Recent strategies in the development of new human cytomegalovirus inhibitors

Structure-Driven Discovery of α,γ-Diketoacid Inhibitors Against UL89 Herpesvirus Terminase

Human cytomegalovirus (HCMV) is an opportunistic pathogen causing a variety of severe viral infections, including irreversible congenital disabilities. Nowadays, HCMV infection is treated by inhibiting the viral DNA polymerase. However, DNA polymerase inhibitors have several drawbacks. An alternative strategy is to use compounds against the packaging machinery or terminase complex, which is essential for viral replication. Our discovery that raltegravir (1), a human immunodeficiency virus drug, inhibits the nuclease function of UL89, one of the protein subunits of the complex, prompted us to further develop terminase inhibitors. On the basis of the structure of 1, a library of diketoacid (α,γ-DKA and β,δ-DKA) derivatives were synthesized and tested for UL89-C nuclease activity. The mode of action of α,γ-DKA derivatives on the UL89 active site was elucidated by using X-ray crystallography, molecular docking, and in vitro experiments. Our studies identified α,γ-DKA derivative 14 able to inhibit UL89 in vitro in the low micromolar range, making 14 an optimal candidate for further development and virus-infected cell assay.

The chemistry and biological potential of azetidin-2-ones

Azetidin-2-ones, commonly referred as β-lactams, represent a unique ring system, with interesting chemistry and great biological potential. Besides its well known antibiotic activity, this ring system exhibits a wide range of activities, attracting the attention of researchers. The biological and pharmacological profile of azetidin-2-ones is reviewed here comprehensively with several examples under fourteen different activity heads. The chemistry and methods of synthesis have also been discussed.

Influence of 6 or 8-substitution on the antiviral activity of 3-phenethylthiomethylimidazo[1,2-a]pyridine against human cytomegalovirus (HCMV) and varicella-zoster virus (VZV)

The synthesis of original imidazo[1,2-a]pyridines bearing a phenethylthiomethyl side chain at the 3 position and a (hetero)aryl substituent on the 6 or 8 position, and their antiviral activities are reported. From the synthesized compounds, the 6-halogeno and 6-phenylimidazo[1,2-a]pyridine derivatives 4c-d and 5b were the most potent against human cytomegalovirus (CMV) and/or varicella-zoster virus (VZV), whereas several other congeners (i.e., 5e, 5g, 5i, 5l, 5n, 5p, 5q, and 5t), while less potent, were equally or more selective in their inhibitory activity against both VZV and CMV. These compounds showed similar activity against thymidine kinase competent (TK(+)) and deficient (TK(-)) VZV strains, demonstrating a mechanism of action independent of the viral thymidine kinase.

Enzymatic activities of human cytomegalovirus maturational protease assemblin and its precursor (pPR, pUL80a) are comparable: [corrected] maximal activity of pPR requires self-interaction through its scaffolding domain

Herpesviruses encode an essential, maturational serine protease whose catalytic domain, assemblin (28 kDa), is released by self-cleavage from a 74-kDa precursor (pPR, pUL80a). Although there is considerable information about the structure and enzymatic characteristics of assemblin, a potential pharmacologic target, comparatively little is known about these features of the precursor. To begin studying pPR, we introduced five point mutations that stabilize it against self-cleavage at its internal (I), cryptic (C), release (R), and maturational (M) sites and at a newly discovered "tail" (T) site. The resulting mutants, called ICRM-pPR and ICRMT-pPR, were expressed in bacteria, denatured in urea, purified by immobilized metal affinity chromatography, and renatured by a two-step dialysis procedure and by a new method of sedimentation into glycerol gradients. The enzymatic activities of the pPR mutants were indistinguishable from that of IC-assemblin prepared in parallel for comparison, as determined by using a fluorogenic peptide cleavage assay, and approximated rates previously reported for purified assemblin. The percentage of active enzyme in the preparations was also comparable, as determined by using a covalent-binding suicide substrate. An unexpected finding was that, in the absence of the kosmotrope Na2SO4, optimal activity of pPR requires interaction through its scaffolding domain. We conclude that although the enzymatic activities of assemblin and its precursor are comparable, there may be differences in how their catalytic sites become fully activated.

