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: Limited clinical experience with continuous subcutaneous mesna administration suggests it is a safe , practical , and economic method of drug delivery that permits ifosfamide to be administered successfully in the outpatient setting .
### Output Text:
Limited clinical experience with continuous subcutaneous <span class="chemical">mesna</span> administration suggests it is a safe , practical , and economic method of drug delivery that permits <span class="chemical">ifosfamide</span> to be administered successfully in the outpatient setting . 