history of development of azole derivates

History of the development of azole derivativesJ. A. Maertens

Department of Haematology, University Hospital Gasthuisberg, Leuven, Belgium

A B S T R A C T

Until the 1940s, relatively few agents were available for the treatment of systemic fungal infections. Thedevelopment of the polyene antifungals represented a major advance in medical mycology. Althoughamphotericin B quickly became the mainstay of therapy for serious infections, its use was associatedwith infusion-related side-effects and dose-limiting nephrotoxicity. The continued search for new andless toxic antifungals led to the discovery of the azoles several decades later. Ketoconazole, the firstavailable compound for the oral treatment of systemic fungal infections, was released in the early1980s. For almost a decade, ketoconazole was regarded as the drug of choice in nonlife-threateningendemic mycoses. The introduction of the first-generation triazoles represented a second majoradvance in the treatment of fungal infections. Both fluconazole and itraconazole displayed a broaderspectrum of antifungal activity than the imidazoles and had a markedly improved safety profilecompared with amphotericin B and ketoconazole. Despite widespread use, however, these agentsbecame subject to a number of clinically important limitations related to their suboptimal spectrum ofactivity, the development of resistance, the induction of hazardous drug–drug interactions, their lessthan optimal pharmacokinetic profile (itraconazole capsules), and toxicity. In order to overcome theselimitations, several analogues have been developed. These so-called ‘second-generation’ triazoles,including voriconazole, posaconazole and ravuconazole, have greater potency and possess increasedactivity against resistant and emerging pathogens, in particular against Aspergillus spp. If the toxicityprofile of these agents is comparable to or better than that of the first-generation triazoles and druginteractions remain manageable, then these compounds represent a true expansion of our antifungalarsenal.

Keywords Antifungal, azole, overview

Clin Microbiol Infect 2004; 10 (Suppl. 1): 1–10

I N T R O D U C T I O N

Despite the implementation of several preventivemeasures and the use of antifungal chemopro-phylaxis, physicians have witnessed an increasedincidence of both mucosal and invasive fungalinfections during the past two decades [1–4]. Thisincrease is linked with progress in medical tech-nology and novel therapeutic options andappears to be multifactorial. The widespread useof quinolone prophylaxis in neutropenic cancerpatients and the availability of broad-spectrumantibacterial agents has virtually eliminated early

death due to bacterial sepsis, thereby setting thestage for fungal colonisation and putting patientsat risk for subsequent mycotic infections. Medicalprocedures have become more invasive andaggressive; the accompanying disruption of pro-tective anatomical barriers as a result of indwell-ing catheters, therapy-induced mucositis, viralinfections, and graft-versus-host disease, or fol-lowing major abdominal surgery or associatedwith extensive burns, allows fungi to reachnormally sterile body sites [5]. In addition, thecommunity of vulnerable patients is continuouslyexpanding as a result of the spread of humanimmunodeficiency virus (HIV) infections, theincreased use of (novel) immunosuppressivedrugs in autoimmune disorders and to preventor treat rejection in the expanding area of trans-plant medicine, the popularity of dose-escalated,often myelo-ablative cytotoxic therapy, the

Corresponding author and reprint requests: J. A. Maertens,MD, Department of Haematology, University Hospital Gasth-uisberg, Herestraat 49, 3000 Leuven, BelgiumTel: + 32 16 34 68 80Fax: + 32 16 34 68 81E-mail: [email protected]

� 2004 Copyright by the European Society of Clinical Microbiology and Infectious Diseases

improved survival rate in premature infants, andthe availability of sophisticated life-saving med-ical techniques [5–7]. Unfortunately, the attribut-able mortality rate of (systemic) fungal infectionsremains high [8,9]. This may partly be explainedby the difficulty of diagnosing these infections atan early stage of their development, becausedefinite proof often requires time-consumingand labour-intensive approaches that cannot al-ways be achieved in these severely ill patients.However, an additional explanation may befound in shortcomings of the current antifungalarmamentarium.

