POSACONAZOLE: Pharmacology and drug interactions

Dr Helen Sambatakou, Research Fellow, Wythenshawe University Hospital, Manchester, UK and Senior Registrar in the Infectious Diseases Unit of the General Hospital “G Gennimatas”, Athens, Greece
E-mail:hsambata@fs1.with.man.ac.uk

Systemic fungal infections have been recognised as a major cause of morbidity and mortality during the last two decades. Moreover, the treatment of serious fungal infections, especially aspergillosis, is still far from satisfactory and although improving, remains poor. Established azole antifungals have several limitations in terms of potency, spectrum, inherent or acquired resistance, pharmacokinetic profile and drug-drug interactions. The diversity of underlying immunocompromised factors of people affected with fungal infections (premature birth, congenital immune deficiencies, such as chronic granulomatous disease, surgery, diabetes, HIV infection, transplantation), as well as the complexity of various treatment regimens these populations receive (chemotherapy, anti-rejection drugs, antiretrovirals etc) make the use of antifungals in these medically complex patients challenging. Several new antifungals, including novel compounds of familiar classes, as well as new classes with new targets of mechanisms are in various stages of development.

Posaconazole [ SCH 56592, Schering-Plough Research Institute (SPRI), Kenilworth, New Jersey, USA] is a broad spectrum third generation triazole with some structural similarities to ITZ (fig 1). It is in the late stages of clinical development (in phase III clinical studies). It has extremely promising potency for many fungi including Aspergillus spp. and a broad spectrum of activity based on accumulated in vitro and murine model data. Posaconazole (POZ) is under investigation for oral administration, and close analogues may allow IV administration in the future.

Pharmacological determinants of response in experimental models

The pharmacokinetic profile of POZ has been studied in murine models and humans, and appears to be favourable, characterised by long terminal half-lives and large volumes of distribution . Dose-related increases in the serum concentrations and excellent bioavailability in several animal species including mice and humans  have been demonstrated. Negligible amounts of unmetabolised drug are found in the urine, suggesting that the major route of elimination is metabolic (SPRI ). Although both Cmax and area under the concentration-time curve (AUC) were found to be increased in the majority of pharmacokinetic studies, preliminary pharmacodynamic analysis suggested that AUC is probably the most important factor in determining the efficacy of POZ in experimental infection models ( Miller et al.,1996).

POZ has been shown effective in several immunocompromised and immunocompetent murine models, as treatment or prophylaxis, of disseminated and pulmonary aspergillosis (Oakley et al.,1997, Graybill et al.,1998) acute pulmonary histoplasmosis (Connolly et al.,1999), invasive candidiasis (Cacciapuoti et al.,2000) and systemic  cryptococcosis (Barchiesi et al.,2001, Perfect et al.,1996), coccidioidomycosis (Lutz et al.,1997), disseminated fusariosis (Lozano-Chiu et al.,1999), as well as in acute and chronic Chagas disease (Molina et al.,2000, Urbina et al.,1998), and cutaneous and visceral leishmaniasis (Al-Abdely et al.,1999). Efficacy was assessed on the basis of animal survivors and/or by reduction of the fungal burden.

Some  pharmacokinetic data in animal models is mentioned below from studies where a correlation of pharmacokinetic parameters with response is established and no studies where simply dose-dependent effect is reported.

