Clinical Pharmacology Approaches to Support Approval of New Routes of Administration for Therapeutic Proteins.

Intravenous or subcutaneous routes of administration (ROAs) are common dosing routes for therapeutic proteins. Eleven therapeutic proteins with approval for one ROA have subsequently received approval for a second ROA. The clinical programs supporting the second ROA consistently leveraged data from the first ROA and included studies that characterized the pharmacokinetics (PKs) of the drug administered by the new ROA to identify an appropriate dosage regimen. The selected dosing regimen was then further evaluated in clinical trials designed with various primary end points. All programs implemented model-informed drug development approaches to ensure that the selected regimens would achieve comparable systemic exposures (PK-based bridging) or pharmacodynamic (PD) responses (PD-based bridging) as the reference ROA. To support the approval of a second ROA, these programs either demonstrated noninferiority in PK, PD, and/or clinical end points for the second ROA, or established efficacy and safety through a comparison to a placebo treatment. The accumulative examples showed that clinical trials which provided the primary evidence to support approvals of the second ROA generally demonstrated noninferiority in the systemic exposures regardless of being specified as an end point or not in the study protocols. The experience to date supports the use of PK- and PD-based bridging approaches not only in the selection of dosing regimens for a second ROA to be tested in clinical studies, but also for providing evidence of effectiveness to support approval, when appropriate.

FDA Approval Summary: Belantamab Mafodotin for Patients with Relapsed or Refractory Multiple Myeloma.

On August 5, 2020, the FDA granted accelerated approval to belantamab mafodotin-blmf (BLENREP; GlaxoSmithKline) for the treatment of adult patients with relapsed or refractory multiple myeloma who have received at least four prior therapies including an anti-CD38 monoclonal antibody, a proteasome inhibitor, and an immunomodulatory agent. Substantial evidence of effectiveness was obtained from the phase II, multicenter DREAMM-2 trial. Patients received belantamab mafodotin 2.5 or 3.4 mg/kg intravenously once every 3 weeks until disease progression or unacceptable toxicity. The trial demonstrated an overall response rate of 31% in the 2.5 mg/kg cohort and 34% in the 3.4 mg/kg cohort. Keratopathy was the most frequent adverse event, occurring in 71% and 77% of patients, respectively. Other ocular toxicities included changes in visual acuity, blurred vision, and dry eye. The U.S. prescribing information for belantamab mafodotin includes a boxed warning for ocular toxicity, and belantamab mafodotin is available only through a restricted program under a Risk Evaluation and Mitigation Strategy. This article summarizes the data and the FDA review process supporting accelerated approval of belantamab mafodotin 2.5 mg/kg intravenously once every 3 weeks. This approval may be contingent upon verification and description of clinical benefit in confirmatory trial(s).

FDA Approval Summary: Revised Indication and Dosing Regimen for Ponatinib Based on the Results of the OPTIC Trial.

On December 18, 2020, US Food and Drug Administration (FDA) approved a supplemental application for ponatinib extending the indication in patients with chronic-phase chronic myeloid leukemia (CP-CML) to patients with resistance or intolerance of at least 2 prior kinase inhibitors. Ponatinib was initially approved in December 2012 but was briefly voluntarily withdrawn due to serious safety concerns including the risk of arterial occlusive events (AOE). It returned to the market in December 2013 with an indication limited to patients with T315I mutation or for whom no other tyrosine kinase inhibitor (TKI) therapy was indicated with revised warnings and precautions. A post-marketing requirement was issued to identify the optimal safe and effective dose for CP-CML. Thus, the OPTIC trial was performed, which randomized patients to 1 of 3 doses, 45 mg, 30 mg, or 15 mg, with a dose reduction to 15 mg on achievement of MR2 (BCR-ABLIS ≤1%). Patients enrolled were treated with at least 2 prior TKIs or had a T315I mutation. Patients with a history of clinically significant, uncontrolled, or active cardiovascular disease were excluded. Efficacy was established on an interim analysis based on the rate of MR2 at 12 months in the modified intent-to-treat population of 261 patients, with 88, 86, and 87 patients in the 45, 30, and 15 mg cohorts, respectively. With a median follow-up of 28 months, the rate of achievement of MR2 at 12 months was 42%, 28%, and 24% in the respective cohorts. The safety profile was consistent with that observed in prior evaluations of ponatinib with notable adverse reactions including pancreatitis, hypertension, hyperlipidemia, liver dysfunction, and AOE. Of patients treated at the 45/15 mg dose, AOEs were seen in 13%, with a higher rate being observed in patients age 65 or older compared to younger patients. A readjudication of AOEs seen on the prior pivotal phase 2 study resulted in a rate of 26%. Overall, the results supported a modification of the recommended dose for patients with CP-CML to 45 mg until the achievement of MR2 followed by a reduction to 15 mg. The expansion of the indication to patients with exposure to 2 prior TKIs was approved given data showing that ponatinib could be successfully used for the treatment of this population with appropriate monitoring and screening for risk factors.

