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: Clinical activity was demonstrated in a single patient , but a comparative trial of standard bolus schedules with the protracted infusion schedule will be necessary to determine if the clinical effectiveness of the drug is maintained .
### Output Text:
Clinical activity was demonstrated in a single patient , but a comparative trial of standard bolus schedules with the protracted infusion schedule will be necessary to determine if the clinical effectiveness of the drug is maintained . 