Nomeir et al (2000) provide extensive pharmacokinetic data of POZ in various animal species.  Pharmacokinetic parameters were determined in mice, rats, rabbits, dogs, and cynomolgus monkeys following IV administration as hydroxypropyl-b- cyclodextrin solution (CD) and orally in a methylcellulose (MC) formulation over a broad range of doses. The rabbits received POZ only p.o. in MC suspension. Three different studies were carried out to evaluate the effect of oral dose on serum drug concentrations: An initial study was carried out with dogs to evaluate the effect of food on oral bioavailability of POZ following a single 10mg/kg dose. The results showed that food significantly improved the bioavailability of POZ, with an increase in both the Cmax and AUC by ~ fourfold in fed  compared to  fasting dogs, while the serum concentration-time profile was not affected by food. Consequently, another study was carried out in fed dogs to evaluate the effect of dose on concentrations in serum while a third study was performed to investigate the pharmacokinetics of POZ in fed dogs following multiple dosing. Results indicated that POZ was orally bioavailable in all species and rapidly distributed following PO administration . The oral bioavailability was higher with the CD solution than with MC suspension, with a ‘species specificity’ grade of absorption. Dose-related increases in serum concentrations were observed in mice (dose range 20-160mg/kg), rats, and fed dogs (10-120mg/kg) following a single oral dose of the MC suspension. In all species, both the Cmax and AUC were linear up to a dose of 40mg/kg. However, as the dose increased, the increase in the Cmax was less than that in the AUC, suggesting that absorption is slowed at higher doses.  The elimination half- life in dogs and monkeys following IV administration was relatively long at 15 and 23h, respectively, and concentrations above the MICs and MFCs were observed at 24h following a single oral dose in all five species studied, suggesting that once-daily dosing in man should provide adequate serum concentrations. In rabbits, the oral half-life was 9h. Additionally, following daily administration of 40mg/kg/d for 8 consecutive days (MC suspension) to fed dogs, serum concentrations were higher than those following a single dose, indicating significant accumulation (~three-fold) upon multiple dosing. However, the AUCo, following a single dose was similar to the AUCo-24 following multiple doses (105mcg.h/ml versus 107mcg.h/ml respectively) indicating no untoward accumulation upon multiple dosing. Steady state serum concentrations were achieved by day 8, since concentrations on days 6, 7, and 9 were similar.

Despite interspecies variability in pharmacokinetic parameters, concentrations above the MICs and MFCs for most organisms were determined at 24h following a single oral dose in MC suspension in all five species studied (20mg/kg for mice, rats and rabbits and 10mg/kg for dogs and monkeys), suggesting that once-daily administration should be an effective dosage regimen.

Similar results were obtained in a persistently neutropenic rabbit model (Petraitiene et al.,2000) of  invasive aspergillosis in terms of treatment and prevention. In this study antifungal efficacy, safety, and pharmacokinetics  of POZ at 2, 6, and 20mg/kg orally, were investigated in comparison with untreated animals and equal dosages of orally administered CD ITZ or 1mg/kg i.v.Amphotericin B (AMB). The antifungal response was observed across all outcome parameters,including microbiologic burden, organism-mediated pulmonary injury, and survival. POZ demonstrated potent dosage-dependent antifungal activity,with a significant decrease in clearance and increase in the elimination half-life with increasing dosages after multiple dosing for more than 10 days. The potency of POZ at a dose of 6mg/kg was comparable to those of i.v. administered AMB and superior to that of orally administered ITZ at equal dosages, in all outcome variables. POZ was well absorbed and achieved concentrations in plasma of 1microg/ml , which exceeded the MIC  for A. fumigatus throughout the 24h dosing interval with all dosages used. Moreover, the relatively long half-life in plasma of 7 to 10h also maintains existing plasma POZ levels well above the MICs for A. fumigatus. As prophylaxis, POZ showed a dose dependent microbiological clearance of A. fumigatus from lung tissue, using the same doses  as for treatment, mentioned above (Fig 2). Significant greater survival was achieved in rabbits treated with POZ than animals treated with ITZ or untreated controls.