FDA Approval Summary: Decitabine and Cedazuridine Tablets for Myelodysplastic Syndromes.

On July 7, 2020, the Food and Drug Administration approved Inqovi (Otsuka Pharmaceutical Co.), an oral fixed-dose combination tablet comprising 35 mg decitabine, a hypomethylating agent, and 100 mg cedazuridine, a cytidine deaminase inhibitor (abbreviated DEC-C) for treatment of adult patients with myelodysplastic syndromes (MDS). Evidence of effectiveness of DEC-C was established in phase III ASTX727-02 (N = 133) in adults with MDS. The study involved a two-sequence crossover comparing DEC-C and intravenous (IV) decitabine 20 mg/m2 once daily for the first 5 days of each 28-day cycle in the first 2 cycles. From cycle 3 onward, patients received DEC-C. Five-day cumulative area under the curve (5-d AUC) of decitabine for DEC-C was similar to that of IV decitabine, with geometric mean ratio 0.99 (90% confidence interval: 0.93-1.06). Clinical benefit was supported by study ASTX727-02 and the similarly designed phase II study ASTX727-01-B (n = 80), with complete remission (CR) of 21% and 18% and median duration of CR 7.5 and 8.7 months, respectively. Adverse reactions were consistent with IV decitabine. Postmarketing assessments were issued to address the effect of cedazuridine on QT prolongation, food effect, moderate and severe hepatic impairment, and severe renal impairment on the pharmacokinetics and safety of DEC-C.

FDA Approval Summary: Belumosudil for Adult and Pediatric Patients 12 Years and Older with Chronic GvHD after Two or More Prior Lines of Systemic Therapy.

On July 16, 2021, the FDA approved belumosudil, a kinase inhibitor, for adult and pediatric patients 12 years and older with chronic GvHD (cGvHD) after failure of at least two prior lines of systemic therapy. Approval was based on the results of Study KD025-213, which included 65 patients with cGvHD treated with belumosudil 200 mg daily in an open-label, single-arm cohort. Efficacy was determined by the overall response rate (ORR) through Cycle 7 Day 1, which included complete response (CR) or partial response (PR) according to the 2014 NIH consensus criteria, and durability of response. The ORR through Cycle 7 Day 1 was 75% [95% confidence interval (CI), 63-85]; 6% of patients achieved a CR, and 69% achieved a PR. The median duration of response was 1.9 months (95% CI, 1.2-2.9), and 62% (95% CI, 46-74) of responding patients remained alive without new systemic therapy for at least 12 months from response. The common adverse reactions were infections, asthenia, nausea, diarrhea, dyspnea, cough, edema, hemorrhage, abdominal pain, musculoskeletal pain, headache, phosphate decreased, gamma-glutamyl transferase increased, lymphocytes decreased, and hypertension. Additional study is warranted to confirm safety with long-term use.

FDA Approval Summary: Selinexor for Relapsed or Refractory Diffuse Large B-Cell Lymphoma.

