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: This complication will be observed even less often in the future as ticlopidine is being replaced by the newer antiplatelet agent clopidogrel .
### Output Text:
This complication will be observed even less often in the future as <span class="chemical">ticlopidine</span> is being replaced by the newer antiplatelet agent <span class="chemical">clopidogrel</span> . 