Current Pharmaceutical Design - Volume 15, Issue 2, 2009

The first targeted therapeutic is presented by penicillin that was discovered by Alexander Fleming in 1928. The first radioactive-tracer experiments were performed as early as in 1913 by Fritz Haber, Ernest Rutherford and George Charles de Hevesy. The term “Magic bullet” was released by Paul Ehrlich and refers to the concept of selectively targeting a distinct bacterium without affecting other organisms. HIV infection which was first described in 1981 led to tremendous advances in drug design resulting in an effective therapeutic agent five years after first description of the disease and two years after identification of the infectious agent, the Human Immunodeficiency Virus (HIV). Essential prerequisite of drug design is the opportunity to perform high-resolution structural analysis by means of structural biochemical procedures as protein crystallography, nuclear magnetic resonance, and computational biochemistry. Based on recent advances in discovering enzymes through genome mining and metagenomics biocatalysis is emerging as transformational technology for drug discovery and production as will be presented by Ningqing Ran et al. in this issue of “Current Pharmaceutical Design”. Why do we need targeted drugs in diagnosis and treatment of infectious diseases and cancer? The answer seems to be simple: To enhance efficacy and to reduce harm of healthy tissue. Are these aims achievable? In part: In case of HIV infection targeted drugs as reverse transcriptase inhibitors (RTIs) - the first-generation treatments of HIV-1 disease- had only limited efficacy based on the interference of these drugs with human cell metabolism. The HIV-1 protease emerged as promising target and led to the development of protease inhibitors (PIs) that are still one of the most effective antiretroviral drugs. However, drug resistance and side effects are major concerns. Recently approved compounds that attack another target, the chemokine receptor CCR5 and the HIV-1 integrase demonstrated to have a more favorable safety profile. In order to overcome the development of drug resistance agents that are synergistic in action, that lack cross-resistance and overlapping toxicity are combined representing the multi-drug regimens that are commonly used in the treatment of HIV infection [1]. As in HIV infection significant progress has been made in the understanding of molecular mechanisms being involved in cancer development and progression leading to similar steps aiming at personalized therapy of cancer patients. Again some concerns with respect to successful therapy are left: Firstly, the molecular mechanisms leading to proliferation of malignant cells and to tumor angiogenesis are complex. Secondly, cancer cells overcome different growth factor signaling pathways by using escape mechanisms for their growth and survival advantage. Thirdly, drug resistance occurs based on mutational changes within the target e.g. the tyrosine kinase as has been discussed recently by Quintas-Gardama and Cortes [2]. Consequences are on the one hand the design of compounds against these mutants and on the other hand the combination of agents of the same or different drug classes as multi-kinase inhibitors (e.g. sorafenib), regimens consisting of Epidermal Growth Factor Receptor (EGFR) and Vascular Endothelial Growth Factor (VEGF) inhibitors as well as combining targeted drugs with conventional chemotherapeutic agents (e.g. erlotinib, gemcitabine and cisplatin; TALENT trial) [3]. Another approach is packaging drugs into nanoparticles resulting in improved bio-availability, bio-compatibility and safety profiles. Targeting particles can signal the presence of the disease site, block a function there, and release a drug to it as will be presented by Paul Debbage in this issue of the journal. Targeting is desirable because many barriers in human's body hinder free access of the drug to its targets as it is common knowledge for the blood brain barrier that lowers the efficacy of therapies. According to the “AIDS epidemic update” published in December 2007 33.2 million people are living with HIV worldwide, 2.5 million have been newly infected in 2007, and 2.1 million people died from AIDS in 2007 [4]. From the very beginning of the AIDS epidemic treatment has dominated and prevention strategies have been marginalized. But, treatment and prevention are inextricable connected in HIV infection as well as in cancer. Prevention in general needs to embrace political, social, as well as economical determinants. In comparison, 1,444,920 new cancer cases were projected in 2007 in the U.S., colorectal and lung cancer representing the most frequent solid tumors in both women and men. However, the U.S. cancer statistics give some hope since the incidence rates of the most frequent solid tumors began to decline since 1992. In contrast a report studying 18 different cancer types in 39 European countries demonstrated an increase from 2.4 million cancer cases in 2004 to 3.2 million in 2006 [5]. The one side of the coin are global statistics the other side is that successful prevention and treatment of solid tumors require the acceptance that these are not single diseases existing as a variety of phenotypes with different response rates especially to targeted drugs. Present chemotherapy regimens reached an efficacy plateau thus leading to the development of targeted therapeutics. An important lesson learned is that kinases are druggable targets for anti-cancer therapy leading to about 90 encoded tyrosine kinases that are major players in human cancers. Basically clinical benefit for patients with cancer requires either improvement of overall survival and of quality of life. Surrogate endpoints in clinical trials should ideally be validated as a predictor of these parameters. Very recently the discussion was raised if these surrogate endpoints are appropriate in case of patients with metastatic renal cell carcinoma. Furthermore ethical considerations arose with respect to crossover in case of disease progression [6, 7]. Additionally to surrogate endpoints appropriate inclusion criteria as well as patient and tumor characteristics that might serve as predictors for treatment efficacy or failure have to be defined.

