Current Pharmaceutical Design

ISSN: 1381-6128

Current Pharmaceutical Design
Volume 12, Number 3, 2006

Contents

Novel Anti-Cancer Drugs and Therapeutic Approaches
Executive Editor: E. Bergmann-Leitner


Editorial Pp. 259-260


Evolution of Resistance to Cancer Therapy Pp. 261-271
F. Michor, M.A. Nowak and Y. Iwasa
[Abstract]


An Update on Overcoming MDR1-Mediated Multidrug Resistance in Cancer Chemotherapy Pp. 273-286
K. Takara, T. Sakaeda and K. Okumura
[Abstract]


Inhibition of Multidrug Resistance of Cancer Cells by Natural Diterpenes, Triterpenes and Carotenoids Pp. 287-311
J. Molnár, N. Gyémánt, M. Tanaka, J. Hohmann, E. Bergmann-Leitner, P. Molnár, J. Deli, R. Didiziapetris and M.J.U. Ferreira
[Abstract]


Transport Mechanism-Based Drug Molecular Design: Novel Camptothecin Analogues to Circumvent ABCG2-associated Drug Resistance of Human Tumor Cells Pp. 313-325
T. Ishikawa, Y. Ikegami, K. Sano, H. Nakagawa and S. Sawada
[Abstract]


New Treatment Strategies for Multiple Myeloma by Targeting BCL-2 and the Mevalonate Pathway Pp. 327-340
N.W.C.J. van de Donk, A.C. Bloem, E. van der Spek and H.M. Lokhorst
[Abstract]


Transforming Growth Factor-β: A Molecular Target for the Future Therapy of Glioblastoma Pp. 341-349
W. Wick, U. Naumann and M. Weller
[Abstract]


TRICOM Vector Based Cancer Vaccines Pp. 351-361
C.T. Garnett, John W. Greiner, Kwong-Yok Tsang, Chie Kudo-Saito, Douglas W. Grosenbach, Mala Chakraborty, James L. Gulley, Philip M. Arlen, Jeffrey Schlom and J.W. Hodge
[Abstract]


Antibodies and their Fragments as Anti-Cancer Agents Pp. 363-378
O. Schaedel and Y. Reiter
[Abstract]


Differentiation-Inducing Therapy for Solid Tumors Pp. 379-385
H. Kawamata M. Tachibana T. Fujimori and Y. Imai
[Abstract]


VEGF Inhibitors in Cancer Therapy Pp. 387-394
A.R. Cardones and L.L. Banez
[Abstract]




Abstracts

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Editorial

The present issue of Current Pharmaceutical Design offers a well-rounded overview over the latest developments in anti-cancer drug design as well as novel approaches of cancer therapy. In this issue, Michor et al. review the evolution of resistance to cancer therapy in a mathematical framework in an attempt to estimate the probability of treatment success [1]. This work shows that there are many aspects to treatment failure. Treatment itself puts selective pressure on the tumor and thus promotes the development of mutant cells within the mass that are able to withstand insult delivered in the form of radio-, chemo- and immunotherapy. Unfortunately, we are facing the challenge that every tumor behaves like an individual making it difficult to predict whether a certain treatment will be efficacious. If the tumor is not completely eradicated, the remaining cells are likely to give rise to a new tumor mass that is resistant to the previously “successful” treatment(s). Thus, novel approaches and novel compounds are urgently needed in instances where conventional drugs or treatments are no longer efficacious. One of the focal points of this issue is the reversal of multidrug resistance in tumors. Takara et al. provide an overview of currently used anticancer drugs designed to overcome chemoresistance of tumor cells, which is mediated by multidrug resistance mediated by MDR1/P-gp-glycoprotein [2], while Molnar et al. summarize their efforts in the search of multidrug-reversing compounds isolated from plants [3]. The rationale was to identify a natural occurring reagent that is able to reverse the action of the ABC transporters and thus reverse the multidrug resistance phenotype. Ishikawa et al. report on the latest in the attempt to bypass drug resistance in tumors mediated by ABCG2 protein that renders cells insensitive to DNA topoisomerase inhibitors such as irinotecan [4].