In June 2020, the U.S. Food and Drug Administration granted accelerated approval to selinexor for the treatment of adult patients with relapsed or refractory diffuse large B-cell lymphoma (DLBCL), not otherwise specified, including DLBCL arising from follicular lymphoma, after at least two lines of systemic therapy. Approval was based on SADAL, a multicenter trial of selinexor monotherapy in patients with DLBCL after two to five systemic regimens. Efficacy was based on independent review committee-assessed objective response rate (ORR) and duration of response using Lugano criteria. In 134 patients treated with the approved dosage (60 mg orally on days 1 and 3 of each week), the ORR was 29% (95% confidence interval, 22-38), with complete response in 13% and with 38% of responses lasting at least 6 months. Gastrointestinal toxicity developed in 80% of patients, hyponatremia in 61%, central neurological toxicity (such as dizziness and mental status changes) in 25%, and ocular toxicity in 18%. New or worsening grade 3 or 4 thrombocytopenia, lymphopenia, neutropenia, anemia, or hyponatremia developed in ≥15%. Adverse reactions led to selinexor dose interruption in 61% of patients, dose reduction in 49%, and permanent discontinuation in 17%, with thrombocytopenia being the leading cause of dose modifications. Postmarketing studies will evaluate reduced dosages of selinexor and further evaluate clinical benefit in patients with relapsed or refractory DLBCL. IMPLICATIONS FOR PRACTICE: Selinexor is a new potential option for adults with relapsed or refractory diffuse large B-cell lymphoma, not otherwise specified, in the third-line setting or beyond. Toxicities are typically manageable but can be difficult to tolerate and necessitate close monitoring and supportive care.

FDA Approval Summary: Gilteritinib for Relapsed or Refractory Acute Myeloid Leukemia with a FLT3 Mutation.

On November 28, 2018, the FDA approved gilteritinib (Xospata; Astellas), a small-molecule FMS-like tyrosine kinase 3 (FLT3) inhibitor, for treatment of relapsed or refractory acute myeloid leukemia with a FLT3 mutation as detected by an FDA-approved test. In the ADMIRAL study, patients were randomized 2:1 to receive gilteritinib or standard chemotherapy and stratified by response to first-line treatment and intensity of prespecified chemotherapy. Efficacy was established on interim analysis on the basis of complete remission (CR) + CR with partial hematologic recovery (CRh) rate, duration of CR + CRh, and conversion from transfusion dependence to transfusion independence in 138 patients in the gilteritinib arm. With median follow-up of 4.6 months [95% confidence interval (CI), 2.8-15.8 months] at interim analysis, the CR + CRh rate was 21% (95% CI, 15%-29%), median duration of CR + CRh was 4.6 months (range, 0.1-15.8+), and conversion from transfusion dependence to transfusion independence was 31%. Revised labeling approved on May 29, 2019 included the results of the final analysis, showing an improvement in overall survival (OS) with gilteritinib compared with chemotherapy (HR, 0.64; 95% CI, 0.49-0.83; one-sided P = 0.0004; median OS, 9.3 vs. 5.6 months). The OS benefit was observed in both high and low chemotherapy intensity subgroups. Labeling includes a boxed warning for differentiation syndrome and warnings for posterior reversible encephalopathy syndrome, QT prolongation, pancreatitis, and embryo-fetal toxicity. Safe use requires frequent monitoring of electrocardiograms and blood chemistries. Assessments of long-term safety are pending.

A Simulation Approach to Evaluate the Impact of Patterns of Bioanalytical Bias Difference on the Outcome of Pharmacokinetic Similarity Studies.

Pharmacokinetic (PK) similarity studies are vital to assess the biosimilarity of a biosimilar to a reference product. Systematic bias in a bioanalytical method that quantify products could be a potential source of error affecting the variability of the data and influencing the outcome of a PK similarity study. We investigated the impact of six varying patterns of bioanalytical bias difference (biasdiff ) between the similar products on the probability passing the PK similarity test. A population PK model was used to simulate concentration-time profiles for a biosimilar and a reference product and added biasdiff ranging from 030%. The probability of achieving the PK similarity criteria (90% confidence interval between 0.8 and 1.25) for the maximum serum concentration (Cmax ) and area under the curve (AUC) was assessed. The data indicate that an increase in absolute biasdiff between products of ≥ 10% would decrease the power to assess the similarity criteria for Cmax and AUC.

Pharmacokinetic/Pharmacodynamic Evaluation of Solithromycin against Streptococcus pneumoniae Using Data from a Neutropenic Murine Lung Infection Model.

