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: It is concluded that advanced pregnancy has a negligible effect on the neurotoxic response to theophylline in rats .
### Output Text:
It is concluded that advanced pregnancy has a negligible effect on the neurotoxic response to <span class="chemical">theophylline</span> in rats . 