A second focal point is the use of targeted therapies aimed at specific pathways that are responsible for tumor survival: van de Donk et al. report treatment strategies that target Bcl-2 expression and the mevalonate pathway to overcome drug resistance in multiple myeloma [5]. Overexpression of anti-apoptotic proteins such as Mcl-1 and Bcl-2 is common in tumor cells and mediates resistance to chemotherapeutic treatment that would induce apoptosis in cells without the overexpression of such proteins. As the authors report, anti-sense oligonucleotides to Bcl-2 and statins show promising in results in Phase I and II studies.

Evasion of immunosurveillance is another important strategy that tumors employ in order to thrive. This issue presents three reports on how to counteract this strategy while using very different approaches. Wick et al. review the various treatments of glioblastoma by targeting transforming growth factor beta (TGF-β) [6]. TGF-β is an interesting molecules as it represents a strong immunosuppressive factor produced by a variety of cancers. Moreover, TGF-β is known to stimulate angiogenesis, migration and invasion and thus metastasis.

C.T. Garnett et al. review their strategy on how to induce an immune response against tumor cells that are naturally either weak or non-immunogenic [7]. This group developed a strategy to overcome the weak immunogenicity of tumor antigens such as carcinoembryonic antigen (CEA) and prostate specific antigen (PSA). Co-immunization with a poxvirus encoding costimulatory molecules resulted in a stronger immune response in patients. The ultimate goal of this therapeutic approach is to increase the number of tumor-specific T lymphocytes in the patients and thus increase the chance for efficacious immunosurveillance.

O. Schaedel et al. review their efforts in designing antibodies and antibody fragments for immunotherapy of patients [8]. This type of immunotherapy has many different modes of action: injection of tumor-specific antibodies labeled with radioactive isotopes allows the detection of even small clusters of tumor cells (micrometastasis) for radio-immunoguided surgery (RIGS) or destruction of this tumor cells by lifting the cloak of the tumor mass and marking the cells for destruction by the immune system. Moreover, antibodies can be used as targeting moieties to deliver radioactive isotopes, toxins, drugs or prodrugs with high specificity to tumor cells.

One of the biggest advantages of immunotherapy certainly is high specificity associated with low toxicity. Furthermore, immunotherapy is attractive because any antigen-expressing cell will be affected regardless of its proliferative state (in contrast to chemotherapy that can only affect actively dividing cells).

A striking phenomenon of tumor cells is their ability to de-differentiate and thus achieve a higher proliferative potential and resistances to various therapeutics. Hence, differentiation-inducing agents are gaining more and more attention. Kawamata et al. review the latest developments in this field [9].

Apart from targeting directly the tumor cells in an attempt to destroy the tumor mass, the field of anti-angiogenetic factors has been another focal point in the development of anti-cancer therapies. Cardones et al. summarize the advances made with such drugs and report on the latest clinical trials that investigate the effects of targeting the vascular endothelial growth factor (VEGF) [10].

I would like to thank all contributing authors for their efforts to provide us with the latest information available in these diverse research areas and illuminating their cutting edge approaches.

References

[1] Michor F, Novak MA, Iwasa Y. Evolution of resistance to cancer therapy. Curr Pharm Design 2006; 12(3): 261-271.

[2] Takara K, Sakaeda T, Okumura K. An update on overcoming MDR1-mediated multidrug resistance in cancer chemotherapy. Curr Pharm Design 2006; 12(3): 273-286.

[3] Molnar J, Gyemant N, Tanaka M, Hochmann J, Bergmann-Leitner E, Molnar P, Deli J, Didiziapetris R, Ferreira MJU. Inhibition of multidrug resistance of cancer cells by natural diterpenes, triterpenes and carotenoids. Curr Pharm Design 2006; 12(3): 287-311.

