Shire plc and Parion Sciences, Inc. have entered an agreement granting Shire exclusive rights to develop and commercialize P-321, an investigational epithelial sodium channel (ENaC) inhibitor in Phase II development for the treatment of dry eye disease.    P-321 is being developed to address tear volume deficiency and promote ocular surface healing and is believed to address dry eye disease by inhibiting ENaC, which is thought to block the absorption of tears. Current prescription treatments ...