Echinocandin Treatment of Pneumocystis Pneumonia in Rodent Models Depletes Cysts Leaving Trophic Burdens That Cannot Transmit the Infection

Abstract

Fungi in the genus Pneumocystis cause pneumonia (PCP) in hosts with debilitated immune systems and are emerging as co-morbidity factors associated with chronic diseases such as COPD. Limited therapeutic choices and poor understanding of the life cycle are a result of the inability of these fungi to grow outside the mammalian lung. Within the alveolar lumen, Pneumocystis spp., appear to have a bi-phasic life cycle consisting of an asexual phase characterized by binary fission of trophic forms and a sexual cycle resulting in formation of cysts, but the life cycle stage that transmits the infection is not known. The cysts, but not the trophic forms, express β -1,3-D-glucan synthetase and contain abundant β -1,3-D-glucan. Here we show that therapeutic and prophylactic treatment of PCP with echinocandins, compounds which inhibit the synthesis of β -1,3-D-glucan, depleted cysts in rodent models of PCP, while sparing the trophic forms which remained in significant numbers. Survival was enhanced in the echincandin treated mice, likely due to the decreased β -1,3-D-glucan content in the lungs of treated mice and rats which coincided with reductions of cyst numbers, and dramatic remodeling of organism morphology. Strong evidence for the cyst as the agent of transmission was provided by the failure of anidulafungin-treated mice to transmit the infection. We show for the first time that withdrawal of anidulafungin treatment with continued immunosuppression permitted the repopulation of cyst forms. Treatment of PCP with an echinocandin alone will not likely result in eradication of infection and cessation of echinocandin treatment while the patient remains immunosuppressed could result in relapse. Importantly, the echinocandins provide novel and powerful chemical tools to probe the still poorly understood bi-phasic life cycle of this genus of fungal pathogens.

Characterization of a Distinct Host Response Profile to Pneumocystismurina Asci during Clearance of Pneumocystis Pneumonia

“…It has long been held that the periplasmic space in the ascus was the repository for glucan, as that space increases throughout maturation. However, staining with a monoclonal antibody (MAb) to ␤-glucan suggests it may be on the surface of the asci as well, which is in keeping with the recognition of these forms via Dectin-1 (7).…”

“…We have shown that treatment of Pneumocystis-infected mice and rats with inhibitors of ␤-glucan synthesis, the echinocandins, dramatically shifted the mixed ascus and trophic form populations to ones made almost exclusively of trophic forms (7). The mortality was markedly reduced in the mice treated with echinocandins, especially anidulafungin, yet large populations of trophic forms were present in their lungs.…”

“…Recent studies have also demonstrated the potential clinical usefulness of ␤-glucan as potential marker for pneumocystis pneumonia (PcP) in serum and bronchoalveolar lavage fluid (BALF) (19)(20)(21)(22)(23)(24)(25). Inhibition of Pneumocystis ␤-glucan synthase may provide, in part, an alternative treatment option for patients with PcP (2,7,(27)(28)(29)(30)(31).…”

Characterization of N-Acetylglucosamine Biosynthesis in Pneumocystis species. A New Potential Target for Therapy

“…To accomplish this, we employed the rat Pc pneumonia model that has been used extensively to analyze drug activity against the organism (6,60,61). Due to limited drug supply, we treated the rats with nikkomycin Z by oral gavage after establishment of fulminate PCP by immune suppressing rats with corticosteroids and inoculating with Pc.…”

“…We and others have shown that the Pneumocystis species contain b-glucan synthetases that can be inhibited by specific b-glucan inhibitors, such as the echinocandins, pneumocandins, and newly generated compounds (3,4,6,7). A number of these drugs are already implemented as antifungal therapies against pathogens (8).…”