Susceptibility of the human pathogenic fungi Cryptococcus neoformans and Histoplasma capsulatum to γ-radiation versus radioimmunotherapy with α- and β-emitting radioisotopes

Fungal diseases are difficult to treat in immunosuppressed patients and, consequently, new approaches to therapy are urgently needed. One novel strategy is to use radioimmunotherapy (RIT) with fungal-binding monoclonal antibodies (mAbs) labeled with radionuclides. However, many fungi manifest extreme resistance to γ-radiation, such that the doses of several thousand gray are required for 90% cell killing, whereas for mammalian cells the lethal dose is only a few gray. We compared the susceptibility of human pathogenic fungi Cryptococcus neoformans (CN) and Histoplasma capsulatum (HC) to external γ-radiation and to the organism-specific mAbs 18B7 and 9C7, respectively, conjugated to ²¹³Bi and ¹⁸⁸Re radionuclides. Methods: CN and HC cells were irradiated with up to 8,000 Gy (¹³⁷Cs source, 30 Gy/min). RIT of CN with ²¹³Bi- and ¹⁸⁸Re-labeled specific mAb and of HC with ¹⁸⁸Re-labeled specific mAb used 0-1.2 MBq per 10⁵ microbial cells. After irradiation or RIT, the cells were plated for colony-forming units (CFUs). Cellular dosimetry calculations were performed, and the pathway of cell death after irradiation was evaluated by flow cytometry. Results: Both CN and HC proved to be extremely resistant to γ-radiation such that significant killing was observed only for doses of >4,000 Gy. In contrast, these cells were much more susceptible to killing by radiation delivered with a specific mAb, such that a 2-logarithm reduction in colony numbers was achieved by incubating them with ²¹³Bi- and ¹⁸⁸Re-labeled mAb 18B7 or with ¹⁸⁸Re-9C7 mAb. Dosimetry calculations showed that RIT was ∼1,000-fold more efficient in killing CN and ∼100-fold more efficient in killing HC than γ-radiation. Both γ-radiation and RIT caused cell death via an apoptotic-like pathway with a higher percentage of apoptosis observed in RIT-treated cells. Conclusion: Conjugating a radioactive isotope to a fungal-specific antibody converted an immunoglobulin with no antifungal activity into a microbicidal molecule. RIT of fungal cells using specific antibodies labeled with α- and β-emitting radioisotopes was significantly more efficient in killing CN and HC than γ-radiation when based on the mean absorbed dose to the cell. These results strongly support the concept of using RIT as an antimicrobial modality.