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: CONCLUSION : Established visual field defects presumed to be due to Vigabatrin therapy did not usually progress in spite of continuing use of the medication .
### Output Text:
CONCLUSION : Established visual field defects presumed to be due to <span class="chemical">Vigabatrin</span> therapy did not usually progress in spite of continuing use of the medication . 