Distinct concentration-dependent effects of the polo-like kinase 1-specific inhibitor GSK461364A, including differential effect on apoptosis.

Polo-like kinase 1 (Plk1) is a conserved serine/threonine kinase that plays an essential role in regulating the many processes involved in mitotic entry and progression. In humans, Plk1 is expressed primarily during late G(2) and M phases and, in conjunction with Cdk1/cyclin B1, acts as master regulatory kinases for the myriad protein substrates involved in mitosis. Plk1 overexpression is strongly associated with cancer and has been correlated with poor prognosis in a broad range of human tumor types. We have identified a potent, selective, reversible, ATP-competitive inhibitor of Plk1, GSK461364A, capable of inhibiting cell growth of most proliferating cancer cell lines tested. We observe distinct cell cycle effects of GSK461364A depending on the dose used. The predominant phenotype for cells treated with GSK461364A is prometaphase arrest with characteristic collapsed polar polo spindle. At high concentrations, GSK461364A delays mitotic entry in G(2) followed by gradual progression into terminal mitosis; in some cell lines, this correlates with decreased apoptosis. Cell culture growth inhibition by GSK461364A can be cytostatic or cytotoxic but leads to tumor regression in xenograft tumor models under proper dose scheduling. Finally, we describe pharmacodynamic biomarkers of GSK461364A activity (pHH3 and Plk1) that are currently being evaluated in human cancer clinical trials.

Biochemical characterization of the interactions of the novel pleuromutilin derivative retapamulin with bacterial ribosomes.

Retapamulin is a semisynthetic pleuromutilin derivative being developed as a topical antibiotic for treating bacterial infections of the skin. It is potent in vitro against susceptible and multidrug-resistant organisms commonly associated with bacterial skin infections. We report detailed mode of action studies demonstrating that retapamulin binds to the bacterial ribosome with high affinity, inhibits ribosomal peptidyl transferase activity, and partially inhibits the binding of the initiator tRNA substrate to the ribosomal P-site. Taken together, these data distinguish the mode of action of retapamulin from that of other classes of antibiotics. This unique mode of action may explain the lack of clinically relevant, target-specific cross-resistance of retapamulin with antibacterials in current use.