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: However , when converting patients to RAPA drug levels should be monitored to avoid over - immunosuppression and adequate antiviral and Pneumocystis carinii pneumonia prophylaxis should be given .
### Output Text:
However , when converting patients to <span class="chemical">RAPA</span> drug levels should be monitored to avoid over - immunosuppression and adequate antiviral and Pneumocystis carinii pneumonia prophylaxis should be given . 