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: To the best of our knowledge , the occurrence of neuroleptic malignant syndrome due to combination therapy with tetrabenazine and tiapride has not been previously reported .
### Output Text:
To the best of our knowledge , the occurrence of neuroleptic malignant syndrome due to combination therapy with <span class="chemical">tetrabenazine</span> and <span class="chemical">tiapride</span> has not been previously reported . 