Snapshot A 45-year-old woman presents to the emergency room with painless vision loss. She has a past medical history of HIV infection and has not been on any anti-retroviral medications due to loss of health insurance last year. Her course has been complicated by CMV esophagitis in the past, which was resistant to ganciclovir. Her last CD4+ count was < 50/uL 1 week ago. On physical exam, she is found to be dehydrated with decreased skin turgor and dry mucous membranes. Funduscopic exam shows yellow-white patches of retinal opacification. After fluid resuscitation, she is started on foscarnet. (CMV retinitis) Introduction Drugs foscarnet Mechanism of action inhibits viral DNA/RNA polymerase and HIV reverse transcriptase by preventing pyrophosphate from binding the drug is a pyrophosphate (pyroFOSphate, FOScarnet) analog does not need to be activated by a kinase can be used in viral strains resistant to acyclovir and ganciclovir Mechanism of resistance mutation of viral DNA/RNA polymerase Clinical use cytomegalovirus (CMV) infections in immunocompromised patients (e.g., HIV) especially if resistant to ganciclovir can also be given in combination with ganciclovir herpes simplex virus infections especially if resistant to acyclovir varicella zoster virus infections Adverse effects nephrotoxic must correct dehydration prior to therapy acute tubular necrosis cardiac effects QT prolongation torsades de pointes electrolyte abnormalities hypocalcemia hypomagnesemia seizures