1. Combine first 8 ingredients in a bowl; mix until combined. Divide panko mixture into 4 portions, shaping each into a 1/2-inch-thick oval patty.
2. Heat a nonstick grill pan over medium-high heat. Coat pan with cooking spray. Add patties to pan; cook 6 minutes on each side or until done. Place 1 patty and 1/2 cup arugula in each pita half.
Tzatziki sauce: Combine 1/2 cup plain low-fat, Greek-style yogurt; 1/4 cup finely chopped seeded cucumber; 1/4 teaspoon salt; and 1/8 teaspoon ground red pepper. Serve with sliced bell pepper and celery sticks.