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: Intravenous injection of the specific platelet - activating factor ( PAF ) antagonist BN 52021 ( 10 mg / kg ) , 30 min before bupivacaine administration ( 2 mg / kg i . v . ) suppressed both the decrease of MBP and HR .
### Output Text:
Intravenous injection of the specific platelet - activating factor ( PAF ) antagonist <span class="chemical">BN 52021</span> ( 10 mg / kg ) , 30 min before <span class="chemical">bupivacaine</span> administration ( 2 mg / kg i . v . ) suppressed both the decrease of MBP and HR . 