Model-based learn and confirm: designing effective treatment regimens against multidrug resistant Gram-negative pathogens.

Over the last decade, there has been a growing appreciation for the use of in vitro and in vivo infection models to generate robust and informative nonclinical PK/PD data to accelerate the clinical translation of treatment regimens. The objective of this study was to develop a model-based "learn and confirm" approach to help with the design of combination regimens using in vitro infection models to optimise the clinical utility of existing antibiotics. Static concentration time-kill studies were used to evaluate the PD activity of polymyxin B (PMB) and meropenem against two carbapenem-resistant Klebsiella pneumoniae (CRKP) isolates; BAA2146 (PMB-susceptible) and BRKP67 (PMB-resistant). A mechanism-based model (MBM) was developed to quantify the joint activity of PMB and meropenem. In silico simulations were used to predict the time-course of bacterial killing using clinically-relevant PK exposure profiles. The predictive accuracy of the model was further evaluated by validating the model predictions using a one-compartment PK/PD in vitro dynamic infection model (IVDIM). The MBM captured the reduction in bacterial burden and regrowth well in both the BAA2146 and BRKP67 isolate (R2 = 0.900 and 0.940, respectively). The bacterial killing and regrowth predicted by the MBM were consistent with observations in the IVDIM: sustained activity against BAA2146 and complete regrowth of the BRKP67 isolate. Differences observed in PD activity suggest that additional dose optimisation might be beneficial in PMB-resistant isolates. The model-based approach presented here demonstrates the utility of the MBM as a translational tool from static to dynamic in vitro systems to effectively perform model-informed drug optimisation.

Predicting the pharmacokinetics and pharmacodynamics of antisense oligonucleotides: an overview of various approaches and opportunities for PBPK/PD modelling.

INTRODUCTION: Advances in research and development (R&D) have enabled many approvals of antisense oligonucleotides (ASOs). Its administration expanded from systemic to local for treating various diseases, where predicting target tissue exposures and pharmacokinetics (PK) and pharmacodynamics (PD) in human can be critical. AREAS COVERED: A literature search for PBPK/PD models of ASOs was conducted using PubMed and Embase (to 1 April 2023). ASO PK and PD in animals and humans and modeling approaches including physiologically based (PB) are summarized; and relevance and impacts of PBPK/PD modeling are assessed. EXPERT OPINION: Allometric scaling and compartmental PK/PD modeling have been successful to predict human ASO PK/PD, addressing most R&D needs. Understanding tissue distribution of ASOs can be crucial for their efficacy and safety especially for intrathecal (IT), pulmonary, or other local routes. PBPK/PD modeling is expected to improve such understanding, for which, efforts have been sporadic. However, developing a PBPK/PD model requires careful review of known biology/pharmacology and thoughtful experimental designs. Resulting models have the potential to predict target/specified tissue exposures and responses in human adults and pediatrics. Ultimately, a PBPK/PD modeling approach can lead to more efficient and rational clinical development, resulting in well-informed decision making and a shortened timeline.

International society of Pharmacometrics Mentorship Program (IMP): feedback survey from the first cohort of mentor-mentee pairs.

The International Society of Pharmacometrics (ISoP) Mentorship Program (IMP) aims to help professionals at all career stages to transition into the pharmacometrics field, move to a different role/area within pharmacometrics, or expand their skillsets. The program connects mentees at various stages of their careers with mentors based on established criteria for mentor-mentee matching. Pairing mentees with appropriate mentors ensures strong alignment between mentees' interests and mentors' expertise as this is critical to the success and continuation of the relationship between the mentor and mentee. Once mentors and mentees are connected, they are strongly encouraged to meet at least once per month for an hour. The mentor and mentee have the freedom to tailor their sessions to their liking, including frequency, duration, and topics they choose to focus on. Mentees are encouraged to clearly define their goals to help direct their mentor-mentee relationship and conversations. Mentees and mentors alike are given the opportunity to provide feedback about the program to the ISoP Education Committee through surveys and testimonials. Due to the program's infancy, structured guidelines for mentor-mentee sessions are still being developed and instituted using the program evaluation described in this paper.

Assessment of the Effect of Organ Impairment on the Pharmacokinetics of 2'-MOE and Phosphorothioate Modified Antisense Oligonucleotides.

