Efficacy, Safety & Pharmacokinetic (PK) Study of HLD200 in Children Aged 4-5 Years with ADHD

Inclusion Criteria:

1. The subject's assigned caregiver must be able to read, write, and/or understand at alevel sufficient to provide informed consent and will provide this written consent (and subject will provide assent, if required by the IRB) before any study-specificactivity is performed.

2. Subjects must be male or female children 4 to 5 years of age at the time of consentand assent (if applicable).

3. Subjects must have a diagnosis of ADHD as defined by the DSM-5 criteria withconfirmation using the Mini - International Neuropsychiatric Interview for Childrenand Adolescents (MINI KID).

4. All subjects must have an ADHD-RS-IV Preschool Version Parent Total Score of ≥28 forboys or ≥24 for girls at Baseline (Visit 2). In addition, subjects not receivingpharmacotherapy for ADHD at time of consent must have an ADHD-RS-IV PreschoolVersion Parent Total Score of ≥28 for boys or ≥24 for girls at Screening; if theADHD-RS-IV total score at Screening differs from the score at Baseline (Visit 2) bygreater than 20% for these subjects (i.e., subjects who are not receiving ADHDpharmacotherapy at time of consent), the Investigator must investigate the reasonsfor the difference in score and determine whether the subject is an appropriatecandidate to participate in the study.

5. Subjects not receiving pharmacotherapy for ADHD at time of consent must have aClinical Global Impression - Severity (CGI-S) score ≥4 at Screening. All subjectsmust have a CGI-S score of ≥4 at Baseline (Visit 2).

6. Subjects must have a Peabody Picture Vocabulary Test 4 (PPVT-4) Standard Score ≥70at Screening.

7. Subject has undergone an adequate course of nonpharmacologic treatment or has asevere enough condition in the opinion of the Investigator to consider enrollmentwithout undergoing prior nonpharmacological treatment.

8. Subject's weight at Screening is between the 5th and 95th percentiles and subject'sheight at Screening is ≥ 5th percentile according to the Centers for Disease Controland Prevention growth charts by age and sex.

9. Subject must have a resting pulse less than 127 bpm, systolic and diastolic bloodpressure below the 95th percentile for age, gender and height according to the 2017American Academy of Pediatrics guidelines, based on the average of 3 measurements 2to 5 minutes apart at Visit 1 and Visit 2 (only a single measurement is required atVisit 2 unless an elevated excursion is noted which requires the averaged value oftriplicate measurements). If a subject's height is above the 95th percentile, thesubject must meet the systolic and diastolic blood pressure criteria for the 95thpercentile height.

10. Subject must be considered clinically appropriate for treatment with HLD200.

11. Subject must be in general good health based upon medical history, physicalexamination, clinical laboratory examinations, vital signs, and 12-leadelectrocardiogram (ECG).

12. Subject and the subject's assigned caregiver are willing and able to participate inall study assessments and procedures, as applicable, and to follow all studyrequirements (including compliance with study drug), as described in the informedconsent form.

Exclusion Criteria:

1. History of, or current, medical or psychiatric condition, including gastrointestinaldisorders (e.g., surgery, malabsorption syndrome, and other similar conditions),open-angle glaucoma, abnormally increased intraocular pressure (IOP), or laboratoryresult that, in the opinion of the Investigator, unfavorably alters the risk-benefitof study participation, may jeopardize subject safety, or may interfere with thesatisfactory completion of the study and study related procedures.

2. Serious structural cardiac abnormalities, cardiomyopathy, serious heart rhythmabnormalities, severe hypertension, untreated thyroid disease, peripheralvasculopathy, known structural cardiac disorders, serious cardiac conditions,serious arrhythmias, known family history of sudden death, or other cardiac problemsthat may place the subject at increased vulnerability to the sympathomimetic effectsof a stimulant drug.

3. History of, or current, seizure disorder (except febrile seizures prior to age 4 andwith last occurrence at least 1 year prior to study participation) or intellectualdisability of mild severity or greater (DSM-5 criteria).

4. Past or current diagnosis of psychosis, bipolar I or II disorder, major depressivedisorder, eating disorder, conduct disorder, obsessive-compulsive disorder, autismspectrum disorder, Tourette's Syndrome or other tic disorders, disruptive mooddysregulation disorder, or confirmed genetic disorder with cognitive and/orbehavioral disturbances. Subjects with oppositional defiant disorder (ODD) oranxiety disorder (e.g., separation anxiety disorder simple phobia disorder) arepermitted to enroll in the study provided these diagnoses are not the primary focusof treatment, and, in the opinion of the Investigator, are not anticipated tointerfere with study assessments or procedures.

5. Subject is currently considered at risk of suicide in the opinion of theInvestigator, or has previously made a suicide attempt, or has a history of, or iscurrently demonstrating active suicidal ideation or behavior, as determined by theC-SSRS at Screening and/or Baseline (Visit 2).

6. History of severe allergic reaction or intolerance to methylphenidate hydrochloride (MPH).

7. Alanine aminotransferase, aspartate aminotransferase, total bilirubin, or creatininegreater than 1.5 times the upper limit of normal. Elevated bilirubin due toGilbert's syndrome is not exclusionary.

8. Use of ADHD stimulant medication within 72 hours (3 days) prior to Baseline (Visit 2); clonidine and guanfacine within 5 days prior to Baseline (Visit 2); psychotropicmedications, including antidepressants (including MAOIs), mood stabilizers, andantipsychotics, within 14 days prior to Baseline (Visit 2); other prescriptionmedications (including atomoxetine, excluding allowed medications) within 7 daysprior to Baseline (Visit 2); and nonprescription/over-the-counter medications (excluding allowed medications) or herbal medications within the 3 days prior toBaseline (Visit 2). Medications not covered in allowed medications or prohibitedmedications must be cleared by the Medical Monitor prior to randomizing the subject.

9. Participation in a clinical trial with an investigational drug within the 30 daysprior to Baseline (Visit 2).

10. Initiation of non-pharmacological treatment within 30 days prior to Baseline (Visit

2. or expected to initiate new non-pharmacological treatment during the study.

11. Use of any other medications that might confound the results of the study orincrease risk to the subject.

12. In the opinion of the Investigator, the subject is well-controlled on his/hercurrent ADHD medication with acceptable tolerability.

13. Subject has a sibling or step-sibling that is concurrently participating in thisstudy or who has previously participated in this study.

14. Subject or caregiver is a participating Investigator, Sub-investigator, studycoordinator, or employee of a participating Investigator, or is an immediate familymember of the aforementioned.

15. Any factor, which in the opinion of the Investigator would jeopardize the evaluationor safety or be associated with poor adherence to the protocol.