Doi:10.1016/j.antiviral.2006.05.002

Antiviral Research 71 (2006) 154–163 Antiviral drugs for cytomegalovirus diseases Department of Clinical Virology, Division of Virology, GlaxoSmithKline Inc., RTP, NC, United States Received 15 March 2006; accepted 4 May 2006 Dedicated to Prof. Erik De Clercq on the occasion of reaching the status of Emeritus-Professor at the Katholieke Universiteit Leuven in September 2006 Cytomegalovirus infections are associated with severe morbidity and mortality is patients at risk for disease because of immune system disabilities; in particular, recipients of stem cell (HSCT) or solid organ (SOT) transplants. There are three systemic drugs approved for CMV treatment:ganciclovir, or its prodrug valganciclovir, foscarnet, and cidofovir. An anti-sense therapeutic, ISIS 2922, is also approved specifically as inintravitreal treatment for CMV retinitis. Ganciclovir, and more recently, valganciclovir, have been useful in proactive approaches of CMV diseasemanagement; in both prophylactic and preemptive regimens in HSCT and SOT populations. The major anti-herpes agent valacyclovir has also beenapproved for prophylaxis of renal transplant recipients, or SOTs outside of the US. These drugs have provided major advances in CMV diseasemanagement, although they are limited by intolerable toxicities, oral bioavailability and efficacy, and risk of drug resistance with extended use.
Several drugs are in early clinical development which may address these limitations; this review will provide an overview of our current arsenalof available drugs, and of those in the early clinical development pipeline.
2006 Elsevier B.V. All rights reserved.
Keywords: CMV; Nucleoside analog; Nucleotide analog; Pyrophosphate analog; Novel benzimidazole riboside antiviral drugs; Alkoxyalkyl esters of cidofovir;Ganciclovir; Valganciclovir; Valaciclovir; Foscarnet ∗ Present address: GlaxoSmithKline, 5 Moore Drive, RTP, NC 27709, United States. Tel.: +1 919 483 9310; fax +1 919 315 5243.
E-mail address: 0166-3542/$ – see front matter 2006 Elsevier B.V. All rights reserved.
doi: K.K. Biron / Antiviral Research 71 (2006) 154–163 Human cytomegalovirus (CMV) is an opportunistic pathogen 3. CMV-associated disease in transplant recipients
associated with significant morbidity and mortality in suscep-tible populations; i.e. those with immature or immunocom- CMV infection is the leading viral cause of morbidity and promised immune systems. Numerous antiviral agents with in mortality facing patients who receive hematopoietic stem cell vitro activity against the various human herpesviruses have been transplant (HSCT) or solid organ transplant (SOT), with both described over the past three decades, yet only a few have been direct adverse effects resulting from viral invasion of organ sys- approved for the treatment or prophylaxis of CMV diseases. This tems and indirect effects on the immune systems that increase article will provide an overview of the diseases caused by this the risk of other infections and promote acute graft rejection ubiquitous virus and a description of approved drug products, (reviewed in CMV viremia is a and will briefly describe several drug candidates that are in early significant predictor for organ involvement and progression to stages of clinical development.
Risk of CMV-associated complications is increased with 1. CMV infection and CMV disease
more potent immunosuppressive regimens, such as many ofthose required for HSCT, and transplant patients are at great- CMV is a double-stranded DNA virus of approximately est risk for CMV-associated disease within the first 100 days 220 kb and is a member of the beta class of human herpesviruses.
post-transplant. For recipients of SOTs, the most vulnerable Cytomegalovirus is easily transmitted, usually through contact patients ("high-risk patients") are CMV-seronegative recipients with bodily fluids or by placental transfer. Seroprevalence rates who receive an organ from a CMV-seropositive donor (D+/R−).
vary by socioeconomic class and geographic location, but the CMV-seropositive recipients of allogeneic stem cell transplants overall seroprevalence in developed countries is estimated to are at risk for reactivation of latent CMV infection.
be in the range of 30–70% (Primary infection In high-risk patients without symptomatic CMV disease, two in immunocompetent individuals is usually benign, with mini- common strategies of disease management are prophylactic and mal or no clinical manifestations (although approximately 10% preemptive therapy, both of which are designed to prevent CMV of mononucleosis syndromes are a result of CMV infection).
