Current Medicinal Chemistry - Volume 11, Issue 4, 2004

Paclitaxel has been proven to be effective against different types of cancer. A delivery system loaded with paclitaxel at tumor site should provide a high local concentration of the drug detrimental to malignant cells, which prevents the re-growth and metastasis of tumor. In this review, paclitaxel formulations for systemic and for intratumoral administration are discussed.

Paclitaxel (Taxol™), a naturally occurring antimitotic agent, has shown significant cell-killing activity against tumor cells through induction of apoptosis. The mechanism by which paclitaxel induces cell death is not entirely clear. Recent studies in our laboratory discovered that glucocorticoids selectively inhibited paclitaxel-induced apoptosis without affecting the ability of paclitaxel to induce microtubule bundling and mitotic arrest. This finding implies that apoptotic cell death induced by paclitaxel may occur via a pathway independent of mitotic arrest. Through analyses of a number of apoptosis-associated genes or regulatory proteins, we found that glucocorticoids and paclitaxel possess opposite regulatory role in the NF- κB / IκBα signaling pathway. Further studies indicate that paclitaxel activates IκB Kinase (IKK), which in turn causes degradation of IκBα and activation of NF-κB, whereas glucocorticoids antagonize paclitaxel-mediated NF-κB activation through induction of IκBα synthesis. These results suggest that the NF-κB / IκBα signaling pathway might play a critical role in the mediation or regulation of paclitaxel-induced cell death. On the other hand, since glucocorticoids (such as dexamethasone) are routinely used in the clinical application of paclitaxel to prevent hypersensitivity reactions and other adverse effects, the inhibitory action of glucocorticoids on paclitaxel-induced apoptosis also raises a clinically relevant question as to whether the pretreatment with glucocorticoids might interfere with the therapeutic efficacy of paclitaxel.

Paclitaxel is one of the best antineoplastic drugs found from nature in the past decades, which has been found effective against a wide spectrum of cancers including ovarian cancer, breast cancer, small and non small cell lung cancer, colon cancer, head and neck cancer, multiple myeloma, melanoma, and Kaposi's sarcoma. Like many other anticancer drugs, it has difficulties in clinical administration due to its poor solubility in water and most pharmaceutical reagents. In its current clinical application, an adjuvant called Cremophor EL has to be employed, which has been found to be responsible for many serious side effects. Nanoparticles of biodegradable polymers can provide an ideal solution to such an adjuvant problem and realize a controlled and targeted delivery of the drug with better efficacy and less side effects. With further development, such as particle size optimization and surface coating, nanoparticle formulation of paclitaxel can promote a new concept of chemotherapy to realize its full efficacy and to improve quality of life of the patients, which includes personalized chemotherapy, local chemotherapy, sustained chemotherapy, oral chemotherapy, chemotherapy across the blood-brain barrier, chemotherapy across the microcirculation barrier, etc. The present research proposes a novel formulation for fabrication of nanoparticles of poly(lactic-co-glycolic acid) (PLGA) by a modified solvent extraction / evaporation technique, in which natural emulsifiers, such as phospholipids, cholesterol and vitamin E TPGS are creatively applied to achieve high drug encapsulation efficiency, desired drug released kinetics, high cell uptake and high cytotoxicity. The nanoparticles composed of various recipes and manufactured under various conditions were characterized by laser light scattering (LLS) for size and size distribution, scanning electron microscopy (SEM) and atomic force microscopy (AFM) for morphological properties, X-ray photoelectron spectroscopy (XPS) and Fourier Transformation Infrared Spectroscopy (FTIR) for surface chemistry, zeta-potential for surface charge, and differential scanning calorimetry (DSC) for the thermogram properties. The drug encapsulation efficiency and the drug release kinetics under in vitro conditions were measured by high performance liquid chromatography (HPLC). It was found that these natural emulsifiers have great advantages for nanoparticle formulation of paclitaxel over the traditional macromolecular emulsifiers, such as polyvinyl alcohol (PVA). Nanoparticles of desired small size and narrow size distribution can be obtained. The drug encapsulation efficiency can be achieved as high as 100 %. The released kinetics can be made under control. The HT-29 cancer cell line experiment showed that after 24 hours of incubation, the cell mortality caused by the drug administered by such nanoparticle formulation could be more than 13 times higher than that caused by the free drug under similar conditions.