The pharmacokinetics (PK) of 2'-O-methoxyethyl and phosphorothioate antisense oligonucleotides (ASOs), with or without N-acetyl galactosamine conjugation, have been well characterized following subcutaneous or intravenous drug administration. However, the effect of organ impairment on ASO PK, primarily hepatic or renal impairment, has not yet been reported. ASOs distribute extensively to the liver and kidneys, where they are metabolized slowly by endo- and exonucleases, with minimal renal excretion as parent drug (<1%-3%). This short review evaluated the effect of organ impairment on ASO PK using 3 case studies: (1) a phase 1 renal impairment study evaluating a N-acetyl galactosamine-conjugated ASO in healthy study participants and study participants with moderate renal impairment, (2) a phase 2 study evaluating an unconjugated ASO in patients with end-stage renal disease; and (3) a phase 3 study evaluating an unconjugated ASO, which included patients with mild hepatic or renal impairment. Results showed that patients with end-stage renal disease had a mild increase (≈34%) in total plasma exposure, whereas mild or moderate renal impairment showed no effect on plasma PK. The effect of hepatic impairment on ASO PK could not be fully evaluated due to lack of data in moderate and severe hepatic impairment study participants. Nonetheless, available data suggest that mild hepatic impairment had no effect on ASO exposure.

Population pharmacokinetic/pharmacodynamic modelling of eplontersen, an antisense oligonucleotide in development for transthyretin amyloidosis.

AIMS: Transthyretin-mediated amyloidosis is a progressive and fatal disease caused by the build-up of misfolded transthyretin (TTR) protein. Eplontersen is a triantennary N-acetyl galactosamine (GalNAc3)-conjugated antisense oligonucleotide targeting TTR messenger ribonucleic acid (mRNA) to inhibit production of both variant and wild-type TTR. We aimed to develop a population pharmacokinetic/pharmacodynamic (PK/PD) model for eplontersen and to evaluate the impact of covariates on exposure and response. METHODS: Plasma eplontersen and serum TTR concentration data were obtained from two phase 1 studies in healthy volunteers (ClinicalTrials.gov: NCT03728634, NCT04302064). Model development was conducted using a nonlinear mixed-effects approach. RESULTS: Eplontersen PK was well described by a two-compartment model. Evaluation of demographics identified significant covariates of lean body mass on clearance and body weight on intercompartmental clearance and volumes of distribution. Population PK modelling showed the absorption rate was 29.6% greater with injection into the abdomen versus the arm. The typical population terminal elimination half-life was 25.5 days. Serum TTR was well described by an indirect response model with inhibition of TTR production by eplontersen. Maximum fractional inhibition (Imax ) was 0.970 (0.549%RSE) and the half maximal inhibitory concentration (IC50 ) was 0.0283 ng/ml (13.3%RSE). Simulations showed subjects with lower weight had higher exposure (AUC, Cmax ), while higher Cmax was observed when comparing site of administration (ratio abdomen/arm = 1.18), but differences in exposure did not significantly impact response at evaluated doses. CONCLUSION: The exposure-response relationship of eplontersen was well characterised by the PKPD model. Weight and injection site were found to affect systemic exposure, but this effect does not seem to result in clinically relevant variation in response.

Pharmacodynamic and immunomodulatory effects of polymyxin B in combination with fosfomycin against KPC-2-producing klebsiella pneumoniae

Abstract Determining the role of the immune response in preventing antimicrobial resistance and optimizing antibiotic regimens against carbapenemase-producing Klebsiella pneumoniae (KPC) is a research gap that exists and needs to be further explored. The objective of this study was to determine the pharmacodynamics and immunomodulatory effects of fosfomycin alone and in combination with polymyxin B against KPC-2-producing K. pneumoniae clinical isolates. Six K. pneumoniae isolates were selected (polymyxin B_MIC: 0.5-64 mg/L; Fosfomycin MIC: 16-128 mg/L) to evaluate the pharmacodynamics of mono- and combination therapies in static time-kill studies. A mechanism based model was used to characterize the joint activity of polymyxin B and fosfomycin. A549 human airway epithelial cells were infected with four isolates to evaluate the immunomodulatory effects of treatment. Our mechanism-based model indicated greater bacterial killing efficacy of fosfomycin with polymyxin B compared to monotherapy. In combination, polymyxin B was assumed to exert an outer membrane effect which resulted in an increase in fosfomycin's ability to reach its target site. The mechanism based model described the data well across all six strains with R2 values ranging from 0.705 to 0.935. The combination reduced K. pneumoniae-induced IL-6 and IL-8 but not TNF-α expression. The reduction in cytokine expression was greater with polymyxin B than fosfomycin alone, and combinations showed significantly greater reductions compared to monotherapies. Our findings suggest that further research is needed to understand immune-mediated killing to identify a strategy which harnesses the power of the immune response against these hard to treat bacteria in an in vivo system.

