol by reducing the
`electron density of the double bond at C-5 through the inductive effects of flu-
`orine (38).
`Fluorination of natural hormones can lead to molecules with enhanced efficacy,
`and also to qualitative differences in activity. The changes in biological activity
`can be complex (pharmacodynamic and pharmacokinetic) and involve several
`chemical factors, including conformational changes in the steroid nucleus and
`altered receptor binding.
`The introduction of fluorine into the 9fi-position, of corticosteroids has a dra-
`matic reduction in some of the major routes of metabolism of cortisol. The most
`striking finding is the lack of oxidation of the 11fl-hydroxyl group to a ketone,
`which occurs rapidly with cortisol and leads to a loss of biological activity. Hence,
`in vivo, there is a shift in the enzymatic equilibrium between the biologically
`inactive 11-oxo compounds and the active reduced 11fl-hydroxy form (39).
`In vitro studies have shown that although cortisone and A-ring–reduced metabolites
`are the major products of cortisol metabolism in liver, 9fi-fluorocortisol undergoes
`preferential 6fl-hydroxylation and 20fl-reduction (40).
`We have investigated how introduction of fluorine may alter the balance be-
`tween metabolism of the A- and D-rings of estrogens (Figure 3). Introduction
`of fluorine into the 2-position can block 2-hydroxylation and bioactivation to a
`quinone (41, 42). Introduction of fluorine into the 16-position blocked not only
`C-16 hydroxylation but also dehydrogenation of the C-17 hydroxyl group (43).
`The restriction on D-ring metabolism was partly compensated for by enhancement
`of glucuronylation and A-ring hydroxylation.
`The 4-fluorophenyl group is usually resistant to aromatic hydroxylation, es-
`pecially at the 4-position, although aromatic hydroxylation at the unsubstituted
`positions does occur. Defluorination of the fluoromethyl group is extremely rare,
`although 5-trifluoromethyluracil is converted into 5-carboxyuracil in man with
`concomitant excretion of inorganic fluoride (44). Metabolism by cytochrome
`P450 enzymes does occur at the ortho and para positions. In molecules such as
`fluphenazine, which contain two rings that are chemically equivalent apart from the
`one being substituted with a CF3 group, hydroxylation occurs in the unsubstituted
`ring because of the deactivating effect of the CF3 group (45).
`The presence of the fluorine group in aromatic systems serves two purposes.
`First, the presence of fluorine per se can block oxidation at a specific position.
`Second, fluorine decreases the rate of reaction of the …-system of the benzene
`ring with activated cytochrome P450(FeO)3C
`(46). Nevertheless, cytochrome P450
`enzymes can attack at positions adjacent to the carbon-fluorine bond. Direct evi-
`dence for such a process comes from the demonstration that 1,4-difluorobenzene
`forms a significant amount of the NIH-shift product 2,4-difluorophenol (Figure 4).
`However, molecular orbital calculations show that increasing the extent of fluorine
`substitution further decreases nucleophilicity and, thus, reduces further the rate of
`reaction of di-, tri-, and tetrafluorobenzenes (46, 47). Evidence for such a pro-
`cess occurring in man is the detection of an NIH-shift metabolite of the novel
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`Figure 3 Differential effects of A- and D-ring fluorination on the regioselective
`metabolism of estradiol (29, 41).
`
`quinoxazoline reverse trancriptase inhibitor GW420867X in human urine, using
`NMR and tandem mass spectrometry (48).
`The blockade of aromatic hydroxylation by the CF3 group may have undesirable
`consequences. Endoperoxides are a new class of antimalarials that destroy para-
`sites by selective bioactivation of the peroxide group, to carbon-centered radi-
`cals, within the parasite. In contrast, bioactivation in mammalian systems is
`extremely limited (49). Arteflene is a synthetic derivative of the natural product,
`which has a promising biological and pharmacokinetic profile. However, blockade
`of aromatic hydroxylation by fluorination promotes extensive bioactivation of the
`peroxide system to potentially toxic metabolites in mammalian systems (50).
`The strategic value of fluorine substitution in rational drug design is neatly il-
`lustrated by the development of the orally active inhibitor of cholesterol absorption
`SCH58235 from SCH48461 (51). Fluorine was introduced to block undesirable
`metabolic transformations and produce a lead compound with 50-fold greater po-
`tency in vivo (Figure 5).
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`Figure 4 Mechanism of aromatic hydroxylation of fluorobenzenes. The NIH shift of
`fluorine (47, 48).
