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: Therapeutic effects were noted at all dose levels , with objective responses in 17 ( two complete and 15 partial regressions ) of 41 previously untreated patients .
### Output Text:
Therapeutic effects were noted at all dose levels , with objective responses in 17 ( two complete and 15 partial regressions ) of 41 previously untreated patients . 