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 report three cases of severe hypoxaemia after ibuprofen administration during a randomised controlled trial of prophylactic treatment of patent ductus arteriosus with ibuprofen in premature infants born at less than 28 weeks of gestation .
### Output Text:
We report three cases of severe hypoxaemia after <span class="chemical">ibuprofen</span> administration during a randomised controlled trial of prophylactic treatment of patent ductus arteriosus with <span class="chemical">ibuprofen</span> in premature infants born at less than 28 weeks of gestation . 