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: T - cell stimulation by therapeutic concentration of ticlopidine was demonstrated in vitro in the patients , but not in healthy controls .
### Output Text:
T - cell stimulation by therapeutic concentration of <span class="chemical">ticlopidine</span> was demonstrated in vitro in the patients , but not in healthy controls . 