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: In 35 patients ( ages 20 to 86 yr ) receiving cimetidine therapeutically two serum samples and all urine formed in the interim were collected for analysis of cimetidine by high - pressure liquid chromatography and for creatinine .
### Output Text:
In 35 patients ( ages 20 to 86 yr ) receiving <span class="chemical">cimetidine</span> therapeutically two serum samples and all urine formed in the interim were collected for analysis of <span class="chemical">cimetidine</span> by high - pressure liquid chromatography and for <span class="chemical">creatinine</span> . 