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: OBJECTIVE : To determine the maximum tolerated dose ( MTD ) of carbetocin ( a long - acting synthetic analogue of oxytocin ) , when administered immediately after vaginal delivery at term .
### Output Text:
OBJECTIVE : To determine the maximum tolerated dose ( MTD ) of <span class="chemical">carbetocin</span> ( a long - acting synthetic analogue of <span class="chemical">oxytocin</span> ) , when administered immediately after vaginal delivery at term . 