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: Immunosuppressive drugs have been used during the last 30 years in treatment of patients with severe rheumatoid arthritis .
### Output Text:
Immunosuppressive drugs have been used during the last 30 years in treatment of patients with severe rheumatoid arthritis . 