14591_PT05_179_212.qxd
188
PART 5
2/14/11
1:23 PM
Page 188
Antiviral Drugs
neurotoxic, and it is reserved for patients who have failed other therapy. Nausea, vomiting, and diarrhea can occur from any of these agents. Foscarnet can also cause penile ulcers. Cidofovir is an uncommonly used agent that also exhibits nephrotoxicity.
■
Important Facts
• Oral ganciclovir has been replaced by valganciclovir, which has much better bioavailability. • Ganciclovir must be carefully dosed by patient weight and renal function. Monitor patients closely for changes in renal function when they are on therapy. • The package insert for valganciclovir specifies dose adjustment for renal dysfunction but not weight. It comes in two strengths: 900 mg and 450 mg. The dose of 900 mg BID is considered to be equivalent to 5 mg/kg q12h of IV ganciclovir, but it may be much more than that for an underweight patient because it is approximately 60% bioavailable. Consider this example for a 50-kg patient: • Ganciclovir dose ⫽ 50 kg ⫻ 5 mg/kg ⫽ 250 mg ganciclovir • Valganciclovir dose ⫽ 900 mg ⫻ 0.60 bioavailability ⫽ 540 mg of active ganciclovir This patient would receive more than double the amount of active ganciclovir if 900 mg BID of valganciclovir are used. It may be worth considering dose reduction in underweight patients, particularly if they are at high risk of toxicity. • Foscarnet has significant nephrotoxicity. This can be somewhat attenuated through hydra-