Solithromycin (CEM-101) is a novel fluoroketolide antimicrobial agent with activity against typical and atypical pathogens associated with community-acquired bacterial pneumonia. Using a neutropenic murine lung infection model, the objectives of this study were to identify the pharmacokinetic/pharmacodynamic (PK/PD) index most closely associated with efficacy and the magnitude of such indices associated with solithromycin efficacy against Streptococcus pneumoniae Plasma and epithelial lining fluid (ELF) samples for pharmacokinetics (PK) were collected serially over 24 hours from healthy mice administered single doses of solithromycin (0.625 to 40 mg/kg). Neutropenic CD-1 mice infected with 108 CFUs of one of five S. pneumoniae isolates were administered solithromycin (0.156 to 160 mg/kg/day) via oral gavage. Doses were administered in a fractionated manner for mice infected with one isolate, while mice infected with the remaining four isolates received solithromycin as either a regimen every 6 hours or every 12 hours. A three-compartment model best described solithromycin PK in the plasma and ELF (r2 = 0.935 and 0.831, respectively). The ratio of total-drug ELF to free-drug plasma area under the concentration-time curve (AUC) from time 0 to 24 hours was 2.7. Free-drug plasma and total-drug ELF AUC to minimum inhibitory concentration ratios (AUC/MIC ratios) were most predictive of efficacy (r2 = 0.851 and 0.850, respectively). The magnitude of free-drug plasma/total-drug ELF AUC/MIC ratios associated with net bacterial stasis and a 1- and 2-log10 CFU reduction from baseline was 1.65/1.26, 6.31/15.1, and 12.8/59.8, respectively. These data provided dose selection support for solithromycin for clinical trials in patients with community-acquired bacterial pneumonia.

Effects of deferasirox dose and decreasing serum ferritin concentrations on kidney function in paediatric patients: an analysis of clinical laboratory data from pooled clinical studies.

BACKGROUND: Serious and fatal deferasirox-induced kidney injury has been reported in paediatric patients. This study aimed to investigate the effects of deferasirox dose and serum ferritin concentrations on kidney function and the effect of impaired kidney function on dose-normalised deferasirox minimum plasma concentration (Cmin). METHODS: We did a case-control analysis using pooled data from ten clinical studies. We identified transfusion-dependent patients with thalassaemia, aged 2-15 years, who were receiving deferasirox and had available baseline and follow-up serum creatinine and ferritin measurements. Cases of acute kidney injury (AKI) were defined according to an estimated glomerular filtration rate (eGFR) threshold of 90 mL/min per 1·73 m2 or less (if baseline eGFR was ≥100 mL/min per 1·73 m2), an eGFR of 60 mL/min per 1·73 m2 or less (if baseline eGFR was <100 mL/min per 1·73 m2), or an eGFR decrease from baseline of at least 25%. Cases were matched to control visits (eGFR ≥120 mL/min per 1·73 m2) on age, sex, study site, and time since drug initiation. We calculated rate ratios for AKI using conditional logistic regression, and evaluated the effect of eGFR changes on Cmin. FINDINGS: Among 1213 deferasirox-treated paediatric patients, 162 cases of AKI and 621 matched control visits were identified. Patients with AKI had a mean 50·2% (SD 15·5) decrease in eGFR from baseline, compared with a 6·9% (29·8) decrease in controls. A significantly increased risk for AKI (rate ratio 1·26, 95% CI 1·08-1·48, p=0·00418) was observed per 5 mg/kg per day increase in deferasirox dispersible tablet dose (equivalent to a 3·5 mg/kg per day dose of film-coated tablets or granules), above the typical starting dose (20 mg/kg per day). An increased risk (1·25, 1·01-1·56, p=0·0400) for AKI was also observed per 250 µg/L decrease in serum ferritin, starting from 1250 µg/L. High-dose deferasirox (dispersible tablet dose >30 mg/kg per day) resulted in an increased risk (4·47, 1·25-15·95, p=0·0209) for AKI when serum ferritin was less than 1000 µg/L. Decreases in eGFR were associated with increased Cmin. INTERPRETATION: Deferasirox can cause AKI in a dose-dependent manner. The increased AKI risk with high-dose deferasirox and lower serum ferritin concentration is consistent with overchelation as a causative factor. Small decreases in eGFR correlate with increased deferasirox Cmin, especially in younger patients. Physicians should closely monitor renal function and serum ferritin, use the lowest effective dose to maintain acceptable body iron burden, and interrupt deferasirox treatment when AKI or volume depletion are suspected. FUNDING: None.