`
`THE USE OF FLUORINE SUBSTITUTION TO PREVENT
`METABOLIC BIOACTIVATION
`
`The physiological role of drug metabolism is that of detoxication. However, cer-
`tain biotransformations, and in particular those catalyzed by cytochrome P450,
`can produce chemically reactive metabolites. Normally enzymes such as epoxide
`hydrolase and glutathione transferase rapidly detoxify such metabolites. However,
`if such reactive metabolites interact with cellular macromolecules, they may cause
`carcinogenicity, apoptosis, necrosis, or hypersensitivity (52). Fluorine substitution
`has been used as a tool to investigate mechanisms of chemical toxicity and in the
`development of safer drugs.
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`Figure 5 Examples of fluorine substitution in rational drug design (51, 64, 65, 67).
`
`Polycyclic aromatic hydrocarbons undergo metabolic bioactivation to arene ox-
`ides, which bind covalently to DNA and thus initiate carcinogenesis. Metabolic ac-
`tivation by the cytochrome P450 system is generally blocked at the carbon to which
`fluorine is attached. Miller & Miller (53) used fluorine substitution as a tool in the
`early mechanistic investigations of carcinogenicity and found that substitution of
`fluorine in the 3-position of 10-methyl-1,2-benzanthracene virtually abolished the
`carcinogenic activity of this hydrocarbon toward mouse skin. This technique has
`been widely used to define the chemistry of carcinogen activation (Figure 6).
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`Figure 6 Use of fluorine substitu-
`tion to define the role of metabolic
`activation in chemical carcinogenic-
`ity (54, 55).
`
`Benzpyrene was the first member of the polycyclic aromatic hydrocarbon class
`of compounds for which the structures of both proximate and ultimate carcino-
`gens were defined. The metabolic pathway is catalyzed by CYP1A enzymes and
`epoxide hydrolysis leading to the ultimate carcinogen, 7.8-dihydroxy-9,10-epoxy-
`tetra-hydrobenzpyrene. Accordingly, it was found that 7-, 8-, 9-, and 10-fluorobenz
`(a)pyrenes were not tumorigenic in mice and that fluorine substitution, at each po-
`sition, had blocked formation of the respective diol-epoxides (54).
`The metabolic activation of 5-methylchrysene has been of interest because this
`carcinogen is typical of the class of methylated polynuclear aromatic hydrocarbons
`and is the most carcinogenic of all the methylchrysene isomers. Hecht et al (55)
`investigated the comparative carcinogenicity of seven derivatives fluorinated at
`the 1-, 3-, 6-, 7-, 9-, 11-, and 12-positions, respectively. Investigation of mouse
`skin tumor formation showed that the 6-, 7-, 9-, and 11-fluorinated derivatives
`were as carcinogenic as 5-methylchrysene, 12-fluoro was significantly less potent,
`and both 1-F and 3-F were inactive as complete carcinogens. The results are
`entirely consistent with the characterization of 5-methylchrysene-1,2-dihydrodiol-
`3,4-epoxide-DNA adducts (56, 57).
`Synthetic and natural estrogens have been associated in humans with a variety
`of vaginal, breast, hepatic, and cervical cancers. It has been suggested that both
`hormonal potency and oxidative metabolism may play a role in the carcinogenic-
`ity of steroid estrogens. The natural estrogens, estrone and estradiol, undergo
`extensive oxidation in the 2- and 4-positions by cytochrome P450 (CYP) en-
`zymes. The resulting catechols are readily oxidized to chemically reactive quinones
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`
`and semiquinones, which form covalent bonds with proteins and DNA. Liehr (58)
`found that 2-fluoro-estradiol was as estrogenic as estradiol but noncarcinogenic
`in the Syrian hamster. In vivo studies (42, 43) showed that the presence of a 2-
`fluoro substituent diverted the metabolism of the steroid from 2-hydroxylation to
`glucuronidation, thus providing direct evidence for the role of bioactivation in the
`model of oestrogen carcinogenicity used by Liehr (58).
`The widely used analgesic paracetamol serves as a paradigm for the role of
`metabolic bioactivation in drug toxicity. Studies in knockout mice have clearly
`shown that formation of a reactive metabolite, N-acetylbenzoquinoneimine, is res-
`ponsible for the hepatic necrosis seen when the drug is taken in overdose (59, 60).
`Introduction of fluorine into the paracetamol molecule alters the oxidation poten-
`tial in a manner dependent on the number and position of the fluorine atoms (26).
`The presence of fluorine at the 2- and 6-positions increased the oxidation potential
`of paracetamol sufficiently to reduce the propensity of the molecule to undergo
`oxidative bioactivation in vivo and thereby reduced hepatotoxicity (61). Introduc-
`tion of fluorine did not affect the important detoxication pathways of glucuroni-
`dation or sulphation. It is interesting that 2,6-fluorine substitution also resulted in
`a loss of analgesic activity, indicating a role for oxidation in the analgesic activity
`of the drug (61).
