Carcinogenic potency of alkylating agents in rodents and humans.

Alkylating agents are known to produce second tumors in cancer patients treated for their primary cancer. Since therapeutic doses are high and the pharmacokinetics of the drugs are thoroughly studied, these agents provide a unique opportunity to compare intrinsic carcinogenic potency between experimental animals and humans. We have examined the carcinogenicity of melphalan, chlorambucil, and cyclophosphamide in causing leukemia in patients treated for cancer or polycythemia vera and lymphosarcoma in rats and mice. A good correlation among species is observed when the carcinogenic potency is based on the total lifetime exposure to active species derived from these drugs.

Predicted and observed effects of antibody affinity and antigen density on monoclonal antibody uptake in solid tumors.

The uptake and binding of monoclonal antibodies (MAbs) in solid tumors after a bolus i.v. injection are described using a compartmental pharmacokinetic model. The model assumes that MAb permeates into tumor unidirectionally from plasma across capillaries and clears from tumor by interstitial fluid flow and that interstitial antibody-antigen interactions are characterized by the Langmuir isotherm for reversible, saturable binding. Typical values for plasma clearance and tumor capillary permeability of a MAb and for interstitial fluid flow and interstitial volume fraction of a solid tumor were used to simulate the uptake of MAbs at various values of the binding affinity or antigen density for a range of MAb doses. The model indicates that at low doses, an increase in binding affinity may lead to an increase in MAb uptake. On the other hand, at doses approaching saturation of antigen or when uptake is permeation limited, an increase in the binding affinity from moderate to high affinity will have only a small effect on increasing MAb uptake. The model also predicts that an increase in antigen density will greatly increase MAb uptake when uptake is not permeation limited. Our experiments on MAb uptake in melanoma tumors in athymic mice after injection of 20 micrograms MAb (initial plasma concentration, about 120 nM) are consistent with these model-based conclusions. Two MAbs differing in affinity by more than 2 orders of magnitude (3.8 x 10(8) M-1 and 5 x 10(10) M-1) but with similar in vivo antigen densities in M21 melanoma attained similar concentrations in the tumor. Two MAbs of similar affinity but having a 3-fold difference in in vivo antigen density in SK-MEL-2 melanoma showed that the MAb targeted to the more highly expressed antigen attained a higher MAb concentration. We also discuss the model predictions in relation to other experiments reported in the literature. The theoretical and experimental findings suggest that, for high dose applications, efforts to increase MAb uptake in a tumor should emphasize the identification of an abundantly expressed antigen on tumor cells more than the selection of a very high affinity MAb.

Effects of time-variant exposure on toxic substance response.

Sources of time-variant exposure to toxic substances are identified and examined for their effects on the estimation of response. It is shown that only time-averaged target tissue concentrations are required to obtain rigorous risk estimates from the one-hit and multihit models. In contrast, detailed concentration histories need to be retained throughout analyses involving two-event models with intermediate-stage clonal growth advantage (clonal two-stage) and multistage models. Cumulative incidence ratios, based on the exact to time-averaged treatment of concentration time dependencies, are evaluated for substances whose toxic responses exhibit moderate (arsenic) and strong (ethylene dibromide) dependence on time of actual exposure. These ratios reveal that time-averaged dose approximations may lead to several orders of magnitude error in both the multistage and clonal two-stage models if exposure periods are short, and that 3.4-fold (arsenic) and 8-fold (ethylene dibromide) errors still exist even when an actual two-thirds lifetime exposure is averaged over a full lifetime. Finally, the effects of time-variant exposure on risk estimation due to migration and birth-death in an epidemiological setting are examined. A residence time distribution calculation shows that, if these effects are ignored for a population orally exposed to arsenic and characterized by an out-migration rate in excess of 5%/yr, response errors will exceed an order of magnitude.

Convection-enhanced selective excitotoxic ablation of the neurons of the globus pallidus internus for treatment of parkinsonism in nonhuman primates.