disease. In the prophylactic approach, therapy is usually initiated Following primary infection, the virus establishes latency, and at the time of stem cell engraftment or solid organ transplant.
viremia is mainly controlled by cell-mediated immunity. Virus The suppressive doses used for prophylaxis are generally lower reactivation occurs when this protective immune surveillance than those instituted for induction treatment of active disease, fails; e.g. as a result of chemotherapy or in patients who have and the suppression of CMV reactivation in specific transplant AIDS or who are immunosuppressed for transplantation pur- populations can be successfully accomplished with a less potent poses. Such reactivation or primary infection in the context of a antiviral agent than would be used for treatment. In the pre- disabled immune system can lead to overt disease. In the case of emptive approach, therapy is initiated in asymptomatic high-risk vertical transmission of CMV to the developing fetus, adverse patients based on diagnostic test results indicating primary CMV outcomes are most commonly associated with primary infec- infection or reactivation of latent virus to a threshold level that tion of the mother, although significant morbidity has also been signals the potenial for disease escalation (blood CMV DNA associated with secondary infection.
load by PCR or pp65 antigenemia). This latter strategy ofteninvolves intermittent therapy, creating conditions thought to pose 2. Congenital CMV infection
a greater risk of selection of resistant virus. However, this riskmay be balanced by the protective effect of restoration of T- In developed countries, congenital CMV infection occurs in cell responses to CMV afforded by the delay in treatment with approximately 1% of live births. The majority of the cases are potent antivirals, particularly with myelosuppressive agents. On asymptomatic, but approximately 5–10% of infants with con- the other hand, the longer duration of drug exposure in the pro- genital CMV will have symptomatic disease, associated with phylactic approach also poses risk of resistance emergence.
profoundly deleterious effects on the central nervous system(CNS), including microcephaly, intracranial calcifications, and 4. CMV retinitis in AIDS patients
ventriculomegaly. Prognosis for neonates with symptomatic dis-ease is poor, with a high likelihood of mental defecits, hearing Although CMV retinitis is a relatively rare manifestation of loss and psychomotor and perceptual handicaps (reviewed in CMV disease in other immunocompromised populations, it is the primary manifestation of CMV infection in patients with It is now recognized that even asymptomatic congenital CMV AIDS, usually resulting from reactivation of latent virus. CMV is associated with increased risk of sensorineural hearing loss retinitis is a disease characterized by progressive, necrotizing (SNHL) (an observation that high- retinitis that can lead to retinal detachment and blindness. Initial lights the importance of identifying infants with congenital symptoms are non-specific, but may include blurred or distorted CMV infection and conducting periodic auditory assessments.
vision, floaters, light flashes, and loss of peripheral vision.
The morbidity and mortality associated with congenital CMV CMV retinitis and other manifestations of CMV disease in infection underscores the need for a vaccine to prevent CMV individuals with HIV-1 infection are opportunistic infections, infection. CMV vaccines currently in preclinical and clinical occurring when CD4+ cell counts are profoundly suppressed development are reviewed in (e.g. <50 cells/␮l). Since the advent of highly active antiretro-

K.K. Biron / Antiviral Research 71 (2006) 154–163 viral therapy for treatment of HIV-1 infection, CMV retinitis is (PCV). Ganciclovir has become the gold standard for manage- a condition rarely seen in developed countries, although aymp- ment of CMV diseases in the majority of patient settings. A tomatic CMV viremia remains a significant risk factor for death series of nucleotide analogs with broad activity across viruses was discovered by DeClercq and colleagues; from this class,cidofovir (CDV) was evaluated for anti-CMV activity. Otherantiviral agents designed to exploit the unusual characteristics 5. Antiviral therapies for CMV
of the herpesviral DNA polymerases are the pyrophosphate ana-log phosphonoacetic acid (PAA) and its analog foscarnet (PFA), The nucleoside analog class of compounds has historically which have broad inhibitory activity across the herpesviruses.
provided the richest source of antiviral agents, originating from These agents have facilitated the management of CMV infec- basic cancer research programs into purine and pyrimidine tions; the next section of this review will highlight their thera- metabolic pathways. These nucleoside analogs have been highly peutic applications and advantages based on key pivotal studies successful due to the potential for chemical diversity within the and experience in broader clinical practice.
class, and the differentiation of target viral DNA polymerasesor reverse transcriptases from host enzymes. The herpesviral-encoded nucleoside kinases (HSV, VZV and EBV thymidine 6. Currently marketed antiviral agents
kinases, and the CMV protein kinase) provided added selectivityin the initial phosphorylation of the various nucleoside analogs.