[4] Ishikawa T, Ikegami Y, Sano K, Nakagawa H, Sawada S. Transport mechanism-based drug molecular design: novel camptothecin analogues to circumvent ABCG-2 associated drug resistance of human tumor cells. Curr Pharm Design 2006; 12(3): 313-325.

[5] van de Donk NWCJ, Bloem AC, van der Spek E, Lokhorst HM. New treatment strategies for multiple myeloma by targeting bcl-2 and the mevalonate pathway. Curr Pharm Design 2006; 12(3): 327-340.

[6] Wick W, Naumann U, Weller M. Transforming growth factor-b: a molecular target for the future therapy of glioblastoma. Curr Pharm Design 2006; 12(3): 341-349.

[7] Garnett CT, Greiner JW, Tsang K-Y, Kudo-Saito C, Grosenbach DW, Chakraborty M, Gulley JL, Arlen PM, Schlom J, Hodge JW. TRICOM vector based cancer vaccines. Curr Pharm Design 2006; 12(3): 351-361.

[8] Schaedel O, Reiter Y. Antibodies and their fragments as anti-cancer agents. Curr Pharm Design 2006; 12(3): 363-378.

[9] Kawamata H, Tachibana M, Fujimori T, Imai Y. Differentiation-inducing therapy for solid tumors. Curr Pharm Design 2006; 12(3): 379-385.

[10] Cardones AR, Banez LL. VEGF inhibitors in cancer therapy. Curr Pharm Design 2006; 12(3): 387-394.

E. Bergmann-Leitner
Department of Immunology
Walter Reed Army Institute of Research
Silver Spring, Maryland
USA


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Evolution of Resistance to Cancer Therapy
F. Michor, M.A. Nowak and Y. Iwasa

Acquired drug resistance is a major limitation for successful treatment of cancer. Resistance emerges due to drug exclusion, drug metabolism and alteration of the drug target by mutation or overexpression. Depending on therapy, the type of cancer and its stage, one or several genetic or epigenetic alterations are necessary to confer resistance to treatment. The fundamental question is the following: if a genetically diverse population of replicating cancer cells is subjected to chemotherapy that has the potential to eradicate it, what is the probability of emergence of resistance? Here, we review a general mathematical framework based on multi-type branching processes designed to study the dynamics of escape of replicating organisms from selection pressures. We apply the general model to evolution of resistance of cancer cells and discuss examples for diverse mechanisms of resistance. Our theory shows how to estimate the probability of success for any treatment regimen.


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An Update on Overcoming MDR1-Mediated Multidrug Resistance in Cancer Chemotherapy
K. Takara, T. Sakaeda and K. Okumura

The intrinsic or acquired resistance to anticancer drugs remains one of the most significant factors impeding the progress of cancer chemotherapy. This phenomenon often involves simultaneous resistance to other anticancer drugs that differ in their chemical structure and mode of action and are not even used in chemotherapy. This phenotype has been called multidrug resistance (MDR). Although the cellular basis underlying MDR is not fully understood, several factors mediating therapy resistance in tumors have been proposed. One of the mechanisms leading to chemoresistance of tumor cells is the increased activity of transporter proteins. The best-characterized transporter protein is MDR1/P-glycoprotein, and a number of clinical investigations have suggested that its intrinsic or acquired overexpression resulted in a poor clinical outcome of chemotherapy. Various types of compounds and techniques for the reversal of MDR1/P-glycoprotein-mediated MDR have been developed, and efforts have concentrated on the inhibition of function and suppression of expression. This review summarizes the current state of knowledge of MDR1/P-glycoprotein and the modulation of MDR by targeting MDR1/P-glycoprotein.