Clearly, progress in the development of newantifungals has lagged behind antibacterialresearch, a fact that can be explained by at leasttwo factors. First, before the HIV-era, the occur-rence of fungal infections was believed to be toolow to warrant aggressive research by the phar-maceutical industry. Second, the ‘apparent’ lackof a highly selective fungal target, not present inother eukaryotic (including mammalian) cells,precluded the development of new agents. Untilrecently, the arsenal that was available for thetreatment of systemic fungal infections was lim-ited in number and consisted mainly of thepolyene antibiotic amphotericin B, some azolederivatives, the allylamines–thiocarbamates and5-flucytosine. With the exception of 5-flucytosine,all other agents acted by interfering with thestructural or functional integrity of the fungalplasma membrane, either by physical disruptionor by blocking the biosynthesis of membranesterols. The past decade, however, has witnessedan expansion of basic and clinical research inantifungal pharmacology and many companieshave launched new compounds, including sev-eral new azole compounds and the candins [5].

M E C H A N I S M O F A C T I O N

For a detailed discussion of the mechanism ofaction, the reader is referred to original work byVanden Bossche et al. [ 10–12] and a recent reviewarticle by White et al. [13]. Azole antifungals aredivided into the imidazoles (e.g. miconazole andketoconazole) and the triazoles (e.g. itraconazole,fluconazole, voriconazole). The latter group hasthree instead of two nitrogen atoms in the azolering. All of the azoles operate via a common modeof action: they prevent the synthesis of ergosterol,the major sterol component of fungal plasma

membranes, through inhibition of the fungalcytochrome P450-dependent enzyme lanosterol14-a-demethylase. The resulting depletion ofergosterol and the concomitant accumulation of14-a-methylated precursors interferes with thebulk function of ergosterol in fungal membranesand alters both the fluidity of the membrane andthe activity of several membrane-bound enzymes(e.g. chitin synthase). The net effect is an inhibi-tion of fungal growth and replication. In addition,a number of secondary effects, such as inhibitionof the morphogenetic transformation of yeasts tothe mycelial form, decreased fungal adherence,and direct toxic effects on membrane phospho-lipids, have been reported [14].

Unfortunately, as a result of the nonselectivenature of the therapeutic target, cross-inhibitionof P450-dependent enzymes involved in mamma-lian biosynthesis has been responsible for sometoxicity, although significantly lower and lesssevere with fluconazole, itraconazole and voric-onazole than with the older compounds. Theimproved toxicity profile of the triazoles com-pared to the imidazoles (especially endocrine-related side-effects) can be explained by theirgreater affinity for fungal rather than mammalianP450-enzymes at therapeutic concentrations [15].

H I S T O R Y O F A Z O L E S

Although the first report of antifungal activity of anazole compound, benzimidazole, was already des-cribed in 1944 by Woolley, it was not until after theintroduction of topical chlormidazole in 1958 thatresearchers became interested in the antifungalactivity of azole compounds [16]. In the late 1960s,three new topical compounds were introduced:clotrimazole, developed by Bayer Ag (Germany),and miconazole and econazole, both developed byJanssen Pharmaceutica (Belgium) [17].

The in-vitro activity of clotrimazole againstdermatophytes, yeasts, and dimorphic as well asfilamentous fungi, is well-established and com-parable to that of amphotericin B for manypathogens [18]. However, unacceptable side-effects following oral administration [19] andunpredictable pharmacokinetics as a result ofthe induction of hepatic microsomal enzymes [20]have limited the use of clotrimazole to the topicaltreatment of dermatophytic infections and super-ficial candida infections, including oral thrushand vaginal candidiasis.

