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 study supports the high frequency of restrictive valve regurgitation in PD patients treated with pergolide and reveals that a significant improvement is usual when the treatment is converted to non - ergot dopamine agonists .
### Output Text:
This study supports the high frequency of restrictive valve regurgitation in PD patients treated with <span class="chemical">pergolide</span> and reveals that a significant improvement is usual when the treatment is converted to non - ergot <span class="chemical">dopamine</span> agonists . 