Antiviral targets of a chromene derivative from Sargassum micracanthum in the replication of human cytomegalovirus

A chromene derivative (1) obtained from a brown alga, Sargassum micracanthum, has been proved to be a potent inhibitor of human cytomegalovirus (HCMV). In the present study, we evaluated its mode of action by various experimental assays. Time-of-addition experiments revealed that 1 was active if applied to cells before viral DNA synthesis, indicating that it inhibited early events of virus replication including virus adsorption and penetration, and a step immediately after viral internalization. Virus attachment and penetration studies suggested that one of the targets for anti-HCMV action of 1 was virus adsorption to cells and to a lesser extent, virus internalization was delayed in the presence of the compound. Pretreatment of virus particles with 1 showed that the compound exerted dose-dependent virucidal action. The chromene derivative and ganciclovir (GCV), an anti-HCMV drug, were synergistic inhibitors when used in combination. The synergistic effect could be explained by inhibition of different steps in HCMV replication cycle produced by 1 and GCV.

Novel yeast cell-based assay to screen for inhibitors of human cytomegalovirus protease in a high-throughput format

The protease encoded by the human cytomegalovirus (HCMV) is an attractive target for antiviral drug development because of its essential function in viral replication. We describe here a cellular assay in the yeast Saccharomyces cerevisiae for the identification of small molecule inhibitors of HCMV protease by conditional growth in selective medium. In this system, the protease cleavage sequence is inserted into the N-(5'-phosphoribosyl)anthranilate isomerase (Trp1p), a yeast protein essential for cell proliferation in the absence of tryptophan. Coexpression of HCMV protease with the engineered Trp1p substrate in yeast cells results in site-specific cleavage and functional inactivation of the Trp1p enzyme, thereby leading to an arrest of cell proliferation. This growth arrest can be suppressed by the addition of validated HCMV protease inhibitors. The growth selection system presented here provides the basis for a high-throughput screen to identify HCMV protease inhibitors that are active in eukaryotic cells.

Design and synthesis of a potent macrocyclic 1,6-naphthyridine anti-human cytomegalovirus (HCMV) inhibitors

A novel class of macrocyclic 1,6-napthyridines designed to adopt the presumed bioactive conformation of anti-HCMV acyclic 1,6-napthyridines are described. Both 14- and 15-membered macrocycles were shown to be highly potent against HCMV HSV-1 and HSV-2.

Good oral absorption prediction on non-nucleoside benzothiadiazine dioxide human cytomegalovirus inhibitors using combined chromatographic and neuronal network techniques

The current drugs available against human cytomegalovirus (HCMV) suffer from a number of shortcomings such as toxic side effect, poor bioavailability and/or risk for emergence of drug-resistance virus strains. Due to these limitations, the development of new drugs against HCMV is of great interest. Taking into account the therapeutic potential of benzothiadiazines dioxides (BTD) derivatives, it is most important to know their oral bioavailability because all the current clinical drugs are poorly absorbed. In this work, the utility of CODES neural networks and biopartitioning micellar chromatography (BMC) in predicting pharmacokinetic properties has been used to estimate the oral absorption of BTD derivatives and their efficacy has been verified. The results indicate higher values for BTD derivatives than the currently licensed anti-viral agents.

Studies of substrate-induced conformational changes in human cytomegalovirus protease using optical biosensor technology

