A Study to Evaluate the Effect of Multiple Doses of Itraconazole, Phenytoin, and Paroxetine on the Single-Dose Pharmacokinetics of Poziotinib in Healthy Adult Participants

Key Inclusion Criteria: * Healthy, adult, male or female (of non-childbearing potential only), 18-55 years of age. * Body mass index (BMI) ≥ 18.0 and ≤ 32.0 kilogram per meter square (kg/m\^2) at screening, and a minimum weight of 50.0 kg and a maximum weight of 100.0 kg at screening. * Continuous non-smoker who has not used nicotine-containing products for at least 3 months prior to the first dosing and throughout the study. * Medically healthy with no clinically significant findings from medical history, physical examination, 12-lead ECG, vital signs, and laboratory evaluations. * Female must be of non-childbearing potential only and must have undergone a sterilization procedure at least 6 months prior to the first dosing, or * Postmenopausal women should have amenorrhea for at least 1 year prior to the first dosing. * A non-vasectomized male participant must agree to use a highly effective method of birth control with female partners of childbearing potential or with pregnant partners during the study and for 120 days following last dosing. * Able to comprehend and willing to sign an Informed Consent Form (ICF) and abide by the clinical protocol, study procedures, and restrictions. * For Part 1 only: Participant must be a CYP2D6 extensive metabolizer or CYP2D6 poor metabolizer as determined by a valid genotyping method. * For Part 2 only: Participant must not be a CYP2C9 and CYP2C19 poor metabolizer as determined by a valid genotyping method and must be a CYP2D6 extensive metabolizer as determined by a valid genotyping method. * For Part 3 only: Participant must be a CYP2D6 extensive metabolizer as determined by a valid genotyping method. Key Exclusion Criteria: * Significant history or clinical manifestation of any metabolic, allergic, dermatological, hepatic, renal, hematological, pulmonary, cardiovascular, gastrointestinal, neurological, respiratory, endocrine, or psychiatric disorder, as determined by the PI (or designee). * History of a developing or established acute event or infection in the prior 2 weeks to screening. * History of significant hypersensitivity, or idiosyncratic reaction to poziotinib, itraconazole, phenytoin, paroxetine, or related drugs, food, or other substances. * Any surgical or medical condition within 6 months prior to first dosing that may potentially alter absorption, distribution, metabolism or excretion of the study drugs, in the opinion of the PI (or designee). * History or presence of alcoholism or drug/chemical abuse within 2 years prior to check-in. * Female participants with a positive pregnancy test result or lactating. * Positive urine drug or alcohol test results at screening or check-in. * Positive hepatitis panel (hepatitis B surface antigen (HBsAg) or hepatitis C virus (HCV) and/or positive human immunodeficiency virus (HIV) test at screening. Automatic reflex Differential and ribonucleic acid testing will be conducted in the event of a reactive antibody/antigen screen. * Seated blood pressure is less than 90/40 millimetre of mercury (mmHg) or greater than 140/90 mmHg at screening. * Seated heart rate is lower than 40 beats per minute (bpm) or higher than 99 bpm at screening. * QT interval with Fridericia's correction (QTcF) interval is \>450 millisecond (msec) (males) or \>470 msec (females). * Unable to refrain from or anticipates the use of: * Any drug, including prescription and non-prescription medications, herbal remedies, or vitamin supplements beginning 14 days prior to the first dosing and throughout the study. Hormone replacement therapy will not be allowed. * Any drugs known to be inhibitors and/or inducers of CYP3A, CYP2D6, CYP2C9, or CYP2C19 enzymes; and/or P-gp; and/or gastric acid reducing agents (proton-pump inhibitors, H2-receptor antagonists, antacids) for 28 days prior to the first dosing and throughout the study (except those required as part of the study). * Has a coagulation test (i.e., prothrombin time and activated partial thromboplastin time) outside of normal ranges at screening or at check-in. * Has platelet, hemoglobin, or hematocrit that are below the lower limit of normal at screening or at check-in. * Has alanine aminotransferase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (ALP), or total bilirubin that are greater than the upper limit of normal at screening or at check-in. * Estimated creatinine clearance \<90 milliliter per minute (mL/min) at screening. * Participation in another clinical study within 30 days prior to the first dosing. * Prior exposure to poziotinib. * For Part 1 Only:History or presence of any of the following, deemed clinically significant by the Investigator (or designee), and as confirmed by the Sponsor: * Ventricular dysfunction or risk factors for Torsades de Pointes (e.g., heart failure, cardiomyopathy, family history of Long QT Syndrome). * Uncorrected hypokalemia (potassium levels \< 3.7) and/or hypomagnesemia (magnesium levels \< 1.9). * Myasthenia gravis. * For Part 2 Only: History of seizure (excluding simple febrile seizure), epilepsy, severe head injury, multiple sclerosis, or other known neurological conditions which the Investigator considers to be clinically significant. * Is at suicidal risk in the opinion of the PI as per the following criteria: * Any suicidal attempts within 12 months prior to screening. * any suicidal intent including a plan or C-SSRS answer of "YES" on suicidal ideation currently or within 3 months.