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: OBJECTIVE : We evaluated the tolerability of CE in a group of patients with documented history of adverse cutaneous reactions to P and N associated or not to classic NSAIDs .
### Output Text:
OBJECTIVE : We evaluated the tolerability of <span class="chemical">CE</span> in a group of patients with documented history of adverse cutaneous reactions to <span class="chemical">P</span> and <span class="chemical">N</span> associated or not to classic NSAIDs . 