OBJECT: Selective treatment of central nervous system (CNS) structures holds therapeutic promise for many neurological disorders, including Parkinson's disease (PD). The ability to inhibit or augment specific neuronal populations within the CNS reliably by using present therapeutic techniques is limited. To overcome this problem, the authors modeled and developed a method in which convection was used to deliver compounds to deep brain nuclei in a reproducible, homogeneous, and targeted manner. To determine the feasibility and clinical efficacy of convective drug delivery for treatment of a neurological disorder, the investigators selectively ablated globus pallidus internus (GPi) neurons with quinolinic acid (QA), an excitotoxin, in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced model of primate parkinsonism. METHODS: After the parameters of convective distribution to the GPi were confirmed by infusion of biotinylated albumin into the GPi of a primate (Macaca mulatta), seven adult monkeys of this species were rendered either fully parkinsonian by intravenous injections of MPTP (five animals) or hemiparkinsonian by a right-sided intracarotid injection of this agent (two monkeys). Using convection-enhanced delivery to the GPi, animals were infused with either QA (three fully parkinsonian, two hemiparkinsonian) or saline (two fully parkinsonian). The three fully parkinsonian animals that underwent GPi lesioning with QA had substantial improvement of PD symptoms, manifested by a marked increase in activity (34 +/- 2.5%; mean +/- standard deviation) and dramatic improvement of parkinsonian clinical scores. In contrast, the control animals did not improve (activity monitor change = -1.5 +/- 0.5%). The two hemiparkinsonian animals that underwent QA lesioning of the GPi had dramatic recovery of extremity use. Histological examination revealed selective neural ablation of GPi neurons (mean loss 87%) with sparing of surrounding gray and white matter structures. No animal developed worsening signs of PD or neurological deficits after infusion. CONCLUSIONS: Convection-enhanced delivery of QA permits selective, region-specific (GPi), and safe lesioning of neuronal subpopulations, resulting in dramatic improvement in parkinsonian symptomatology. The properties of convection-enhanced delivery indicate that this method could be used for chemical neurosurgery for medically refractory PD and that it may be ideal for cell-specific therapeutic ablation or trophic treatment of other targeted structures associated with CNS disorders.

Convective delivery of macromolecules into the naive and traumatized spinal cords of rats.

OBJECT: Many macromolecules have the potential to enhance recovery after injury and other lesions of the spinal cord, but because of the limited penetration of these compounds across the blood-spinal cord barrier, they cannot be used effectively. To determine if convective delivery could be used in a common animal model to investigate potential therapeutic macromolecules and to examine the effects of trauma on convective delivery in that model, the authors examined the distribution of a macromolecule in naive and traumatized rat spinal cords. METHODS: Using convection, various infusion volumes ([Vi]; 1, 2, and 4 microl) of 14C-albumin were infused into the dorsal columns of 13 naive and five traumatized rat spinal cords. Volume of distribution (Vd), homogeneity, percentage of recovery, and anatomical location were determined using quantitative autoradiography, scintillation analysis, calculation of kurtosis (K) value, and histological analysis. In the nontraumatized group, Vd was linearly proportional (R2 = 0.98) to Vi (Vd/Vi, 4.3+/-0.6; mean +/- standard deviation), with increases in Vd resulting from linear expansion (R2 = 0.94) primarily in the craniocaudal dimension. In the traumatized spinal cords, the Vd/Vi ratio (3.7+/-0.5) was smaller (p<0.02) and distributions were less confined to the craniocaudal dimension, with significantly larger cross-sectional distributions in the region of injury (p<0.02) compared to the noninjured spinal cords. Histological analysis revealed that after infusion into the dorsal columns, albumin distribution in naive cords was limited to the dorsal white matter, but in the traumatized cords there was penetration into the central gray matter. The distribution of the infusate was homogeneous in the nontraumatized (K = -1.1) and traumatized (K = -1.1) spinal cords. Recovery of radioactivity was not significantly different (p>0.05) between the nontraumatized (84.8+/-6.8%) and traumatized (79.7+/-12.1%) groups. CONCLUSIONS: Direct convective delivery of infusate can be used to distribute macromolecules in a predictable, homogeneous manner over significant volumes of naive and traumatized rat spinal cord. These characteristics make it a valuable tool to investigate the therapeutic potential of various compounds for the treatment of injury and spinal cord disease.

