The oral splicing modifier RG7800 increases full length survival of motor neuron 2 mRNA and survival of motor neuron protein: Results from trials in healthy adults and patients with spinal muscular atrophy

Spinal muscular atrophy (SMA) is a rare genetic and progressively debilitating neuromuscular disease. It is the leading genetic cause of death among infants. In SMA, low levels of survival of motor neuron (SMN) protein lead to motor neuron death and muscle atrophy as the SMN protein is critical to motor neuron survival. SMA is caused by mutations in, or deletion of, the SMN1 gene. A second SMN gene, SMN2, produces only low levels of functional SMN protein due to alternative splicing which excludes exon 7 from most transcripts, generating truncated, rapidly degraded SMN protein. Patients with SMA rely on limited expression of functional SMN full-length protein from the SMN2 gene, but insufficient levels are generated. RG7800 is an oral, selective SMN2 splicing modifier designed to modulate alternative splicing of SMN2 to increase the levels of functional SMN protein. In two trials, oral administration of RG7800 increased in blood full-length SMN2 mRNA expression in healthy adults and SMN protein levels in SMA patients by up to two-fold, which is expected to provide clinical benefit.

Levofloxacin-Induced QTc Prolongation Depends on the Time of Drug Administration

Understanding the factors influencing a drug's potential to prolong the QTc interval on an electrocardiogram is essential for the correct evaluation of its safety profile. To explore the effect of dosing time on drug‐induced QTc prolongation, a randomized, crossover, clinical trial was conducted in which 12 healthy male subjects received levofloxacin at 02:00, 06:00, 10:00, 14:00, 18:00, and 22:00. Using a pharmacokinetic‐pharmacodynamic (PK‐PD) modeling approach to account for variations in PKs, heart rate, and daily variation in baseline QT, we find that the concentration‐QT relationship shows a 24‐hour sinusoidal rhythm. Simulations show that the extent of levofloxacin‐induced QT prolongation depends on dosing time, with the largest effect at 14:00 (1.73 (95% prediction interval: 1.56–1.90) ms per mg/L) and the smallest effect at 06:00 (−0.04 (−0.19 to 0.12) ms per mg/L). These results suggest that a 24‐hour variation in the concentration‐QT relationship could be a potentially confounding factor in the assessment of drug‐induced QTc prolongation.

Colistin: Revival of an Old Polymyxin Antibiotic

Colistin (polymyxin E) is a positively charged deca-peptide antibiotic that disrupts the integrity of the outer membrane of the cell wall of gram-negative bacteria by binding to the lipid A moiety of lipopolysaccharides, resulting in cell death. The endotoxic activity of lipopolysaccharides is simultaneously inhibited. Colistin is increasingly being prescribed as rescue treatment for infections with multidrug-resistant bacilli. Nephrotoxicity and, to a lesser degree, neurotoxicity occur often during systemic colistin therapy, and have severely limited its application in the past. However, these side effects are largely reversible and can be managed through close monitoring. The prodrug colistimethate sodium (CMS) is less toxic and is, therefore, the preferred formulation for parenteral administration. Importantly, resistance to colistin seems to emerge often unless it is combined with another antibiotic, but further studies into this phenomenon are necessary. Pharmacokinetic and pharmacodynamic properties have received little attention, partly because of the physicochemical peculiarities of polymyxin antibiotics, especially their propensity to stick to other molecules and surfaces. The ratio between the area under the curve of free colistin and the pathogen's Minimal Inhibitory Concentration (MIC) best predicts microbiological and clinical responses, but more studies are needed in this area. Likewise, further standardization is needed in production and labeling of colistin formulations, and in the way the susceptibility of bacteria to colistin is determined.

Identifying 24 h variation in the pharmacokinetics of levofloxacin: a population pharmacokinetic approach

AIM

The objective of this study was to investigate whether the pharmacokinetics of orally administered levofloxacin show 24 h variation. Levofloxacin was used as a model compound for solubility and permeability independent absorption and passive renal elimination.

