PK/PD: New Microbial Diseases and Model Systems

Tawanda Gumbo, MD

Associate Professor of Medicine,Division of Infectious Diseases,

University of Texas Southwestern Medical Center, Dallas, Texas

• Log-phase growth bacilli (LPG): Killed rapidly by INH during early bactericidal activity. Termed BACTERICIDAL effect.

• Slowly replicating bacilli under acidic conditions (SRB): Killed by PZA during ~8 weeks of therapy. Believed mostly in macrophages

• Non-replicating persistent bacilli (NRP): Killed by rifamycins during short bursts of metabolism but needs up to 6 months to be eradicated. Size SRB>>NRP

Mitchison’s three population model hypothesis

Mitchison DA. The basic mechanisms of chemotherapy. Chest 1979; 76: 771-81

Models for PK/PD work in TB

• Require that one of each of the three Mtb populations be emulated

• Require that free drug pharmacokinetics of anti-TB at site of infection be correctly recapitulated

• Mouse models traditionally used. However, it is becoming clear that the bacillary populations in mice are different from cavitary TB

M. tuberculosis in the hollow fiber system

Gumbo T, et al. J Infect Dis 2006;195:194-201

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GROWTH OF MYCOBACTERIUM TUBERCULOSIS IN THE HOLLOW FIBER SYSTEM: log-phase growth population

• 5 week experiment

• 104 initial inoculum

• Log-phase growth pattern during days 1 through 8

• Bacterial burden relatively stable thereafter

• Isoniazid has the greatest bactericidal activity, but ceases to kill after 2-3 days, believed due to depleting M. tuberculosis in log phase growth

ISONIAZIDBackground

Is the cessation of isoniazid microbial kill due to depletion of M. tuberculosis in

exponential growth phase?

ISONIAZIDQuestions

• Slow and fast acetylators simulated

• Doses studied for each acetylation phenotype:

– 0, 25, 100, and 300 mg a day

• In addition, a dose of 600 mg a day for fast acetylators

ISONIAZID AGAINST M. TUBERCULOSISExperimental Design

ISONIAZID AGAINST M. TUBERCULOSIS Resistance, Not Extinction Responsible For Effect Cessation

Gumbo T, Louie A, Liu W, Bhavnani S, Ambrose P,Brown D, Drusano GL. (2006). Journal of Infectious Diseases 195:194-201

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Simulated total population

Simulated isoniazid-susceptible population

Simulated isoniazid-resistant population

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Observed isoniazid-resistant population

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PZA dose-effect & dose scheduling studies• All media acidified to pH 5.8 for entire study, and Mtb

grew slowly in the HFS, to simulate SRB under acidic conditions

• Half-life =11hr

• Doses mimicked: 0, 7.5, 15, 30, 60, 90, 120 mg/kg daily for 28 days (dose effect)

• EC20, EC40, EC60 then examined in dose-scheduling studies

• Checked microbial kill and actual PK parameters achieved

Relationship between PZA AUC/MIC and kill

• The PK/PD index most closely associated with pyrazinamide sterilizing effect was AUC/MIC

• E=6.52-(2.89 x AUC/MIC1.21 34.22 1.2 +AUC/MIC1.21

r2 =0.93; p<0.01

BIG QUESTION

• Where in pulmonary lesions is this large population that PZA kills?– The majority, starting with Mackeness &

McDermott in the 1950s and 1960s believe it is within macrophages

• Few voices here and there: extra-cellular

Rates of sterilizing effect with standard dosing

• Standard dose of 2grams PZA per day in patients produces 0.1 log10CFU/ml/day in 90% of patients

• From our inhibitory sigmoid Emax relationship, the AUC0-24/MIC associated with such a kill rate of 0.1log10CFU/ml/day is 120

Population PK data• 100,000 patients simulated using ADAPT II & ADAPT

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• Pop PK: Wilkins J et al. Eur. J Clin. Pharmacol. 2006; 62:727-735.

– Serum clearance= 3.4 L/h, Volume=30 L.

• PZA penetration in ELF and alveolar macrophages Conte et al. Antimicrob. Agents Chemother. 1999; 43, 1329-1333

Alveolar macrophage/serum ratio=0.52 to 1.14 Epithelial lining fluid/serum ratio=13.9-21.7

• MICs at pH 5.8: Salfinger M & Heifets LB. Antimicrob Agents Chemother 1988;32:1002-4

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Monte Carlo simulation assuming intracellular location

• MCS targeting alveolar macrophage PZA concentration associated with AUC0-24/MIC of 120.

• Processing….• Processing….• Processing… • Processing….

• …..in 0.07% of patients!

• But we know that 2G of pyrazinamide has good sterilizing effect rates in patients.

Monte Carlo simulation assuming extracellular location

• MCS targeting ELF associated with AUC0-24/MIC of 120.

• Processing….• Processing….• Processing… • Processing….

• …..in 80.0-90.0% of patients!• Match!

PZA conclusions

• Based on these simulations, seems that the majority opinion is in this case likely incorrect

• In fact, in reviewing Robert Koch’s autopsy reports that he published, he commented that most Mtb in pulmonary cavities was extracellular

Wasana Siyambalapitiyage, Crystal Norton, Sandirai Musuka, Shashikant Srivasatava, Carleton Sherman