Connolly et al (1999) demonstrated, in a pulmonary model of histoplasmosis in mice , that POZ was more effective than ITZ and the activity of POZ was similar to that of AMB for sterilisation of the lungs and spleens, whereas the in vitro activity of POZ was greater compared to AMB against H. capsulatum. The MIC90 of the clinical isolates used in this experimental study for POZ was 0.019mcg/ml compared to 0.5mcg/ml for AMB and < or =0.019mcg/ml for ITZ. POZ was as effective as AMB in preventing death following exposure to a lethal inoculum. All mice treated with 5,1 and 0.25mg/kg/d of POZ or with 2.5mg every other day of AMB survived over a 29-days period. Both drugs were equally effective at lowering fungal burden during the course of a sublethal infection following exposure to an inoculum of 10⁴ organisms. The ability of both regimens to reduce fungal burdens was dosage-dependent. POZ was considerably more effective than ITZ for the treatment of experimental histoplasmosis, although the MIC90s for ITZ and POZ were similar. POZ at a dose of 0.25mg/kg/d totally prevented mortality following lethal exposure; compared with approximately half of the animals treated with ITZ at 5mg/kg/d. POZ also was at least 10 times more effective than ITZ in reducing fungal burdens in lung and spleen tissues following exposure to a lower inoculum.  Serum drug concentrations were measured, by both bioassay and HPLC, 3 and 24h after the last dose in several groups of mice, each treated for 7 days with POZ (10 and 1mg/kg/d) and ITZ (75 and 10mg/kg/d). ITZ at 75 mg/kg/d produced the highest mean peak serum drug concentration (22.53mcg/ml and 7.53mcg/ml by bioassay and HPLC respectively), but levels were not sustained throughout the 24h dose interval, resulting in undetectable levels by either method at 24h. POZ at 10mg produced higher peak concentrations than ITZ at the same dose. POZ given at the low dose of 1mg/kg/d produced detectable peak serum concentrations, 0.54mcg/ml and 0.70mcg/ml by bioassay and HPLC respectively, but was still detectable at 24h only by HPLC (0.023mcg/ml). The investigators suggest that improved efficacy may have been demonstrated if ITZ had been administered twice daily as POZ at 10mg/kg could still be detected 24h following the last gavage treatment, whereas ITZ at 10mg/kg/d had undetectable levels at 24h, by both methods.

Basic pharmacology and pharmacokinetics of POZ in humans

The pharmacokinetic profile of orally administered POZ has been evaluated in Phase I escalating single and multiple dose studies over a broad range of dose, including mostly healthy volunteers. The mean maximum plasma concentration was dose proportional from 50-800mg for single doses of POZ; dose-related increases in absorption occurred with multiple dosing from 100-800mg/d, given twice daily for 14 days. The mean terminal phase half-life of POZ was ~25h, dose-dependent and there was extensive tissue distribution of 343 to 486L. Steady state POZ plasma concentrations were reached within 7-10 days (SPRI, data on file).

The main pharmacokinetics parameters of the commercially available oral formulation in adults are as follows: Tmax: 4-6h; T1/2: 19.2-31h; Volume of distribution (Vd): 4.9-6.9L/kg; clearance: 10.3-13.9L/h (Anonymous,Drugs R&D 1999). The mean half-life increase of the drug was dose-dependent, suggesting a saturable mechanism of elimination (Laughlin et al.,1997).

The effect of food on the bioavailability of POZ has been evaluated in several studies. There is an approximate fourfold increase in absorption of oral suspension taking with food, especially a high fat meal, suggesting that dose adjustments may be needed for patients with poor food intake. No influence of food was found using tablet formulation. The oral suspension is~35% more bioavailable than the solid dosage form. Split dosing under fasted conditions improved the absorption of POZ. In healthy volunteers a high-fat meal increased the bioavailability of POZ by 40% (Radwanski et al.,1997).

POZ is highly protein bound. Penetration into the CSF is poor, however POZ has demonstrated activity against CNS infection, as with ITZ which is also barely detectable in CSF (Stevens, 2000). The majority of POZ is excreted in the faeces; approximately 10% of the orally administered dose is recovered in the urine (as multiple metabolites).  

In a study in patients with liver insufficiency a minimal effect of hepatic dysfunction on the pharmacokinetic profile of POZ was found. Therefore no dose adjustment is suggested for patients with liver dysfunction, although probably drug monitoring and dosage adjustment is desirable in patients with severe liver disease.

Although there are no data available for the pharmacokinetic disposition of the drug in patients with renal failure, the clearance through the enterohepatic circulation is consistent with minor influence of drug disposition in renal insufficiency. Moreover the low water solubility, high protein binding, high tissue distribution predict minimal effect even in end-stage patients undergoing haemodialysis, since a small fraction of free drug is available for dialysis and lipophilic drugs are not dialysed.

In consideration of the likely use of POZ in the setting of chemotherapy-induced neutropenia and mucositis, and the expected differences compared to healthy volunteers, a pharmacokinetic (PK) study assessing bioavailability in patients undergoing autologous bone marrow transplantation was undertaken. POZ was initiated after chemotherapy with the onset of neutropenia and/or mucositis. Three dose regimens were evaluated: 200mg QD, 400mg QD, and 200mg QID. The PK profile was assessed on day one and on the final day of dosing. Preliminary analysis of the PK profile indicated that on the final day of dosing, the mean Cmax for evaluation of the dose regimens were~250, 320 and 500ng/ml, respectively. The relatively low plasma levels achieved with the first two dose schedules were possibly related to the poor food intake of these patients (SPRI, data on file).

