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: The results suggest a strain - dependent effect of amantadine on motility and indicate a differential response to the acute and multiple dose regimens used .
### Output Text:
The results suggest a strain - dependent effect of <span class="chemical">amantadine</span> on motility and indicate a differential response to the acute and multiple dose regimens used . 