The Hedgehog signalling pathway plays a critical role in controlling growth, especially during development, but is often over-activated in tumourigenesis. It has recently emerged as an important target for anticancer drugs, with several compounds in clinical trials. It subsequently presents the discovery and development of drugs targeting this pathway. The initial focus is on cyclopamine - the first compound discovered that could inhibit the Hedgehog pathway - and selected cyclopamine analogues, including a review of the development of IPI
|Published (Last):||23 May 2006|
|PDF File Size:||15.44 Mb|
|ePub File Size:||7.32 Mb|
|Price:||Free* [*Free Regsitration Required]|
Aptamer-Based radiopharmaceuticals for diagnostic imaging and targeted radiotherapy of epithelial tumors. In the continuous search for earlier diagnosis and improved therapeutic modalities against cancer, based on our constantly increasing knowledge of cancer biology, aptamers hold the promise to expand on current antibody success, but overcoming some of the problems faced with antibodies as therapeutic or delivery agents in cancer.
However, as the first aptamer reached the market as an inhibitor against angiogenesis for the treatment of macular degeneration, aptamers have found only limited applications or interest in oncology, and even less as radiopharmaceuticals for diagnostic imaging and targeted radiotherapy of tumours.
Yet, the chemistry for the labelling of aptamers and the options to alter their pharmacokinetic properties, to make them suitable for use as radiopharmaceuticals is now available and recent advances in their development can demonstrate that these molecules would make them ideal delivery vehicles for the development of targeted radiopharmaceuticals that could deliver their radiation load with accuracy to the tumour site, offering improved therapeutic properties and reduced side effects.
Key words: Aptamer, radiopharmaceutical, diagnostic imaging, radiotherapy, epithelial tumor. Our knowledge on cancer has changed dramatically over the last few years and, with it, our approach to diagnosis and therapy. A lot has become known about the causes, onset and spread of the disease, the different features of individual cancers and their origin.
Scientific knowledge and technological development are constantly improving the outlook on cancer. Yet, even though prevention methods, through changes in lifestyle, vaccinations etc. Thus, there is clearly a need for improved therapeutic approaches to make these cancers treatable conditions, instead of lifethreatening or debilitating diseases.
There is, thus, a continuous effort to develop novel therapeutic approaches, based on surgical improvements and novel chemotherapeutic or radiotherapeutic approaches. These may attempt to improve on previous molecules or techniques, find new ways to target previous targets, deliver radiation more specifically and use different kinds of radiation with improved properties, or generate novel agents that are targeting new markers identified from large proteomics efforts Missailidis, When chemotherapy first became established, there were predictions that, within a few years, radiotherapy would be a thing of the past.
Improvements to existing techniques, and development of new ones, are happening at a rapid rate. One of the progress areas in radiotherapy is the use of radiopharmaceuticals and molecular targeted radiotherapy approaches to deliver radiation specifically to the cancer site, improving clinical outcome and reducing side effects.
Radiopharmaceuticals have traditionally only been used when there is a high uptake by a particular part of the body, such as Iodine by the thyroid. Otherwise, the use of a radiopharma-ceutical would be dangerous, often causing more damaging than therapeutic effect.
So, the majority of radiotherapy approaches are focused on external beam therapy, which is now quite accurate. However, research on new ways of targeting cancers with radiopharmaceuticals is likely to result in the wider use of unsealed source radiotherapy, with the possibility that treatment may be individually tailored to the patient's cancer.
Such molecular targeted radiotherapy approaches are evolving into using targeted therapeutics to deliver radiotherapy specifically to the tumour site, where a seed can not be introduced. With the use of appropriate targeting agents, such as antibodies and aptamers, a new generation of targeted radiopharmaceuticals has emerged, with conjugated antibodies already in the market, such as Zevalin, and others in clinical development. Aptamers used as delivery agents for radiotherapy are currently at preclinical stage Borbas et al.
The development of coupling techniques to such delivery agents led to the use of new chelators and different metals that could emit alpha or beta particles for cancer radiotherapy and can now be directed specifically at the tumour site.