A more water soluble prodrug of POZ, SCH 59884 has been developed which can be administered intravenously and may overcome potential problems of tolerance or bioavailability through enterohepatic circulation. SCH 59884 is a carboxylate  ester of SCH 56592 with gammabutyric acid phosphate. It has no intrinsic antifungal activity. Following IV administration of SCH 59884, the regimen is rapidly dephosphorylated to SCH 207962 which then is hydrolysed to POZ.  SCH 207962 is an active metabolite and its transformation to the final compound POZ takes place primarily in liver and serum (Nomeir et al.,1998).

Overall, the results of pharmacokinetic evaluations of single and multiple doses of POZ, using a broad range of doses have demonstrated that adequate plasma levels can be achieved which will generally exceed the MIC of clinically relevant fungal pathogens, including most of those that are resistant to other azoles.

Drug-drug interactions:

One potential limitation of the azole antifungal drugs, as a class, is the frequency of interactions with concurrently administered drugs, resulting in undesirable clinical consequences. One form of  interaction may have as a result the decrease of azole plasma concentration, through competition in absorption or metabolism. Another kind of azole-drug interaction may lead to occurrence of toxic side effects of the coadministered drug, relating to the ability of azoles to increase plasma concentrations of other drugs, through inhibition of cytochrome P-450 system.

As with other azole antifungals, POZ is an inhibitor of the CYP3A4 enzyme system.  Therefore, the potential exists for interactions with drugs also metabolised by the same system. The magnitude of the interaction depends on the dosage and on the CYP isoform involved in the metabolism of the coadministered drug. Although  azole antifungals are all inhibitors of CYP isoenzymes in vitro, the grade of inhibition is extremely varied  depending on their chemical structure and the CYP isoforms involved.

However, not all drug interactions are of clinical significance.

The PK of POZ has  been evaluated in a limited number of drug interaction studies. In addition several clinically other significant interactions are expected to occur based on the known pathway of metabolism of the class of azoles and the accumulated experience of drug interactions with older azoles, although formal PK studies have not performed, Current recommendations for POZ might be modified in the future when more data will be available.

Numerous pharmacokinetic studies have reported clinically significant interactions during coadministration of the established azoles with several classes of drugs, including benzodiazepines, opioid analgesics, immunosuppressive and anticonvulsants.

Azole antifungal agents impair the clearance of benzodiazepines, since some of them are extensively metabolised by CYP3A4. Ketoconazole (KTZ), ITZ, FLZ, were found to consistently impair the clearance of midazolam ( Backman et al.,1998, Olkkola et al.,1996, Ahonen et al.,1995, Olkkola et al.,1994), triazolam (Varhe et al.,1994, Varhe et al.,1996), and to a lesser degree of alprazolam (Greenblatt et al.,1998), increasing the AUC and resulting in a severe prolongation of sedation. A similar effect is likely with POZ and SPRI suggests coadministration should be avoided until more data is available.

Several case reports and pharmacokinetic studies documenting significant interactions between older azoles and cyclosporine, causing an increase in cyclosporine levels (Sugar et al.,1989, Collignon and Hurley,1989, Kramer et al.,1990, Albengres and Tillement,1992, Gomez et al.,1995) and to a variable extent of tacrolimus (Floren et al.,1997, Osowski et al.,1996, Billaud et al.,1998), depending on the route of administration. Since cyclosporine is metabolised by the CYP3A4 metabolic pathway, which is inhibited by POZ, an increase in cyclosporine levels is predictable and supported by limited clinical data. In an open–label, multiple-dose study conducted in 4 adult heart transplant patients administration of POZ (200mg POZ daily for 10 days) to subjects on a stable dose of cyclosporine led to a reduction in the required dose of  cyclosporine (Statkevich et al.,2001a). Reduction of cyclosporine dose ranged from 14.3-28.6% and dose modification was required for three of the four patients studied. The individual steady-state  cyclosporine clearance values were 16-33% lower on day 10 (POZ and cyclosporine co-administered) compared to day 1 (cyclosporine only). Although the dosage adjustments were considered low, monitoring of cyclosporine levels is recommended (Table I). Tacrolimus should be coadministered with caution, with plasma drug levels and/ or clinical monitoring and dosage adjustment, when required.

