Clinical pharmacodynamics of antifungals

A Randomized, Double-blind Study of Amorolfine 5% Nail Lacquer with Oral Fluconazole Compared with Oral Fluconazole Alone in the Treatment of Fingernail Onychomycosis

Background

It is a challenge to treat onychomycosis due to frequent treatment failures and relapses. Systemic and topical therapies need to be combined to improve cure rates. Antifungal susceptibility might play a role in the treatment resistance of onychomycosis.

Aims

To compare the safety and effectiveness of amorolfine 5% nail lacquer + oral fluconazole versus only oral fluconazole in the treatment of fingernail onychomycosis.

Methodology

In this double-blind trial (CTRI/2015/02/005369), patients were randomized (1:1) into amorolfine 5% nail lacquer + fluconazole and dummy lacquer + fluconazole. Treatment was given for 3 months with monthly follow-ups. Antifungal sensitivity was carried out for Candida. Effectiveness was assessed by reduction in the number and percentage area of nails involved and mycological cure. At the end of 3-month treatment period, the association between drug sensitivity and treatment response was explored for the Candida infections.

Results

Among 30 study participants, the combination group showed significantly lower number of nail involvement (P = 0.004) and percentage nail involvement (P = 0.005) than only fluconazole group. Pretreatment fungal culture showed a comparable number of dermatophytes, Candida, Aspergillus in both the groups. Sensitivity testing was done for the isolated Candida species. Antifungal sensitivity for Candida (n = 11) was tested, and 8 (72.7%) of the organisms were sensitive to fluconazole (minimum inhibitory concentration [MIC] 1.25 ± 1.19 μg/ml), 100% were sensitive to itraconazole (MIC 0.0726 ± 0.021 μg/ml), and 3 (27.3%) were susceptible-dose dependent (S-DD) to fluconazole (MIC 16 μg/ml). Fluconazole only group patients with Candida who showed resistance to fluconazole did not respond to therapy; however, patients in the combination group showed moderate improvement (reduction in area involvement = 55.56 ± 35.36%).

Conclusion

The combination of amorolfine/fluconazole achieved a higher cure rate not only for sensitive fungus but also for those which were S-DD to fluconazole.

Antifungal pharmacokinetics and pharmacodynamics

Successful treatment of infectious diseases requires choice of the most suitable antimicrobial agent, comprising consideration of drug pharmacokinetics (PK), including penetration into infection site, pathogen susceptibility, optimal route of drug administration, drug dose, frequency of administration, duration of therapy, and drug toxicity. Antimicrobial pharmacokinetic/pharmacodynamic (PK/PD) studies consider these variables and have been useful in drug development, optimizing dosing regimens, determining susceptibility breakpoints, and limiting toxicity of antifungal therapy. Here the concepts of antifungal PK/PD studies are reviewed, with emphasis on methodology and application. The initial sections of this review focus on principles and methodology. Then the pharmacodynamics of each major antifungal drug class (polyenes, flucytosine, azoles, and echinocandins) is discussed. Finally, the review discusses novel areas of pharmacodynamic investigation in the study and application of combination therapy.

Treatment of Aspergillus terreus infections: a clinical problem not yet resolved

Despite the use of recommended therapies, invasive infections by Aspergillus terreus show a poor response. For years, investigative studies on the failure of therapy of fungal infections have focused on in vitro susceptibility data. However, it is well known that low minimum inhibitory concentrations (MICs) are not always predictive of response to therapy despite a correct dosage schedule. Many experimental and clinical studies have tried to establish a relationship between MICs and outcome in serious fungal infections but have come to contradictory and even surprising conclusions. The success or failure of treatment is determined by many factors, including the in vitro susceptibility of the causative fungal isolate, the pharmacokinetics/pharmacodynamics of the drug used for treatment, pharmacokinetic variability in the population, and the underlying disease that patients suffer. To try to understand this poor response to treatment, available data on the in vitro susceptibility of A. terreus, the experimental and clinical response to amphotericin B, triazoles and echinocandins, and the pharmacokinetics/pharmacodynamics of these antifungals have been reviewed. Of special interest are the fungistatic activites of these drugs against A. terreus and the high interpatient variability of serum drug levels observed in therapy based on triazoles, which make monitoring of infected patients necessary.

