An orally bioavailable inhibitor of Janus-associated kinase 1 (JAK1) with potential antineoplastic activity. Upon oral administration of JAK1 inhibitor INCB039110, this agent selectively inhibits the phosphorylation and activity of JAK1, which may result in inhibition of JAK1-mediated signaling, induction of apoptosis, and reduction of cell proliferation in JAK1-expressing tumor cells. JAK1 is overexpressed in a variety of tumor cell types and plays a key role in tumor cell proliferation.