It is not surprising that the above developments in molecular radiotherapy came at a time when the biggest boom in anticancer therapeutics has been in the area of biological medicines. Antibodies and nucleic acid therapeutics have been obtaining FDA approval faster than ever before, and dozens of these reagents are now in clinical trials. Antibodies are the most well-established biological agents, with more than 40 monoclonal antibodies currently in clinical trials against various forms of cancer Belimezi, Similarly, various classes of nucleic-acid therapeutics, including aptamers, antisense, antigene and siRNA agents, are also being developed as tumour-specific therapies.
Aptamers are oligonucleotides with a defined conformation that allows them to bind specifically to target molecules, including proteins, peptides, enzymes, antibodies and cell surface receptors Bacher and Ellington, Aptamers vary in size between 25 and 50 bases and are identified from combinatorial libraries through selective targeting and affinity maturation.
They offer unique benefits compared to other targeting agents, in that they bind with high affinity and selectivity, are easily and quickly synthesised using in vitro techniques, are stable and consistent Jayasena, , making them alternatives to antibodies and peptides for diagnostic assays Hesselberth et al.
As aptamers are not immunogenic or toxic, they also have potential as therapeutic and imaging agents Borbas et al. Aptamers have been primarily used for their ability to act as inhibitors by binding to specific receptors, thus blocking a cellular pathway, which has led to clinical trials of aptamers for the treatment of cancer and macular degeneration and the first FDA approval for an aptamer against AMD, named Pegaptanib.
The two year safety results demonstrated that all the doses of Pegaptanib were well tolerated and any adverse events were momentary owing to the injection preparation and procedures rather than the Pegaptanib drug D'Amico et al. The demonstration that aptamers, either through incorporation of fluorescent bases in the selection process Jhaveri et al. However, aptamers have not yet reached their full potential, partly because their labelling has not been fully explored, particularly with regards to radio-and MRI labelling that would allow them specific delivery of radiotherapy, with minimised side effects and could significantly increase definition in gamma-camera or MRI imaging, by carrying radionuclides or contrast agents respectively, specifically at the disease site.
Aptamers in the Literature. Aptamers as therapeutic or diagnostic agents in cancer have been recently reviewed Khan and Missailidis, ; Makwana et al.
Yet, references to aptamers in radiotherapy or as radiopharmaceuticals have been sporadic at best and the field is clearly lacking in development. To evaluate the current state of research in the field of aptamers, alone and in relationship to cancer, therapy, diagnosis, imaging, radiotherapy and radiopharmaceuticals over the past 18 years, since the first aptamer paper appeared, we have used two of the largest scientific databases, PubMed and Web of Science, with some interesting results.
ThedatawereobtainedonAugust14 th , Our analysis of the scientific literature on aptamers since , has been very interesting see Table 1. First of all, we note that, although the first papers on aptamers appeared almost simultaneously in by two independent groups, led by Gold and Ellington respectively Tuerk and Gold, ; Ellington and Szostak, , there is no reference on aptamers appearing on either database till This, however, may be related to the term 'aptamer' not being coined and used widely until later on.
Furthermore, although PubMed appears to be more widely accessible, Web of Science presented consistently a larger number of hits on aptamers and additional keywords we searched for, with exception to diagnosis. This clearly indicates that aptamers have found wider applications in other areas of research, with only limited attention to their great potential in cancer.
Furthermore, few papers are related to aptamers and radiopharmaceuticals or aptamers and radiotherapy. These are spread throughout the years, with one or that saw an increase to 4 papers. Yet, as 3 out of 4 two papers each year, with the exception of papers are reviews, there seem to be only limited research on this field, with only one research paper this year. Given the success of aptamers in disease diagnosis and therapy, their radiolabelling potential remains an unexplored field.
High affinity and specificity aptamers are, primarily, generated via an in vitro selection process referred to as SELEX. However, recently, alternative selection processes have also been developed Drabovich et al. The basic SELEX protocol is an evolutionary, iterant stringent process involving a combinatorial library of randomized nucleic acid sequences, with structural variations of more than 10 15 different molecules, flanked by primers that allow PCR amplification.
This is subjected to the selected molecular target for a series of events of binding, partitioning of unbound aptamers from the bound, followed by elution and amplification of the bound aptamers, which are subsequently further subjected to the target.
This process is repeated for several rounds, typically from 8 to 12, to obtain, through competitive binding, one or few aptamer sequences with high specificity and affinity to the chosen target. Selected aptamers are cloned and sequenced, to reveal the binding sequences. Many alterations can be made to the SELEX protocol, such as on the approach employed to present the target or the manner in which the aptamer-target complexes are partitioned.
