Lanthanide Chemistry for Diagnosis and Therapy

Objective

A. GENERAL BACKGROUND

A1: Why a COST action for this topic?

Lanthanide (III) chelates are becoming increasingly important in biomedical diagnosis. The proportion of Magnetic Resonans Imaging (MRI) scans made after the administration of a Ln(III) chelate is steadily increasing and nowadays is about 50%. A continuing demand for more effective and specific MRI contrast agents exists. The use of lanthanide complexes as luminescent labels for analyses in biological media has attracted a great deal of attention, in particular as an alternative to radioimmunoassay. Furthermore, progress in the application of Ln(III) chelates in therapy is impressive; it is expected that within two years, the first Ln(III) chelates for application in photodynamic therapy of cancer and arterial plaque will be introduced on the market.

Lanthanide(III) ions are unique in many respects. The 4f valence electrons are shielded by the 5s and p electrons and, consequently, the interactions between Ln(III) ions and ligands are largely determined by electrostatic and steric effects. In this way, the chemistry differs significantly from the d-block elements, where covalent binding plays an important role. Although the chemical differences between the various Ln(III) ions are small, the physical differences are dramatic. This can be ascribed to the presence of various amounts of unpaired electrons (0-7), which leads to peculiar paramagnetic properties as reflected in magnetic resonance. Furthermore, long luminescence lifetimes and line-like emission bands make these ions unique among the luminescent species. The exploitation of these special properties in medical diagnosis and therapy requires a multidisciplinary approach, in which, chemists, physicists, biologists, and physicians combine their complementary insights.

Presently, the European research on contrast agents for Magnetic Resonance Imaging (MRI) is, to a large extent, coordinated in the COST D8 Working Group "Rational Design of Lanthanide Chelates for Biomedical Applications" (D8/0001/97). This Working Group consists of 11 academic partners from 9 different European countries, whereas the relevant industries are linked via collaborations with individual academic groups. The cooperations among the participating groups are intensive. For example, more than ten exchanges took place in 1997-98, and 14 co-authored joint publications were produced. An annual workshop is organised, where members of the WG, representatives of the industry and prominent scientists from outside COST present their work. The most recent conferences had about 50 attendants, among which many were junior scientists. Furthermore, 6 academic and 2 industrial groups are collaborating in the development of new contrast agents for MRI angiography in the BIOMED 2 network MACE of the European Union 4th framework program for research.

A2: Status of Research in the Field

In the course of the last decade Gd(III) chelates have become commonplace in medical diagnosis. Nowadays, about 50% of the exams with Magnetic Resonance Imaging (MRI) are performed after administration of a Gd(III) chelate. These compounds can be considered as one of the most safe drugs known. At the same time magnetic resonance spectroscopy is also evolving into a clinic modality. It may be expected that the information obtained with that technique can be increased substantially by application of lanthanide chelates as shift reagents. A few examples of in vivo applications of lanthanide shift reagents have been published. The fast evolution of the MRI and MRS technique gives rise to an increasing demand of more effective and more specific contrast and shift reagents and, consequently, to a large research activity in this field. Molecular recognition is an important issue in much of the research directed towards higher specificity.

Molecular recognition, which is the heart of life processes, implies the treatment of essential information at a supramolecular level. The fixation of a guest onto a specifically designed host, natural or artificial, results in changes in electronic, ionic, optic, magnetic and conformational properties which translate into the generation of signals. The latter can be used to detect the completion of the recognition process. On the other hand, external stimuli such as chemical potential, light, electron flow, magnetic field, are known to regulate the action of a supramolecular recognition device, for instance by initiating or stopping its action.

One of the more convenient vectors to convey signals is light because it travels almost instantaneously and can easily reach regions of a complex edifice which are not accessible to molecular messengers. Moreover, light carries a substantial amount of energy which can be transmitted to the device, inducing the desired effect. Finally, light is easily detected by very sensitive devices and techniques, which makes signalling or analytical processes based on the emission of light among the most sensitive known.

The analysis of biological material is usually based on a two-step procedure. The first involves a recognition process using specific biochemical or supramolecular reactions. In the second step, some counting has to be made, either by specific biochemical methods detecting a change in the activity of the substrate (the analyte) or of the reactant, or by physicochemical means. If a labeled reactant is used, the emission of light by this sensor provides an extremely sensitive and accurate way of determining the number of labeled molecules.

