Pharmacokinetics, Efficacy and Safety of Abatacept Administered Subcutaneously (SC) in Children and Adolescents With Active Polyarticular Juvenile Idiopathic Arthritis (pJIA) and Inadequate Response (IR) to Biologic or Non Biologic Disease Modifying Anti-rheumatic Drugs (DMARDs)

Primary Outcome Measures

1. Abatacept Trough Concentration (Cmin) in Participants Ages 6 to 17
   Trough concentration of abatacept (reported as geometric mean of Cmin) in all pharmacokinetic (PK)-evaluable participants. Cmin is reported in microgram per milliliter (µg/mL). Desired target therapeutic Cmin should be \>= 10 µg/mL.

Secondary Outcome Measures

1. Percentage of Participants (Ages 6 to 17) Achieving American College of Rheumatology Pediatric 30 Response (ACRp30)
   ACRp30 is defined as ≥30% improvement in at least 3 of the 6 juvenile idiopathic arthritis (JIA) core set variables: 1. number of active joints 2. number of joints with limitation of motion (LOM) 3. physician global assessment of disease activity 4. parent global assessment of patient overall well-being 5. functional ability as measured by the Children's Health Assessment Questionnaire (CHAQ) 6. C-reactive protein (CRP). In addition to the above condition, to be considered a responder participants cannot have ≥30% worsening in more than 1 of the 3 remaining JIA core set variables for which improvement was not observed.
2. Abatacept Trough Concentration (Cmin) in Participants Ages 6 to 17 by Weight Tier Dose
   Evaluation of the trough concentration of abatacept (reported as geometric mean of Cmin) in all pk-evaluable participants at Days 57, 85 and 113. Weight-tiered dosing groups are based on the first dose the participant received. Cmin is reported in microgram per milliliter (µg/mL). Here 'n' number analyzed signifies participants who were evaluable for each time point.
3. Number of Participants With Adverse Events (AEs), Deaths, Serious AEs (SAEs) and AEs Leading to Discontinuation in the Short-Term Period for the 6-17 Year Age-Group Cohort
   An AE is defined as any new untoward medical occurrence or worsening of a preexisting medical condition in a clinical investigation subject administered an investigational (medicinal) product and that does not necessarily have a causal relationship with this treatment. An AE can therefore be any unfavorable and unintended sign (such as an abnormal laboratory finding), symptom, or disease temporally associated with the use of investigational product, whether or not considered related to the investigational product. A SAE is any untoward medical occurrence that at any dose which results in death, is life threatening, requires inpatient hospitalization or causes prolongation of existing hospitalization, results in persistent or significant disability/incapacity, is a congenital anomaly/birth defect.
4. Number of Participants With Adverse Events (AEs), Deaths, Serious AEs and AEs Leading to Discontinuation in the Cumulative Period
   An AE is defined as any new untoward medical occurrence or worsening of a preexisting medical condition in a clinical investigation subject administered an investigational (medicinal) product and that does not necessarily have a causal relationship with this treatment. An AE can therefore be any unfavorable and unintended sign (such as an abnormal laboratory finding), symptom, or disease temporally associated with the use of investigational product, whether or not considered related to the investigational product. A SAE is any untoward medical occurrence that at any dose which results in death, is life threatening, requires inpatient hospitalization or causes prolongation of existing hospitalization, results in persistent or significant disability/incapacity, is a congenital anomaly/birth defect.
5. Number of Participants With Positive Immunogenicity Response in the Short-Term Period for the 6-17 Year Age-Group Cohort
   Overall number of participants with either a positive immunogenicity response for 'CTLA4 and possibly Ig' or 'Ig and/or Junction Region' relative to baseline. Sample draws for immunogenicity were scheduled at specific study days while on treatment for all subjects and at follow-up visits 28, 85, and 168 days after the last abatacept dose for those subjects who discontinued from the short term (ST) period (initial 4-month treatment period) or completed the ST study without continuing abatacept treatment.
6. Number of Participants With Positive Immunogenicity Response in the Cumulative Period
   Overall number of participants with either a positive immunogenicity response for 'CTLA4 and possibly Ig' or 'Ig and/or Junction Region' relative to baseline. Sample draws for immunogenicity were scheduled at specific study days while on treatment for all subjects and at follow-up visits 28, 85, and 168 days after the last abatacept dose regardless of whether they discontinued early in the short term (ST) or long term (LT) period, elected not to enter the LT period, or completed both ST and LT periods.