Anticytomegaloviral activity of methotrexate associated with preferential accumulation of drug by cytomegalovirus-infected cells

Pemetrexed inhibits Kaposi's sarcoma-associated herpesvirus replication through blocking dTMP synthesis

Kaposi's sarcoma-associated herpesvirus (KSHV) is the etiologic agent of Kaposi's sarcoma, primary effusion lymphoma, and multicentric Castleman's disease. In immunocompromised patients, KSHV infection is capable of causing severe and fatal diseases. Current antiviral treatments for KSHV infections consist mostly of nucleoside analogs, all of which target viral polymerases and are associated with adverse effects and drug resistance. By screening an FDA-approved drug library, we identified pemetrexed as a potent anti-KSHV agent, with an IC₅₀ of 90 nM. Characterization of the antiviral properties of pemetrexed revealed that it interferes with the lytic replication of viral DNA, resulting in the reduction of infectious virions. The antiviral effect of pemetrexed depends on the dTMP synthesis pathway that requires the folate-dependent enzymes. Besides, pemetrexed shows a broad spectrum of anti-herpes virus activity. Thus, our findings suggest that pemetrexed inhibits the lytic replication of KSHV DNA by blocking dTMP synthesis. Pemetrexed has the potential to be utilized as an anti-KSHV agent.

Selective modification of adenovirus replication can be achieved through rational mutagenesis of the adenovirus type 5 DNA polymerase

Mutations that reduce the efficiency of deoxynucleoside (dN) triphosphate (dNTP) substrate utilization by the HIV-1 DNA polymerase prevent viral replication in resting cells, which contain low dNTP concentrations, but not in rapidly dividing cells such as cancer cells, which contain high levels of dNTPs. We therefore tested whether mutations in regions of the adenovirus type 5 (Ad5) DNA polymerase that interact with the dNTP substrate or DNA template could alter virus replication. The majority of the mutations created, including conservative substitutions, were incompatible with virus replication. Five replication-competent mutants were recovered from 293 cells, but four of these mutants failed to replicate in A549 lung carcinoma cells and Wi38 normal lung cells. Purified polymerase proteins from these viruses exhibited only a 2- to 4-fold reduction in their dNTP utilization efficiency but nonetheless could not be rescued, even when intracellular dNTP concentrations were artificially raised by the addition of exogenous dNs to virus-infected A549 cells. The fifth mutation (I664V) reduced biochemical dNTP utilization by the viral polymerase by 2.5-fold. The corresponding virus replicated to wild-type levels in three different cancer cell lines but was significantly impaired in all normal cell lines in which it was tested. Efficient replication and virus-mediated cell killing were rescued by the addition of exogenous dNs to normal lung fibroblasts (MRC5 cells), confirming the dNTP-dependent nature of the polymerase defect. Collectively, these data provide proof-of-concept support for the notion that conditionally replicating, tumor-selective adenovirus vectors can be created by modifying the efficiency with which the viral DNA polymerase utilizes dNTP substrates.

The multi-targeted kinase inhibitor sorafenib inhibits human cytomegalovirus replication

Human cytomegalovirus (HCMV) is a major pathogen in immunocompromised individuals. Here, non-toxic concentrations of the anti-cancer kinase inhibitor sorafenib were shown to inhibit replication of different HCMV strains (including a ganciclovir-resistant strain) in different cell types. In contrast to established anti-HCMV drugs, sorafenib inhibited HCMV major immediate early promoter activity and HCMV immediate early antigen (IEA) expression. Sorafenib is known to inhibit Raf. Comparison of sorafenib with the MEK inhibitor U0126 suggested that sorafenib inhibits HCMV IEA expression through inhibition of Raf but independently of signaling through the Raf downstream kinase MEK 1/2. In concordance, siRNA-mediated depletion of Raf but not of MEK-reduced IEA expression. In conclusion, sorafenib diminished HCMV replication in clinically relevant concentrations and inhibited HCMV IEA expression, a pathophysiologically relevant event that is not affected by established anti-HCMV drugs. Moreover, we demonstrated for the first time that Raf activation is involved in HCMV IEA expression.

The thymidylate synthase inhibitor ZD1694 potently inhibits murine and human cytomegalovirus replication in quiescent fibroblasts

Tomudex (ZD1694) is a quinazoline-based folate analog and a powerful inhibitor of cellular thymidylate synthase and is approved in Europe for use in oncology. Here the first evidence of its activity against murine and human cytomegalovirus (MCMV and HCMV) is reported. ZD1694 irreversibly inhibited the replication and DNA synthesis of both viruses in quiescent fibroblasts. The corresponding 50% effective concentrations were 0.006 and 0.002 microM respectively, whereas the 50% cytotoxic concentration was >10 microM for both murine and human quiescent fibroblasts. A similar antiviral effect was observed against two ganciclovir-resistant HCMV strains isolated from AIDS patients. Taken as a whole these results demonstrate that cellular thymidylate synthase plays an essential role in viral replication and that ZD1694 merits further investigation as anticytomegaloviral agent.