The highlights of the present day developments of lanthanide(III) chelates in diagnostics and therapy are summarised in the following topics:
1.Relationships between structure and activity of Ln(III) chelates relevant to MRI and Magnetic Resonance Spectroscopy (MRS). The understanding is being deepened by studying relevant lanthanide complexes with a variety of techniques, including NMR, NMRD, EPR, kinetic measurements, and molecular modelling
2.Targeting MRI contrast agents and MRS shift reagents. Several contrast agents are now known that target organs (e.g. liver, blood).
3.Intelligent MRI and MRS responsive systems. Recently, the first examples of lanthanide(III) probes that sense their biochemical environment have been reported.
4.Ln(III) chelates as luminescent bioassays. Recently, promising examples of emissive lanthanide complexes have emerged wherein the metal-based luminescence is a function of pH, pO2 and/or the concentration of certain bioactive anions. Such complexes are addressed at wavelengths in excess of 355 nm by incorporation of appropriate antenna chromophores. Light is emitted with millisecond delays, obviating the problems of autofluorescence and light-scattering that beset many conventional fluorescent assays. The intensity, lifetime or polarisation of the metal-based emission may be measured to signal changes in the concentration of species that perturb either the antenna singlet or triplet, or the excited lanthanide ion itself.
5.Lanthanide(III) chelates for therapy. Various Ln(III) chelates with a net positive charge have been developed that can be utilised for specific hydrolytic cleavage of RNA and DNA. Other Ln(III) chelates are under clinical tests as photosentisizers for treatment of atherosclerosis and as tumour selective radiation sensitizer.

A3: Relationship with other European Programmes

There is no particular specific European programme devoted at present to lanthanide chemistry for diagnosis and therapy exclusively, except, that COST Action D8 on "Chemistry of Metals in Medicine" is complementary to, and has initiated this new Action. ( The COST Action D8 is ending in 2001; it is covered with 16 working groups). In fact the proposed new Action is developed out of the COST D8 Working Group "Rational Design of Lanthanide Chelates for Biomedical Applications", consisting of 11 teams. Indeed, it may be noted that the field of research of that Working Group differs significantly from that of the others in action D8, both because of the chemistry (f- versus d-block transition metal ions) and because of the strong physical component (NMR, luminescence) of the research.

Positive complementarity is foreseen with the new COST action B12 "Radiotracers for in vivo Assessment of Biological Function".

Lanthanide chemistry is often neglected in the academic curriculum. The proposed network may be a good starting point for the establishment of training networks in this area.

The significant interest in the field of lanthanide chemistry in diagnosis and therapy is exemplified by the inclusion of this topic at almost every major international conference on (bio)inorganic chemistry (Recent examples: International Conference on f-elements, Paris 1997; XXXIIII ICCC, Florence 1998; 5th International Symposium on Applied Bioinorganic Chemistry, Corfu 1999).

B. OBJECTIVES OF THE ACTION AND SCIENTIFIC CONTENT

B1: Main Objective

The main objective of the Action is an increase of the knowledge of the chemistry of lanthanide(III) chelates and to apply this knowledge to the development of novel diagnostic agents and to therapy by an interdisciplinary approach by chemists, physicists, biologists, and physicians. Strong links between academics and industries will reinforce these efforts. The goals will lead to more effective diagnosis and therapy in health care, particularly of the ageing population
B2: Sub-Topics

1.Relationships between structure and activity of Ln(III) chelates relevant to MRI and Magnetic Resonance Spectroscopy (MRS)

Efficient agents are based first on a rational design of the ligand. Both applications share a common requirement: a) the ligand must efficiently wrap the Ln(III) ion to form highly stable complexes which prevent the loss of harmful Ln(III) ions; b) the complexes must be highly water soluble in order to allow their administration in the form of concentrated solutions. MRI Relaxation Agents : the relaxation enhancement of water protons is the result of a dipolar interaction with the large magnetic moment of the Gd(III) ion, which occurs by the exchange of water molecules in the inner and outer coordination spheres with the bulk solvent molecules. A number of parameters is responsible for the relaxation enhancement promoted by a paramagnetic complex. The relaxation enhancements found to date are noticeably lower than the values expected on the grounds of the available theory of paramagnetic relaxation. It is still necessary to address the problem of getting an in-depth understanding of the structural, dynamic and electronic factors determining the relaxation enhancement of Gd(III) complexes. MRS Shift Agents : the prototype application deals with the separation (and quantification) of the NMR signals of species present in the inner- and outer-cellular compartments. The optimisation of the shift reagent properties may be pursued by endowing the Ln(III) complex with suitable electric charge distribution and molecular recognition capabilities. The availability of more efficient shift reagents will allow the investigation of intra-/extra-cellular distribution of a number of cationic, anionic and zwitterionic species.

2.Targeting and organ specific MRI contrast agents and MRS shift reagents

Up to now, many efforts in MR contrast agent development have been channelled in certain directions by the relative ease of chemical synthesis rather than by the goal of specific medical applications. Thus, the contrast agents available for clinical routine examinations today are safe and good enhancers, but unspecific. Development of new contrast agents for unspecific indications is no longer worthwhile. What is wished and needed are new compounds with higher specificity and, preferably, higher sensitivity.

