Phenotypic characterization of two naturally occurring human Cytomegalovirus sequence polymorphisms located in a distinct region of ORF UL56 known to be involved in in vitro resistance to letermovir

Cytomegalovirus drug resistance mutations in transplant recipients with suspected resistance

Resistant CMV infections are challenging complications after SOT and HSCT. Prompt recognition of ARMs is imperative for appropriate therapy. 108 plasma samples from 96 CMV + transplant recipients with suspected resistance were analysed in CNM in a retrospective nationwide study from January 2018 to July 2022 for resistance genotyping. ARMs in UL97 and UL54 were found in 26.87% (18/67) and 10.60% (7/66) of patients, respectively. Patients' ARM distribution in UL97 was as follows: L595S n = 3; L595S/M460I n = 1; L595S/N510S n = 1; L595W n = 1; C603W n = 4; A594V n = 3; A594E n = 1; C607Y n = 1; L397R/T409M/H411L/M460I n = 1; L397I n = 1; H520Q n = 1; four patients showed ARMs in UL54 as well (F412C n = 1; T503I n = 2; P522S n = 1), whereas three patients exhibited ARMs in UL54 only (L501I/T503I/L516R/A834P n = 1; A987G n = 2). L516R in UL54 and L397R/I and H411L in UL97 have been found for the first time in a clinical sample. L595S/W was the most prevalent ARM found to lend resistance to GCV. In UL54 all ARMs lent resistance to GCV and CDV. In addition, A834P, found in one patient, also lent resistance to FOS. CMV load did not differ significantly in patients with or without ARMs, and no differences were found either between patients with ARMs in UL97 or in UL97 and UL54. Despite extensive use of classical antivirals for the treatment of CMV infection after HSCT and SOT, ARMs occurred mainly in viral UL97 kinase, which suggests that CDV and mostly FOS continue to be useful alternatives to nucleoside analogues after genotypic detection of ARMs.

Relative frequency of cytomegalovirus UL56 gene mutations detected in genotypic letermovir resistance testing

Genotypic testing for letermovir (LMV) resistance was performed by Sanger sequencing of cytomegalovirus terminase gene UL56 (codons 202-412) in 1165 diagnostic specimens, disclosing 36 sequence variants among 173 (14.8%) of the specimens, including one or more LMV resistance mutations in 134 specimens. Codon 325 mutations (C325Y/F/W/R) were the most common (108 specimens), followed by those at codon 369 (R369 S/G/T/K, 13 specimens) and V236M (11 specimens). Mutations V231L, N232Y, Q234R, L257F and V363I were detected in 1-3 specimens each. Combinations of codon 325 mutation and those at codons 236 or 369 were found in 6 specimens. Eleven novel sequence variants were phenotyped, validating Q234R, V363I and R369K as conferring 2- to 5-fold increased LMV 50% inhibitory concentrations (EC50). These findings indicate that UL56 codon 325 mutations conferring >3000-fold LMV EC50 are detected much more frequently in clinical practice than those conferring lower grade resistance, and suggest that a single step mutation to absolute LMV resistance is an ongoing concern in its therapeutic use.

De Novo Genotypic Heterogeneity in the UL56 Region in Cytomegalovirus-Infected Tissues: Implications for Primary Letermovir Resistance

BACKGROUND

Letermovir, an inhibitor of unique long (UL)56-encoded cytomegalovirus (CMV)-terminase, shows prophylactic effects with low-grade adverse events in hematopoietic stem cell transplant recipients. Despite few case reports on acquired letermovir resistance, the frequency of de novo amino acid (A.A.) changes encoded by UL56 in CMV-infected tissues is unclear.

METHODS

We analyzed CMV UL56 sequences between the conserved region IV and variable region I in 175 formalin-fixed, paraffin-embedded tissues obtained from 147 patients showing positive CMV immunochemical staining between November 2012 and October 2016. Nucleotides 552-1330 of the open reading frame of UL56 were amplified with 5 primers and sequenced by a dideoxy fluorescence-based cycle.

