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: Randomized clinical trials comparing VNR with other drugs in the treatment of cancer were searched in Medline , Embase , Evidence - based Medicine Reviews databases and the Cochrane library from 1987 to 2002 .
### Output Text:
Randomized clinical trials comparing <span class="chemical">VNR</span> with other drugs in the treatment of cancer were searched in Medline , Embase , Evidence - based Medicine Reviews databases and the Cochrane library from 1987 to 2002 . 