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Title:
NEW USE OF L-HISTIDINE AND DERIVATIVES THEREOF
Document Type and Number:
WIPO Patent Application WO/2013/017475
Kind Code:
A1
Abstract:
The present invention discloses the use of L-Histidine to reduce Gadolinium accumulation i nto target org ans, preventing its toxic effects, after administration of gadolinium based contrast agents. The invention thus, also disclose the use of L-Histidine to prevent the NSF syndrome in patients with renal functionality impairment. A kit of part is also disclosed by the use of which toxicity associated to free Gd3+, deriving from GDBCA, is prevented by oral administration of L- histidine before administration of the contrast agent. A method for the prevention of toxic metal accumulation which consists in the ad min istration, preferably by the oral route, of L-Histidine is also disclosed. L-Histidine is preferably administered a s a single oral dose comprised from 0.2-20 g.

Inventors:
ZSOLT BARANYAI (HU)
BRUECHER ERNOE (HU)
UGGERI FULVIO (IT)
MAIOCCHI ALESSANDRO (IT)
BUSSI SIMONA (IT)
Application Number:
PCT/EP2012/064489
Publication Date:
February 07, 2013
Filing Date:
July 24, 2012
Export Citation:
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Assignee:
BRACCO IMAGING SPA (IT)
ZSOLT BARANYAI (HU)
BRUECHER ERNOE (HU)
UGGERI FULVIO (IT)
MAIOCCHI ALESSANDRO (IT)
BUSSI SIMONA (IT)
International Classes:
A61K31/4172; A61P13/12; A61P39/04; A61P43/00
Domestic Patent References:
WO2008066862A22008-06-05
Foreign References:
US20090148432A12009-06-11
US20020004072A12002-01-10
Other References:
"Membership of the Working Party, Guidelines Safety in magnetic Resonance units: an update", ANAESTHESIA, vol. 65, 2010, pages 766 - 770
S. BUSSI ET AL., EXP. TOXICOL. PATHOL., vol. 58, 2007, pages 323 - 330
GADOLINIUM-BASED CONTRAST AGENTS (GBCAA) AND THE NSF RISK: REGULATORY UPDATE, 21 January 2011 (2011-01-21), Retrieved from the Internet
CANAVESE ET AL., J. NEPHROL., vol. 21, 2008, pages 324 - 336
BROOME D R, EUR. J. RADIOL., vol. 66, 2008, pages 230 - 234
ALTUN E. ET AL., ACAD. RADIOL, vol. 16, 2009, pages 897 - 905
SCHECHTER PJ; PRAKASH, J. AM. J. CLIN. NUTRITION, vol. 32, 1979, pages 1011 - 1014
GERBER DA ET AL., J. AM. J. CLIN. NUTRITION, 1971, pages 1382 - 1383
CARAVAN P. ET AL., INORG. CHEM, vol. 40, 2001, pages 2170 - 2176
MORCOS S K ET AL., EUR J. RADIOL., vol. 66, 2008, pages 175 - 179
TWEEDLE ET AL., MAGN RESON IMAGING, vol. 9, 1991, pages 409 - 415
G. E. JACKSON; S. WYNCHANK; M. WOUDENBERG, MAGN. RESON. MED., vol. 16, 1990, pages 57 - 66
L. SARKA; L. BURAI; E. BRUCHER, CHEM. EUR. J., vol. 6, 2000, pages 719 - 724
WEDEKING P.; TWEEDLE, NUCL. MED. BIOL., vol. 15, 1988, pages 395 - 402
VANDER ET AL., INVEST RADIOL, vol. 36, 2001, pages 115 - 122
SITTON N G, ANN RHEUM DIS, vol. 47, 1988, pages 48 - 52
"The handbook of pharmaceutical excipients", 2003, PHARMACEUTICAL PRESS
P. M. MAY; P. W. LINDE; D. R. WILLIAMS, J. CHEM. SOC. DALTON TRANS., 1977, pages 588 - 595
Attorney, Agent or Firm:
MERLI, Silvia (Via XXV Aprile No. 4, San Donato Milanese, IT)
Download PDF:
Claims:
CLAIMS

I. L-Histidine or derivatives thereof for use in the prevention of a condition associated to Gadolinium (Gd3+) accumulation in a patient's target organ after a GDBCA (Gadolinium Based Contrast Agent) administration.

2. The use according to claim 1 wherein said L-Histidine derivative is selected from the g rou p consisti ng of: L-Histidine di-hydrochloride, L- Histidine monohydrochloride monohydrate.

3. The use according to any one of claims 1-2, wherein said target organ is selected from the group consisting of: bones, skin, blood, plasma and liver. 4. The use according to any one of claims 1-2, wherein said GDBCA is a ionic or non-ionic macrocyclic or linear Gd chelating agent selected in the g rou p consisti ng of : Gad overseta mide, Gadod iam ide, Gad opentetate dimeglumine, Gadoxetic acid or Gadoxetate disodium, Gadobutrol, Gadofosfeset, Gadobenate dimeglumine, Gadoteridolo, Gadoteric acid.

5. The use according to claim 4, wherein said GDBCA is selected in the g rou p consisti n g of : Gad od ia m ide, Gadoversetamide, Gadopentetate dimeglumine, Gadobenate dimeglumine and Gadoxetic acid or Gadoxetate disodium.

6. The use according to any one of claims 1-5 wherein said condition is a dermopathy.

7. The use according to any one of claims 1-5 wherein said condition is NSF (Nephrogenic systemic fibrosis).

8. The use according to any one of claims 1-7 where, in said patient, the renal function is impaired .

9. A composition comprising L-Histidine or derivatives thereof for use in a method the prevention of a condition associated to accumulation of Gadolinium (Gd3+) in patient's target organs after a GDBCA (Gadolinium Based Contrast Agent) administration.

