Pharmacodynamics of Isavuconazole in a Dynamic In Vitro Model of Invasive Pulmonary Aspergillosis

Therapeutic Drug Monitoring and Biomarkers; towards Better Dosing of Antimicrobial Therapy

Due to variability in pharmacokinetics and pharmacodynamics, clinical outcomes of antimicrobial drug therapy vary between patients. As such, personalised medication management, considering both pharmacokinetics and pharmacodynamics, is a growing concept of interest in the field of infectious diseases. Therapeutic drug monitoring is used to adjust and individualise drug regimens until predefined pharmacokinetic exposure targets are achieved. Minimum inhibitory concentration (drug susceptibility) is the best available pharmacodynamic parameter but is associated with many limitations. Identification of other pharmacodynamic parameters is necessary. Repurposing diagnostic biomarkers as pharmacodynamic parameters to evaluate treatment response is attractive. When combined with therapeutic drug monitoring, it could facilitate making more informed dosing decisions. We believe the approach has potential and justifies further research.

Population Pharmacokinetics, Exposure-Safety, and Probability of Target Attainment Analyses for Isavuconazole in Japanese Patients With Deep-Seated Mycoses

Isavuconazonium sulfate is the water-soluble prodrug of the novel, broad-spectrum, triazole antifungal agent isavuconazole. Its pharmacokinetics (PK) and exposure-response relationship have been well investigated, but not in a Japanese patient population. The objectives of this analysis were to (1) develop a population PK model for Japanese patients with deep-seated mycoses and healthy subjects, and to identify significant covariates; (2) determine the probability of PK-pharmacodynamic (PK-PD) target attainment in Japanese patients by a clinical dosing regimen; and (3) evaluate the exposure-safety relationship of isavuconazole in Japanese patients. Data from 2 phase 1 studies and 1 phase 3 study in Japanese patients were pooled to develop the population PK model using NONMEM. The PK of isavuconazole in Japanese patients was best described as a 2-compartment model with a Weibull absorption function and first-order elimination. The identified covariates on clearance were creatinine clearance and lean body mass. The probability of target attainment showed that >90% of simulated Japanese patients would achieve the PK-PD target, an exposure index corresponding to 50% survival of nonneutropenic infected mice, with minimal inhibitory concentration values of ≤1 mg/L according to Clinical and Laboratory Standards Institute methodology and of ≤2 mg/L according to European Committee on Antimicrobial Susceptibility Testing methodology by the clinical dosing regimen. No apparent relationships were found for any of the exposure parameters of isavuconazole with any assessed safety end points in Japanese patients. Taken together, the clinical dosing regimen is appropriate for the treatment of Japanese patients with deep-seated mycoses.

Assessment of Antifungal Pharmacodynamics

Pharmacokinetic-pharmacodynamic (PK-PD) analysis is of central importance to the progress of an antifungal agent into clinical use. It is crucial to ensure that preclinical studies give the best possible prediction of the way drugs are likely to behave in a clinical setting. This review details the last 30 years of progress in terms of disease model design, efficacy outcome selection and translational modelling in antifungal PK-PD studies. The principles of how PK-PD parameters inform current clinical practice are also discussed, including a review of how these apply to existing and novel agents.

NIH4215: A mutation-prone thiamine auxotrophic clinical Aspergillus fumigatus isolate

