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: On the basis that only free drug in the systemic circulation will elicit a pharmacological response target , free concentrations in plasma were selected to mimic the free drug exposures in man .
### Output Text:
On the basis that only free drug in the systemic circulation will elicit a pharmacological response target , free concentrations in plasma were selected to mimic the free drug exposures in man . 