Pharmacokinetic-Pharmacodynamic Evaluation of Gepotidacin against Gram-Positive Organisms Using Data from Murine Infection Models.

Gepotidacin (formerly called GSK2140944) is a novel triazaacenaphthylene bacterial topoisomerase inhibitor with in vitro activity against conventional and biothreat pathogens, including Staphylococcus aureus and Streptococcus pneumoniae Using neutropenic murine thigh and lung infection models, the pharmacokinetics-pharmacodynamics (PK-PD) of gepotidacin against S. aureus and S. pneumoniae were characterized. Candidate models were fit to single-dose PK data from uninfected mice (for doses of 16 to 128 mg/kg of body weight given subcutaneously [s.c.]). Dose fractionation studies (1 isolate/organism; 2 to 512 mg/kg/day) and dose-ranging studies (5 isolates/organism; 2 to 2,048 mg/kg/day; MIC ranges of 0.5 to 2 mg/liter for S. aureus and 0.125 to 1 mg/liter for S. pneumoniae) were conducted. The presence of an in vivo postantibiotic effect (PAE) was also evaluated. Relationships between the change from baseline in log10 CFU at 24 h and the ratio of the free-drug plasma area under the concentration-time curve (AUC) to the MIC (AUC/MIC ratio), the ratio of the maximum concentration of drug in plasma (Cmax) to the MIC (Cmax/MIC ratio), and the percentage of a 24-h period that the drug concentration exceeded the MIC (%T>MIC) were evaluated using Hill-type models. Plasma and epithelial lining fluid (ELF) PK data were best fit by a four-compartment model with linear distributional clearances, a capacity-limited clearance, and a first-order absorption rate. The ELF penetration ratio in uninfected mice was 0.65. Since the growth of both organisms was poor in the murine lung infection model, lung efficacy data were not reported. As determined using the murine thigh infection model, the free-drug plasma AUC/MIC ratio was the PK-PD index most closely associated with efficacy (r2 = 0.936 and 0.897 for S. aureus and S. pneumoniae, respectively). Median free-drug plasma AUC/MIC ratios of 13.4 and 58.9 for S. aureus, and 7.86 and 16.9 for S. pneumoniae, were associated with net bacterial stasis and a 1-log10 CFU reduction from baseline, respectively. Dose-independent PAE durations of 3.07 to 12.5 h and 5.25 to 8.46 h were demonstrated for S. aureus and S. pneumoniae, respectively.

Pharmacokinetics-Pharmacodynamics of Tazobactam in Combination with Piperacillin in an In Vitro Infection Model.

We have previously demonstrated the pharmacokinetic-pharmacodynamic (PK-PD) index best associated with the efficacy of tazobactam when used in combination with ceftolozane to be the percentage of the dosing interval during which tazobactam concentrations remained above a threshold value (%time>threshold). Using anin vitroinfection model and the same isogenic CTX-M-15-producingEscherichia colitriplet set genetically engineered to transcribe different levels ofblaCTX-M-15, herein we describe dose fractionation studies designed to evaluate the PK-PD index associated with tazobactam efficacy, when given in combination with piperacillin, and the impact of the presence of a different ß-lactam agent, or differentblaCTX-M-15transcription levels, on the magnitude of the tazobactam PK-PD index necessary for efficacy. The recombinant strains demonstrated piperacillin MIC values of 128, >256, and >256 µg/ml for the low-, moderate-, and high-level CTX-M-15-producingE. colistrains, respectively. The MIC value for piperacillin in the presence of 4 µg/ml of tazobactam was 2 µg/ml for all three strains. The PK-PD index associated with tazobactam efficacy was confirmed to be %time>threshold, regardless of ß-lactamase transcription (r(2)= 0.839). The tazobactam concentration thresholds, however, changed with the CTX-M-15 transcription level and were 0.25, 0.5, and 2 µg/ml for the low-, moderate-, and high-level CTX-M-15-producing strains, respectively (r(2)= 0.921, 0.773, and 0.875, respectively). The %time>threshold values for tazobactam necessary for net bacterial stasis and a 1- and 2-log10-unit CFU/ml decrease from baseline at 24 h were 44.9, 62.9, and 84.9%, respectively. In addition to verifying our previous study results, these results also demonstrated that the magnitude of bacterial-cell killing associated with a ß-lactam-ß-lactamase inhibitor combination is dependent on the amount of ß-lactamase produced. These data provide important information for the development of ß-lactam-ß-lactamase inhibitor combination agents.

