A Pharmacokinetic Drug-Drug Interaction (DDI) Study Between Sitaxsentan And Sildenafil, And Between Sitaxsentan And Tadalafil After Multiple Doses

Phase 1

Terminated

Interventions: Drug: sitaxsentan
Drug: tadalafil
Drug: sitaxentan
Drug: sildenafil

Brief Summary: Sitaxsentan has a low drug-drug interaction potential and it did not have a clinically relevant effect on pharmacokinetics of sildenafil (a CYP3A sensitive substrate and PDE5 inhibitor).

Tadalafil did not have clinically relevant effect on pharmacokinetics of bosentan and ambrisentan. Based on overall clinical drug-drug interaction profiles, and in vitro CYP enzymes and transporter data, a clinically relevant drug-drug interaction between sitaxsentan and tadalafil is not expected. Sildenafil is not expected to affect sitaxsentan pharmacokinetics (PK), as sitaxsentan is a substrate of CYP3A4 and CYP2C9, where sildenafil did not show clinically relevant effect on PK of substrates of CYP3A4 and CYP2C9.

Recruitment & Eligibility

Inclusion Criteria

Healthy male subjects and women of non-child bearing potential between the ages of 21 and 55 years, inclusive. Healthy is defined as no clinically relevant abnormalities identified by a detailed medical history, full physical examination, including blood pressure and pulse rate measurement, 12-lead ECG and clinical laboratory tests.
Body Mass Index (BMI) of 17.5 to 30.5 kg/m2; and a total body weight >45 kg (99 lbs).
An informed consent document signed and dated by the subject or a legally acceptable representative.
Subjects who are willing and able to comply with scheduled visits, treatment plan, laboratory tests, and other study procedures.

Exclusion Criteria

Evidence or history of clinically significant hematological, renal, endocrine, pulmonary, gastrointestinal, cardiovascular, hepatic, psychiatric, neurologic, or allergic disease (including drug allergies, but excluding untreated, asymptomatic, seasonal allergies at time of dosing) or clinical findings at Screening.
A positive urine drug screen.
Subjects with hepatic dysfunction, defined as aspartate aminotransferase (AST) and/or alanine aminotransferase (ALT) >1.5 times the upper limit of the normal range at Screening. A retest may be done if AST and/or ALT within 1.5- to 2- times the upper limit of the normal range at Screening, and the average of the first and repeated test values should be used to decide the eligibility.

Arm && Interventions

Group	Intervention	Description
Treatment D	sitaxsentan	sitaxsentan 100 mg QD co-administered with sildenafil 20 mg TID for 6 days
Treatment D	sildenafil	sitaxsentan 100 mg QD co-administered with sildenafil 20 mg TID for 6 days
Treatment C	sitaxsentan	sitaxsentan 100 mg QD co-administered with tadalafil 40 mg QD for 6 days
Treatment B	tadalafil	tadalafil 40 mg QD for 6 days
Treatment A	sitaxentan	sitaxsentan 100 mg QD for 6 days (Treatment A)
Treatment C	tadalafil	sitaxsentan 100 mg QD co-administered with tadalafil 40 mg QD for 6 days

Primary Outcome Measures

Name	Time	Method
Maximum Observed Plasma Concentration (Cmax)	Day 6 (predose and 0.5, 1, 1.5, 2, 3, 4, 6, 8, 12, 16, and 24 hours post dose)
Time to Reach Maximum Observed Plasma Concentration (Tmax)	Day 6 (predose and 0.5, 1, 1.5, 2, 3, 4, 6, 8, 12, 16, and 24 hours post dose)
Trough Plasma Concentrations (Ctrough)	Day 6 (predose and 0.5, 1, 1.5, 2, 3, 4, 6, 8, 12, 16, and 24 hours post dose)	Minimum or "trough"concentrations
Area Under the Curve of the 24 Hour Dosing Interval (AUC24)	Day 6 (predose and 0.5, 1, 1.5, 2, 3, 4, 6, 8, 12, 16, and 24 hours post dose)
Apparent Oral Clearance (CL/F)	Day 6 (predose and 0.5, 1, 1.5, 2, 3, 4, 6, 8, 12, 16, and 24 hours post dose)	Clearance of a drug is a measure of the rate at which a drug is metabolized or eliminated by normal biological processes. Clearance obtained after oral dose (apparent oral clearance) is influenced by the fraction of the dose absorbed. Clearance was estimated from population pharmacokinetic (PK) modeling. Drug clearance is a quantitative measure of the rate at which a drug substance is removed from the blood.
Volume of Distribution at Steady State (Vss)	Day 6 (predose and 0.5, 1, 1.5, 2, 3, 4, 6, 8, 12, 16, and 24 hours post dose)	Volume of distribution is defined as the theoretical volume in which the total amount of drug would need to be uniformly distributed to produce the desired blood concentration of a drug. Steady state volume of distribution (Vss) is the apparent volume of distribution at steady-state.

Secondary Outcome Measures

Name	Time	Method
Plasma Decay Half-Life (t1/2)	Day 6 (predose and 0.5, 1, 1.5, 2, 3, 4, 6, 8, 12, 16, and 24 hours post dose)	Plasma decay half-life is the time measured for the plasma concentration to decrease by one half.