2 Clinical Microbiology and Infection, Volume 10 Supplement 1, 2004

� 2004 Copyright by the European Society of Clinical Microbiology and Infectious Diseases, CMI, 10 (Suppl. 1), 1–10

Miconazole, a phenethyl imidazole synthesisedin 1969, was the first azole available for parenteraladministration (although not before 1978). Likeother azoles, it interferes with the biosynthesis offungal ergosterol, but at high concentrations,miconazole may also cause direct membranedamage that results in leakage of cell constituents.The drug has a limited spectrum of activityincluding dermatophytes, Candida species, dimor-phic fungi, and Pseudallescheria boydii. The agenthas proven to be an effective topical antifungalagent, but toxicity associated with the vehicleused for intravenous administration has limitedits parenteral use [21], although it has been usedsuccessfully in the treatment of systemic candidainfections, pseudallescheriasis and some refract-ory cases of cryptococcal meningitis [22,23].Miconazole has recently been withdrawn fromthe market.

In 1981, the Food and Drug Administration(FDA) approved the systemic use of ketoconaz-ole, an imidazole derivative synthesised anddeveloped by Janssen Pharmaceutica [24]. Foralmost a decade it would be regarded as thestandard and was the only available oral agentfor the treatment of systemic fungal infections.Until the introduction of the triazoles, ketocon-azole was indicated as the drug of choice inchronic mucocutaneous candidiasis [25] and asan effective alternative to amphotericin B in lesssevere (nonimmunocompromised) cases of blas-tomycosis [26], histoplasmosis [26], and paracoc-cidioidomycosis [27]; in coccidioidomycosis, therelapse rate after discontinuation of the drug washigh [28]. Ketoconazole has not been adequatelyevaluated in deep-seated candida infections orcryptococcosis and was ineffective in aspergil-losis and mucormycosis.

Over the years, a number of clinically relevantshortcomings of this compound became evident:• The absorption of orally administered ketocon-

azole showed considerable interindividual vari-ation and was markedly influenced by gastricpH [29].

• An intravenous formulation was not available.• The drug penetrated the blood–brain barrier

poorly and could therefore not be recommen-ded for the treatment of fungal meningitis[30,31].

• Ketoconazole was largely fungistatic andproved to be less effective in immunocompro-mised patients [17].

• The use of ketoconazole was associated withseveral dose-related (gastrointestinal) side-effects [26]; in addition, ketoconazole couldcause symptomatic, even fatal, drug-inducedhepatitis [32].

• When given in doses exceeding 400 mg daily,ketoconazole might reversibly inhibit the syn-thesis of testosterone and cortisol, resulting in avariety of endocrine disturbances, includingrare cases of adrenal insufficiency [33].

• A number of clinically important, often unpre-dictable, drug interactions (e.g. to cyclosporine)have been reported [34].Thus, the poor response rates and frequent

recurrences of major fungal infections, as well asthe toxicity associated with ketoconazole therapy,led to the search for a second chemical group ofazole derivatives, namely the triazoles. In general,the triazoles demonstrate a broader spectrum ofantifungal activity and reduced toxicity whencompared with the imidazole antifungals. Terc-onazole, the first triazole marketed for humanuse, was active in the topical treatment of vaginalcandidiasis and dermatomycoses.

Fluconazole (Figure 1a), a broad-spectrumtriazole antifungal developed by Pfizer andapproved for use in early 1990, covers many ofthe shortcomings of the imidazoles. In contrast toketoconazole, fluconazole is highly water solubleand can be given intravenously to seriously illpatients. After oral administration, absorption isessentially complete (� 90% bioavailability) andnot influenced by gastric pH [35]. In contrastto ketoconazole, fluconazole enters the cerebro-spinal fluid (CSF) extremely well, with CSF levelsof almost 80% of the corresponding serum levels[36]. The serum half-life allows once-daily dos-ing, and, also in contrast to ketoconazole, renalclearance is the major route of elimination offluconazole, with 70–80% of unchanged drugexcreted in the urine [37]. Given this favourablepharmacokinetic profile (Table 1), fluconazolehas been studied extensively in various clinicalsettings, both in prophylaxis and in therapy.The drug is approved for the treatment oforopharyngeal, oesophageal, vaginal, peritonealand genito-urinary candida infections, dissem-inated candidiasis (including chronic dissemin-ated candidiasis) and cryptococcal meningitis.Fluconazole also has good activity against coc-cidioidomycosis and is a good alternative to keto-conazole in chronic mucocutaneous candidiasis.

