Pharmacokinetic/Pharmacodynamic Based Breakpoints of Polymyxin B for Bloodstream Infections Caused by Multidrug-Resistant Gram-Negative Pathogens

The latest PK/PD findings have demonstrated negligible efficacy of intravenous polymyxins against pulmonary infections. We investigated pharmacokinetic/pharmacodynamic (PK/PD)-based breakpoints of polymyxin B for bloodstream infections and the rationality of the recent withdrawal of polymyxin susceptibility breakpoints by the CLSI. Polymyxin B pharmacokinetic data were obtained from a phase I clinical trial in healthy Chinese subjects and population pharmacokinetic parameters were employed to determine the exposure of polymyxin B at steady state. MICs of 1,431 recent clinical isolates of Pseudomonas aeruginosa, Acinetobacter baumannii and Klebsiella pneumoniae collected from across China were determined. Monte-Carlo simulations were performed for various dosing regimens (0.42–1.5 mg/kg/12 h via 1 or 2-h infusion). The probability of target attainment, PK/PD breakpoints and cumulative fraction of response were determined for each bacterial species. MIC90 of polymyxin B was 1 mg/L for P. aeruginosa and 0.5 mg/L for A. baumannii and K. pneumoniae. With the recommended polymyxin B dose of 1.5–2.5 mg/kg/day, the PK/PD susceptible breakpoints for P. aeruginosa, A. baumannii and K. pneumoniae were 2, 1 and 1 mg/L respectively for bloodstream infection. For Chinese patients, polymyxin B dosing regimens of 0.75–1.5 mg/kg/12 h for P. aeruginosa and 1–1.5 mg/kg/12 h for A. baumannii and K. pneumoniae were appropriate. Breakpoint determination should consider the antimicrobial PK/PD at infection site and delivery route. The recent withdrawal of polymyxin susceptible breakpoint by CLSI primarily based on poor efficacy against lung infections needs to be reconsidered for bloodstream infections.

De novo centromere formation on chromosome fragments with an inactive centromere in maize (Zea mays)

The B chromosome of maize undergoes nondisjunction at the second pollen mitosis as part of its accumulation mechanism. Previous work identified 9-Bic-1 (9-B inactivated centromere-1), which comprises an epigenetically silenced B chromosome centromere that was translocated to the short arm of chromosome 9(9S). This chromosome is stable in isolation, but when normal B chromosomes are added to the genotype, it will attempt to undergo nondisjunction during the second pollen mitosis and usually fractures the chromosome in 9S. These broken chromosomes allow a test of whether the inactive centromere is reactivated or whether a de novo centromere is formed elsewhere on the chromosome to allow recovery of fragments. Breakpoint determination on the B chromosome and chromosome 9 showed that mini chromosome B1104 has the same breakpoint as 9-Bic-1 in the B centromere region and includes a portion of 9S. CENH3 binding was found on the B centromere region and on 9S, suggesting both centromere reactivation and de novo centromere formation. Another mini chromosome, B496, showed evidence of rearrangement, but it also only showed evidence for a de novo centromere. Other mini chromosome fragments recovered were directly derived from the B chromosome with breakpoints concentrated near the centromeric knob region, which suggests that the B chromosome is broken at a low frequency due to the failure of the sister chromatids to separate at the second pollen mitosis. Our results indicate that both reactivation and de novo centromere formation could occur on fragments derived from the progenitor possessing an inactive centromere.

