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: On an intention - to - treat basis , 55 % of the patients achieved at least partial response , including 19 % CR and 35 % achieved at least very good partial response .
### Output Text:
On an intention - to - treat basis , 55 % of the patients achieved at least partial response , including 19 % CR and 35 % achieved at least very good partial response . 