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: Our results failed to demonstrate an important response rate to single agent thalidomide in indolent lymphomas and contrast with the higher activity level reported with the second generation immunomodulatory agent , lenalidomide .
### Output Text:
Our results failed to demonstrate an important response rate to single agent <span class="chemical">thalidomide</span> in indolent lymphomas and contrast with the higher activity level reported with the second generation immunomodulatory agent , <span class="chemical">lenalidomide</span> . 