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 two metabolic phenotypes , extensive ( EM ) and poor metabolizers ( PM ) , show different stereoselective metabolism , resulting in apparently higher beta - 1 adrenoceptor antagonistic potency of racemic metoprolol in EMs .
### Output Text:
The two metabolic phenotypes , extensive ( EM ) and poor metabolizers ( PM ) , show different stereoselective metabolism , resulting in apparently higher beta - 1 adrenoceptor antagonistic potency of racemic <span class="chemical">metoprolol</span> in EMs . 