Antiviral drug resistance of human cytomegalovirus

Antiviral drug resistance in clinical HCMV isolates was initially observed mostly in patients with HIV/AIDS, but the incidence in this population has greatly declined following the introduction of combination antiretroviral therapy. Currently, resistance is reported most frequently among transplant recipients, especially solid organ recipients with a D+/R- serostatus. Drug resistance generally appears after weeks to months of antiviral drug exposure during periods of immunosuppression and may manifest clinically as rising viral loads or progressive disease despite therapy. Because these clinical findings are not specific for drug resistance, laboratory testing is needed for confirmation. However, the ability to detect resistance is low if antiviral treatment has been in place for less than 6 weeks. The three systemic antiviral drugs currently approved for treatment of HCMV (GCV, FOS, and CDV) all target the DNA polymerase, and GCV anabolism requires an additional viral product, the UL97 kinase. Resistance mutations can arise in the gene sequences encoding one or both enzymes, depending on the drug exposure. Because GCV is the drug of choice for prophylactic and preemptive therapy, UL97 resistance mutations are detected most frequently and are consistently found in a limited number of codons. There are a much larger number of pol mutations, which tend to be selected only after extended periods of treatment with GCV or FOS, and all known GCV resistance mutations in pol confer cross-resistance to CDV and/or FOS. The original identification of drug resistance in clinical HCMV isolates depended on phenotypic methods, which are too time-consuming to provide therapeutically relevant results. They have been replaced by rapid genotypic assays for resistance mutations in viral sequences directly amplified from blood or other clinical specimens without use of cell cultures. HCMV drug resistance genotyping is increasingly available through reference and clinical laboratories. However, accurate interpretation of these assays is dependent on validation of resistance mutations by a phenotypic or recombinant phenotypic method. The complexity of recombinant phenotyping requires the expertise of research laboratories, which have played a key role in the development of HCMV genotypic assays, in particular the differentiation of true resistance mutations from baseline sequence variation. Diagnostic laboratories should be careful to identify confirmed resistance mutations and not confuse them with uncharacterized sequence variants. Timely results of resistance testing are useful for making clinical decisions. If no drug resistance is identified, clinical management may focus on improving host defenses rather than switching antivirals. If there is confirmed genotypic evidence of resistance, the specific mutation(s), host immune status, and disease severity should all factor into these decisions, such as to continue or intensify current treatment, to switch to a non-cross-resistant drug, to use drug combinations, or to try experimental drugs. Management algorithms have been proposed by several groups (17, 113, 162, 193). The study of HCMV drug resistance mutations is an important component of antiviral drug development as well as basic virological research. Early on, GCV resistance led to the discovery of the important role of UL97 in GCV anabolism, followed by mapping of pUL97 functional domains as additional mutations were identified. Mutations conferring resistance to each of the approved drugs have served to map the functional domains of pol. More recent work with MBV has identified resistance mutations in UL27 and new mutations in UL97 in functional domains distinct from those associated with GCV resistance. As new antiviral drug targets are explored, mutations identified in the targeted genes may offer new insights into virological functions and interactions of HCMV with host cells.