Ovarian cancer patients with paclitaxel-resistance have been reported to respond to a weekly schedule of the same drug. In this report, two cases with long progression free interval by weekly paclitaxel (T) are presented. Case 1. A 41-year-old Japanese woman, gravida 2, para 0, was referred to our hospital in September 16, 1998, because of abdominal mass accompanying large amount of ascites with elevated CA125 (8400 U / ml) and CA19-9 (770 U / ml). Exploratory laparotomy (tumor biopsy plus partial omentectomy) was performed September 21, 1998. After the surgery, the tumor was diagnosed as serous cystadenocarcinoma of the ovary (stage IV) and 6 cycles of treatment consisting of cyclophosphamide, adriamycin and cisplatin (CAP) were performed. The CA 125 level (8400 U / ml) rapidly declined to 150 U / ml by this CAP therapy. After second cytoreductive surgery (SRS) (total hysterectomy and bilateral salpingo-oophorectomy), residual tumor was less than 2 cm. Although 7 cycles of CAP was added, ascites and elevation of CA 125 (5100 U / ml) were observed. Therefore, treatment with single weekly T was performed and CA 125 levels remained between 70-90 U / ml during 13 cycles of this therapy (progression free interval; more than 1 year). Thereafter, she is alive with disease and followed-up. Case 2. A 48-year-old Japanese woman, gravida 3, para 2, was referred to our hospital in July 22, 1998, because of abdominal swelling and pain. Computing tomography (CT) and magnetic resonance imaging (MRI) revealed large amount of ascite and pelvic mass (9 x 7 x 7 cm), and low density area (3 x 3 cm) suggesting metastasis in right lobe of liver. Serum CA 125 level elevated to 5100 U / ml. Bilateral salpingo-oophorectomy and infracolic omentectomy were performed on August 5, 1998. The tumor was diagnosed as endometrioid adenocarcinoma of the ovary, stage IV and chemotherapy with CAP was initiated on September 5, 1998. After 6 cycles of CAP, SRS was performed. After SRS, 3 cycles of CAP were added and changed to weekly T because of damage of renal function. The CA 125 level returned within normal range during weekly T. Total 13 cycles of weekly T were performed and progression free interval was about 18 months. Thereafter, she received treatments with gamma knife and CAP for brain metastasis. She is alive without disease and followed-up. Side effects by weekly T were mild and tolerable despite of long term treatment. In addition, weekly T can be safely used in outpatient setting and even in patients with poor performance status (PS), and warrant long time to progression.

The basic principle of the targeted delivery approach is that the conjugation of a drug to a tumorspecific molecule renders the drug inactive until it reaches the target site. Monoclonal antibodies (mAbs), which have shown high binding specificity for tumor-specific antigens, could be used as targeting agents. Paclitaxel has brought significant impact on the current cancer chemotherapy, but seriously suffers from the lack of tumor specificity. A series of paclitaxel-monoclonal antibody conjugates via C-2' ester linkage were reported. Taking into account the fact that the cytotoxicity of paclitaxel is not good enough and thus not applicable to this target delivery prodrug approach, new taxoids bearing methyldisulfanyl(alkanoyl) groups were designed, synthesized, and their activities evaluated. A highly cytotoxic C-10 methyldisulfanylpropanoyl taxoid, was conjugated to monoclonal antibodies recognizing the epidermal growth factor receptor (EGFR). These conjugates were shown to possess remarkable target-specific antitumor activity in vivo against EGFR-expressing A431 tumor xenografts in SCID mice, resulting in complete inhibition of tumor growth in all the treated mice without any noticeable toxicity to the animals.