Maertens History of azole derivatives 3


Fluconazole has no clinically meaningful activityin infections caused by filamentous fungi. Inaddition, the drug is relatively safe (even atdaily doses up to 1600 mg) and does not inter-fere with the synthesis of testosterone or cortisol.Also, fluconazole has fewer drug interactionsthan does ketoconazole. The clinical experiencewith fluconazole is reviewed by Dr Pfaller inthis issue.

The initial enthusiasm for fluconazole, how-ever, has been challenged by two recent develop-ments: the evolving spectrum of fungal pathogensand the development of azole-resistance.

The evolving spectrum of fungal pathogens

In virtually every North American and Europeanmedical centre, hospital-acquired infectionscaused by Candida species—both superficial anddeep-seated forms—have increased substantiallyover the past two decades. According to theNational Nosocomial Infection Surveillance Pro-gram from the Centers for Disease Control andPrevention (CDC) and the Surveillance and Con-trol of Pathogens of Epidemiological Importance(SCOPE) study, Candida species now account for7–8% of all nosocomial bloodstream infectionsin the USA [38,39]. Although Candida albicansremains the most commonly encountered patho-genic yeast, a number of recent studies havedemonstrated a shift towards infection with

‘non-albicans’ Candida species. In the SCOPEstudy, 50% of isolates from bloodstream infec-tions were non-albicans species, including C.tropicalis, C. glabrata, C. krusei, C. parapsilosis andC. lusitaniae [39]. Some species are considered lessvirulent than C. albicans but are, on the otherhand, inherently less susceptible to fluconazole.Recent data, however, have revealed importantdifferences between patient groups at risk (e.g.,oncology versus nononcology), between institu-tions and between countries [9]. Although thecause of this changing spectrum is multifactorialand has not been evaluated systematically, theselective pressure resulting from the widespreaduse of fluconazole has most probably contributedto the higher proportion of non-albicans species[40]. Nevertheless, links between spectral changesand prior use of fluconazole have not yet beenfirmly proved; additional factors such as institu-tion-related differences in anti-infective protocolsand treatment-specific factors may be equallyimportant for fungal colonisation and subsequentinfection [41].

In addition, major observational and autopsysurveys, both in Europe and the USA, haveshown that the incidence of invasive aspergillusinfections—a pathogen not covered by flucona-zole—has increased dramatically over the pasttwo decades [2,3].

Fungal infections have been important compli-cations among patients with HIV from the begin-

Fig. 1. Chemical structures of oldand new triazoles.



ning of the pandemic. Population-based surveil-lance studies, conducted by the CDC and others,have clearly reflected the impact of highly activeantiretroviral therapy (HAART) on the incidenceand spectrum of these infections in HIV-positivepatients. The incidence of cryptococcal meningitis(from 10% to < 3%), mucosal candidiasis, asper-gillosis, and many other fungal infections (histo-plasmosis, penicilliosis marneffei) has decreasedspectacularly with the use of protease inhibitors[42,43].

Although Candida and Aspergillus species stillrepresent the vast majority of fungal isolatesencountered in human pathology, a batteryof new species—both yeasts and filamentousfungi—is increasingly recognized as opportunis-tic pathogens. Of particular concern is the fact thatmany of these so-called ‘emerging’ pathogens arenot covered by fluconazole, including Trichospo-ron spp., Fusarium spp., Scedosporium prolificans,members of the Mucoraceae, and dematiaceous ordarkly pigmented fungi [44,45].

