Susceptibility testing new agents.

1
Susceptibility testing new agents.

• Dr. Ian Morrissey
• Chief Executive
• GR Micro Ltd.
• London, UK.

2
New Agents

• Linezolid (Pfizer)
• Tigecycline (Wyeth)
• Daptomycin (Novartis)
• Dalbavancin (Pfizer)

3
Basic principles still important.
4
Linezolid

• Approved in 2000 in the USA.
• Community-acquired and nosocomial pneumonia.
• Complicated and uncomplicated skin and
  soft-tissue infections.
• Methicillin-resistant Staphylococcus aureus and
  vancomycin-resistant enterococci.
• Protein synthesis inhibitor
• Bacteriostatic agent
• Oral and parenteral formulations.

5
Linezolid susceptibility

• Non-susceptibility is rare
• Some development signs in Enterococci esp. E.
  faecium.
• Occasional cases in S. aureus.
• Very rare in S. pneumoniae.

6
CLSI modified guidelines

• QC ranges changed from 27-31 mm to 25-32 mm after
  4-5 yrs of use.
• Bacteriostatic agents may be more problematical
  when measuring zones or MICs by Etest.

Biedenbach Jones 2003 CMI 91035
7
Tigecycline

• Launched 2005 in USA for SSTI.
• First glycylcycline
• Derivative of minocycline
• Evades Tet A-E K efflux pumps
• But not those of Proteae or Pseudomonas
• Unique amongst newer agents having activity
  against Gram-negative bacteria.
• But is active against MRSA and VRE.
• Acinetobacter baumannii
• Parenteral administration only.
• Bacteriostatic.

8
Tigecycline susceptibility

• High against Gram-positive bacteria, E. coli K.
  oxytoca.
• Some resistance found in Gram-negs
• K. pneumoniae (92-95 susceptibility)
• E. aerogenes (96 susceptibility)
• E. cloacae (93 susceptibility)
• S. marcescens (97 susceptibility)

Waites et al 2006 AAC 50 3479
9
Tigecycline breakpoints
10
Medium batch Tigecycline activity

• Inconsistencies were noticed in QC data.
• Linked to medium batch variation.
• Tetracyclines are prone to inactivation by
  oxidation.
• Studies carried out to investigate.
• Petersen Bradford 2005 AAC 493910
• Bradford et al 2005 AAC 493903.

11
Old vs new
12
Resolved by addition of Oxyrase
13
Chromatographic evidence
H2O
H2O Oxyrase
Aged MHB
14
Medium effect

• Important for agar dilution or broth dilution.
• Media gt12h old affected.
• Does not effect disc diffusion or Etest methods.
• If using broth microdilution frozen panels are
  also not affected.
• Solved by boiling or addition of oxyrase.

15
Daptomycin

• First glycolipopeptide
• Launched in 2003 in the USA 2006 in Europe
• SSTI
• Endocarditis in the USA
• Gram-positives only
• Parenteral application only
• Rapidly bactericidal

16
Daptomycin Susceptibility

• Non-susceptibility rare.
• Some reports but case studies only.

CLSI only
17
Daptomycin requires Calcium
18
Calcium effect
MIC at zero Ca 64 mg/L
GR Micro Ltd, data on file.
19
Daptomycin diffusion assays

• Daptomycin discs supplemented with 50 mg/L Ca
  have been developed.
• Only for CLSI methodology (i.e. MHB)
• Discontinued in 2005
• Isosensitest discs have proved problemmatical and
  have never been available.
• Why are dapto Ca discs no longer available
  even for CLSI?

20
Daptomycin treatment failure
Susceptible BP was 16 mm
Hayden et al 2005 JCM 435285.
21
Multicentre evaluation
Jevitt et al 2006 JCM 443098
22
Etest to the rescue!
Jevitt et al 2006 JCM 443098
23
Dalbavancin

• New lipoglycopeptide
• Derived from teicoplanin
• Extended half-life
• Once weekly dosing
• Bactericidal
• 8-16-fold more active than vancomycin
• No resistance found to date
• Except for cross-resistance to vanA Enterococci.
• Not clinically available but has FDA Approval

24
Dalbavancin Susceptibility

• Breakpoints not currently set.
• Broth microdilution requires addition of
  polysorbate-80 into wells for accurate and
  reproducible results.
• Agar diluton has not been proposed as a standard
  method
• Fritsche et al 2006 JCM 442988
• Problems with disc development
• Poor agar diffusion
• Jones et al 2006 JCM 442622
• Etest is available.
• Correct interpretation is essential.

25
Dalbavancin Etest
Biedenbach et al 2007 JCM 45998
26
BSAC agar dilution method

• BSAC method compares well with CLSI (NCCLS)
• Mustaq et al 2004 JAC 54617.
• Agar dilution commonly 1 dilution higher

27
Surrogate marker for dalbavancin
Jones et al 2006 JCM 442622
28
Summary

• Linezolid
• Continued use has ironed-out previous QC issues
• Tigecycline
• For broth fresh medium or oxyrase is essential
• Daptomycin
• Discs not available but Etests work.
• Ca supplement required
• Dalbavancin
• Some question over agar dilution but BSAC seems
  OK.
• Polysorbate-80 supplement in broth.
• No disc but Etest is available.