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: The efficacy and tolerability of VAL / HCTZ combinations were maintained during the extension ( 797 patients ) .
### Output Text:
The efficacy and tolerability of <span class="chemical">VAL</span> / <span class="chemical">HCTZ</span> combinations were maintained during the extension ( 797 patients ) . 