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: These results suggested that KF17837 is a centrally active adenosine A2A receptor antagonist and that the dopaminergic function of the nigrostriatal pathway is potentiated by adenosine A2A receptor antagonists .
### Output Text:
These results suggested that <span class="chemical">KF17837</span> is a centrally active <span class="chemical">adenosine</span> A2A receptor antagonist and that the dopaminergic function of the nigrostriatal pathway is potentiated by <span class="chemical">adenosine</span> A2A receptor antagonists . 