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 undertook an open - label study using thalidomide 100 mg once daily for up to 6 months in 20 men with androgen - independent prostate cancer .
### Output Text:
We undertook an open - label study using <span class="chemical">thalidomide</span> 100 mg once daily for up to 6 months in 20 men with <span class="chemical">androgen</span> - independent prostate cancer . 