RESULTS

Six (3.4%) tissues from 4 (2.7%) patients harbored A.A. substitutions. There were no known potent resistant mutations. However, we found C325Y in 2 tissues from 1 patient, along with other mutations. Four novel A.A. changes, which have not been observed in previous in vitro experiments, were identified (T244I, S301T, G312V, and M434I). Most (9 of 11, 81.8%) of the A.A. changes occurred between the codons 301 and 325 present between the conserved regions V and VI.

CONCLUSIONS

The treatment difficulties associated with letermovir resistance in a clinical setting need to be verified before its widespread use.

DNA Encapsidation and Capsid Assembly Are Underexploited Antiviral Targets for the Treatment of Herpesviruses

Although there are effective nucleoside analogs to treat HSV, VZV, and HCMV disease, herpesvirus infections continue to contribute to significant morbidity and mortality. Acyclovir is the drug of choice for HSV encephalopathy, yet there is an estimated 6-19% mortality rate with half of the survivors experiencing moderate to severe chronic neurological deficits. For VZV, current treatments are inadequate to prevent acute and persistent pain due to zoster. Treatment of HCMV with GCV requires close monitoring particularly in patients with impaired renal function and there are no approved treatments for congenital HCMV infections. New therapeutic options to control cytomegalovirus reactivation in bone marrow and stem cell transplant patients are needed to improve patient outcome. No successful chemotherapeutic options are available for EBV, HHV-6, 7, and 8. Drug resistance is a concern for HCMV, HSV, and VZV since approved drugs share common mechanisms of action. Targeting DNA encapsidation or capsid assembly provide additional options for the development of non-nucleoside, small molecule anti-herpesviral drugs.

Letermovir and its role in the prevention of cytomegalovirus infection in seropositive patients receiving an allogeneic hematopoietic cell transplant

Cytomegalovirus (CMV) reactivation is one of the most common infections affecting allogeneic hematopoietic cell transplant recipients. Although available anti-CMV therapies have been evaluated for the prevention of CMV reactivation, their toxicity profile makes them unfavorable for use as primary prophylaxis; thus, they are routinely reserved for the treatment of CMV viremia or CMV end-organ disease. Pre-emptive CMV monitoring strategies have been widely accepted, and although they have been helpful in early detection, they have not affected the overall morbidity and mortality associated with CMV. Letermovir is a novel agent that was approved for primary prophylaxis in CMV-seropositive adult allogeneic hematopoietic cell transplant recipients. This review focuses on letermovir's novel mechanism; clinical trials supporting its United States Food and Drug Administration (FDA) approval and subsequent follow-up analyses; clinical considerations, with an emphasis on pharmacology; and lessons learned from solid organ transplant recipients, as well as potential future directions.

Advances in the genotypic diagnosis of cytomegalovirus antiviral drug resistance

Cytomegalovirus (CMV) drug resistance mutation maps are updated with recent information for polymerase inhibitors, the terminase inhibitor letermovir and the UL97 kinase inhibitor maribavir. Newly mapped mutations and their phenotypes provide more detail on cross-resistance properties and suggest the need to expand the CMV gene regions covered in diagnostic testing. Next-generation deep sequencing technology offers a more sensitive, higher resolution view of emerging antiviral resistance and is recommended for use in clinical trials. Issues of standardization and diagnostic utility in comparison with traditional Sanger sequencing remain unresolved. Quality control is important for the accurate and reproducible detection of mutant viral populations in clinical specimens.

Hinge Region in DNA Packaging Terminase pUL15 of Herpes Simplex Virus: A Potential Allosteric Target for Antiviral Drugs

