Current Pharmaceutical Design - Volume 10, Issue 12, 2004

This issue of Current Pharmaceutical Design is dedicated to delivery of therapeutics to the central nervous system (CNS). Some of the most devastating diseases facing mankind are those of the CNS. A great number of substances are candidates for treatment of CNS diseases, but therapeutic development in this area faces a special challenge: the blood-brain barrier (BBB). Almost as great as the actual BBB, is the barrier of knowledge, as right perceptions and wrong perceptions about the BBB battle each other in the minds of those interested in drug delivery. Here, we present 6 reviews by eminent BBB researchers. Their reviews cover much traditional ground, but more importantly, offer fresh insights into the mysteries of the BBB. The first article by David J. Begley is a most timely review about one of the hottest area in BBB [1]. The ABC transporters include the efflux systems of the multi-drug resistance and p-glycoprotein families. This area has developed rapidly and independently in many different fields. Dr. Begley's article clarifies terminology, anatomical locations, and function. It also offers brilliant insights on the topics of efflux and of BBB in general. One area in which efflux has been shown to be of particular importance is that of treatment of AIDS, especially neuroAIDS. Most HIV-1 antivirals are transported out of the brain, creating a reservoir for virus. Dr. Sarah A. Thomas has written a wonderful review of this area [2]. This article, in combination with that of Begley's, give insight into the great role which BBB efflux systems will play in CNS therapeutics. One of the most promising groups of potential CNS therapeutics is that of the VIP / PACAP / Secretin family. This family is comprised of peptides which have a great number of CNS effects, including that of acting as powerful neurotrophins. Unlike most peptides or regulatory proteins, we know a great deal about how these peptides cross the BBB and their effects once in the CNS. Ak et al review this topic thoroughly in a format that could act as a roadmap for work on other families of peptides and regulatory proteins [3]. The review by Rautio and Chikhale addresses one of the special areas of therapeutic delivery across the BBB, that of treatment of brain tumors [4]. A number of new and novel approaches are being investigated which deal with the BBB in a variety of ways. The review by Kabanov and Batrakova [5] addresses similar issues to drug delivery, but across the normal BBB. This review addresses three relatively new approaches to drug delivery based on fundamental principles, such as negating efflux, of BBB drug delivery. Finally, a review by William A. Banks of a more speculative nature raises the question of whether the extracellular pathways may be useful for delivery of therapeutics [5]. Two candidate therapeutics, antibodies directed against amyloid beta protein for use in Alzheimer's disease and erythropoietin used in treatment of strokes, likely enter the brain by this mechanism. Overall, this issue addresses important aspects of therapeutic delivery across the BBB. I thank the contributors for their hard work and excellent reviews.

This issue of Current Pharmaceutical Design is dedicated to the characterization of potential anticancer agents. In this respect, a plethora of approaches are currently being investigated to design or discover novel compounds that are capable to effectively and selectively kill cancer cells, sparing normal cells. These approaches vary from the selection of possible cancer-specific protein targets and the design and synthesis of their potential inhibitors, to the selection and chemical modification of natural products or known cytotoxic agents with anticancer activity, to computational approaches, just to mention a few. Thought not nearly close to be a comprehensive review of the possible current approaches to cancer drug discovery, the four reviews reported here represents excellent examples of as many different methodologies. The first article by Kazuhiro Irie, Yu Nakagawa and Hajime Ohigashi [1] reports on the derivation of novel compounds based on indolactams and benzolactams targeting protein kinase C (PKC) isozymes. As described in the review, the design of agents with not only PKC isozyme selectivity but also the C1 domain selectivity of each PKC isozyme is of fundamental importance for understanding the precise mechanism of tumor promotion and developing novel therapeutic compounds. An emerging field in cancer drug discovery is the search for compounds capable to sensitize cancer cells to chemotherapy (chemosensitizers). Most of the activities in this area are related to the inhibition of anti-apoptotic Bcl-2 family proteins that, over expressed in most human cancers, prevent cancer cells to die despite radiation or chemotherapy. The review by Maurizio Pellecchia and John Reed [2] reports on recent findings that link anticancer and chemopreventive activity of certain natural polyphenols to their direct inhibition of anti-apoptotic Bcl-2 family proteins. A discussion on possible improvements of such compounds as well as their potential uses as chemosensitizers in combination therapy is reported. The search for novel anticancer agents often requires the synthesis of many compounds or the screen of large libraries of compounds. Both approaches are costly and time consuming. Computational chemistry may reduce these efforts by increasing the likelihood of a positive outcome from such studies. In the review by Steven Ren and Eric Lien [3] the contributions of different physicochemical parameters to the tumor cell growth inhibitory activities of several anticancer agents are evaluated by a quantitative structure-activity relationship approach (QSAR). Intriguing results are shown demonstrating that compounds' molecular weight (MW) is the most important contributor to the tumor cell growth inhibitory activities among the physicochemical parameters examined, indicating that MW should be taken into account in anticancer drug design. Also, the results obtained may be used as a filter to design smaller focused libraries from a large number of compounds to be experimentally tested. A traditional way to develop anticancer compounds capable of discriminating between cancer and normal cells is to exploit protein targets involved in cell division. The crucial role played by Tubulin in cell-division has made this protein a target for natural product-based chemical defense mechanisms. Such antimitotic compounds have also great potential as anticancer agents. Peter Wipf, Jonathan Reevs and Billy Day [4] report on elegant synthetic approaches to derive novel analogues of the antimitotic marine natural product curacin A aimed at improving anticancer activity and chemical-physical properties. Low water-solubility and lack of chemical stability have prevented the use of curacin A for clinical development. Therefore the synthetic schemes and analogs proposed in the review might prove very useful in probing the efficacy of such agents in cancer therapy. Overall, the current issue addresses in remarkable detail different aspects of current approaches to cancer drug discovery. We thank all the contributors for their excellent reviews.

