Transcript Prevention of Emergence of Resistance: A Pharmacodynamic

Prevention of Emergence of
Resistance:
A Pharmacodynamic Solution
G.L. Drusano, M.D.
Professor and Director
Division of Clinical Pharmacology
Clinical Research Institute
Albany Medical College &
NYSDOH
Prevention of Emergence of Resistance:
A Pharmacodynamic Solution
• Currently, the therapeutic armamentarium is
amazingly limited for many Gram-negative
pathogens
• Discovery programs do not promise any
relief for at least 5 years
• We must learn to use available drugs more
intelligently to preserve the susceptibility of
the infecting flora to current agents
Prevention of Emergence of Resistance:
A Pharmacodynamic Solution
Prevention of Emergence of Resistance:
A Pharmacodynamic Solution
• Many organisms have resistance
mechanisms that occur as a function of
single point mutations
• Examples are stable derepression of type I
beta lactamases for 3rd generation
cephalosporins and target mutations or
pump upregulation for fluoroquinolones
• As these occur at a frequency of 1/108 or
greater, infection site populations exceed
this frequency, often by multiple logs
Prevention of Emergence of Resistance:
A Pharmacodynamic Solution
• Consequently, such total populations do not
behave as a single, sensitive population, but
as a mixture of two populations of differing
drug susceptibility
• This raises an important question:
Prevention of Emergence of Resistance:
A Pharmacodynamic Solution
Can a drug exposure be
identified that will prevent the
resistant subpopulation from
taking over the total
population?
The Team
N. L. Jumbe, A. Louie, W. Liu,V. Tam, T. Fazili, R.
Leary, C. Lowry, M.H. Miller and
G. L. Drusano
Prevention of Emergence of Resistance:
A Pharmacodynamic Solution
Let Us Compare and Contrast the
Pharmacodynamics of Levofloxacin
for Streptococcus pneumoniae and
Pseudomonas aeruginosa in a Mouse
Thigh Infection Model
S. pneumoniae outcome studies
P. aeruginosa outcome studies
Rf in vitro
Rfin vivo
2.35x10-6
2.2x10-6
MIC (g/mL)
0.8
MBC (g/mL)
1.6
•
•
•
•
Prevention of Emergence of Resistance:
A Pharmacodynamic Solution
Clearly, Pseudomonas and Pneumococcus
differ in their response
Pneumococcus has no inoculum effect;
Pseudomonas has a major inoculum effect
The explanation probably rests in the
mutational frequency to resistance
Pseudomonas has a high frequency, while
Pneumococcus has a frequency that is not
measurable at the bacterial densities used in
these experiments with Levofloxacin
Peripheral (thigh)
Compartment (Cp)
kcp
IP
injection
+
Bacteria
(XT/R)
kpc
Central Blood
Compartment (Cc)
ke
f(c)
dXS=KGS x XS x L - fKS(CcH ) x XS
dt
dXR= KGR x XR x L- fKR(CcH ) x XR
dt
[1] dCc= kaCa+kpcCp-kcpCc-keCc
dt
[2] dCp = kcpCc - kpc Cp
dt
f(CcH)=
[4]
[5]
L = (1-(XS+XR)/POPMAX)
Kmax  CcH 
[3]
, =K and  = S,R
[6]
C H 50+CcH 
Y1=XT=XS+XR
[7]
Y2=XR
[8]
Mean Parameter Estimates of the Model.
KmaxGS
KmaxKS
HKS
C50KS
0.117
94.01
6.26
123.5
KmaxGR
KmaxKR
HKR
C50KR
0.163
12.16
2.37
129.8
Popmax = 3.6 x 1010
KmaxG
KmaxK
C50K
HK
Popmax
-maximum growth rate (hr-1) in the presence of drug
-maximum kill rate (hr-1)
-drug concentration (g/mL) to decrease kill rate by half
-rate of concentration dependent kill
-maximal population size
Prevention of Emergence of Resistance:
A Pharmacodynamic Solution
• All regimens were
simultaneously fit in a
large population model
• The displayed graph is
the predicted-observed
plot for the total
population after the
Maximum Aposteriori Probability
(MAP) Bayesian step
Prevention of Emergence of Resistance:
A Pharmacodynamic Solution
• All regimens were
simultaneously fit in a
large population model
• The displayed graph is
the predicted-observed
plot for the resistant
population after the
Maximum Aposteriori Probability
(MAP) Bayesian step
Prevention of Emergence of Resistance:
A Pharmacodynamic Solution
Prevention of Emergence of Resistance:
A Pharmacodynamic Solution
• We wished to evaluate the
model prospectively
• Models, to be useful, need
to predict the future
• We simulated a dose not
previously studied that
would encourage selection
of resistance
• The study was carried out
for 48, not 24 hours
• The model predicted the
change in the resistant
mutant population well
Prevention of Emergence of Resistance:
A Pharmacodynamic Solution
• In this experiment, a dose
was selected to generate an
exposure that would
prevent emergence of
resistance
• As this was at the limit of
detection, the measured
population sometimes had
“less than assay detectable”
for the colony count
• These were plotted at the
detection limit
Prevention of Emergence of Resistance:
A Pharmacodynamic Solution
• We were able to determine how the overall
(sensitive plus resistant) population responds to
pressure from Levofloxacin
• More importantly, we were able to model the
resistant subpopulation and choose a dose based
on simulation to suppress the resistant mutants
• The prospective validation demonstrated that the
doses chosen to encourage and suppress the
resistant mutants did, indeed, work
Prevention of Emergence of Resistance:
A Pharmacodynamic Solution
• Now, for Pneumococcus
• We were unable to recover resistant mutants
with levofloxacin as the selecting pressure
in the mouse thigh model
• However, we then examined ciprofloxacin
as the selecting agent
• Now, selecting mutants was straightforward
Study Design: Mouse Thigh Infection ModelCiprofloxacin Studies [50mg/kg BID ~
AUC/MIC 100:1]
1. Microbial eradication
-2 hr
Infect
0 hr
Begin therapy
BID
24 hr
2. Selection of resistance
Sacrifice, harvest,
homogenize muscle
+ 2xMIC Cipro
- Drug
+ 4xMIC Cipro
+ 3xMIC Levo
Prevention of Emergence of Resistance:
A Pharmacodynamic Solution
Drug
#58
RC2
Cipro/±Reserpine
0.6/0.6
3.5/1.0
Levo/±Reserpine
0.6/0.6
0.6/0.6
Prevention of Emergence of Resistance:
A Pharmacodynamic Solution
• Strain 58, the RC2 and RC4 mutants were sequenced
through Gyr A, Gyr B, Par C & Par E.