The development of azole-resistance

Until the 1990s, acquired resistance to azoleantifungals was uncommon. In recent years,

however, particularly as a result of the liberaluse of fluconazole in immunosuppressed andcritically ill patients, clear patterns of resistancehave emerged. However, the lack of an estab-lished definition of resistance remains problem-atic when analysing the scope of this problem.Clearly, the continuous or intermittent adminis-tration of any antifungal agent may select forovergrowth of intrinsically resistant or less sus-ceptible strains or species, as has been docu-mented in oncology patients receiving fluconazoleprophylaxis. This phenomenon, however, was notreported in a very large study of female patientswith, or at risk of, HIV infection. Unfortunately,resistance has too often been defined as ‘clinicallyresistant’, referring to a patient whose infectionhas progressed or persisted despite antifungaltherapy. Classical resistance refers to treatmentfailure in association with high or rising mini-mum inhibitory concentrations (MICs) for thesame fungal strain while receiving therapy: adefinition not used by many publications. Be-sides, key features such as pharmacodynamicparameters, fungal virulence factors, host factorsand differences in susceptibility testing methodsfurther impair the analysis of a possible linkbetween MIC and outcome [46]. Therefore, the

Table 1. Comparative data of avail-able triazole antifungal agents Parameter Fluconazole Itraconazole Voriconazole

Formulation oral suspension oral solution tabletstablets capsules intravenousintravenous intravenous

Oral bioavailability (%) > 90 55 > 85Influence of food none › (capsules) flfl (solution)Influence of › pH none fl (capsules

only)none

Tmax (h) 1–2 1.5–4.0 < 2Steady-state plasmaconcentration (mg ⁄L)

10 1 2(200 mg bid)

Protein binding (%) < 10 > 95 60Vd (L ⁄ kg) 0.7–0.8 10.7 2Principal route of elimination renal hepatic hepaticElimination half-life 27–37 h 21–64 h 6–9 hCl (mL ⁄min.kg) 0.23 3.80 � 3Unchanged drug in urine (%) 80 < 1 < 5Relative CSF levels (%) > 60–80 < 1 > 50Important drug interactions + + + + + + + +ToxicityNephrotoxicity – – –Hepatotoxicity + + +Relevant other toxicity – – visualDose reduction in renal failure yes no ?Dialysable yes no ?

CSF ¼ cerebrospinal fluid; Cl ¼ clearance.



National Committee for Clinical LaboratoryStandards has recently developed a standardizedand reproducible method for the in vitro suscep-tibility testing of yeasts to azoles (measuring truemicrobial resistance) [47].

Apart from this, it is obvious from the literaturethat in-vitro resistance for C. albicans has emergedrapidly during the pre-HAART era in HIV ⁄acquired immune deficiency syndrome (AIDS)patients with oropharyngeal or oesophageal can-didiasis [48]. In this setting, up to 20% of C. albicansstrains have become resistant to fluconazole,depending on the duration and total cumulativedose of fluconazole given, the frequency of expo-sure and the patient’s CD4+ cell count. Fortu-nately, at least in developed countries, theintroduction of HAART and the subsequent hostimmune reconstitution has led to a marked reduc-tion in the number of HIV-associated opportunis-tic fungal infections; as such, azole-resistantisolates from AIDS patients are now seldomencountered, although it remains to be seenwhether this improvement will be maintained. Inaddition, it is critical to recognize the concept ofdose dependency in azole-susceptibility whenapproaching the therapy of these infections [49].For example, AIDS patients with oropharyngealcandidiasis caused by strains with low MICs willtypically respond to low doses of fluconazole. Incontrast, patients with ‘susceptible, dose-depend-ent’ strains may still respond to fluconazoletherapy provided that higher doses of 400–800 mg ⁄day are given. In general, yeast in-vitroresistance remains manageable in HIV ⁄AIDSpatients.