Exploration of the Pharmacokinetic-Pharmacodynamic Relationships for Fosfomycin Efficacy Using an In Vitro Infection Model.

Fosfomycin, a phosphonic class antibiotic with a broad spectrum of antibacterial activity, has been used outside the United States since the early 1970s for the treatment of a variety of infections. In the United States, an oral (tromethamine salt) formulation is used for uncomplicated urinary tract infections. Recently, there has been interest in the use of an intravenous solution (ZTI-01) for the treatment of a broad range of infections associated with multidrug-resistant bacteria. In this era of multidrug-resistant bacteria with few treatment options, it is critical to understand the pharmacokinetic-pharmacodynamic (PK-PD) determinants for fosfomycin efficacy. Since such data are limited, a one-compartment in vitro infection model was used to determine the PK-PD index associated with efficacy and the magnitude of this measure necessary for various levels of effect. One challenge isolate (Escherichia coli ATCC 25922, for which the fosfomycin agar MIC is 0.5 mg/liter and the broth microdilution MIC is 1 mg/liter) was evaluated in the dose fractionation studies, and two additional clinical E. coli isolates were evaluated in the dose-ranging studies. Mutation frequency studies indicated the presence of an inherently fosfomycin resistant E. coli subpopulation (agar MIC = 32 to 64 mg/liter) within the standard starting inoculum of a susceptibility test. Due to the presence of this resistant subpopulation, we identified the percentage of the dosing interval that drug concentrations were above the inherent resistance inhibitory concentration found at baseline to be the PK-PD index associated with efficacy (r(2) = 0.777). The magnitudes of this PK-PD index associated with net bacterial stasis and 1- and 2-log10 CFU/ml reductions from baseline at 24 h were 11.9, 20.9, and 32.8, respectively. These data provide useful information for modernizing and optimizing ZTI-01 dosing regimens for further study.

Evaluation of the pharmacokinetics and pharmacodynamics of liposomal amikacin for inhalation in cystic fibrosis patients with chronic pseudomonal infections using data from two phase 2 clinical studies.

The pharmacokinetic-pharmacodynamic (PK-PD) relationships between serum exposure measures of liposomal amikacin for inhalation (LAI) and the change in pulmonary function test (PFT) measures and number of CFU from baseline were evaluated in cystic fibrosis (CF) patients chronically infected with Pseudomonas aeruginosa. A dose of 70, 140, 280, or 560 mg of LAI or placebo was administered to CF patients once daily for 28 days. PFTs and sputum samples for microbiology were assessed on days 7, 14, 21, 28, 35 (for log10 CFU), and 56 (for PFTs). Serum, urine, and sputum samples were collected for PK evaluation. The relationships between efficacy endpoints (relative change in forced expiratory volume in 1 s [FEV1 {expressed in liters}] and FEV1% predicted and the absolute change in log10 CFU of P. aeruginosa from baseline) and exposure measures (dose, day 1 area under the curve [AUC], dose/MIC ratio, and day 1 AUC/MIC ratio) and baseline MIC value were assessed. The serum and urine PK data were best fit by a 3-compartment model (lung, serum, and urine) with linear clearance and interoccasional variation on total and renal clearance. Significant univariable relationships between dose or day 1 AUC and the relative change in PFT measures (P≤0.017) or the absolute change in log10 CFU from baseline (P≤0.037) on the study days were identified. Repeated-measures mixed-effects models, which showed dose- and AUC-related improvements for each efficacy endpoint (P≤0.041), predicted the observed data well. The increases in the relative change in FEV1 and FEV1% predicted of 11% and 9.9%, respectively, and a 1.23-log10 CFU reduction per 560 mg of LAI estimated on day 7 were comparable to the observed increases of 10.7% and 10.3%, respectively, and a 1.24-log10 CFU reduction on the same day. The model-estimated PFT effects were predicted to be sustained to day 28. An additional 0.451-log10 CFU reduction (P=0.022) was estimated on day 14 relative to day 7, with a persistence of effect predicted to day 35.

