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: Based on 290 subjects , there was an average difference of 0 . 50 dB between subjects treated with DFMO plus sulindac compared with those treated with placebo ( 95 % confidence interval , - 0 . 64 to 1 . 63 dB ; P = 0 . 39 ) , adjusted for baseline values , age , and frequencies .
### Output Text:
Based on 290 subjects , there was an average difference of 0 . 50 dB between subjects treated with <span class="chemical">DFMO</span> plus <span class="chemical">sulindac</span> compared with those treated with placebo ( 95 % confidence interval , - 0 . 64 to 1 . 63 dB ; P = 0 . 39 ) , adjusted for baseline values , age , and frequencies . 