The blood-brain barrier (BBB) and the blood-cerebrospinal fluid barrier (BCSFB) form a very effective barrier to the free diffusion of many polar solutes into the brain. Many metabolites that are polar have their brain entry facilitated by specific inwardly-directed transport mechanisms. In general the more lipid soluble a molecule or drug is, the more readily it will tend to partition into brain tissue. However, a very significant number of lipid soluble molecules, among them many useful therapeutic drugs have lower brain permeability than would be predicted from a determination of their lipid solubility. These molecules are substrates for the ABC efflux transporters which are present in the BBB and BCSB and the activity of these transporters very efficiently removes the drug from the CNS, thus limiting brain uptake. Pglycoprotein (Pgp) was the first of these ABC transporters to be described, followed by the multidrug resistanceassociated proteins (MRP) and more recently breast cancer resistance protein (BCRP). All are expressed in the BBB and BCSFB and combine to reduce the brain penetration of many drugs. This phenomenon of “multidrug resistance” is a major hurdle when it comes to the delivery of therapeutics to the brain, not to mention the problem of cancer chemotherapy in general. Therefore, the development of strategies for bypassing the influence of these ABC transporters and for the design of effective drugs that are not substrates and the development of inhibitors for the ABC transporters becomes a high imperative for the pharmaceutical industry.

The introduction, in 1995, of highly active antiretroviral therapy (HAART) dramatically reduced the morbidity and mortality of HIV-infected patients. However, the brain remains a site of viral replication for HIV and thus is still an important target for antiretroviral agents. Consequently, a clear understanding of how the current anti-HIV drugs reach the CNS, and interact at the level of the blood-brain barrier and blood-CSF barrier, is important if we are to maximise viral suppression and improve clinical outcome. It would also contribute to the development of new anti-HIV drugs and the identification of transport inhibitors that could be used as adjuvant therapies. In this review we focus on the role of the blood-brain and blood-CSF barriers in the delivery of the main classes of approved anti-HIV drugs. Among these groups, the CNS distribution of the nucleoside reverse transcriptase inhibitors is the best characterised. It involves probenecid efflux transport mechanisms, which limit their brain delivery and probably their, neurological efficacy. Nevirapine and efavirenz, the commonly prescribed non-nucleoside reverse transcriptase inhibitors, can readily enter the CSF, however, it remains to be seen if a transport system is involved in their distribution. The protease inhibitors have only a limited ability to reach the CNS, with the majority of this class of drugs not even being detected in human CSF after administration. This is partly the result of their removal from the CNS by the efflux transporters; P-glycoprotein, and possibly multi-drug resistance associated protein (MRP).

