Breathing brief, moderate bouts of low oxygen trigger (low oxygen therapy, LOT) spinal plasticity (the ability of the nervous system to strengthen neural pathways based on new experiences), and improve walking after spinal cord injury (SCI). The greatest improvements in walking ability occur when LOT is administered prior to skill-based walking practice (WALK). However, the enduring benefits of LOT on walking recovery may be undermined by the accumulation of LOT-induced increase in extracellular adenosine. The goal of the study is to understand the extent to which istradefylline (adenosine 2a receptor antagonist) may limit the competing mechanisms of adenosine on LOT-induced walking recovery following SCI.

This randomized, placebo-controlled clinical trial will examine the efficacy of a selective adenosine 2a antagonist (istradefylline) to enhance the beneficial effects of LOT-related gains on overground walking performance after spinal cord injury (SCI).

Participants will be randomly assigned to a combinatorial intervention: istradefylline+LOT, placebo+LOT, istradefylline+SHAM. Participants will be asked to avoid caffeine-containing substances for 48 hrs (> 5* half-life of ~7 hrs) before the start of the study. They also will refrain from consuming caffeine during the 4-week combinatorial intervention.

Participants enrolled in istradefylline+AIH will receive 20mg of istradefylline orally for 28 consecutive days. After 14 days of istradefylline treatment, the participants will receive 2 weeks (4 sessions/week) of LOT prior to 45min of skill-based walking practice (WALK) within the INSPIRE Lab. A single session of LOT will consist of 15 episodes of breathing 90s low levels of oxygen (10% O2) with 60s intervals at room air (21% O2).

Participants enrolled in istradefylline+SHAM will receive 20mg of istradefylline orally for 28 consecutive days. After 14 days of istradefylline treatment, the participants will receive 2 weeks (4 sessions/week) of SHAM therapy prior to 45min of skill-based walking practice (WALK) within the INSPIRE Lab. A single session of SHAM therapy will consist of 15 episodes of breathing 90s of normal levels of oxygen (21% O2) with 60s intervals at room air (21% O2).

Participants enrolled in placebo+AIH will receive 20mg of placebo (dextrose) treatment orally for 28 consecutive days. After 14 days of placebo treatment, the participants will receive 2 weeks (4 sessions/week) of LOT prior to 45min of skill-based walking practice (WALK) within the INSPIRE Lab. A single session of LOT will consist of 15 episodes of breathing 90s low levels of oxygen (10% O2) with 60s intervals at room air (21% O2).

Blood samples will be collected at baseline, and at the end of week 2, week 4 to assess for potential confounding effects of systemic inflammation and caffeine on responsiveness to the combinatorial interventions.

The study will assess functional outcomes, vital signs, and symptoms before and after each intervention. For our primary outcome measure, the study will assess walking speed (10-meter walk test, 10MWT) relative to baseline at the end of day 5 (D5), and 8 (F1) and 14 days (F2) post-treatment. This study also will assess leg strength, walking distance, and coordination on D5, F1, and F2 as secondary outcome measures. A linear mixed model will be used to compare differences in 10MWT with treatment and time as main effects and participants as random effects. This study will follow the Consolidated Standards of Reporting Trials.

Inclusion Criteria:

1. age 18 and 75 years (the latter to reduce the likelihood of heart disease) medical clearance to participate
2. lesion at or below C2 and above T12 with non-progressive etiology
3. classified as motor-incomplete with visible volitional leg movement
4. injury greater than 12 months
5. ability to advance one step overground without human assistance

Exclusion Criteria:

1. Concurrent severe medical illness (i.e., infection, cardiovascular disease, ossification, recurrent autonomic dysreflexia, unhealed decubiti, and history of pulmonary complications)
2. Pregnant women because of the unknown effects of AIH on pregnant women and fetus
3. History of seizures, brain injury, and/or epilepsy
4. Undergoing concurrent physical therapy
5. Diabetes
6. Cirrhosis Caffeine and/or NSAID allergies or intolerances