Three of the antiviral agents mentioned earlier, GCV, CDV, The triphoshorylated forms ultimately served as competitive and FOS have received marketing approval for the inhibitors of, and substrates for, the viral DNA polymerases, thus systemic treatment of CMV infection. ACV has also received reducing the amount of viral DNA synthesized in infected cells.
marketing approval in various European countries for prophy- Nucleoside analogs with variable anti-tumor cell activity, laxis of CMV disease in solid organ transplant (SOT) recipients, notably adenosine arabinoside (ara A), cytosine arabinoside but lacks sufficient potency to be used for treatment of active (ara C), and trifluorothymidine (TFT), were among those first CMV disease. An anti-sense RNA (fomivirsen) is approved for described to have anti-HSV and anti-VZV activity. Acyclovir (9- local treatment of CMV retinitis by intraocular injection.
[(2-hydroxyethoxy)methyl]guanine) was the first really selec-tive nucleoside analog (with potent activity 6.1. Ganciclovir against HSV 1 and 2, VZV and EBV, and moderate activityagainst CMV in vitro. Acyclovir (ACV) and its prodrug, the l- GCV was the first antiviral agent approved for treatment of valyl ester valacyclovir, have become the standard of care for CMV disease, and remains the first-line treatment for CMV prophylaxis and treatment of the most common diseases caused infection and CMV disease in transplant recipients by HSV and VZV, and both have provided benefit in CMV dis- GCV is an acyclic nucleoside analogue of eases in certain transplant populations.
2-deoxyguanosine (In a multi-step process dependent The discovery of ACV was quickly followed by discovery on both viral and cellular enzymes, ganciclovir is converted of related purine analogs ganciclovir (GCV) and penciclovir to ganciclovir triphosphate, the chemical form that is active Fig. 1. Approved anti-CMV drugs. (a) Ganciclovir; (b) valganciclovir; (c) foscarnet; (d) cidofovir; (e) valacyclovir. Not shown is fomivirsen.
K.K. Biron / Antiviral Research 71 (2006) 154–163 against CMV. The initial phosphorylation is catalyzed by an 1994 for treatment of CMV retinitis, but only as maintenence unusual protein kinase homolog encoded by the CMV UL97 therapy, as the low bioavailability (approximately 5%) of the oral open reading frame (Cellular enzymes formulation was considered insufficient for induction therapy.
generate the triphosphate form. Ganciclovir triphosphate com- A sustained-release GCV intraocular implant (Vitrasert®; petitively inhibits DNA synthesis catalyzed by the viral DNA developed by Chiron, now marketed by Bausch and Lomb) for polymerase (encoded by the UL54 gene), with slower chain treatment of CMV retinitis in AIDS patients was approved in elongation resulting from incorporation of ganciclovir triphos- 1996. In a clinical study comparing the implant to IV GCV in phate in place of dGTP into the growing viral DNA chain.
