MR imaging properties and pharmacokinetics of MnDPDP in healthy volunteers

Purpose: Thirteen male volunteers were studied to evaluate the MR imaging properties and pharmacokinetics of 10 mM mangafodipir trisodium infusion (MnDPDP, Teslascan).

Material and Methods: Doses of 5 and 10 μmol/kg b.w. were administered by bolus injection (<1 min) to 5 subjects, and by infusion (20 min) to 8 subjects, with a 3-week wash-out between doses. Infusion subjects underwent MR imaging.

Results: At 1 h after infusion, the plasma concentration of Mn was reduced to ∼15% of the maximum value. Fifteen to 20% of Mn was recovered in the urine, and 50–60% was recovered in the faeces. The rapid initial plasma clearance of Mn is consistent with both rapid tissue uptake and rapid renal elimination. Increases in signal intensity were apparent on T1-weighted images of the liver, pancreas, spleen, renal cortex and the renal medulla, but not in regions of the brain protected by an intact blood-brain barrier. Increases were seen in the choroid plexus and pituitary. Contrast-related adverse events, only flushing of moderate intensity, occurred in bolus injection subjects.

Conclusion: At 5 and 10 μmol/kg, mangafodipir produces relatively long-lasting enhancement of several abdominal organs, including the liver, pancreas and kidney.

Plasma pharmacokinetics, tissue distribution and excretion of MnDPDP in the rat and dog after intravenous administration

Purpose: To investigate distribution and excretion of mangafodipir (MnDPDP, Teslascan) in the rat and dog.

Material and Methods: Formulations of either 14C-MnDPDP or 54MnDPDP were injected intravenously at near clinical doses in rats and dogs.

Results: The manganese (Mn) moiety is rapidly removed from plasma with an elimination half-life of less than 25 min in both species, reflecting a rapid distribution to the tissues and an early excretion. The plasma clearance of the DPDP moiety is slower than that of Mn and it appears to distribute into the extracellular fluid. Mn is distributed largely to the liver, pancreas and kidneys, and in pregnant rats, also to foetal liver and bones. No transplacental passage of DPDP could be detected. The metal is mainly excreted by the faecal route, with a small fraction eliminated early in the urine. DPDP is rapidly and essentially completely excreted in the urine, consistent with the glomerular filtration rate.

Conclusion: The ligand does not appear to facilitate the transport of Mn into any organ except the kidney for subsequent excretion, and it reduces distribution to the heart. The Mn is taken up by those organs indicated for MR imaging, primarily liver and pan-creas.

Metabolism and pharmacokinetics of MnDPDP in man

Purpose: To study the metabolism and pharmacokinetics of mangafodipir trisodium injection, 0.01 mmol/ml (Teslascan), in healthy male volunteers.

Material and Methods: Eight volunteers received mangafodipir trisodium as an infusion over 20 min, and 5 received it as an injection (≤ min). Both groups received 5 and 10 μmol/kg b.w. with a wash-out period of 3 weeks between doses. Metabolites were measured in plasma, total manganese and zinc were measured in plasma and urine and total manganese was measured in faeces.

Results: The parent compound MnDPDP (manganese dipyridoxyl diphosphate) and 5 metabolites; MnDPMP (manganese dipyridoxyl monophosphate), MnPLED (manganese dipyridoxyl ethylenediamine) and the corresponding zinc compounds ZnDPDP, ZnDPMP and ZnPLED, were detected in plasma. ZnPLED was the only detectable metabolite 8 h after dosing. The apparent volume of distribution of manganese exceeded the interstitial body fluids. The volume of distribution of the ligand indicated distribution to the extracellular fluid only, with the plasma clearance close to the glomerular filtration rate. The manganese was incompletely excreted during the 4 days after treatment with the major part in faeces and less than 20% of the dose in the urine.

Conclusion: Dephosphorylation and simultaneous transmetallation with zinc are the main metabolic pathways of MnDPDP in man.

NMR relaxation studies with MnDPDP

Purpose: Our studies were designed to compare the efficacy of mangafodipir triso-dium (MnDPDP, Teslascan) as a tissue-specific MR agent with that of manganese chloride (MnCl2), to compare the efficacy of different doses and rates of administration of MnDPDP, and to collect the data needed for predicting optimum pulse sequences.

Material and Methods: The dose response for the relaxation rates R1 and R2 at 0.47 T, and the manganese (Mn) concentrations in rat liver and in the liver, pancreas, heart and adrenals of pigs was determined for both MnDPDP and MnCl2 administered i.v. Computer simulations were carried out to model the effects of different tissue Mn concentrations and TR on signal intensities and contrast-to-noise ratios.

Results: In rat liver and pig organs both compounds produced a positive dose-response in R1 and tissue Mn concentration, and only small or no response in R2. The Mn concentration in rat liver was positively correlated with R1, regardless of the form in which Mn was given, or the rate of administration. Optimal imaging parameters are therefore expected to be different pre- and post-MnDPDP administration.

Conclusion: The added cardiovascular safety of MnDPDP compared with MnCl2 does not result in loss of efficacy in increasing R1 at the intended clinical dose of 5 μmol/kg MnDPDP. The changes in R2 were too small to affect T2-weighted images. The data give the basis for choosing the appropriate pulse sequences for MnDPDP-en-hanced MR imaging.

Comparison of manganese biodistribution and MR contrast enhancement in rats after intravenous injection of MnDPDP and MnCl2

Purpose: To compare the time course of the MR enhancing properties and biodistri-bution of manganese (Mn) in rats given i.v. Mn dipyridoxyl diphosphate (MnDPDP) or Mn chloride (MnCl2).

Material and Methods: Twenty-four adult rats were injected i.v. with 5 μmol/kg MnDPDP or MnCl2, or with 0.5 ml/kg saline. High resolution T1-weighted MR imaging was performed during early (10 min), mid (2 h) and late (24 h) phases after injection. Mn concentrations in major organs were measured by using an ICP-AES technique, and correlated with MR findings.

Results: Variable degrees of signal enhancement of major organs observed in MR images corresponded with the amount of Mn uptake after injection of MnDPDP or MnCl2. A prominently lower cardiac, pancreatic and hepatic uptake of Mn was seen at 10 min in rats injected with MnDPDP compared with those given MnCl2 and this was reflected in a difference in signal intensity (SI) in the MR images. At 2 h, the Mn content and SI in the major organs were similar with both MnDPDP and MnCl2. An overall Mn clearance was achieved at 24 h without any important organ retention, with kidney excretion of Mn seen only with MnDPDP.

Conclusion: With both MnDPDP and MnCl2, the Mn uptake correlates with the SI enhancement in tissues. The reduced initial cardiac uptake of Mn after MnDPDP treatment compared to MnCl2 may account for the favourable cardiovascular safety of the contrast agent. These data contribute to an understanding of SI enhancement by MnDPDP, and are consistent with other studies showing that at a dose of 5 μmol/kg, MnDPDP can be safely used as a potent MR organ-specific contrast agent.

General toxicology of MnDPDP

Purpose: To investigate the general toxicology of mangafodipir trisodium (MnDPDP, Teslascan).

Material and Methods: Studies were performed in accordance with standard methods and in compliance with regulations current at the time of conduct.