Variables affecting convection-enhanced delivery to the striatum: a systematic examination of rate of infusion, cannula size, infusate concentration, and tissue-cannula sealing time.

OBJECT: Although recent studies have shown that convection can be used to distribute macromolecules within the central nervous system (CNS) in a homogeneous, targeted fashion over clinically significant volumes and that the volume of infusion and target location (gray as opposed to white matter) influence distribution, little is known about other factors that may influence optimum use of convection-enhanced distribution. To understand the variables that affect convective delivery more fully, we examined the rate of infusion, delivery cannula size, concentration of infusate, and preinfusion sealing time. METHODS: The authors used convection to deliver 4 microl of 14C-albumin to the striatum of 40 rats. The effect of the rate of infusion (0.1, 0.5, 1, and 5 microl/minute), cannula size (32, 28, and 22 gauge), concentration of infusate (100%, 50%, and 25%), and preinfusion sealing time (0 and 70 minutes) on convective delivery was examined using quantitative autoradiography, National Institutes of Health image analysis software, scintillation analysis, and histological analysis. Higher rates of infusion (1 and 5 microl/minute) caused significantly (p < 0.05) more leakback of infusate (22.7+/-11.7% and 30.3+/-7.8% [mean+/-standard deviation], respectively) compared with lower rates (0.1 microl/minute [4+/-3.6%] and 0.5 microl/minute [5.2+/-3.6%]). Recovery of infusate was significantly (p < 0.05) higher at the infusion rate of 0.1 microl/minute (95.1+/-2.8%) compared with higher rates (85.2+/-4%). The use of large cannulae (28 and 22 gauge) produced significantly (p < 0.05) more leakback (35.7+/-8.1% and 21.1+/-7.5%, respectively) than the smaller cannula (32 gauge [5.2+/-3.6%]). Varying the concentration of the infusate and the preinfusion sealing time did not alter the volume of distribution, regional distribution, or infusate recovery. CONCLUSIONS: Rate of infusion and cannula size can significantly affect convective distribution of molecules, whereas preinfusion sealing time and variations in infusate concentration have no effect in this small animal model. Understanding the parameters that influence convective delivery within the CNS can be used to enhance delivery of potentially therapeutic agents in an experimental setting and to indicate the variables that will need to be considered for optimum use of this approach for drug delivery in the clinical setting.

Convection-enhanced distribution of large molecules in gray matter during interstitial drug infusion.

Many novel experimental therapeutic agents, such as neurotrophic factors, enzymes, biological modifiers, and genetic vectors, do not readily cross the blood-brain barrier. An effective strategy to deliver these compounds to the central nervous system is required for their application in vivo. Under normal physiological conditions, brain interstitial fluid moves by both bulk flow (convection) and diffusion. It has recently been shown that interstitial infusion into the white matter can be used to increase bulk flow, produce interstitial convection, and efficiently and homogeneously deliver drugs to large regions of brain without significant functional or structural damage. In theory, even more uniform distribution is likely in gray matter. In the current study, four experiments were performed to examine if convection-enhanced delivery could be used to achieve regional distribution of large molecules in gray matter. First, the volume and consistency of anatomical distribution of 20 microliters of phaseolus vulgaris-leukoagglutinin (PHA-L; molecular weight (MW) 126 kD) after continuous high-flow microinfusion into the striatum of five rats over 200 minutes were determined using immunocytochemistry and quantified with image analysis. Second, the concentration profile of 14C-albumin (MW 69 kD) infused under identical conditions was determined in four hemispheres using quantitative autoradiography. Third, the volume of distribution after convection-enhanced infusion of 250 or 500 microliters biotinylated dextran (b-dextran, MW 10 kD), delivered over 310 minutes into the caudate and putamen of a rhesus monkey from one (250 microliters) or two (500 microliters) cannulas, was determined using immunocytochemistry and quantified with image analysis. Finally, the ability to target all dopaminergic neurons of the nigrostriatal tract via perfusion of the striatum with subsequent retrograde transport was assessed in three experiments by immunohistochemical analysis of the mesencephalon following a 300-minute infusion of 27 microliters horseradish peroxidase-labeled wheat germ agglutinin (WGA-HRP) into the striatum. Convection-enhanced delivery reproducibly distributed the large-compound PHA-L throughout the rat striatum (the percent volume of the striatum perfused, Vs, was 86% +/- 5%; mean +/- standard deviation) and produced a homogeneous tissue concentration in the perfused region (concentration of 14C-albumin relative to infusate concentration 30% +/- 5%). In the monkey, the infusion widely distributed b-dextran within the striatum using one cannula (caudate and putamen Vs = 76% and 76%) or two cannulas (Vs = 90% and 71%).(ABSTRACT TRUNCATED AT 400 WORDS)