Since protein kinases are frequently mutated or otherwise deregulated in human malignancies, they serve as a target for differentiating between tumor cells and normal tissues. Imatinib mesylat (IM), an inhibitor of the BCR-ABL tyrosine kinase was introduced in 2001 and has revolutionized the treatment of patients with chronic myeloid leukemia (CML). Since 2005 a second generation of tyrosine kinase inhibitors is to follow in Imatinib's footsteps: The development of these new small molecules was promoted by the identification of potential target kinases within the cellular signaling apparatus. Modern biochemical tools provide relevant amounts of these target kinases necessary for high throughput screening (HTS) campaigns and for elucidation of their 3-D structure by crystallography. Supported by computational chemistry the resulting data have enabled rational drug design. In this review low molecular weight inhibitors used for the CML treatment are summarized, pointing out their chemical similarities and differences.

Pharmaceuticals have historically been produced by either chemical synthesis or whole cell fermentation. The former is applied to synthetic small molecules while the latter to natural products. As a result of recent advances in rapid discovery of enzymes through genome mining and metagenomics, and their tunability in functions and stability through directed evolution, biocatalysis is emerging to be a transformational technology for drug discovery and production. Enzymes can catalyze reactions otherwise challenging by chemical approaches. Furthermore, enzymatic catalysis is a powerful tool for green chemistry development. This manuscript gives a brief overview of current status in integrating chemical and biological transformations for the synthesis of small molecular therapeutics.

Packaging small-molecule drugs into nanoparticles improves their bio-availability, bio-compatibility and safety profiles. Multifunctional particles carrying large drug payloads for targeted transport, immune evasion and favourable drug release kinetics at the target site, require a certain minimum size usually 30-300 nm diameter, so are nanoparticles. Targeting particles to a disease site can signal the presence of the disease site, block a function there, or deliver a drug to it. Targeted nanocarriers must navigate through blood-tissue barriers, varying in strength between organs and highest in the brain, to reach target cells. They must enter target cells to contact cytoplasmic targets; specific endocytotic and transcytotic transport mechanisms can be used as trojan horses to ferry nanoparticles across cellular barriers. Specific ligands to cell surface receptors, antibodies and antibody fragments, and aptamers can all access such transport mechanisms to ferry nanoparticles to their targets. The pharmacokinetics and pharmacodynamics of the targeted drug-bearing particle depend critically on particle size, chemistry, surface charge and other parameters. Particle types for targeting include liposomes, polymer and protein nanoparticles, dendrimers, carbon-based nanoparticles e.g. fullerenes, and others. Immunotargeting by use of monoclonal antibodies, chimeric antibodies and humanized antibodies has now reached the stage of clinical application. High-quality targeting groups are emerging: antibody engineering enables generation of human/like antibody (fragments) and facilitates the search for clinically relevant biomarkers; conjugation of nanocarriers to specific ligands and to aptamers enables specific targeting with improved clinical efficacy. Future developments depend on identification of clinically relevant targets and on raising targeting efficiency of the multifunctional nanocarriers.