HIV-1-infected patients with CMV retinitis, the implant was Resistance to GCV arises from mutations in either the UL97 significantly more efficacious in treating CMV retinitis in the or the UL54 genes. The point mutations or small deletions in the affected eye, but patients treated with the implant alone were UL97 protein kinase gene lead to changes at codons 460 or 520, at significantly greater risk for developing CMV disease in the or changes clustered in codons 590–607 (regions attributed to contralateral eye or in other organ systems than were patients ATP-binding and substrate recognition, respectively) that appar- who received systemic GCV (The addition ently do not prevent the protein kinase function ( of oral ganciclovir to the implant controlled the systemic CMV The resistance mutations associated with GCV in the pol gene UL54 generally occur in specific conserved Oral GCV represented a major advance in treatment options subdomains, and may confer cross-resistance to CDV or less for maintenence therapy and prophylaxis. However, the low commonly, to FOS ( bioavailability and the high pill burden from the t.i.d. regi- The side effects of GCV include hematologic abnormali- men were limitations. In addition, there were concerns that ties (primarily neutropenia, anemia, and thrombocytopenia) and, inadequate viral suppression resulting from the lower systemic based on preclinical toxicologic studies, probable long-term exposure from oral GCV could lead to emergence of drug resis- reproductive toxicity (In tance. Development of prodrugs has been one valuable strategy animal studies GCV was both carcinogenic and teratogenic and in circumventing problems of poor solubility or low bioavail- ability (reviewed in Based on the GCV as an intravenous (IV) formulation (Cytovene-IV®, model exemplified by valacyclovir (see below), a prodrug was Roche) was approved in 1989 for treatment of CMV retinitis developed to improve the bioavailability of oral GCV.
in AIDS patients. The IV formulation was later approved for Valganciclovir (Valcyte®, Roche) is the l-valyl ester of gan- prevention of CMV disease in SOT recipients and in individu- ciclovir (After oral administration, valganciclovir is als with advanced HIV infection at risk for CMV disease. The rapidly metabolized to the active form (ganciclovir) in the intesti- pivotal clinical trials for the transplant indication included two nal wall and liver. Valganciclovir has an oral bioavailability of double-blind, placebo-controlled trials evaluating the incidence around 60% Once-daily admin- of CMV disease in D+/R− or D+/R+ heart transplant recipients istration of valganciclovir 900 mg produces systemic exposure (or in allogeneic bone marrow transplant to GCV that is equivalent to that produced with once-daily recipients with positive CMV cultures administration of IV GCV 5 mg/kg, and 1.7-fold greater than In the heart transplant study, patients were given study drug the systemic exposure produced by oral GCV 1000 mg given for 28 days post-transplant. The incidence of CMV illness at t.i.d. (In principle, the val- post-transplant day 120 was assessed, with the results strati- ganciclovir formulation could be used to deliver the exposures fied based on the recipient's serostatus. For seropositive recip- demonstrated to be efficacious in various transplant populations ients, significantly fewer patients given GCV had CMV illness treated with IV or high-dose oral GCV, barring absorption defi- during the first 120 days post-transplant compared to patients ciencies associated with GI disruption, as in graft-versus-host- given placebo (5/56 patients, 9% versys 26/56 patients, 46%; disease. Indeed, registrational studies and multiple comparative p < 0.001). For D+/R− transplant recipients there was no sig- trials have been reported ( nificant difference among treatment groups in the incidence of Valganciclovir was approved in 2000 for treatment of CMV disease during the first 120 days.
CMV retinitis in AIDS patients (reviewed in In the bone marrow transplant study, patients were given and was later approved for prophylactic treat- study drug from the time of engraftment until 100 days post- ment of CMV in certain SOT recipients. The first study of transplant. The incidence of CMV disease occuring within the valganciclovir for CMV prophylaxis in SOT recipients com- first 100 days post-transplant was significantly less in the GCV pared valganciclovir to oral GCV in a randomized, double-blind, arm (1/37 patients, 3%) compared to the placebo arm (15/35 double-dummy study of 364 D+/R− SOT recipients ( patients, 43%; p < 0.00001). In addition, patients receiving GCV Patients were randomized 2:1 to receive valganciclovir had significantly greater overall survival than the placebo group 900 mg once-daily or oral GCV 1000 mg t.i.d. for 100 days post- at both 100 and 180 days post-transplantion (p = 0.041 and 0.027, transplant. The study was designed to show equivalence, rather than superiority of valganciclovir compared to oral GCV. The To circumvent the risks and inconvenience associated with incidence of CMV disease by 12 months were comparable in the the need for an indwelling catheter for intravenous administra- two treatment groups, and the safety profiles were similar tion, an oral formulation was developed. Oral GCV (250 and Valganciclovir has now replaced oral ganciclovir 500 mg GCV capsules; Cytovene®, Roche) was approved in in clinical practice.