Aspergillus fumigatus is the main cause of life-threatening invasive aspergillosis. Despite the availability of various antifungals, therapy remains challenging and requires further studies. Accordingly, the clinical A. fumigatus isolate NIH4215 deriving from a fatal case of human pulmonary aspergillosis has frequently been used in drug efficacy studies. Unexpectedly, our initial attempts to generate a bioluminescent reporter of strain NIH4215 for in vivo drug efficacy studies failed, as NIH4215 was unable to grow on defined minimal medium. Subsequent analyses discovered a previously undescribed thiamine auxotrophy of strain NIH4215 and transformation with thiamine biosynthesis genes from A. fumigatus strain Af293 identified the nmt1 gene as cause of the thiamine auxotrophy. Sequencing of the defective nmt1 gene revealed the loss of a cysteine codon within an essential iron-binding motif. Subsequently, the wild-type nmt1 gene was successfully used to generate a bioluminescent reporter strain in NIH4215 by simultaneously deleting the akuB locus. The resulting bioluminescent ΔakuB strains showed a high frequency of homologous integration as confirmed by generation of pyrG and niaD deletion mutants. When tested in a Galleria mellonella infection model, neither thiamine auxotrophy nor the deletion of the akuB locus had a significant effect on virulence. However, besides thiamine auxotrophy, sectors with altered morphology and albino mutants frequently arose on colony edges of strain NIH4215 and its derivatives, and stable albino mutants were successfully isolated. A proposed increased mutation rate of NIH4215 was confirmed by screening for spontaneous occurrence of fluoorotic acid resistant mutants. Independent mutations in the pyrG and pyrE gene were identified in the fluoroorotic acid resistant NIH4215 isolates and the frequency of mutation was by at least one order of magnitude higher than that observed for the clinical A. fumigatus isolate CBS144.89. In summary, despite its virulence in animal models, strain NIH4215 is a thiamine auxotroph and prone to accumulate mutations. Our results suggest that thiamine biosynthesis is dispensable for host infection and mutation-prone strains such as NIH4215 could potentially facilitate the evolution of azole resistant strains as increasingly observed in the environment.

Antifungal Resistance and the Role of New Therapeutic Agents

Purpose of Review

Advances in health care over time have led to an evolution in the epidemiology of invasive fungal infections. There is an increasing concern for antifungal resistance and emergence of less common fungal species for which optimal therapies are not well defined. The purpose of this review is to describe mechanisms of antifungal resistance and to evaluate the modern role of new and investigational antifungals.

Recent Findings

Isavuconazole and ibrexafungerp represent the two newest antifungal agents. Evidence from in vivo and in vitro studies has been published recently to help define their place in therapy and potential roles in treating resistant fungi. Isavuconazole is a broad-spectrum triazole antifungal with evidence to support its use in invasive aspergillosis and mucormycosis. Its utility in treating voriconazole-resistant Candida should be confirmed with susceptibility testing if available. Ibrexafungerp is an oral glucan synthase inhibitor with little cross-resistance among currently available antifungals, including echinocandins. It is a promising new agent for invasive candidiasis, including azole-resistant Candida species, and in combination therapy with voriconazole for aspergillosis. Multiple antifungals, some with novel mechanisms, are in development, including rezafungin, oteseconazole, olorofim, fosmanogepix, and opelconazole.

Summary

Both isavuconazole and ibrexafungerp are welcome additions to the arsenal of antifungals, and the prospect of more antifungal options in the future is encouraging. Such an array of antifungals will be important as antifungal resistance continues to expand alongside evolving medical practices. However, managing resistant fungal infections will grow in complexity as the unique role of each new agent is defined.

Efficacy and Associated Drug Exposures of Isavuconazole and Fluconazole in an Experimental Model of Coccidioidomycosis