• The sequences examined were: GyrA (ORF 822 aa) aa 4229; Gyr B (ORF 648 aa) aa 346-579; ParC (ORF 823 aa)
aa 1-178; ParE (ORF 647 aa) aa 359-561. No differences
were seen between parent and the RC2 daughter strain.
• This, coupled with the decrement in Ciprofloxacin MIC
with reserpine exposure (3.5 mg/L  1.0 mg/L), implies
RC2 is a pump mutant.
• For RC4, a mutation was found in parC (aa 79, sertyr)
and this strain also decreased its MIC with addition of
reserpine.
-2 hr
0 hr
Infect
Begin therapy
BID
24 hr
Sacrifice, harvest,
homogenize muscle
- Drug
+ 4xMIC Cipro
+ 3xMIC Levo
#58-RC2
+ 2xMIC Cipro
Study Design: Second
Passage of First Stage
Ciprofloxacin Resistant
S. pneumoniae
-2 hr
#58-WT
Infect
0 hr
Begin therapy
BID
24 hr
Sacrifice, harvest,
homogenize muscle
+ 3xMIC Levo
- Drug
+ 3xMIC Cipro
Total Counts
Cipro Resistance
Levo Resistance
* = no colonies detected in any
sample. Sample size  4 animals
Prevention of Emergence of Resistance:
A Pharmacodynamic Solution
• What next?
• We are currently examining the RC2 mutant in the
mouse thigh model
• In preliminary data, exposures to levofloxacin that
would kill the wild-type isolate did not kill the
mutant, even though the MIC has not changed
• This finding has been recreated with another later
generation fluoroquinolone in a hollow fiber
model
• This implies that, counter to the output of
Resistance 2000, sometimes newer drugs preserve
the sensitivity of the flora better than older drugs
Prevention of Emergence of Resistance:
A Pharmacodynamic Solution
• Are there other factors that can alter the
probability of emergence of resistance?
• The most likely is duration of therapy
• Fluoroquinolones induce an SOS response
• This resembles a “hypermutator phenotype”
• Therapy intensity and therapy duration
should influence the probability of having
the resistant population becoming ascendant
Prevention of Emergence of Resistance:
A Pharmacodynamic Solution
• A 10 day hollow fiber
experiment was performed
for MSSA and MRSA (CS)
for 6 regimens
• The time to complete
replacement of the population
with resistant organisms was
recorded
• CART was employed to look
for a breakpoint in the
exposure
• > 200/1 AUC/MIC ratio was
identified
Prevention of Emergence of Resistance:
A Pharmacodynamic Solution
• A stratified Kaplan-Meier
analysis was performed
with this breakpoint
• The breakpoint was
significant (Mantel test p =
0.0007); Tarone-Ware and
Breslow Gahan tests were
also significant
• To prevent resistance, hit
hard (> 200 AUC/MIC)
and stop early (< 7 days)
Prevention of Emergence of Resistance:
A Pharmacodynamic Solution
CONCLUSIONS
• Probability of emergence of resistance is
impacted upon by the intensity of therapy
and by the duration of therapy
• Short duration therapy trials should examine
an endpoint of resistance frequency
• As importantly, doses should be chosen to
provide resistance counterselection
exposures for a large fraction of the
population. An example follows:
Prevention of Emergence of Resistance:
A Pharmacodynamic Solution
Target Attainment
Prevention of Emergence of Resistance:
A Pharmacodynamic Solution
Target Attainment
Prevention of Emergence of Resistance:
A Pharmacodynamic Solution
• While this example is for microbiological
outcome, a similar analysis could (and
should!) be performed for a prevention of
resistance target
• Such a dose choice, coupled with short
duration therapy will yield the highest
probability of a good clinical and
microbiological outcome and the lowest
probability of the resistant subpopulation
taking over the whole population