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,

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.Pleuromutilin is a tricyclic, diterpene natural product first identified from the basidiomycete bacterial species Pleurotus mutilus (now referred to as Clitopilus scyphoides) that possesses modest antibacterial activity against primarily gram-positive bacterial organisms (8). Early studies on the mode of action of tiamulin and other semisynthetic pleuromutilin derivatives showed that it interfered with cell-free protein synthesis and formylmethionine-puromycin synthesis (5). Further work has shown that the binding of these compounds to the bacterial 50S ribosomal subunit is displaced by puromycin and chloramphenicol (6). Competitive rRNA footprinting experiments also indicated that tiamulin and another analogue, valnemulin, can bind concurrently with the macrolide antibiotic erythromycin. In contrast, these compounds compete with the peptidyl transferase inhibitor carbomycin (13). Additional evidence for the binding of pleuromutilins to the peptidyl transferase center has come from recent x-ray crystallographic data (17) which show tiamulin having interactions with both the ribosomal A-and P-sites. The interaction of pleuromutilins also appears to involve ribosomal protein L3 near the peptidyl transferase center, since Brachyspira sp. isolates with reduced tiamulin susceptibility have recently been identified with mutations in L3 (14). Thus, it is becoming clear that pleuromutilins have a mechanism of action which involves nucleotides within the peptidyl transferase center.Despite the discovery and development of tiamulin and valnemulin, which have become important veterinary agents to treat swine disease, there has been little progress in the identification of pleuromutilin derivatives to treat bacterial infections in humans. Hence, there has been a renewed interest in the exploitation of novel pleuromutilin derivatives for deployment in human clinical practice (2,7,20). Retapamulin (Fig. 1A) is a selective, prokaryotic protein synthesis inhibitor being developed as a topical antibiotic for treating bacterial infections of the skin. This semisynthetic pleuromutilin has potent in vitro activity against susceptible and multidrug-resistant organisms commonly associated with bacterial skin infections (15, 16). Furthermor...

Additions to Supplement Table 3 from Distinct Concentration-Dependent Effects of the Polo-like Kinase 1–Specific Inhibitor GSK461364A, Including Differential Effect on Apoptosis

Supplementary Tables 1-3 from Distinct Concentration-Dependent Effects of the Polo-like Kinase 1–Specific Inhibitor GSK461364A, Including Differential Effect on Apoptosis

Supplementary Figures 1-4 from Distinct Concentration-Dependent Effects of the Polo-like Kinase 1–Specific Inhibitor GSK461364A, Including Differential Effect on Apoptosis