AIM: Hepatic metabolism of sildenafil uses the same metabolic pathway as the calcineurin inhibitors (cyclosporine/tacrolimus), through the CYP3A4 isoenzyme. The aim of this pilot study was to evaluate the potential interaction between sildenafil therapy and circulating levels of cyclosporine and tacrolimus in a group of steady-state renal transplant recipients with erectile dysfunction. MATERIAL AND METHODS: A prospective pilot study of sildenafil interactions was carried out in 9 stable male renal transplant recipients with severe erectile dysfunction (mean age 50 +/- 8 years, range 38-64). All patients were receiving therapy with calcineurin inhibitors (5 with cyclosporine and 4 with tacrolimus). Erectile dysfunction was evaluated by clinical history, physical examination, International Index of Erectile Function (IIEF) questionnaire and the nocturnal penile tumescence test (RigiScan). Each patient received a first dose of 50 mg of sildenafil, one hour before sexual activity and a second dose at 72 hours of 50 or 100 mg according to the clinical response to the first dose. We evaluated the efficacy and safety of sildenafil and the evolution of cyclosporine-tacrolimus levels. Cyclosporine and tacrolimus trough whole blood concentrations were determined in basal conditions (before starting sildenafil) and on days 1, 4 and 7 after sildenafil therapy. RESULTS: Eighty-nine percent of patients (n = 8) required a complete 100 mg dose of sildenafil. There was a positive clinical response in two-thirds of cases (6 patients). In 5 patients (55%) sildenafil administration produced a complete response, in one patient the response was incomplete, and in the remaining 3 cases (33%) no clinical response was observed. Associated side effects included self-limited tachycardia in one patient and mild visual disturbances in another. Cyclosporine and tacrolimus levels remained stable in all patients. There were no significant differences in circulating levels of cyclosporine (basal 120 +/- 47; day 1: 116 +/- 55; day 4: 123 +/- 56 and day 7: 121 +/- 56 ng/ml p = NS) or tacrolimus (basal 11.6 +/- 1.3; day 1: 11.9 +/- 1.3; day 4: 11.1 +/- 1.0 and day 7: 11.8 +/- 0.9 ng/ml p = NS) over the study period. CONCLUSIONS: Sildenafil therapy is safe and effective for the treatment of erectile dysfunction in renal transplant recipients. Recommended therapeutic doses of sildenafil did not modify cyclosporine and tacrolimus trough blood levels

AIMS: To characterize the cytochrome P450 (CYP) enzymes responsible for the N-demethylation of sildenafil to its main metabolite, UK-103 320, to investigate the potential inhibitory effects of sildenafil on CYP enzymes and to evaluate the potential of selected drugs to affect sildenafil metabolism. METHODS: The metabolic pathways of sildenafil N-demethylation were studied using human liver microsomes, as well as microsomes expressing individual human CYP enzymes. Further studies to identify the individual enzymes were performed at 2.5 and 250 microM sildenafil, and employed a combination of chemical inhibition, correlation analysis, and metabolism by expressed recombinant CYP enzymes. In addition, the effect of sildenafil on the activity of the six major drug metabolizing enzymes was investigated. RESULTS: Sildenafil conversion was found to be mediated by at least two CYP enzymes, for which the mean kinetic parameters were Km1 = 6(+/-3 microM), Km2 = 81(+/-45 microM), Vmax1 = 22(+/-9 pmol) and Vmax2 = 138(+/-77 pmol) UK-103 320 formed min(-1) mg(-1). At 250 microM sildenafil, N-demethylation was primarily mediated through the low-affinity, high-Km enzyme (approximately 83%), whilst at 2.5 microM there was a greater role for the high-affinity, low-Km enzyme (approximately 61%). Ketoconazole strongly inhibited metabolism at both sildenafil concentrations and was the only significant inhibitor at 250 microM sildenafil. At the lower sildenafil concentration, sulphaphenazole and quinidine also inhibited formation of UK-103 320. Overall, 75% or more of the N-demethylation of sildenafil at any concentration is probably attributable to CYP3A4. These results were supported by experiments using expressed human CYP enzymes, in which only CYP3A4 and CYP2C9 exhibited substantial sildenafil N-demethylase activity (respective Km values of 221 microM and 27 microM). Sildenafil metabolism was inhibited by potent CYP3A4 inhibitors which are used clinically, but was found to be only a weak inhibitor of drug metabolizing enzymes itself, the strongest inhibition occurring against CYP2C9 (Ki = 80 microM). CONCLUSIONS: Evidence is provided for CYP3A4 and to a lesser extent CYP2C9-mediated metabolism of sildenafil. There is the possibility that elevated plasma concentrations of sildenafil could occur with coadministration of known inhibitors of CYP2C9 or CYP3A4. Since peak plasma concentrations of clinical doses of sildenafil are only 200 ng ml(-1) ( approximately 0.4 microM) it is very unlikely that sildenafil will significantly alter the plasma concentration of other compounds metabolized by cytochrome P450 enzymes

