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: This , together with our previous findings that allopurinol failed to prevent adrenocorticotrophic hormone induced hypertension , suggests that XO activity is not a major determinant of GC - HT in the rat .
### Output Text:
This , together with our previous findings that <span class="chemical">allopurinol</span> failed to prevent adrenocorticotrophic hormone induced hypertension , suggests that XO activity is not a major determinant of GC - HT in the rat . 