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S. Balakrishnan
Department of Pharmacology,
Pondicherry Institute of Medical Sciences
Absorption
How long it takes after absorption till drug is detectable? (lag time or tlag).
How long it takes before peak – serum or plasma concentration are achieved (tmax).
What is the peak serum concentration? (Cmax)
Absorption problems 1
Vomiting patient
Ketoconazole needs acid
Patients on proton pump inhibitors
(PPI), H2 blockers
Take with Coca Cola
Absorption problems-2
Quinolones (ciprofloxacin) Bind to antacids, sucralfate
Solution: PPI or H2 blocker
Didanosine (ddI) unstable in acid; so: antacid in the tablet
Drugs taken with (out) food
Distribution
Changing Vd
Gentamicin distributes into space resembling extracellular fluid (ECF)
ECF larger in shock, drops with recovery
Gentamicin levels lower in shock, rise with recovery
Limited distribution-1
Most antibiotics well distributed, but ...
Not always intracellular
Not always to:Central nervous systemEye Prostate BonePlacenta Breast milk
Limited distribution-2
Meningitis: Higher doses to get adequate CNS levels
Prostatitis: Prefer trimethoprim-sulfamethoxazole, quinolones
Intracellular penetration
pH- only basic drugs penetrate
Beta lactams ansd AGs- NO
Quinolones and macrolides-YES
Distribution to placenta & breast milk
Hard to predict
Practical matter: look up data on a drug
Plasma protein binding
Unbound drug exerts effect.
Unbound drug diffuses into extra vascular sites.
Slows rate of elimination - & t½ - longer dosing interval.
Significant only if > 80%
Extensive protein binding
“Good”: Allows slow, steady release of heavily bound drug, e.g. ceftriaxone
“Bad”: since less “free” drug available for bacteria, e.g. Ceftriaxone
Reality: Only one factor
Protein binding perinatal issue
Sulfonamide displaces unconjugated bilirubin from serum protein
Perinatally, high unbound bilirubin causes kernicterus & brain damage
Don’t use sulfonamides in 3rd trimester, neonate
Biotransformation
Phase I
Phase II
Biotransformation: CYP 450
Often hepatic microsomal enzymes (CYP 450)
Rates vary up to 6-fold from one person to the next
Enzymes genetically determined
Biotransformation: HIV & TB
Rifampin (for TB) induces CYP450 3A4 & reduces levels of indinavir (for HIV)
Indinavir inhibits CYP450 3A4 & increases levels of rifampin
Solution: Low dose rifabutin, high dose indinavir
Biotransformation: ketoconazole, erythromycin
Ketoconazole, erythomycin inhibit CYP450 3A4
Slows metabolism of cisapride, levels rise, causes torsade de pointes, death
Cisapride highly restricted
Bioavailability
IV to oral switch
Elimination
Renal vs non renal clearanceElimination t1/2
General concept: Elimination t1/2
Half-life
Time for serum concentration to fall
50%
Constant if a person is stable
Varies from person to person
Concentration- time curve
Extravascularconcentration
Extracellular sites reached via diffusion from blood
Intracellular fluid
Extracellular sites with restrictive barriers
Urine
General concept: Clearance
Quantitative measure of body’s
ability to eliminate the drug
Includes various forms of excretion
Antimicrobial concept: MIC, MBC
MIC: Minimum inhibitory concentration (to inhibit growth in vitro)
MBC: Minimum bactericidal concentration (to kill in vitro)
MIC90: Inhibits 90% of strains
Break point
Is in part concentration which can be achieved at the site of infection
Susceptible: MIC < breakpoint
Resistant: MIC > breakpoint
Post-antibiotic effect
Persistence of effect (inhibition of
growth or killing) after drug
removed (or level below MIC)
“PAE” + pharmacokinetics affects
dosing strategy
Post-antibiotic effect
Post antibiotic sub – MIC effects
Post antibiotic – leukocyte effects
Important PK/PD ParametersImportant PK/PD Parameters
Time above MIC
Time
An
tib
ioti
c c
on
cen
trati
on
(u
g/m
l)
2
