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 this principle a 27 - year old woman , classified as being in the high - risk group ( Goldstein and Berkowitz score : 11 ) , was treated with multiple cytotoxic drugs .
### Output Text:
Based on this principle a 27 - year old woman , classified as being in the high - risk group ( Goldstein and Berkowitz score : 11 ) , was treated with multiple cytotoxic drugs . 