Coccidioides spp. are important pathogens in regions where they are endemic, and new treatment options are needed. Here, isavuconazonium sulfate (ISAVUSULF) and fluconazole (FLU) were evaluated in experimental disseminated coccidioidomycosis to characterize drug exposures associated with efficacy. Broth macrodilution was performed on Coccidioides isolates to measure minimal effective concentrations (MEC) and minimal fungicidal concentrations (MFC). Mice were inoculated with Coccidioides posadasii (Silveira strain). Treatment started 4 days postinoculation. In model 1, mice were treated for 19 days, followed by 30 days of off-therapy observation, measuring survival through day 49 and residual fungal burden. Treatments included ISAVUSULF (prodrug; 186, 279, or 372 mg/kg twice daily), FLU (20 or 100 mg/kg once daily), and no treatment. Model 2 included 7-day treatment with ISAVUSULF (prodrug; 74.4, 111.6, or 148.8 mg/kg twice daily), FLU (20 or 100 mg/kg once daily), and no treatment. Serial plasma and tissues samples were obtained for pharmacokinetics (PK) and fungal burden measurement, respectively. Fifty percent minimal effective concentration (MEC50) values were 0.39 mg/liter (isavuconazole [ISAV]) and 12.5 mg/liter (FLU). Treatment with ISAVUSULF186 or with either FLU dose resulted in higher survival compared to that in the untreated group. Treatment with ISAVUSULF186 or ISAVUSULF279 twice daily or FLU100 reduced fungal burden in all organs (model 1). In model 2, a >1 log10 CFU/organ reduction was demonstrated, with ISAV area under the concentration-time curve (AUC) values achieved with 111.6 mg/kg twice daily (56.8 mg · h/liter) in the spleen and liver. FLU AUC values of 100 and 500 mg·h/liter for 20 and 100 mg/kg doses, respectively, resulted in a >1 log10 CFU/organ mean reduction in all organs. ISAVUSULF and FLU improved survival and reduced fungal burden. Increasing plasma drug exposures resulted in decreases in fungal burden.

Breathing in vitro: Designs and applications of engineered lung models

The aim of this review is to provide a systematic design guideline to users, particularly engineers interested in developing and deploying lung models, and biologists seeking to identify a suitable platform for conducting in vitro experiments involving pulmonary cells or tissues. We first discuss the state of the art on lung in vitro models, describing the most simplistic and traditional ones. Then, we analyze in further detail the more complex dynamic engineered systems that either provide mechanical cues, or allow for more predictive exposure studies, or in some cases even both. This is followed by a dedicated section on microchips of the lung. Lastly, we present a critical discussion of the different characteristics of each type of system and the criteria which may help researchers select the most appropriate technology according to their specific requirements. Readers are encouraged to refer to the tables accompanying the different sections where comprehensive and quantitative information on the operating parameters and performance of the different systems reported in the literature is provided.

Antifungal drugs: An updated review of central nervous system pharmacokinetics

Invasive fungal infections (IFIs) in the central nervous system (CNS) are particularly hard to treat and are associated with high morbidity and mortality rates. Four chemical classes of systemic antifungal agents are used for the treatment of IFIs (eg meningitis), including polyenes, triazoles, pyrimidine analogues and echinocandins. This review will address all of these classes and discuss their penetration and accumulation in the CNS. Treatment of fungal meningitis is based on the antifungal that shows good penetration and accumulation in the CNS. Pharmacokinetic data concerning the entry of antifungal agents into the intracranial compartments are faulty. This review will provide an overview of the ability of systemic antifungals to penetrate the CNS, based on previously published drug physicochemical properties and pharmacokinetic data, for evaluation of the most promising antifungal drugs for the treatment of fungal CNS infections. The studies selected and discussed in this review are from 1990 to 2019.

Pharmacodynamics of the Novel Antifungal Agent F901318 for Acute Sinopulmonary Aspergillosis Caused by Aspergillus flavus

Background

Aspergillus flavus is one of the most common agents of invasive aspergillosis and is associated with high mortality. The orotomides are a new class of antifungal agents with a novel mechanism of action. An understanding of the pharmacodynamics (PD) of the lead compound F901318 is required to plan safe and effective regimens for clinical use.

Methods

The pharmacokinetics (PK) and PD of F901318 were evaluated by developing new in vitro and in vivo models of invasive fungal sinusitis. Galactomannan was used as a pharmacodynamic endpoint in all models. Mathematical PK-PD models were used to describe dose-exposure-response relationships.