Direct convective delivery of macromolecules to peripheral nerves.

OBJECT: Although many macromolecules have treatment potential for peripheral nerve disease, clinical use of these agents has been restricted because of limitations of delivery including systemic toxicity, heterogeneous dispersion, and inadequate distribution. In an effort to overcome these obstacles, the authors examined the use of convection to deliver and distribute macromolecules into peripheral nerves. METHODS: For convective delivery, the authors used a gas-tight, noncompliant system that provided continuous flow through a small silica cannula (inner diameter 100 microm, outer diameter 170 microm) inserted into a peripheral nerve. Increases in the volume of infusion (Vi) (10, 20, 30, 40, and 80 microl) of 14C-labeled (nine nerves) or gadolinium-labeled (two nerves) albumin were infused unilaterally or bilaterally into the tibial nerves of six primates (Macaca mulatta) at 0.5 microl/minute. The volume of distribution (Vd), percentage recovery, and delivery homogeneity were determined using quantitative autoradiography, an imaging program developed by the National Institutes of Health, magnetic resonance (MR) imaging, scintillation counting, and kurtosis (K) analysis. One animal that was infused bilaterally with gadolinium-bound albumin (40 microl to each nerve) underwent MR imaging and was observed for 16 weeks after infusion. The Vd increased with the Vi in a logarithmic fashion. The mean Vd/Vi ratio over all Vi was 3.7+/-0.8 (mean+/-standard deviation). The concentration across the perfused region was homogeneous (K=-1.07). The infusate, which was limited circumferentially by the epineurium, followed the parallel arrangement of axonal fibers and filled long segments of nerve (up to 6.8 cm). Recovery of radioactivity was 75.8+/-9%. No neurological deficits arose from infusion. CONCLUSIONS: Convective delivery of macromolecules to peripheral nerves is safe and reliable. It overcomes obstacles associated with current delivery methods and allows selective regional delivery of putative therapeutic agents to long sections of nerve. This technique should permit the development of new treatments for numerous types of peripheral nerve lesions.

Direct convective delivery of macromolecules to the spinal cord.

OBJECT: Because of the limited penetration of macromolecules across the blood-spinal cord barrier, numerous therapeutic compounds with potential for treating spinal cord disorders cannot be used effectively. The authors have developed a technique to deliver and distribute macromolecules regionally in the spinal cord by using convection in the interstitial space. METHODS: The authors designed a delivery system connected to a "floating" silica cannula (inner diameter 100 microm, outer diameter 170 microm) that provides for constant volumetric inflow to the spinal cord. A solution containing albumin that was either unlabeled or labeled with carbon-14 or gadolinium was infused at various volumes (3, 6, 10, 20, 40, or 50 microl) at a rate of 0.1 microl/minute into the spinal cord dorsal columns of nine swine and into the lateral columns of three primates (Macaca mulatta). Volume of distribution (Vd), concentration homogeneity, and percentage of recovery were determined using scintillation analysis, kurtosis calculation (K), and quantitative autoradiography (six swine), magnetic resonance imaging (one swine and three primates), and histological analysis (all animals). Neurological function was observed for up to 3 days in four of the swine and up to 16 weeks in the three primates. The Vd of 14C-albumin was linearly proportional (R2=0.97) to the volume of infusion (Vi) (Vd/Vi=4.4+/-0.5; [mean+/-standard deviation). The increases in Vd resulting from increases in Vi were primarily in the longitudinal dimension (R2=0.83 in swine; R2=0.98 in primates), allowing large segments of spinal cord (up to 4.3 cm; Vi 50 microl) to be perfused with the macromolecule. The concentration across the area of distribution was homogeneous (K=-1.1). The mean recovery of infused albumin from the spinal cord was 85.5+/-5.6%. Magnetic resonance imaging and histological analysis combined with quantitative autoradiography revealed the albumin infusate to be preferentially distributed along the white matter tracts. No animal exhibited a neurological deficit as a result of the infusion. CONCLUSIONS: Regional convective delivery provides reproducible, safe, region-specific, and homogeneous distribution of macromolecules over large longitudinal segments of the spinal cord. This delivery method overcomes many of the obstacles associated with current delivery techniques and provides for research into new treatments of various conditions of the spinal cord.

