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: Patients with large pelvic tumor loads are , according to existing classifications , at high risk to die and to develop drug resistance .
### Output Text:
Patients with large pelvic tumor loads are , according to existing classifications , at high risk to die and to develop drug resistance . 