Accumulated  data supports clinically significant interactions between phenytoin  and azoles, especially with FLZ, and particularly at dosages>200mg/d (Cadle et al.,1994) ,causing increase in both AUC and trough concentration of  phenytoin, since phenytoin is metabolised mainly by CYP2C9 and CYP2C19 isozymes and  FLZ is a potent inhibitor of their activity. In contrast, KTZ had no significant effect on phenytoin concentrations (Touchette et al.,1992). ITZ was found to cause a clinically insignificant increase of phenytoin AUC, but phenytoin extensively decreased the AUC of ITZ by more than 90% (Ducharme et al.,1995). In an interaction study (C96-201) of POZ with phenytoin, a shift in both POZ and phenytoin concentrations was observed (Table I).

In an open-label, randomised, multiple-dose study, conducted in 36 healthy adult volunteers (male and female), the potential drug interaction of POZ with phenytoin was investigated (Statkevich et al.,2001b). When POZ was given alone, Cmax and AUC (0-24h) on day 10 were ~2-fold higher than those on day 1, and the sready state clearance on day 10 was 30.3 L/h. In the presence of phenytoin, POZ  steady state clearance increased by~90% and the Cmax and AUC of POZ on day 10 were similar to those on day 1. As about phenytoin pharmacokinetics, although Cmax and AUC results indicated no statistically significant differences after single and multiple dose administration of phenytoin in the presence or absence of POZ, the large variability in the values of the above parameters, due to clinically significant increases in exposure of phenytoin in some individuals should be carefully considered in terms of unpredictable effect of POZ in phenytoin levels. Based on these results, at this time co-administration of POZ with phenytoin is not recommended.

Several other drugs, based on the data with older azoles, are expected to be of clinical significance: digoxin-related toxicity during ITZ co-administration (Cone et al.,1996, Mc Clean and Sheehan,1994) and increased degree and duration of anticoagulant effect during FLZ and warfarin concurrent use (Black et al.,1996) have been reported. Other controlled pharmacokinetic studies demonstrated that azoles-mediated inhibition of CYP activity may impair the clearance of cardiovascular drugs, especially calcium antagonist felodipine (Jalava et al.,1997) and the antiarrhythmic quinidine (Kaukonen et al.,1997) and recent case reports revealed azoles-nifedipine interactions (Kremens et al.,1999, Tailor et al.,1996). Theophylline disposition may be affected by POZ, since POZ is an inhibitor of CYP-1A2, which is a major pathway of  theophylline metabolism. Such an influence has not been demonstrated with KTZ and FLZ (Heusner et al.,1987), due to lack of an inhibitory effect of these drugs on CYP-1A2 .

Some corticosteroids are metabolised through cytochrome P450 3A4 isoenzyme and thus when used concurrently with POZ , they might produce an enhanced corticosteroid effect. Therefore, clinical monitoring is warranted.

Since pharmacokinetic data is not available with POZ and the majority of above mentioned drugs, no definite conclusions can be drawn regarding the safety of these drug combinations, but their usage with TDM-guided adjustment of dosage should be advised, until more  information is available.

In a few instances azole-mediated reduction of concurrently used drug clearance was found to be several-fold higher after oral than IV administration . Such  examples are the studies performed in healthy volunteers with coadministration of oral KTZ  with cyclosporine or tacrolimus respectively. In these cases, reduction of clearance of coadministered with KTZ drugs was found to be 3-fold higher after oral than intravenous administration. This interaction suggests a major impairment of CYP3A4 activity at the intestinal rather than at the hepatic level and/or an involvement of intestinal p-glycoprotein. This is an important consideration for POZ as it is currently available in an oral formulation only.

SPRI has performed some interactions studies mentioned below:                                                                                                                                                             

A summary of drug interaction studies is presented in Table I.