Chronic interstitial infusion of protein to primate brain: determination of drug distribution and clearance with single-photon emission computerized tomography imaging.

High-flow interstitial infusion into the brain, which uses bulk fluid flow to achieve a relatively homogeneous drug distribution in the extracellular space of the brain, has the potential to perfuse large volumes of brain. The authors report reproducible long-term delivery of 111In-diethylenetriamine pentaacetic acid-apotransferrin (111In-DTPA-Tf) (molecular mass 81 kD) to Macaca mulatta brain and monitoring with single-photon emission computerized tomography (SPECT). The 111In-DTPA-Tf was infused at 1.9 microl/minute over 87 hours into the frontal portion of the centrum semiovale using a telemetry-controlled, fully implanted pump. On Days 1, 3, 4, 8, 11, and 15 after beginning the infusion, planar and SPECT scans of 111In-DTPA-Tf were obtained. Spread of protein in the brain ranged from 2 to 3 cm and infusion volumes ranged from 3.9 to 6.7 cm3. Perfusion of over one-third of the white matter of the infused hemisphere was achieved. From brain SPECT images of (99m)Tc-hexamethylpropyleneamine oxime, which was administered intravenously before each 111In scan, the authors also found that blood perfusion in the infused region was reduced by less than 5% relative to corresponding noninfused regions. Histological examination at 30 days revealed only mild gliosis limited to the area immediately surrounding the needle tract. These findings indicate that long-term interstitial brain infusion is effective for the delivery of drugs on a multicentimeter scale in the primate brain. The results also indicate that it should be possible to perfuse targeted regions of the brain for extended intervals to investigate the potential utility of neurotrophic factors, antitumor agents, and other materials for the treatment of central nervous system disorders.

Steady-state theory for quantitative microdialysis of solutes and water in vivo and in vitro.

A mathematical framework was developed to provide a quantitative basis for either in vivo tissue or in vitro microdialysis. Established physiological and mass transport principles were employed to obtain explicit expressions relating dialysate concentration to tissue extracellular concentration for in vivo applications or external medium concentrations for in vitro probe characterization. Some of the important generalizations derived from the modeling framework are: (i) the microdialysis probe can perturb the spatial concentration profile of the substance of interest for a considerable distance from the probe, (ii) for low molecular weight species the tissue is generally more important than the probe membrane in determining the dialysate-to-tissue concentration relationship, (iii) metabolism, intracellular-extracellular and extracellular-microvascular exchange, together with diffusion, determine the role of the tissue in in vivo probe behavior, and, consequently, (iv) in vitro "calibration" procedures could be useful for characterizing the probe, if properly controlled, but have limited applicability to in vivo performance. The validity of the proposed quantitative approach is illustrated by the good agreement obtained between the predictions of a model developed for tritiated water ([3]H2O) in the brain and experimental data taken from the literature for measurements in the caudoputamen of rats. The importance of metabolism and efflux to the microvasculature is illustrated by the wide variation in predicted tissue concentration profiles among [3]H2O, sucrose and dihydroxyphenylacetic acid (DOPAC).

Transport of cisplatin in rat brain following microinfusion: an analysis.

