Prospects for the use of radiolabeled antiviral drugs in the diagnosis of herpes simplex encephalitis

Molecular imaging of influenza and other emerging respiratory viral infections

Research on the pathogenesis and therapy of influenza and other emerging respiratory viral infections would be aided by methods that directly visualize pathophysiologic processes in patients and laboratory animals. At present, imaging of diseases, such as swine-origin H1N1 influenza, is largely restricted to chest radiograph and computed tomography (CT), which can detect pulmonary structural changes in severely ill patients but are more limited in characterizing the early stages of illness, differentiating inflammation from infection or tracking immune responses. In contrast, imaging modalities, such as positron emission tomography, single photon emission CT, magnetic resonance imaging, and bioluminescence imaging, which have become useful tools for investigating the pathogenesis of a range of disease processes, could be used to advance in vivo studies of respiratory viral infections in patients and animals. Molecular techniques might also be used to identify novel biomarkers of disease progression and to evaluate new therapies.

Radiolabeled antiviral drugs and antibodies as virus-specific imaging probes

A number of small-molecule drugs inhibit viral replication by binding directly to virion structural proteins or to the active site of a viral enzyme, or are chemically modified by a viral enzyme before inhibiting a downstream process. Similarly, antibodies used to prevent or treat viral infections attach to epitopes on virions or on viral proteins expressed on the surface of infected cells. Such drugs and antibodies can therefore be thought of as probes for the detection of viral infections, suggesting that they might be used as radiolabeled tracers to visualize sites of viral replication by single-photon emission computed tomography (SPECT) or positron emission tomography (PET) imaging. A current example of this approach is the PET imaging of herpes simplex virus infections, in which the viral thymidine kinase phosphorylates radiolabeled thymidine analogues, trapping them within infected cells. One of many possible future applications might be the use of a radiolabeled hepatitis C protease inhibitor to image infection in animals or humans and provide a quantitative measure of viral burden. This article reviews the basic features of radionuclide imaging and the characteristics of ideal tracer molecules, and discusses how antiviral drugs and antibodies could be evaluated for their suitability as virus-specific imaging probes. The use of labeled drugs as low-dose tracers would provide an alternative application for compounds that have failed to advance to clinical use because of insufficient in vivo potency, an unsuitable pharmacokinetic profile or hepato- or nephrotoxicity.

Synthesis of brain-targeted 1-(2-deoxy-2-fluoro-beta-D-ribofuranosyl)-(E)-5-(2-iodovinyl)uracil coupled to a dihydropyridine <---> pyridinium salt redox chemical-delivery system

1-(2-Deoxy-2-fluoro-beta-D-ribofuranosyl-(E)-5-(2-iodovinyl)uracil (IVFRU) was coupled to a dihydropyridine <---> pyridinium salt redox chemical-delivery system (CDS) via a cleavable sugar-ester linkage as a site-directed approach to increase diffusion of the parent nucleoside into the central nervous system. Treatment of 1-(2-deoxy-2-fluoro-beta-D-ribofuranosyl)uracil with Bu(t)Me(2)SiCl in the presence of imidazole in DMF yielded the protected 5-O-tert-butyldimethylsilyl derivative. Subsequent reaction with nicotinoyl chloride hydrochloride in pyridine afforded 1-[5-O-tert-butyldimethylsilyl-2-deoxy-2-fluoro-3-O-(3-pyridylcarbony l )-beta-D-ribofuranosyl]uracil. Reaction with iodine monochloride in methanol simultaneously cleaved the silyl ether moiety and iodinated the uracil ring at the 5-position. Coupling with (E)-Bu(3)Sn-CH = CH-SiMe(3) in the presence of (Ph3P)2Pd2(II)Cl2 in THF gave 1-[2-deoxy-2-fluoro-3-O-(3-pyridylcarbonyl)-beta-D-ribofuranosyl]- (E)-5-(2-trimethylsilylvinyl)uracil. Quaternization with iodomethane in acetone yielded the N-methylpyridinium iodide salt. Ionation of the reactive (E)-trimethylsilylvinyl moiety with iodine monochloride in acetonitrile and reduction of the quaternary pyridinium iodide salt with sodium dithionite in the presence of sodium hydrogen carbonate was carried out as a one-pot procedure to afford 1-[2-deoxy-2-fluoro-3-O-(1-methyl-1,4-dihydropyridyl-3-carbonyl)-b eta-D-ribofuranosyl]-(E)-5-(2-iodovinyl)uracil (IVFRU-CDS). This synthetic strategy is readily amenable to the high specific-activity radioiodination of IVFRU.

HM-PAO-imaging and herpes encephalitis

Selective uptake of the cerebral blood-flow imaging agent 99mTc-hexamethylpropyleneamine oxime (HM-PAO) by Human Herpesvirus 1 (HSV-1) infected cells was investigated in vivo and in vitro. No specific uptake of HM-PAO was observed either in encephalitic rats (by brain scintigraphic imaging or by immunoperoxidase staining/autoradiography of brain sections) or in HSV-1 infected Vero cells.

Altered in vitro uptake of the radiolabelled antiviral imaging "probe" E-5-(2-125iodovinyl)-2'-deoxyuridine following administration of acyclovir

Current developments in the use of radiolabelled antiviral drugs as specific "probes" for virus-infected cells in vivo may allow the specific neuroradiological diagnosis of herpes encephalitis. As "blind therapy" with the antiviral drug acyclovir may precede specific neuroradiological diagnosis, the aim of the present study was to investigate whether or not acyclovir interferes with the uptake of the radioprobe E-5-(2-125Iodovinyl)-2'-deoxyuridine (rIVDU) by virus-infected cells in vitro. Acyclovir treatment (0.1 to 10 micrograms/ml) was shown to increase initial radioprobe uptake by virus-infected cells. However, with continued incubation in the presence of acyclovir, intracellular radioactivity decreased to a level not significantly different from that associated with noninfected cells. A mechanism to explain these results is proposed. It was concluded that concurrent acyclovir therapy could interfere with neuroradiological diagnosis using rIVDU, although this may not occur with all the candidate radioprobes currently under investigation.

