title: Critical concentrations in the dilution of oral self-microemulsifying drug delivery systems.
abstract: BACKGROUND
  Self-microemulsifying drug delivery systems provide a key technology to formulate challenging drugs. These formulations are commonly screened in early development by simple in vitro dilution tests. However, there is often a lack of rationale of how these tests are performed; so this article aims to improve this situation by studying critical concentrations in the dilution of self-microemulsifying formulations.
  
  
  METHODS
  Dynamic laser light backscattering, conductivity measurements, and electron paramagnetic resonance spectroscopy were conducted.
  
  
  RESULTS
  All model formulations exhibited profound changes at a similar aqueous dilution, which was interpreted as a percolation threshold of the formulation in water. It marked the change of a bicontinuous microemulsion to discrete micelles. The systems exhibited at this point maximal particle dispersion with a threshold of polydispersity. A marked change was also observed in the paramagnetic resonance spectra and with the conductivity measurements. This altered microenvironment can be relevant for solubilized drugs.
  
  
  CONCLUSIONS
  Future dilution tests should include a formulation-to-water ratio of roughly 1:5 (w/w), which is in the proximity of the individual threshold concentration. Additional dilutions may be tested below and clearly above this value to reflect the physiological dilution process.
authors.affiliation: Institute of Pharma Technology, University of Applied Sciences Northwestern Switzerland, Muttenz, Switzerland. martin.kuentz@fhnw.ch
authors.forename: Martin
authors.initials: M
authors.lastname: Kuentz
authors.forename: Martin
authors.initials: M
authors.lastname: Cavegn
mesh_terms: D000284:Administration, Oral; D002626:Chemistry, Pharmaceutical / Q000379:methods*; D016503:Drug Delivery Systems / Q000379:methods*; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods; D045703:Emulsifying Agents / Q000008:administration & dosage / Q000737:chemistry*; D012995:Solubility
chemical_list: D045703:Emulsifying Agents