Current Pharmaceutical Design - Volume 10, Issue 13, 2004

A variety of therapeutic strategies in oncology are focused on the inhibition of tumor-induced angiogenesis. Thus, there is a keen interest in methods which allow non-invasive monitoring of molecular targets involved in angiogenesis which would support information for planning and controlling corresponding therapies. Moreover, such techniques would provide an insight into the formation of new sprouting blood vessels, the involved processes and regulatory mechanisms in patients. At the moment, development of radiotracer based techniques is mainly concentrated on three different targets which include peptidic and non-peptidic αvβ3-integrin binding antagonists, matrix metalloproteinase inhibitors and single chain anti-fibronectin antibody fragments. Development of radiolabeled MMP inhibitors is based on either the decapeptide Cys- Thr-Thr-His-Trp-Gly-Phe-Thr-Leu-Cys resulting from a phage display library or small molecular weight compounds. The in vitro data for these tracers are very promising. However, more detailed in vivo data are necessary to evaluate the potency of MMP-inhibitors for in-vivo imaging. The radiolabelled anti-ED-B single chain antibody fragment scFv L-19 shows selective accumulation in the tumor vasculature in a murine tumour model. In a first patient study a selective localisation of the 123I-labeled tracer in lesions of different tumours was found. On the basis of the lead structure cyclo(- Arg-Gly-Asp-DPhe-Val) a variety of different radiolabeled RGD-peptides has been developed for the non-invasive determination of the αvβ3 expression. These developments include peptides labeled with minimum structural alteration, peptide carbohydrate conjugates, peptidomimetics based on the RGD-structure as well as heterodimeric, homodimeric and homotetrameric ligand systems. Many of the tracers show high αvβ3-affinity and selectivity in vitro and receptor selective tumour accumulation with high image contrast in different murine tumour models. Further studies have to demonstrate that this approach can be translated to clinical settings allowing visualisation of αvβ3-positive tumours and αvβ3 expression during tumour-induced angiogenesis in patients.

A deficiency or an excess of programmed cell death (apoptosis) is an integral component of autoimmune disorders, organ and bone marrow transplant rejection, and cancer. A technique to image programmed cell death would be useful in the development of drugs to treat these and others diseases, and to monitor the effectiveness of therapy. The most widely studied agent for the in vivo study of apoptosis is radiolabeled annexin V, an endogenous protein labeled with technectium-99m, now undergoing clinical trials in both Europe and the United States. While annexin V has been studied extensively in humans the precise mechanism(s) of uptake of this agent in vivo is unclear and needs further study. Other agents are also underdevelopment including radiolabeled forms of Z-VAD.fmk, a potent inhibitor of the enzymatic cascade intimately associated with apoptosis. MR imaging techniques and tracers also hold promise as methods to monitor apoptotic cell death. In this article we will review these and other imaging technologies for the non-invasive imaging of cell death. The mechanism (s) and latest data on the conditions in which cellular stress and early apoptosis occur will also be discussed in detail including potential new strategies for the targeting and novel therapeutic interventions of tissues and organs undergoing stress or apoptosis when cell salvage is still possible.

The development of radioligands to image β-amyloid (Aβ) plaques and neurofibrillary tangles (NFTs) in vivo in the aging human brain is an important and active area of radiopharmaceutical design. When used in combination with positron emission tomography (PET) or single photon emission computed tomography (SPECT), amyloid-imaging tracers could facilitate the evaluation of the efficacy of anti-amyloid therapies currently under intense development by many major pharmaceutical companies throughout the world. Amyloid-imaging agents could also serve as surrogate markers in early diagnosis and neuropathogenesis studies of Alzheimer's disease and other aging-related neurodegenerative disorders. In this review article, the design and biological evaluation of amyloid-imaging agents are discussed. The structures of these agents vary from large proteins and peptides such as radiolabeled Aβ peptides and monoclonal antibodies to small molecules derived from Congo red, Chrysamine-G, thioflavin-T, and Acridine Orange. In vitro studies indicate that amyloid plaques contain multiple binding sites that can accommodate structurally diverse compounds, providing flexibility for radiopharmaceutical design of amyloid imaging agents. Compared to large biomolecules, small molecule radiotracers are often readily accessible through chemical synthesis and can display superior brain permeability. Several small molecule amyloid-imaging radioligands display high binding affinities to Aβ and sufficient brain penetration for imaging studies. Recent studies demonstrate the feasibility of imaging amyloid plaques in vivo in human subjects with PET. Imaging NFTs, separately or in concert with Ab plaques, is not as far advanced as imaging Aβ plaques and remains to be fully characterized and demonstrated.

