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 stronger antiarrhythmic effect of ( - ) - cibenzoline indicates that ( - ) - isomer may have an effect nearly 5 - 20 times stronger in suppressing Na channels , but effects of both drugs on Ca channels may be almost equipotent .
### Output Text:
The stronger antiarrhythmic effect of ( - ) - <span class="chemical">cibenzoline</span> indicates that ( - ) - isomer may have an effect nearly 5 - 20 times stronger in suppressing <span class="chemical">Na</span> channels , but effects of both drugs on <span class="chemical">Ca</span> channels may be almost equipotent . 