The clinical motivation for the work is substantial. HSV-1 causes painful gingivostomatitis and, in severe cases, life-threatening herpes simplex encephalitis. HCMV infection can produce severe virus-associated pneumonia or colitis, and congenital HCMV infection can lead to microcephaly, sensorineural hearing loss, and motor and mental developmental disorders. EBV is commonly associated with infectious mononucleosis and with a number of lymphocyte- or epithelial-cell-specific malignancies, including Burkitt lymphoma and nasopharyngeal carcinoma. Despite this burden of disease, therapeutic and vaccination options remain limited. Current antiviral drugs suffer from side effects, the emergence of resistance, and a lack of broad-spectrum antiherpesviral efficacy. The identification of new targets for antiviral therapy and improved treatment strategies is therefore described by the authors as urgently required, and combination therapy is one of the most promising avenues for achieving greater potency while limiting toxicity and resistance development.

The four compounds selected for the study represent distinct mechanistic classes and different levels of clinical maturity. Abemaciclib (ABE) is an approved CDK4/6 inhibitor originally developed for hormone receptor-positive, HER2-negative breast cancer, which has recently been shown to possess antiviral activity against HCMV and murine cytomegalovirus. Brincidofovir (BCV), a developmental candidate, is a putative broad-spectrum antiherpesviral that targets the viral DNA polymerase. LDC4297 is a highly selective inhibitor of the host cyclin-dependent kinase CDK7 with antitumoral activity. Finally, P-020 is an experimental, pharmacophore-derived prototype that blocks the viral nuclear egress complex, a conserved structure required for the exit of viral nucleocapsids from the nucleus. This selection deliberately covered both direct-acting antivirals (DAAs), which target viral proteins, and host-directed antivirals (HDAs), which target cellular factors co-opted by the viruses during replication.

Methodologically, the study relied on quantitative, cell culture-based reporter systems. HSV-1 expressing a VP22-green fluorescent protein reporter was analyzed in primary human foreskin fibroblasts, HCMV strain AD169 carrying a GFP reporter was measured in human fibroblast cultures, and a luciferase-expressing MHV-68 was assessed in Vero 76 cells, which the authors identified as the most sensitive and reliable cell type for quantitative gamma-herpesvirus drug testing. Viral replication was quantified through fluorescence or luminescence readouts, while cytotoxicity was monitored in parallel using Neutral Red uptake assays. In single-drug treatments, all four compounds inhibited all three viruses in a concentration-dependent manner. Mean effective half-maximal concentrations fell in the micromolar range for abemaciclib and P-020, in the submicromolar range for LDC4297, and in the nanomolar to subnanomolar range for brincidofovir. Against HCMV, EC50 values ranged from approximately 0.0007 to 19.7 micromolar, while potency against HSV-1 and MHV-68 was somewhat lower but remained within comparable ranges.

To characterize the temporal dimension of drug activity, the researchers performed time-of-addition experiments against HCMV, applying four treatment schedules: pretreatment starting three days before infection, simultaneous addition with the virus inoculum, delayed addition two days after infection, and exclusive prophylactic pretreatment. The antiviral potency of the compounds proved largely robust across these schedules, indicating that the drugs generally showed moderate or no difference in activity whether administered before, after, or throughout the infection period. One notable exception was abemaciclib, which displayed enhanced activity when added with a two-day delay, suggesting that a peak level of the drug early in infection may be particularly relevant. Brincidofovir, abemaciclib, and LDC4297 all showed strong activity under exclusive pretreatment conditions, pointing to rapid drug uptake into fibroblasts and consistent inhibitory activity across multiple rounds of viral replication. P-020, consistent with its role as a late-phase inhibitor of nuclear egress, was effective whether present early or late, and its highest activity was observed with very early presence, hinting at high compound stability and possible intranuclear accumulation over time.

Before testing drug combinations, the team addressed a critical safety question: whether cotreatment might mutually reinforce cytotoxicity. They exposed nine different cell types, including primary human fibroblasts and mouse embryonic fibroblasts, immortalized monolayer lines such as 293T, ARPE-19, and MRC-5, suspension cells including Raji, BJAB, and J-Jhan, and a simian Vero 76 line, to brincidofovir alone or in combination with abemaciclib or LDC4297. Although minor variability in drug-induced effects was observed between cell types, no marked enhancement of cytotoxicity was identified under the chosen conditions. This finding made it improbable that any antiviral synergy measured subsequently was an indirect consequence of combinatorial cytotoxic enhancement, strengthening the pharmacological interpretation of the interaction data.