K.K. Biron / Antiviral Research 71 (2006) 154–163 Debate continues over the most appropriate antiviral treat- disorders, including seizures, that in several cases resulted in ment for prevention of CMV disease in high-risk SOT recipients (D+/R−) and whether prophylactic therapy or preemptive ther- FOS is considered second-line therapy, but is the preferred apy should be used for asymptomatic high-risk patients drug for patients who are failing GCV therapy due to viral In one study late-onset CMV disease was more preva- resistance, or those who cannot be treated with GCV due to lent in in D+/R− liver transplant recipients who had received dose-limiting neutropenia or leucopenia prophylaxis versus preemptive therapy, suggesting that perhaps In one study, FOS was compared to IV GCV as a pre- prophylactic treatment with a potent antiviral agent interfered emptive therapy in a large, prospective, randomized, open-label with development of cell-mediated immune response study in HSCT patients. FOS and IV GCV were equally effective Similar outcomes have been reported in HSCT popula- in prevention of CMV disease and mortality within 180 days of tions (The International HSCT FOS has also been used in combi- Herpes Management Forum issued a set of guidelines in 2004 nation with IV GCV, each at half dose, and the combination was for the use of ganciclovir and valganciclovir as either prophylac- compared to IV GCV alone in SOT patients. The outcome was tic or preemptive therapy in SOT and HSCT patients ( unfavorable for the combination in terms of virologic response This consensus opinion provided recommen- dations for quantitative monitoring of CMV load to optimize thetiming, duration and intensity of therapy. The guidelines also 6.3. Cidofovir compare the efficacy of several regimens for first- and second-line viral control, as well as for treatment of established disease.
Cidofovir (Vistide®, Gilead) is an acyclic nucleoside phos- To date (2006), no anti-CMV agent has been approved for phonate, with the chemical name 1-[(S)-3-hydroxy-2-(phos- treatment of congenital CMV disease, although the Collabora- phonomethoxy)propyl]cytosine dihydrate (HPMPC, tive Antiviral Study Group (CASG) of the National Institute of CDV is a broad-spectrum antiviral agent with potency against Allergy and Infectious Diseases has conducted Phase II and III both herpesviruses and other DNA viruses, such as smallpox trials evaluating twice-daily dosing of 6 mg/kg IV GCV for treat- virus (Host kinases convert CDV to ment of infants with symptomatic congenital CMV involving the active diphosphoryl form, and cidofovir disphosphate then the CNS. Study results showed clear benefits that for neonates acts as a competitive inhibitor of the viral DNA polymerase, with severe CMV disease outweighed the risks of acute tox- causing premature chain termination in viral DNA synthesis.
icities and long-term reproductive toxicities associated with Resistance to CDV has been difficult to select in the laboratory intravenous ganciclovir ( (Resistant isolates have not been reported A large multicenter trial is under- in the clinic, although this may reflect the shorter treatment reg- way to assess the safety and efficacy of valganciclovir syrup on imens and the limited use of this agent. Several GCV-resistant infants with symptomatic congenital CMV disease. However, CMV clinical isolates or laboratory-selected strains with specific while the oral formulation would avoid the considerable risks pol gene mutations are clearly cross-resistant to CDV ( and disadvantages associated with IV administration, the hema- tologic and reproductive toxicities would remain, limiting the CDV received US marketing approval in 1996 for treatment usefulness of the therapy for all but the most severely affected of CMV retinitis in AIDS patients. CDV is available only as an IV formulation; its oral bioavailability is less than 5%. One ofthe distiguishing features of CDV, and others in this nucleotide 6.2. Foscarnet analog class, is the stability of the active form in cells. Theintracellular half-life of CDV-DP is reported to be >24 h, and In 1991 foscarnet (Foscavir®, AstraZeneca) became the sec- efficacy in both animal models and in humans can be achieved ond drug approved for treatment of CMV retinitis in AIDS with infrequent dosing (Recom- patients. Foscavir®, or foscarnet sodium, is the trisodium salt mended treatment for CMV retinitis in AIDS patients consists of phosphormophonic acid, a pyrophosphonate analogue. Fos- of 5 mg/kg administered over a 1-h period once a week for two carnet (FOS) inhibits activity of the viral DNA polymerase by consecutive weeks (induction phase), followed by 5 mg/kg once binding to the pyrophosphate binding site and blocking cleav- every 2 weeks (maintenance phase).
