Tenofovir, PMPA
Valacyclovir (Valtrex) Interactions
Following administration of valacyclovir to healthy patients taking cimetidine, probenecid, or a combination of both, an additive increase in acyclovir AUC and peak serum concentration was observed, primarily due to a reduction in the clearance of acyclovir. The clinical significance of these pharmacokinetic interactions is unknown; however, no dosage adjustments are recommended for patients with normal renal function.
In a single case report, the addition of acyclovir to a regimen of phenytoin and valproate led to a clinically significant decrease in phenytoin serum concentrations and loss of seizure control. Adjustments to the phenytoin dose were necessary when acyclovir was added and discontinued. Acyclovir did not appear to affect valproate concentrations in this report. Until more data are known, clinicians should be prepared to make adjustments in phenytoin or fosphenytoin dosing if acyclovir or valacyclovir therapy is added or discontinued.
Patients receiving potentially nephrotoxic drugs together with valacyclovir may have an increased risk of renal dysfunction.
Because entecavir is primarily eliminated by the kidneys and valacyclovir can affect renal function, concurrent administration with valacyclovir may increase the serum concentrations of entecavir and adverse events. The manufacturer of entecavir recommends monitoring for adverse effects when these drugs are coadministered.
Since tenofovir, PMPA is primarily eliminated by the kidneys, concurrent administration of tenofovir disoproxil with valacyclovir may increase serum concentrations of tenofovir via competition for renal tubular secretion.
[ Last revised: 5/18/2005 4:59:00 PM ]
References
. Parmeggiani A, Riva R, Posar A, et al. Possible interaction between acyclovir and antiepileptic treatment. Ther Drug Monit 1995;17:312 - 5.
. DeBony F, Tod M, Bidault M, et al. Multiple interactions of cimetidine and probenecid with valacyclovir and its metabolite acyclovir. Antimicrob Agents Chemother 2002;46:458 - 63.
. Viread® (tenofovir) package insert. Foster City, CA: Gilead; 2005 May.
. Baraclude™ (entecavir) package insert. Princeton, NJ: Bristol-Myers Squibb Company; 2005 Mar.
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