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: Further long - term analysis of renal allograft studies using sirolimus as de novo immunosuppression along with clinical and laboratory studies will refine these issues in the future .
### Output Text:
Further long - term analysis of renal allograft studies using <span class="chemical">sirolimus</span> as de novo immunosuppression along with clinical and laboratory studies will refine these issues in the future . 