Synergism : what does it mean?

JW MoutonDept 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 DEFINITIONSTIME 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 DEFINITIONSTIME 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

0 4 8 12 16 20 2410 1

10 2

10 3

10 4

10 5

10 6

10 7

10 8

10 9

10 10

A

B

A+B

Control

time (h)

C F

U /

m l

0 4 8 12 16 20 2410 1

10 2

10 3

10 4

10 5

10 6

10 7

10 8

10 9

10 10 ControlA

B

A+B

time (h)

C F

U /

m l

0 4 8 12 16 20 24101

102

103

104

105

106

107

108

109

1010

A

B

A+B

Control

time (h)

C F

U /

m l

Synergism Indifference Antagonism

Reasons for Combination Therapy

antibacterial antiviral antifungal

spectrum resistance synergismresistance toxicitykilling killingtoxicitysynergism

Synergism by E-testWhite 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 d1 Conc d2

Where Conc d is concentration that would individually produce the same effect as the combination

URSAUniversal Response Surface Approach

• Loewe constraints

• Berenbaum constraints

• URSA possible if effect measured quantitatively (Greco, 1990)

URSA

1 =

C1

EC50E

Emax E

1

1

C2

EC50E

Emax E

1

2

I

Bliss Independence Effects Equation

E EmaxD1

Ec50 1

1 D2

Ec50 2

2

*

1D1

Ec50 1

11

D2

Ec50 2

2

Zero Surface Substraction Bliss

-1.000 -0.900 -0.800 -0.700 -0.600 -0.500 -0.400 -0.300 -0.200 -0.100 0.000 0.100 0.200 0.300 0.400 0.500 0.600 above

Data: 817pAMFL.STA 10v * 186c

exp2

SYNERGISM DEFINITIONScheckerboard

• 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 EXPRESSIONScheckerboard

• 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 EFFICACYcombination 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 tobramycinpseudomonas

-6 -5 -4 -3 -2 -1 0 1 2 3 4-6

-4

-2

0

2

4

predicted : t > 0.25 * mic ticarcillin and log auc tobramycin

Slope

Y-intercept

X-intercept

1/slope

0.9285 ± 0.07826

-2.815 ± 0.1250

3.032

1.077

r² 0.7830

dcfu predicted

dcf

u m

easu

red

0 100 200 300 400 500-6

-4

-2

0

2

sum auc/mic of ticarcillin and tobramycin

Slope

Y-intercept

X-intercept

1/slope

-0.005566 ± 0.002515

-2.360 ± 0.3898

-424.0

-179.7

r² 0.1115

sum aucmicd

cfu

measu

red

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