Pharmacological basis of ß-lactamase inhibitor therapeutics: tazobactam in combination with Ceftolozane.

We recently investigated the pharmacokinetics-pharmacodynamics (PK-PD) of tazobactam in combination with ceftolozane against an isogenic CTX-M-15-producing Escherichia coli triplet set, genetically engineered to transcribe different levels of blaCTX-M-15. The percentage of the dosing interval that tazobactam concentrations remained above a threshold (%Time>threshold) was identified as the PK-PD exposure measure that was most closely associated with efficacy. Moreover, the tazobactam concentration was dependent upon the enzyme transcription level. Given that the aforementioned strains were genetically engineered to transcribe a single ß-lactamase enzyme and that clinical isolates typically produce multiple ß-lactamase enzymes with various transcription levels, it is likely that the tazobactam threshold concentration is isolate/enzyme dependent. Our first objective was to characterize the relationship between the tazobactam %Time>threshold in combination with ceftolozane and efficacy using clinical isolates in an in vitro PK-PD infection model. Our second objective was to identify a translational relationship that would allow for the comodeling across clinical isolates. The initial challenge panel included four well-characterized ß-lactamase-producing E. coli strains with variable enzyme expression and other resistance determinants. As evidenced by r(2) values of ranging from 0.90 to 0.99 for each clinical isolate, the observed data were well described by fitted functions describing the relationship between the tazobactam %Time>threshold and change in log10 CFU from baseline; however, the data from the four isolates did not comodel well. The threshold concentration identified for each isolate ranged from 0.5 to 4 mg/liter. We identified an enabling translational relationship for the tazobactam threshold that allowed comodeling of all four clinical isolates, which was the product of the individual isolate's ceftolozane-tazobactam MIC value and 0.5. As evidenced by an r(2) value of 0.90, the transformed data were well described by a fitted function describing the relationship between tazobactam %Time>threshold and change in log10 CFU from baseline. Due to these findings, the challenge panel was expanded to include three well-characterized ß-lactamase-producing Klebsiella pneumoniae strains with variable enzyme expression and other resistance determinants. The translational relationship for the tazobactam threshold that allowed for the comodeling of the four E. coli isolates performed well for the expanded data set (seven isolates in total; four E. coli and three K. pneumoniae), as evidenced by an r(2) value of 0.84. This simple translational relationship is especially useful as it is directly linked to in vitro susceptibility test results, which are used to guide the clinician's choice of drug and dosing regimen.

Relationship between ceftolozane-tazobactam exposure and drug resistance amplification in a hollow-fiber infection model.

In an era of rapidly emerging antimicrobial-resistant bacteria, it is critical to understand the importance of the relationships among drug exposure, duration of therapy, and selection of drug resistance. Herein we describe the results of studies designed to determine the ceftolozane-tazobactam exposure necessary to prevent the amplification of drug-resistant bacterial subpopulations in a hollow-fiber infection model. The challenge isolate was a CTX-M-15-producing Escherichia coli isolate genetically engineered to transcribe a moderate level of blaCTX-M-15. This organism's blaCTX-M-15 transcription level was confirmed by relative quantitative reverse transcription-PCR (qRT-PCR), ß-lactamase hydrolytic assays, and a ceftolozane MIC value of 16 mg/liter. In these studies, the experimental duration (10 days), ceftolozane-tazobactam dose ratio (2:1), and dosing interval (every 8 h) were selected to approximate those expected to be used clinically. The ceftolozane-tazobactam doses studied ranged from 125-62.5 to 1,500-750 mg. Negative- and positive-control arms included no treatment and piperacillin-tazobactam at 4.5 g every 6 h, respectively. An inverted-U-shaped function best described the relationship between bacterial drug resistance amplification and drug exposure. The least- and most-intensive ceftolozane-tazobactam dosing regimens, i.e., 125-62.5, 750-375, 1,000-500, and 1,500-750 mg, did not amplify drug resistance, while drug resistance amplification was observed with intermediate-intensity dosing regimens (250-125 and 500-250 mg). For the intermediate-intensity ceftolozane-tazobactam dosing regimens, the drug-resistant subpopulation became the dominant population by days 4 to 6. The more-intensive ceftolozane-tazobactam dosing regimens (750-375, 1,000-500, and 1,500-750 mg) not only prevented drug resistance amplification but also virtually sterilized the model system. These data support the selection of ceftolozane-tazobactam dosing regimens that minimize the potential for on-therapy drug resistance amplification.

