This study is being conducted to assess the relative bioavailability of naproxen and esomeprazole from a single dose of PN 400 (delayed-release naproxen 500mg / immediate-release esomeprazole 20mg), enteric-coated naproxen 500mg co-administered with enteric-coated esomeprazole 20mg, enteric-coated naproxen 500mg administered alone, and enteric-coated esomeprazole 20mg administered alone.