The rate-limiting isozyme of guanosine biosynthesis, , was identified as an essential gene in Merkel cell carcinoma (MCC) but the consequences of its functional disruption were unclear. Inhibition of IMPDH2 led to reduced MCC cell viability, independent of functional p53 or Merkel cell polyomavirus status, but dependent on depletion of guanylate nucleotides. In contrast to other cancer models, inhibition of IMPDH2 in MCC led to rapid ablation of nascent DNA synthesis and the onset of replication stress without a significant effect on total or ribosomal RNA biosynthesis. Combining IMPDH inhibitors with ataxia telangiectasia mutated and Rad3-related (ATR) inhibitors significantly increased levels of replication stress and reduced tumor growth . These findings support replication stress as the dominant consequence of IMPDH2 inhibition in MCC and, when combined with ATR inhibition, indicate a potential therapeutic strategy.