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: Switching the immunosuppressive regimen from tacrolimus to cyclosporine did not improve the clinical situation .
### Output Text:
Switching the immunosuppressive regimen from <span class="chemical">tacrolimus</span> to <span class="chemical">cyclosporine</span> did not improve the clinical situation . 