Synergism : what does it mean? JW Mouton Dept Medical Microbiology Canisius Wilhelmina Hospital Nijmegen, The Netherlands

RATIONALE FOR ANTIMICROBIAL COMBINATIONS Extension of spectrum Minimization of resistance Minimization of toxicity Synergism

Classical definition of Synergism A combination of two drugs is synergistic if the effect of the combination is more than it would be if the concentration of the second drug is replaced by the first drug.

SYNERGISM Increase killing rate Potentiation of a drug Prevention of elimination –metabolic (enzyme inhibitors) –renal function (e.g. probenicide) Significant better outcome in vivo

SYNERGISM : IN VITRO METHODS Checkerboard titrations Time-kill curves Diffusion methods –disks –paperstrips –E-test

SYNERGISM DEFINITIONS TIME KILL CURVES >100 fold killing then most active agent after 6 or 24h >1000 fold killing then most active agent after 6 or 24h significant earlier reduction to 99% or 99.9% cfu

SYNERGISM DEFINITIONS TIME KILL CURVES >100 fold killing then most active agent after 24h or 48h >1000 fold killing then most active agent after 24h or 48h significant earlier reduction to 99% or 99.9% cfu

Synergism, time kill

Reasons for Combination Therapy

Synergism by E-test White et al, 1996

Functions of drug concentration Two fold checkerboard dilutions, FIC –advantage : easy to perform –disadvantage : very crude, hardly informative only with steep effect curves Effect function –advantage : uniformly applicable if effect function is known –disadvantage : extensive calculations

Synergy, Checkerboard FICi = FICa + FICb MIC of drug A, tested in combination MIC of drug B, tested in combination MIC of drug A, tested alone MIC of drug B, tested alone +

Simple Isobole Constraints 1= [drug1] + [drug2] Conc d1Conc d2 Where Conc d is concentration that would individually produce the same effect as the combination

URSA Universal Response Surface Approach Loewe constraints Berenbaum constraints URSA possible if effect measured quantitatively (Greco, 1990)

URSA 1 =

Bliss Independence Effects Equation

Zero Surface Substraction Bliss exp2

SYNERGISM DEFINITIONS checkerboard FIC index –<0.5 to <1 for synergism –>1 to >2 for antagonism –at least 3 different methods to calculate FICi Isoboles –by eye –math expression Universal Response Surface

SYNERGISM EXPRESSIONS checkerboard FIC index –<0.5 to <1 for synergism –>1 to >2 for antagonism Isoboles –by eye –math expression Function of Effect –Universal Response Surface (Greco, based on Loewe and Berenbaum) –Interaction (Bliss)

PREDICTION OF EFFICACY combination regimens determine slope and intercept of the pdi best explaining efficacy for each single drug –time > (0.25 *) MIC for beta-lactams –log (AUC) for aminoglycosides and quinolone calculate efficacy of the combination as a function of the two slopes and intercepts determine the coefficient of determination between predicted and measured values

Predicted efficacy of ticarcillin and tobramycin pseudomonas Mouton et al, AAC 1999

CONCLUSIONS Pharmacodynamic indices can be used to predict efficacy of combination therapy in vivo These are the same indices as during single drug therapy These indexes vary over classes, one index for all classes is not proper