Virus resistance to antiviral therapies is an increasing concern that makes the development of broad-spectrum antiviral drugs urgent. Targeting of the viral envelope, a component shared by a large number of viruses, emerges as a promising strategy to overcome this problem. Natural and synthetic porphyrins are good candidates for antiviral development due to their relative hydrophobicity and pro-oxidant character. In the present work, we characterized the antiviral activities of protoprophyrin IX (PPIX), Znprotoporphyrin IX (ZnPPIX), and mesoporphyrin IX (MPIX) against vesicular stomatitis virus (VSV) and evaluated the mechanisms involved in this activity. Treatment of VSV with PPIX, ZnPPIX, and MPIX promoted dose-dependent virus inactivation, which was potentiated by porphyrin photoactivation. All three porphyrins inserted into lipid vesicles and disturbed the viral membrane organization. In addition, the porphyrins also affected viral proteins, inducing VSV glycoprotein cross-linking, which was enhanced by porphyrin photoactivation. Virus incubation with sodium azide and ␣-tocopherol partially protected VSV from inactivation by porphyrins, suggesting that singlet oxygen ( 1 O 2 ) was the main reactive oxygen species produced by photoactivation of these molecules. Furthermore, 1 O 2 was detected by 9,10-dimethylanthracene oxidation in photoactivated porphyrin samples, reinforcing this hypothesis. These results reveal the potential therapeutic application of PPIX, ZnPPIX, and MPIX as good models for broad antiviral drug design.KEYWORDS photoactivation, porphyrin, singlet oxygen, vesicular stomatitis virus, viral inactivation D espite the increasing number of antiviral drugs available nowadays, therapeutics for viral diseases often fail due to drug resistance development by the target viruses. In the search for antiviral agents with a broad spectrum of activity against viruses, antiviral agents targeting the viral envelope, a component shared by a large number of viruses, emerge as promising compounds able to overcome the drug resistance problem. Porphyrins, amphipathic molecules able to interact with membranes, appear to be good candidates for the development of such antivirals. Porphyrins are formed by four pyrrole rings linked by methine bonds. The nitrogen atoms in the center of the ring form a pocket able to coordinate metallic ions, conferring to porphyrin the ability to absorb light at additional wavelengths and act in catalytic transformations (1). These activities allow porphyrins to be involved in important biological processes, such as respiration (heme) and photosynthesis (chlorophyll and bacteriochlorophyll) (2).

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With current drug treatments failing due to toxicity, low efficacy and resistance; leishmaniasis is a major global health challenge that desperately needs new validated drug targets. Inspired by activity of the natural chalcone 2’,6’-dihydroxy-4’-methoxychalcone (DMC), the nitro-analogue, 3-nitro-2’,4’,6’- trimethoxychalcone (NAT22, 1c) was identified as potent broad spectrum antileishmanial drug lead. Structural modification provided an alkyne containing chemical probe that labelled a protein within the parasite that was confirmed as cytosolic tryparedoxin peroxidase (cTXNPx). Crucially, labelling is observed in both promastigote and intramacrophage amastigote life forms, with no evidence of host macrophage toxicity. Incubation of the chalcone in the parasite leads to ROS accumulation and parasite death. Deletion of cTXNPx, by CRISPR-Cas9, dramatically impacts upon the parasite phenotype and reduces the antileishmanial activity of the chalcone analogue. Molecular docking studies with a homology model of in-silico cTXNPx suggest that the chalcone is able to bind in the putative active site hindering access to the crucial cysteine residue. Collectively, this work identifies cTXNPx as an important target for antileishmanial chalcones.

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