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Inhibition of Multidrug Resistance of Cancer Cells by Natural Diterpenes, Triterpenes and Carotenoids
J. Molnár, N. Gyémánt, M. Tanaka, J. Hohmann, E. Bergmann-Leitner, P. Molnár, J. Deli, R. Didiziapetris and M.J.U. Ferreira

The multidrug resistance (MDR) proteins are member of the ATP-binding cassette superfamily and are present in a majority of human tumors. Their activity is a crucial factor leading to therapeutic failure. It is likely that compounds which inhibit the function of the MDR-efflux proteins such as MDR1 will improve the cytotoxic action of anticancer chemotherapy. Therefore, a search for MDR reversing compounds was conducted among three classes of plant derived compounds such as diterpenes, triterpenes and carotenoids in a hope to find inhibitors without adverse effects in these natural compounds.

The inhibition of efflux activity was determined by measuring the accumulation of substrate analogues such as rhoda-mine in tumor cells in the presence of potential inhibitors. Thus we determined the effect of structurally unrelated diter-penes, triterpenes and carotenoids on reversal of multidrug resistance in MDR-1 gene-transfected L1210 mouse lym-phoma cells and MDR mediated multidrug resistance of human breast cancer cells MDA-MB-231 (HTB-26) and MCF-7.

The majority of diterpenes, cycloartane triterpenes and carotenoids isolated from vegetables and medicinal plants were able to enhance rhodamine 123 accumulations of MDR-cells. Synergistic interaction was found between epirubicine and resistance modifier terpenoids in vitro. It is supposed that these MDR modulators bind into transmembrane domains and the action of ABC transporters is inhibited by induced conformational changes.


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Transport Mechanism-Based Drug Molecular Design: Novel Camptothecin Analogues to Circumvent ABCG2-associated Drug Resistance of Human Tumor Cells
T. Ishikawa, Y. Ikegami, K. Sano, H. Nakagawa and S. Sawada

Acquired and intrinsic drug resistance in cancer is the major obstacle to long-term, sustained patient response to chemotherapy. Irinotecan (CPT-11) is a widely-used potent antitumor drug that inhibits mammalian DNA topoisomerase I (Topo I). However, overexpression of ABCG2 (BCRP/MXR/ABCP) reportedly confers cancer cells resistance to SN-38, the active form of CPT-11. To circumvent the ABCG2-associated drug resistance, we have synthesized and char-acterized a total of fourteen new camptothecin (CPT) analogues with respect to both the inhibition of Topo I and the substrate specificity of ABCG2. While the lactone E ring is a prerequisite for anticancer activity, modifications of the A or B rings do not significantly affect Topo I inhibition activity. In this context, we have synthesized new CPT analogues with different substitutions at positions 10 or 11 of the A ring. All of the tested CPT analogues strongly inhibited the Topo I activity in a cell-free system. Accordingly, we have examined ATP-dependent transport of those CPT analogues by using plasma membrane vesicles prepared from ABCG2-overexpressing cells. Based on the substrate specificity of ABCG2 thus evaluated, it is strongly suggested that CPT analogues with a hydroxyl group at position 10 or 11 of the A ring are good substrates for ABCG2 and therefore effectively extruded from cancer cells. Thus, hydrogen bond formation is considered to be involved in substrate recognition and/or transport processes of ABCG2. The present study provides a practical approach to discover new CPT-based drugs for the chemotherapy of drug-resistant human cancer.


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New Treatment Strategies for Multiple Myeloma by Targeting BCL-2 and the Mevalonate Pathway
N.W.C.J. van de Donk, A.C. Bloem, E. van der Spek and H.M. Lokhorst

Insight into the mechanisms of primary or acquired drug resistance of (hematological) malignancies is critical for the development of new treatment strategies. This review will focus on Bcl-2 and the mevalonate pathway as targets for reversal of drug resistance in multiple myeloma. The Bcl-2 protein is highly expressed in myeloma patients and in vitro studies have shown its role in the regulation of chemosensitivity, which makes Bcl-2 an attractive target for treatment. Statins are widely used for the treatment of hypercholesteremia. Several in vitro studies have shown that statins may also kill hematological malignant cells including myeloma cells. We found that lovastatin induced apoptosis in myeloma and lymphoma cells by inhibition of geranylgeranylation and subsequent down regulation of Mcl-1, probably the most important anti-apoptotic protein in myeloma. Phase 1 and 2 studies have been performed with Bcl-2 antisense oligonucleotides and high dose simvastatin in combination with chemotherapy in heavily pre-treated myeloma patients. Encouraging results from these studies may provide the framework for the future application of new treatment strategies for myeloma


