In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: Animal and clinical studies have suggested that N - methyl - D - aspartate ( NMDA ) antagonists , such as ketamine , may be effective in improving opioid analgesia in difficult pain syndromes , such as neuropathic pain .
### Output Text:
Animal and clinical studies have suggested that <span class="chemical">N - methyl - D - aspartate</span> ( <span class="chemical">NMDA</span> ) antagonists , such as <span class="chemical">ketamine</span> , may be effective in improving opioid analgesia in difficult pain syndromes , such as neuropathic pain . 