Antiviral treatment of cytomegalovirus infection: an update

Abstract

This editorial summarizes recent developments in the management of ganciclovir-resistant human cytomegalovirus (HCMV) infections. All current drugs available for systemic treatment, including ganciclovir (GCV), valganciclovir, foscarnet and cidofovir, target the viral polymerase. However, all such compounds are hampered by dose-related toxicities and the emergence of resistance. Different approaches (e.g., PCR-based direct sequencing, pyrosequencing, mass spectrometry-based comparative sequencing) allow the fast detection of resistant HCMV and are well suited to therapy monitoring. However, more studies are required on the dynamic of mixed HCMV populations under drug pressure. Alternate antiviral compounds with new mechanisms of action, such as artesunate, leflunomid, letermovir and maribavir, are now being investigated in clinical studies. An advantage of some of the new substances is lesser toxicity issues, which might lead to new prophylactic and treatment strategies.

Keywords::

• antiviral treatment
• cytomegalovirus
• ganciclovir
• resistance
• virus evolution

1. Background

Human cytomegalovirus (HCMV) infection remains a significant cause of morbidity and occasional mortality in transplant recipients. All current anti-HCMV agents available for systemic treatment like ganciclovir (GCV), its oral prodrug valganciclovir (ValGCV), foscarnet (FOS) and cidofovir (CDV) finally target the viral DNA polymerase (pUL54). However, all those compounds are hampered by dose-related toxicities, emergence of resistance and in worst cases by cross-resistance due to a similar mode of action. Relevant factors for the emergence of drug-resistant virus variants involve, for example, HCMV serostatus of donor/recipient, lack of T-cells, severe immunosuppression, high levels of viral replication, inadequate dosing of antiviral compounds and prolonged antiviral therapy. During the last couple of years new antiviral compounds with new mechanisms of action have been developed and are now being investigated in clinical studies.

This review is intended to line out the latest developments in prevention and management of ganciclovir-resistant cytomegalovirus infections which have been described previously [1].

2. Recent studies on the emergence of resistant cytomegalovirus in the clinical setting

Antiviral prophylaxis and pre-emptive therapy are the main strategies aiming to prevent cytomegalovirus-associated disease in transplant recipients. However, both strategies have their benefits and limitations.

Guidelines in solid-organ transplantation recommend GCV-prophylaxis for 100 – 200 days, although pre-emptive therapy remains a valid alternative [2]. However, late infection and disease remain problems, especially in the setting of short-term GCV-prophylaxis (< 100 days). These complications seem to be reduced after 200 days in renal transplant patients. An advantage of the more selective pre-emptive therapy might be the potential earlier occurrence of HCMV-specific T-cell reconstitution, which could reduce the incidence of HCMV-disease and antiviral resistance.

Recent clinical studies revealed higher incidences of drug resistance in patients, particularly in high-risk patients (donor HCMV seropositive/recipient HCMV seronegative), on pre-emptive management than in those given prophylaxis [3-7]. In general, if antiviral resistance is suspected, the first recommended therapeutic change is to decrease immunosuppressive therapy to the lowest feasible amount. However, antiviral therapy may be insufficient to suppress viral replication due to, for example, immune impairment or inadequate drug levels, strictly independent from virus-borne mutations. This clinical treatment failure has to be distinguished from real virological resistance. In a French multicenter study ∼ 50% of the suspected antiviral resistances due to clinical treatment failure were not based on virological resistance [3]. UL97 and UL54 genotyping was performed if viral replication persisted for > 21 days of appropriate antiviral treatment. Persistent viraemia at day 21, but not at day 14, was shown to be a risk factor for the emergence of resistance. In this study > 20% of resistant isolates harbored pUL97 mutations combined with pUL54 mutations or pUL54 alone (n = 1/18). Another study reported a high incidence of UL97-resistance, but no UL54 gene mutations were detected [4]. Foscarnet treatment rapidly and safely eradicated HCMV DNA-emia. However, patients, in whom immunosuppressive medication was reduced and continued on ValGCV treatment, eventually, cleared the virus.

3. New aspects for early detection of resistant cytomegalovirus and virus dynamic in treated patients

Different genotypic approaches show limited sensitivity in detecting minor resistant virus populations. Pyrosequencing demonstrated superiority for the monitoring of resistance-associated mutations in the HCMV UL97 gene, which were not detected by conventional DNA sequencing [8,9]. Thereby, four different HCMV variants carrying resistance-associated mutations each representing 11 – 17% of the total HCMV population were found. The power of pyrosequencing has been confirmed by another study, which strikingly revealed the presence and dynamic of complex HCMV mixtures of up to six different HCMV genotypes in the same patient specimen. The relative levels of replication of the individual HCMV genotypes changed within a mixed-genotype population upon reemergence of the virus [10]. However, pyrosequencing is important for research, but the clinical significance of the HCMV minority variants should be elucidated in further studies. The implication of the increased sensitivity in clinical settings is still unclear. In addition, the application of mass spectrometry-based comparative sequencing was successfully used to detect ganciclovir-resistant HCMV [11]. In conclusion, more studies are required on the dynamic of virus populations under drug pressure. Clinicians have to face the decision as to whether risking a re-exposure to the antiviral compound in relapses is reasonable or whether an evolution of HCMV-resistant virus in long-term patients should be feared [12,13].

Finally, as a large number of new mutations have been reported, consequently a validated and continuously updated public database of the impact of new mutations on the viral phenotype should be intensified [14-16].