In recent years, VIP / PACAP / secretin family has special interest. Family members are vasoactive intestinal peptide (VIP), pituitary adenylate cyclase-activating polypeptide (PACAP), secretin, glucagon, glucagon like peptide-1 (GLP1), GLP2, gastric inhibitory peptide (GIP), growth hormone releasing hormone (GHRH or GRF), and peptide histidine methionine (PHM). Most of the family members present both in central nervous system (CNS) and in various peripheral tissues. The family members that are released into blood from periphery, especially gut, circulate the brain and they can cross the blood brain barrier. On the other hand, some of the members of this family that present in the brain, can cross from brain to blood and reach the peripheral targets. VIP, secretin, GLP1, and PACAP 27 are transported into the brain by transmembrane diffusion, a non-saturable mechanism. However, uptake of PACAP 38 into the brain is saturable mechanism. While there is no report for the passage of GIP, GLP2, and PHM, there is only one report that shows, glucagon and GHRH can cross the BBB. The passage of VIP / PACAP / secretin family members opens up new horizon for understanding of CNS effects of peripherally administrated peptides. There is much hope that those peptides may prove to be useful in the treatment of serious neurological diseases such as Alzheimer's disease, amyotropic lateral sclerosis, Parkinson's disease, AIDS related neuropathy, diabetic neuropathy, autism, stroke and nerve injury. Their benefits in various pathophysiologic conditions undoubtly motivate the development of a novel drug design for future therapeutics.

Brain tumors are one of the most lethal forms of cancer. They are extremely difficult to treat. Although, the rate of brain tumor incidence is relatively low, the field clearly lacks therapeutic strategies capable of overcoming barriers for effective delivery of drugs to brain tumors. Clinical failure of many potentially effective therapeutics for the treatment of brain tumors is usually not due to a lack of drug potency, but rather can be attributed to shortcomings in the methods by which a drug is delivered to the brain and into brain tumors. In response to the lack of efficacy of conventional drug delivery methods, extensive efforts have been made to develop novel strategies to overcome the obstacles for brain tumor drug delivery. The challenge is to design therapeutic strategies that deliver drugs to brain tumors in a safe and effective manner. This review provides some insight into several potential techniques that have been developed to improve drug delivery to brain tumors, and it should be helpful to clinicians and research scientists as well.

The blood-brain barrier (BBB) efficiently restricts penetration of therapeutic agents to the brain from the periphery. Therefore, discovery of new modalities allowing for effective delivery of drugs and biomacromolecules to the central nervous system (CNS) is of great need and importance for treatment of neurodegenerative disorders. This manuscript focuses on three relatively new strategies. The first strategy involves inhibition of the drug efflux transporters expressed in BBB by Pluronic® block copolymers, which allows for the increased transport of the substrates of these transporters to the brain. The second strategy involves the design of nanoparticles conjugated with specific ligands that can target receptors in the brain microvasculature and carry the drugs to the brain through the receptor mediated transcytosis. The third strategy involves a rtificial hydrophobiz ation of peptides and proteins that facilitates the delivery of the se pe ptides a nd pr oteins ac r oss BBB. This review discusses the current state, advantages and limitations of each of the three technologies and outlines their future prospects.

Most drugs with central nervous system (CNS) activity enter the brain either by diffusing across the membranes which comprise the blood-brain barrier (BBB) or by being transported by carrier systems across those membranes. Substances which cannot cross the BBB by one of these mechanisms, like serum albumin, are virtually excluded from the CNS. However, this exclusion is not absolute. Cerebrospinal fluid (CSF) levels of albumin, for example, are about 0.5% those of serum levels. Albumin enters the CNS through a variety of pathways collectively termed the extracellular pathways. Any circulating substance can, in theory, use these pathways to enter the CNS. But, traditional drug development has ignored this pathway. To approach even the CSF / serum ratio of 0.5%, a candidate therapeutic would need to meet several criterion: long half-life in blood, small volume of distribution, high potency in the CNS, and absence of brain-to-blood efflux. Two emerging therapeutics which are likely exerting their CNS effects by way of the extracellular pathways are antibodies directed against amyloid beta protein (ABP) and erythropoietin (Epo) used in the treatment of stroke. These examples suggest that the extracellular pathways are an option for the delivery of certain therapeutics to the brain.

