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: Immediate but gradual improvement followed withdrawal of amiodarone and treatment with prednisolone .
### Output Text:
Immediate but gradual improvement followed withdrawal of <span class="chemical">amiodarone</span> and treatment with <span class="chemical">prednisolone</span> . 