Pharmacokinetics-pharmacodynamics of tazobactam in combination with ceftolozane in an in vitro infection model.

Despite ß-lactamase inhibitors being available for clinical use for nearly 30 years, a paucity of data exists describing the pharmacokinetic-pharmacodynamic (PK-PD) determinants of efficacy for these agents. Herein, we describe dose fractionation studies designed to determine the exposure measure most predictive of tazobactam efficacy in combination with ceftolozane and the magnitude of this measure necessary for efficacy in a PK-PD in vitro infection model. The challenge organism panel was comprised of an isogenic CTX-M-15-producing Escherichia coli triplet set, genetically engineered to transcribe different levels of bla(CTX-M-15). These recombinant strains exhibited ceftolozane MIC values of 4, 16, and 64 µg/ml representing low, moderate, and high levels of CTX-M-15, respectively. Different bla(CTX-M-15) transcription levels were confirmed by relative quantitative real-time PCR (qRT-PCR) and ß-lactamase hydrolytic assays. The exposure measure associated with efficacy was the percentage of the dosing interval that tazobactam concentrations remained above a threshold (%Time>threshold), regardless of enzyme expression (r(2) = 0.938). The threshold concentrations identified were 0.05 µg/ml for low and moderate and 0.25 µg/ml for the high-ß-lactamase expression strain constructs. The magnitudes of %Time>threshold for tazobactam associated with net bacterial stasis and a 1- and 2-log10 CFU reduction in bacteria at 24 h were approximately 35, 50, and 70%, respectively. These data provide an initial target tazobactam concentration-time profile and a paradigm to optimize tazobactam dosing when combined with ceftolozane.

Population pharmacokinetics of fusidic acid: rationale for front-loaded dosing regimens due to autoinhibition of clearance.

The objectives of this analysis were to develop a population pharmacokinetic (PK) model to describe the absorption and disposition of fusidic acid after single and multiple doses and to determine the effect of food on the rate and extent of bioavailability. Plasma PK data from three phase 1 studies (n = 75; n = 14 with and without food) in which healthy subjects received sodium fusidate (500 to 2,200 mg) as single or multiple oral doses every 8 h (q8h) or q12h for up to 7 days were modeled using S-ADAPT (MCPEM algorithm). Accumulation of fusidic acid after multiple doses was more than that predicted based on single-dose data. The PK of fusidic acid was best described using a time-dependent mixed-order absorption process, two disposition compartments, and a turnover process to describe the autoinhibition of clearance. The mean total clearance (% coefficient of variation) was 1.28 liters/h (33%) and the maximum extent of autoinhibition was 71.0%, with a 50% inhibitory concentration (IC(50)) of 46.3 mg/liter (36%). Food decreased the extent of bioavailability by 18%. As a result of the autoinhibition of clearance, steady state can be achieved earlier with dosing regimens that contain higher doses (after 8 days for 750 mg q12h and 1 day for 1,500 mg q12h on day 1 followed by 600 mg q12h versus 3 weeks for 500 mg q12h). Given that large initial doses autoinhibit the clearance of fusidic acid, this characteristic provides a basis for the administration of front-loaded dosing regimens of sodium fusidate which would allow for effective concentrations to be achieved early in therapy.