Population Pharmacokinetics and Exposure-Response Analysis for the Phase 3 COSMIC-311 Trial of Cabozantinib for Radioiodine-Refractory Differentiated Thyroid Cancer

BACKGROUND AND OBJECTIVE

In the USA, cabozantinib was approved for the treatment of patients aged ≥ 12 years with radioiodine-refractory differentiated thyroid cancer (DTC) who progressed on prior vascular endothelial growth factor (VEGFR)-targeted therapy based on the Phase 3 COSMIC-311 trial, which evaluated cabozantinib 60 mg/day versus placebo. Approved dosing is 60 mg/day for adults and for pediatric patients aged ≥ 12 years with body surface area (BSA) ≥ 1.2 m², and 40 mg/day for pediatric patients aged ≥ 12 years with BSA < 1.2 m². This report describes a population pharmacokinetic (PopPK) and exposure-response analysis of COSMIC-311.

METHODS

A PopPK model was developed using concentration-time data from COSMIC-311 and 6 other cabozantinib studies. The final (full) PopPK model was used to simulate the effect of sex, body weight, race, and patient population. For exposure-response analysis, derived datasets from COSMIC-311 were constructed for time-to-event analyses of progression-free survival (PFS) and safety endpoints.

RESULTS

The PopPK analysis included 4746 cabozantinib PK samples from 1745 patients and healthy volunteers. Body weight had minimal impact on cabozantinib exposure but increasing body weight was associated with increased apparent volume of distribution. Based on model-based simulation, adolescents < 40 kg had higher maximum plasma concentration at steady state of cabozantinib 60 mg/day compared to adults. Allometric scaling simulation in adolescents < 40 kg demonstrated higher exposure with 60 mg/day relative to adults receiving the same dose, while exposure with 40 mg/day in adolescents < 40 kg was similar to 60 mg/day in adults. The exposure-response analysis included 115 patients. There was no clear relationship between PFS or dose modification and cabozantinib exposure. A statistically significant relationship was demonstrated for cabozantinib exposure and hypertension (Grade ≥ 3) and fatigue/asthenia (Grade ≥ 3).

CONCLUSIONS

These results support the dosing strategy implemented in COSMIC-311 and the BSA-based label recommendations for adolescents. The cabozantinib dose should be reduced to manage adverse events as indicated.

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 bla_KPC-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 doripenem against heteroresistant Acinetobacter baumannii: pharmacodynamics of new dosing strategies

OBJECTIVES

Polymyxin B is being increasingly utilized as a last resort against resistant Gram-negative bacteria. We examined the pharmacodynamics of novel dosing strategies for polymyxin B combinations to maximize efficacy and minimize the emergence of resistance and drug exposure against Acinetobacter baumannii.

METHODS

The pharmacodynamics of polymyxin B together with doripenem were evaluated in time-kill experiments over 48 h against 10⁸ cfu/mL of two polymyxin-heteroresistant A. baumannii isolates (ATCC 19606 and N16870). Pharmacokinetic/pharmacodynamic relationships were mathematically modelled using S-ADAPT. A hollow-fibre infection model (HFIM) was also used to simulate clinically relevant polymyxin B dosing strategies (traditional, augmented 'front-loaded' and 'burst' regimens), together with doripenem, against an initial inoculum of 10⁹ cfu/mL of ATCC 19606.

RESULTS

In static time-kill studies, polymyxin B concentrations >4 mg/L in combination with doripenem 25 mg/L resulted in rapid bactericidal activity against both strains with undetectable bacterial counts by 24 h. The mathematical model described the rapid, concentration-dependent killing as subpopulation and mechanistic synergy. In the HFIM, the traditional polymyxin B combination regimen was synergistic, with a >7.5 log₁₀ reduction by 48 h. The polymyxin B 'front-loaded' combination resulted in more rapid and extensive initial killing (>8 log₁₀) within 24 h, which was sustained over 10 days. With only 25% of the cumulative drug exposure, the polymyxin B 'burst' combination demonstrated antibacterial activity similar to traditional and 'front-loaded' combination strategies. The polymyxin B 'front-loaded' and 'burst' combination regimens suppressed the emergence of resistance.

CONCLUSIONS

Early aggressive dosing regimens for polymyxin combinations demonstrate promise for treatment of heteroresistant A. baumannii infections.

Colistin and doripenem combinations against Pseudomonas aeruginosa: profiling the time course of synergistic killing and prevention of resistance

OBJECTIVES

Colistin is an 'old' drug, which is being increasingly utilized due to limited therapeutic options. However, resistance emergence during monotherapy is concerning. Here, our objective was to optimize colistin combinations against Pseudomonas aeruginosa by profiling the time course of synergistic killing and prevention of resistance.

METHODS

Hollow-fibre infection models over 10 days simulated clinically relevant dosage regimens of colistin and doripenem against two heteroresistant P. aeruginosa strains (MIC 1 mg/L) and one resistant (MIC 128 mg/L) strain (inoculum 10(9.3) cfu/mL). New mathematical mechanism-based models (MBMs) were developed using S-ADAPT.

RESULTS

Against heteroresistant P. aeruginosa strains, colistin monotherapy resulted in initial killing (up to 2.64 log10 cfu/mL) within 24 h followed by regrowth. High-intensity combinations involving free steady-state colistin concentrations of 5 mg/L achieved complete eradication (>9.3 log10 killing) within 48 h. These combinations achieved synergy with up to 9.38 log10 greater killing compared with the most active monotherapy. Against the colistin-resistant strain, the combination yielded marked initial synergy with up to 6.11 log10 cfu/mL bacterial reductions within 72 h followed by regrowth. The MBMs quantified total and resistant subpopulations and the proposed synergy between colistin and doripenem.

CONCLUSIONS

Our findings provide insight into optimal antibiotic treatment and may serve as a framework for new drug combinations and combination modelling.

New dosing strategies for an old antibiotic: pharmacodynamics of front-loaded regimens of colistin at simulated pharmacokinetics in patients with kidney or liver disease

Increasing evidence suggests that colistin monotherapy is suboptimal at currently recommended doses. We hypothesized that front-loading provides an improved dosing strategy for polymyxin antibiotics to maximize killing and minimize total exposure. Here, we utilized an in vitro pharmacodynamic model to examine the impact of front-loaded colistin regimens against a high bacterial density (10(8) CFU/ml) of Pseudomonas aeruginosa. The pharmacokinetics were simulated for patients with hepatic (half-life [t1/2] of 3.2 h) or renal (t1/2 of 14.8 h) disease. Front-loaded regimens (n=5) demonstrated improvement in bacterial killing, with reduced overall free drug areas under the concentration-time curve (fAUC) compared to those with traditional dosing regimens (n=14) with various dosing frequencies (every 12 h [q12h] and q24h). In the renal failure simulations, front-loaded regimens at lower exposures (fAUC of 143 mg · h/liter) obtained killing activity similar to that of traditional regimens (fAUC of 268 mg · h/liter), with an ∼97% reduction in the area under the viable count curve over 48 h. In hepatic failure simulations, front-loaded regimens yielded rapid initial killing by up to 7 log10 within 2 h, but considerable regrowth occurred for both front-loaded and traditional regimens. No regimen eradicated the high bacterial inoculum of P. aeruginosa. The current study, which utilizes an in vitro pharmacodynamic infection model, demonstrates the potential benefits of front-loading strategies for polymyxins simulating differential pharmacokinetics in patients with hepatic and renal failure at a range of doses. Our findings may have important clinical implications, as front-loading polymyxins as a part of a combination regimen may be a viable strategy for aggressive treatment of high-bacterial-burden infections.