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: Re ( 300 mg / kg ) inhibited OIH in both the thermal sensitivity test and the acetic acid - induced writhing test .
### Output Text:
<span class="chemical">Re</span> ( 300 mg / kg ) inhibited OIH in both the thermal sensitivity test and the <span class="chemical">acetic acid</span> - induced writhing test . 