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 found 19 trials , involving 2441 patients treated by VNR and 2050 control patients .
### Output Text:
We found 19 trials , involving 2441 patients treated by <span class="chemical">VNR</span> and 2050 control patients . 