The post-microinfusion transport of cis-diamminedichloroplatinum(II) (cisplatin) in rat brain has been modeled as a linear diffusion-reaction-permeation process. The model has been used to analyze the experimental data of Kroin and Penn to obtain the macromolecular binding constant of cisplatin in the brain, k = 0.0050 +/- 0.0023 min-1, and the capillary permeability, p = (9.0 +/- 4.4) X 10(-7) cm/s. Inclusion of saturation effects led to the same p value and a higher k value of 0.007 min-1. The corresponding diffusion length is 0.8 mm. The reaction constant is similar to those reported for plasma (0.008 min-1) and muscle (0.004 min-1), and the permeability value is within the range predicted by correlation with the permeability-octanol/water partition coefficient. Fits to data were accomplished with mathematical expressions giving the average total platinum concentration in saggital cerebellar sections which were not subdivided. Both time-dependent and steady-state solutions were obtained for the transport model, the former predicting a half-time to steady state of 3 h. Boundary effects were also investigated. Concentration profiles, calculated for a point source and for a 23-gauge cannula, were shown to differ by 7%. Similar comparisons between two profiles, one computed for an infinite diffusion range and another computed for drug diffusion into a flowing cerebrospinal fluid (CSF) at a finite range of 3 mm, showed differences of less than 3%. Free and bound drug forms, protein turnover, and CSF uptake have been accounted for as well as the percent infusate recoveries at 100 and 160 h reported by Kroin and Penn.(ABSTRACT TRUNCATED AT 250 WORDS)

A kinetic analysis of substance P trafficking.

The potential for administering substance P (SP) nocitoxins for the treatment of chronic pain has been identified. To characterize treatment protocols for the spinal cord or elsewhere, binding/internalization of these compounds at the cellular targets must be understood quantitatively. Thus, a kinetic model of SP binding and intracellular trafficking has been developed from data. The eight differential equation model describes surface binding between SP and neurokinin 1 receptor, clathrin-mediated endocytosis followed by spatial translation to a perinuclear endosome where SP is sorted from its receptor, SP degradation in late endosomes/early lysosomes, and return of sorted receptor to plasma membrane via recycling endosomes. With suitably optimized parameters, the model accounts for the kinetics of total, membrane-associated, and internalized SP in cells continuously exposed to SP, as well as the fractions of internalized SP remaining intact at 30 and 60 min. Simultaneously, the model accounts for the kinetics of internalization and receptor recycling after SP preloading of membrane and subsequent exposure to SP-free media. Rate constants (min(-1)) are: 0.034 +/- 0.004 (receptor off-rate), 0.15 +/- 0.03 (internalization), 0.048 +/- 0.003 (exit from sorting endosome), 0.062 +/- 0.008 (exit of labeled SP amino acids from prelysosome), and 0.029 +/- 0.004 (receptor return from recycling endosome to plasma membrane). The SP kinetics resemble those of transferrin and its receptor at the internalization step, but are several-fold slower in the sorting and recycling steps.

Labeled kynurenine pharmacokinetic modeling studies in gerbils. Nonequilibrium between infused and endogenous kynurenine.

In order to complete pharmacokinetic studies on the central vs. peripheral origin of several tryptophan metabolites, we infused gerbils with labelled kynurenine (2H4 or 15N2). Osmotic minipumps charged with kynurenine solutions were surgically implanted subcutaneously in adult female gerbils (50-60 g). After a variable number of hours, the gerbils were sacrificed and organs taken for determination of labelled/unlabelled kynurenine ratios using mass spectrometric assay of a pentafluorobenzyl derivative as described previously. Surprisingly high ratios of 2H to 1H-kynurenine were measured in the kidney (0.25-0.40) and urine (4.0-8.0), although the ratio of deuterium labelled to endogenous kynurenine remained below detection limits (< 0.05) in serum and other tissues. Infusion of greater quantities of 2H4-kynurenine confirmed these observations in gerbils in which ratios of 2H4-to-1H kynurenine were measurable in serum and tissues. Synthesis and infusion of 15N2-kynurenine demonstrated that these effects were not due to deuterium isotope substitution. The data demonstrate a non-equilibrium between infused and endogenous kynurenine, which is related to differential rates of protein binding and the rapid clearance of free, infused kynurenine by kidney.

Image-guided, direct convective delivery of glucocerebrosidase for neuronopathic Gaucher disease.

