Effect of CYP2B6 Genotype and Efavirenz on the Disposition and Pharmacodynamic of Methadone and Tizanidine in Healthy Volunteers
The main goal of this clinical study is to test how CYP2B6 genetic variations and efavirenz (cornerstone in HIV-1 therapy) dictate the disposition (PK) of CYP2B6 substrate (methadone) and PK and effect (PD) of CYP1A2 substrate (tizanidine). Specifically, the investigators will test whether efavirenz produces CYP2B6 genotype dependent unanticipated DDIs with CYP2B6 (methadone) and CYP1A2 (tizanidine), leading to lack of efficacy or increased toxicity. Healthy volunteers genotyped for CYP2B6\*6 and \*18 alleles will be grouped in to three genotype predicted phenotype groups: 20 normal metabolizer (NM) (CYP2B6\*1/\*1); 20 intermediate metabolizer (IM) (\*1/\*6, or \*1/\*18); and 20 poor metabolizer (PM) (\*6/\*6, \*6/\*18 or \*18/\*18). Each phenotype group will receive methadone and tizanidine (separated by a washout period) on two occasions: at baseline (control) and after treatment with efavirenz (600 mg/day for 17 days).
⁃ Subjects will be included in the study if participants:
• are male and female (approximately 1:1) volunteers between the age of 18 and 65 years old
• are judged healthy without any significant medical condition as determined by and decided from a pre-enrollment screening session that include medical history, laboratory tests such as blood and urine tests, vital signs, and an electrical tracing of the heartbeat (electrocardiogram, EKG). The pre-enrollment screening will be done no more than six weeks before the start of the study.
• are able and willing to adhere to the study medication restrictions two weeks before initiating the study and during the conduct of the entire study. These will include refraining from taking any prescriptions medications, over-the-counter medications, and herbal, dietary, and alternative supplements that may interact with the metabolism of those study drugs at least 2 weeks prior to the start of the study and until study completion.
• are nonsmoker or individuals willing to refrain from smoking or use of tobacco or marijuana for at least two weeks prior to and until the completion of the study.
• are willing to commit the time requested for this study.