An orally bioavailable, small-molecule HDM2 antagonist with potential antineoplastic activity. HDM2 antagonist JNJ-26854165 inhibits the binding of the HDM2–p53 complex to the proteasome, blocking the degradation of p53; p53 signalling and p53-mediated induction of tumour cell apoptosis may thus be restored. In addition to p53, degradation of other HDM2 client proteins may be inhibited. HDM2 (human homolog of double minute 2), a zinc finger protein, is a negative regulator of the p53 pathway; often overexpressed in cancer cells, this oncoprotein has been implicated in cancer cell proliferation and survival.