Approximately 80% of adults are infected with a member of the herpesviridae family. Herpesviruses establish life-long latent infections within neurons, which may reactivate into lytic infections due to stress or immune suppression. There are nine human herpesviruses (HHV) posing health concerns from benign conditions to life threatening encephalitis, including cancers associated with viral infections. The current treatment options for most HHV conditions mainly include several nucleoside and nucleotide analogs targeting viral DNA polymerase. Although these drugs help manage infections, their common mechanism of action may lead to the development of drug resistance, which is particularly devastating in immunocompromised patients. Therefore, new classes of drugs directed against novel targets in HHVs are necessary to alleviate this issue. We analyzed the conservation rates of all proteins in herpes simplex virus 1 (HHV-1), a representative of the HHV family and one of the most common viruses infecting the human population. Furthermore, we generated a full-length structure model of the most conserved HHV-1 protein, the DNA packaging terminase pUL15. A series of computational analyses were performed on the model to identify ATP and DNA binding sites and characterize the dynamics of the protein. Our study indicates that proteins involved in HHV-1 DNA packaging and cleavage are amongst the most conserved gene products of HHVs. Since the packaging protein pUL15 is the most conserved among all HHV-1 gene products, the virus will have a lower chance of developing resistance to small molecules targeting pUL15. A subsequent analysis of the structure of pUL15 revealed distinct ATP and DNA binding domains and the elastic network model identifies a functionally important hinge region between the two domains of pUL15. The atomic information on the active and allosteric sites in the ATP- and DNA-bound model of pUL15 presented in this study can inform the structure-based drug discovery of a new class of drugs to treat a wide range of HHVs.

An Influenza Virus Hemagglutinin-Based Vaccine Platform Enables the Generation of Epitope Specific Human Cytomegalovirus Antibodies

Human cytomegalovirus (CMV) is a highly prevalent pathogen with ~60%–90% seropositivity in adults. CMV can contribute to organ rejection in transplant recipients and is a major cause of birth defects in newborns. Currently, there are no approved vaccines against CMV. The epitope of a CMV neutralizing monoclonal antibody against a conserved region of the envelope protein gH provided the basis for a new CMV vaccine design. We exploited the influenza A virus as a vaccine platform due to the highly immunogenic head domain of its hemagglutinin envelope protein. Influenza A variants were engineered by reverse genetics to express the epitope of an anti-CMV gH neutralizing antibody that recognizes native gH into the hemagglutinin antigenic Sa site. We determined that the recombinant influenza variants expressing 7, 10, or 13 residues of the anti-gH neutralizing antibody epitope were recognized and neutralized by the anti-gH antibody 10C10. Mice vaccinated with the influenza/CMV chimeric viruses induced CMV-specific antibodies that recognized the native gH protein and inhibited virus infection. In fact, the influenza variants expressing 7–13 gH residues neutralized a CMV infection at ~60% following two immunizations with variants expressing the 13 residue gH peptide produced the highest levels of neutralization. Collectively, our study demonstrates that a variant influenza virus inserted with a gH peptide can generate a humoral response that limits a CMV infection.

Role of letermovir for prevention of cytomegalovirus infection after allogeneic haematopoietic stem cell transplantation

PURPOSE OF REVIEW

Cytomegalovirus (CMV) infection is a common opportunistic infection after allogeneic haematopoietic stem cell transplantation (HSCT). CMV surveillance-preemptive therapy is the current preferred approach for preventing CMV disease after HSCT. In contrast, antiviral prophylaxis is not commonly used due to myelosuppressive effects of valganciclovir. In this article, the role of the newly approved antiviral compound, letermovir, is reviewed.

RECENT FINDINGS

Letermovir inhibits CMV by interfering viral terminase complex. In a phase 3 randomized placebo-controlled clinical study that enrolled 495 CMV-seropositive HSCT recipients, the primary end point of clinically significant CMV infection was significantly reduced by letermovir prophylaxis. Letermovir was well tolerated without risk of myelotoxicity and nephrotoxicity. Experimental in-vitro data suggest that letermovir may possess low genetic barrier to resistance. In prophylaxis trials, two breakthrough infections were reported due to selection of CMV UL56 V236M mutation.

SUMMARY

Letermovir is an important addition to the current strategies for CMV prevention after allogeneic HSCT. Its favourable efficacy and safety profile re-opens door for antiviral prophylaxis another first-line option, similar to CMV surveillance-preemptive therapy, for preventing CMV in allogeneic HSCT recipients.