Drug A
Drug B
A
B
4
6
8
0
Important PK/PD Parameters
Drug A
Drug B
B
Proportion of the dosing interval when the drug concentration exceeds the MIC
Time above MIC:
Important PK/PD Parameters
An
tib
ioti
c
con
cen
trati
on
Time
Area under the curve over MIC
PEAK
AUC/MIC is the ratio of the AUC to MICPeak/MICis the ratio of the peak concentration to MIC
PK/PD and Antimicrobial Efficacy
2 main patterns of bacterial killing Concentration dependent
Aminoglycosides, quinolones, macrolides, azalides, clindamycin, tetracyclines, glycopeptides, oxazolidinones
Correlated with AUC/MIC , Peak/MIC
Time dependent with no persistent effectBeta lactamsCorrelated with Time above MIC(T>MIC)
Goal of therapy based on PK/PD
Pattern of Activity
Antimicrobials Goal of therapy and relevant PK/PD
Parameter
Concentration dependent killing
AGs, Quinolones, Daptomycin, ketolides, Macrolides, azithro-mycin, clindamycin,streptogramines,tetracyclines, glycopeptides, oxazolidinones
Maximise concentrations; AUC/MIC, peak/MICUse high doses; daily dosing for some agents
Time dependent killing with no persistent effects
Beta lactams Maximise duration of exposure; T>MICUse more frequent dosing; longer infusion times including continuous infusion
Pattern of Activity Antibiotics Goal of
Therapy PK/PD
Parameter
Type IConcentration-dependent killing andProlonged persistent effects
AminoglycosidesDaptomycinFluoroquinolonesKetolides
Maximize concentrations
24h-AUC/MICPeak/MIC
Type IITime-dependent killing and Minimal persistent effects
CarbapenemsCephalosporinsErythromycinLinezolidPenicillins
Maximize duration of exposure
T>MIC
Type IIITime-dependent killing andModerate to prolonged persistent effects.
AzithromycinClindamycinOxazolidinonesTetracyclinesVancomycin
Maximize amount of drug
24h-AUC/MIC
Magnitude of PK/PD measures predictive of efficacy for select antibiotic classes versus some pathogens
Drug PK/PD variable Magnitude of variable correlated with efficacy
Beta lactams Time > MIC >40-50% of dosing interval
Quinolones vs Gram –ve bacteria
24- hour AUC:MIC >90-125
Quinolones vs S. pneumoniae
24-hour AUC:MIC >30-40
Aminoglycoside pharmacodynamics in vivo
Vancomycin Outcome vs 24h-AUC/ MIC ratio
24h-AUC/MIC ratio
Satisfactory Unsatisfactory
< 125 4 (50%) 4
> 125 71 (97%) 2
24h-AUC/MIC ratio
Microbiological Response
< 33.7 (64%)
> 33.7 (100%)
Fluoroquinolone PK/PD vs S. pneumoniae
PK/PD of beta-lactams and macrolides in otitis media
Concentration dependent killing….azithromycin
•24 hour AUC/ 25-immunocompetent patients
•24 hour AUC/ 125- immnocompromised
patients
•24 hour AUC mg.h/ L -3 mg.h/L
•Macrolide susceptible S.pneumoniae MIC90 0.12
mg/L
•H. Influenzae MIC90 1-2mg/L
•Macrolide resistant S. pneumoniae MIC90
>8mg/L
PK/PD breakpoints of parenteral beta-lactams based on serum concentrations present for >40-50% of dosing regimens shown and MIC90 values of isolates of S. pneumoniae
Drug Dosing regimen
S.Pneumoniae MIC90 mg/L
PK/PD breakpoint mg/L
Pen G 2 X 106 U qid 4 4
Ampicillin 1 g qid 4 2
Cefuroxime 0.75 g tid 8 4
Cefotaxime 1 g tid 2 2
Ceftriaxone 1 g qd 2 2
Cefepime 1 g bid 4 4
Ceftazidime 1 g tid 32 8
Meropenem 0.5 g tid 2 1
Dosage Adjustment Needed in Renal Impairment I
Acyclovir ethambutol
aminoglycosides, Penicillins (except antistaph)
aztreonam, Quinolones
cephalosporins (except cefaperazone & ceftriaxone)
clarithromycin, Carbapenems
daptomycin, Vancomycin
doripenem, emtricitabine,
famiclovir, ertapenem,
flucytosine, ganciclovir,
imipenem, meropenem,
lamivudine, foscarnet,
fluconazole,
Dosage Adjustment Needed in Renal Impairment II
C/I in renal failure
MethanamineNalidixic acidNitrofurantoinSulfonamidesTetracyclines except doxy &
minocycline
Dosage adjust in hepatic impairment
Chloramphenicol
Clindamycin
Erythromycin
Metronidazole
Tigecycline
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