title: Increased hydrophobic interactions of iclaprim with Staphylococcus aureus dihydrofolate reductase are responsible for the increase in affinity and antibacterial activity.
abstract: OBJECTIVE
  Iclaprim is a novel 2,4-diaminopyrimidine that exhibits potent, rapid bactericidal activity against major Gram-positive pathogens, including methicillin-susceptible Staphylococcus aureus and methicillin-resistant S. aureus, and is currently in clinical development for the treatment of complicated skin and skin structure infections. An understanding of the known mechanism of resistance to trimethoprim led to the design of this new inhibitor, with improved affinity towards dihydrofolate reductase (DHFR) from S. aureus and clinically useful activity against S. aureus including isolates resistant to trimethoprim. The objective of this study was to characterize the mode of action of iclaprim and its inhibitory properties against DHFR.
  
  
  METHODS
  The mode of action of iclaprim was assessed by enzymatic analysis, direct binding studies, macromolecular synthesis profiles, synergy and antagonism studies to define its role as an inhibitor of DHFR. The binding properties of iclaprim to DHFR were compared with those of trimethoprim by X-ray crystallography.
  
  
  RESULTS
  The enzymatic properties, direct binding and X-ray crystallographic studies delineated the mode of interaction with DHFR and the reason for the increased affinity of iclaprim towards the enzyme. The effect of iclaprim on bacterial physiology suggests that iclaprim behaves as a classical antibacterial DHFR inhibitor, as previously documented for trimethoprim.
  
  
  CONCLUSIONS
  Iclaprim binds and inhibits bacterial DHFR in a similar manner to trimethoprim. However, the increased hydrophobic interactions between iclaprim and DHFR account for increased affinity and, unlike trimethoprim, enable iclaprim to inhibit even the resistant enzyme with nanomolar affinity, thus overcoming the mechanism of trimethoprim resistance. The increased antibacterial activity and lower propensity for resistance make iclaprim a clinically promising and useful inhibitor.
authors.affiliation: Arpida AG, Duggingerstrasse 23, CH-4153 Reinach, Switzerland.
authors.forename: Christian
authors.initials: C
authors.lastname: Oefner
authors.forename: Monica
authors.initials: M
authors.lastname: Bandera
authors.forename: Andreas
authors.initials: A
authors.lastname: Haldimann
authors.forename: Heike
authors.initials: H
authors.lastname: Laue
authors.forename: Henk
authors.initials: H
authors.lastname: Schulz
authors.forename: Seema
authors.initials: S
authors.lastname: Mukhija
authors.forename: Sandro
authors.initials: S
authors.lastname: Parisi
authors.forename: Laurent
authors.initials: L
authors.lastname: Weiss
authors.forename: Sergio
authors.initials: S
authors.lastname: Lociuro
authors.forename: Glenn E
authors.initials: GE
authors.lastname: Dale
mesh_terms: D000900:Anti-Bacterial Agents / Q000378:metabolism / Q000494:pharmacology*; D018360:Crystallography, X-Ray; D004791:Enzyme Inhibitors / Q000378:metabolism / Q000494:pharmacology*; D006801:Humans; D057927:Hydrophobic and Hydrophilic Interactions; D007700:Kinetics; D008826:Microbial Sensitivity Tests; D008958:Models, Molecular; D015394:Molecular Structure; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D011743:Pyrimidines / Q000378:metabolism / Q000494:pharmacology*; D013211:Staphylococcus aureus / Q000187:drug effects* / Q000201:enzymology*; D013762:Tetrahydrofolate Dehydrogenase / Q000378:metabolism*; D014295:Trimethoprim / Q000378:metabolism / Q000494:pharmacology
chemical_list: D000900:Anti-Bacterial Agents; D004791:Enzyme Inhibitors; D011743:Pyrimidines; C486448:iclaprim; D014295:Trimethoprim; D013762:Tetrahydrofolate Dehydrogenase