Results

F901318 minimum inhibitory concentrations (MICs) ranged from 0.015 to 0.06 mg/L. F901318 induced a concentration-dependent decline in galactomannan. In the in vitro model, a minimum concentration:MIC of 10 resulted in suppression of galactomannan; however, values of approximately 10 and 9–19 when assessed by survival of mice or the decline in galactomannan, respectively, were equivalent or exceeded the effect induced by posaconazole. There was histological clearance of lung tissue that was consistent with the effects of F901318 on galactomannan.

Conclusions

F901318 is a potential new agent for the treatment of invasive infections caused by A flavus with PDs that are comparable with other first-line triazole agents.

Comparative in vitro pharmacodynamic analysis of isavuconazole, voriconazole, and posaconazole against clinical isolates of aspergillosis, mucormycosis, fusariosis, and phaeohyphomycosis

We compared the in vitro pharmacodynamics of isavuconazole, voriconazole, and posaconazole against 92 clinical isolates from documented cases of invasive aspergillosis, mucormycosis, fusariosis, and phaeohyphomycosis. Whereas inhibitory and fungicidal concentrations of these triazoles were predictably similar with the exception of Mucorales, isavuconazole appeared to have improved pharmacodynamics against Fusarium solani.

Recent Advances in the Treatment of Scedosporiosis and Fusariosis

Species of Scedosporium and Fusarium are considered emerging opportunistic pathogens, causing invasive fungal diseases in humans that are known as scedosporiosis and fusariosis, respectively. These mold infections typically affect patients with immune impairment; however, cases have been reported in otherwise healthy individuals. Clinical manifestations vary considerably, ranging from isolated superficial infection to deep-seated invasive infection—affecting multiple organs—which is often lethal. While there have been a number of advances in the detection of these infections, including the use of polymerase chain reaction (PCR) and matrix-assisted laser desorption ionization/time-of-flight mass spectrometry (MALDI-TOF MS), diagnosis is often delayed, leading to substantial morbidity and mortality. Although the optimal therapy is controversial, there have also been notable advances in the treatment of these diseases, which often depend on a combination of antifungal therapy, reversal of immunosuppression, and in some cases, surgical resection. In this paper, we review these advances and examine how the management of scedosporiosis and fusariosis may change in the near future.

Pharmacokinetic/Pharmacodynamic Analysis of Isavuconazole Against Aspergillus spp. and Candida spp. in Healthy Subjects and Patients With Hepatic or Renal Impairment by Monte Carlo Simulation

The aim of this pharmacokinetic/pharmacodynamic (PK/PD) study is to evaluate the efficacy of various isavuconazole dosing regimens for healthy individuals and patients with hepatic or renal impairment against Aspergillus spp. and Candida spp. Monte Carlo simulations were conducted using pharmacokinetic (PK) parameters and pharmacodynamics (PD) data to determine the probabilities of target attainment and cumulative fractions of response in terms of area under the concentration curve/minimum inhibition concentration (AUC/MIC) targets of isavuconazole. A clinically recommended dosage regimen of isavuconazole (200 mg qd) obtained high cumulative fraction of response values of > 90% for all subjects against A. fumigatus, A. flavus, A. nidulans, A. terreus, A. versicolor, C. parapsilosis and C. tropicalis. For patients with mild or moderate hepatic impairment, the dosage should be halved only when treating invasive fungal infections caused by C. albicans, C. parapsilosis or C. tropicalis. However, dose adjustment is unlikely to be required in mild to severe renal impairment patients because all cumulative fraction of response values were similar to those of comparing with healthy subjects. Notably, all isavuconazole dosing regimens were not effective against C. glabrata and C. krusei in all subjects. These PK/PD-based simulations rationalize and optimize the dosage regimens of isavuconazole for healthy individuals and patients with hepatic or renal impairment against Aspergillus spp. and Candida spp.