The interaction between human cytomegalovirus (HCMV) protease and a peptide substrate was studied using a surface plasmon resonance (SPR)-based biosensor. Immobilization of the enzyme to the sensor chip surface by amine coupling resulted in an active enzyme with a higher catalytic efficiency than the enzyme in solution, primarily due to a lower K(m) value. The interaction between immobilized protease and substrate was characterized by a biphasic SPR signal. Rate constants for the formation of the initial enzyme-substrate complex could be determined from the sensorgrams. Simulated binding curves based on the determined k(cat) and the rate constants indicated that the complex binding signal did not originate from the accumulation of intermediates in the catalytic reaction. By chemical crosslinking of the immobilized HCMV protease, which was shown to limit the enzyme's structural flexibility, it was revealed that the obtained sensorgrams were composed of a signal caused by substrate binding and considerable structural alterations in the immobilized enzyme. Furthermore, HCMV protease was inactivated by chemical crosslinking, indicating that structural flexibility is essential for this enzyme. Parallel experiments with immobilized alpha-chymotrypsin revealed that it does not undergo similar conformational changes on peptide binding and that crosslinking did not inactivate the enzyme. The simultaneous detection of binding and conformational changes using optical biosensor technology is expected to be of importance for further characterization of the enzymatic properties of HCMV protease and for identification of inhibitors of this enzyme. It can also be of use for studies of other flexible proteins.

Design of translactam HCMV protease inhibitors as potent antivirals

Human cytomegalovirus (HCMV) is an important pathogen for which there is a significant unmet medical need. New HCMV antivirals, active against novel molecular targets, are undoubtedly needed as the currently available drugs ganciclovir, cidofovir, and foscarnet, which are all viral DNA inhibitors, suffer from limited effectiveness, mainly due to the development of drug resistance, poor bioavailability, and toxicity. One of the newer molecular targets that has been exploited in the search for better drug candidates is HCMV protease. Our deltaAla HCMV protease (wild type variant with the internal cleavage site deleted) was cloned and expressed in E. coli. This viral enzyme was used to develop HCMV protease assays to evaluate potential inhibitors. The chirally pure (SRS)-alpha-methyl pyrrolidine-5,5-trans-lactam template was synthesized, which together with the natural substrate requirements of HCMV protease and detailed SAR, was used to design potent and selective mechanism based inhibitors of HCMV protease. The mechanism of action of these inhibitors of HCMV protease was investigated by ESI/MS, and the X-ray crystal structure of the HCMV protease was used to refine our selective viral enzyme inhibitors to obtain plasma stable antivirals. A novel ELISA antiviral assay was developed which, together with a cytotoxicity assay, enabled us to discover anti-HCMV drug candidates equivalent in potency to ganciclovir that had good pharmacokinetics in the dog and good brain and ocular penetration in the guinea pig.

Synthesis and anti-HCMV activity of 1-acyl-beta-lactams and 1-acylazetidines derived from phenylalanine

Different Phe-derived 1-acyl-beta-lactams, analogous to a series of 2-azetidinones acting as HCMV serine protease inhibitors, were synthesized. Some of these compounds were modest inhibitors of the HCMV replication. Interestingly, removal of the carbonyl group of the beta-lactam ring, most likely acting as the serine trap, resulted in an azetidine derivative with anti-HCMV activity comparable to that of the reference compound ganciclovir.

Gastrointestinal Infections Caused by Cytomegalovirus

Cytomegalovirus (CMV) is an important human pathogen, causing a variety of syndromes ranging from asymptomatic infections to life-threatening end-organ disease. Infections of the gastrointestinal tract are an especially common form of disease caused by CMV and are usually manifest as luminal infections, such as esophagitis or colitis. Solid organ disease caused by CMV is also known to occur, with hepatitis being the most common syndrome. The large majority of cases of tissue-invasive disease caused by CMV have been reported to occur in immunocompromised hosts. Patients at highest risk for serious CMV-mediated disease are commonly HIV infected, recipients of transplants, or are receiving potent immunosuppressive medications. Intriguingly, several recent reports have discussed the previously underappreciated role of serious forms of CMV-mediated disease occurring in immunocompetent patients. This paper reviews several recent reports addressing important aspects related to the diagnosis, treatment, and management of CMV infections of the gastrointestinal tract.