age of pyrophosphate from the terminal nucleoside triphosphate The major limitation of CDV as an antiviral agent is severe added to the growing DNA chain. Resistance to FOS in the lab- renal toxicity An anion trans- oratory or clinical setting has been mapped to point mutations in porter located in the convoluted proximal tubules binds to the pol gene UL54. Cross-resistance has been observed between CDVwith high affinity, leading to the accumulation of CDV in GCV and FOS in several laboratory and clinical isolates with the renal cortex The major route of drug elim- both phenotypic and genotypic resistance.
ination occurs through the kidney. Patients receiving IV CDV The major dose-limiting toxicity of FOS is renal impair- must be given oral probenecid to protect against kidney failure, ment, underscoring the importance of adequate hydration and and must be prehydrated before infusion. Neutropenia is another frequent monitoring of serum creatinine levels in patients receiv- toxicity associated with CDV, and CDV was shown to be both ing FOS. Mineral and electrolyte abnormalities resulting from carcinogenic and teratogenic in preclinical toxicological studies renal impairment can lead to a number of cardiac or neurologic K.K. Biron / Antiviral Research 71 (2006) 154–163 Due to the risks of renal damage, CDV remains a second-line Valacyclovir prophylaxis significantly therapy. In the transplant setting, CDV has been used primary reduced the incidence of CMV disease among transplant recip- in preemptive strategies to rescue allogeneic HSCT patients ients at 6 months post-transplant, at 1% versus 6% for seropos- following therapy with GCV, FOS, or both drugs, with suc- itive patients and 16% versus 45% for seronegative patients cesses ranking from 62% to 66% In randomized to valacyclovir versus placebo, respectively. Sim- this same retrospective analysis, patients with established CMV ilar results were seen in incidence of active CMV infection disease were also treated, and 50% (of a total of 20 subjects) and graft rejection. Although not potent enough for treatment responded clinically and virologically. However, 25% of these of established CMV disease, valacyclovir has been approved patients experienced renal toxicity, which was irreversible in in several countries for prophylaxis of CMV infection and approximately half of the affected patients. More recently, sim- CMV disease in renal or heart transplant recipients or SOT ilar efficacy was seen following preemptive administration of CDV in pediatric HSCT patients failing GCV or FOS, with rea-sonable success achieved in the management of renal function 6.5. Fomivirsen in these patients ( The usual treatment regimen in these studies involves an Fomivirsen (Vitravene®; developed by Isis Pharmaceuticals, induction dosage of 1–5 mg/kg/week, followed by a mainte- licensed to Novartis Ophthalmics) is a 21-nucleotide anti-sense nance dose every other week. The toxicities of CDV or the RNA (5-GCG TTT GCT CTT CTT CTT GCG-3), specifi- adjunct probenicid limit the utility of this interesting and potent cally targeted against the mRNA from the major immediate- antiviral drug. The potential of CDV prodrugs to avoid renal early transcriptional unit of CMV. Fomivirsen is administered tubular uptake and concentration is under evaluation.
by intraocular injection. Fomivirsen was approved in 1998 asa second-line therapy for local treatment of CMV retinitis in 6.4. Acyclovir AIDS patients. Recommended treatment consists of a 4-weekinduction phase, with a single injection every other week (i.e.
