Possible false-negative results on therapeutic drug monitoring of phenytoin using a particle enhanced turbidimetric inhibition immunoassay in a patient with a high level of IgM.

: In this report, the authors described the unusual case of a patient in whom the plasma phenytoin concentration was unexpectedly not detected on a particle-enhanced turbidimetric inhibition immunoassay (PETINIA) technique, a typical immunoassay for phenytoin. The plasma concentration was measured using PETINIA and high-performance liquid chromatography in a 69-year-old male patient treated with fosphenytoin intravenously at the standard dose for 7 days. Although the plasma concentration of phenytoin was below the limit of detection (<0.5 mcg/mL) on PETINIA after the administration of fosphenytoin, the trough plasma concentration was estimated to be between 5 and 10 mg/L on high-performance liquid chromatography. When the plasma concentrations of IgM and IgG were measured using an enzyme-linked immunosorbent assay, the plasma IgG level was within the reference range, whereas the plasma IgM level was 2-3 times higher than the upper limit of the reference range. We concluded that the PETINIA method yielded a possible false-negative result regarding the phenytoin level in this patient, perhaps because of some hindrance to the measurement process by IgM. This case suggests that false-negative results should be considered when therapeutic drug monitoring reveals abnormally low values using PETINIA and that it is necessary to evaluate the plasma IgM level.

Superior plasma retention of a cross-linked human serum albumin dimer in nephrotic rats as a new type of plasma expander.

Human serum albumin (HSA) is used clinically as a plasma expander in patients with hypoalbuminemia and can also function as a drug carrier. However, the administered HSA is readily eliminated from the blood circulation under pathological conditions, especially the nephrotic syndrome. In this study, we present data on the pharmacokinetics of a structurally defined HSA dimer [two HSA molecules that are cross-linked by reaction with 1,6-bis(maleimido)hexane via Cys34] in nephrotic rats and its superior circulation persistence, owing to the molecular size effect. The half-time (t(1/2)) of the HSA dimer persisted in the circulation 1.3 times longer than that of monomeric HSA in normal rats, primarily because of the suppression of the accumulation of the HSA dimer in the skin and muscle. In nephrotic rats, the t(1/2) of the HSA monomer decreased considerably, whereas the HSA dimer remained unaltered in the blood stream, similar to that for normal rats. As a result, the t(1/2) of the HSA dimer was 2-fold longer than that of the HSA monomer. This longer t(1/2) can be attributed to the fact that accumulation in the kidney and urinary excretion of the HSA dimer were significantly suppressed. The cross-linked HSA dimer shows a longer blood circulation than native HSA monomer in nephrotic rats, which can be attributed to the suppression of renal filtration and leakage into the extravascular space. This HSA dimer has the potential for use as a drug carrier, new plasma expander, and an artificial albumin-based oxygen carrier under a high glomerular permeability condition such as nephrosis.

Pharmacokinetic study of enclosed hemoglobin and outer lipid component after the administration of hemoglobin vesicles as an artificial oxygen carrier.

The hemoglobin vesicle (HbV) is an artificial oxygen carrier that encapsulates a concentrated Hb solution in lipid vesicles (liposomes). The pharmacokinetic properties of HbV were investigated in mice and rats. With use of HbV in which the internal Hb was labeled with (125)I ((125)I-HbV) and cell-free (125)I-Hb, it was found that encapsulation of Hb increased the half-life by 30 times, accompanied by decreased distribution in both the liver and kidney. The half-life of HbV was increased, and the uptake clearance for the liver and spleen were decreased with increasing doses of HbV. In an in vitro study, the specific uptake and degradation of HbV in RAW 264.7 cells were found, but this was not the case for parenchymal and endothelial cells. The pharmacokinetics of HbV components (internal Hb and liposomal lipid) were also investigated using (125)I-HbV and (3)H-HbV (liposomal cholesterol was radiolabeled with tritium-3). The time courses for the plasma concentration curves of (125)I-HbV, (3)H-HbV, and iron derived from HbV suggest that HbV maintain an intact structure in the blood circulation up to 24 h after injection. (125)I-HbV and (3)H-HbV were distributed mainly to the liver and spleen. Internal Hb disappeared from both the liver and spleen 5 days after injection, and the liposomal cholesterol disappeared at approximately 14 days. Internal Hb was excreted into the urine and cholesterol into feces via biliary excretion. These results suggest that the HbV has a reasonable blood retention and metabolic and excretion performance and could be used as an oxygen carrier.

Hemoglobin vesicles, polyethylene glycol (PEG)ylated liposomes developed as a red blood cell substitute, do not induce the accelerated blood clearance phenomenon in mice.

The hemoglobin vesicle (HbV) is an artificial oxygen carrier encapsulating a concentrated hemoglobin solution in a liposome of which the surface is covered with polyethylene glycol (PEG). It was recently reported that repeated injections of PEGylated liposomes induce the accelerated blood clearance (ABC) phenomenon, in which serum anti-PEG IgM plays an essential role. To examine this issue, we investigated whether HbV induces the ABC phenomenon in mice at a dose of 0.1 mg Hb/kg, a dose that is generally known to induce the ABC phenomenon, or at 1400 mg Hb/kg, which is proposed for clinical use. At 7 days after the first injection of nonlabeled HbV (0.1 mg Hb/kg), the mice received HbV in which the Hb had been labeled with (125)I. After a second injection, HbV was rapidly cleared from the circulation, and uptake clearances in liver and spleen were significantly increased. In contrast, at a dose of 1400 mg Hb/kg, the pharmacokinetics of HbV was negligibly affected by repeated injection. It is interesting to note that IgM against HbV was produced 7 days postinjection at both of the above doses, and their recognition site was determined to be 1,2-distearoyl-sn-glycero-3-phosphatidylethanolamine-N-PEG in HbV. These results suggest that a clinical dose of HbV does not induce the ABC phenomenon, and that suppression of ABC phenomenon is caused by the saturation of phagocytic processing by the mononuclear phagocyte system. Thus, we conclude that induction of the ABC phenomenon would not be an issue in the dose regimen used in clinical settings.

Cross-linked human serum albumin dimer has the potential for use as a plasma-retaining agent for the fatty acid-conjugated antidiabetic drugs.

OBJECTIVES: The half-life of fatty acid-conjugated antidiabetic drugs are prolonged through binding to albumin, but this may not occur in diabetic patients with nephropathy complicated with hypoalbuminemia. We previously showed that human serum albumin (HSA) dimerized at the protein's Cys34 by 1,6-bis(maleimido)hexane has longer half-life than the monomer under high permeability conditions. The aim of this study was to investigate the superior ability of this HSA dimer as a plasma-retaining agent for fatty acid conjugated antidiabetic drugs. METHODS: The diabetic nephropathy rat model was prepared by administering a single injection of streptozotocin (STZ) intravenously, and the pharmacokinetic properties of HSA monomer and dimer were evaluated. Site-specific fluorescent probe displacement experiments were performed using warfarin and dansylsarcosine as site I and site II specific fluorescent probes, respectively. KEY FINDINGS: The half-life of the HSA dimer in STZ-induced diabetic nephropathy model rats was 1.5 times longer than the HSA monomer. The fluorescent probe displacement experiment results for HSA monomer and dimer were similar, where fatty acid-conjugated antidiabetic drugs displaced dansylsarcosine but not warfarin in a concentration-dependent manner. CONCLUSIONS: The HSA dimer shows potential for use as a plasma-retaining agent for antidiabetic drugs due to its favourable pharmacokinetic properties.