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: The function of P2X3 receptor and NK1 receptor antagonists on cyclophosphamide - induced cystitis in rats .
### Output Text:
The function of P2X3 receptor and NK1 receptor antagonists on <span class="chemical">cyclophosphamide</span> - induced cystitis in rats . 