Chronic herpes simplex virus and varicella zoster virus infection

After defining such terms as persistent and chronic infection, latency, recurrence, recrudescence, and exogenous reinfection they are applied to infections with HSV and VZV. Possible factors determining pathogenicity are discussed, and an overview is given of the wide range of illnesses and case reports ascribed to HSV and VZV infections. Various types of infection afford different diagnostic procedures. Besides virus isolation supplemented by viral antigen identification IgG antibody tests (increase in titer) may be useful. IgG subtype and IgA antibody determinations appear to be of limited value. Despite the rather large number of available tests, there are still considerable shortcomings in their ultimate significance as to the patient's disease. Thus, some new experimental approaches are mentioned.

Specific neuro-radiological diagnosis of herpes encephalitis in an animal model

The potential of utilizing a radio-labelled derivative of the antiviral drug (E)-5-(2-iodovinyl)-2'-deoxyuridine (IVDU) for the specific, non-invasive, in vivo diagnosis of Herpes simplex virus encephalitis (HSVE) was investigated in a rat model of the disease. Following pharmacological disruption of the blood brain barrier radiolabelled IVDU was administered by intra-carotid injection. Brain radioactivity was compared between control and infected animals via gamma camera scintigraphy. After clearance of non-metabolized drug, markedly higher levels of activity were found in infected brain. Post-mortem studies of cryostat sections of brain examined by autoradiography and immunochemical staining showed the radioactivity selectively accumulated in areas of virus infection. These results indicate that radio-labelled derivatives of antiviral drugs may allow the specific neuro-radiological diagnosis of HSVE.

Trifluorothymidine: potential non-invasive diagnosis of herpes simplex infection using 19F nuclear magnetic resonance in a murine hepatitis model

Trifluorothymidine (TFT) is known to be concentrated in herpes simplex virus (HSV) infected cells in vitro in the form of phosphorylated derivatives. We studied a murine hepatitis model of HSV infection to determine whether this in vitro observation would also be demonstrable in vivo. Following i.v. injection of 100 or 160 mg/kg TFT, TFT was found in significantly higher concentrations in the livers of HSV-2 infected mice than in the livers of uninfected mice, mice infected with murine hepatitis virus (MHV-A59) or mice with hepatitis from carbon tetrachloride treatment. Neither altered renal function, nor altered pharmacokinetics could account for this difference. 19F Nuclear Magnetic Resonance spectroscopy readily detected the 19F from TFT in both liver extracts and whole livers, particularly at higher tissue levels, i.e. greater than 50 micrograms/g tissue. If further studies with living animals support these preliminary observations, clinical application could be pursued.

Effect of acyclovir on the uptake of 131I-labelled 1-(2'fluoro-2'-deoxy-beta-D-arabinofuranosyl)-5-iodouracil in herpes infected cells

Selective uptake of nucleoside analogues by herpes simplex virus infected cells may serve as the basis for a specific non-invasive diagnostic test for herpes simplex encephalitis. We have examined the effect of acyclovir on the selective uptake of [131I] 1-(2'-fluoro-2'-deoxy-beta-D-arabinofuranosyl)-5-iodouracil in herpes simplex virus infected primary rabbit kidney cells. Infected cells treated with acyclovir continued to concentrate [131I] 1-(2'-fluoro-2'-deoxy-beta-D-arabinofuranosyl)-5-iodouracil for up to 24 h after the addition of the antiviral agent. These results indicated that therapy with acyclovir for as long as 24 h would not prevent the selective trapping of nucleoside analogues. This has important implications for the use of nucleoside analogues in diagnostic brain scans to detect herpes simplex encephalitis.

Simple quantitative radioiodination of (E)-5-(2-iodovinyl)-2'-deoxyuridine (IVDU) by exchange labelling

The synthesis of [125I]-(E)-5-(2-iodovinyl)-2'-deoxyuridine [( 125I]IVDU) by a halogen isotope exchange reaction has been monitored in various reaction conditions. Quantitative incorporation of radioiodine into IVDU was obtained within 20 min by a simple and safe method that is suitable for application in any nuclear medicine laboratory. The radiolabelled antiviral agent can be used without further purification.

Diagnostic imaging of herpes simplex virus encephalitis using a radiolabeled antiviral drug: autoradiographic assessment in an animal model

To develop a new approach to the diagnosis of herpes simplex encephalitis, we used a radiolabeled antiviral drug, 2'-fluoro-5-methyl-1-beta-D-arabinosyluracil labeled with carbon 14 ([14C]FMAU), as a probe for selectively imaging brain infection in a rat model by quantitative autoradiography. A high correlation was found between focal infection, as defined by immunoperoxidase viral antigen staining, and increased regional [14C]FMAU uptake in brain sections. Two potential sources of false-positive imaging were defined: high concentrations of drug in the choroid plexus because of its higher permeability compared with brain, and drug sequestration by proliferating uninfected cell populations. Our results support the soundness of the proposed strategy of using a labeled antiviral drug that is selectively phosphorylated by herpes simplex virus type 1 thymidine kinase in conjunction with scanning methods for human diagnosis, and also define some of the factors that must be taken into account when planning clinical application.