Stabilization of radiosynthetic intermediates

The present invention relates to methods for the synthesis of 18 F radiolabeled aromatic amino acid derivatives and more particularly to a method for stabilizing the radiosynthetic intermediates used in said methods toward their decomposition caused by beta and gamma radiations.

Positron Emission Tomography

Positron emission tomography (PET) is an imaging method for obtaining quantitative molecular and biochemical information of physiological processes in the body. The most common PET radiotracer in use today is [18F]-fluorodeoxyglucose ([18F]-FDG), a radiolabeled glucose molecule. PET imaging with [18F]-FDG allows to visualize glucose metabolism and has a broad range of clinical indications. Other PET radiotracers are about to enter the clinical use. Among positron emitters, 18 F is the most widely used today in the clinical environment.

Decomposition of Radiochemicals—Radiolysis

Radiotracers are obtained after single or multiple step organic synthesis, among which the fluorination with 18 F-fluorides. radiosynthetic intermediates are generally involved in such radiosyntheses. These radiosynthetic intermediates can be exposed to high levels of radioactivity and high dose rates which results in some decomposition processes commonly named radiolysis. These side reactions can consume those radiosynthetic intermediates or react with the 18 F-fluorides and are detrimental to having high radiochemical yields.

The modes by which radiosynthetic intermediates decompose and their corresponding methods of control were classified in 1960 (Bayly, R. J. and Weigel, H., Self-decomposition of compounds labeled with radioactive isotopes. Nature, 188, 384-387 (1960).) (see Table I below).

The compound itself and/or its immediate surroundings will absorb the energy from the radiation. This energy excites the molecules, which can break up or react with other species or compounds. The excited molecular fragments may also react with other labeled compounds producing impurities. Energy absorbed by the immediate surroundings (mainly the solvent) can also produce reactive species, often free radicals, which can subsequently cause destruction of the molecules of radiolabeled compound. Whilst this is occurring, chemical decomposition often takes place as well, as these reactions occur in solution where chemical stability is known to be far more limited.

Secondary Decomposition

This is commonly the greatest path for the decomposition of radiochemicals, and it arises from the interaction of, for example, free radicals created by the radiation energy, with surrounding molecules including the radiolabeled molecules. It is by far the most difficult mode of decomposition to control and it is easily influenced by tiny changes to the environmental conditions. The low chemical content of radiolabeled compounds, particularly at high specific activity, amplifies the problems.

Secondary Decomposition in Water Solutions

The action of ionizing radiation on water is well documented (Thomas, J. K., Elementary processes and reactions in the radiolysis of water. Advances in Radiation Chemistry, 1, 103-198 (1969)). Ionization is known to occur along paths of the beta particles in discrete compartments known as “spurs” (Collison, E. and Swallow, A. J., The action of ionizing radiations on organic compounds. Quarterly Reviews of the Chemical Society, 9, 311-327 (1955)). The most damaging of the reactive species believed to be formed is thought to be the hydroxyl radical (Evans, E. A., Tritium and its Compounds. 2nd edition, Butterworths, London, pp. 642-782 (1974)). This was supported by the hydroxylation reaction of carbon-14 or tritium-labeled phenylalanine, to produce tyrosine and dihydroxyphenylalanine (Waldeck, B., [3H]Dopa in [3H]tyrosine with high specific activity. Journal of Pharmacy and Pharmacology, 23, 64-65 (1971)). In order to lower the decomposition, it is necessary to avoid or lessen the interactions between the damaging radicals and the surrounding molecules including the radiolabeled molecules. This can be achieved by lowering the temperature, diluting the radioactive concentration, and by adding radical scavengers. Ethanol is a common radical scavenger (typically as a 2% solution in water).

Secondary Decomposition in Organic Solvents

The detailed mechanism of decomposition of radiochemicals in organic solvents is not well known, and is expected to be complex. The effect of radiation energy on organic solvents is expected to be very different than that of aqueous media and would produce different forms of reactive species. The chemical purity of the solvent is for sure a critical parameter and well purified or very high quality purchased solvents ought to be used. The presence of peroxide in the solution may cause total destruction of the surrounding molecules including the radiolabeled molecules.

[18F]-Radiolabeled Aromatic Amino Acid

Radiolabeled aromatic amino acid derivatives, such as 18F-FDOPA, 18F-FTYR, 18F-FmTYR, etc. are often used to monitor the metabolism in the dopaminergic system. These radiotracers can be indicated to monitor Parkinson disease (PD), Alzheimer disease (AD) and some neurodegenerative diseases. These radiotracers have also shown interest in medical imaging of neuroendocrine tumors (NET).