Results: Single-dose studies in rodents and dogs showed that MnDPDP was tolerated at doses of approximately 2000 μmol/kg, approximately 400 times a single imaging dose of 5 μmol/kg. The single dose tolerance of MnDPDP was approximately 10 times greater than MnCl2. A good safety profile of MnDPDP was also shown in repeat-dose studies (3 weeks), in which the no-observed-adverse-effect level for the rat, monkey and dog was 116, 29 and 10 μmol/kg, respectively. The local tolerance studies indicated that no adverse local tissue reactions are likely to occur after i.v. injection. Other studies indicate that accidental spillage of MnDPDP onto the skin is not expected to lead to significant systemic exposure, or to local irritation or hypersensitivity. MnDPDP was not genotoxic in a battery of several different tests.

Conclusion: MnDPDP was shown to have a good safety profile suitable as an hepatobiliary MR contrast agent for i.v. administration.

Tissue distribution and general safety of MnDPDP in male beagle dogs, with or without total common bile duct obstruction

Purpose: Evaluation of the tissue distribution of manganese (Mn) and general safety in normal and cholestatic male beagle dogs after i.v. administration of mangafodipir trisodium (MnDPDP, Teslascan).

Material and Methods: Male beagle dogs, with or without surgical obstruction of the common bile duct, received a single i.v. bolus injection of saline (control), or MnDPDP at doses of 10 or 50 μmol/kg b.w. and were sacrificed 1 or 7 days after treatment. Tox-icity was assessed and tissue concentrations of Mn were measured.

Results: Increased tissue Mn concentrations were found in all dogs treated with MnDPDP and were greatest in those with biliary obstruction. Although Mn concentrations decreased with time in most tissues in each of the treated groups, this was not the case for the brain and adrenal glands in dogs with total biliary obstruction in which further increases in Mn concentrations were seen at the later time point. This suggested a re-distribution of Mn from the major body depots such as the liver. There were no effects of MnDPDP on clinical sign/behaviour, organ weights, histomorphology or clinical biochemistry.

Conclusion: These findings indicate that a single clinical dose of 5 μmol/kg MnDPDP is likely to be well tolerated in patients with cholestasis.

Biomedical aspects of pyridoxal 5'-phosphate availability

The biologically active form of vitamin B6, pyridoxal 5'-phosphate (PLP), is a cofactor in over 160 enzyme activities involved in a number of metabolic pathways, including neurotransmitter synthesis and degradation. In humans, PLP is recycled from food and from degraded PLP-dependent enzymes in a salvage pathway requiring the action of pyridoxal kinase, pyridoxine 5'-phosphate oxidase and phosphatases. Once pyridoxal 5'-phosphate is made, it is targeted to the dozens different apoenzymes that need it as a cofactor. The regulation of the salvage pathway and the mechanism of addition of PLP to the apoenzymes are poorly understood and represent a very challenging research field. Severe neurological disorders, such as convulsions and epileptic encephalopathy, result from a reduced availability of pyridoxal 5'-phosphate in the cell, due to inborn errors in the enzymes of the salvage pathway or other metabolisms and to interactions of drugs with PLP or pyridoxal kinase. Multifactorial neurological pathologies, such as autism, schizophrenia, Alzheimer's disease, Parkinson's disease and epilepsy have also been correlated to inadequate intracellular levels of PLP.

Pharmacokinetic comparison of a delayed-release combination of doxylamine succinate and pyridoxine hydrocholoride (Diclectin) and oral solutions of these drugs in healthy women of childbearing age

BACKGROUND

The delayed-release combination of doxylamine succinate and pyridoxine hydrochloride was the most commonly used antiemetic (Bendectin) approved by FDA for nausea and vomiting of pregnancy (NVP) until its removal of the market in 1983. The drug is widely used today in Canada (Diclectin). The pharmacokinetics of Diclectin has never been described in humans.

OBJECTIVES

To compare the pharmacokinetics of Diclectin to oral solutions of its two components.

SUBJECTS AND METHODS

A randomized, cross over, open label design, comparing the pharmacokinetics of Diclectin to those of the oral solutions of the two components in 18 healthy adult, non pregnant women of childbearing age.

RESULTS

Diclectin exhibited similar oral bioavailability to those of the oral solutions. In contrast, the time-to-peak, (Tmax), reflecting the rate of absorption, was 3-6 times longer for the two components of the delayed-release drug confirming its delayed-release characteristics.

CONCLUSION

The pharmacokinetic profile of Diclectin well explains its documented delayed efficacy.

Contrast enhanced cartilage imaging: Comparison of ionic and non-ionic contrast agents

Our objective was to compare relaxation effects, dynamics and spatial distributions of ionic and non-ionic contrast agents in articular cartilage at concentrations typically used for direct MR arthrography at 1.5T. Dynamic MR-studies over 11h were performed in 15 bovine patella specimens. For each of the contrast agents gadopentetate dimeglumine, gadobenate dimeglumine, gadoteridol and mangafodipir trinatrium three patellae were placed in 2.5mmol/L contrast solution. Simultaneous measurements of T(1) and T(2) were performed every 30min using a high-spatial-resolution "MIX"-sequence. T(1), T(2) and DeltaR(1), DeltaR(2) profile plots across cartilage thickness were calculated to demonstrate the spatial and temporal distributions. The charge is one of the main factors which controls the amount of the contrast media diffusing into intact cartilage, but independent of the charge, the spatial distribution across cartilage thickness remains highly inhomogeneous even after 11h of diffusion. The absolute DeltaR(2)-effect in cartilage is at least as large as the DeltaR(1)-effect for all contrast agents. Maximum changes were 5-12s(-1) for DeltaR(1) and 8-15s(-1) for DeltaR(2). This study indicates that for morphologically intact cartilage only the amount of contrast agents within cartilage is determined by the charge but not the spatial distribution across cartilage thickness. In addition, DeltaR(2) can be considered for quantification of contrast agent concentrations, since it is of the same magnitude and less time consuming to measure than DeltaR(1).

An apparent unidirectional influx constant for manganese as a measure of myocardial calcium channel activity

PURPOSE

To develop an in vivo MR method for evaluation of myocardial calcium channel activity through quantification of apparent unidirectional manganese influx constants following manganese dipyridoxyl-diphosphate (MnDPDP) infusions.

MATERIALS AND METHODS

A total of 10 healthy volunteers were divided in two groups, and received 5 micromol of MnDPDP per kg of body weight intravenously in a 1.5 Tesla scanner over five or 30 minutes, respectively. A fast inversion recovery gradient echo sequence was used to estimate pre- and postcontrast R1 values and to measure signal changes following infusions. By assuming equal longitudinal relaxivity (r1) of the contrast in all tissue compartments, signal changes in blood and myocardial tissue yielded temporal input and tissue contrast concentrations respectively. Through a two-tissue compartment model, apparent unidirectional influx constants (Ki) for myocardial manganese accumulation were estimated.

RESULTS

Consistent values for Ki in left ventricular wall were found, with a mean value of 5.96 mL/100 g/minute (SD=0.49; N=10). No statistical significant differences in Ki were found between the two infusion groups.

CONCLUSION

Since unidirectional manganese accumulation depends upon intact myocyte membranes with functioning calcium channels, the use of unidirectional manganese influx rates may be a valuable research tool for in vivo studies of myocyte functioning in myocardial disease.