Using pharmacokinetics and pharmacodynamics to optimise dosing of antifungal agents in critically ill patients: a systematic review

The prevalence of invasive fungal infections (IFIs) caused by Candida spp. is increasing in critically ill patients. Recent development of new antifungal agents has significantly contributed to the successful treatment of IFIs. However, the pharmacokinetics of antifungal agents can be altered in a number of disease states, including critical illness. Therefore, doses established in healthy volunteers and other patient groups may not be appropriate for the critically ill. Moreover, inadequate dosing may contribute to treatment failure and the emergence of resistance. This systematic review provides a critical analysis of the pharmacokinetics of antifungal agents in the critically ill and their relevance to dosing requirements in clinical practice. Based on the limited data available, dosing of some antifungal agents may have to be adjusted in critically ill patients with conserved renal function as well as in those requiring renal replacement therapy. Further research to confirm the appropriateness of current dosing strategies to attain the appropriate pharmacodynamic targets is recommended.

Current concepts in antifungal pharmacology

The introduction of new antifungal agents (eg, echinocandins, second-generation triazoles) in the past decade has transformed the management of invasive mycoses to the point that drug toxicity is no longer the major limiting factor in treatment. Yet, many of these newer antifungal agents have important limitations in their spectrum of activity, pharmacokinetics, and unique predisposition for pharmacokinetic drug-drug interactions and unusual toxicities associated with long-term use. This article reviews key pharmacological aspects of systemic antifungal agents as well as evolving strategies, such as pharmacokinetic-pharmacodynamic optimization and therapeutic drug monitoring, to improve the safety and efficacy of systemic antifungal therapy.

Multilaboratory testing of two-drug combinations of antifungals against Candida albicans, Candida glabrata, and Candida parapsilosis

There are few multilaboratory studies of antifungal combination testing to suggest a format for use in clinical laboratories. In the present study, eight laboratories tested quality control (QC) strain Candida parapsilosis ATCC 22019 and clinical isolates Candida albicans 20533.043, C. albicans 20464.007, Candida glabrata 20205.075, and C. parapsilosis 20580.070. The clinical isolates had relatively high azole and echinocandin MICs. A modified CLSI M27-A3 protocol was used, with 96-well custom-made plates containing checkerboard pairwise combinations of amphotericin B (AMB), anidulafungin (AND), caspofungin (CSP), micafungin (MCF), posaconazole (PSC), and voriconazole (VRC). The endpoints were scored visually and on a spectrophotometer or enzyme-linked immunosorbent assay (ELISA) reader for 50% growth reduction (50% inhibitory concentration [IC(50)]). Combination IC(50)s were used to calculate summation fractional inhibitory concentration indices (FICIs) (ΣFIC) based on the Lowe additivity formula. The results revealed that the IC(50)s of all drug combinations were lower or equal to the IC(50) of individual drugs in the combination. A majority of the ΣFIC values were indifferent (ΣFIC = 0.51 to 2.0), but no antagonism was observed (ΣFIC ≥ 4). Synergistic combinations (ΣFIC ≤ 0.5) were found for AMB-PSC against C. glabrata and for AMB-AND and AMB-CSP against C. parapsilosis by both visual and spectrophotometric readings. Additional synergistic interactions were revealed by either of the two endpoints for AMB-AND, AMB-CSP, AMB-MCF, AMB-PSC, AMB-VRC, AND-PSC, CSP-MCF, and CSP-PSC. The percent agreements among participating laboratories ranged from 37.5% (lowest) for AND-CSP and POS-VOR to 87.5% (highest) for AMB-MCF and AND-CSP. Median ΣFIC values showed a wide dispersion, and interlaboratory agreements were less than 85% in most instances. Additional studies are needed to improve the interlaboratory reproducibility of antifungal combination testing.

