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: Given its excellent tolerance profile and low toxicity , further evaluation of VNB in combination therapy is warranted .
### Output Text:
Given its excellent tolerance profile and low toxicity , further evaluation of <span class="chemical">VNB</span> in combination therapy is warranted . 