MC-1 (pyridoxal 5'-phosphate): novel therapeutic applications to reduce ischaemic injury

Despite the overall efficacy of mechanical reperfusion therapies, such as percutaneous coronary intervention and coronary artery bypass graft surgery, in reducing the morbidity and mortality that is associated with acute ischaemic syndromes, many of the treated patients develop ischaemia-reperfusion injury due to impaired microvascular integrity, embolisation of atherothrombotic debris and/or disrupted end-organ metabolism. MC-1 is an investigational drug from Medicure, Inc. In preclinical models of ischaemia and ischaemia-reperfusion injury, treatment with MC-1 has demonstrated significant cardio- and neuroprotective effects. Although the pharmacological activity of MC-1 may involve multiple mechanisms, research suggests that at least part of the protective effect may be mediated through its actions on purinergic receptors. Early clinical experience with MC-1 also appears to be promising: in a recent Phase II evaluation, treatment with MC-1 was associated with a statistically significant reduction in periprocedural infarct size (as measured by area under the curve creatine kinase-myocardial band) among high-risk patients undergoing elective percutaneous coronary intervention. Based on these findings, larger, randomised trials to confirm the safety and efficacy of MC-1 in the setting of coronary artery revascularisation with coronary artery bypass graft, acute coronary syndromes and stroke are ongoing or in development. These forthcoming evaluations should clarify the safety and efficacy of MC-1 and improve the understanding regarding its potential therapeutic role in a variety of clinical settings and indications.

Effects of consecutive high-dose alcohol administration on the utilization of sulfur-containing amino acids by rats

In this study, we attempted to evaluate changes in sulfur-containing amino acid (SCAA) metabolism after short-term high-dose alcohol ingestion. At the beginning of the study, six animals were sacrificed as the baseline group and then other animals in the experiment were consecutively gavaged with alcohol (30%, 3 g/kg) for 7 days. Animals (n=6 each) were subsequently sacrificed at the time points of Days 1 (Group E1), 3 (Group E3) and 7 (Group E7). Blood samples and selected tissues were collected at each time interval. SCAA, pyridoxal phosphate (PLP) and glutathione (GSH) levels were analyzed. Results showed that taurine levels of tissues (brain, liver, heart and kidneys) all declined after the ethanol intervention and continued to decrease in selected tissues except the brain during the experiment. Furthermore, the trends of plasma taurine and PLP contents were highly correlated (r=.98, P=.045). A similar utilization pattern of plasma taurine and PLP indicated that transsulfuration preferred taurine production to GSH synthesis. The trend of plasma taurine levels being positively correlated with PLP levels reveals that dramatic transsulfuration occurred to meet the urgent demand for taurine by brain cells. In conclusion, we reported that continual alcohol ingestion alters SCAA utilization, especially by depletion of taurine and hypotaurine and by elevation of S-adenosyl homocysteine in the selected organs.

Relaxation enhancing properties of MnDPDP in human myocardium

PURPOSE

To assess magnitude and duration of changes in myocardial longitudinal relaxation rate (R1) in humans following infusion of the manganese (Mn) releasing contrast agent MnDPDP (Mn-dipyridoxyl-diphosphate).

MATERIALS AND METHODS

Fifteen healthy volunteers were divided into three groups. After initial myocardial and liver R1 measurements using an inversion recovery (IR) turbo fast low-angle shot (FLASH) sequence at 1.5 Tesla, the groups were given different doses of intravenous MnDPDP: 5, 10 and 15 micromol/kg body weight, respectively, over 30 minutes. R1 measurements were then repeated at 1, 2, 4, 8, and 24 hours after the infusion ended.

RESULTS

The left ventricular wall R1 prevalue was 0.98 second(-1) (+/-0.04). R1 increased on average (all 15 subjects) 0.41 second(-1) (+/-0.09). The increase was present one hour after the end of the infusion, remained relatively constant the next two hours, and then declined gradually. After 24 hours, there was still a moderate R1 elevation present, with an average R1-value of 1.16 (+/-0.05). There were only small differences in myocardial R1 responses between the three doses investigated, which was contrasted by a marked dose-response in liver tissue.

CONCLUSION

MnDPDP gave a significant and prolonged rise in myocardial R1 even at a dose of 5 micromol/kg. The R1-values in the myocardium did not increase linearly with higher doses.

Manganese toxicity upon overexposure

Manganese (Mn) is a required element and a metabolic byproduct of the contrast agent mangafodipir trisodium (MnDPDP). The Mn released from MnDPDP is initially sequestered by the liver for first-pass elimination, which allows an enhanced contrast for diagnostic imaging. The administration of intravenous Mn impacts its homeostatic balance in the human body and can lead to toxicity. Human Mn deficiency has been reported in patients on parenteral nutrition and in micronutrient studies. Mn toxicity has been reported through occupational (e.g. welder) and dietary overexposure and is evidenced primarily in the central nervous system, although lung, cardiac, liver, reproductive and fetal toxicity have been noted. Mn neurotoxicity results from an accumulation of the metal in brain tissue and results in a progressive disorder of the extrapyramidal system which is similar to Parkinson's disease. In order for Mn to distribute from blood into brain tissue, it must cross either the blood-brain barrier (BBB) or the blood-cerebrospinal fluid barrier (BCB). Brain import, with no evidence of export, would lead to brain Mn accumulation and neurotoxicity. The mechanism for the neurodegenerative damage specific to select brain regions is not clearly understood. Disturbances in iron homeostasis and the valence state of Mn have been implicated as key factors in contributing to Mn toxicity. Chelation therapy with EDTA and supplementation with levodopa are the current treatment options, which are mildly and transiently efficacious. In conclusion, repeated administration of Mn, or compounds that readily release Mn, may increase the risk of Mn-induced toxicity.

Uptake of MnCl2and mangafodipir trisodium in the myocardium: A magnetic resonance imaging study in pigs

PURPOSE

To examine the changes in the longitudinal relaxation times (DeltaR1) induced in pig myocardium and blood following injections of 5, 10, and 15 micromol mangafodipir trisodium (Mn-DPDP) or MnCl2/kg of body weight (b.w.).

MATERIALS AND METHODS

Twelve pigs were divided into two groups, one group receiving MnCl2 and the other receiving Mn-DPDP. Three consecutive doses of contrast agent (5, 10, and 15 micromol/kg of b.w.) were injected in each animal with a 40-minute time interval between each dose. Measurements of T1 in blood and myocardium were made 5, 15, 25, and 35 minutes after each injection. Additionally, relaxivity measurements in blood samples were performed.

RESULTS

An increase in myocardial R1 was observed for both contrast agents at all concentration levels tested. This increase peaked 5 minutes after injection and then declined. An increase could still be detected 35 minutes after injection. The effect was larger when using MnCl2 than when using Mn-DPDP.

CONCLUSION

The dissociation kinetics of Mn2+ from the DPDP ligand limits the relaxation increase of Mn-DPDP relative to that of MnCl2. On the other hand, the toxicity of MnCl2 may exclude it from clinical use.

Biliary tract depiction in living potential liver donors: comparison of conventional MR, mangafodipir trisodium-enhanced excretory MR, and multi-detector row CT cholangiography--initial experience

PURPOSE

To compare biliary tract depiction in living potential liver donors at conventional magnetic resonance (MR), mangafodipir trisodium-enhanced excretory MR, and multi-detector row computed tomographic (CT) cholangiography.

MATERIALS AND METHODS

Eight living potential liver donors underwent iodipamide meglumine-enhanced CT cholangiography. Eight different potential liver donors then underwent conventional MR cholangiography and mangafodipir trisodium-enhanced excretory MR cholangiography. Two readers independently scored all first-, second-, and third-order biliary branches with a four-point scale from 0 (not seen) to 3 (excellent visualization). Interobserver agreement was calculated by using the weighted kappa statistic. Scores were compared between imaging modalities by using generalized estimating equations. Imaging findings of second-order biliary tract anatomy were compared with intraoperative findings for nine patients.

