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: Since the administration of BN 52021 , at all doses studied , did not alter MBP and HR at the doses used , the bulk of these results clearly demonstrate a protective action of BN 52021 , a specific antagonist of PAF , against bupivacaine - induced cardiovascular toxicity .
### Output Text:
Since the administration of <span class="chemical">BN 52021</span> , at all doses studied , did not alter MBP and HR at the doses used , the bulk of these results clearly demonstrate a protective action of <span class="chemical">BN 52021</span> , a specific antagonist of PAF , against <span class="chemical">bupivacaine</span> - induced cardiovascular toxicity . 