Permeability of the blood-brain barrier (BBB) is one of the factors determining the bioavailability of therapeutic drugs. The BBB only allows entry of lipophilic compounds with low molecular weights by passive diffusion. However, many lipophilic drugs show negligible brain uptake. They are substrates for transporters such as P-glycoprotein (P-gp), multidrug-resistance associated protein (MRP) and organic anion transporting polypeptides (OATPs). The action of these carrier systems results in rapid efflux of xenobiotics from the central nervous system (CNS). Classification of candidate drugs as substrates or inhibitors of such carrier proteins is of crucial importance in drug development. Positron emission tomography (PET) can play an important role in the screening process by providing in vivo information, after the putative drug has passed in vitro tests. Although radiolabeled probes for MRP and OATP function are not yet available, many radiotracers have been prepared to study P-glycoprotein function in vivo with PET. These include alkaloids (11C-colchicine), antineoplastic agents (11Cdaunorubicin, 18F-paclitaxel), modulators of L-type calcium channels (11C-(±)verapamil, 11C-R(+)-verapamil), ßadrenoceptor antagonists (11C-(S)-carazolol, 18F-(S)-1'-fluorocarazolol, 11C-carvedilol), serotonin 5-HT1A receptor antagonists (18F-MPPF), opioid receptor antagonists (11C-loperamide, 11C-carfentanyl) , and various 64Cu-labeled copper complexes. Studies in experimental animals have indicated that it is possible to assess P-glycoprotein function in the BBB and its effect on the uptake and binding of drugs within the intact CNS, using suitable P-gp modulators labeled with positron emitters. Provided that radiopharmaceuticals (and P-gp modulators) can be developed for human use, several exciting fields of study may be explored, viz. (i) direct evaluation of the effect of modulators on the cerebral uptake of therapeutic drugs; (ii) assessment of mechanisms underlying drug resistance in epilepsy; (iii) examination of the role of the BBB in the pathophysiology of neurodegenerative and affective disorders; and (iv) exploration of the relationship between polymorphisms of transporter genes and the pharmacokinetics of test compounds within the CNS.

Several cholinesterase (ChE) inhibitors have been labeled with carbon-11 for visualizing binding sites on acetylcholinesterase (AChE) by positron emission tomography (PET). Following intravenous injection of 1,2,3,4- tetrahydro-9-[11C]methylaminoacridine or [11C]donepezil, however, the radioactivity distribution does not reflect the regional distribution of AChE in the brain of animals, probably because these compounds have high non-specific binding and / or other specific binding sites in vivo in the brain. PET studies with [11C]physostigmine and [11C]CP-126,998 in the brain of healthy subjects have shown a radioactivity distribution corresponding to the regional distribution of AChE activity measured in postmortem human brains. These radiotracers may be useful for measuring the occupancy of binding sites on AChE by AChE inhibitors, and for investigating the cerebral pharmacokinetics of such therapeutic drugs. An alternative approach to map AChE is the use of acetylcholine analogue substrates. We have developed Nmethylpiperidinyl esters labeled with carbon-11 for quantitative measurement of AChE activity. Currently, two N- [11C]methylpiperidine esters, N-[11C]methylipiperidin-4-ylacetate (MP4A) and N-[11C]methylpiperidin-4-yl propionate (MP4P or PMP), have been used for clinical studies of Alzheimer's disease and other neurodegenerative diseases. Both [11C]MP4A- and [11C]MP4P-PET have demonstrated not only the reduction of AChE activity in the cerebral cortex of patients with Alzheimer's disease (AD) but also the inhibitory effects of donepezil and rivastigmine on AChE activity in the brain of AD patients. AChE imaging should prove useful for therapeutic monitoring of the effects of ChE inhibitors, including determination of the appropriate clinical doses of newly developed compounds, and can thus prompt the development of novel drugs targeting AChE.

