Drug Delivery Based on Blood Brain Barrier Disruption

Another invasive strategy for drug delivery to the brain is the temporary physi-cochemical disruption of endothelial integrity. Experimental barrier opening for low molecular weight tracers and macromolecules (Evans blue-albumin) was demonstrated with intracarotid infusions of membrane active agents like bile salts (24), oleic acid (25), the cytostatic drugs etoposide (26) and melpha-lan (27), and cytochalasin B (28). Intracarotid low pH buffer infusion also opens the BBB (29).

1. Hyperosmolar Barrier Opening

Most studies, however, are available with hyperosmolar solutions, a principle that was described by Broman (30) and extended by Rapoport (31). Techni cally, the procedure requires general anesthesia and high flow, short-term infusion of 25% mannitol or arabinose. Hypertonic disruption is under clinical evaluation for enhanced delivery of small molecular weight cytostatic agents to brain tumors (32). The underlying mechanism is a sequela of endothelial cell shrinkage, disruption of tight junctions, and vasodilation by osmotic shift. Morphological studies in rats, where the induction of neuropathological changes by osmotic opening was examined, provided evidence of brain uptake of macromolecules: the extravasation of plasma proteins such as fibrinogen and albumin was shown immunohistochemically at the light microscopic level (33). Electron microscopy also revealed ultrastructural changes such as swelling of astrocytic processes and severe mitochondrial damage in neurons (34). Infusion of albumin-gold complexes after BBB disruption by intracarotid hyperosmolar arabinose was used to visualize the cellular mechanism in rats at the electron microscopic level (35). In addition to opening of junctional complexes and the formation of interendothelial gaps, transendothelial openings and tracer passage through the cytoplasm of injured endothelial cells were observed. In response to hyperosmotic barrier disruption, there was also evidence of prolonged (24 h) cellular stress or injury in neurons and glia, as expressed by the induction of heat shock protein (HSP-70) (36).

It has been shown that the barrier opening for high molecular weight compounds is of shorter duration than that for small molecules (37). When the degree of barrier opening is measured with methods that are suitable for a regional evaluation (autoradiography in animal studies, positron emission tomography in man), there is a characteristic difference in the degree of barrier opening in the tumor versus normal brain. This opening was consistently found to be more pronounced for the normal BBB (38, 39). While the nonspecific opening of the BBB to plasma proteins has a potential to elicit neuropathologi-cal changes, osmotic disruption has been tested as a strategy for the brain delivery of macromolecular drugs such as monoclonal antibodies, nanopar-ticles, and viruses. Quantitative uptake studies after hyperosmolar BBB opening in animals and humans were performed with radiolabeled monoclonal antibodies and their antigen binding fragments against various tumor antigens (40-42). In normal rat brain, a 25- to 100-fold relative accumulation in the BBB-disrupted hemisphere of a radioiodinated rat monoclonal antibody (IgM) against human small-cell carcinoma of the lung was reported (40). The mean PS value at 10 minutes after BBB opening and intracarotid antibody infusion was calculated as 8.36 X 10-6 s-1 (= 0.5 |il min^g-1). In patients with in-tracranial melanoma metastasis, 131I-labeled antigen binding fragments of melanoma-specific antibodies were infused intravenously in conjunction with BBB disruption (41). Brain uptake was measured by gamma camera imaging and was used to calculate PS values at 3 hours. A mean PS value of 1.16 X

10~6 s_1 was estimated for the treated hemisphere, which was threefold higher than for the nonperfused hemisphere. Owing to the transient nature of BBB opening, the calculated PS values might represent only rough estimates. At any rate, no specific enhancement of tracer uptake in tumor versus normal brain was seen in the patient study (41). The ability of osmotic disruption to deliver 20 nm iron oxide particles to normal brain was postulated in another study (43). Similarly, recombinant adenovirus or herpesvirus was delivered by intracarotid administration to normal brain tissue (44) and to tumor xeno-grafts in nude rats (45).

2. Biochemical Barrier Opening

BBB opening may also be achieved by receptor-mediated mechanisms. The vasoactive compounds prostaglandins, histamine, serotonin, leukotriene C4 (LTC4), and bradykinin have all been shown to induce BBB leakage (46). The effects of LTC4 and bradykinin are more pronounced on the blood-tumor barrier than on the normal BBB (47, 48). In the case of LTC4, that difference is ascribed to the presence of an enzymatic barrier in normal brain tissue due to the endothelial expression of y-glutamyltransferase (y-GT). The enzyme metabolizes and inactivates LTC4 to LTD4 (49). In contrast, tumor vessels are unable to express equivalent activities of y-GT, a fact that may be exploited for selective opening of the tumor barrier by intracarotid administration of LTC4. However, the effect is restricted to small molecules, as indicated by the absence of any increase in the tumor accumulation of a dextran tracer of molecular weight 70 kDa (47). On the other hand, bradykinin opens the barrier in the high molecular weight range, too. It acts on endothelial cells through B2 receptors located on the abluminal side. Normal brain tissue is protected from barrier opening by bradykinin in the vascular lumen because the peptide cannot access these receptors. In tumor vessels the barrier integrity is sufficiently compromised to allow for a bradykinin-mediated additional opening at low peptide concentrations (48). The effect shows a rapid desensitization within 60 minutes. An increase in intracellular Ca2+ concentration has been shown, which in turn transiently disrupts intercellular tight junctions (50). In addition, the nitric oxide-cyclic GMP pathway is involved. While bradykinin itself requires intracarotid administration, an analogue with prolonged halflife (RMP-7) is effective after intracarotid (51) or intravenous (52) application. The drug is evaluated in the therapy of human malignant gliomas to enhance delivery of carboplatin to the tumor. Recently, a four- to fivefold increase in the delivery of the cytokines interferon y, tumor necrosis factor a, and interleu-kin 2 to experimental brain tumors (RG2 glioma) after intracarotid infusion of RMP-7 was demonstrated (53).

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