RESULTS

Interobserver agreement for overall biliary tract visualization was good for CT, conventional MR, and excretory MR cholangiography (with weighted kappa values of 0.76, 0.66, and 0.79, respectively). The mean second-order biliary branch visualization scores for readers 1 and 2, respectively, were significantly higher at CT cholangiography (2.81 and 2.75) than at conventional MR (1.84 and 1.75, P <.001), excretory MR (2.00 and 2.06, P <.001), and combined conventional and excretory MR cholangiography (2.31 and 2.25, P <.01). At CT, conventional MR, and excretory MR cholangiography, respectively, second-order biliary branching anatomy was discernible in eight, five, and seven patients, with second-order biliary branch variants seen in three, two, and two patients. Surgical findings confirmed the pattern of second-order biliary branching seen at CT in five patients, that seen at conventional MR imaging in one patient, and that seen at excretory MR cholangiography in three patients. At surgery, one case of variant biliary anatomy was found to have been missed at CT cholangiography.

CONCLUSION

In living potential liver donors, CT cholangiography enables significantly better biliary tract visualization than conventional or excretory MR cholangiography either alone or in combination.

[Organ specific MRI contrast media in general practice]

Conventional MR imaging with non-specific gadolinium contrast agents is useful in the detection and staging of malignancies. With the development of organ-specific MR contrast agents, MR imaging is assuming an even larger role in body imaging. Two different classes of liver-specific agents, hepato-biliary agents targeted to the hepatocytes, and reticulo-endothelial agents (or Kupffer cell agents) targeted to the reticuloendothelial system in the liver and spleen, have been developed. MRI with liver-specific contrast agents is superior to helical CT for detection of liver metastases. Mangafodipir, which was originally developed as a liver-specific agent, has been shown to accumulate in the pancreas as well. Recent studies have revealed that mangafodipir-enhanced MRI is at least equal to spiral CT in the staging of pancreatic cancer and may be superior to CT in the detection of small cancers. Blood-pool contrast agents with a long blood half-life, such as ultrasmall iron oxide particles (USPIO), have been developed for MR angiography. After the clearance from the vascular system, USPIO are taken up by phagocytes and accumulate in normal lymph, but not in metastatic nodes. This effect can be exploited to increase the accuracy of MRI in lymph node staging of cancer of head-neck, lung or pelvis. In conclusion, various organ-specific MR contrast agents with a wide range of clinical applications have been developed. Future studies will demonstrate the role of contrast-enhanced MRI in comparison with multi-detector array CT.

T1 efficacy of EVP-ABD: a potential manganese-based MR contrast agent for hepatic vascular and tissue phase imaging

PURPOSE

To evaluate the T1 efficacy of EVP-ABD, a new manganese (Mn)-based contrast agent, for vascular and liver tissue enhancement in comparison with currently approved agents.

MATERIALS AND METHODS

Ten Yorkshire pigs (body weight, 26 -46 kg) were used for the efficacy evaluation, nine for kinetic T1 evaluation (three each agent) and one for post EVP-ABD imaging. With a fast imaging scheme to monitor T1 values of blood and liver, 10 micromol/kg EVP-ABD was injected intravenously and compared with gadopentetate dimeglumine (Magnevist, GdDTPA) and mangafodipir trisodium (Teslascan, mangafodipir trisodium) at routine clinical dosages. All were imaged with 3D T1 Gradient Recalled Echo (GRE) sequence (TR/TE/alpha = 3.8/1.6/25 degrees ) prior to and 10 minutes post injection using a 1.5-T whole-body scanner. Additional high-resolution 2D liver images (TR/TE/alpha = 50/4.6/40 degrees ) and arterial phase images of the upper aorta were acquired from the pig for post EVP-ABD imaging.

RESULTS

At 10 micromol/kg, EVP-ABD provided a dramatic decline in blood T1, comparable to 0.1 mmol/kg GdDTPA, followed by a rapid return to blood baseline T1 values. In addition to the blood enhancement phase, EVP-ABD achieved a 70% reduction in liver T1 within 2 minutes postadministration, with an imaging window of at least 2 hours. A substantially improved signal-to-noise ratio (SNR) was observed in both the 2D and 3D liver images postcontrast.

CONCLUSION

EVP-ABD demonstrated peak vascular enhancement similar to GdDTPA and prolonged specific liver enhancement exceeding mangafodipir trisodium. EVP-ABD has favorable T1 enhancing characteristics with the potential to allow for a comprehensive liver evaluation.

Role of exocrine cells in pancreatic enhancement using Mn-DPDP-enhanced MR imaging

OBJECTIVE

To investigate role of exocrine cells in the pancreatic enhancement images at Manganese (II) N, N'-dipyridoxylethlenediamine-N, N'-diacetate 5, 5'-bisc (Mn-DPDP)-enhanced magnetic resonance (MR) imaging.

METHODS

Artificial pancreatic leakage was constructed in six dogs using a fistula tube inserted into the duodenum papillae. Pancreatic juice was collected before and after intravenous infusion of 2 ml/kg of Mn-DPDP at a rate of 2 - 3 ml/min. The Mn content of pancreatic juice was measured by atomic absorption spectroscopy. T(1)-weighted spin-echo images and T(1)-weighted spoiled phase gradient-echo (SPGR) images were obtained prior and approximately 30 min after the administration of Mn-DPDP at 1.5T.

RESULTS

The Mn content of pancreatic secretion increased 60.47 +/- 21.83 micro g/dl after the administration of Mn-DPDP (t = 6.785, P < 0.01). The signal/noise ratio (S/N) of the pancreas increased 53 percent +/- 49 percent and 62 percent +/- 44% on T(1)W spin echo images and SPGR images, respectively.

CONCLUSIONS

Exocrine cells of the pancreas can absorb manganese and excrete it through the pancreatic juice. Exocrine cells play an important role in the enhancement of the pancreas in MR imaging with Mn-DPDP.

Stability and transmetallation of the magnetic resonance contrast agent MnDPDP measured by EPR

MnDPDP [manganese(II) N, N'-dipyridoxylethylenediamine- N, N'-diacetate-5,5'-bis(phosphate)] is the active component of Teslascan, a contrast medium for magnetic resonance imaging of the liver. It has previously been shown that MnDPDP is rapidly dephosphorylated to the monophosphate MnDPMP and the non-phosphorylated MnPLED, and that all these substances are rapidly transmetallated to the corresponding Zn complexes. In the present study we used EPR at 9 and 230 GHz to show that no free Mn(2+) ions can be detected in the product or in a mixture of MnDPDP and human serum. Competition experiments between MnDPDP and Zn(2+), Ca(2+), and Mg(2+) ions revealed approximately 15% transmetallation with Zn(2+) in a buffer system containing metal ion concentrations similar to that in serum, whereas approximately 10% transmetallation was obtained with Ca(2+) and only negligible transmetallation was obtained with Mg(2+) under these conditions. Binding experiments with Mn(2+) added to human albumin and human serum indicate that albumin accounts for most of the protein-bound Mn(2+) in serum.