Benzothiadiazine dioxide human cytomegalovirus inhibitors: synthesis and antiviral evaluation of main heterocycle modified derivatives

The benzothiadiazine dioxide derivatives are potent non-nucleoside human cytomegalovirus (HCMV) inhibitors. As part of our comprehensive structure-activity relationship (SAR) study of these compounds, we have now proposed structural modifications on the heterocyclic moiety both on the number and the nature of the fused heterocycle and on the kind of heteroatoms present on it. Synthesis of these new compounds (benzyl derivatives of thiadiazines, thienothiadiazines, benzothienothiadiazines and quinazolines) and the antiviral evaluation against HCMV has been performed. SAR investigation on this class of compounds has defined the structural requirements for potency and toxicity. They have revealed two important clues: i) a fused ring to the thiadiazine framework is necessary to maintain the anti-HCMV action, and ii) the sulfamido moiety in the main heterocycle is crucial to avoid cytotoxicity.

Congenital Cytomegalovirus Infection

Intrauterine infection with cytomegalovirus (CMV), a betaherpesvirus, remains the most frequent congenital virus infection in many regions of the world. Although most CMV-infected newborns lack signs of CMV infection, approximately 10% have signs that can consist of low birth weight, jaundice, hepatosplenomegaly, skin rash, microcephaly, and chorioretinitis. Neonates with signs of CMV infection at birth have high rates of audiologic and neurodevelopmental sequelae. Although postnatal therapy with ganciclovir transiently reduces virus shedding and may lessen the audiologic consequences of CMV in some infected infants, additional strategies are needed to prevent congenital CMV disease and to improve the neurodevelopmental prognosis of infants infected with CMV in utero. Some cases of intrauterine infections can be prevented in susceptible women by avoiding contact with the urine or saliva of young children who may be shedding CMV. Vaccines against CMV remain in the experimental stages of development. Termination of pregnancy can be offered to women whose infants have evidence of intrauterine CMV infection and sonographic signs of central nervous system damage. Infants who survive symptomatic intrauterine infections have high rates of neurodevelopmental sequelae and require comprehensive evaluation and therapy through center and home-based early intervention programs.

Non-nucleoside inhibitors of herpesviruses

While the treatment of herpes simplex virus with acyclovir and similar nucleoside analogues was one of the first success stories in antiviral chemotherapy, substantial unmet medical needs remain for herpesvirus diseases. In particular, the increasing numbers of immunosuppressed people due to AIDS, transplantation, cancer and aging has driven the need for improved antivirals to treat diseases caused by human cytomegalovirus (HCMV). Currently available drugs for the treatment of HCMV diseases are less than ideal agents due to issues of toxicity, modest efficacy and poor oral bioavailability. High throughput screening of large compound collections for inhibitors of specific viral enzymes or inhibition of viral growth in cell culture have identified a number of new HCMV inhibitors at several pharmaceutical companies. These compounds act by inhibition of novel molecular targets such as the viral protein kinase, viral protease and viral proteins involved in DNA cleavage/packaging. In addition, novel non-nucleoside inhibitors of the herpesvirus DNA polymerase have recently been described. This review will summarise some of these research efforts and will focus on non-nucleoside compounds that directly inhibit a viral process.

Cytomegalovirus infection: perinatal implications

Cytomegalovirus (CMV), a member of the herpes virus family, is the most common cause of congenital infection in humans, affecting 0.5-3% of all newborns worldwide. Congenital cytomegalovirus infection is the leading infectious cause of deafness, learning disabilities, and mental retardation in children. The high prevalence of cytomegalovirus in the general population, unpredictability of transmission, and asymptomatic nature of the disease in otherwise healthy women challenge prevention and treatment efforts.

Anti-HIV-1 activity of benzothiadiazine dioxide

Antiviral assays carried out on the potent benzothiadiazine dioxide (BTD) human cytomegalovirus (HCMV) inhibitors have led us to find marginal but selective anti-HIV-1 activity. Specific pharmacological studies, such as time of addition experiments and assays on specific viral strains with mutations on its reverse transcriptase, have indicated that BTD compounds act as non-nucleoside reverse transcriptase inhibitors. Theoretical calculations showed a butterfly conformation for the active derivatives that are compatible with their mechanism of action. Therefore, BTD derivatives can be considered as potential lead compounds for the treatment of opportunistic HCMV infections in immunocompromised individuals such as AIDS patients.