METHODS

In this single centre, crossover, open label study, 12 healthy subjects received an oral dose of 1000 mg levofloxacin at six different time points equally divided over the 24 h period. Population pharmacokinetic modelling was used to identify potential 24 h variation in the pharmacokinetic parameters of this drug.

RESULTS

The pharmacokinetics of levofloxacin could be described by a one compartment model with first order clearance and a transit compartment to describe drug absorption. The fit of the model was significantly improved when the absorption rate constant was described as a cosine function with a fixed period of 24 h, a relative amplitude of 47% and a peak around 08.00 h in the morning. Despite this variation in absorption rate constant, simulations of a once daily dosing regimen showed that tmax , Cmax and the area under the curve at steady-state were not affected by the time of drug administration.

CONCLUSION

The finding that the absorption rate constant showed considerable 24 h variation may be relevant for drugs with similar physicochemical properties as levofloxacin that have a narrower therapeutic index. Levofloxacin, however, can be dosed without taking into account the time of day, at least in terms of its pharmacokinetics.

[Practical guideline for the use of colistin]

Colistin (polymyxin E) binds to the cell wall of gram-negative bacteria, leading to osmotic destruction of the cell. Since its introduction in 1959, colistin has been little used parenterally due to a high incidence of reversible nephrotoxicity and, to a lesser extent, neurotoxicity. Colistin use remained limited to combating Pseudomonas aeruginosa in cystic fibrosis patients. In addition, oral colistin is part of the recently introduced regime of selective digestive tract decontamination in ICU patients. Intravenous administration of colistin is now increasingly prescribed for the control of multi-resistant microorganisms. Colistin monotherapy, however, rapidly selects resistant subpopulations. Therefore, only combination therapy is advised. The prodrug colistimethate sodium is less toxic and is hydrolyzed in vivo to active colistin; colistin is renally cleared. Clinical practice remains hampered by lack of uniformity and standardization of names, dosage units, dosing recommendations and methods of concentration and susceptibility testing.

Exosomal transmission of functional aquaporin 2 in kidney cortical collecting duct cells

Exosomes are vesicles released following fusion of endosomes with the plasma membrane. Urine contains exosomes that are released from the entire length of the nephron and change in composition with kidney disease. Exosomes can shuttle information between non-renal cells via transfer of protein and RNA. In this study murine kidney collecting duct (mCCDC11) cells were used to demonstrate that exosomes can act as a signalling mechanism between cells. First, the release of exosomes by mCCDC11 cells was confirmed by multiple approaches. Following isopynic centrifugation, exosomal proteins flotillin-1 and TSG101 were identified in fractions consistent with exosomes. Electron microscopy demonstrated structures consistent in size and shape with exosomes. Exposure of mCCDC11 cells to the synthetic vasopressin analogue, desmopressin, did not affect exosomal flotillin-1 or TSG101 but increased aquaporin 2 (AQP2) in a dose- and time-dependent manner that was highly correlated with cellular AQP2 (exosomal AQP2 vs. cellular AQP2, Pearson correlation coefficient r = 0.93). To test whether the ratio of exosomal AQP2/flotillin-1 is under physiological control in vivo, rats were treated with desmopressin. The ratio of AQP2/flotillin-1 in the urinary exosome was significantly increased. Inter-cellular signalling by exosomes was demonstrated: exosomes from desmopressin-treated cells stimulated both AQP2 expression and water transport in untreated mCCDc11 cells (water flow across cells: control exosome treatment 52.8 ± 11 μl cm(-2); AQP2-containing exosomes 77.4 ± 4 μl cm(-2), P = 0.05, n = 4). In summary, the amount of AQP2 in exosomes released from collecting duct cells is physiologically regulated and exosomal AQP2 closely reflects cellular expression. Exosomes can transfer functional AQP2 between cells and this represents a novel physiological mechanism for cell-to-cell communication within the kidney.