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: In contrast with the literature , serum levels of angiogenesis factors did not change significantly by pegylated interferon and ribavirin therapy .
### Output Text:
In contrast with the literature , serum levels of angiogenesis factors did not change significantly by <span class="chemical">pegylated interferon</span> and <span class="chemical">ribavirin</span> therapy . 