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: By using pharmacokinetic pharmacodynamic modeling the pharmacodynamics of racemic metoprolol and the active S - isomer , were quantitated in EMs and PMs in terms of IC50 values , representing metoprolol plasma concentrations resulting in half - maximum receptor occupancy .
### Output Text:
By using pharmacokinetic pharmacodynamic modeling the pharmacodynamics of racemic <span class="chemical">metoprolol</span> and the active S - isomer , were quantitated in EMs and PMs in terms of IC50 values , representing <span class="chemical">metoprolol</span> plasma concentrations resulting in half - maximum receptor occupancy . 