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Transforming Growth Factor-β: A Molecular Target for the Future Therapy of Glioblastoma
W. Wick, U. Naumann and M. Weller

The median survival of patients with glioblastoma treated by surgery, radiotherapy and chemotherapy is in the range of 12 months. These limits in the efficacy of current treatment modalities call for the development of novel therapeutic approaches targeting the specific biological features of this type of cancer. Glioblastomas are a rich source of immunosuppressive molecules which may interfere with immune recognition and rejection as well as clinical strategies of active immunotherapy. The most prominent glioblastoma-associated immunosuppressant is the cytokine, transforming growth factor (TGF)-β, a multifunctional cytokine which not only interferes with multiple steps of afferent and efferent immune responses, but also stimulates migration, invasion and angiogenesis. The complex regulation of TGF-β bioavailability includes its synthesis as a proprotein, proteolytic processing by furin-like proteases, assembly in a latent complex, and finally liberation from latency by multiple effector mechanisms, a process collectively referred to as activation. Several in vitro paradigms and rodent glioma models have been used to demonstrate that the antagonism of TGF-β holds promise for the treatment of glioblastoma, employing antisense strategies, inhibition of pro-TGF-β processing, scavenging TGF-β by decorin, or blocking TGF-β activity by specific TGF-β receptor (TGF-βR) I kinase antagonists. Moreover, the local application of TGF-β₂ antisense oligonucleotides is currently evaluated in a randomized clinical trial for recurrent malignant glioma. In summary, we propose that TGF-β-antagonistic treatment strategies are among the most promising of the current innovative approaches for glioblastoma, particularly in conjunction with novel approaches of cellular immunotherapy and vaccination.


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TRICOM Vector Based Cancer Vaccines
C.T. Garnett, John W. Greiner, Kwong-Yok Tsang, Chie Kudo-Saito, Douglas W. Grosenbach, Mala Chakraborty, James L. Gulley, Philip M. Arlen, Jeffrey Schlom and J.W. Hodge

For the immune system to mount an effective antitumor T-cell response, an adequate number of T-cells specific for the antigens expressed by the malignancy must be activated [1]. Since most antigens expressed by tumors are “self”-antigens, tumor antigens often lack endogenous immunogenicity and thus do not sufficiently activate T-cells to levels that can mediate tumor eradication. In addition, virtually all solid tumor cells lack the costimulatory molecules necessary to activate tumor-specific T-cells. Approaches that stimulate immune responses to these tumor antigens have the potential to alter this poor responsiveness. This theory has promoted the use of active immunotherapy to generate immune responses against tumor-associated antigens (TAAs) for the treatment of cancer. As one such vaccine strategy, we have utilized poxviruses as delivery vehicles for TAAs in combination with T-cell costimulatory molecules. Initial studies have demonstrated that the insertion of costimulatory molecule trangenes into viral vectors, along with a TAA transgene, greatly enhances the immune response to the antigen. Using this approach, a TRIad of COstimulatory Mole-cules (TRICOM; B7-1, ICAM-1 and LFA-3) has been shown to enhance T-cell responses to TAAs to levels far greater than any one or two of the costimulatory molecules in combination. In this article, preclinical findings and recent clinical applications of TRICOM-based vaccines as a cancer immunotherapy are reviewed.



