Delivery of Therapy to Brain Tumors: Problems and Potentials

The Blood-Brain Barrier

The BBB consists of a layer of endothelial cells that line the blood vasculature throughout the brain. These endothelial cells are held together by tight junctions ^, and are supported by neighboring astrocytes. Certain molecules, particularly large water-soluble molecules, are prevented from diffusing through the gaps between these cells. Small, lipophilic molecules passively diffuse, but their distribution is also limited because of high tissue clearance rates. The BBB limits the delivery of many common chemotherapeutic agents to the CNS, and its permeability to most molecules can be predicted on the basis of each agent’s octanol/water partition coefficient and the unidirectional transfer coefficient (K _in ). The octanol/water partition coefficient is a measure of solute lipophilicity, whereas K _in is a quantitative measure of the ability of a drug to pass from the plasma into the brain. K _in is determined largely by lipid solubility because agents must first dissolve in the lipid membranes of the BBB to cross by lipid-mediated diffusion. K _in can be plotted against the partition coefficient, and 20 reference permeability markers that bind minimally to plasma proteins and cross the BBB by passive diffusion are used to formulate the plot ( Fig. 141.1 ). In this plot, many of the chemotherapeutic agents fall below the line predicted for BBB passive diffusion.

Active or facilitated transport is used for substances with low partition coefficients. This type of transport is dependent on ion channels, specific transporters, energy-dependent pumps, and receptor-mediated endocytosis. Glucose, amino acids, and small intermediate metabolites are carried into the brain via facilitated transport, whereas substances with larger molecules, such as insulin and transferrin, are carried across the endothelial layer via receptor-mediated endocytosis.

Specific factors contribute to the poor chemotherapeutic uptake across the BBB; these include plasma protein binding, solute molecular weight, and active efflux transport. Most chemotherapeutic agents are bound to plasma proteins, which reduce the free fraction of the drug in the plasma that is available to cross the BBB. The molecular weights of most of these agents exceed 400 Da, which is significantly larger than what normally would leak passively through the BBB. ^, Another component of the BBB that makes it difficult for chemotherapeutics to function is its expression of high levels of drug efflux pumps, such as P-glycoprotein. These pumps actively remove chemotherapeutic drugs from the brain. A number of newer chemotherapeutic agents, such as gemcitabine, docetaxel, pemetrexed, irinotecan, and topotecan, show promising antitumor activity against systemic tumors but have limited delivery across the BBB because of active efflux transport and plasma protein binding. The tyrosine kinase inhibitor imatinib binds heavily to plasma proteins and is a substrate for active efflux pumps, which is also suspected to be the case for erlotinib and gefinitib.

The organic anion transporters and glutathione-dependent multidrug resistance–associated proteins (MRPs) also contribute to the efflux of organic anions from the brain and CSF. Receptor-mediated transport modulation, vasomodulators, nanoparticle delivery methods, and osmotic BBB disruption are all techniques that have been used to increase BBB permeability for chemotherapeutics. Unfortunately, these techniques do not ensure specific delivery to tumor tissue alone, and neurotoxicity can occur as a consequence of the enhanced CNS penetration of some of these agents. Interstitial drug administration via convection-enhanced delivery (CED) is another CNS delivery method; it circumvents the BBB, thereby potentially avoiding systemic toxicity while enabling delivery to targeted areas of the brain.

Lipophilic Analogues

The BBB allows the diffusion of small, nonionic, lipid-soluble molecules, whereas larger, ionic, more water-soluble molecules do not readily cross it ( Table 141.1 ). To improve drug delivery to brain tumors, certain strategies have been formulated in which lipophilic analogues are used to enable passive drug uptake into the brain. Carmustine is an alkylating agent used to treat brain tumors and other well-known malignancies. Multiple carmustine analogues studied in clinical trials have demonstrated decreased alkylating activity and increased dose-limiting toxicity in comparison with carmustine. This result was probably caused by factors affecting drug-receptor interactions or increased binding to plasma proteins, which resulted in lower drug concentrations available for diffusion into the brain. Moreover, lipophilic analogues are less soluble in the brain interstitial fluid, which limits their activity against tumor cells. Although the use of lipophilic analogues has met a number of obstacles, ^, the concept of lipophilic transport across the BBB has been expanded upon in the form of liposomal and nanoparticle transport, ^, cellular transport, and vector-mediated transport strategies. ^,

