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: METHODS AND RESULTS : The present study is a multicenter , randomized , double - blind comparison of iopamidol and iodixanol in patients with chronic kidney disease ( estimated glomerular filtration rate , 20 to 59 mL / min ) who underwent cardiac angiography or percutaneous coronary interventions .
### Output Text:
METHODS AND RESULTS : The present study is a multicenter , randomized , double - blind comparison of <span class="chemical">iopamidol</span> and <span class="chemical">iodixanol</span> in patients with chronic kidney disease ( estimated glomerular filtration rate , 20 to 59 mL / min ) who underwent cardiac angiography or percutaneous coronary interventions . 