Antiviral prophylaxis for cytomegalovirus infection in allogeneic hematopoietic cell transplantation

Patients treated with allogeneic hematopoietic cell transplantation (HCT) are at risk of cytomegalovirus (CMV) reactivation and disease, which results in increased morbidity and mortality. Although universal antiviral prophylaxis against CMV improves outcomes in solid organ transplant recipients, data have been conflicting regarding such prophylaxis in patients undergoing allogeneic HCT. We conducted a systematic review of randomized trials of prophylactic antivirals against CMV after allogeneic HCT to summarize the evolution of the field over the last 35 years and evaluate the prophylactic potential of antiviral agents against CMV after allogeneic HCT. Electronic databases were queried from database inception through 31 December 2017. For included studies, incidence of CMV infection and all-cause mortality were collected as primary outcomes; CMV disease incidence, use of preemptive therapy, and drug toxicities were collected as secondary outcomes. Nineteen clinical trials conducted between 1981 and 2017 involving a total of 4173 patients were included for review. Prophylactic strategies included use of acyclovir, valacyclovir, ganciclovir, maribavir, brincidofovir, and letermovir compared with placebo or a comparator antiviral. Fourteen trials that compared antiviral prophylaxis with placebo demonstrated overall effectiveness in reducing incidence of CMV infection (odds ratio [OR], 0.49; 95% confidence interval [CI], 0.42-0.58), CMV disease (OR, 0.56; 95% CI, 0.40-0.80), and use of preemptive therapy (OR, 0.51; 95% CI, 0.42-0.62; 6 trials); however, none demonstrated reduction in all-cause mortality (OR, 0.96; 95% CI, 0.78-1.18) except the phase 3 trial of letermovir (week-24 OR, 0.59; 95% CI, 0.38-0.98). Additional research is warranted to determine patient groups most likely to benefit from antiviral prophylaxis and its optimal deployment after allogeneic HCT.

[Modern ethiotropic chemotherapy of human cytomegalovirus infection: clinical effectiveness, molecular mechanism of action, drug resistance, new trends and prospects. Part 2.]

A number of synthetic compounds, such as the nucleoside analog ganciclovir, its L-valine ester (a metabolic precursor of ganciclovir) and pyrophosphate analog foscarnet, are permitted for the treatment of HCMVrelated diseases in the WHO European Region. The viral DNA- polymerase is used by all these drugs as a biotarget. However, the usage of standard anti-CMV therapy is accompanied by severe side effects, as well as the development of drug resistance in the virus, mainly in conditions of immunodefciency. In this review, we focused on viral proteins of interest as new potential targets and their inhibitors, such as the inhibitor of human CMV terminology, lethermovir, which showed great activity in the third phase of clinical trials, inhibitors of viral cyclin-dependent kinase (maribavir, cyclopropavir) and a number of compounds exhibiting anti-HCMV-activity, undergoing only preclinical trials in the experiment. Inclusion of new anti-CMV agents that are active against GСV/PFA/CDV-resistant strains of CMV into standard prophylactic and therapeutic regimens, will allow to increase the effectiveness of anti-CMV therapy, including in cases when standard therapy is ineffective. Areas covered: the international databases such as A MEDLINE, PubMed, eLIBRARY.RU, ClinicalTrials.gov., etc. with the purpose of obtaining information on compounds showing selective action against the human cytomegalovirus, the most promising for the development of drugs.

Pharmacokinetic drug evaluation of letermovir prophylaxis for cytomegalovirus in hematopoietic stem cell transplantation

INTRODUCTION

Letermovir is a new antiviral approved to prevent cytomegalovirus infection in hematopoietic stem cell transplant recipients. It has a distinct mechanism of action as it acts as a terminase complex inhibitor, and shows some advantages compared to the current treatment options for cytomegalovirus infection. Areas covered: This review focuses on the efficacy, safety, pharmacokinetics, pharmacodynamics, and drug-drug interactions of letermovir. Expert opinion: Letermovir is a new antiviral to prevent cytomegalovirus infection. Unlike the currently used polymerase inhibitors, it has a distinct mechanism of action with better safety, limited resistance, and no cross-resistance. Although a lot of research on pharmacokinetics and drug-drug interactions has already been performed, it might be useful to clarify the effect of letermovir on voriconazole exposure, the drug-drug interaction between caspofungine and letermovir and the effect of statins on letermovir exposure. Also, the lack of an exposure-response relationship should be confirmed in large real-life post-marketing studies in order to be able to lower the intravenous dose of letermovir.