The in vitro biotransformation of sildenafil to its major circulating metabolite, UK-103,320, was studied in human liver microsomes and in microsomes containing heterologously expressed human cytochromes. In human liver microsomes, the mean K(m) (+/-S.E. ) was 14.4 +/- 2.0 microM. A screen of the chemical inhibitors omeprazole (10 microM), quinidine (10 microM), sulfaphenazole (10 microM), and ketoconazole (2.5 microM) only revealed detectable inhibition with ketoconazole. Sildenafil biotransformation (36 microM) was inhibited by increasing concentrations of ketoconazole and ritonavir (IC(50) values less than 0.02 microM), which are established cytochrome P450 (CYP) 3A4 inhibitors. Using microsomes containing cDNA-expressed cytochromes, UK-103,320 formation was found to be mediated by four cytochromes: CYP3A4, -2C9, -2C19, and -2D6. Estimated relative contributions to net intrinsic clearance were 79% for CYP3A4 and 20% for CYP2C9; for CYP2C19 and -2D6, estimated contributions were less than 2%. These results demonstrate that CYP3A4 is the primary cytochrome mediating UK-103,320 formation and that drugs that inhibit CYP3A4 are likely to impair sildenafil biotransformation

OBJECTIVES: To study the pharmacokinetics of the combined use of sildenafil (which may provide an effective treatment for patients with erectile dysfunction after kidney transplantation) and tacrolimus, as interactions between them are expected because of a common elimination pathway. METHODS: Ten male patients (age 29 to 52 years) were included. Because of its importance in transplant recipients, medication remained unchanged. On day 1, tacrolimus was administered routinely, and blood samples for tacrolimus assays were drawn at predefined times. On day 2, sildenafil was added and blood was collected for assays of tacrolimus, sildenafil, and the sildenafil metabolite UK103,320 (UK) at the indicated times. Blood pressure was monitored on both study days. Sildenafil and UK were assessed by high-pressure liquid chromatography and tacrolimus was assessed by microparticle enzyme immunoassay. RESULTS: No effects of sildenafil on the tacrolimus pharmacokinetics were found. However, in the patients studied, the sildenafil and UK pharmacokinetics were altered compared with the results of previous studies. The mean peak concentration of sildenafil was higher by 44% and the area under the concentration-time data increased by 90%. The elimination half-life was prolonged (4.7 hours compared with 3 hours in healthy volunteers). The area under the concentration-time data for UK was about threefold larger than in healthy volunteers, and the half- life was prolonged from 3.8 hours to 11.4 hours. Pronounced blood pressure drops were observed. CONCLUSIONS: Tacrolimus or the concomitant medication or the disease itself might have altered the sildenafil and UK pharmacokinetics. Because of the drop in blood pressure, sildenafil therapy should start at the lowest dose and any antihypertensive medication should be adjusted

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