Alterations in the presentation of the target can entail a method of counter selection or ToggleSELEX, whereas variations in partitioning include photocrosslinking and capillary electrophoresis CE reviewed by Hamula et al. Chemistry and Labelling. Oligonucleotides can now be purchased with a number of modifications from various providers of customised oligonucleotides.
Such modifications can facilitate the interaction of aptamers with moieties of interest in labelling and to improve their pharmacokinetic properties. Thus, an amino modification at the aptamer terminus either 3' or 5' can facilitate the interaction of the aptamer with a chelator. Ideally, however, this interaction would be post-labelling of the chelator with a metal Fig. Improving Pharmacokinetic Properties. Two major disadvantages have plagued aptamers since their initial development stage.
The first major obstacle has been the degradation of oligonucleotides by nucleases. However, this issue has been resolved by a number of techniques. First of all, DNA aptamers have been shown to be far more resistant to nuclease degradation than their RNA counterparts.
One such easy modification we have used on the design of an aptamer radiopharmaceutical has been an inverted base on the unlabelled end of the aptamer Borbas et al. The second issue is aptamers' pharmacokinetic properties. Thus, pegylation of the aptamer on one end with radiolabelling on the other can quickly and confidently increase the half life of the molecule in serum, whilst conferring nuclease resistance. PEG molecules of various molecular weights have been used with aptamers, but only one with a radiolabelled aptamer Hicke et al.
Aptamer Radiopharmaceuticals Concluding Remarks. Aptamers have a great potential in the disease diagnosis and therapy. They have the ability to bind their targets with unrivalled affinity and specificity, they have shown no side effects in treatments they have already been used, such as the nucleolin or the VEGF aptamer, they have no immunogenicity and, in the case of cancer, great tumour penetration Borbas et al.
Furthermore, through labelling and PEGylation has the possibility to act as diagnostic, imaging or therapeutic agents. A few years ago, an agreement between a major aptamer company and the NCI had been reached to develop and test aptamers against all major known tumour markers. This is consistent with the great commercial interest of aptamers. Research publications on aptamers as radiopharmaceuticals remain limited and, resulting mostly from research of our group and collaborators worldwide.
Bacher, J. Belimezi, M. In- Anticancer Therapeutics , Missailidis S editor. Wiley and Sons Ltd, United Kingdom. Berezovski, M.
J Am Chem Soc. Borbas, K. D'Amico, D. Ophthalmology, , Drabovich, A. Ellington, A. Nature, , Brazilian Arch Biol Technol. Ferreira, C. Hamaguchi, N. Hamula, C. Trac-Trends Anal Chem. Hesselberth, J. Hicke, B. J Nucl Med. Jayasena, S.
You are currently using the site but have requested a page in the site. Would you like to change to the site? Sotiris Missailidis. Clearly structured throughout, the book not only provides information on currently used molecular treatment approaches, but also describes the various agents that are currently at various stages of development and clinical trials, thus making them the drugs of tomorrow. The book goes on to present current therapeutic regimes including their indications and side effects, as well as their position in the international market in terms of sales and development costs.
Aptamer-Based radiopharmaceuticals for diagnostic imaging and targeted radiotherapy of epithelial tumors. In the continuous search for earlier diagnosis and improved therapeutic modalities against cancer, based on our constantly increasing knowledge of cancer biology, aptamers hold the promise to expand on current antibody success, but overcoming some of the problems faced with antibodies as therapeutic or delivery agents in cancer. However, as the first aptamer reached the market as an inhibitor against angiogenesis for the treatment of macular degeneration, aptamers have found only limited applications or interest in oncology, and even less as radiopharmaceuticals for diagnostic imaging and targeted radiotherapy of tumours. Yet, the chemistry for the labelling of aptamers and the options to alter their pharmacokinetic properties, to make them suitable for use as radiopharmaceuticals is now available and recent advances in their development can demonstrate that these molecules would make them ideal delivery vehicles for the development of targeted radiopharmaceuticals that could deliver their radiation load with accuracy to the tumour site, offering improved therapeutic properties and reduced side effects. Key words: Aptamer, radiopharmaceutical, diagnostic imaging, radiotherapy, epithelial tumor. Our knowledge on cancer has changed dramatically over the last few years and, with it, our approach to diagnosis and therapy.