Antibodies and anticancer drug-antibody conjugates used in experimental cancer research or clinically must be freeze-dried for preserving the activity and storage at room temperature. This often results in some denaturation and loss of activity . We describe a recovery of the cytotoxic activity of a paclitaxel-mAb immunoconjugate after freeze-drying. The paclitaxel-antibody conjugate specific for ovarian cancer was tested both for its cytotoxicity in vitro and immunological activity after freeze drying in the presence of various preservatives. Results show that the inclusion of trehalose as a stabilizer at concentrations varying from the 0.25 and 0.40M protected the antibody and saved the pharmacological activity. When PEG alone or with trehalose was used, the immunological and cytotoxic activities recovery were lower. Albumin was not protective. This study shows that the addition of trehalose for freeze drying labile drug is a promising method for storage of large quantities of the immunoconjugates for experimental and therapeutic use.

All types of thymic cells are able to produce cytokines either spontaneously or after stimulation. The main producers of cytokines in the thymus are thymic epithelial cells (TEC) and thymocytes. Thymic cytokines act at short distance and their effects are limited by the internal space of the organ. The spectrum of biological effects of thymic cytokines is determined by the expression of cytokine receptors on the thymic cell surface. Some cytokines produced by the thymic cells of one type are supplied to cells of other types; other cytokines act as autocrine factors. Examples of paracrine thymic cytokines are IL-7 (produced by TEC or stromal fibroblasts induces CD4-CD8- thymocyte growth and differentiation) and INFγ (produced by thymocytes, induces TEC activation). An example of an autocrine factor is IL-2, for which the producers and targets are thymocytes. The ability of thymocytes to produce cytokines and express cytokine receptors is gradually reduced as they mature from the stage of CD44+CD3-CD4-CD8- precursor cells to the stage of CD3loCD4+CD8+ cortical thymocytes; in the latter stage both these capacities become completely blocked. This change reflects the decrease of cytokine dependence of the respective processes. After the completion of the selection process, the capacity of thymocytes to produce cytokines and respond to their action is restored. Some differences in the function of the cytokine system in thymus and peripheral compartments of the immune system can be noted. 1. Unlike the periphery, where cytokine production and receptor expression are inducible, the synthesis of cytokines and expression of their receptors in the thymus has mainly a “spontaneous” character (or it is induced by cell-cell interactions). 2. Cytokines tightly interact, forming a cytokine network both at the periphery and in the thymus, but the structure of the peripheral and intrathymic cytokine network is different. The latter can be termed as a “minor cytokine network”. Some peptide hormonelike factors play a significant role in the intrathymic cytokine network. 3. The principal role of thymic cytokines is to provide constitutive processes (migration and development of thymocytes, regulation of cell number in the cell populations, etc.), but not inducible ones (inflammation, immune response, etc.) as in the periphery. 4. The functions of some cytokines in the thymus can be significantly different from those in the periphery of the immune system. For example, proinflammatory cytokines act in the thymus as factors or cofactors of thymocyte or TEC activation, proliferation or differentiation. The key cytokines of Th1 and Th2 cells - IFNγ and IL-4 - do not participate in the immune response but mediate the dialogue between thymocyte and TEC and play a role in autoregulating the thymocyte population. The functions of many cytokines in the thymus are not established up to now. Detailed analysis of the “minor cytokine network” and intrathymic cytokine effects will reveal some unknown events of thymus physiology.

Sorbitol dehydrogenase (SDH), a member of the medium-chain dehydrogenase / reductase protein family and the second enzyme of the polyol pathway of glucose metabolism, converts sorbitol to fructose strictly using NAD+ as coenzyme. SDH is expressed almost ubiquitously in all mammalian tissues. The enzyme has attracted considerable interest due to its implication in the development of diabetic complications and thus its tertiary structure may facilitate the development of drugs for the treatment of diabetes sufferers. Modelling studies suggest that SDH is structurally homologous to mammalian alcohol dehydrogenase with respect to conserved zinc binding motif and a hydrophobic substrate-binding pocket. Recently, the three-dimensional (3- D) structure of a mammalian SDH was solved, and it was found that while the overall 3-D structures of SDH and alcohol dehydrogenase are similar, the zinc coordination in the active sites of the two enzymes is different. The available structural and biochemical information of SDH are currently being utilized in a structure-based approach to develop drugs for the treatment or prevention of the complications of diabetes. This review provides an overview of the recent advances in the structure, function and drug development fields of sorbitol dehydrogenase.