Drugs that should not be administered with POZ:

Medications that should have plasma/blood concentrations monitored by therapeutic drug monitoring (TDM) and / or clinically and may require dosage adjustments:

Table I.  Drug-drug interaction studies of  Posaconazole

Concurrently used drug	Class of drug	Effect On   POZ Bio- availability	Effect on coadministered  drug	Comments
Mylanta (C96-120)	Antacid gastric acid buffer	No clinically significant effect	Not determined	Coadministration has no clinically significant effect
Cimetidine (C96-173)	Histamine H2 receptor  antagonist	50% reduction	Not determined	Effects plasma concentration of POZ
Cyclosporine  (C96-247)	Immunosuppressive	Not determined	Increase	Monitor cyclosporine plasma levels
Phenytoin (C96-201)	Anticonvulsant	2-fold reduction	Increase (large interpatient variability)	Effects plasma concentrations of POZ and phenytoin
Rifabutin (I96-207)	Antimycobacterial agent	2-fold reduction	Increase	Effects plasma concentrations of   POZ and rifabutin
Zidovudine Lamivudine (C96-190)	Nucleoside analogues (reverse transcriptase inhibitors)	Not determined	No significant    interaction	Coadministration has no clinically significant effect
Ritonavir Indinavir (C96-190)	Protease inhibitors	Not  determined	No significant increase in Ritonavir	Concurrent use has no clinically significant effect
Dextromethorphan, Caffeine, Tolbutamide, and    Chlorzoxazone	Markers of activity of the P450 isozymes 1A2, 2C8, 2C9, 2D6, 2E1 and 3A4	Not determined	POZ is a hepatic CYP 3A4 inhibitor	Effects plasma concentrations of POZ and drugs metabolised by the CYP 3A4 pathway when coadministered

PHARMACOLOGICAL CONSIDERATIONS  AND FUTURE   PERSPECTIVES IN THE ERA OF NEW ANTIFUNGAL AGENTS

   Selection of an effective antifungal regimen is a complex process that depends on accumulative clinical data and a general knowledge of the in vitro activity and pharmacokinetics of antifungal drugs. Considerable controversies exist about the reliability and clinical utility of several assays used, such as inhibitory-fungicidal activity, measurement of serum levels. Moreover, the development of new antifungals and the emergence of resistant strains make the antifungal management more complicated. Since recently most important pathogenic fungi had predictable antifungal susceptibility and limited therapeutic options. In the era of new antifungal drugs, a better understanding of the overall mechanisms of antifungal drugs and a critical judgement of clinicians who manage people with systemic fungal infections is crucial to avoid much variability in physician practices. In clinical practice, issues concerning determination of optimal dose and duration of antifungal drugs remain unknown, It is uncertain whether the common therapeutic failure of systemic fungal infections is attributable to suboptimal pharmacokinetics, pharmacodynamics, intrinsic or de novo emergence of resistance or to the immunological or genetic status of the host and it is unknown how all these parameters interact, resulting in the final outcome. Some unanswered questions and difficulties of interpretation of pharmacology of POZ and antifungal drugs generally are discussed in a brief overview below:

1)  IN VITRO VERSUS IN VIVO STUDIES:

Although recent attempts for reproducibility of the in vitro tests and clinical relevant interpretative breakpoints, they are still far from the reliable in vitro tests of antimicrobial agents. The antifungal azoles present perhaps the greatest problem in susceptibility testing, with a broad range of variations to MICs  and inoculum dependent results. Wide ranges of reported MICs reflect problems with standardisation and interpretation. There are only few clear correlations of in vitro sensitivity among azoles with in vivo effectiveness, and most of these are in animal models rather than humans. Therefore preclinical evaluation of the in vitro activity of new antifungals is necessary and is also important in some clinical cases in directing therapy.

However, is there a well defined break-point of MIC or MFC or there is a broad range of values, a continuum dependent primarily from the underlying immune status of the host?

How reliable is the determination of PAFE compared to antimicrobial PAE, especially for fungistatic drugs?

