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: We recommend careful use of ifosfamide in patients pretreated with nephrotoxic chemotherapy and inadequate renal perfusion .
### Output Text:
We recommend careful use of <span class="chemical">ifosfamide</span> in patients pretreated with nephrotoxic chemotherapy and inadequate renal perfusion . 