Dennis R. Groothuis

The blood-brain and blood-tumor barriers: A review of strategies for increasing drug delivery

Drug delivery to brain tumors has been a controversial subject. Some believe the blood-brain barrier is not important, while others believe it is the major obstacle in treatment and have devised innovative approaches to circumvent it. These approaches can be divided into two categories: those that attempt to increase drug delivery of intravascularly administered drugs by manipulating either the drugs or capillary permeability, and those that attempt to increase drug delivery by local administration. Several strategies have been developed to increase the fraction of intravascular drug reaching the tumor, including intra-arterial administration, barrier disruption, new ways of packaging drugs, and, most recently, inhibiting drug efflux from tumor. When given intravascularly, all drugs have a common drawback: the body acts as a sink, and, even in the best situations, only a small fraction of administered drug actually reaches the tumor. A consequence is that systemic toxicity is usually the dose-limiting factor. When given locally, such as into the cerebrospinal fluid or directly into the tumor, 100% of an administered dose is delivered to the target site. However, local delivery is associated with variable and unpredictable spatial distribution and variation in drug concentration. The major dose-limiting factor of most local delivery methods will be neurotoxicity. The relative advantages and disadvantages of the different methods of circumventing the blood-brain barrier are presented in this review, and special attention is given to convection-enhanced delivery, which has particular promise for the local delivery of large therapeutic agents such as monoclonal antibodies, antisense oligonucleotides, or viral vectors.

Permeability of Different Experimental Brain Tumor Models to Horseradish Peroxidase

The permeability of different brain tumor models to horseradish peroxidase (HRP) was examined by determining the fraction of tumor that contained HRP after intravenous administration. The intracerebral tumor models studied were Avian Sarcoma Virus (ASV)-induced tumors and tumors from transplanted RG-2, S69-C1-5, and 9L cell lines. The average fraction of RG-2 tumors permeable to HRP was .95; of S69-C1-5 tumors, .699; of ASV-induced tumors, .63; and of 9L tumors, .52. Except for the RG-2 tumors, there was considerable regional variation in HRP permeability, which was most marked in the ASV-induced tumors. In ASV-induced tumors, HRP permeability did not correlate with tumor histo ogical classification, size, or anatomic location within the brain. The subcutaneous tumor models studied were RG-2-, S69-C1-5, and 9L-transplanted tumors in rats, and human glioblastoma cell lines transplanted into nude mice. All were completely permeable to HRP. These results indicate that significant differences in permeability to HRP exist among brain tumor models when the tumors are intracerebral, and that all subcutaneous tumors from transplanted glial cell lines are completely permeable to HRP. These variables must be considered in future studies of permeability in experimental brain tumors. Care must be exercised in extrapolating results about permeability from one brain tumor model to another

Pleomorphic xanthoastrocytoma. Ultrastructural and immunohistochemical study of a case with a rapidly fatal outcome following surgery.

In 1979, researchers described a series of young patients with clinically and histologically distinctive supratentorial gliomas which were designated pleomorphic (meningocerebral) xanthoastrocytomas (PXA). Significantly, patients with these neoplasms were reported to have a relatively favorable prognosis. The authors present a new case of PXA in a 32‐year‐old man. This case is unique for two reasons: (1) a relatively rapid fatal outcome with death 21 months after diagnosis; and (2) the presence, at autopsy, of extensive recurrent tumor with features of a malignant astrocytoma. Detailed electron microscopic and immunohistochemical studies, supporting the proposed subpial astrocytic origin of PXA, are presented. Literature pertaining to PXA is reviewed. This report illustrates the unique features of PXA and demonstrates its potential for aggressive behavior. Cancer 52:2055‐2063, 1983.

The entry of antiviral and antiretroviral drugs into the central nervous system

The ability of antiviral and antiretroviral drugs to enter the brain is a critical issue in the treatment of many viral brain diseases, including HIV-related neurologic disease. Much of the literature concerning nucleoside analog entry into the nervous system focuses on drug levels in the cerebrospinal fluid (CSF), equating these with drug levels in the brain extracellular fluid (ECF) as though the two compartments intermix freely. We review the anatomic and physiologic aspects of drug entry into CSF and into brain ECF, as well as the exchange processes between these two compartments. In most instances drug concentrations in the CSF and ECF compartments bear little relationship to one another and using CSF concentrations to extrapolate brain ECF concentrations may significantly overestimate the latter. Accepted terminology and methodology for making measurements of blood-brain barrier function are discussed. Studies of brain uptake that express results as brain:plasma ratios, or that have used microdialysis, may overestimate the amount of drug reaching the brain. Using published data, we present an estimate of the time course of Zidovudine (AZT) concentrations in brain ECF and show that brain concentrations of AZT will likely be below that necessary to inhibit HIV-1 replication when AZT is administered systemically. Antiviral nucleosides and oligonucleotides appear to have limited entry into the brain when given systemically, which may hinder therapy of viral brain diseases, while some of the protease inhibitors may enter the brain more readily. Alternative methods for increasing antiviral and antiretroviral drug delivery to brain are discussed.