It is evident that the same phenomenon of azoleresistance could also arise in deep-seated candidainfections and candidaemia. However, evidenceof emerging resistance in this setting remainslargely equivocal. Based on a number of largesurveillance studies in developed countries, thereis no evident trend towards an increased acquiredresistance to fluconazole for bloodstream isolatesof C. albicans [46]. Although an increased preval-ence of C. krusei and C. glabrata bloodstreaminfections has been seen in oncology patients,bone marrow transplant recipients and patients inintensive care units, it remains debatable whetherthis is (solely) the result of the introduction offluconazole or not.

In 1992, itraconazole (Figure 1b), a broad-spectrum triazole from Janssen Pharmaceutica,

was approved by the Food and Drug Adminis-tration. Compared to ketoconazole, itraconazolewas less toxic and showed a broad spectrum ofactivity against Candida spp., Aspergillus spp.,Cryptococcus neoformans, Coccidioides immitis, His-toplasma capsulatum, Blastomyces dermatitidis, Para-coccidioides brasiliensis, Sporothrix schenckii andsome phaeohyphomycetes [50]. Since its intro-duction, itraconazole has gradually replacedketoconazole as the treatment of choice for non-meningeal, nonlife-threatening cases of histoplas-mosis, blastomycosis and paracoccidioidomycosis(amphotericin B for severe or meningeal cases).Itraconazole also has considerable spectral advan-tages over fluconazole with greater activityin aspergillosis and sporotrichosis [51]. However,fluconazole demonstrates a more favourablepharmacological and toxicity profile.

Unlike fluconazole, itraconazole is highly lipidsoluble and when first introduced, itraconazolewas formulated in capsular form only. Thisformulation became widely used for the treat-ment of onychomycosis, superficial fungal infec-tions, endemic systemic infections, systemicaspergillus infections, and to a much lesser extent,systemic candida infections. The prophylacticefficacy has been demonstrated in a number oftrials in patients with neutropenia or with HIVinfection [52]. However, because absorption ofitraconazole capsules can be erratic and low bloodconcentrations (< 500 ng ⁄mL) have been asso-ciated with treatment failure, many cliniciansdisapprove of its use in patients who weresuffering from therapy-induced mucositis orwho were receiving antacids [53]. A novel oralformulation of itraconazole, containing the solu-bilizing excipient hydroxypropyl-b-cyclodextrin,has therefore been developed (FDA approved in1997). When the capsules and oral solution arecompared, the bioavailability of the solution isapproximately 60% greater than that of thecapsules. This value was obtained in healthyvolunteers [54], as well as in various patientgroups, including neutropenic patients [55], HIVpatients [56], and patients following autologousand allogeneic transplantation [57].

Recently, an intravenous formulation has beendeveloped. In patients with haematological malig-nancy, patients with advanced HIV infection, orthose in intensive care units, high and steady-state plasma concentrations can be achievedwithin 2–3 days (as opposed to 1–2 weeks when



using capsules) using intravenous itraconazole,400 mg ⁄day for 2 days followed by 200 mg ⁄dayfor 5 days. Subsequent administration of itracon-azole oral solution or capsules, 400 mg daily, willmaintain these high plasma levels [58]. Thus, theavailability of new formulations has introducedmore flexibility in the use of itraconazole. How-ever, when compared with fluconazole, clinicalexperience remains limited, especially with theintravenous formulation.

Although the availability over the past decadeof both fluconazole and itraconazole represents amajor advance in the management of systemicfungal infections, these triazole antifungal drugshave some important limitations.

