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: Higher metoprolol / alpha - hydroxymetoprolol ratios in PMs were predictive for higher R - / S - isomer ratios of unchanged drug .
### Output Text:
Higher <span class="chemical">metoprolol</span> / <span class="chemical">alpha - hydroxymetoprolol</span> ratios in PMs were predictive for higher R - / S - isomer ratios of unchanged drug . 