Acyclovir is an analogue of 2-deoxyguanosine. Like GCV, two doses), followed by a maintenence phase, in which a sin- acyclovir must be phosphorylated in a multi-step process in the gle injection is administered every 4 weeks. The most frequent host cell to the active triphosphate form. The CMV-encoded pro- adverse effect is ocular inflammation (uveitis), which can be tein kinase pUL97 catalyzes the initial phosphorylation step of managed by treatment with topic corticosteroids and by delay- this purine analog, which, like GCV monophosphate, is subse- ing additional injections.
quently di- and tri-phosphorylated by host kinases. ACV is a lessefficient substrate than GCV, which in part explains the lower 7. Anti-CMV drugs in clinical development
in vitro potency of ACV compared to GCV in CMV-infectedcells. Another factor that clearly differentiates ACV and GCV New drugs, preferably in oral formutions, are needed for is the four- to five-fold shorter half-life of ACV-TP compared treatment of CMV disease, especially congenital disease in to GCV-TP in infected cells, resulting in the lower intracellular neonates. The currently approved systemic drugs have an unfa- levels of the active ACV-TP. As with GCV, drug resistance to vorable safety profile, with severe acute and long-term toxi- ACV results from mutations in the viral DNA polymerase or cities. Of special concern for a pediatric population is long- term reproductive toxicity and carcinogenicity. Moreover, the Oral acyclovir has approximately 6–10% bioavailability, approved systemic drugs share a similar mechanism of action, which increases to approximately 55% with administration targeting the viral DNA polymerase. As a consequence, viral of valacyclovir (Valtrex®, GlaxoSmithKline), the l-valyl ester cross-resistance is a potential problem with the current drug of acyclovir The primary use of high-dose oral ACV or its prodrug valacyclovir in trans- Despite the medical need for new drugs, relatively few plant patients has been for suppression of HSV reactivation.
research programs currently focus on anti-CMV drug develop- However, prophylactic treatment can also significantly reduce ment. One reason may be the reduction in CMV retinitis in AIDS the incidence of CMV infection and CMV disease in SOT patients following the introduction of HAART.
patients. In a meta-analysis of 12 randomized trials enrolling1574 SOT patients, a 56% decrease 7.1. Maribavir in the risk of CMV infection (p < 0.001), a 59% reductionin CMV disease (p < 0.001), and a 30% reduction in oppor- One of the most promising anti-CMV drugs in clinical devel- tunistic infections (p < 0.009) in patients receiving prophy- opment is maribavir (1-(␤-l-ribofuranosyl)-2-isopropylamino- lactic therapy with high-dose oral acyclovir or valacyclovir, 5,6-dichlorobenzimidazole), also known as GW1263W94 compared to patients receiving placebo or no prophylactic Although this drug is a riboside analog, it does not act as such: it is not anabolized in infected cells, nor do its phospho- The safety and efficacy of valacyclovir for prevention of rylated forms directly inhibit the viral DNA polymerase ( CMV disease was evaluated in 408 R+ and 208 R−/D+ renal Maribavir is a potent and selective, orally bioavail- transplant patients, randomly assigned to treatment with either able drug with a novel mechanism of action against only two 2000 g valacyclovir or placebo q.i.d. for 90 days post-transplant of the human herpesviruses: CMV and EBV. Maribavir inhibits

K.K. Biron / Antiviral Research 71 (2006) 154–163 Fig. 2. Anti-CMV drugs in clinical development. (a) Maribavir; (b) BAY 38-4766; (c) GW275175X; (d) CMX001.