Prodrugs

As alternatives to analogues, prodrugs require a chemical or biochemical transformation to achieve the active form within the body. ^, Prodrugs are designed to overcome pharmaceutical and pharmacokinetic limitations of the parent molecule to better penetrate the BBB. Lipophilic ester prodrugs of the anticancer agent chlorambucil have been developed to increase efficacy in the treatment of brain tumors After equimolar doses of chlorambucil and chlorambucil–tertiary butyl ester, the brain delivery of the ester was 35-fold greater than that of chlorambucil. However, despite an enhanced CNS delivery ratio, none of the prodrugs demonstrated anticancer activity superior to that of the equimolar administration of chlorambucil against a brain-sequestered carcinosarcoma in rats. Other examples of lipophilic prodrugs have shown excellent CNS and glioma penetration in animal models. A different strategy for prodrug conversion consists of using stem cell–mediated conversion. Neural stem cells transduced with cytosine deaminase have been shown to effectively convert the prodrug 5-fluorocytosine to 5-fluorouracil following direct intracranial administration in recurrent high-grade glioma patients.

Antibody- and Gene-directed Enzyme Prodrug Therapy

To enhance prodrug activity at the site of action with minimal systemic toxicity, antibody-directed enzyme prodrug therapy (ADEPT) can be utilized. With ADEPT, enzymes that activate prodrugs are directed to human tumor xenografts by conjugating them to tumor-selective monoclonal antibodies. Once the enzyme is conjugated with an antitumor antibody, it is delivered to the tumor via intravenous infusion. When the conjugate is cleared from the blood, a prodrug is then delivered. The prodrug can now be activated by the tumor-associated enzyme.

Unfortunately, this strategy has limitations, and most directed prodrug therapy is being pursued in the form of gene-directed enzyme prodrug therapy (GDEPT). With GDEPT, an inactive prodrug can be activated to release a cytotoxic drug by an enzyme that has been delivered to the tumor for expression. Specific enzymes utilized include thymidine kinase, nitroreductase, cytosine deaminase, and cytochrome P-450. One example is the thymidine kinase gene, which is inserted into the herpes simplex virus and whose administration is followed by combination treatment with the prodrug ganciclovir. The cytotoxicity of these prodrug/enzyme combinations can be quite effective under in vitro conditions. However, this approach does not specifically target the invasive tumor cell populations. Improving the efficacy of delivery of these so-called suicide genes to infiltrating tumor cells has been one goal associated with research into stem cell or viral vector delivery systems. This combination of GDEPT with cell-targeting technologies exemplifies the synergistic approach to neuro-oncology that is evolving. A phase 2 study in which thymidine kinase introduced via viral vectors in conjunction with valacyclovir into high-grade gliomas in conjunction with the standard of care has been shown to demonstrate survival benefit.

Receptor- and Vector-mediated Drug Targeting

In carrier-mediated drug delivery, the facilitative endogenous transport systems that are present in brain endothelial cells are used. Specific transport systems for the brain include those of glucose, amino acids, choline, vitamins, low-density lipoproteins, and nucleosides. Glucose and the large neutral amino acids have a high transport capacity, so they are mostly used for drug delivery to the brain. Several amino acid–mimetic drugs in clinical use, such as levodopa, α-methyldopa, and baclofen, are readily taken up into the brain by these transport systems. The coupling of nontransportable therapeutic molecules to a drug-transport vector has improved CNS delivery significantly.

Similarly, drugs targeted for transport through olfactory and respiratory epithelium are receiving increased attention, with promising results in early phase 1 and phase 2 clinical trials. Intranasal delivery is a practical, noninvasive method for delivering therapeutic agents to the brain because of the unique anatomic connection provided by the olfactory and trigeminal nerves. Intranasally administered drugs reach the parenchymal tissues of the brain and spinal cord or CSF within minutes via an extracellular route through perineural channels. In addition to its bypassing the BBB, the advantages of intranasal delivery include rapid delivery to the CNS, avoidance of hepatic first-pass drug metabolism, and elimination of the need for systemic delivery, which thereby reduce systemic side effects. Various molecules have been investigated as vectors to facilitate the intranasal route whereby lipid solubility is relied on in addition to intraneural and perineural transport mechanisms. ^,