4. Alternative compounds for future cytomegalovirus treatment

Maribavir (MBV), a competitive inhibitor of the UL97 protein, showed limitations in recent Phase III clinical trials [17]. Proposed explanations include an inadequate dosing regimen and choice of study end point. Obviously, MBV deserves extra systematic evaluation, and its optimal dose has to be determined. However, the first patient with MBV resistance during therapy has been reported [18,19].

In a very recent, retrospective, single-center study, leflunomide, an immunosuppressive agent, was investigated in patients who had failed other therapies. The study revealed that leflunomide may work slowly and it may take weeks to months for HCMV DNA clearance. Therefore, the optimal duration of therapy and the determining factors for the use of the compound alone or in combination have to be defined [20].

An orally bioavailable ester formulation of CDV, CMX-001, has the same mechanism of action as the parent compound and may therefore not offer a novel rescue strategy in case of resistance. Clinical trials are on the way for the prevention of HCMV infection after allogeneic stem cell transplantation. However, as CMX-001 is reported to have lesser toxicity, it might be considered for the substitution of CDV [21].

Cyclopropavir (CPV) is a methylenecyclopropane nucleoside analogue that requires initial phosphorylation by the UL97 protein. Therefore, CPV selects similar mutations as observed for GCV, conferring partial cross-resistance [22,23].

It has been demonstrated that artesunate, an antimalaria drug, possesses antiviral activity against HCMV. In a recent study, viral kinetics of pre-emptive artesunate treatment was examined in stem cell transplant recipients. However, overall, a divergent antiviral efficacy was observed, which seemed to be dependent on the virus baseline growth dynamics [24]. Therefore, additional dose escalation studies are needed. However, occasionally case reports have found that artesunate is ineffective against GCV-resistant HCMV [25].

The compound letermovir (known as AIC246) interacts with the viral UL56 subunit, which is involved in viral DNA processing and/or packaging [26,27]. Therefore, this compound designates a new mechanism of action. Letermovir is at present undergoing a clinical Phase IIb trial for prophylactic use. Promising results offer the possibility of successful treatment of refractory multidrug-resistant HCMV [28]. So far, no resistance in patients has been observed.

5. Alternative strategies to overcome active cytomegalovirus infections

Besides antiviral therapy, immunological cell therapies are promising strategies to reconstitute HCMV-specific T-cell responses. Various techniques have been used in patients after allogeneic stem cell transplantation. HCMV-specific T-cells can be selected of a donor and then transferred to patients. Other strategies use an in vitro stimulation of antigen-presenting cells with HCMV-specific proteins or peptides [29]. The streptamer technology has been used successfully for adoptive transfer of cytomegalovirus-specific CD8+ T-cells in two patients with recurrent high HCMV antigenemia after allogeneic stem cell transplantation [30].

6. Expert opinion

In the next 5 years we expect new antiviral substances for the management of HCMV infections. Most probably these compounds will be administered for prophylaxis. Several of these substances display new modes of action, distinct from the viral polymerase as target. This will also open new options for the treatment of resistant virus in patients at risk. Advantages of some of the new substances are lesser toxicity issues, which might lead to new prophylactic and treatment strategies. Analogous to HIV, HBV or HCV treatment, the administration of the currently approved and new antiviral compounds in combination therapy should improve the management of HCMV infections.

However, it should be appropriate to amend further fast genotyping for the detection of the relevant mutations before a treatment failure is evident. If clinical treatment failure is suspected, genotyping should be performed to exclude virological resistance (caused by a mutation). Genotyping should be done among patients who have had an initial good response to therapy but who exhibit ongoing persistent or increasing viral replication for > 14 – 21 days, despite compliance with appropriately dosed antiviral therapy. Fast genotypic assays allow analysis of HCMV DNA directly from the patient's specimens, comprising all UL97 mutations known to confer GCV resistance. An advantage of direct genotyping is that reliable results can generally be obtained within several hours (routinely 1 – 2 working days). Therefore, this approach is well suited to monitoring of GCV therapy. Until now, fast genotyping of the viral polymerase requires 5 – 7 working days, but owing to several limitations it is not yet established for the routine diagnostic setting. However, despite suspected symptoms and/or signs of HCMV disease by clinicians, HCMV DNA could not be detected in ∼ 20% of patients [1,3,12]. This often-observed discrepancy of treatment failure due to suspected viral resistance, but lacking of resistance mutations in genotypic assays or even viral DNA or pp65 antigenaemia, points to the requirement for well-directed diagnostic tools and better defined clinical symptoms of HCMV-associated disease.

However, genotyping should be mandatory directly from the patient specimens before any adjustment of antiviral therapy. UL97 mutations conferring to lesser degrees of resistance (e.g., C592G, C603F, C607F) may permit the continued use of GCV at higher doses, but genotyping of the UL54 is suggested. However, if genotypic resistance testing reveals a major UL97 mutation (e.g., M460V, H520Q, A594V, L595S), a switch to FOS is suggested. The implementation of new methods in the routine diagnostic for the management of transplant recipients should contribute to a faster and more sensitive detection of therapy-resistant HCMV. The simultaneous detection of UL97 and UL54 GCV resistance mutations or detection of UL54 resistance mutations after UL97 mutations, is worthy of more investigation, for multiple recurrences of resistant HCMV can pose a difficult medical problem and can also be a psychologically debilitating experience for the patient. Therefore, more studies are required for the better understanding of HCMV evolution in patients consecutively treated with different antiviral compounds.

In conclusion, the ultimate goal for the forthcoming years should be the introduction of alternative compounds with new mechanisms and therefore the achievement of new treatment algorithms for the benefit of the patients.

Declaration of interest

D Michel is a consultant for diagnostic viral evaluations for AiCuris GmbH Co.KG. The authors have received no payment in preparation of this manuscript.