Eli Lilly and Co. plans to buy Centessa Pharmaceuticals plc for $6.3 billion in up-front cash and another potential $1.5 billion through contingent value rights, gaining access to a pipeline of orexin receptor 2 agonists for sleep disorders. The Indianapolis-based pharma is re-entering a field that has multibillion-dollar potential and one that could emulate the success seen with its obesity program. Centessa, of Boston and London, has a lead candidate, cleminorexton (formerly ORX-750), with positive phase IIa data in narcolepsy types 1 and 2 and idiopathic hypersomnia.