Dexamethasone Effects on [125I]Albumin Distribution in Experimental RG-2 Gliomas and Adjacent Brain

A total of 72 RG-2 transplanted gliomas were studied in 58 rats at three time points (1, 30, 240 min) after intravenous injection of [125I]radioiodinated serum albumin ([125I]RISA). The animals were divided into two groups: a control group that received no treatment and a second group that was treated with five doses of 1.5 mg/kg of dexamethasone over 2.5 days. Local tissue concentrations of [125I]RISA were measured with quantitative autoradiography based on morphological features of the tumors and used to calculate the tissue distribution space. Two models were used to analyze the data. A two compartment model yielded estimates of local blood-to-tissue influx constants (K1), lower limit extracellular volumes (Ve), and plasma vascular volumes (Vp) in different tumor regions. Treatment with dexamethasone consistently reduced the RISA distribution space in the RG-2 tumors; the reduction in Ve was statistically significant in almost all tumor regions: whole tumor Ve (mean ± SE) was reduced from 0.14 ± 0.02 ml g−1 in control animals to 0.08 ± 0.01 ml g−1 in dexamethasone treated animals. K1 and Vp were also decreased in all tumor regions after treatment with dexamethasone (whole tumor K1 decreased from 2.36 ± 0.89 to 0.83 ± 0.29 μl g−1 min−1 and Vp decreased slightly from 0.016 ± 0.013 to 0.010 ± 0.005 ml g−1 after dexamethasone treatment), but these changes were not statistically significant. A comparison of the tumor influx constants in control animals and the aqueous diffusion constants of two different size molecules (RISA and aminoisobutyric acid) suggests that the “pores” across RG-2 capillaries are large and may not restrict the free diffusion of RISA (estimated minimum pore diameter > 36 nm) and that the total pore area is ∼6.2 × 10−5 cm2 g−1 in RG-2 tumor tissue. The second model, which allows for diffusion and solvent drag of RISA across tumor capillaries and through the tissue, was used to analyze the distribution profiles of RISA in peripheral tumor and adjacent brain. This analysis was consistent with a small bulk flow of plasma-derived edema fluid (capillary filtration rate ≈ 0.8 μl g−1 min−1) and a larger component of free diffusion of RISA (K ≈ 2 μl g−1 min−1) through pores in the tumor vessels of control animals. Dexamethasone treatment markedly reduced or eliminated the filtration of plasma-derived fluid across tumor capillaries and the movement of RISA through the extracellular space by solvent drag. These effects may be mediated by a reduction in the size of the extracellular space and/or a decrease in the pore size of tumor capillaries and could represent an important mechanism for corticosteroid control of tumor and peritumoral brain edema.

Hematopoietic growth factors pass through the blood-brain barrier in intact rats

We have previously demonstrated that receptors for hematopoietic growth factors, stem cell factor (SCF) and granulocyte-colony stimulating factor (G-CSF) are expressed in the neurons and the neural progenitor cells (NPCs) of the adult rat brain, and that systemic administration of SCF and G-CSF in the first week after induction of cortical brain ischemia (3 h-7 days post-ischemia) significantly improves functional outcome, augments NPC proliferation, and reduces infarct volume in rats. The purpose of the present study is to determine whether SCF and G-CSF pass through the blood-brain barrier (BBB) in intact rats. The growth factors were labeled with iodine (I(125)), a radioactive compound. I(125)-SCF and I(125)-G-CSF were intravenously administered and the concentrations of I(125)-SCF and I(125)-G-CSF in the blood plasma and the brain were determined at 10, 30, 60, and 120 min after injection. We observed that both SCF and G-CSF were slowly and continuously transported from the blood stream to the brain in the same rate. In addition, both immunofluorescent staining and Western blots showed that receptors for SCF and G-CSF were expressed in the capillaries of the adult rat brain, suggesting that SCF and G-CSF entry to the brain may be mediated via receptor-mediated transport, one of the endogenous transports in the BBB. These data indicate that both SCF and G-CSF were able to pass through the BBB in intact animals. This observation will help in further exploring the physiological role of peripheral SCF and G-CSF in the brain and therapeutic possibility to chronic stroke.