The results observed in the experimental study conducted by Petraitiene et al , suggest more potent activity than is reflected by the MICs and MFCs ( MICs and MFCs demonstrate a consistent fourfold difference in in vitro potencies between POZ and ITZ). This greater disparity between the in vitro and in vivo potencies of POZ was also observed by Oakley et al (1997) in a transiently neutropenic murine model of disseminated aspergillosis. Similarly, although in vitro studies showed that POZ had modest activity, inferior to amphotericin B but substantially better than FLZ against leishmanial promastigotes, this in vitro activity probably may not correlate with its activity against intracellular amastigotes, since in vivo studies (Al-Abdely et al., 1999) demonstrated that POZ was efficacious in the treatment of experimental cutaneous leishmaniasis. Moreover, in Connolly et al study POZ demonstrated greater in vitro activity against H. capsulatum than did AMB and similar with ITZ; however both POZ and AMB were equally effective at lowering tissue fungal burden and preventing death whereas POZ was considerably more effective than ITZ for the treatment of experimental histoplasmosis.

Such discrepancies are not surprising since have been reported repeatedly between in vitro and in vivo activity by using animals models, especially for azole antifungal agents. Among the possible explanations for this difference are tissue penetration, rates of microbicidal activity, and nonlinear pharmacodynamics . .Moreover it has been suggested that the superior intrinsic antifungal and anti-protozoal activity of POZ could probably be associated with the higher affinity of this triazole to its biochemical target, cytochrome P-450-dependent C14 sterol demethylase (Urbina et al.,1998).

2) ANIMAL MODELS

Animal models allow estimation of many variables, like absorption, distribution, clearance, drug interactions, toxicity, reproducing host immune defects and systemic infections ,which in vitro studies do not allow

Despite many inherent problems, experimental infections in animals may provide useful data for antifungal therapy, but the  extrapolation from animal to human disease is a problematic issue. Variables such as pharmacokinetics and drug metabolism differ not only between animals and humans, but also among different animals used in murine models.

Moreover, direct comparison from one study to another is unrealistic since different dosage schedules, different methods producing infection, in terms of route and timing of administration, are used.

In the Nomeir study, species specificity revealed broad ranges of pharmacokinetics parameters, such as bioavailability (100%in mice,52%in monkeys for CD solution, and 47%,14%for MC suspension respectively)while the iv terminal-phase half lives were 7h in mice and rats, while in dogs and monkeys 15 and 23h respectively.

In addition to species specificity of animal  models, genetic variability among fungal isolates used in vivo experiment make difficult the intepretation  and generalisation of results in the clinical practice. In an experimental  study of pulmonary mouse aspergillosis  conducted by Cacciapuoti et al (2000), POZ appeared to be more effective in clearing the infection from the lungs of mice infected with A. fumigatus (65%) than those of mice infected with A.flavus (3%) at 25 and 10mg/kg (P<0.01). This differential effect against A. fumigatus and A. flavus was also evident at the lower doses.

Even pharmacokinetic studies in humans, usually are performed in healthy volunteers, for  a short period of time and as monotherapy, which does not reflect the situation in the immunocompromised host with the long term administration of complex  treatment. 

3) ORAL ANTIFUNGALS IN SYSTEMIC FUNGAL INFECTIONS: DIFFICULTIES IN INTEPRETATION OF PHARMACOLOGICAL ISSUES

It is very interesting that preclinical  and early stages clinical studies suggest great efficacy of orally administered new antifungals, as POZ. Oral formulations are cost effective and make possible the outpatients clinic management of stable cases.

Since POZ is a lipophilic regimen, poor absorption should be expected. However animal and human studies performed demonstrate high bioavailability. Moreover, the influence of food in the absorption of POZ probably reflects the need for an individualised approach in the majority of immunocompromised patients, since malnutrition, chemotherapy related nausea are often reported in critically ill patients. Another important issue that must be considered is about enterohepatic circulation impact on the pharmacokinetic disposition of an orally administered drug. The biotransformation of a drug is underwent by liver P450 enzyme system, but in a lesser extent in the enterocytes by both intestinal P450 enzyme system and  p-gp glycoprotein. The latter is a protein which acts as a “drug efflux transporter” and is a product of the mdr 1 gene and initially had been reported as the aetiologic factor in emergence of resistance in chemotherapeutic agents, but recently increased expression of p-gp has been correlated with the therapeutic failure of many classes of drugs, such as antiretrovirals, antiepileptics. Antagonists of p-gp have been used to overcome the emergence of resistance.