Pharmacodynamic and immunomodulatory effects of polymyxin B in combination with fosfomycin against KPC-2-producing Klebsiella pneumoniae.

Determining the role of the immune response in preventing antimicrobial resistance and optimising antibiotic regimens against carbapenemase-producing Klebsiella pneumoniae is a research gap that exists and needs to be further explored. The objective of this study was to determine the pharmacodynamic and immunomodulatory effects of fosfomycin alone and in combination with polymyxin B against KPC-2-producing K. pneumoniae clinical isolates. Six K. pneumoniae isolates were selected (polymyxin B MIC, 0.5-64 mg/L; fosfomycin MIC, 16-128 mg/L) to evaluate the pharmacodynamics of monotherapy and combination therapies in static time-kill studies. A mechanism-based model was used to characterise the joint activity of polymyxin B and fosfomycin. A549 human airway epithelial cells were infected with four isolates to evaluate the immunomodulatory effects of treatment. Our mechanism-based model indicated greater bacterial killing efficacy of fosfomycin with polymyxin B compared with monotherapy. In combination, polymyxin B was assumed to exert an outer membrane effect that resulted in an increase in the ability of fosfomycin to reach its target site. The mechanism-based model described the data well across all six strains, with R2 values ranging from 0.705-0.935. Combination therapy reduced K. pneumoniae-induced IL-6 and IL-8 but not TNFα expression. The reduction in cytokine expression was greater with polymyxin B than fosfomycin alone; combination therapy showed significantly greater reduction compared to either monotherapy. Our findings suggest that further research is needed to better understand immune-mediated killing in order to identify a strategy which harnesses the power of the immune response against these hard-to-treat bacteria.

Evaluation Strategies for Triple-Drug Combinations against Carbapenemase-Producing Klebsiella Pneumoniae in an In Vitro Hollow-Fiber Infection Model

Mounting antimicrobial resistance to carbapenemase-producing Klebsiella pneumoniae (CPKP) highlights the need to optimize currently available treatment options. The objective of this study was to explore alternative dosing strategies that limit the emergence of resistance to preserve the utility of last-line antibiotics by: (i) evaluating the pharmacodynamic (PD) killing activity of simulated humanized exposures to monotherapy and two-drug and three-drug combinations against CPKP bacterial isolates with different resistance mechanisms; and (ii) optimizing polymyxin B (PMB) exposure simulated in the three-drug combination regimens to maximize the killing activity. Two CPKP clinical isolates (BAA2146 (NDM-1) and BRKP76 (KPC-2)) were evaluated over 168 hours using a hollow-fiber infection model simulating clinically relevant PMB, fosfomycin, and meropenem dosing regimens. PMB-based three-drug combinations were further optimized by varying the initial exposure (0­24 hours) or maintenance dose received over the duration of treatment. The area under the bacterial load-versus-time curve (AUCFU) was used to determine PD activity. Overall reductions in PMB exposure ranged from 2 to 84%. BAA2146 and BRKP76 had median (range) AUCFUs of 11.0 (10.6­11.6) log10 CFU hour/mL and 7.08 (7.04­11.9) log10 CFU hour/mL, respectively. The PMB "front loaded" 2.5 mg/ kg/day + 0.5 mg/kg maintenance dose in combination with meropenem and fosfomycin was a promising regimen against BRKP76, with an overall reduction in PMB exposure of 56% while still eradicating the bacteria. Tailored triple combination therapy allows for the optimization of dose and treatment duration of last-line agents like PMB to achieve adequate drug exposure and appropriate PD activity while minimizing the emergence of resistance.

Moving From Point-Based Analysis to Systems-Based Modeling: Integration of Knowledge to Address Antimicrobial Resistance Against MDR Bacteria.