Kinetics of antagonist actions at rat P2X2/3 heteromeric receptors

1. Currents through heteromeric P2X(2/3) receptors were evoked by applying alpha,beta-methylene-ATP to human embryonic kidney cells transfected with cDNAs encoding the P2X(2) and P2X(3) subunits. The concentration of alpha,beta-methylene-ATP were < or =30 microM because higher concentrations can activate homomeric P2X(2) receptors. The kinetics of action of three structurally unrelated antagonists were studied; these were 2', 3'-O-(2,4,6,trinitrophenyl)-ATP (TNP-ATP), pyridoxal-5-phosphate-6-azophenyl-2',4'-disulphonate (PPADS) and suramin. The association and dissociation rate constants were determined by pre-applying the antagonist for various periods prior to the co-application of agonist and antagonist, or by changing the solution from one containing only the agonist to one containing both agonist and antagonist. The high affinity of TNP-ATP for the P2X(2/3) receptor (K(D) approximately 2 nM) results from fast binding (k(+1) approximately 100 microM(-1) s(-1)) rather than slow unbinding (k(-1) approximately 0.3 s(-1)). For suramin (K(D) approximately 1 microM) the association rate constant ( approximately 1 microM(-1) s(-1)) was 100 times slower than that of TNP-ATP but the dissociation rate constant was similar (k(-1) approximately 1 s(-1)). PPADS (K(D) approximately 0.1 microM) associated and dissociated some 100 - 10,000 times more slowly than the other antagonists.

A model of stepwise isovolaemic blood exchange in anaesthetised, spontaneously breathing rats to evaluate the oxygen transport efficiency of artificial oxygen carriers

Our research pursues the production of hypo-oncotic artificial oxygen carriers, based on artificial covalently cross-linked hyperpolymeric mammalian haemoglobins. To evaluate their in vivo efficiency in oxygen delivery to the tissue we developed a small animal model of stepwise isovolaemic blood exchange in anaesthetised, spontaneously breathing rats. With the aid of a two-way respiratory micro valve for small animals the overall oxygen uptake by the tissue of the animal can be determined. Measurements of oxygen contents in arterial and mixed venous blood and of some further blood parameters together with known oxygen-binding characteristics of artificial and native oxygen carriers, permits the determination of the way the two oxygen carriers contribute to the overall oxygen uptake. These so-called partial oxygen net to transport rates (i.e. partial oxygen uptakes), related to the corresponding intravascular mass flow of the transporters, are characteristic measures of the efficiency of the oxygen transporter, the so-called oxygen transport quality. Other biological indicators for an adequate oxygen supply are oxygen-dependent changes of ventilation, cardiac output, heart rate, and systemic vascular resistance. The performance of artificial oxygen carriers is elucidated by a comparison with experimental results from the analogous treatment of rats with non oxygen-transporting plasma expanders.

Mechanisms of action of liver contrast agents: impact for clinical use

New contrast agents for magnetic resonance imaging are continually being developed by pharmaceutical companies in order to better image the liver. These agents can be divided into hepatobiliary agents directed to the hepatocytes and nanoparticulate agents directed to the reticulo-endothelial system. After intravenous injection, all these agents concentrate in the liver and induce profound changes in signal intensity. Particulate agents induce predominantly a darkening of the liver parenchyma, while hepatobiliary agents induce a brightening. In both cases, liver-lesion conspicuity is enhanced, leading to a better visualization of the lesion. After a brief description of the principal characteristics of the agents, this paper will attempt to summarize the utility of these agents for the detection and characterization of focal liver disease.

Cardiac metal contents after infusions of manganese. An experimental evaluation in the isolated rat heart

RATIONALE AND OBJECTIVES

Manganese dipyridoxyl diphosphate (MnDPDP), a contrast agent for liver MRI, releases free Mn2+ in a graded manner. The aim of the study was to compare the effects of brief versus prolonged infusions of MnDPDP and manganese chloride (MnCl2) on cardiac function, metabolism, Mn accumulation, and tissue metal content.

METHODS

Isolated perfused rat hearts received 1-minute or 10-minute infusions of MnDPDP (100 microM, 1000 microM) or of MnCl2 (10 microM, 100 microM). Physiologic indices were measured intermittently, and tissue high-energy phosphate compounds and Ca/Fe/Mg/Mn/Zn contents were measured after a standardized Mn washout.

RESULTS

One-minute and 10-minute infusions induced, respectively, minor and marked depressions of contractile function and corresponding elevations in myocardial Mn content. MnCl2 was markedly more potent than MnDPDP. Ten-minute infusions of the highest concentration of MnDPDP and MnCl2 lowered tissue Mg and elevated tissue Ca (MnCl2), whereas high-energy phosphates were unaffected.

CONCLUSIONS

Mn uptake after Mn infusion is strongly related to the duration, concentration, and dose of free Mn ions. Differences in Mn accumulation between MnDPDP and MnCl2 were more pronounced after the 10-minute infusion.

Uptake of mangafodipir trisodium in liver metastases from endocrine tumors

The purpose of the study was to investigate retrospectively whether mangafodipir trisodium (MnDPDP) can enhance the liver metastases from endocrine tumors. Thirteen patients with endocrine tumors and liver metastases underwent T1-weighted spin-echo (SE) and turbo gradient-echo (GRE) MRI conducted before and 20 to 60 minutes after i.v. infusion of MnDPDP. Additional 24-hour-delay scans were performed in 8 of 13 patients. MR signal intensity (SI) was measured in liver parenchyma and metastases, which was then related to that of paraspinal muscle. A total of 30 lesions on precontrast and postcontrast images and 18 lesions on 24-hour-delay images were measured. An enhancement by 49% in SE and 40% in GRE images (P = .0001) was observed in tumor tissues after MnDPDP infusion. In 24-hour-delay images, the SI of the lesions remained relatively high, but in liver parenchyma, it decreased significantly, and the tumor-liver tissue contrast was reduced.

Dynamic evaluation of the hepatic uptake and clearance of manganese-based MRI contrast agents: a 31P NMR study on the isolated and perfused rat liver

This spectroscopic study compares the mechanisms of the hepatic uptake of manganese chloride (MnCl2) and manganese dipyridoxyl diphosphate (MnDPDP). Alterations of the phosphorus-31 (31P)-NMR spectrum of the intracellular adenosine 5'-triphosphate (ATP) are used to monitor the internalization of manganese by the isolated and perfused rat liver. Mn2+ delivered as MnCl2 in the perfusate rapidly enters the hepatocytes, where it strongly interacts with ATP, inducing a broadening of the 31P lines. The inhibition of the process by nifedipine confirms that manganese ions cross the cellular membrane at least partly through Ca2+ channels. MnDPDP induces weaker but still significant changes of the ATP spectrum. The inability of pyridoxine to compete for the uptake of manganese confirms that the vitamin B6 carrier is not involved in the internalization process of the paramagnetic complex. Finally, preincubation of MnDPDP with blood does not increase the extent of the dissociation. The alterations of the 31P spectrum of the liver subsequent to the administration of MnDPDP are attributable to a fraction of free Mn2+ released by the chelate and delivered to the hepatocytes.

Effects of MnDPDP, DPDP--, and MnCl2 on cardiac energy metabolism and manganese accumulation. An experimental study in the isolated perfused rat heart

RATIONALE AND OBJECTIVES

Recent studies indicate that manganese dipyridoxyl diphosphate (MnDPDP) may function as a slow release agent for manganese ions (Mn++) and that MnDPDP is approximately 10 times less potent than manganese chloride (MnCl2) in depressing cardiac function. The authors examined the possibility that MnDPDP and MnCl2 may influence cardiac metabolism and enzyme release and lead to a tissue accumulation of Mn.