Several azole–drug interactions may result inhazardous and unpredictable toxicity, especiallyin patients receiving chemotherapy (e.g. vincris-tine), transplant recipients (e.g. cyclosporine A,tacrolimus) and AIDS patients (e.g. indinavir,ritonavir). The potential for drug interactions wasgreater with ketoconazole, but similar interactionshave been described with the triazoles. One typeof azole–drug interaction may result in decreasedplasma concentrations of the azole, either as aresult of decreased absorption or increased meta-bolism of the azole. A second type of interactionmay result in increased toxicity of the coadmin-istered drug via interference with the cytochromeP450 systems that are involved in the metabolismof many drugs [59].

Their antifungal spectrum remains suboptimal,especially when considering the growing diversityof offending species. The activity of fluconazole islimited to dermatophytes, C. neoformans, C. albicansand the dimorphic fungi. Although itraconazoledisplays a broader spectrum of activity, includingAspergillus spp. and some yeast strains that areintrinsically resistant to fluconazole, neither com-pound has documented activity against some ofthe emerging pathogens, such as Fusarium spp.,Scedosporium spp. and the Zygomycetes.

Clinical resistance associated with microbiolo-gical resistance has been reported, both for fluc-onazole (mainly C. albicans) and itraconazole(including Aspergillus spp.) [60]. Meanwhile, sev-eral mechanisms of azole-resistance have beenidentified, including enhanced efflux of the azoleby up-regulation of multidrug efflux pumps,alterations in the cellular target of azoles (Erg11p),and modification of the ERG11 gene at themolecular level [61]. Of note is that these mech-

anisms mediate cross-resistance amongst theazoles [62].

Given these shortcomings, the characteristics ofthe ideal azole can easily be deduced: the agentshould be available in oral and intravenousdosage forms, demonstrate a broad spectrum ofactivity, covering both yeast as well as classic andemerging filamentous fungi, be fungicidal, dis-play a good pharmacokinetic profile with min-imal drug interactions, be stable to resistance andbe cost-effective. Whether the ‘second-generation’triazole derivatives that are currently under clin-ical development or that have recently beenlaunched on the market will eliminate or reducethese shortcomings remains to be seen.

Voriconazole (Figure 1c), structurally related tofluconazole, was developed by Pfizer Pharmaceu-ticals as part of a programme designed to enhancethe potency and spectrum of activity of flucona-zole [63]. Voriconazole displays wide-spectrumin-vitro activity against fungi from all clinicallyimportant pathogenic groups such as Candidaspp., Aspergillus spp., C. neoformans, dimorphicfungi, dermatophytes, and some of the emergingmould pathogens including Fusarium spp., Peni-cillium, Scedosporium, Acremonium and Trichospo-ron. Members of the zygomycetes still appear tobe resistant. Compared to reference triazoles,voriconazole is several-fold more active thanfluconazole and itraconazole against Candidaspp. However, C. albicans isolates with decreasedsusceptibility to fluconazole and itraconazole alsodemonstrate significantly higher MICs for voric-onazole, and isolates (Candida as well as Aspergil-lus) that are highly resistant to both fluconazoleand itraconazole show apparent cross-resistanceto voriconazole. The drug is orally and parenteral-ly active but exhibits complex pharmacokinetics.Interestingly, animal studies have revealed goodpenetration into the CSF and central nervoussystem. The promising in-vitro activity has beenconfirmed in a range of infections in immuno-suppressed animal models where voriconazoleproved to be more effective than amphotericin B,fluconazole and itraconazole. Data from phase IIand III clinical trials indicate that voriconazole is apromising agent for the treatment of oropharyn-geal candidiasis in AIDS patients, oesophagealcandidiasis, and acute and chronic invasiveaspergillosis, including cerebral aspergillosis. Anumber of cases have reported activity in unusualmould infections, such as scedosporiosis and



fusariosis. However, voriconazole is not yet theideal azole, because the agent is not devoid of theclassical azole–drug interactions and class-relatedside-effects (including severe cases of hepaticdysfunction [64]). Furthermore, dose-related tran-sient visual disturbances (without morphologicalcorrelates) have been reported in up to 10% ofpatients receiving this agent. Hence, additionalwork regarding (visual) safety, drug–drug inter-actions, metabolism and exposure (given thegenetic polymorphism of CYP2C19), and emer-gence of (cross)-resistance is needed. The currentstatus of voriconazole is reviewed by Dr Donnellyand Dr De Pauw in this issue.