Animal models: an important tool in mycology

Animal models of fungal infections are, and will remain, a key tool in the advancement of the medical mycology. Many different types of animal models of fungal infection have been developed, with murine models the most frequently used, for studies of pathogenesis, virulence, immunology, diagnosis, and therapy. The ability to control numerous variables in performing the model allows us to mimic human disease states and quantitatively monitor the course of the disease. However, no single model can answer all questions and different animal species or different routes of infection can show somewhat different results. Thus, the choice of which animal model to use must be made carefully, addressing issues of the type of human disease to mimic, the parameters to follow and collection of the appropriate data to answer those questions being asked. This review addresses a variety of uses for animal models in medical mycology. It focuses on the most clinically important diseases affecting humans and cites various examples of the different types of studies that have been performed. Overall, animal models of fungal infection will continue to be valuable tools in addressing questions concerning fungal infections and contribute to our deeper understanding of how these infections occur, progress and can be controlled and eliminated.

Pharmacokinetic/pharmacodynamic profile of voriconazole

Voriconazole is the first available second-generation triazole with potent activity against a broad spectrum of clinically significant fungal pathogens, including Aspergillus,Candida, Cryptococcus neoformans, and some less common moulds. Voriconazole is rapidly absorbed within 2 hours after oral administration and the oral bioavailability is over 90%, thus allowing switching between oral and intravenous formulations when clinically appropriate. Voriconazole shows nonlinear pharmacokinetics due to its capacity-limited elimination, and its pharmacokinetics are therefore dependent upon the administered dose. With increasing dose, voriconazole shows a superproportional increase in area under the plasma concentration-time curve (AUC). In doses used in children (age < 12 years) voriconazole pharmacokinetics appear to be linear. Steady-state plasma concentrations are reached approximately 5 days after both intravenous and oral administration; however, steady state is reached within 24 hours with voriconazole administered as an intravenous loading dose. The volume of distribution of voriconazole is 2-4.6 L/kg, suggesting extensive distribution into extracellular and intracellular compartments. Voriconazole was measured in tissue samples of brain, liver, kidney, heart, lung as well as cerebrospinal fluid. The plasma protein binding is about 60% and independent of dose or plasma concentrations. Clearance is hepatic via N-oxidation by the hepatic cytochrome P450 (CYP) isoenzymes, CYP2C19, CYP2C9 and CYP3A4. The elimination half-life of voriconazole is approximately 6 hours, and approximately 80% of the total dose is recovered in the urine, almost completely as metabolites. As with other azole drugs, the potential for drug interactions is considerable. Voriconazole shows time-dependent fungistatic activity against Candida species and time-dependent slow fungicidal activity against Aspergillus species. A short post-antifungal effect of voriconazole is evident only for Aspergillus species. The predictive pharmacokinetic/pharmacodynamic parameter for voriconazole treatment efficacy in Candida infections is the free drug AUC from 0 to 24 hour : minimum inhibitory concentration ratio.

Les relations pharmacocinétique-pharmacodynamique des antifongiques. Conséquences pour le suivi thérapeutique

Despite pharmacokinetic-pharmacodynamic relationships were clearly evidenced for antifungal drugs by the use of experimental models, few target plasma concentrations could be determined from studies performed in patients. The main causes explaining this lack of data are reviewed and the possible use in humans of the parameters obtained from animal models is discussed.

Interpretive breakpoints for fluconazole and Candida revisited: a blueprint for the future of antifungal susceptibility testing

Developing interpretive breakpoints for any given organism-drug combination requires integration of the MIC distribution, pharmacokinetic and pharmacodynamic parameters, and the relationship between in vitro activity and outcome from both in vivo and clinical studies. Previously, the Subcommittee for Antifungal Testing of the Clinical and Laboratory Standards Institute (CLSI [formerly National Committee for Clinical Laboratory Standards]) proposed MIC interpretive breakpoints for fluconazole and Candida spp. These breakpoints were considered to be somewhat weak, because the clinical data supporting them came largely from mucosal infections and there were very few infections involving strains with elevated fluconazole MICs. We readdress the issue of fluconazole breakpoints for Candida by using published clinical and microbiologic data to provide further validation of the breakpoints proposed by the CLSI in 1997. We also address interpretive breakpoints for agar disk diffusion testing of fluconazole. The MIC distribution for fluconazole was determined with a collection of 13,338 clinical isolates. The overall MIC at which 90% of the isolates were inhibited was 8 microg/ml: 91% were susceptible (S) at a MIC of or= 64 microg/ml). Similar results were obtained for 2,190 isolates from randomized clinical trials. Analysis of available data for 1,295 patient-episode-isolate events (692 represented mucosal infections and 603 represented invasive infections) from 12 published clinical studies demonstrated an overall success rate of 77%, including 85% for those episodes in which the fluconazole MIC was or= 64 microg/ml) isolates. Pharmacodynamic analysis demonstrated a strong relationship between MIC, fluconazole dose, and outcome. A dose/MIC ratio of approximately 25 was supportive of the following susceptibility breakpoints for fluconazole and Candida spp.: S, MIC or= 64 microg/ml. The corresponding disk test breakpoints are as follows: S, >or=19 mm; SDD, 15 to 18 mm; R, Collapse