The low molecular weight contrast extracellular fluid agents available for clinical routine today can only be considered as targeted to the kidney because they are excreted by glomerular filtration. Targeting in this way is described as "passive" targeting, which also includes targeting of the liver, spleen, lymph nodes, and bone marrow by introducing contrast agents into the reticulo endothelial system. During the forthcoming years, an important goal of contrast agent development for MRI is the identification of specific tissue- or pathology-seeking compounds which would target pathological sites "actively".

From the point of view of the doses of contrast material required by the modality, MR imaging is by far less demanding than X-ray but more than nuclear medicine. However, the development of new contrast agents for magnetic resonance imaging and tracers for radiopharmaceuticals reflects a far wider spectrum of ideas than for computed tomography.

Summarizing, at least three parameters have to be optimised for the development of targeting agents:
.The local concentrations as a result of selective distribution in the body (organ- or pathology- specific tracers). These concentrations should be as high as possible.
.The tolerance. Although tolerance is of the present agents is already very good, a further improvement is desirable in the future. This includes chemical and biological inertness as well as complete elimination from the body, soon after the medical exam.
. The enhancing effect (see also subtopic 1).

On a somewhat longer term, the expertise obtained on targeting and organ specific contrast agents can be exploited for the development of this type of compounds as shift reagents for MRS. There, particularly reduction of the dose required to bring about a significant effect is an important issue.

3.Intelligent MRI and MRS responsive systems

There is a need to devise lanthanide complexes in which the relaxivity is a function of a defined chemical /biochemical variable or combination of variables. The more important parameters are time, pH - allowing for example the definition of the more extracellular environment around tumour tissue - and the concentration of certain bio-active ions (e.g. intracellular Ca2+, Na+ and HCO3-). Useful information may also be gleaned from parallel magnetic resonance spectroscopy measurements using 1H, 31P and 23Na NMR. Time dependent systems may emerge from the controlled hydrolysis (in vivo) of a high relaxivity agent giving stable, low-relaxivity products. Such a complex would be of immediate use in selective arterial imaging, i.e. without the venal background. This information is of particular importance in patients suffering from stroke, heart-disease and atherosclerosis.

pH Dependent systems may emerge from the controlled modulation of relaxivity via ligand or metal-based processes. Examples include reversible protonation of a bound or proximate ligand donor leading to pronounced changes in inner and second-sphere relaxivity contributions. Alternatively, the pH-dependent displacement of a natural analyte - such as hydrogencarbonate - may lead to an increase in the relaxivity. Promising early examples of such behaviour have recently been defined by the Torino and Durham groups in molecular and macromolecular systems.

4.Responsive Luminescent Complexes

It is proposed to study:

a)Screening of macrocyclic and supramolecular self-assembled nanometric luminescent edifices incorporating lanthanide ions to maximise the ligand to metal energy transfer and the protection of the excited ion against unwanted de-excitation processes.
b)Design of specific responsive luminescent devices working in physiological conditions, optimised for various light emission in the visible (europium, terbium) or in the infra-red (neodymium, erbium, ytterbium), and triggered by photonic, electric or chemical excitation.
c)Extension of the above-mentioned devices to provide responses depending on pH, on anion concentration (mainly halides), or on pO2.
d)Charge neutral complexes which respond to changes in endogenous cations (including Zn2+, Ca2+, and Mg2+), allowing the intracellular concentrations of some of these important secondary messengers to be assessed.
e)Development of luminescent sensors using enantioselective interactions between photo (or electro-) excited chiral lanthanide chelates and biomolecules.
f)Elaboration of bimetallic sensors for imaging purposes.

5.Lanthanide(III) chelates for therapy

Lanthanides have many properties that make them very suitable for a variety of therapeutic applications. Their strong Lewis acidity make them good catalysts for the hydrolytic cleavage of phosphate esters in RNA and DNA. Particularly, 3+ charged Ln(III) chelates have been demonstrated to be very effective. Selective cleavage has been achieved with the use of anti-sense technology. Photochemical properties of Ln(III) chelates can, for example, be exploited in photodynamic therapy of tumours and arterial plaque. Similarly, other Ln(III) chelates are being tested as radiation sensitizer in cancer therapy and seem to be very promising. Another new technique is neutron capture therapy with chelates of 155/157Gd, 149Sm or 151Eu. Much has to be done to make the latter technique clinically applicable. For instance, it will be necessary to localise a large number of metal ions at the tumour site, probably with the help of dendrimeric structures. Since the above isotopes are stable ones, this new development belongs more to the field of the proposed action than that of the new Action B12. Generally speaking, all physicochemical studies carried out in the present project have a direct impact on the developments of radionuclide chelates for therapy.

The knowledge on Ln(III) chelates in general and, particularly, the insight on interactions between Ln(III) chelates and biosystems that is being obtained in the present and proposed research could form a very useful basis for the further exploitation of Ln(III) chelates in therapy. A strong European initiative should give impetus to this research.

