Acanthamoeba is a protist pathogen that can cause serious human
infections, including blinding
keratitis and a granulomatous amoebic
encephalitis that almost always results in death. The current treatment for these
infections includes a mixture of drugs, and even then, a recurrence can occur.
Photochemotherapy has shown promise in the treatment of
Acanthamoeba infections; however, the selective targeting of pathogenic Acanthamoeba has remained a major concern. The
mannose-binding protein is an important adhesin expressed on the surface membranes of pathogenic Acanthamoeba organisms. To specifically target Acanthamoeba, the overall aim of this study was to synthesize a photosensitizing compound (
porphyrin) conjugated with
mannose and test its efficacy in vitro. The synthesis of
mannose-conjugated
porphyrin was achieved by mixing
benzaldehyde and
pyrrole, yielding
tetraphenylporphyrin.
Tetraphenylporphyrin was then converted into mono-nitrophenylporphyrin by selectively nitrating the para position of the phenyl rings, as confirmed by nuclear magnetic resonance (NMR) spectroscopy. The mono-nitrophenylporphyrin was reduced to mono-aminophenylporphyrin in the presence of
tin dichloride and confirmed by a peak at m/z 629. Finally, mono-aminoporphyrin was conjugated with
mannose, resulting in the formation of an
imine bond.
Mannose-conjugated
porphyrin was confirmed through spectroscopic analysis and showed that it absorbed light of wavelengths ranging from 425 to 475 nm. To determine the antiacanthamoebic effects of the derived product, amoebae were incubated with
mannose-conjugated
porphyrin for 1 h and washed 3 times to remove extracellular compound. Next, the amoebae were exposed to light of the appropriate wavelength for 1 h. The results revealed that
mannose-conjugated
porphyrin produced potent trophicidal effects and blocked excystation. In contrast, Acanthamoeba castellanii incubated with
mannose alone and
porphyrin alone did not exhibit an antiamoebic effect. Consistently, pretreatment with
mannose-conjugated
porphyrin reduced the A. castellanii-mediated host cell cytotoxicity from 97% to 4.9%. In contrast, treatment with
porphyrin,
mannose, or
solvent alone had no protective effects on the host cells. These data suggest that
mannose-conjugated
porphyrin has application for the targeted
photodynamic therapy of
Acanthamoeba infections and may serve as a model in the development of therapeutic interventions against other eukaryotic
infections.