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: PURPOSE : The purpose of the study is to explore the function of P2X3 and NK1 receptors antagonists on cyclophosphamide ( CYP ) - induced cystitis in rats .
### Output Text:
PURPOSE : The purpose of the study is to explore the function of P2X3 and NK1 receptors antagonists on <span class="chemical">cyclophosphamide</span> ( <span class="chemical">CYP</span> ) - induced cystitis in rats . 