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: Their ester prodrugs 6 and 8 were orally active in three models of pain : reversal of formalin - induced paw licking , carrageenan - induced thermal hyperalgesia , and capsaicin - induced mechanical hyperalgesia .
### Output Text:
Their ester prodrugs 6 and 8 were orally active in three models of pain : reversal of <span class="chemical">formalin</span> - induced paw licking , <span class="chemical">carrageenan</span> - induced thermal hyperalgesia , and <span class="chemical">capsaicin</span> - induced mechanical hyperalgesia . 