OBJECTIVE: To determine if convection-enhanced delivery (CED) of glucocerebrosidase could be used to treat targeted sites of disease progression in the brain and brainstem of a patient with neuronopathic Gaucher disease while monitoring enzyme distribution using MRI. METHODS: A CED paradigm in rodents (n = 8) and primates (n = 5) that employs co-infusion of a surrogate MRI tracer (gadolinium diethylenetriamine penta-acetic acid [Gd-DTPA]) with glucocerebrosidase to permit real-time monitoring of distribution was developed. The safety and feasibility of this delivery and monitoring paradigm were evaluated in a patient with type 2 Gaucher disease. RESULTS: Animal studies revealed that real-time, T1-weighted, MRI of Gd-DTPA accurately tracked enzyme distribution during CED. Targeted perfusion of clinically affected anatomic sites in a patient with neuronopathic Gaucher disease (frontal lobe and brainstem) with glucocerebrosidase was successfully performed. Real-time MRI revealed progressive and complete filling of the targeted region with enzyme and Gd-DTPA infusate. The patient tolerated the infusions without evidence of toxicity. CONCLUSIONS: Convection-enhanced delivery can be used to safely perfuse large regions of the brain and brainstem with therapeutic levels of glucocerebrosidase. Co-infused imaging surrogate tracers can be used to monitor and control the distribution of therapeutic agents in vivo. Patients with neuronopathic Gaucher disease and other intrinsic CNS disorders may benefit from a similar treatment paradigm.

The kinetics of methotrexate polyglutamation in human breast cancer cells.

The polyglutamation kinetics of methotrexate (MTX) in MCF-7 human breast cancer cells have been formulated mathematically. The model takes account of glutamation and hydrolysis kinetics up through the pentaglutamate level, increased synthesis of dihydrofolate reductase following exposure to drug, reversible tight-binding to reductase, and membrane transport of polyglutamates. The glutamation, hydrolysis, and efflux parameters have been determined from fits to experimental MTX polyglutamate uptake and efflux data. The preferred substrate for folypolyglutamyl synthase in the intact cell appears to be MTX diglutamate, on average being two to three times as reactive as either the parent drug or the triglutamate. Hydrolysis rate constants range from 0.03 to 0.19 h-1, but no clear trend with chain length is observable given the large uncertainty of each parameter estimate. However, the efflux of MTX polyglutamates from MCF-7 cells does show a trend with chain length decreasing with increasing length as expected. The best characteristic time of MTX diglutamate efflux is 4.1 h, about one-third that of the higher polyglutamate species, in agreement with observations on the MDA.MB.436 breast cancer cell line. The model shows quantitative agreement with the fraction of MTX polyglutamates found still to be bound to reductase in MCF-7 cells following 24 h of efflux, and qualitative agreement with the time dependence of bound MTX-polyglutamate concentration profiles obtained on the ZR-75 breast cancer line.

Quantification of local de novo synthesis versus blood contributions to quinolinic acid concentrations in brain and systemic tissues.

The source of the neurotoxin quinolinic acid (QUIN) in brain and systemic tissues under normal and pathologic circumstances reflects either de novo synthesis from L-tryptophan and other precursors, or entry of QUIN itself from the blood. To quantify the relative contributions of blood- versus tissue-derived QUIN, [13C7]-QUIN was infused subcutaneously via osmotic pumps (0.55 microliter/h, 30 mM) in gerbils, and the fraction of QUIN in tissue (Tl; measured in tissue homogenates) derived from blood (Bl; measured in serum) was calculated by the formula ([13C7]QUINTi/QUINTi)/([13C7]QUINBl/ QUINBl). In controls, blood QUIN contributed 38-49% of QUIN in brain, 70% in CSF, between 40 and 70% in kidney, heart, and skeletal muscle, but < 5% in spleen, lung, liver, and intestine. Systemic endotoxin (450 micrograms/kg) increased blood, brain, CSF, and systemic tissue QUIN levels. Notably, the relative proportion of QUIN derived from blood in brain, spleen, lung, and intestine was unchanged by endotoxin, but increased in kidney, heart, and skeletal muscle. In contrast, cerebral ischemic injury (10 min of bilateral carotid artery occlusion) increased regional brain QUIN concentrations at 4 days post ischemia, with a proportional increase in the amount of QUIN derived from de novo synthesis by brain tissue. In the blood and systemic tissues of postischemic gerbils, there were no changes in systemic tissue or blood QUIN levels, or changes in the relative proportions of blood- versus systemic tissue-derived QUIN. These results establish that the brain normally synthesizes QUIN, that the blood is a significant source of QUIN in controls and during acute systemic immune activation, and that the rate of QUIN formation by brain tissue increases in conditions of brain and systemic immune activation.

