The purpose of this study is to collect and compare information on how and when adolescent heart transplant recipients take their prescribed medication. The investigators want to find out if regular use of 'an app' on cell phones, called the Teen Pocket PATH® (TPP), can help adolescents take their medication according to their prescribed dosing schedule. This may then help reduce complications of transplant, such as rejection. The investigators also want to find out if how adolescent heart transplant recipients take their medications affects the development of antibodies in their blood. Antibodies are small proteins in the blood that may develop after heart transplantation, and which can sometimes damage a new heart.

This is a multi-center, randomized, open label clinical trial of a novel mobile application. The intervention in this randomized controlled trial is the provision of a novel mobile health application (Teen Pocket PATH® [TPP]). Participants will include heart recipients 11-17 years of age transplanted at one of the listed study sites (refer to Contacts and Locations Section of this ClinicalTrials.gov record).

It is important that heart transplant recipients take their prescribed heart transplant medication. One of the challenges for adolescent (children ages 11 to 17) heart transplant recipients is being able to follow their medication dosing schedule consistently. The purpose of this study is to collect and compare information on how and when adolescent heart transplant recipients take their prescribed medications. The investigators want to find out if regular use of 'an app' on the participant's cell phone, called the Teen Pocket PATH® (TPP), can help adolescent heart transplant recipients take their medication according to their prescribed dosing schedule. This may then help reduce complications of transplant, such as rejection. The investigators also want to evaluate whether how one takes their medications affects the development of antibodies in their blood. Antibodies are small proteins in the blood that may develop after heart transplantation, and which can sometimes damage a recipient's new heart.

The TPP app provides reminders to the participant and their parent/guardian, includes a list of their medications, and maintains a log to help track medication taking. The parent(s) will receive a text message when their participating child enters into the TPP app that s/he has taken his/her medication. The parent(s) will also be notified via text if their participating adolescent child does not enter information into the TPP app regarding their medication adherence and related timing information. The information collected on the TTP app will be available to the study investigators.

Eligible participants will be randomly selected (randomized) to be in one of two groups: this means that each participant will have an equal but random chance of being assigned to:

• the Experimental Group - participants will receive standard of care and use the TPP app to monitor how they take their medication or
• the Control Group - participants will receive standard of care and will not not receive the TPP app.

Both groups will have the same number of scheduled research visits and will receive the usual treatment and medication provided by their doctor for their heart transplant. Research visits will coincide with your routine clinic visits.

The study will not change the care that participants receive prior to, during or after transplantation. The study will collect information about participants, their medication dosing schedule, and their medical condition and care. In addition, the investigators will collect some blood at certain times for research. This blood will be used to measure antibodies in the blood.

Inclusion Criteria:

• Enrolled (consented) prior to hospital discharge from heart transplantation or within one month after discharge, but not later than 3 months post-transplantation;
• English as primary language or fluent in English; and
• Informed consent and assent obtained.

Exclusion Criteria:

• Positive donor-specific cytotoxicity requiring different immunosuppressive regimen from standard care;
• Inability or unwillingness of a participant to give written informed consent or comply with study protocol;
• Re-transplant or multi-organ transplant recipient;
• Condition or characteristic which in the opinion of the investigator makes the participant unlikely to complete at least one year of follow-up; or
• Current participation in other research studies that would, or might, interfere with the scientific integrity or safety of current study (e.g. by interference with immunosuppression management guidelines and study end-points).