Protein kinase C (PKC) isozymes (α, βI, βII, γ, δ, ε, η, Θ) are major receptors of tumor promoters and also play a crucial role in cellular signal transduction via the second messenger, 1,2-diacyl-sn-glycerol (DG). Each isozyme of PKC is involved in diverse biological events, indicating that it serves as a novel therapeutic target. Since PKC isozymes contain two possible binding sites of tumor promoters and DG (C1A and C1B domains), the design of agents with binding selectivity for individual PKC C1 domains is a pressing need. We developed a synthetic C1 peptide library of all PKC isozymes for high-throughput screening of new ligands with such binding selectivity. This peptide library enabled us to determine that indolactam and benzolactam compounds bound to the C1B domains of novel PKC isozymes (δ, ε, eegr, Θ) in some selective manner, unlike phorbol esters and DG. Simpler in structure and higher in stability than the other potent tumor promoters, a number of indolactam and benzolactam derivatives have been synthesized to develop new PKC isozyme modulators by several groups. We focused on the amide function of these compounds because recent investigations revealed that both the amide hydrogen and carbonyl oxygen of indolactam-V (ILV) are involved in hydrogen bonding with the C1B domains of PKCd. Synthesis of several conformationally fixed analogues of ILV led to the conclusion that the trans-amide restricted analogues with a hydrophobic chain at an appropriate position (2,7) are promising leads with a high binding selectivity for novel PKC isozyme C1B domains. We also developed a new lactone analogue of benzolactam-V8 (17) which shows significant binding selectivity for the C1B domains of PKCe and PKCh. Furthermore, our synthetic approach with the PKC C1 homology domains clarified that diacylglycerol kinase b and g are new targets of phorbol esters.

Amongst the most promising chemopreventive agents, certain natural polyphenols have recently received a great deal of attention from the scientific community, nutritionists, the pharmaceutical industry and the public, due to their demonstrated inhibitory activity against tumorigenesis. In view of their anticancer properties, these compounds also hold great promise as potential chemotherapeutic agents. However, to translate these chemopreventive agents into chemotherapeutic compounds, their exact mechanism of action must be delineated. The aim of this manuscript is to review recent findings suggesting that certain natural products bind and antagonize the anti-apoptotic effects of Bcl-2 family proteins such as Bcl-xL and Bcl-2. We will summarize recent studies that were aimed at the identification of the molecular targets of natural polyphenols and at the characterization of their mechanism of action on a molecular level. We will emphasize the importance of these findings that resides not only in the opportunity for the development of novel cancer treatments with these compounds, but also in the structural information that can be used for the design and development of novel and more effective semi-synthetic analogues. The finding reviewed here should encourage the study of possible direct effects of other dietary compounds on Bcl-2 family proteins.

The tumor cell growth inhibitory activities (log 1 / GI50) of 166 anticancer agents studied at the National Cancer Institute (NCI) in vitro anticancer screening program have allowed us to analyze the relative importance of physicochemical parameters in influencing the inhibitory activities. Increased molecular weight, as measured by the logarithm of molecular weight (log MW), is found to be an important contributor to the tumor cell growth inhibitory activities. The tumor cell growth inhibitory activities in different subpanels of the tumor cells are highly inter-correlated with each other. A simple binary quantitative structure-activity relationship (QSAR) model was derived from the 166 anticancer drugs, based on the tumor cell growth inhibitory activities transformed into a binary (active or inactive) data format. The model obtained can be tested with additional new data, and may be useful to identify active compounds from a large compound library to be included in high throughput screening.

Many natural and synthetic compounds bind to tubulin, an ubiquitous globular protein that provides the building blocks for the cellular microtubule network that controls chromosome segregation during mitosis, vesicle movements, intracellular transport of organelles, ciliar and flagellar movement, and maintenance of cell shape. Since the isolation of the antimitotic marine natural product curacin A in 1994, synthetic work on this colchicine-site binding agent has been intense, but only recently have synthetic derivatives been identified that match its potency for tubulin polymerization inhibition and its high level of growth inhibition in cancer cell lines. In addition to several total synthesis efforts, combinatorial libraries were constructed using solution phase and fluorous scavenging approaches. Low watersolubility and lack of chemical stability represent strong detriments for the clinical development of curacin A, but synthetic analogs with improved bioavailability might ultimately probe the paradigm for anticancer efficacy of colchicinesite tubulin binding agents.