Investigation on interaction between tacrolimus and sildenafil in kidney-transplanted patients with erectile dysfunction.

ABSTRACT

OBJECTIVE: Sildenafil may provide an effective treatment for erectile dysfunction, frequently observed in uremic patients and after kidney transplantation. Pharmacokinetic interactions between sildenafil and tacrolimus are to be expected due to a common elimination pathway via cytochrome P450 3A4. Therefore, the pharmacokinetics during combined use of these agents were studied over 9 days. MATERIAL AND METHODS: Nine male patients (age 29-52 years) were included, who had previously participated in a recent interaction study with sildenafil given as a single dose. Comedication remained unchanged in order to avoid introducing confounding factors. In the previous study in the patients, tacrolimus blood levels with and without sildenafil were measured for pharmacokinetic analysis. In the present study, 25 mg sildenafil were coadministered daily over 9 days and tacrolimus levels were assessed at sampling times optimized using simulation. In addition, laboratory parameters and blood pressure changes were measured and adverse effects monitored. RESULTS: Terminal half-lives of tacrolimus did not differ significantly and trough levels did not change when sildenafil was coadministered daily over 9 days. Mean arterial blood pressure was lower after sildenafil intake. Two patients had to reduce their antihypertensive treatment, 6 patients reported mild side effects. In 1 case, there was an asymptomatic, temporary increase in the serum concentration of gamma-GT. CONCLUSIONS: There was no evidence obtained for a change in elimination half-life or average concentration of tacrolimus during repeated coadministration of sildenafil. Since blood pressure decreased, a starting dose of 25 mg sildenafil and, if necessary, adjustment of the dose of antihypertensive drugs on days when sildenafil is given has to be considered. With respect to the observed blood pressure changes, pharmacokinetic/pharmacodynamic interaction studies with other antihypertensive drugs are of critical importance in these patients.