C.SCIENTIFIC PROGRAMME

The scientific programme will depend on the projects submitted by individual research teams. The working group projects will be selected according to the objectives outlined above. At this stage there is no specific scientific programme suggested for this action in order to place no limitations on the invited proposals. The selection will strictly occur according to the outlined objectives.

D.ORGANISATION AND TIMETABLE

D1: Organisation

Research projects fitting in the sub-topics described in section C will be submitted by scientists to the Management Committee members. This Committee will establish contacts between scientists.

The Management Committee has responsibilities for:

1.Drawing up the inventory during the first year, organisation of workshops and start of the activity; existing contacts will be used which should greatly facilitate this task.
2. The co-ordination of the joint activities with other COSTS Actions; joint meetings are likely to result from this activity.
3.Exploration of wider participation and exchange of information with EC-specific programmes, ESF, etc.
4.The planning of the intermediate report, the final report and the concluding symposium.

Progress in each of the projects will also be reported by the respective participants in their own countries within the framework of existing programmes.

D2: Reports

The progress of the programme will be monitored by brief annual reports from each of the participating scientists which will describe the results of research obtained through concerted action. A milestone report will be prepared by the Management Committee after 2 years of joint activities. The report will be presented to the COST Technical Committee for Chemistry for their review.
A final report will be published to inform non-participating scientists and research workers interested in the results about the scientific achievements of the Action. It is expected that some reviews by participants which describe the progress made and state of the field will be published in International Journals. To conclude the COST Action, a symposium will be held after 5 years which will be accessible to other scientists.

D3: Timetable

The Action will last five years and comprise the following four stages:

Stage 1:After the first meeting of the Management Committee, a detailed inventory of ongoing research and existing plans of the participating groups to begin joint projects will be made. This will result in a discussion document which will allow further planning to occur.
Stage 2:It will be evident which projects are closely related and would benefit from joint activities. Researchers (and co-workers) will set up (and continue) joint collaborative projects and exchange their recent research results. It may be appropriate to explore wider collaboration with other European countries during this stage.
Stage 3: An intermediate progress report will be prepared after 2 years for review by the COST Technical Committee for Chemistry and by the COST Senior Officials Committee.
Stage 4:This final phase will begin after 4 years and will involve the evaluation of the results obtained. It may include the organisation of a symposium for all the participants and co-workers.

E.ECONOMIC DIMENSIONS

The economic dimension of the Action (initial estimate of total costs = personnel + operational + running + commission costs) is: Euro 80 million.

The human effort in the area of "Lanthanide Chemistry for Diagnosis and Therapy", as described in this document, amounts to 600 man-years (120 researchers during 5 years), being equivalent to Euro 60 million approximately.
E1: Personnel costs

Estimates of personnel costs (research + administration) will depend on the rates applicable for various EU countries (estimate ECU 60 million).

Estimates of personnel costs (research + administration) are as follows:

Sub-topic 1:in about 5 countries a total of 120 man-years, totalling Euro 12 million
Sub-topic 2:in about 5 countries a total of 120 man-years, totalling Euro 12 million
Sub-topic 3:in about 5 countries a total of 120 man-years, totalling Euro 12 million
Sub-topic 4:in about 5 countries a total of 120 man-years, totalling Euro 12 million
Sub-topic 5:in about 5 countries a total of 120 man-years, totalling Euro 12 million

E2: Operational and running costs

The estimate of the total operational and running costs including costs of instruments and materials is Euro 20 million.

E3: Co-ordination costs

The costs of co-ordination to be covered by the COST budget are estimated to be Euro 80 000 per year, i.e. a total of Euro 400 000 for the five year duration of the project ( 0.5%).

F.DISSEMINATION OF SCIENTIFIC RESULTS

All publications arising from research carried out under COST Action D18 will credit COST support and the Management Committee will encourage and promote all co-authored papers. Results of research carried out by the working groups under COST Action D18 will be submitted to international scientific journals and reviews.

Joint meetings among different working groups in COST Action D18 and with working groups from other COST Actions, particularly with those of COST Actions D8 and B12, will be organised in such a way as to best promote interdisciplinary communication.

The Management Committee (MC), in conjunction with the working groups (WG) of the Action, will meet every year with the main aim of presenting results to the MC as a whole and, where possible, the MC will invite potential users and interested parties to this meeting.

The Management Committee (MC) will, during the first year of the Action, also set up a work-plan for interdisciplinary events for the dissemination of results of the Action COST D18.

Programme(s)

IC-COST - European cooperation in the field of scientific and technical research (COST), 1971-

Topic(s)

13 - Chemistry

Call for proposal

Data not available

Funding Scheme

Data not available

Coordinator

N/A

EU contribution

No data

Address

Total cost