Letermovir for the prevention of cytomegalovirus infection in adult cytomegalovirus-seropositive hematopoietic stem cell transplant recipients

INTRODUCTION

Allogeneic hematopoietic cell transplants (allo-HCT) recipients are at the high-risk of reactivation of cytomegalovirus (CMV), and reactivation is associated with significant morbidity and mortality. Although available anti-CMV therapies may be effective for the prevention of CMV, they are plagued by unacceptable toxicities that prohibit their use in the post-transplant period. Recently studied CMV-active agents, such as maribavir and brincidofovir, failed to reduce the incidence of CMV infection in HCT recipients. Letermovir represents the first agent in the non-nucleoside 3,4 dihydro-quinazoline class of CMV viral terminase complex inhibitors, with activity solely against CMV. The positive results from the recently published Phase III study of letermovir for prevention of CMV infection in CMV-seropositive allo-HCT recipients led to its approval as a prophylactic agent for CMV in multiple countries. Areas covered: In this review, we will evaluate this novel agent with a focus on letermovir mechanism of action, pharmacokinetics and metabolism, clinical efficacy, and safety and toxicities. Expert commentary: With the introduction of letermovir, prevention of CMV infection in allo-HCT recipients may shift considerably, from a predominantly preemptive strategy to one that utilizes this novel therapy for prophylaxis.

Drug Susceptibility and Replicative Capacity of Multidrug-Resistant Recombinant Human Cytomegalovirus Harboring Mutations in UL56 and UL54 Genes

Letermovir is an investigational antiviral agent with a novel mechanism of action involving the viral terminase (pUL56). We evaluated the impact of the V236M mutation in the UL56 gene alone and in combination with the E756K mutation in the UL54 gene on drug susceptibility and viral replicative capacity of recombinant human cytomegalovirus. The double mutant exhibited at least borderline resistance to all antivirals tested (ganciclovir, foscarnet, cidofovir, brincidofovir, and letermovir) and replicated less efficiently than the wild-type virus in vitro.

Letermovir for the management of cytomegalovirus infection

INTRODUCTION

Cytomegalovirus (CMV) is a major cause of morbidity and mortality in immunocompromised patients. Available antivirals are fraught with adverse effects and risk for the development of CMV resistance. Letermovir is a novel antiviral in the late stages of drug development for the treatment and prevention of CMV. Areas covered: A MEDLINE search of the MeSH terms 'letermovir,' 'cytomegalovirus,' 'hematopoietic stem cell transplant,' and 'solid organ transplant,' was last conducted on 15 August 2016. Articles were selected on the basis of their contribution to current knowledge about letermovir. Expert opinion: Letermovir's mechanism of action, pharmacokinetic and pharmacodynamic profile, and favorable efficacy and safety make it an attractive option for both the prevention and treatment of CMV in immunocompromised patients. The lack of cross-resistance with other antivirals and the absence of myelosuppression are two prominent characteristics of letermovir that could support broad use of this product following FDA-approval. One major limitation is its lack of activity against other herpesviruses, which are commonly seen in immunocompromised hosts. We believe that with additional clinical efficacy data, this medication could emerge as a primary option for the prevention and treatment of CMV in the immunocompromised patient population.

Treatment of alpha and beta herpesvirus infections in solid organ transplant recipients

INTRODUCTION

Human herpesviruses frequently cause infections in solid organ transplant (SOT) recipients. Areas covered: We provide an overview of the clinical impact of alpha and beta herpesviruses and highlight the mechanisms of action, pharmacokinetics, clinical indications, and adverse effects of antiviral drugs for the management of herpes simplex virus, varicella zoster virus and cytomegalovirus. We comprehensively evaluated key clinical trials that led to drug approval, and served as the foundation for management guidelines. We further provide an update on investigational antiviral agents for alpha and beta herpesvirus infections after SOT. Expert commentary: The therapeutic armamentarium for herpes infections is limited by the emergence of drug resistance. There have been major efforts for discovery of new drugs against these viruses, but the results of early-phase clinical trials have been less than encouraging. We believe, however, that more antiviral drug options are needed given the adverse side effects associated with current antiviral agents, and the emergence of drug-resistant virus populations in SOT recipients. Likewise, optimized use and strategies are needed for existing and novel antiviral drugs against alpha and beta-herpesviruses in SOT recipients.

