Beyond Antifungal MICs

Fungicidal activity, biofilms and tissue distribution

Russell E. Lewis, Pharm.D.
Associate Professor- Infectious Diseases
Department of Medical and Surgical Sciences
University of Bologna

Agenda

• How do deep-seated infections and biofilms alter drug penetration and fungal killing?
• How can we adapt antifungal selection an dosing strategies to account for these changes?

My job in this presentation is to address a couple of the areas of pharmacology that may confound the translation and application of antifungal susceptibility testing results in patient care.

Specifically I will adress how deep-seated infections and biofilms alter fungal killing and what are the current challenges of adating antifungal dosing and selection strategies for the treatment of deep-seated and biofilm-assocaited infections

deep-seated, adj

“Being so far below the surface as to be non-susceptible to
superficial examination, study or treatment” ¹

“…very fixed and strong; difficult to change or destroy.” ²

What is exactly defines a “deep-seated” infection?

Standard definitions of “deep-seated” that one finds in non-medical dictionaries indicate two key characteristics

1. Being so far below the surface as to be non-susceptible to superficial examination, study or treatment- i.e. more difficult to diagnose

2. very fixed and strong; difficult to change or destroy- i.e. more difficlt to treat

1. Oxford English Dictionary 2021

2. Merriam-Webster.com

Deep-seated vs. “uncomplicated” infections

In mycology textbooks, deepseated infections are classified when the invasive fungal pathogen involved the brain, heart, lungs , kidney , spleen, liver abdominal viscous or other internal organs. Catheter-associated bloodstream infections may not be considered deep-seated, although there is obviously some area of overlap. Cutaneous infections, while technically invasive in the sense the fungi are invading tissue, hair follicles or nails, or mucous membranes, are not deep-seated, even if these infections can be refratory to treatment

PK/PD of antifungal therapy

(Theuretzbacher, 2012)

So in the classic pharmacology viewpoint, the question is how does the pharmacokinetic and pharmacodynamics of antifungal therapy change with deep-seated or biofilm-associated infections?

I would like to start by focusing on the pharmacodynamics question and specifically the anticipated micrpbiologic effects and clinical outcome

How important is fungicidal activity?

Disease	Evidence	References
Cryptococcal meningitis	Early fungicidal activity (EFA) > 0.2 log10/day in CSF is a surrogate marker for 18 week survival	(Bicanic et al., 2009; Jarvis et al., 2022; Pullen et al., 2020; Rhein et al., 2016)
Invasive candidiasis	AMB or echinocandins achieve a higher probability of early therapeutic success and decreased risk of relapse	(Andes et al., 2012; Kumar et al., 2018)
Invasive aspergillosis	Poorer outcomes reported with fungistatic echinocandin monotherapy in EORTC probable/proven aspergillosis	(Ullmann et al., 2018; Viscoli et al., 2009)

A common point of discussion is whether the antimicrobials or antifungals produce cidal versus static activity- and in fact the importance of having bactericidal activity for some infections has been recently questioned in the literature.
In the case of treating invasive fungal infection, I think the evidence of role of fungicidal activity is more consistent and stronger, especially for cryptococcal meningitis where early fungicidal activity defined as > 0.2 log10 CFU reduction per day in the CSF is associated with improved 18-week survival. So we actually have a PD biomarker that is predictive of outcomes.
In the case of invasive candidiasis, the evidence is less clear, but certainly evidence from major clinical trials have suggested a higher probability of clinical success and lower risk of relapse when mostly bloodstream infections are treated with agents such as echinocandins and amphotericin B, that at least in vitro, display more rapidly fungicidal effects vs. triazoles
Invasive aspergillosis is an area where the evidence is even more difficult to parse and the microbiological definition of fungicidal is more complicated for a multicellular hypahe, but there is some suggestion that drugs that produce more rapid global fungal cell death produce better clinical outcomes that echinocandin monotherapy, that only cause cell lysis at the fungal tips.

Biofilm-mediated resistance

(Ciofu et al., 2022; Mitchell et al., 2015)

How do fungal cells survive fungicidal drug exposure?

• Increased drug sequestration by biofilm matrix (glucan-mannans)
• Up-regulation of drug efflux pumps
• Immune cell evasion
• Persister cells

(Nett and Andes, 2020; Wuyts et al., 2018)

Persister cells survive fungicidal drug exposures

(Nett and Andes, 2020; Wuyts et al., 2018)

Antifungal susceptibility in biofilms

(Chatzimoschou et al., 2011)

Biofilms aid escape from the host immune response

(Kernien et al., 2018)

Biofilms aid immune-evasion during invasive aspergillosis

(Loussert et al., 2010; Subroto et al., 2022)

Biofilms modulate innate host immune response against Aspergillus

(Subroto et al., 2022)

Aspergillus biofilms are adapted to environmental conditions and host responses

(Subroto et al., 2022)

Rapid laboratory methods for detection of
high-biofilm producers

(De Carolis et al., 2019)

PK/PD of antifungal therapy

(Theuretzbacher, 2012)

Antifungal tissue distribution patterns

(Felton et al., 2014)

Inflammation and transporters affect antifungal distribution into the CNS

Single measurements of tissue homogenates are crude and potentially misleading

(Felton et al., 2014)

Matrix-assisted desorption ionization mass spectrometry imaging

(Prideaux and Stoeckli, 2012; Zhao et al., 2017)

Drug distribution in infected liver tissue after a single dose of micafungin

(Zhao et al., 2017)

Continuous monitoring of blood or interstitial fluid (ISF) antifungal concentrations

(Rawson et al., 2021)

Barriers to current antifungal TDM approaches

(Rawson et al., 2021)

Summary

• Timely (early) fungicidal antifungal activity provides the best opportunity for reducing deep-seated (biofilm-associated) infections

• Antifungal therapy should be ideally selected not only its basis to kill fungal cells in vitro, but also its ability to penetrate the fungal extracellular matrix and achieve optimal fungicidal concentrations in tissue

• Emerging technologies have the potential to address knowledge gaps of how to optimally dose antifungals for deep-tissue infections

Acknowledgements

All figures, unless otherwise cited, were created at: www.biorender.com

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