There is an overlapping in the function of P450 enzyme system and the p-gp in the intestine and there seems to  have a similar specificity, although their exact mechanism of action is unclear. Inhibition of both increase the bioavailability of a drug, as a result of reduction in “first pass metabolism”. Genetic diversity of both is well established and although there is no data at this time for POZ, variations in bioavailability due to  interpatient genetic diversity, probably will be observed as accumulated clinical data  and results of ongoing clinical studies become available.

In addition to interpatient variability, intrapatient differences in bioavailability should be expected with oral treatment of certain patients at different times evaluated due to several variables which change through the time; diet (quality, quantity of food taken), stage of underlying disease, concurrent medications. Whether the consequences of these inevitable variations can be overcome by the in vitro potency and other pharmacokinetic and pharmacodynamic characteristics of a regimen remains to be established.

Besides the potential problems in absorption, compliance is another critical issue, with oral treatment if long-term therapy is required for difficult to treat fungal infections and probably is an unrealistic approach for medically exhausted patients, receiving chemotherapeutics, antiretrovirals and various other regimens.

However, the potent antifungal activity of oral  POZ against invasive pulmonary and disseminated aspergillosis in experimental models is impressive. It is of note that an orally administered antifungal compound would attain this level of potent activity even in profoundly and persistently neutropenic host. That POZ demonstrates fungicidal properties against A. fumigatus at relatively low MICs of 0.125mg/ml that were exceeded in all dosage cohorts used may explain, at least in part this level of activity

4) SERUM ANTIFUNGAL LEVELS AND PHARMACOKINETIC PARAMETERS:IS THERE ANY CORRELATION WITH THE CLINICAL OUTCOME?

Several reports suggest a correlation between breakthrough infections and clinical failure when suboptimal levels of ITZ or FLZ are achieved. In contrast, for ketoconazole no correlation of blood levels with clinical efficacy in either animal models or humans has been found. In terms of toxicity, for Amphotericin B there is a well established relationship between serum levels and emergence of toxicity or clinical outcome, minimizing the value for routine monitoring measurement of drug levels, but this is not so clear for other antifungals, such as ITZ. Therefore the correlation between  serum levels of antifungal drugs and clinical outcome is not so clear.

In Petraitiene and Connolly  studies, the cyclodextrin solution of ITZ was selected for comparison with POZ  in order to optimise oral bioavailability. ITZ in a dose of 6mg/kg achieved concentrations in plasma above the MIC throughout the dosing interval. However, despite similar AUCs and plasma concentration-time curves of POZ and CD ITZ in Petraitiene study (Fig 3), differences in efficacy were observed with superiority of POZ. Among the possible explanations for this disparity are differences in  tissue distribution, in vitro potency, pharmacokinetics, pharmacodynamics.

Similarly, in Conolly et al study of experimental  histoplasmosis, POZ was at least 10 times more effective than CD ITZ in reducing fungal burdens in lung and spleen tissues, although the serum concentrations were almost the same (somewhat lower ITZ levels).The rapid clearance of ITZ was suggested to be one of the reasons for the inferior activity of ITZ  compared to POZ in this study. This disparity in antifungal efficacy  may be explained in part by differences in their potencies as well .However, MICs of POZ and ITZ for H.capsulatum were reported to be similar in this study. Time-kill assays may prove to be helpful in understanding the differences in rates of kill of A. fumigatus.

Based on a dose-dependent pharmacokinetic, variability in absorption, potential drug-drug interactions of established and new oral azole antifungals, is reasonable to propose TDM under several conditions or in a routine basis?

Is individualisation and optimisation of dose in the era of new oral azole antifungal agents  a crucial issue?

Many concerns arise in terms of TDM:

However despite many difficulties for interpretation of in vitro and in vivo studies, overall, POZ demonstrates potent broad-spectrum antifungal activity in vitro which translates to good efficacy in the laboratory models of fungal infections. The pharmacokinetics and pharmacodynamics predict that this oral regimen should be sufficient to treat serious fungal infections with once-daily dosing. This encouranging profile supports the continued development of POZ and further evaluation in multicenter clinical studies.

Acknowledgments I thank Professor Alan Sugar and Dr David Denning for their review of   this article.

 REFERENCES