The emerging discipline of Quantitative Systems Pharmacology (QSP) enables the integration of quantitative experimental data describing the interactions between the various biological processes within the system using mathematical modeling to gain better insights into the factors that drive disease pathogenicity and influence antibiotic pharmacokinetics (PKs)/pharmacodynamics (PDs). Through our perspective we consider the evolution from PK/PD models to mechanism-based and systems-based models and then finally QSP. We further emphasize the need to invest in ambitious research that takes into consideration: (i) the antibiotic PK/PD activity, (ii) the time course of the host immune response to understand the progression of the infection, (iii) and a growing appreciation of the cellular and molecular networks using multi-omics analysis to understand the modulation of antimicrobial therapy at a true systems level.

Evaluation Strategies for Triple-Drug Combinations against Carbapenemase-Producing Klebsiella Pneumoniae in an In Vitro Hollow-Fiber Infection Model.

Mounting antimicrobial resistance to carbapenemase-producing Klebsiella pneumoniae (CPKP) highlights the need to optimize currently available treatment options. The objective of this study was to explore alternative dosing strategies that limit the emergence of resistance to preserve the utility of last-line antibiotics by: (i) evaluating the pharmacodynamic (PD) killing activity of simulated humanized exposures to monotherapy and two-drug and three-drug combinations against CPKP bacterial isolates with different resistance mechanisms; and (ii) optimizing polymyxin B (PMB) exposure simulated in the three-drug combination regimens to maximize the killing activity. Two CPKP clinical isolates (BAA2146 (NDM-1) and BRKP76 (KPC-2)) were evaluated over 168 hours using a hollow-fiber infection model simulating clinically relevant PMB, fosfomycin, and meropenem dosing regimens. PMB-based three-drug combinations were further optimized by varying the initial exposure (0-24 hours) or maintenance dose received over the duration of treatment. The area under the bacterial load-versus-time curve (AUCFU) was used to determine PD activity. Overall reductions in PMB exposure ranged from 2 to 84%. BAA2146 and BRKP76 had median (range) AUCFUs of 11.0 (10.6-11.6) log10  CFU hour/mL and 7.08 (7.04-11.9) log10 CFU hour/mL, respectively. The PMB "front loaded" 2.5 mg/kg/day + 0.5 mg/kg maintenance dose in combination with meropenem and fosfomycin was a promising regimen against BRKP76, with an overall reduction in PMB exposure of 56% while still eradicating the bacteria. Tailored triple-combination therapy allows for the optimization of dose and treatment duration of last-line agents like PMB to achieve adequate drug exposure and appropriate PD activity while minimizing the emergence of resistance.

Comparison of the composition and in vitro activity of polymyxin B products.

A number of companies manufacture polymyxin B using United States Pharmacopeia (USP) metrics, rather than chemical composition, to report biological activity. Given that polymyxin B contains several different components, it is unknown whether pharmacokinetic and pharmacodynamic variability exists between the different brands and whether USP metrics capture this variability. Here we investigated the composition of polymyxin B obtained from four manufacturers (Sigma-Aldrich, AK Scientific, USP and MP Biomedicals) and evaluated their rate and extent of killing against multidrug-resistant Acinetobacter baumannii and Klebsiella pneumoniae using in vitro static time-kill experiments. Ultraviolet (UV) fingerprinting and liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis revealed similarities and differences between component distributions. The significant differences between products, based on UV fingerprinting and LC-MS/MS, did not translate into pharmacodynamic differences at the three concentrations evaluated. The aggregate polymyxin B concentration, rather than that of the individual components, influences overall bacterial killing.

Mechanism-Based Disease Progression Model Describing Host-Pathogen Interactions During the Pathogenesis of Acinetobacter baumannii Pneumonia.

The emergence of highly resistant bacteria is a serious threat to global public health. The host immune response is vital for clearing bacteria from the infected host; however, the current drug development paradigm does not take host-pathogen interactions into consideration. Here, we used a systems-based approach to develop a quantitative, mechanism-based disease progression model to describe bacterial dynamics, host immune response, and lung injury in an immunocompetent rat pneumonia model. Previously, Long-Evans rats were infected with Acinetobacter baumannii (A. baumannii) strain 307-0294 at five different inocula and total lung bacteria, interleukin-1beta (IL-1ß), tumor necrosis factor-α (TNF-α), cytokine-induced neutrophil chemoattractant 1 (CINC-1), neutrophil counts, and albumin were quantified. Model development was conducted in ADAPT5 version 5.0.54 using a pooled approach with maximum likelihood estimation; all data were co-modeled. The final model characterized host-pathogen interactions during the natural time course of bacterial pneumonia. Parameters were estimated with good precision. Our expandable model will integrate drug effects to aid in the design of optimized antibiotic regimens.

