title: Photodynamic inactivation of bacteria using polyethylenimine-chlorin(e6) conjugates: Effect of polymer molecular weight, substitution ratio of chlorin(e6) and pH.
abstract: OBJECTIVE
  Antimicrobial photodynamic therapy (APDT) is a novel technique to treat local infections. Previously we reported that the attachment of chlorin(e6) to polyethylenimine (PEI) polymers to form PEI-ce6 conjugates is an effective way to improve ce6 PDT activity against bacteria. The aim of this work was to explore how the polymer molecular weight, substitution ratio (SR) of ce6 and pH value affect the PDT efficacy.
  
  
  METHODS
  We have synthesized PEI-ce6(10) (MW = 60,000, SR = 1) and PEI-ce6(11) (MW = 60,000, SR = 5) and compared these with the previous PEI-ce6(9) (MW = 10,000, SR = 1). We tested the PDT efficacy of these three conjugates against Gram-negative E. coli and Gram-positive bacteria (S. aureus and E. fecalis) at three different pH values (5.0, 7.4, 10.0) that may affect the charge on both the bacterial cells and on the conjugate (that has both basic and acidic groups).
  
  
  RESULTS
  PEI-ce6(9) and PEI-ce6(10) were the most effective against these tested bacteria. The PDT effect of all three conjugates depended on pH values. The effective order was pH = 10.0 > pH = 7.4 > pH = 5.0 on E. coli. For S. aureus and E. fecalis the order was pH = 5.0 > pH = 10.0 > pH = 7.4. PEI-ce6(11) PDT activity was worse than PEI-ce6(10) activity which is probably connected to the fact that ce6 molecules are self-quenched within the PEI-ce6(11) molecule. Ce6 quenching within the PEI-ce6 molecules was proved by analyzing fluorescence spectra of PEI-ce6 conjugates at different pH values. There were no differences in bacterial uptake between different pH values in three PEI-ce6 conjugates.
  
  
  CONCLUSIONS
  We assume high pH (rather than low pH as was hypothesized) disaggregates the conjugates, so the higher pH was more effective than the lower pH against E. coli. But for Gram-positive bacteria, low pH was more effective possibly due to more overall positive charge on the conjugate.
authors.affiliation: Wellman Center for Photomedicine, Massachusetts General Hospital, Boston, Massachusetts 02114, USA.
authors.forename: Liyi
authors.initials: L
authors.lastname: Huang
authors.forename: Timur
authors.initials: T
authors.lastname: Zhiyentayev
authors.forename: Yi
authors.initials: Y
authors.lastname: Xuan
authors.forename: Dulat
authors.initials: D
authors.lastname: Azhibek
authors.forename: Gitika B
authors.initials: GB
authors.lastname: Kharkwal
authors.forename: Michael R
authors.initials: MR
authors.lastname: Hamblin
mesh_terms: D002713:Chlorine / Q000494:pharmacology; D006090:Gram-Negative Bacteria / Q000187:drug effects* / Q000302:isolation & purification; D006094:Gram-Positive Bacteria / Q000187:drug effects*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D008970:Molecular Weight; D010778:Photochemotherapy / Q000379:methods*; D011094:Polyethyleneimine / Q000494:pharmacology*; D011108:Polymers / Q000494:pharmacology; D011838:Radiation-Sensitizing Agents / Q000494:pharmacology*; D012680:Sensitivity and Specificity; D013050:Spectrometry, Fluorescence
chemical_list: D011108:Polymers; D011838:Radiation-Sensitizing Agents; D002713:Chlorine; D011094:Polyethyleneimine