Prognostic value of galactomannan: current evidence for monitoring response to antifungal therapy in patients with invasive aspergillosis

Galactomannan (GM) is a polysaccharide present in the cell wall of Aspergillus spp. that is released during growth of the organism. It has been successfully used to aide in the diagnosis of invasive aspergillosis allowing for earlier recognition of disease compared to conventional methods. Since its implementation in the clinic as a diagnostic tool, GM has been used in experimental models to measure therapeutic response. Several clinical studies describe the prognostic value of GM. Herein, we review the evidence supporting the utilization of GM antigen as a biomarker to measure response to systemic antifungal therapy.

Isavuconazole for the treatment of invasive aspergillosis and mucormycosis: current evidence, safety, efficacy, and clinical recommendations

The majority of invasive mold infections diagnosed in immunocompromised cancer patients include invasive aspergillosis (IA) and mucormycosis. Despite timely and effective therapy, mortality remains considerable. Antifungal agents currently available for the management of these serious infections include triazoles, polyenes, and echinocandins. Until recently, posaconazole has been the only triazole with a broad spectrum of anti-mold activity against both Aspergillus sp. and mucorales. Other clinically available triazoles voriconazole and itraconazole, with poor activity against mucorales, have significant drug interactions in addition to a side effect profile inherent for all triazoles. Polyenes including lipid formulations pose a problem with infusion-related side effects, electrolyte imbalance, and nephrotoxicity. Echinocandins are ineffective against mucorales and are approved as salvage therapy for refractory IA. Given that all available antifungal agents have limitations, there has been an unmet need for a broad-spectrum anti-mold agent with a favorable profile. Following phase III clinical trials that started in 2006, isavuconazole (ISZ) seems to fit this profile. It is the first novel triazole agent recently approved by the United States Food and Drug Administration (FDA) for the treatment of both IA and mucormycosis. This review provides a brief overview of the salient features of ISZ, its favorable profile with regard to spectrum of antifungal activity, pharmacokinetic and pharmacodynamic parameters, drug interactions and tolerability, clinical efficacy, and side effects.

Pharmacodynamics for antifungal drug development: an approach for acceleration, risk minimization and demonstration of causality

The treatment of invasive fungal diseases constitutes a significant unmet medical need. There are relatively few antifungal agents in clinical development and a paucity of novel targets. Morbidity and mortality remain high and clinical outcomes are compromised by submaximal efficacy, emergence of drug resistance and drug-related toxicity. Thus, new antifungal agents are urgently required. A deep understanding of exposure-response relationships underpins the development of safe and effective clinical regimens of any therapeutic agent. Pharmacokinetics (PK) and pharmacodynamics (PD) is increasingly recognized as a vital tool in the development of new antimicrobial agents and maximizes the probability that the right dose will be studied the first time. There is currently no information or agreement as to what constitutes an adequate PK/PD package for the development of a new antifungal agent. This review provides a summary of the achievements of antifungal PK/PD for the treatment of invasive candidiasis, invasive aspergillosis and cryptococcal meningoencephalitis, and outlines the necessary components of a PK/PD package for a new antifungal agent. Such information is critical for the accelerated and efficient development of new agents and enables improved clinical outcomes to be secured.

Isavuconazole in the treatment of invasive aspergillosis and mucormycosis infections

We have a limited arsenal with which to treat invasive fungal infections caused by Aspergillus and Mucorales. The morbidity and mortality for both pathogens remains high. A triazole antifungal, isavuconazole, was recently granted approval by the US Food and Drug Administration and the European Medicines Agency for the treatment of invasive aspergillosis and mucormycosis. A randomized double-blind comparison trial for the treatment of invasive aspergillosis found isavuconazole noninferior to voriconazole. A separate, open-label study evaluating the efficacy of isavuconazole in the treatment of mucormycosis found comparable response rates to amphotericin B and posaconazole treated historical controls. The prodrug isavuconazonium sulfate is commercially available in both an oral and intravenous formulation and is generally well tolerated. Isavuconazole’s broad spectrum of activity, limited side effect profile, and favorable pharmacokinetics will likely solidify its place in therapy.