Delivering a drug to a specific target in the body is comparable to the “magic bullet principle” applied in Nuclear Medicine. If clinical medicine today found treatment options by targeting specific receptors, proteins or enzymes by “small-molecule drugs” it utilizes concepts that have been initially described by Nobel Laureate George von Hevesy as “tracer principle”. This article is going to show that molecular imaging probes in Nuclear Medicine can be regarded as proof of principle of many of recent trends in diagnosis and therapy and offers exciting opportunities for further developments. Radioiodine therapy of benign and malignant thyroid disease has been established in Nuclear Medicine over six decades ago and is a fine example for using the same highly specific probe for diagnosis and treatment of a given disease. The use of radio labeled monoclonal antibodies against surface receptors of tumor cells (e.g. CEA) dominated diagnostic Nuclear Medicine in the eighties and sees a recent revival in lymphoma treatment radioimmunotherapy. Finally Nuclear Medicine has shown that it may advance drug development by visualizing its biodistribution and site of action. On the other hand some drugs like somatostatin analogues have been reinvented as diagnostic and therapeutic probes over a decade after their initial introduction as therapeutics. Molecular Imaging and targeted therapy are merging and potentate their individual strength. Nuclear Medicine has ample experience in applying Molecular Imaging in clinical research and practice and has a bright future in this exciting field.

Targeted therapies have improved and will continue to improve the outcome of lung cancer. Current strategies focus on the blockade of growth factor receptors and the inhibition of angiogenesis. Epidermal growth factor receptor (EGFR)-directed tyrosine kinase inhibitors (TKIs) have already been established as a treatment option in patients with advanced non-small cell lung cancer (NSCLC) progressing after prior treatment with chemotherapy. EGFR-directed monoclonal antibodies in combination with platinum-based first-line chemotherapy have shown promising efficacy in phase II trials. In a phase III trial, cetuximab combined with cisplatin/vinorelbine resulted in superior survival compared to chemotherapy alone in patients with advanced EGFR-positive NSCLC. Inhibition of angiogenesis has also been successfully applied as a new treatment strategy. Bevacizumab added to palliative chemotherapy has improved progression-free survival in two phase III trials and overall survival in one of these trials in selected patients with advanced non-squamous cell lung cancer. Bevacizumab is now approved for selected patients with advanced NSCLC in combination with platinum-based chemotherapy. Other targeted therapies including dual and multi-kinase inhibitors are in earlier stages of clinical development. In small cell lung cancer (SCLC), targeted therapies have also been studied but no clinical benefit could be demonstrated for these agents.

It is now exactly 100 years ago (1908) that Paul Ehrlich, who is regarded as the inventor of the concept of targeted therapy, received the Nobel Prize for Medicine. His initial perception leading to this theory was derived from observations that certain substances are capable of selectively staining either tissues or microorganisms. These observations culminated in the discovery of the inorganic mercury compound arsphenamine (Salvarsan®) by Sahachiro Hata in the laboratory of Paul Ehrlich. Salvarsan® might be regarded as the first effective “targeted” treatment for syphilis at that time. Tamoxifen (Nolvadex®), an anti-estrogen, which was introduced in the early 1970s, was one of the first rationally designed targeted anti-tumour drugs. Since the 1970s a dramatic development of new molecular technologies occurred, culminating, for example, into the Human Genome Project and the public availability of various gene and protein databases, such as the Cancer Genome Anatomy Program established by the National Cancer Institute. Genomics, proteomics, structural genomics, transcriptomics, and high-throughput screening technologies for identification of targeted drugs are now available, which were almost unimaginable only a few years ago. Over 500 kinases are known of which about 250 have been cloned and are available to directly evaluate the activity of novel drug candidates. These technologies in conjunction with bio-informatic and chemical tools allow us to design novel molecules, and consequently tailor drug therapy to specific targets within tumours.