San Diego-based Ligand Pharmaceuticals reported Monday that it has acquired Connecticut-based Neurogen, a developer of small-molecule drugs to improve the lives of patients suffering from psychiatric and neurological disorders. Ligand said it would pay approximately $11M for the firm in common stock in the merger. In addition, Ligand said the sale would entitle Neurogen shareholders to cash payments for sale or licensing of certain assets, and reaching specific milestones. Neurogen has a partnership with Merck to develop treatments for acute and chronic pain, in addition to other programs.