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: A comparison of individual selective and unselective cyclooxygenase inhibitors suggests substance - specific differences , which may depend on differences in pharmacokinetic parameters or inhibitory potency and may be contributed by prostaglandin - independent effects .
### Output Text:
A comparison of individual selective and unselective <span class="chemical">cyclooxygenase inhibitors</span> suggests substance - specific differences , which may depend on differences in pharmacokinetic parameters or inhibitory potency and may be contributed by <span class="chemical">prostaglandin</span> - independent effects . 