We report on the synthesis and in vitro biological evaluations of a nanomolar protein kinase inhibitor (PKI) and its β-glucuronidase-responsive albumin-binding prodrug. The highly potent PKI is 400-3400 times more cytotoxic than the well-known PKI Roscovitine. The prodrug is able to bind covalently to human serum albumin through Michael addition and release the cytotoxic PKI in the presence of β-glucuronidase, an enzyme over-expressed in the microenvironment of solid tumours.