Radiolabeled aromatic amino acid derivatives can be synthesized following different methods or paths such as for example:

However, most of these syntheses show drops in the radiochemical yields at high level of radioactivity due to the instability of the benzylic and/or phenolic radiosynthetic intermediates.

Benzylic and Phenolic Radiosynthetic Intermediates

The instability toward radioactivity and especially beta and gamma radiations may be attributed to the presence of the aromatic ring (see above reactivity toward hydroxyl radicals), but also to the benzylic or phenolic positions that are present whatever the synthesis path (as shown above) chosen for radiolabeled aromatic amino acid synthesis.

These reactive sites can be subjected to side reactions that will consume the radiosynthetic intermediates, including cold intermediates such as the precursor to be radiolabelled, that become unavailable for the subsequent chemistry steps. This is detrimental for having high radiosynthesis yields in the radiopharmacies where starting activity is in the range of 1-30 Ci, i.e. radioactivity concentrations from 0.5 to 15 Ci/ml.

Document WO 2005/061415 A1 discloses the preparation of a resin-bound benzylic intermediate (example 13). This compound is used as an intermediate in the synthesis of 18F-labeled FDOPA (example 11). 2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl (TEMPO) is used as a stabilizing agent. In this context ascorbate is also known to be a free radical trap.

The present invention aims at avoiding decomposition reactions, i.e. radiolysis resulting from high radioactivity concentrations, of radiosynthetic intermediates used in the synthesis of 18 F labeled aromatic amino acids by the use of radical scavengers or reductants or antioxidants during the crucial steps of the radiochemical synthesis. In other words, the invention aims at stabilizing the radiochemical yields of radiochemical synthesis of aromatic amino acids whatever the level of starting radioactivity used.

The present invention relates to the stabilization toward radiolysis of radiosynthetic intermediates, including cold intermediates such as the precursor to be radiolabelled, by the use of radical scavengers or reductants or antioxidants during the steps where high radioactivity concentrations are involved. The decomposition is especially important when water is present, where the presence of highly reactive hydroxyl radicals decomposes the radiosynthetic intermediates.

The present invention focuses on radiosynthetic intermediates involved in the synthesis of 18 F radiolabeled aromatic amino acids derivatives, including cold molecules such as the prescursor to be radiolabelled, because of the instability of the aromatic ring and of the benzylic and phenolic species involved in these syntheses.

The general structure of the radiosynthetic intermediates that needs to be stabilized in the context of the present invention can be as follows:

with R 1 , R 2 , R 3 , R 4 , R 5 and L as follows:

R 1 , R 2 , R 3 , R 4 ═H, OMe, OBn, Oallyl, H, OH; L=NO 2 , NMe 3 + , F, 18 F, IO 2

and

R 5 ═OH, I, Br, Cl

Due to the side reactions that are involved during the decomposition of the radiosynthetic intermediates, the “stabilizer” used during the radiosynthetic process according to the present invention can either be:

a radical scavenging species; a reductant species; an antioxidant species; a species with more than one of the characteristics above.

In some preferred embodiments of the present invention, carbonate (CO 3 2− ), nitrite, thiosulfate, thiosulfite, phosphate, phosphite or hypophosphite is used for the stabilization of the radiosynthetic intermediate.

In some preferred embodiments of the present invention, phosphorous acid is used for the stabilization of the radiosynthetic intermediate.

In some preferred embodiments of the present invention, a Fe(II) derivative or a Sn(II) derivative is used for the stabilization of the radiosynthetic intermediate.

In some preferred embodiments of the present invention, a phenol derivative is used for the stabilization of the radiosynthetic intermediate.

In some preferred embodiments of the present invention, iodide is used for the stabilization of the radiosynthetic intermediate.

In some preferred embodiments of the present invention, the compound used for the stabilization is also involved in the radiosynthetic process as a reagent or as a solvent.

In some preferred embodiments of the present invention, HI is used as for the stabilization of the radiosynthetic intermediate but also as a reagent and can be involved in chemical reactions such as halogenation or hydrolysis/deprotection.

In some preferred embodiments of the present invention, toluene is used for as a solvent for the stabilization of the radiosynthetic intermediate.

In some preferred embodiments of the present invention, dichloromethane is used for as a solvent for stabilization of the radiosynthetic intermediate.

In these examples it will be demonstrated how the right choice of the additives and reagents allows to stabilize the yields toward increasing the starting activity.