Evaluation of Activity and Emergence of Resistance of Polymyxin B and ZTI-01 (Fosfomycin for Injection) against KPC-Producing Klebsiella pneumoniae.

ZTI-01 (fosfomycin for injection) is a broad-spectrum antibiotic with a novel mechanism of action and is currently under development in the United States for treatment of complicated urinary tract infections. Globally, fosfomycin and polymyxin B are increasingly being used to treat multidrug-resistant Gram-negative infections. The objectives were to evaluate the pharmacodynamic activity of polymyxin B and fosfomycin alone and in combination against KPC-producing Klebsiella pneumoniae and to assess the rate and extent of emergence of resistance to different antibiotic regimens. Two clinical isolates, BRKP26 (MIC of polymyxin B[MICPMB], 0.5 mg/liter; MIC of fosfomycin [MICFOF], 32 mg/liter) and BRKP67 (MICPMB, 8 mg/liter; MICFOF, 32 mg/liter) at an initial inoculum of 107 CFU/ml, were evaluated over 168 h in a hollow-fiber infection model simulating clinically relevant polymyxin B (2.5-mg/kg loading dose as a 2 h-infusion followed by 1.5-mg/kg dose every 12 h [q12h] as a 1-h infusion) and fosfomycin (6 g q6h as a 1-h or 3-h infusion) regimens alone and in combination. Population analysis profiles (PAPs) and MIC testing were performed to assess emergence of resistance. Polymyxin B or fosfomycin monotherapy was ineffective and selected for resistance by 24 h. Polymyxin B plus a fosfomycin 1-h infusion demonstrated sustained bactericidal activity by 4 h, with undetectable colony counts beyond 144 h. Polymyxin B plus a fosfomycin 3-h infusion demonstrated bactericidal activity at 4 h, followed by regrowth similar to that of the control by 144 h. PAPs revealed resistant subpopulations by 120 h. The combination of polymyxin B and a fosfomycin 1-h infusion is a promising treatment option for KPC-producing K. pneumoniae and suppresses the emergence of resistance. Further evaluation of novel dosing strategies is warranted to optimize therapy.

In Vitro Assessment of Combined Polymyxin B and Minocycline Therapy against Klebsiella pneumoniae Carbapenemase (KPC)-Producing K. pneumoniae.

The multidrug resistance profiles of Klebsiella pneumoniae carbapenemase (KPC) producers have led to increased clinical polymyxin use. Combination therapy with polymyxins may improve treatment outcomes, but it is uncertain which combinations are most effective. Clinical successes with intravenous minocycline-based combination treatments have been reported for infections caused by carbapenemase-producing bacteria. The objective of this study was to evaluate the in vitro activity of polymyxin B and minocycline combination therapy against six KPC-2-producing K. pneumoniae isolates (minocycline MIC range, 2 to 32 mg/liter). Polymyxin B monotherapy (0.5, 1, 2, 4, and 16 mg/liter) resulted in a rapid reduction of up to 6 log in bactericidal activity followed by regrowth by 24 h. Minocycline monotherapy (1, 2, 4, 8, and 16 mg/liter) showed no reduction of activity of >1.34 log against all isolates, although concentrations of 8 and 16 mg/liter prolonged the time to regrowth. When the therapies were used in combination, rapid bactericidal activity was followed by slower regrowth, with synergy (60 of 120 combinations at 24 h, 19 of 120 combinations at 48 h) and additivity (43 of 120 combinations at 24 h, 44 of 120 combinations at 48 h) against all isolates. The extent of killing was greatest against the more susceptible polymyxin B isolates (MICs of ≤0.5 mg/liter) regardless of the minocycline MIC. The pharmacodynamic activity of combined polymyxin B-minocycline therapy against KPC-producing K. pneumoniae is dependent on polymyxin B susceptibility. Further in vitro and animal studies must be performed to fully evaluate the efficacy of this drug combination.

Polymyxin B in combination with meropenem against carbapenemase-producing Klebsiella pneumoniae: pharmacodynamics and morphological changes.