The core of the study is the quantitative analysis of drug-drug interactions using the Loewe additivity fixed-dose assay, in which combination indices were calculated at multiple levels of viral inhibition and weighted to classify interactions as synergistic, additive, or antagonistic. The results revealed a striking virus-specific pattern. The combination of LDC4297 with brincidofovir was antagonistic against all three viruses. The authors propose a mechanistic explanation: because brincidofovir targets the viral polymerase, whose expression and function may depend on CDK7-mediated transcriptional activity, inhibiting CDK7 with LDC4297 could suppress the very viral early proteins that the polymerase inhibitor requires, producing an antagonistic outcome. In contrast, the combination of abemaciclib with LDC4297, which targets the host kinases CDK4/6 and CDK7 respectively, showed true synergy against HCMV, consistent with an earlier report, and the present study extended this synergy to HSV-1 and MHV-68 as well. The authors attribute this to a broad, herpesvirus replication-supportive role of the two host kinases, operating through distinct effector events such as promotion of the G1/S cell cycle transition and the onset of transcription, so that blocking both pathways yields a synergistic inhibitory effect.

A third combination, abemaciclib plus brincidofovir, produced a more nuanced picture. This pairing was synergistic against HCMV and MHV-68, but antagonistic against HSV-1, a result confirmed independently using a Bliss independence checkerboard assay analyzed with MacSynergy II software. The authors hypothesize that combined targeting of host CDK4/6 and the viral polymerase is critical for the replication of beta- and gamma-herpesviruses but less critical, or even counterproductive, for the alpha-herpesvirus HSV-1, possibly reflecting a so-far unknown regulatory interplay between these two targets that is specific to the alpha-herpesvirus. The weighted combination indices reported for the various pairings quantified these differences precisely, ranging from strongly synergistic values around 0.2 to clearly antagonistic values above 4. The virus-specific nature of these interactions underscores that broad-spectrum combination therapy cannot be assumed to work uniformly across herpesvirus subfamilies and must be validated for each pathogen.

The authors conclude that their findings support novel options for broad-spectrum intervention based on combined antiherpesviral drug treatment. The synergistic pairing of abemaciclib with LDC4297 across all three subfamilies, and the synergy of abemaciclib with brincidofovir against beta- and gamma-herpesviruses, suggest that combining host-directed and direct-acting antivirals could enhance antiviral potency while allowing drug concentrations to be restricted to low-dosage ranges, without a proportional increase in cytotoxic effects or antiviral resistance. The confirmed absence of reinforced cytotoxicity under combinatorial exposure further supports developmental potential. The researchers caution that clinical relevance remains an open question, since many synergistic combinations measured in cultured cells have not translated into clinical utility. Nevertheless, they argue that novel combination treatments are urgently needed for clinical situations in which monotherapy is insufficient or dose-limited by toxicity, particularly in immunocompromised patients. The combination of abemaciclib and brincidofovir, demonstrated here for a gamma-herpesvirus model, may offer new options for conditions such as Epstein-Barr virus-associated post-transplant lymphoproliferative disorder, and the authors call for further mechanistic research to clarify the molecular basis of the observed synergies and to advance these strategies toward antiviral treatment opportunities.

Subject of Research: Combined antiviral drug treatment against alpha-, beta-, and gamma-herpesviruses

Article Title: Assessment of combined antiviral drug treatment against selected α-, β-, and γ-herpesviruses

Article References: Fortelny, L., Wangen, C., Obergfäll, D., Dhotre, K., Hahn, F., Tillmanns, J., & Marschall, M. (2026). Assessment of combined antiviral drug treatment against selected α-, β-, and γ-herpesviruses. Virology Journal, 23(1), Article 219. https://doi.org/10.1186/s12985-026-03303-1

Image Credits: AI Generated

DOI: 10.1186/s12985-026-03303-1

Keywords: herpesviruses, antiviral drugs, drug synergy, abemaciclib, brincidofovir, LDC4297, HCMV, HSV-1, MHV-68, CDK inhibitors, combination therapy, nuclear egress