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: Laitinen ' s procedure improved most symptoms in drug - resistant PD , which engendered wide interest in the neurosurgical community .
### Output Text:
Laitinen ' s procedure improved most symptoms in drug - resistant PD , which engendered wide interest in the neurosurgical community . 