Many immune responses are dependent on the four members of the leukocyte integrin family of surface receptors. Organ transplantation triggers a range of immune responses aimed at rejecting the donor tissue. Engagement of cells of the immune system including T cells, B cells, and antigen presenting cells with each other or with the donor organ is an essential element of the rejection cascade, and this engagement is mediated by adhesions through the leukocyte integrins and their ligands. This review will first focus on the molecular mechanisms that regulate rapid activation of the leukocyte integrins by inside-out signaling that results in changes in receptor clustering on the leukocyte membrane and ligand affinity. Longer lasting changes in receptor expression through transcriptional mechanisms will also be discussed. The role of these receptors in organ transplant rejection, graft-versus-host disease, and ischemic reperfusion injury will also be reviewed. The potential of anti-adhesion therapy to prolong organ transplant survival will be discussed in both animal and patient studies.

Conformational change plays an important role in the life of all proteins, starting from when they fold, through their function and often their fate. For an increasing number of proteins inappropriate conformational change leads to a chain of events, which culminate in the deposition of proteinacious aggregates and disease. In this review we consider the current literature on a number of proteins which form part of the Conformational Disease family. We describe here two types of aggregate that can be formed, Type I aggregates are typified by the Serpin superfamily and consist of non-fibrillar polymeric species. Type II aggregates are of the classical fibrillar form formed by a diverse range of proteins. Through biochemical and biophysical analyis of the aggregation reaction of members of these two classes we show that they form these aggregates through highly similar pathways. Essentially, the whole process can be summed up in two key stages. Firstly, the existence of conditions which increase the conformational flexibility of the protein, enabling it to adopt a partially folded state. Secondly, the propensity of this intermediate conformer to form intermolecular linkages leads to multimeric forms, a step often mediated via hydrophobic or β-strand interactions. Our understanding of these structural changes has facilitated the rationale design of specific aggregation inhibitors. We will discuss the successes and pitfalls of such approaches to demonstrate how similar approaches may be applied to any misfolding protein.

Bile formation is an osmotic process driven by the vectorial transport of actively transferred biliary components across the basolateral (sinusoidal) and apical (canalicular) hepatocyte membranes, the latter being the rate-limiting step of the overall blood-to-bile transfer. The ATP-binding cassette (ABC) superfamily of membrane transporters comprises novel ATP-dependent carriers that mediate canalicular transfer of several endogenous and exogenous substrates, and therefore play a key role in bile formation. Gene expression, as well as the balance between vesicular targeting and internalization of these transporters to / from the canalicular membrane are highly regulated processes. This balance is affected in several models of hepatocellular cholestasis, and these alterations may either initiate or perpetuate the cholestatic manifestations. This review describes the regulation of the normal activity of hepatocellular ABC transporters, focusing on the involvement of transcription factors and signaling pathways in the regulation of carrier synthesis, dynamic localization and phosphorylation status. Its alteration in different experimental models of cholestasis, such as those induced by estrogens, lipopolysaccharide (endotoxin), monohydroxylated bile salts and oxidative stress, is also reviewed. Finally, several experimental therapeutic approaches based upon the administration of compounds known / thought to induce carrier synthesis (e.g., protein synthesis inducers), to counteract etiological factors responsible for the cholestatic disease (e.g., corticoids in lipopolysaccharide-induced cholestasis) or to stimulate exocytic insertion of canalicular transporters (e.g., cAMP, silymarin or tauroursodeoxycholate) are described with respect to their ability to prevent cholestatic alterations; the role of signaling molecules as putative downstream mediators of their effects are also discussed.