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: OBJECTIVES : The risk of acute myocardial infarction ( AMI ) with COX - 2 inhibitors may offset their gastrointestinal ( GI ) benefit compared with non - selective ( NS ) non - steroidal anti - inflammatory drugs ( NSAIDs ) .
### Output Text:
OBJECTIVES : The risk of acute myocardial infarction ( AMI ) with <span class="chemical">COX - 2 inhibitors</span> may offset their gastrointestinal ( GI ) benefit compared with non - selective ( NS ) <span class="chemical">non - steroidal anti - inflammatory drugs</span> ( NSAIDs ) . 