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Antibodies and their Fragments as Anti-Cancer Agents
O. Schaedel and Y. Reiter

The recent developments in the field of recombinant DNA, protein engineering and cancer biology, have let us gain insight into many cancer-related mechanisms. Moreover, novel techniques have facilitated tools allowing unique distinction between malignantly transformed cells, to regular ones. This understanding has paved the way for the rational design of a new age of pharmaceuticals; monoclonal antibodies and their fragments. Antibodies can select anti-gens on both a specific and high affinity account, and further implementation of these qualities is used to target cancer cells by specifically identifying exogenous antigens of cancer cell populations. The structure of the antibody provides plasticity resonating from its functional sites. Upon binding to the Fc Receptor on effector cells, the crystallisable frag-ment (Fc) region elicits the onslaught of Antigen Dependant Cell-mediated Cytotoxicity (ADCC) and the plasma-native Complement Dependant Cytotoxicity (CDC) response and apoptosis. The progenitor form of the antibody can evolve in to a tailored therapeutic molecule with the help of recombinant DNA technology. Recombinant antibodies may be linked to potent toxins or radio-labeled fragments, conferring a high killing capability. Other recombinant techniques such as ADEPT, conjugate the specificity of antibodies to a prodrug-catalytic subunit thus creating a high local concentration of an activated chemotherapeutic. Antibodies can be used to recruit the adaptive immune response by binding the antibody fragment to a recombinant MHC molecule displaying a highly immunogenic peptide. Apart from their therapeutic capabilities antibodies are powerful detection tools as observed in the operating theater in a procedure known as Radio-immuno-guided Surgery (RIGS).


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Differentiation-Inducing Therapy for Solid Tumors
H. Kawamata M. Tachibana T. Fujimori and Y. Imai

Treating malignant tumor through the induction of cell differentiation has been an attractive concept, but clinical development of differentiation–inducing agents to treat malignant tumor, especially for solid tumors has been limited to date. Nerve growth factor, all trans retinoic acid, dimethyl sulfoxide, active form vitamin D₃, peroxisome proliferator-activated receptorγ, 12-0-tetradecanoylphorbol 13-acetate, hexamethylene-bis-acetamide, transforming growth factor-β, butyric acid, cAMP, and vesnarinone are known to have a differentiation-inducing capability on solid tumors in vitro and/or in vivo. Moreover some of the differentiation-inducing agents have been used for treating patients with solid tumor, but the therapeutic effect of the differentiation-inducing agents on solid tumor is not strong when compared with that of conventional chemotherapeutic agents. However, because most of the differentiation-inducing agents can potenti-ate the effect of conventional chemotherapy or radiation therapy, combination of differentiation-inducing therapy with conventional chemotherapy or radiation therapy might be used as a second- or third-line therapy in patients with ad-vanced cancer. Furthermore, analysis of the molecular mechanisms of the tumor differentiation therapy might provide selective and targeted molecules for novel cancer therapy.


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VEGF Inhibitors in Cancer Therapy
A.R. Cardones and L.L. Banez

Vascular endothelial growth factor (VEGF)-mediated angiogenesis is thought to play a critical role in tumor growth and metastasis. Consequently, anti-VEGF therapies are being actively investigated as potential anti-cancer treatments, either as alternatives or adjuncts to conventional chemo or radiation therapy. Among the techniques used to block the VEGF pathway are: 1) neutralizing monoclonal antibodies against VEGF or its receptor, 2) small molecule tyrosine kinase inhibitors of VEGF receptors, 3) soluble VEGF receptors which act as decoy receptors for VEGF, and 4) ribozymes which specifically target VEGF mRNA. Recent evidence from phase III clinical trials led to the approval of bevacizumab, an anti-VEGF monoclonal antibody, by the FDA as first line therapy in metastatic colorectal carcinoma in combination with other chemotherapeutic agents. However, may challenges still remain, and the role of anti-VEGF ther-apy in the treatment of other solid tumors remains to be elucidated. The aim of this article is to review the progress of clinical investigations involving VEGF inhibitors in the treatment of different types of solid tumors.