Pharmacokinetics and Concentration-Dependent Efficacy of Isavuconazole for Treatment of Experimental Invasive Pulmonary Aspergillosis

We studied the pharmacokinetics and efficacy of the broad-spectrum triazole isavuconazole for the treatment of experimental invasive pulmonary aspergillosis (IPA) in persistently neutropenic rabbits. Treatment started 24 h after endotracheal administration of Aspergillus fumigatus inoculum; study subjects included rabbits receiving orally administered prodrug isavuconazonium sulfate (BAL8557) equivalent to active moiety isavuconazole (ISA; BAL4815) at 20 (ISA20), 40 (ISA40), and 60 (ISA60) mg/kg (of body weight)/day, with an initial loading dose of 90 mg/kg (ISA90), and untreated rabbits (UC). There were significant concentration-dependent reductions of residual fungal burden (log CFU/gram) and of organism-mediated pulmonary injury, lung weights, and pulmonary infarct scores in ISA40- and ISA60-treated rabbits in comparison to those of UC (P < 0.001). ISA20-treated (P < 0.05), ISA40-treated, and ISA60-treated (P < 0.001) rabbits demonstrated significantly prolonged survival in comparison to that of UC. ISA40- and ISA60-treated animals demonstrated a significant decline of serum (1→3)-β-d-glucan levels (P < 0.05) and galactomannan indices (GMIs) during therapy following day 4 in comparison to progressive GMIs of UC (P < 0.01). There also were significantly lower concentration-dependent GMIs in bronchoalveolar lavage (BAL) fluid from ISA40- and ISA60-treated rabbits (P < 0.001). There was a direct correlation between isavuconazole plasma area under the concentration-time curve from 0 to 24 h (AUC0-24) and residual fungal burdens in lung tissues, pulmonary infarct scores, and total lung weights. In summary, rabbits treated with isavuconazole at 40 and 60 mg/kg/day demonstrated significant dose-dependent reduction of residual fungal burden, decreased pulmonary injury, prolonged survival, lower GMIs in serum and BAL fluid, and lower serum (1→3)-β-d-glucan levels.

Pharmacodynamics of isavuconazole in experimental invasive pulmonary aspergillosis: implications for clinical breakpoints

OBJECTIVES

Isavuconazole, a novel triazole antifungal agent, has broad-spectrum activity against Aspergillus spp. and other pathogenic fungi. The isavuconazole exposure-response relationship in experimental invasive pulmonary aspergillosis using galactomannan index (GMI) suppression as a marker of disease clearance was explored.

METHODS

The impact of exposure on GMI suppression in persistently neutropenic rabbits treated with isavuconazonium sulphate (isavuconazole-equivalent dosages of 20, 40 or 60 mg/kg every 24 h, after a 90 mg/kg loading dose) for 12 days was linked using mathematical modelling. Bridging to humans using population pharmacokinetic (PK) data from a clinical trial in invasive aspergillosis was performed using Monte Carlo simulations.

RESULTS

Mean plasma isavuconazole AUC/MIC (EC50) of 79.65 (95% CI 32.2, 127.1) produced a half-maximal effect in GMI suppression. The inhibitory sigmoid Emax curve dropped sharply after an AUC/MIC of ≥30 and was near maximum (EC80) at ∼130. Bridging the experimental PK/pharmacodynamic (PD) target to human population PK data was then used to return to the rabbit model to determine a clinically relevant PD endpoint. The clinical dosing regimen used in the trial would result in a mean GMI of 4.3 ± 1.8, which is a 50% reduction from the starting GMI in the experiment.

CONCLUSIONS

The clinical trial results showing the non-inferiority of isavuconazole to voriconazole for all-cause mortality further support the PK-PD endpoint, thereby demonstrating the usefulness of the rabbit model and endpoint for isavuconazole and implications on interpretive breakpoints. Importantly, the analysis supports this model as an important tool for development of antifungal agents.