Aspergillus fumigatus Infection Models

Aspergillus fumigatus (A. fumigatus) is a ubiquitous environmental mold and the leading cause of invasive mold infection in humans. Its airborne spores (conidia) are small enough to reach the alveoli and are inhaled by nearly everyone every day. A healthy immune system clears them, but in people with neutropenia, hematologic malignancy, transplants, advanced HIV, or severe influenza, the fungus germinates and invades, producing the disease spectrum known as aspergillosis. The most consequential form is invasive pulmonary aspergillosis, an angioinvasive lung infection with a high mortality rate. Chronic pulmonary aspergillosis, allergic bronchopulmonary aspergillosis, and – rarely – cutaneous disease round out the range. 

Two features drive the clinical complexity of A. fumigatus: the fungus is thermotolerant and grows readily at body temperature, and the azole antifungals that are the mainstay of treatment are losing ground to resistance. That combination is why the WHO placed A. fumigatus in the critical-priority group of its 2022 Fungal Priority Pathogens List, and why the CDC lists azole-resistant A. fumigatus on the Watch List of its 2019 Antibiotic Resistance Threats Report. TransPharm Preclinical Solutions evaluates candidates against A. fumigatus in vivo in pulmonary and systemic infection models and produces the quantitative burden and survival data an antifungal program needs.

The scientific problem in an A. fumigatus study begins with the host. Unlike a typical bacterial challenge, invasive aspergillosis does not establish in a fully immunocompetent animal. The model depends on a defined immunosuppression regimen, and the choice of regimen shapes how the disease progresses and how a candidate should be read. 

Strain choice is the second variable: a wild-type reference isolate and an azole-resistant clinical isolate carrying a cyp51A mutation answer different development questions, particularly for a candidate positioned against resistant disease. Finally, because A. fumigatus grows as invasive hyphae rather than dividing cells, fungal burden does not always track linearly with colony counts, so the burden readout has to be matched to the question. 

TransPharm Preclinical Solutions maintains an on-site library of hundreds of validated pathogens and can source or validate characterized A. fumigatus isolates, including defined resistant phenotypes, to a program’s specifications.

Every study parameter is configurable to the scientific question: challenge strain, host species and strain, immunosuppression regimen, route of administration, comparator agents, and dosing schedule. The core readouts are quantitative fungal burden in target tissues, organs, and blood, with survival and mortality tracking where applicable.

The following are available as add-ons to any study:

• Pharmacokinetic profiling and PK/PD analysis
• Blood chemistry
• Additional tissue harvests
• Histopathology

Inhaled  A. fumigatus conidia establish invasive pulmonary aspergillosis in a susceptible host, and it is the setting in which most antifungal candidates need to demonstrate activity. The pulmonary model is the appropriate choice when the development question is reduction of fungal burden in the lung, survival after airway challenge, or delivery of an inhaled or systemically dosed agent to infected lung. Animals are challenged by intranasal or intratracheal instillation. Test article may be delivered by nebulization, oral gavage, intranasal or intratracheal instillation, or by intravenous, intraperitoneal, intramuscular, or subcutaneous injection. Endpoints include survival and fungal burden in lung and other harvested tissues. Validated in mice and rats. Validated A. fumigatus isolate: ATCC 13073.

The fungemia model delivers A. fumigatus intravenously to produce disseminated infection, and is used to screen systemic activity and to rank candidates on survival and on organ burden. Animals are challenged by intravenous injection in the lateral tail vein. Test article may be administered by oral gavage or by subcutaneous, intravenous, or intraperitoneal injection. Endpoints include survival (mortality tracking) and fungal burden in harvested organs and/or blood. Validated in mice, with rats available on request. Validated A. fumigatus isolate: ATCC 13073.

In vitro antifungal susceptibility testing complements the in vivo models. For a mold, the reference method is broth microdilution MIC (CLSI M38), which can be paired with time-kill and checkerboard analysis to characterize a candidate and its combinations. The latter is particularly relevant for azole-resistant isolates, where the treatment options are narrow.

Aspergillosis is treated primarily with triazole antifungals, which are the only oral drugs available for the disease. That makes emerging azole resistance a direct threat to the treatable population. Resistance is most often conferred by mutations in the cyp51A gene, which encodes the azole target; the dominant pattern, TR34/L98H, combines a tandem repeat in the promoter with an amino-acid substitution and confers cross-resistance across the triazole class. 

A substantial share of resistance arises in the environment rather than the patient: azole fungicides used widely in agriculture are chemically similar to the medical triazoles, and A. fumigatus growing on treated crop debris is selected for the same mutations – a One Health dynamic that seeds resistant spores into the air people breathe. Because only a few antifungal classes exist (azoles, echinocandins, and polyenes), a resistant isolate sharply narrows an already limited arsenal, and the resistance profile of the challenge strain determines what an efficacy study can show.