the replication of both CMV and EBV in cell culture by interfer- 7.2. BAY 38-4766 ing with viral DNA synthesis (In CMV-infected cells, maribavir has also been shown BAY 38-4766 (Bayer Pharmaceuticals), or 3-hydroxy-2,2- to interfere with viral nucleocapsid egress from the nucleus, thereby reducing the yield of infectious CMV no)-phenyl]propanamide, represents a novel class of non- A key target for maribavir's action in the CMV life cycle nucleoside antiviral agents. BAY 38-4766 is a highly selective is the viral-encoded protein kinase, pUL97, a finding that was inhibitor of CMV in vitro ( based on the genetics of resistance, direct protein kinase inhi- In an immunodeficient mouse model the compound shows bition studies (and on phenotypic similarity anti-CMV activity against human CMV similar to that of GCV of maribavir-treated, CMV-infected cells and cells infected with (BAY 38-4766 had a favorable safety and the pUL97-deleted virus ( efficacy profile in a guinea pig model of CMV, and measurable Inhibition of viral DNA synthesis is likely a consequence of a amounts of drug were detected in fetal blood, indicating that block in the phosphorylation of the polymerase accessory pro- the compound crosses the placenta in pregnant guinea pigs tein, pUL44, by the pUL97 protein kinase ( BAY 38-4766 was active against strains resistant to currently Clearly the mechanism of action of maribavir against CMV approved anti-CMV agents and no has not been fully elucidated, since the role of the pUL97 protein cross-resistance to these agents was seen in virus selected for kinase in CMV replication or disease pathogenesis is still under resistance to BAY 38-4766 (Antivi- study. Moreover, laboratory-generated resistant mutations also ral activity of BAY 38-4766 results from inhibition of DNA map to the CMV UL27 gene, a protein of unknown function maturation, and mutations conferring drug resistance map in the UL89 and UL56 genes, which encode subunits of the viral termi- Maribavir preclinically shows advantages over existing anti- nase (This mechanism of action is similar CMV drugs in its in vitro potency, bioavailability, safety profile to that of the original benzimidazole riboside leads, BDCRB and in acute, chronic and genetic toxicology testing, and the lack of cross-resistance inherent in its novel mechanism of action. The vation that is interesting in view of the differences in structures drug has completed several Phase 1 clinical studies between these unrelated chemical series and the complexity of and its potential for efficacy the drug target.
was demonstrated in a 28-day study in HIV-infected subjects BAY 38-4766 entered clinical development and showed a that showed a reduction in viral shedding in the semen and urine favorable safety profile in healthy male volunteers at single oral This drug is currently in a prophylaxis doses up to 2000 mg. However, no recent reports have revealed study in allogeneic stem cell transplants, with results expected the current status of clinical development of this compound or related compounds in the series.
K.K. Biron / Antiviral Research 71 (2006) 154–163 7.3. GW275175X drug has provided a reduction in the risk of diseases caused byother herpesviruses, as well as bacterial and fungal infections, Another interesting clinical candidate to emerge from the which could reflect the direct activity of the broad spectrum anti- novel benzimidazole riboside class of CMV inhibitors is the ␤-d- herpetic, as well as the indirect benefits of suppressing CMV pyranosyl sugar analog of the original leads BDCRB and TCRB Additional progress in reducing the consequences of CMV infection in all the susceptible populations will be made with the benzimidazole), this molecule addressed the in vivo lability of introduction of new drugs with greater efficacy and safety, and the glycosidic linkage of BDCRB by substitution of the six- long term management of infection will be facilitated by drugs membered sugar ring for the ␤-d-ribose moiety. GW275175X with non-overlapping mechanisms of action.
retains the mechanism of action of the parent compoundBDCRB; that of blocking the maturational cleavage of highmolecular weight CMV DNA by interaction with pUL56 and pUL89, the two subunits of the viral terminase complex Balfour Jr., H.H., 1979. Cytomegalovirus: the troll of transplantation [edito- rial]. Arch. Intern. Med. 139, 279–280.
GW275175X was advanced through a Phase 1 single-escal- Beadle, J.R., Hartline, C., Aldern, K.A., Rodriguez, N., Harden, E., Kern, ating dose trial of safety, tolerability and pharmacokinetics, but E.R., Hostetler, K.Y., 2002. Alkoxyalkyl esters of cidofovir and cyclic was then shelved in favor of the advancement of maribavir. The cidofovir exhibit multiple-log enhancement of antiviral activity against clinical potential of this early candidate is yet to be determined.
cytomegalovirus and herpesvirus replication in vitro. Antimicrob. AgentsChemother. 46, 2381–2386.
Biron, K.K., Harvey, R.J., Chamberlain, S.C., Good, S.S., Smith III, 7.4. Cidofovir esters A.A., Davis, M.G., Talarico, C.L., Miller, W.H., Ferris, R., Dornsife,R.E., Stanat, S.C., Drach, J.C., Townsend, L.B., Koszalka, G.W., 2002.
Renal toxicity associated with CDV treatment limits the use- Potent and selective inhibition of human cytomegalovirus replication by ful of the drug, despite its efficacy as an anti-CMV agent.
1263W94, a benzimidazole l-riboside with a unique mode of action.
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