The clinical relevance of intragenic NRXN1 deletions

BackgroundIntragenic NRXN1 deletions are susceptibility variants for neurodevelopmental disorders; however, their clinical interpretation is often unclear. Therefore, a literature study and an analysis of 43 previously unpublished deletions are provided.MethodsThe literature cohort covered 629 heterozygous NRXN1 deletions: 148 in controls, 341 in probands and 140 in carrier relatives, and was used for clinical hypothesis testing. Exact breakpoint determination was performed for 43 in-house deletions.ResultsThe prevalence of exonic NRXN1 deletions in controls was ~1/3000 as compared with ~1/800 in patients with neurodevelopmental/neuropsychiatric disorders. The differential distribution of deletions across the gene between controls and probands allowed to distinguish distinct areas within the gene. Exon 6–24 deletions appeared only twice in over 100000 control individuals, had an estimated penetrance for neurodevelopmental disorders of 32.43%, a de novo rate of 50% and segregated mainly with intellectual disability (ID) and schizophrenia. In contrast, exon 1–5 deletions appeared in 20 control individuals, had an estimated penetrance of 12.59%, a de novo rate of 32.5% and were reported with a broad range of neurodevelopmental phenotypes. Exact breakpoint determination revealed six recurrent intron 5 deletions.ConclusionExon 6–24 deletions have a high penetrance and are mainly associated with ID and schizophrenia. In contrast, the actual contribution of exon 1–5 deletions to a neurodevelopmental/neuropsychiatric disorder in an individual patient and family remains very difficult to assess. To enhance the clinical interpretation, this study provides practical considerations for counselling and an interactive table for comparing a deletion of interest with the available literature data.

Interstitial 14q31.3-q32.13 deletion

Aim: With the exception of ring chromosome 14 or translocations, interstitial deletions of the long arm of chromosome 14 are very rare. All patients with these deletions share common phenotypic characteristics, primarily mild dysmorphia and developmental delay. Molecular karyotyping (array CGH) enabled the precise breakpoint determination and improved the analysis of genotype-phenotype correlations. Case presentation: In a 7-year-old girl, array CGH was performed due to developmental delay. The array CGH study showed 8.3Mb de novo interstitial deletion of the 14q31.3–q32.13 region. Conclusions: Comparison of our patient´s phenotype with previously reported chromosome 14q interstitial deletion cases confirmed the presence of common clinical features and highlights the utility of array CGH as a diagnostic tool in clarifying the developmental delay etiology.

Optimizing Echinocandin Dosing and Susceptibility Breakpoint Determination viaIn VivoPharmacodynamic Evaluation against Candida glabrata with and withoutfksMutations

ABSTRACTEchinocandins are a preferred therapy for invasive candidiasis due to their potency and broad spectrum. Resistance, especially inCandida glabrata, is an emerging threat to their use. Pharmacodynamic (PD) studies examining reduced susceptibility secondary tofksmutations inC. glabrataare lacking. The current study explored PD targets for anidulafungin, caspofungin, and micafungin in anin vivoinvasive candidiasis model against 11C. glabrataisolates with known or putativefksmutations. The PD targets were compared to those of 8 wild-type (WT) isolates. The MIC ranges in the WT group were 0.03 to 0.25 mg/liter for anidulafungin, 0.03 to 0.25 mg/liter for caspofungin, and 0.01 to 0.06 mg/liter for micafungin. The MIC ranges for mutants were 0.06 to 4, 0.25 to 16, and 0.13 to 8 mg/liter for the same compounds, respectively. The mean free drug 24-h area under the concentration-time curve (AUCf)/MIC ratio associated with a stasis endpoint for the WT group was 13.2 for anidulafungin, 2.04 for caspofungin, and 6.78 for micafungin. Comparative values for mutants were 3.43, 2.67, and 0.90, respectively. Pharmacokinetic data from patients suggest that theC. glabrataPD targets needed for success in this model could be achieved based on MIC values of 0.25 mg/liter for anidulafungin, 2 mg/liter for caspofungin, and 0.5 mg/liter for micafungin. These values are higher than recently identified epidemiology cutoff values (ECVs). The results suggest that drug-specific MIC breakpoints could be increased for caspofungin and micafungin againstC. glabrataand could include organisms with mutations infks-1andfks-2. While identification of genetic mutants is epidemiologically important, the phenotype (MIC) provides a better predictor of therapeutic efficacy.