Beta-adrenoceptors are predominantly located in the cerebral cortex, nucleus accumbens and striatum. At lower densities, they are also present in amygdala, hippocampus and cerebellum. Beta-2 sites regulate glial proliferation during ontogenic development, after trauma and in neurodegenerative diseases. The densities of beta-1 adrenoceptors are changed by stress, in several mood disorders (depression, excessive hostility, schizophrenia) and during treatment of patients with antidepressants. A technique for beta-adrenoceptor imaging in the human brain is not yet available. Although 24 (ant)agonists have been labeled with either 11C or 18F and some of these are successful myocardial imaging agents, only two (S-1'-18Ffluorocarazolol and S-1'-18F-fluoroethylcarazolol) could actually visualize ß-adrenoceptors within the central nervous system. Unfortunately, these radiopharmaceuticals showed a positive Ames test. They may be mutagenic and cannot be employed for human studies. Screening of more than 150 beta-blockers described in the literature yields only two compounds (exaprolol and L643,717) which can still be radiolabeled and evaluated for ß-adenoceptor imaging. However, other imaging techniques could be examined. Cerebral ß-adrenoceptors might be labeled after temporary opening of the blood-brain barrier (BBB) and simultaneous administration of a hydrophilic ligand such as S-11C-CGP12388. Another approach to target ß-adrenoceptor ligands to the CNS is esterification of a myocardial imaging agent (such as 11C-CGP12177), resulting in a lipophilic prodrug which can cross the BBB and is split by tissue esterases. BBB opening is not feasible in healthy subjects, but the prodrug approach may be successful and deserves to be explored.

Angiogenesis, the sprouting of new blood vessels from preexisting ones, is a phenomenon associated to several human pathologies, including different potentially blinding ocular disorders, rheumatoid arthritis, and cancer. Indeed, ongoing angiogenesis is a characteristic feature of the majority of aggressive solid tumors, and is also a pre-requisite for the progression towards the metastatic phenotype. One established marker of angiogenesis is represented by an isoform of the oncofoetal fibronectin (FN), containing an additional domain inserted by alternative splicing of the FN pre-mRNA and called extra-domain B (ED-B). This isoform has been found to be present almost exclusively in the modified extra-cellular matrix surrounding newly-formed blood vessels in tumors (and other animal models of ocular pathologies), being completely absent from the normal vasculature in adult organs. This article reviews the recombinant antibodies raised against ED-B and the different methodologies used for the generation of these antibodies. Moreover, new diagnostic and therapeutic applications based on the delivery of bioactive molecules to tumor blood vessels by means of ED-B targeting will be discussed.

Type IV secretion systems (T4SSs) are bacterial multiprotein organelles specialised in the transfer of (nucleo)protein complexes across cell membranes. They are essential for conjugation, bacterial-induced tumour formation in plant cells, as observed in Agrobacterium, toxin secretion, like in Bordetella and Helicobacter, cell-to-cell translocation of virulence factors, and intracellular activity of mammalian pathogens like Legionella. By enabling conjugative DNA delivery, these systems contribute to the spread of antibiotic resistance genes among bacteria. These translocons are made up by 10-15 proteins that are analogous to Vir proteins of Agrobacterium and traverse both membranes and the periplasmic space in between in Gram-negative bacteria. Their secretion substrates range from single-stranded DNA / protein complexes to multicomponent toxins and they are assisted by integral inner-membrane coupling factors, the multimeric type-IV coupling proteins (T4CPs), to connect the macromolecular complexes to be transferred with the secretory conduit. To do so, these T4CPs may be required to localise close to the secretion machinery within the donor cell. The T4CP structural prototype is the hexameric protein TrwB of Escherichia coli conjugative plasmid R388, closely related to Agrobacterium VirD4 protein. It is responsible for coupling the relaxosome with the DNA transport apparatus during cell mating. T4CP family members are related to SpoIIIE / FtsK proteins, essential for DNA pumping during sporulation and cell division. These features suggest possible mechanisms for conjugal T4CP function: as a simple coupler between two molecular machines, as a rotating device to pump DNA through the type-IV transport pore, or as a DNA injector, whereby its central channel would function as part of the transport pore.