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: PURPOSE : This retrospective study used a pharmaceutical company ' s global safety database to determine the reporting rate of hyponatremia and / or syndrome of inappropriate secretion of anti - diuretic hormone ( SIADH ) among vincristine - treated patients and to explore the possibility of at - risk population subgroups .
### Output Text:
PURPOSE : This retrospective study used a pharmaceutical company ' s global safety database to determine the reporting rate of hyponatremia and / or syndrome of inappropriate secretion of anti - diuretic hormone ( SIADH ) among <span class="chemical">vincristine</span> - treated patients and to explore the possibility of at - risk population subgroups . 