Combination therapy provides a useful therapeutic approach to overcome resistance until new antibiotics become available. In this study, the pharmacodynamics, including the morphological effects, of polymyxin B (PMB) and meropenem alone and in combination against KPC-producing Klebsiella pneumoniae clinical isolates was examined. Ten clinical isolates were obtained from patients undergoing treatment for mediastinitis. KPCs were identified and MICs were measured using microbroth dilution. Time-kill studies were conducted over 24 h with PMB (0.5-16 mg/L) and meropenem (20-120 mg/L) alone or in combination against an initial inoculum of ca. 106 CFU/mL. Scanning electron microscopy (SEM) was employed to analyse changes in bacterial morphology after treatment, and the log change method was used to quantify the pharmacodynamic effect. All isolates harboured the blaKPC-2 gene and were resistant to meropenem (MICs ≥8 mg/L). Clinically relevant PMB concentrations (0.5, 1.0 and 2.0 mg/L) in combination with meropenem were synergistic against all isolates except BRKP28 (polymyxin- and meropenem-resistant, both MICs >128 mg/L). All PMB and meropenem concentrations in combination were bactericidal against polymyxin-susceptible isolates with meropenem MICs ≤16 mg/L. SEM revealed extensive morphological changes following treatment with PMB in combination with meropenem compared with the changes observed with each individual agent. Additionally, morphological changes decreased with increasing resistance profiles of the isolate, i.e. increasing meropenem MIC. These antimicrobial effects may not only be a summation of the effects due to each antibiotic but also a result of differential action that likely inhibits protective mechanisms in bacteria.

Polymyxin B in Combination with Rifampin and Meropenem against Polymyxin B-Resistant KPC-Producing Klebsiella pneumoniae.

Safe and effective therapies are urgently needed to treat polymyxin-resistant KPC-producing Klebsiella pneumoniae infections and suppress the emergence of resistance. We investigated the pharmacodynamics of polymyxin B, rifampin, and meropenem alone and as polymyxin B-based double and triple combinations against KPC-producing K. pneumoniae isolates. The rates and extents of killing with polymyxin B (1 to 128 mg/liter), rifampin (2 to 16 mg/liter), and meropenem (10 to 120 mg/liter) were evaluated against polymyxin B-susceptible (PBs) and polymyxin B-resistant (PBr) clinical isolates using 48-h static time-kill studies. Additionally, humanized triple-drug regimens of polymyxin B (concentration at steady state [Css] values of 0.5, 1, and 2 mg/liter), 600 mg rifampin every 12 or 8 h, and 1 or 2 g meropenem every 8 h dosed as an extended 3-h infusion were simulated over 48 h by using a one-compartment in vitro dynamic infection model. Serial bacterial counts were performed to quantify the pharmacodynamic effect. Population analysis profiles (PAPs) were used to assess the emergence of polymyxin B resistance. Monotherapy was ineffective against both isolates. Polymyxin B with rifampin demonstrated early bactericidal activity against the PBs isolate, followed by regrowth by 48 h. Bactericidal activity was sustained at all polymyxin B concentrations of ≥2 mg/liter in combination with meropenem. No two-drug combinations were effective against the PBr isolate, but all simulated triple-drug regimens showed early bactericidal activity against both strains by 8 h that was sustained over 48 h. PAPs did not reveal the emergence of resistant subpopulations. The triple-drug combination of polymyxin B, rifampin, and meropenem may be a viable consideration for the treatment of PBr KPC-producing K. pneumoniae infections. Further investigation is warranted to optimize triple-combination therapy.

. In Vitro assessment of combined Polymyxin B and minocycline therapy against klebsiella pneumoniae carbapenemase (KPC)-producing K. pneumoniae