Treatment of CMV infection after allogeneic hematopoietic stem cell transplantation

Despite a remarkable reduction in the past decades, cytomegalovirus (CMV) disease in allogeneic hematopoietic stem cell transplant (HSCT) recipients remains a feared complication, still associated with significant morbidity and mortality. Today, first line treatment of CMV infection/reactivation is still based on dated antiviral compounds Ganciclovir (GCV), Foscarnet (FOS) and Cidofovir (CDF) with their burdensome weight of side effects. Maribavir (MBV), Letermovir (LMV) and Brincidofovir (BDF) are three new promising anti-CMV drugs without myelosuppressive properties or renal toxic effects that are under investigation in randomized phase II and III trials. Adoptive T-cell therapy (ATCT) in CMV infection possesses a strong rationale, demonstrated by several proof of concept studies; its feasibility is currently under investigation by clinical trials. ATCT from third-party and naïve donors could meet the needs of HSCT recipients of seronegative donors and cord blood grafts. In selected patients such as recipients of T-cell depleted grafts, ATCT, based on CMV-specific host T-cells reconstitution kinetics, would be of value in the prophylactic and/or preemptive CMV treatment. Vaccine-immunotherapy has the difficult task to reduce the incidence of CMV reactivation/infection in highly immunocompromised HSCT patients. Newer notions on CMV biology may represent the base to flush out the Troll of transplantation.

Rapid In Vitro Evolution of Human Cytomegalovirus UL56 Mutations That Confer Letermovir Resistance

Letermovir (LMV) is an experimental cytomegalovirus terminase inhibitor undergoing phase 3 clinical trials. Viral mutations have been described at UL56 codons 231 to 369 that confer widely variable levels of LMV resistance. In this study, 15 independent experiments propagating an exonuclease mutant viral strain in escalating LMV concentrations replicated 6 of the 7 published UL56 mutations and commonly elicited additional resistance-conferring mutations at UL56 codons 231, 236, 237, 244, 257, 261, 325, and 329. Mutations were first detected earlier in LMV (median, 3 passages) than in 8 parallel experiments with foscarnet (median, 15 passages). As LMV concentrations increased, the typical initial UL56 change F261L, which confers low-grade resistance, combined or was replaced with mutations conferring higher-grade resistance, eventually enabling normal viral growth in 30 μM LMV (>5,000-fold the 50% effective concentration [EC50] for the wild type). At high LMV concentrations, the UL56 changes C325F/R were commonly detected, as well as a combination of changes at codons 236, 257, 329, and/or 369. Recombinant viruses containing individual UL56 mutations and combinations were constructed to confirm their resistance phenotypes and normal growth in cell culture. Several double and triple mutants showed much higher LMV resistance than the respective single mutants, particularly those including changes at both codons 236 and 257. The multiplicity of pathways to high-grade LMV resistance with minimal viral growth impact suggests a low viral genetic barrier and the need for close monitoring during treatment of active infection.

Letermovir and inhibitors of the terminase complex: a promising new class of investigational antiviral drugs against human cytomegalovirus

Infection with cytomegalovirus is prevalent in immunosuppressed patients. In solid organ transplant and hematopoietic stem cell transplant recipients, cytomegalovirus infection is associated with high morbidity and preventable mortality. Prevention and treatment of cytomegalovirus with currently approved antiviral drugs is often associated with side effects that sometimes preclude their use. Moreover, cytomegalovirus has developed mutations that confer resistance to standard antiviral drugs. During the last decade, there have been calls to develop novel antiviral drugs that could provide better options for prevention and treatment of cytomegalovirus. Letermovir (AIC246) is a highly specific antiviral drug that is currently undergoing clinical development for the management of cytomegalovirus infection. It acts by inhibiting the viral terminase complex. Letermovir is highly potent in vitro and in vivo against cytomegalovirus. Because of a distinct mechanism of action, it does not exhibit cross-resistance with other antiviral drugs. It is predicted to be active against strains that are resistant to ganciclovir, foscarnet, and cidofovir. To date, early-phase clinical trials suggest a very low incidence of adverse effects. Herein, we present a comprehensive review on letermovir, from its postulated novel mechanism of action to the results of most recent clinical studies.