Chemical Structure and Mechanism of Action
Structurally, the drug is a guanine derivative. It differs from natural deoxyguanosine by an open (acyclic) chain, and from its relative acyclovir by the presence of an additional hydroxymethyl group.
To become active, the drug must undergo activation (phosphorylation) inside the infected cell:
- Initial phosphorylation. In cells infected with herpes simplex virus, this process is initiated by viral thymidine kinase. However, cytomegalovirus (CMV) lacks this enzyme. In CMV, activation is carried out by a specific viral phosphotransferase encoded by the UL97 gene. This specific feature makes the drug effective against cytomegalovirus.
- Further activation. Conversion to di- and triphosphate is carried out by host cell kinases.
The resulting ganciclovir triphosphate is the active form. It competes with the natural substrate (deoxyguanosine triphosphate), incorporates into the elongating viral DNA chain, and blocks its further elongation. The drug accumulates in infected cells at concentrations 10–30 times higher than in healthy tissues and maintains its activity for several days.
Pharmacokinetics: The Absorption Challenge
Ganciclovir itself is very poorly absorbed when taken orally, with a bioavailability of only about 10%. Because of this, it must be administered intravenously or taken orally strictly with food while accepting low absorption efficiency.
To solve this problem, a prodrug was developed — valganciclovir. It is the valyl ester of the parent compound. When taken orally, it is well absorbed (bioavailability reaches 60%) and is transformed into active ganciclovir during first-pass metabolism in the liver.
Indications for Use
The drug has a relatively broad spectrum of activity: it is active against herpes simplex viruses, Epstein-Barr virus, and Varicella-zoster. However, in practice, it is used exclusively for the treatment of cytomegalovirus infection, because other, less toxic agents (e.g., acyclovir) are virtually ineffective against CMV.
Main indications include severe infections in immunocompromised states (including AIDS):
- CMV retinitis (infection of the retina);
- Gastrointestinal manifestations (colitis, esophagitis);
- CMV pneumonia;
- Generalized forms of infection.
- The drug is also prescribed for the prevention of cytomegalovirus disease in newborns and in organ and tissue transplant recipients.
Toxicity and Side Effects
The restricted use of the drug is due to its ability to partially incorporate into the DNA of healthy human cells, inhibiting cellular DNA polymerase. The active metabolite persists in the host organism longer and in larger quantities than acyclovir metabolites.
The most severe complication is myelotoxicity (bone marrow suppression). This manifests as severe hematologic disorders:
- Decreased neutrophil count (neutropenia);
- Reduced platelet count (thrombocytopenia);
- Anemia;
- Pancytopenia (simultaneous deficiency of all blood cell lines).
In addition, systemic reactions may occur: neurological disorders (ataxia, paresthesias), arrhythmias, diarrhea, hepatic and renal impairment, and retinal detachment. Intravenous administration frequently causes local phlebitis.
Drug Resistance
During treatment, cytomegalovirus can develop resistance to therapy. This most commonly occurs due to mutations in the UL97 gene encoding phosphotransferase. Without this enzyme, the drug simply cannot undergo initial phosphorylation and remains inactive. Less commonly, mutations occur in the viral DNA polymerase itself, reducing its affinity for the drug.
If resistance develops, patients are prescribed alternative agents that do not require viral enzymes for activation (e.g., foscarnet or cidofovir).