The multidrug resistance profiles of Klebsiella pneumoniae carbapenemase (KPC) producers have led to increased clinical polymyxin use. Combination therapy with polymyxins may improve treatment outcomes, but it is uncertain which combinations are most effective. Clinical successes with intravenous minocycline-based combination treatments have been reported for infections caused by carbapenemase-producing bacteria. The objective of this study was to evaluate the in vitro activity of polymyxin B and minocycline combination therapy against six KPC-2-producing K. pneumoniae isolates (minocycline MIC range, 2 to 32 mg/liter). Polymyxin B monotherapy (0.5, 1, 2, 4, and 16 mg/liter) resulted in a rapid reduction of up to 6 log in bactericidal activity followed by regrowth by 24 h. Minocycline monotherapy (1, 2, 4, 8, and 16 mg/liter) showed no reduction of activity of >1.34 log against all isolates, although concentrations of 8 and 16 mg/liter prolonged the time to regrowth. When the therapies were used in combination, rapid bactericidal activity was followed by slower regrowth, with synergy (60 of 120 combinations at 24 h, 19 of 120 combinations at 48 h) and additivity (43 of 120 combinations at 24 h, 44 of 120 combinations at 48 h) against all isolates. The extent of killing was greatest against the more susceptible polymyxin B isolates (MICs of ≤0.5 mg/liter) regardless of the minocycline MIC. The pharmacodynamic activity of combined polymyxin B-minocycline therapy against KPC-producing K. pneumoniae is dependent on polymyxin B susceptibility...

Polymyxin B in combination with meropenem againstcarbapenemase-producing Klebsiella pneumoniae: pharmacodynamicsand morphological changes

Combination therapy provides a useful therapeutic approach to overcome resistance until new antibiotics become available. In this study, the pharmacodynamics, including the morphological effects, ofpolymyxin B (PMB) and meropenem alone and in combination against KPC-producing Klebsiella pneumoniaeclinical isolates was examined. Ten clinical isolates were obtained from patients undergoing treatmentfor mediastinitis. KPCs were identified and MICs were measured using microbroth dilution. Time–killstudies were conducted over 24 h with PMB (0.5–16 mg/L) and meropenem (20–120 mg/L) alone or incombination against an initial inoculum of ca. 106 CFU/mL. Scanning electron microscopy (SEM) was employedto analyse changes in bacterial morphology after treatment, and the log change method was usedto quantify the pharmacodynamic effect. All isolates harboured the blaKPC-2 gene and were resistant tomeropenem (MICs ≥8 mg/L). Clinically relevant PMB concentrations (0.5, 1.0 and 2.0 mg/L) in combinationwith meropenem were synergistic against all isolates except BRKP28 (polymyxin- and meropenemresistant,both MICs >128 mg/L). All PMB and meropenem concentrations in combination were bactericidalagainst polymyxin-susceptible isolates with meropenem MICs ≤16 mg/L. SEM revealed extensive morphologicalchanges following treatment with PMB in combination with meropenem compared with thechanges observed with each individual agent. Additionally, morphological changes decreased with increasingresistance profiles of the isolate, i.e. increasing meropenem MIC. These antimicrobial effects maynot only be a summation of the effects due to each antibiotic but also a result of differential action thatlikely inhibits protective mechanisms in bacteria...

Combinatorial pharmacodynamics of polymyxin B and tigecycline against heteroresistant Acinetobacter baumannii.

The prevalence of heteroresistant Acinetobacter baumannii is increasing. Infections due to these resistant pathogens pose a global treatment challenge. Here, the pharmacodynamic activities of polymyxin B (PMB) (2-20 mg/L) and tigecycline (0.15-4 mg/L) were evaluated as monotherapy and in combination using a 4 × 4 concentration array against two carbapenem-resistant and polymyxin-heteroresistant A. baumannii isolates. Time Kill Experiments was employed at starting inocula of 10(6) and 10(8) CFU/mL over 48 h. Clinically relevant combinations of PMB (2 mg/L) and tigecycline (0.90 mg/L) resulted in greater reductions in the bacterial population compared with polymyxin alone by 8 h (ATCC 19606, -6.38 vs. -3.43 log10 CFU/mL; FADDI AB115, -1.38 vs. 2.08 log10 CFU/mL). At 10× the clinically achievable concentration (PMB 20 mg/L in combination with tigecycline 0.90 mg/L), there was bactericidal activity against FADDI AB115 by 4 h that was sustained until 32 h, and against ATCC 19606 that was sustained for 48 h. These studies show that aggressive polymyxin-based dosing in combination with clinically achievable tigecycline concentrations results in early synergistic activity that is not sustained beyond 8 h, whereas combinations with higher tigecycline concentrations result in sustained bactericidal activity against both isolates at both inocula. These results indicate a need for optimised front-loaded polymyxin-based combination regimens that utilise high polymyxin doses at the onset of treatment to achieve good pharmacodynamic activity whilst minimising adverse events.