METHODS

Manganese DPDP, DPDP--, or MnCl2 (1000 microM) was infused in isolated rat hearts, which were freeze-clamped at various time intervals during infusion (5 minutes) and recovery (14-minute washout). Enzyme (lactate dehydrogenase) release, tissue high energy phosphates, Mn contents, and physiologic indices were measured at various time intervals.

RESULTS

No significant differences were noted for: lactate dehydrogenase in the treated groups; tissue creatine phosphate (CrP) and adenosine triphosphate in MnDPDP, DPDP--, and control groups; and tissue Mn in DPDP-- and control groups. Manganese-chloride and MnDPDP-treated hearts accumulated and retained Mn in an 8:1 ratio. Manganese chloride depressed cardiac function more effectively than MnDPDP.

CONCLUSIONS

The study has shown that: heart tissue uptake and retention of Mn++ is rapid and effective; MnCl2 is approximately eight times more potent than MnDPDP in promoting these effects; and a rise in tissue Mn content to eight to nine times (MnDPDP) or 60 to 70 times (MnCl2) the normal level does not lead to acute side effects on cardiac energy metabolism, function, and enzyme release. The study indicates that MnDPDP may act like a slow release compound for Mn++ ions.

The uptake of Mn-DPDP by hepatocytes is not mediated by the facilitated transport of pyridoxine

Manganese-dipyridoxal diphosphate (Mn-DPDP) is a liver-selective contrast agent selectively taken up by the hepatocytes. Because of the analogy of structure with pyridoxine (vitamin B6), it was previously suggested that this compound can be selectively taken up by the facilitated transport of vitamers B6. To understand the uptake mechanism, an in vivo binding study was performed based on a competition between 54Mn-DPDP and pyridoxine on the one hand, and Mn-DPDP and [3H]pyridoxine on the other. We found that the [3H]pyridoxine levels in the liver were not significantly different 5 min after intravenous administration of several doses of Mn-DPDP (5 nmol/kg to 50 mumol/kg): 5.0 +/- 0.3% of the injected dose/g tissue. The content of 54Mn (administered as 54Mn-DPDP) in the liver was not affected by a saturation dose of pyridoxine (1 mmol/kg) and was found to be constant (+/- 10% of the injected dose/g tissue) for 60 min. These experiments showed that the uptake of Mn-DPDP is not mediated by the transporter of pyridoxine.

Evidence for the dissociation of the hepatobiliary MRI contrast agent Mn-DPDP

These experiments assessed and quantitated the release of free manganese Mn++ from the hepatobiliary contrast agent Mn-DPDP (manganese dipyridoxal diphosphate), using several magnetic resonance techniques (EPR spectroscopy, 31P-NMR spectroscopy, and relaxometry) to differentiate between free Mn++ and Mn++ in complexes in various preparations. The presence of calcium and magnesium in physiological concentrations in aqueous solutions induced the release of Mn++ from the complex, as did incubation of the complex in liver homogenates. After intravenous injection of 15 mumol/kg of Mn-DPDP, both EPR and 31P-NMR spectroscopy demonstrated that Mn-DPDP is partly dissociated (approximately 25%) in the liver. By comparing in vitro and ex vivo data from the liver, we concluded that the dissociation of Mn-DPDP occurs primarily in the liver, whereas a minor portion of the dissociated. Mn found in the liver comes from dissociation of the complex in the blood. Most of the dissociated Mn in liver becomes bound to macromolecules and is responsible for the enhancement of relaxivity observed with this agent.

Distribution of [3H]alpha,beta-methylene ATP binding sites in rat brain and spinal cord

The distribution of [3H]alpha,beta-methylene ATP ([3H]alpha,beta-MeATP) binding sites in rat brain and spinal cord was examined using autoradiography. It was shown that many structures in the CNS are densely labelled, and that the binding is displaced by both beta,gamma-methylene ATP and the P2X-purinoceptor antagonist, pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), indicating that the binding sites are P2X-purinoceptors. Image analysis revealed that the nuclei of the thalamus have the highest density of labelling. Other densely labelled structures include: the amygdaloid, substantia nigra, cerebral cortex, hypothalamus, caudate putamen, geniculate nuclei, medial habenula and the intermediate zone of grey matter in the spinal cord. These results are consistent with the limited amount of electrophysiological evidence for the existence of P2X-purinoceptors in the CNS, and provoke the need for extensive physiological studies to establish the roles of P2X-purinoceptors in the CNS.

Variability in absorption lag time of pyridoxal phosphate under fasting and pre- and post-meal conditions

Inter-individual variations in the absorption lag time of pyridoxal phosphate were determined after administration of an enteric-coated tablet (EC) or a plain capsule (PC) to 113 healthy volunteers under fasting, pre-meal, and post-meal conditions. The absorption lag time of pyridoxal phosphate was assessed from the urinary excretion of 4-pyridoxic acid after administration of EC and PC. Significantly larger lag times after administration of both formulations were observed under post-meal conditions than under pre-meal conditions (0.477 +/- 0.315 h versus 0.081 +/- 0.086 h for PC and 1.995 +/- 1.345 h versus 1.064 +/- 1.327 h for EC), indicating that the mean gastric emptying rates of both a solution and a tablet were delayed after food intake. The lag time for PC showed little inter-individual variation with (0-1.2 h) or without food (0-0.25 h), whereas that for EC showed markedly large inter-individual variation, from 0.25 to 2.63 h (median, 1.5 h) in the fasting condition, from 0.25 to > 5.5 h (median 0.25 h) under pre-meal conditions, and from 0.25 to > 5.5 h (median 1.25 h) under post-meal conditions. The effect of food on the gastric emptying rate of a solution appears to be almost uniform, whereas that for a tablet is so unpredictable that a reliable absorption rate for an enteric-coated tablet cannot be expected, particularly under pre- and post-meal conditions.

[Metabolism and catabolism to CO2 of [14C] gamma-aminobutyric acid conjugates with pyridoxal phosphate, nicotinate and biotin]

Pharmacokinetics and catabolism to CO2 in the mice organism and metabolic transformation by the tissues homogenates of [1-14C] GABA and its conjugates with nicotinate, pyridoxal phosphate and biotin have been studied. The permeability of nicotinoyl-GABA through the hemato-encephalic barrier was 10 times as much as the corresponding value for GABA and its conjugates with other vitamins. PLP-GABA is eliminated more rapidly from the brain in comparison with GABA, biotinyl-GABA is retained to a higher degree in kidneys, the entero-hepatic recycling takes place more actively for nicotinoyl-GABA. The latter, in contrast to biotinyl-GABA, remains unaffected by the liver, intestine mucose membrane and brain proteases and is catabolized to CO2 to a considerably lower extent as compared with GABA, perhaps due to the intestine bacterial microflora.

[Distribution of 1-[14C] GABA and its conjugates with nicotinate, pyridoxal phosphate and biotin in tissues of guinea pigs in closed systems]

Distribution of the total radioactive label in the blood, different parts of the brain (the whole cortex and its lobes, bulbi olfactorii, hippocamp, striopallidum, hypothalamus, thalamus, peduculi cerebri, cerebellum, pons, medulla oblongata), eye (retina, hyaloid and vascular membrane), hypophysis and liver of guinea pig males has been studied after the hypodermic injection of [14C] GABA and products of its conjugation with pyridoxal phosphate, nicotinic acid and biotin in the equimolar dose (50 nmol/g) under normal conditions and in case of action of the closed volume factors. Accumulation of the total label in the hypophysis after injection of [14C] GABA, PLP-[14C] GABA and nicotinoyl-[14C] GABA was 3.6, 3.1 and 2.7 times (respectively) as much as that in the blood. The content of the labelled compounds in the cortical structures of the telencephalon and in the diencephalon was the greatest in comparison with other parts of the brain. The permeability through a hemato-ophthalmic barrier was 3-5 times as much as that through a hemato-encephalic barrier. The closed volume factors lead to higher accumulation of the label as compared with the control after injection of [14C] GABA, PLP-[14C] GABA and biotinyl-[14C] GABA.

