Table 2.

Hypothetical Targets for Molecular Imaging in the Replication Cycle of Influenza A Viruses and the SARS Coronavirus

	Potential imaging methods
Target	Influenza A viruses	SARS coronavirus
Binding of virion to cell-surface receptor	PET or SPECT imaging using radiolabeled lectins or viral HA molecules as probes.BLI of infection by a GFP- or luciferase-encoding virus [32].	BLI of infection by a GFP- or luciferase-encoding virus [33].
Fusion and entry into cell	PET or SPECT imaging using radiolabeled adamantanes, which bind specifically within the M2 ion channel [34].	PET or SPECT imaging using radiolabeled peptide inhibitors of viral membrane fusion [35].
Transcription and genome replication	PET or SPECT imaging using a recombinant virus encoding the HSV TK, or using a radiolabeled non-nucleoside RNA polymerase inhibitor as tracer.	PET or SPECT imaging using a recombinant virus encoding the HSV TK, or using a radiolabeled non-nucleoside RNA polymerase inhibitor as tracer.
Processing of viral proteins and virion assembly		PET or SPECT imaging using a radiolabeled protease inhibitor as a tracer [36].
Budding and exit of virus particles	PET or SPECT imaging using radiolabeled antibodies that bind to viral antigens on the cell surface, or a radiolabeled neuraminidase inhibitor.	PET or SPECT imaging using radiolabeled antibodies that bind to viral antigens on the cell surface [37].

NOTE. For BLI, sites of replication can be identified through the expression of a virus-encoded luciferase, GFP or other reporter molecule, as has been shown for an influenza virus encoding a chimeric NS1-GFP protein [31]. Virus-specific imaging with PET or SPECT might make use of a radiolabeled probe that binds with high affinity to a virus-specific molecule, or is selectively modified by a virus-encoded enzyme, causing it to be retained within infected cells.