Folate cycle kinetics in human breast cancer cells.

A mathematical description of polyglutamated folate kinetics for human breast carcinoma cells (MCF-7) has been formulated based upon experimental folate, methotrexate (MTX), purine, and pyrimidine pool sizes as well as reaction rate parameters obtained from intact MCF-7 cells and their enzyme isolates. The schema accounts for the interconversion of highly polyglutamated tetrahydrofolate, 5-methyl-FH4, 5-10-CH2FH4, dihydrofolate (FH2), 10-formyl-FH4 (FFH4), and 10-formyl-FH2 (FFH2), as well as formation and transport of the MTX polyglutamates. Inhibition mechanisms have been chosen to reproduce all observed non-, un-, and pure competition inhibition patterns. Steady state folate concentrations and thymidylate and purine synthesis rates in drug-free intact cells were used to determine normal folate Vmax values. The resulting average-cell folate model, examined for its ability to predict folate pool behavior following exposure to 1 microM MTX over 21 h, agreed well with the experiment, including a relative preservation of the FFH4 and CH2FH4 pools. The results depend strongly on thymidylate synthase (TS) reaction mechanism, especially the assumption that MTX di- and triglutamates inhibit TS synthesis as greatly in the intact cell as they do with purified enzyme. The effects of cell cycle dependence of TS and dihydrofolate reductase activities were also examined by introducing G- to S-phase activity ratios of these enzymes into the model. For activity ratios down to at least 5%, cell population averaged folate pools were only slightly affected, while CH2FH4 pools in S-phase cells were reduced to as little as 10% of control values. Significantly, these folate pool dynamics were indicated to arise from both direct inhibition by MTX polyglutamates as well as inhibition by elevated levels of polyglutamated FH2 and FFH2.

Quinolinic acid in vivo synthesis rates, extracellular concentrations, and intercompartmental distributions in normal and immune-activated brain as determined by multiple-isotope microdialysis.

Quinolinic acid (QUIN) kills neurons by activation of NMDA receptors that are accessed via the extracellular fluid (ECF). In vivo microdialysis was employed to quantify the dynamics of ECF QUIN levels. [(13)C7]QUIN was perfused through the probe for in vivo calibration to accurately quantify ECF QUIN concentrations. Osmotic pumps infused [(2H)3]QUIN subcutaneously to quantify blood contributions to ECF and tissue levels. Local QUIN production rates and influx and efflux rates across the blood-brain barrier were calculated from the extraction fraction of [(13)C7]QUIN, probe geometry, tissue diffusion coefficients, the extracellular volume fraction, and [(2)H3]QUIN/QUIN ratios in blood and dialysates. In normal brain, 85% of ECF QUIN levels (110 nM) originated from blood, whereas 59% of tissue homogenate QUIN (130 pmol/g) originated from local de novo synthesis. During systemic immune activation (intraperitoneal injection of endotoxin), blood QUIN levels increased (10.2-fold) and caused a rise in homogenate (10.8-fold) and ECF (18.5-fold) QUIN levels with an increase in the proportions of QUIN derived from blood. During CNS inflammation (local infusion of endotoxin), increases in brain homogenate (246-fold) and ECF (66-fold) QUIN levels occurred because of an increase in local synthesis rate (146-fold) and a reduction in efflux/influx ratio (by 53%). These results demonstrate that brain homogenate measures are a reflection of ECF concentrations, although there are quantitative differences in the values obtained. The mechanisms that maintain ECF QUIN levels at low values cannot do so when there are large increases in local brain synthesis or when there are large elevations in blood QUIN concentrations.