The primary objective of this research study is to test the hypothesis that late sodium current blocking drugs (mexiletine or lidocaine) can attenuate the effect of hERG potassium channel blocking drugs (dofetilide) on ventricular repolarization (QTc) by shortening early repolarization (J-Tpeakc). The secondary object is to assess the ability of calcium channel block (diltiazem) to reduce the QTc prolongation associated with hERG block (moxifloxacin).

This is a randomized, double-blind, 5-period crossover study in healthy male and female subjects, 18 to 35 years of age, to compare the electrophysiological response of hERG potassium channel blocking drugs with and without the addition of late sodium or calcium channel blocking drugs. The 5 treatment periods are 1) dofetilide alone, 2) mexiletine with and without dofetilide, 3) lidocaine with and without dofetilide, 4) moxifloxacin with and without diltiazem and 5) placebo. During each treatment period, 12 blood samples for pharmacokinetic measurements are obtained with matched 12-lead ECG recordings.

Inclusion Criteria:

1. Subject is a healthy man or woman, 18 to 35 years of age, inclusive, who weighs at least 50 kg (110 pounds), no more than 85 kg (197 pounds) and has a body mass index of 18 to 27 kg/m2, inclusive, at Screening.
2. Subject has normal medical history findings, clinical laboratory results, vital sign measurements, 12 lead ECG results, and physical examination findings at Screening or, if abnormal, the abnormality is not considered clinically significant (as determined and documented by the investigator or designee).
3. Male or female subjects must agree to practice 2 highly effective methods of birth control (as determined by the investigator or designee; one of the methods must be a barrier technique) from Screening until 30 days after the last dose of study drug.

Exclusion Criteria:

• 1. Subject has a 12 lead safety ECG result at Screening or Check in of Period 1 with evidence of any of the following abnormalities:

  • QT corrected interval (QTc) using Fridericia correction (QTcF) >430 milliseconds (ms)

  • PR interval >220 ms or <120 ms

  • QRS duration >110 ms

  • Second- or third-degree atrioventricular block

  • Complete left or right bundle branch block or incomplete right bundle branch block

  • Heart rate <50 or >90 beats per minute

  • Pathological Q-waves (defined as Q wave >40 ms)

  • Ventricular pre-excitation

    2. Subject has more than 12 ectopic beats during the 3 hour Holter ECG at Screening.

    3. Subject has a history of unexplained syncope, structural heart disease, long QT syndrome, heart failure, myocardial infarction, angina, unexplained cardiac arrhythmia, torsades de pointes, ventricular tachycardia, or placement of a pacemaker or implantable defibrillator. Subjects will also be excluded if there is a family history of long QT syndrome (genetically proven or suggested by sudden death of a close relative due to cardiac causes at a young age) or Brugada syndrome.

    4. Subject has a history or current evidence of any clinically significant (as determined by the investigator) cardiovascular, dermatologic, endocrine, gastrointestinal, hematologic, hepatic, immunologic, metabolic, neurologic, psychiatric, pulmonary, renal, urologic, and/or other major disease or malignancy (excluding nonmelanoma skin cancer). The investigator may allow exceptions to these criteria (e.g., stable mild joint disease [that will not interfere with or influence the activities required by the protocol, in the opinion of the investigator], cholecystectomy, childhood asthma) following discussion with the medical monitor.

    5. Subject has a history of thoracic surgery.

    6. Subject has any condition possibly affecting study drug absorption (e.g., gastrectomy, Crohn's disease, irritable bowel syndrome).

    7. Subject has a skin condition likely to compromise ECG electrode placement.

    8. Subject is a female with breast implants.

    9. Subject's laboratory test results at Screening or Check in of Period 1 are outside the reference ranges provided by the clinical laboratory and considered clinically significant (as determined and documented by the investigator or designee).

    10. Subject's laboratory test results at Screening or Check in of Period 1 indicate hypokalemia, hypocalcemia, or hypomagnesemia according to lower limits of the reference ranges provided by the clinical laboratory.

    11. Subject's laboratory test results at Screening or Check in of Period 1 are >2 × the upper limit of normal (ULN) for alanine aminotransferase or aspartate aminotransferase, >1.5 × ULN for bilirubin, or >1.5 × ULN for creatinine.

    12. Subject has a positive test result at Screening for human immunodeficiency virus, hepatitis C antibodies, or hepatitis B surface antigen.

    13. Subject has a mean systolic blood pressure <90 or >140 mmHg or a mean diastolic blood pressure <50 or >90 mmHg at either Screening or Check in of Period 1.