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: Two prodrugs of potent and selective GluR5 kainate receptor antagonists actives in three animal models of pain .
### Output Text:
Two prodrugs of potent and selective GluR5 <span class="chemical">kainate</span> receptor antagonists actives in three animal models of pain . 