Posaconazole (Figure 1d), a hydroxylated ana-logue of itraconazole developed by Schering-Plough Research Institute, possesses potentbroad-spectrum activity against opportunisticand endemic fungal pathogens, including zygo-mycetes and some of the dematiaceous moulds.In vitro, the drug is highly active against Asperg-illus spp. and at least eightfold more potent thanfluconazole against Candida spp. Similar to voric-onazole, posaconazole was more effective thanamphotericin B, fluconazole and itraconazole inanimal studies [65]. Ravuconazole (Figure 1e),another derivative of fluconazole developed byBristol-Myers Squibb, represents the third second-generation triazole. Ravuconazole has a broaderantifungal spectrum than fluconazole and itrac-onazole, particularly against strains of C. kruseiand C. neoformans. In vitro, ravuconazole is notactive against Fusarium spp. and Pseudallescheriaboydii [65]. Both posaconazole and ravuconazoleare currently undergoing phase II and III clinicaltrials.

C O N C L U S I O N

Until the 1940s, relatively few agents wereavailable for the treatment of systemic fungalinfections. The development of the polyeneantifungals, first nystatin and later amphotericinB, represented a major advance in medicalmycology. Although amphotericin B quicklybecame the mainstay of therapy for seriousinfections, its use was associated with a numberof infusion-related side-effects and dose-limitingnephrotoxicity. The continued search for newand less toxic antifungals led to the discovery ofthe azoles several decades later. Ketoconazole,the first available compound for the oral

treatment of systemic fungal infections, wasreleased in the early 1980s. For almost a decade,ketoconazole was regarded as the drug of choicein nonlife-threatening endemic mycoses. Theintroduction of intravenous and oral fluconazolein 1990 and oral itraconazole in 1992 representeda second major advance in the treatment offungal infections. Both first-generation triazolesdisplayed a broader spectrum of antifungalactivity than the imidazoles and had a markedlyimproved safety profile compared with ampho-tericin B and ketoconazole. Fluconazole wasused frequently for prophylaxis and treatmentof candidal and cryptococcal infections, especi-ally in the pre-HAART era, whereas itraconazole,despite its erratic absorption, became the drug ofchoice for the treatment of less severe forms ofhistoplasmosis and blastomycosis, and an attract-ive alternative to amphotericin B for the treat-ment of select cases of invasive aspergillosis.Although expanded uses have been suggestedfor both agents—in prophylaxis as well as intherapy—they also became subject to a numberof clinically important limitations, includingsuboptimal spectrum of activity, developmentof resistance, induction of hazardous drug–druginteractions, less than optimal pharmacokineticprofile (itraconazole capsules), and toxicity. Toovercome these limitations, several structuralanalogues have been developed and tested invarious stages of clinical development. Three ofthese so-called ‘second-generation’ triazoles,including voriconazole, posaconazole and ravuc-onazole, appear to have greater potency andpossess increased activity against resistant andemerging pathogens. All three agents are activefollowing oral administration and have demon-strated promising antifungal activity in vitro andin animal models. These second-generation triaz-oles seem particularly promising for the treat-ment of Aspergillus infections and for unusual(but emerging) opportunistic infections that areotherwise covered only by amphotericin B ornot covered at all. However, further clinicalinvestigation is warranted to identify the role ofthese agents in the future treatment of systemicfungal infections. If the toxicity profile of theseagents is comparable to or better than that of thefirst-generation triazoles and drug interactionsremain manageable, then these compounds rep-resent a true expansion of our antifungalarsenal.



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history of development of azole derivates

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