Changes in blood-brain barrier permeability associated with insertion of brain cannulas and microdialysis probes

Blood-brain barrier (BBB) transcapillary transport was studied after insertion of cannulas and microdialysis probes into the brains of three groups of rats. Quantitative autoradiography was used to measure changes in BBB permeability around the insertion site. In the first group, BBB function was measured with 14C-sucrose at times from immediately, and up to 28 days, after insertion of a microdialysis probe. BBB function was disrupted biphasically: a 19-fold increase in the influx constant (K1) of sucrose occurred immediately after insertion with a second 17-fold increase at 2 days, followed by a slow decline to 5 times normal values at 28 days. In the second group, 14C-dextran (70 kDa) was used to measure BBB transcapillary transport; K1 was increased 90-fold after probe insertion. In the 3rd group, 14C-AIB (alpha-aminoisobutyric acid) was used to evaluate BBB transport after insertion of a 27 gauge cannula, which was used to infuse 1 microliter of saline over 5 min. The K1 of AIB was increased 25 times control values. We conclude that BBB transcapillary transport function is disturbed in response to insertion of brain cannulas and/or microdialysis probes, that BBB dysfunction is maximal at the cannula or probe tip, varies with time after insertion, may persist for at least 28 days after insertion, and occurs over a wide molecular range of solutes. These results suggest caution when using microdialysis as a method to study normal BBB function, and suggest that microdialysis may overestimate the rate of transfer into and out of the brain.

Comparison of cytosine arabinoside delivery to rat brain by intravenous, intrathecal, intraventricular and intraparenchymal routes of administration

We evaluated the delivery of 14C-cytosine arabinoside (AraC) to rat brain by: 1) intravenous (IV) bolus, by 2) intrathecal (IT) and 3) intraventricular (IVT) infusion, and by 4) convection-enhanced delivery (CED) into the caudate nucleus. Plasma and brain AraC metabolites were measured with HPLC, and distribution and concentration of 14C-AraC in brain sections were measured by quantitative autoradiography. After IV administration, the alpha and beta plasma half-lives were 1.9 and 46.5 min, respectively. The blood-to-brain transfer constant of AraC was 2.5+/-1.4 microliter g(-1) min(-1), compatible with high water solubility. After IT and IVT administration, tissue levels were high at the brain and ventricular surfaces, but declined exponentially into brain. After CED, maximum brain levels were up to 10,000 times higher than the IV group, and the distribution pattern was one of high 14C-AraC concentration in the convective component, with exponentially declining concentrations outside this region. The rate loss constant from brain was 0.002+/-0.0004 min(-1), suggesting that AraC was accumulating in brain cells. AraC was metabolized into uracil arabinoside within the brain. 14C-AraC was infused into 1 dog and distributed widely in the ipsilateral hemisphere. These studies suggest that delivery of AraC to brain parenchyma by the IV, IT or IVT routes will be subtherapeutic. Delivery by CED can achieve, and maintain, therapeutic levels of AraC in the brain, and should be further evaluated as a potential method of drug delivery.

Regional cerebral blood flow in the beagle puppy model of neonatal intraventricular hemorrhage Studies during systemic hypertension

The newborn beagle puppy serves as an animal model for intraventricular hemorrhage (IVH) of the premature infant. Since increased systemic blood pressure has been implicated in the genesis of IVH in both babies and puppies, we studied regional cerebral blood flow in control and hypertensive puppies. Hypertension significantly increased blood flow to all structures. The largest increases occurred in gray matter, especially deep cerebral and brainstem nuclei. Blood flow also increased to deep hemispheric white matter, but the magnitude of the increase was smaller. Hypertension also increased blood flow to the subependymal germinal matrix (GM). The magnitude of the increase to most of the GM was small and similar to deep hemispheric white matter. The increase to the most rostral GM was higher and equal to the mean increase seen in gray matter. This rostralcaudal gradient of hypertension-induced hyperperfusion may explain the tendency for IVH to occur in rostral GM in premature babies. However, the failure to find a disproportionate increase in blood flow to GM during hypertension implies that additional factors besides hypertension-induced GM hyperperfusion may be involved in the pathogenesis of IVH.

Brain tumors and the blood—brain barrier

Abstract Brain tumors have blood vessels with permeability characteristics different from those of normal brain, as demonstrated clinically by the uptake of iodinated contrast agents used in CT scanning. The structural alterations of capillaries in brain tumors may account for increased permeability to blood-borne substances. This abnormal permeability may have profound implications for chemotherapy, but the problem is not simple and brain tumor chemotherapy has so far been a disappointment. Since it is generally agreed that the future of chemotherapy will play an important role in the treatment of malignant brain tumors, strategies will depend mainly upon quantitative studies of transcapillary exchange in both experimental and human brain tumors.