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: There have been concerns that the risk of cardiovascular thrombotic events may be higher with cyclooxygenase ( COX ) - 2 - specific inhibitors than nonselective nonsteroidal antiinflammatory drugs ( NSAIDs ) .
### Output Text:
There have been concerns that the risk of cardiovascular thrombotic events may be higher with cyclooxygenase ( COX ) - 2 - specific inhibitors than nonselective nonsteroidal antiinflammatory drugs ( NSAIDs ) . 