A 28-year-old man with HIV/AIDS (CD4 count 45 cells/μL) presents with corneal opacity, anterior chamber reaction, and keratic precipitates. Confocal microscopy shows intranuclear and intracytoplasmic inclusions. He has failed topical acyclovir therapy. What is the most appropriate antiviral agent for this case?

Explanation

## Acyclovir-Resistant Viral Keratitis in Immunocompromised Hosts ### Clinical Context This patient has **cytomegalovirus (CMV) keratitis** in the setting of severe immunosuppression (CD4 < 50 cells/μL). The intranuclear and intracytoplasmic inclusions are characteristic of CMV infection. Failure of acyclovir suggests either CMV infection or acyclovir-resistant HSV. **Key Point:** CMV keratitis occurs almost exclusively in immunocompromised patients, particularly those with CD4 counts < 50 cells/μL. It presents with anterior chamber inflammation and stromal/endothelial involvement rather than epithelial ulceration. ### Drug of Choice: Ganciclovir **High-Yield:** Ganciclovir is the first-line antiviral for CMV keratitis and acyclovir-resistant HSV keratitis in immunocompromised hosts. | Feature | Acyclovir | Ganciclovir | |---|---|---| | **Mechanism** | Viral TK-dependent activation | Viral protein kinase (UL97) activation; more potent | | **Spectrum** | HSV, VZV; poor CMV activity | CMV, HSV, VZV; superior CMV activity | | **Resistance** | TK-deficient mutants | UL97 or DNA polymerase mutants (rare) | | **Formulation** | Topical, oral, IV | IV, intravitreal, topical (limited) | | **Toxicity** | Low | Myelosuppression, nephrotoxicity | ### Mechanism of Action 1. Ganciclovir is a guanosine analogue requiring viral protein kinase (UL97) for activation. 2. Phosphorylated ganciclovir inhibits viral DNA polymerase more potently than acyclovir-triphosphate. 3. Achieves higher intracellular concentrations in CMV-infected cells. **Clinical Pearl:** In CMV keratitis, systemic IV ganciclovir (5 mg/kg twice daily) is preferred over topical application due to poor corneal penetration and the need for systemic viral suppression in immunocompromised patients. ### Treatment Algorithm for Resistant Keratitis ```mermaid flowchart TD A[Viral keratitis]:::outcome --> B{Immunocompetent?}:::decision B -->|Yes| C[Acyclovir topical/oral]:::action B -->|No| D[CD4 < 50?]:::decision D -->|Yes| E[Consider CMV]:::outcome D -->|No| F[Acyclovir IV]:::action E --> G[Ganciclovir IV]:::action C --> H{Response at 7-10 days?}:::decision H -->|Yes| I[Continue therapy]:::action H -->|No| J[Reassess diagnosis]:::decision J -->|CMV suspected| G J -->|Resistant HSV| G ``` **Warning:** Ganciclovir causes myelosuppression (neutropenia, thrombocytopenia) and nephrotoxicity. Monitor CBC and renal function closely. Immune recovery uveitis (IRU) may occur as CD4 count recovers with antiretroviral therapy.