Some pharmacokinetic considerations about homeostatic equilibrium of endogenous substances

Endogenous substances in the body are controlled through simple, very effective mechanisms, that preserve an optimum homeostatic equilibrium of baseline concentration and restore it when impaired. When planning a pharmacokinetic investigation of an endogenous substance exogenously administered, it is imperative to carefully ascertain the above mechanisms as well as the baseline value and their possible variations associated with daily rhythm, food, age, sex, menstrual cycle. Often the control mechanisms operate through non-linear processes, therefore a non-compartmental analysis or a tailored model may be more appropriate than the compartmental models used in standard pharmacokinetic analysis. Some specific examples of endogenous substances are discussed here on the basis of the data from the literature and personal experience.

Pharmacokinetics of a sustained release formulation of pyridoxal phosphate of buflomedil after single or repeated oral doses in healthy volunteers

The pharmacokinetics of a sustained release (SR) formulation of pyridoxal phosphate of buflomedil (Pirxane retard) has been studied after oral administration to healthy volunteers using among else a gaschromatographic dosage method. After oral administration of 400 mg of the SR formulation, pyridoxal phosphate of buflomedil has a much slower kinetics compared to the normal formulation (tmax:approx. 1.5 h) reaching the maximum plasma concentration, which was about 467 ng/ml, in about 3 h. After 24 h the concentrations were still about 1/10 (48 ng/ml) the maximum value. 24-h urinary excretion was about 21% of the administered dose. Repeated administration of the SR formulation for 7 days in single daily doses of 400 mg gave steady state plasma levels (ca. 250 ng/ml) 12 h after the administration without statistically significant variations. The plasma concentrations of the drug measured daily after reaching the steady state were similar one to the other. The tolerability was very good and no local or systemic side effects of any kind were reported.

Metabolism of [14C]- and [32P]pyridoxal 5'-phosphate and [3H]pyridoxal administered intravenously to pigs and goats

To gain more information about the kinetics of vitamin B-6 metabolism in vivo, the metabolism of tracer was examined after the simultaneous intravenous administration of [32P] and [14C]pyridoxal phosphate and [3H]pyridoxal in two 93-kg pigs and two 60-kg goats. In the pigs, [14C] removal was monophasic with T1/2 of 16 and 18 min and clearance of 165 and 248 mL/min. In the goats, [14C] removal was biphasic with T1/2 of 49 and 114 min for 0-30 min and 209 and 227 min for 0.5-6 h (clearance 20 and 17 mL/min). Uptake of pyridoxal phosphate by liver and resecretion into the plasma were too small to cause a detectable decrease in the [32P]:[14C] ratio. Pyridoxal removal from plasma was similar in both species, with a half-life of approximately 12 min from 0-30 min and approximately 50 min for 0.5-3 h. Clearance of [3H]pyridoxal in the four animals ranged from 412 to 2258 mL/min. Little [14C] entered the erythrocytes. The [3H] entered readily but was converted to pyridoxal phosphate faster in the pigs than in the goats. [14C] and [3H] were excreted as pyridoxic acid at the same rate. However, during the 54 h after injection the goats excreted approximately 60% of the [14C] doses in the urine compared with approximately 30% in the pigs. About 5-10% of the [14C] and [3H] doses were recovered in goat milk over 54 h. Pyridoxal kinase activity was higher in lactating mammary tissue than in liver, kidney or muscle of goats.

The role of the kidneys in the excretion of chemically modified hemoglobins

Stroma-free hemoglobin (SFHb) can be chemically modified to prolong the intravascular retention (prevent renal filtration), and to improve oxygen delivering capability for use as a red cell substitute. Hb derivatives radioactively tagged with tritium [3H] or 14C were used to study their metabolic fate following clearance from the circulation. Fully conscious, chronically cannulated rats were treated by exchange transfusion (ET). Hb solutions tested were: PLPHb (Hb monovalently reacted with pyridoxal 5'-phosphate); HbXL (Hb crosslinked beta-beta with 2-nor-2-formylpyridoxal 5'-phosphate, or with bis-pyridoxal tetraphosphate); alpha alpha Hb (Hb cross-linked between the alpha-chains using bis-3,5-dibromosalicyl fumarate); and polyHb (polymerized with glutaraldehyde or o-raffinose). Plasma retention (T1/2) was significantly affected by dose and the degree of cross-linking. Urine flow rates all increased transiently above normal. In rats treated with any 64 kDa interdimerically cross-linked Hb, mild hemoglobinuria was evident and kidney tissue had the highest label concentration at all time points (1, 5, 10, 24, 48 hr, 7 d, and 14 days post-ET). For polymerized Hb derivatives, the amount of radioactivity in urine and kidneys was inversely related to the MW of the polyHb molecules. In all rats, regardless of the Hb derivative tested, the majority of radioactivity (dpm's) was excreted in urine. About 75% of all renal excretion of radioactivity occurred from 12-60 hours post-ET. This provided evidence that catabolism of cross-linked Hb's began early, and that the kidneys are primarily responsible for excreting smaller degradation fragments.

Clearance of differentially labelled infused hemoglobin and polymerized hemoglobin from dog plasma and accumulation in urine and selected tissues

Pyridoxalated hemoglobin polymerized with glutaraldehyde has been proposed as a hemoglobin based blood substitute. The preparations contain significant amounts of unpolymerized hemoglobin. We have prepared polymerized pyridoxalated hemoglobin labelled with 14C by reductive methylation free of unpolymerized hemoglobin and pyridoxalated hemoglobin labelled with 3H by reductive methylation to compare the handling of the two forms after infusion into dogs. Four dogs were examined sequentially. After three hours, 52.4 +/- 8.9% of the 3H label had disappeared from plasma whereas 21.7 +/- 5.8 of the 14C label had disappeared. The decrease of both labels occurred in a very close to linear fashion over the time period examined. From radioactivity in collected urine, it was calculated that 30.7 +/- 6.3% of the 3H and 9.0 +/- 2.7 of the 14C that had been cleared from plasma appeared in urine. The ratio of the specific radioactivity in tissue to the specific radioactivity of plasma indicated that extravascular accumulation of 3H label from unpolymerized hemoglobin occurred in kidney, heart and liver, with the kidney cortex exhibiting a very high concentration of the label. The specific radioactivity of both 3H and 14C label in liver suggested the substantial involvement of the reticuloendothelial system in the removal of both unpolymerized and polymerized hemoglobin from the circulation.

Effect of polymerization on clearance and degradation of free hemoglobin

In the present study we investigated the mechanism of prolongation of the plasma retention of free hemoglobin by polymerization. Polymerization of intramolecularly crosslinked hemoglobin with glutaraldehyde yields a mixture of large polymers, small polymers and monomers. In exchange transfusion experiments in rats we analyzed plasma samples by gel filtration to determine the clearance of polymers of different size. A positive correlation was found between polymer size and vascular retention. Furthermore, the clearance of large polymers appeared to be highly dose-dependent: after 20% and 70% exchange transfusions, we observed for large polymers a plasma half-life of 12 and 26 hours, respectively, whereas the half-life for 64 kD monomers was 4 hours in both cases. The degradation of hemoglobin was followed by measuring the bilirubin excretion. The infused heme was recovered as bilirubin within 72 hours. The delay between the disappearance of free hemoglobin from the plasma and the recovery as bilirubin was about six hours and was not affected by polymerization or dose. We conclude that polymerization prevents the operation of certain clearance mechanisms, while still allowing a route of clearance that is easily saturated. The intracellular degradation of heme into bilirubin is not affected by the modifications of hemoglobin and is not easily saturated.

