The nitrates which can undergo the first-pass metabolism:

Organic nitrates are lipid-soluble: well absorbed from buccal mucosa, intestines and skin. Ingested orally, all except isosorbide mononitrate undergo extensive and variable first pass metabolism in liver.They are rapidly denitrated by a glutathione reductase and a mitochondrial aldehyde dehydrogcnase.   Reference: Essentials of Medical Pharmacology Eighth Edition KD TRIPATHI page no 587