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: While ( - ) - baclofen ( 1 - 3 mg / kg ) proved to be an effective antagonist of muscimol , its ( + ) - isomer ( 5 - 20 mg / kg ) lacked this property .
### Output Text:
While ( - ) - <span class="chemical">baclofen</span> ( 1 - 3 mg / kg ) proved to be an effective antagonist of <span class="chemical">muscimol</span> , its ( + ) - isomer ( 5 - 20 mg / kg ) lacked this property . 