Pyridoxal-5'-phosphate and pyridoxal biokinetics in male Wistar rats fed graded levels of vitamin B-6

Biokinetic parameters of plasma pyridoxal-5'-phosphate (PLP) and pyridoxal (PL) disposition were studied in male Wistar rats (age 8 mo) fed a purified diet containing less than 0.5, approximately 3 or approximately 6 mg pyridoxine.HCl/kg diet from weaning, with animals fed the 6 mg/kg diet serving as the control group. Basal plasma PLP concentration was lower in both the less than 0.5 and 3 mg/kg diet groups than in control animals (98 +/- 12, 314 +/- 40 and 514 +/- 56 nmol/L, respectively). Basal plasma PL concentration was lower in the less than 0.5 mg/kg diet group only [60 nmol/L (measured in pooled samples), 190 +/- 73 and 235 +/- 63 nmol/L for less than 0.5, 3 and 6 mg/kg diet groups, respectively]. In both the less than 0.5 and 3 mg/kg diet groups, PLP clearance was lower than in control rats (0.158 +/- 0.025, 0.131 +/- 0.040 and 0.240 +/- 0.051 L.h-1.kg body weight-1, respectively). In the less than 0.5 mg/kg diet group, PLP synthesis was more efficient than in control animals (34.7 +/- 9.3, 12.1 +/- 2.5 and 16.7 +/- 11.4% for less than 0.5, 3 and 6 mg/kg diet groups, respectively). In both the less than 0.5 and 3 mg/kg diet groups, volume of distribution of PLP as well as of PL was larger than in controls. It is concluded that B-6 vitamer metabolism is influenced by vitamin B-6 status. The metabolic pathway involved (PLP synthesis and/or PLP degradation) was observed to depend on degree of vitamin B-6 deficiency.

[The transhemenation process of bovine and human hemoglobin polymers under conditions simulating the situation in the circulatory bed]

It was shown that transhemenation, i.e. the transfer of heme from hemoglobin to human serum albumin may have the great importance in metabolism of hemoglobin. It was noted that precipitation in the case of human pyridoxal-5'-phosphate modified polyhemoglobin met-derived incubation.

Elimination of pyridoxylated polyhemoglobin after partial exchange transfusion in chimpanzees

Partial exchange transfusion with 8.5% pyridoxylated polyhemoglobin solution [PolyHb-PPa] was performed in five male chimpanzees weighing 22-30 kg. Serial blood and urine samples were obtained for 3 days. Percutaneous liver biopsies were performed on the 3rd to 4th, and the 9th to 11th days after PolyHb-PPa administration. Mean exchange volume was 42.5 +/- 10.7 ml/kg BW (26.8-54.6 ml/kg), mean Hb dose 3.7 +/- 0.9 g PolyHb-PPa/kg BW (2.4-4.8 g/kg), mean exchange rate 56.7 +/- 7.1% (48.2-67.4%). All animals survived long-term. Analysis of the plasma Hb concentration-time data showed a first order decline at a plasma level of 3.7 +/- 0.9 g PolyHb-PPa/kg BW. Mean intravascular half-life was 14.6 +/- 3.2 h. Total renal elimination of PolyHb-PPa was about 7%. PolyHb-PPa was absorbed and stored by Kupffer cells and transformed into hemosiderin. Siderosis of Kupffer cells and renal tubules had largely subsided 10 days after PolyHb-PPa indicating subsequent in vivo degradation and metabolization of the polymerized Hb fractions.

[The effect of the degree of hemoglobin polymerization on its interaction with the body]

Solutions of pyridoxylated polyhemoglobin (PPHb) with the different degree of polymerization (0-53%) were injected intravenously into rats (1 g/kg). The quantity of PPHb, circulating in plasma, being excreted with the urine and accumulated in the organs have been estimated during the first 6 hours after the infusion. During this period 25-40% of the injected PPHb have been found in the urine and 10-25% have been found in plasma, the percentage depending on the degree of polymerization. For all the substances studied 50% accumulation in organs have been observed, the fact of accumulation being proved by the pathologicoanatomic studies. In the present study the localization and the quantity of hemosiderin granules in the organs was the same for the modified hemoglobins with the different degree of polymerization.

Intestinal hydrolysis of pyridoxal 5'-phosphate in vitro and in vivo in the rat. Effect of amino acids and oligopeptides

The first step in the intestinal absorption of phosphorylated forms of vitamin B6 is intraluminal hydrolysis mediated by alkaline phosphatase. The present studies were performed to evaluate the effect of amino acids and oligopeptides, the products of protein digestion, on the hydrolysis of pyridoxal 5'-phosphate. Models utilized rats and included a cell-free in vitro system and the in vivo, single-pass, perfused jejunal segment. In vitro all amino acids and oligopeptides tested significantly inhibited pyridoxal 5'-phosphate decay (hydrolysis). The degree of inhibition of decay was dependent on the particular compound used, the concentration of that compound, and the pH of the medium. Similar effects for L-lysine concentration and perfusate pH were demonstrated in perfused segments in vivo; by contrast, L-lysine had no effect on pyridoxine uptake. These studies demonstrate that intraluminal hydrolysis of phosphorylated vitamin B6 may be modulated by yet other intraluminal constituents and conditions.

In vivo distribution and elimination of hemoglobin modified by intramolecular cross-linking with 2-nor-2-formylpyridoxal 5'-phosphate

Modified hemoglobin solutions have potential application as plasma expanders with oxygen-transporting capacity. In a previous study it was found that modification of hemoglobin by intramolecular cross-linking with 2-nor-2-formylpyridoxal 5'-phosphate (NFPLP) improves the vascular retention time by a factor of three, and it also improves the oxygen-transporting properties. In the present study we investigated in rats how, after exchange transfusion of a clinically relevant dose, the modified hemoglobin (HbNFPLP) was distributed in the body compared with how the unmodified hemoglobin was distributed. By using a new technetium 99m labeling technique, we found in a scintigraphic study that accumulation of hemoglobin in the kidneys was greatly diminished by the intramolecular cross-linking with NFPLP. These findings were confirmed by light-microscopic observations after diaminobenzidine staining. It was concluded that the impairment of kidney function caused by blockade of the tubuli is not to be expected from HbNFPLP. In the liver and spleen, where the free HbNFPLP is possibly eliminated, some accumulation of 99mTc label was observed, but the major part of the extravascular label was diffusely spread throughout the body. This led to the conclusion that important accumulation of undegraded HbNFPLP does not occur in the liver and spleen. Rapid appearance of both hemoglobin and HbNFPLP in the lymph showed that cross-linking with NFPLP does not prevent the distribution of hemoglobin over the interstitial space in the first hours after administration. However, pharmacokinetic analysis demonstrated that transcapillary transfer contributes only to a limited extent to the disappearance from the circulation. During 24-hour infusions of HbNFPLP, a steady state with a constant plasma concentration was easily reached. The latter experiment indicated that the eliminating system does not become saturated during prolonged administration of large doses of HbNFPLP.