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MESH Headings

• Antifungal Agents/administration & dosage
• Antifungal Agents/pharmacology
• Antifungal Agents/therapeutic use
• Candida/drug effects
• Candidiasis/drug therapy
• Clinical Trials as Topic
• Dose-Response Relationship, Drug
• Drug Resistance, Fungal
• Fluconazole/pharmacokinetics
• Fluconazole/pharmacology
• Fluconazole/therapeutic use
• Humans
• Microbial Sensitivity Tests/standards
• Treatment Outcome

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Affiliation(s)

M A Pfaller Department of Pathology, Medical Microbiology Division, C606 GH, University of Iowa College of Medicine, Iowa City, 52242, USA.
D J Diekema
D J Sheehan

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In vitro pharmacodynamics of rapid versus continuous infusion of amphotericin B deoxycholate against Candida species in the presence of human serum albumin

BACKGROUND

Recent open label studies have suggested that dosing amphotericin B (AMB) by continuous infusion (CI) may reduce drug-associated infusion reactions and nephrotoxicity. In vitro and in vivo pharmacodynamic (PD) data, however, do not consistently support the concept of CI dosing based on the concentration-dependent activity of this agent and in vitro studies with AMB rarely account for the drug's high degree of protein binding. Therefore, we compared the PD activity of simulated continuous versus rapid infusion strategies of AMB in killing of AMB-susceptible and -resistant Candida species using an in vitro pharmacodynamic model.

METHODS

Time-kill curves were performed with Candida albicans (Etest MIC 0.38 mg/L) and Candida lusitaniae (MIC 1.5 mg/L) at AMB concentrations between 0 and 16 mg/L in the absence and presence of 4 and 8% human serum albumin (HSA). A one-compartment in vitro pharmacodynamic model was used to simulate the steady-state PK parameters of bolus and CI AMB.

RESULTS

The fungicidal activity of AMB was attenuated by the presence of HSA for both Candida species tested. The EC50 for each isolate significantly increased in the presence of 4% HSA (P<0.05), and fungicidal activity was completely abated for C. lusitaniae when HSA concentrations were increased to 8%. No substantial differences in the rate or extent of AMB killing were observed between rapid infusion or CI dosing and neither regimen produced fungicidal activity in the presence of HSA.

CONCLUSIONS

The presence of HSA changes the in vitro PD of AMB. In our model, CI and rapid infusion dosing of AMB exhibited similar activity when attempts were made to correct for protein binding that is likely to occur in vivo.

Pharmacokinetics/pharmacodynamics of echinocandins

The novel class of echinocandins represents a milestone in antifungal drug research that has further expanded our therapeutic options. The favorable pharmacokinetic profile of the echinocandins has been elucidated in animal and human studies. The echinocandins are targeted for once-daily dosing and are not metabolized through the cytochrome P450 enzyme system, and they are generally well tolerated due to lack of mechanism-based toxicity. Little is known, however, about the disposition of these compounds in tissues and body fluids and the relationships between dosage, concentrations in the body, and antifungal efficacy in vivo. Many unanswered questions remain, including the importance of the high protein binding and the concentrations of free antifungal agents at target sites. Although recent attempts have been made to ensure the reproducibility of in vitro tests, the clinical usefulness of these tests is still unreliable and their relevance remains controversial. In vitro activity must be correlated with achievable concentrations at the site of infection. As little is known about the relationship between the pharmacokinetics and the pharmacodynamics of the echinocandins, increased incorporation of these principles in experimental and clinical studies is an important objective that will benefit the treatment and prophylaxis of life-threatening invasive fungal infections in immunocompromised patients.