`The 4-aminoquinoline antimalarial amodiaquine also contains the 4-aminophe-
`nol function. It is thought that the hemotoxicity and hepatotoxicity of this drug
`may be a result of oxidative metabolism to a chemically reactive quinoneimine
`metabolite (62, 63). The effect of systematic fluorine substitution of the aminop-
`henol ring was investigated with respect to (a) increasing the oxidative stability of
`the ring system and (b) reducing in vivo oxidation to potentially toxic quinoneimine
`metabolites. As was the case for paracetamol, 6-difluorination of the aminophe-
`nol ring of amodiaquine produced an analogue with significantly raised oxidation
`potential and reduced bioactivation in vivo. Isosteric replacement of the hydroxyl
`function with fluorine provided an analogue, fluoroamodiaquine, which had similar
`antimalarial potency to the parent drug but was not bioactivated to toxic metabolites
`in vivo. In contrast to the reduction in biological activity observed in the fluori-
`nated paracetamol series, fluorinated analogues of amodiaquine retained pharma-
`cological activity, which suggested that oxidation to quinonimine metabolites is
`not required for antimalarial activity (64). Fluorinated 4-aminoquinolines and
`8-aminoquinolines are being assessed as lead compounds for drug development
`(65, 66) (Figure 5).
`The effect of fluorine substitution on the metabolism and toxicity of drugs and
`use of such knowledge for the development of safer therapeutic agents is neatly
`illustrated by the flurane group of anaesthetics (68) (Figure 7). Methoxyflurane
`was widely used in clinical anaesthesia during the 1960s, until it was discovered
`that there was an association with nephrotoxicity. A high urine output syndrome
`leading to dehydration, and in some cases fatal renal failure, was related to the
`extensive (40%) metabolism of methoxyflurane and high serum concentrations of
`
`

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`P1: FUM/GBC
`February 22, 2001
`12:24
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`QC: aaa
`Annual Reviews
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`AR126-18
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`456
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`PARK ¥ KITTERINGHAM ¥ O’NEILL
`
`Figure 7 The toxicity and metabolism of general anesthetics.
`
`

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`12:24
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`QC: aaa
`Annual Reviews
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`AR126-18
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`METABOLISM OF FLUORINATED DRUGS
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`457
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`inorganic fluoride. This theory has been questioned in recent years, since the intro-
`duction of sevoflurane, which also produces high serum fluoride but is not reported
`to cause nephrotoxicity (69). Methoxyflurane has been shown to be metabolized
`in man and animals to oxalic acid and free fluoride (70). Studies to date have shown
`only modest inorganic fluoride levels with enflurane and isoflurane, and there are
`only occasional reports of nephrotoxicity for enflurane and none at all for isoflu-
`rane (68, 71, 72). Increasing fluorine substitution results in a reduction in overall
`metabolism, and in specifically biotransformations leading to defluorination.
`In man, enflurane is metabolized to the fluoride ion, difluoromethoxydifluo-
`roacetic acid, and an unidentified acid metabolite. Metabolism is catalyzed by
`CYP2E1 and is stereoselective (73). The predicted products of oxidation of the
`difluoromethyl group, chlorofluoroacetic acid and oxalic acid, have not been
`detected (74). Inorganic fluoride and trifluoroacetic acid have been identified as
`end products of isoflurane metabolism (75). These products are thought to arise
`by a sequence that begins with insertion of an active oxygen atom into the bond
`connecting hydrogen to the ethyl fi-carbon and which is catalyzed by CYP2E1 in
`man (76, 77). Desflurane, in which the chlorine atom in isoflurane is replaced by
`a further fluorine atom, is excreted by the lungs and appears resistant to biotrans-
`formation. The oil gas partition coefficient for desflurane (18.7) is considerably
`less than that of isoflurane (91), which explains the more rapid rates of onset and
`offset of anesthesia of the former (78, 79).
`The first of the modern fluorinated anesthetics was halothane; however, its
`clinical use is now limited because of metabolism-associated liver toxicity. The
`National Halothane Study (80) identified two distinct forms of hepatotoxicity: The
`first, a mild form of transaminitis, occurred in 20% of cases but was reversible.
`In contrast, a small group (approximately 1 in 35,000) suffered severe hepatitis,
`characterized by massive liver necrosis, which could be fatal. The incidence of
`hepatitis increased on reexposure (to 1 in 3,700), indicative of an immunological
`aetiology. Two pathways of halothane metabolism occur (Figure 8): a reductive
`pathway, mediated by CYP3A4 and CYP2A6 (81), and oxidation, principally
`involving CYP2E1 (82). The reductive pathway generates free radicals, which,
`following further reduction and fluorine elimination, gives