10. The composition according to claim 9 wherein the dose unit comprises 0.2-20 g/L-Histidine.

I I. A kit of pa rts for administration of a GDBCA, comprising a first container with a composition wherein the active ingredient is L-Histidine or derivatives thereof and a container comprising a GDBCA, said kit for use in a method fo r the prevention of a condition due to accumulation of Gadolinium after a GDBCA administration.

12. The kit of parts according to claim 11 wherein said GDBCA is selected in the group as defined in any one of claims 4 or 5.

13. The kit of parts according to any one of claims 11-12 wherein the GDBCA is comprised in a prefilled syringe.

14. The kit of parts according to any one of claims 11-13 wherein said composition is an oral composition comprising multiple or single dosage unit(s) of L-Histidine.

15. The kit of parts according to any one of claims 11-14 wherein said single unit dose comprises 0.2-20 g L-Histidine or derivatives thereof.

16. L-Histidine or derivatives thereof, as a powder, as a solid or as a liquid composition for use in a MRI imaging method wherein a Gadolinium Based

Contrast Agent (GDBCA) is administered to a patient.

17. L-Histidine according to claim 16 wherein the Gadolinium Based

Contrast Agent is selected in the group as defined in any one of claims 4 or

5.

Description:
NEW USE OF L-HISTIDINE AND DERIVATIVES THEREOF FIELD OF THE INVENTION

The present invention relates to the use of L-Histid ine and histid ine derivatives for preventing Gadolinium Based Contrast Agent toxicity and free gadolinium (Gd 3+ ) accumulation in tissues, which might occur after the use of such CAs.

STATE OF THE ART

Clinical diagnosis has dramatically evolved in the last few decades, taking advantage from magnetic resonance Imaging (MRI).

Paramagnetic contrast media are injected intravenously to enhance the signal from water protons, thus allowing recognition of tissues with different water content.

Molecules used in contrast media have been shown to be remarkably safe in cl i n ica l use, but they sti l l reta i n a d eg ree of toxicity th ou g ht to be responsible of adverse reactions observed in a small percentage of patients (M em bersh i p of the Wo rki n g Pa rty, G u i de l i nes Safety i n m a g neti c Resonance units: an update, Anaesthesia, 2010, 65 : 766-770).

One of the mechanism which has been proposed to explain the toxicity of paramagnetic gadolinium complexes used as contrast agents for the MRI is the in vivo dissociation and /or metabolism of the complex itself to yield metal and free ligands.

Both free ligands and metal were shown to possess a certain degree of toxicity, considered to be at least 20 times higher than that of their respective complexes, the free ligand by acting as a chelator of physiological positive ions (Ca 2+ , Zn 2+ ) and the metal by taking place of those needed ions in body tissues, through a transmetallation process.

However, while the newly formed complexes are excreted by the urinary system, the metal typically accumulates in tissues, thus being a suitable indicator of complex dissociation.

In this regard, the in vivo stability of three gadolinium complexes used for MRI, MultiHance ® , Omniscan ® and Gadovist ® has been further investigated in an animal model (rats) (S. Bussi et al . Exp. Toxicol . Pathol . 2007, 58 : 323-330) and the pattern of gadolinium accumulation after release from the chelated compounds was estimated and compared to the one observed after administration of gadolinium acetate (free form) after single or repeated (3- weeks) administrations. Even thoug h gadolinium release from chelated compounds has been confirmed unlikely to occur after a single injection of paramagnetic contrast media, used in this study at doses 10-fold higher than those recommended for diagnostic purposes, the highest gadolinium content due to complexed-Gd was observed in the kidneys, while spleen, femur and brain were mainly targeted by free gadolinium (mimicked by GdAc).

It's this aliquot of free Gadolinium released from GDBCA thought to be one of the causes of NSF (Nephrogenic Systemic Fibrosis) occurring to patients with renal insufficiency. NSF (see: "Gadolinium-based contrast agents (G BCAa ) a n d the N S F risk : Reg u l ato ry u pd ate" J a n u a ry 21 , 20 1 1, http ://www.fda.gov/downloads/AdvisoryCommittees/CommitteesMeetin gMa terials/Drugs/PeripheralandCentralNervousSystemDrugsAdvisory Committee/ UCM241072.pdf) was first recognized in 1997 in 15 dialyzed patients and described in 2000. This rare and highly debilitating disorder, that according to preclinical data has been attributed to an immunologic cascade triggered b y f re e G d 3+ (Ca n avese et a l . J . N ep h ro l . , 2008, 21 : 324-336) is characterized by an extensive thickening and hardening of the skin associated with skin-colored to erythematous papules that coalesce into erythematous to brawny pl aq ues with a "peau d 'orange" appearance . Nodules are sometimes also described . Joint contractures may develop, with patients progressively becoming wheelchair-dependent. Patients often complain of pruritus, causalgia and sharp pains. The distal extremities are the most common area of involvement (with a distribution from ankles to mid-thighs and from wrists to mid-upper arms), followed by the trunk. The lesions are typically symmetrical . It is worth noting that the face and neck are virtually never involved .

NSF can occur in all age-groups and there is no predilection for a geographic region, race or gender. So far, there is no recognized treatment for NSF. It has been suggested that improving renal function may slow down the development of the disease and, in some cases, may reverse its course. In 2006, two European teams independently suggested a link between the administration of gadolinium chelates used as contrast media for mag netic resona nce imag ing ( M RI) a nd the occurrence of NSF in patients with renal failure. Numerous retrospective analyses rapidly followed and confirmed this temporal link (Broome D R, Eur. J. Radiol., 2008, 66: 230-234). NSF has not been reported in patients with normal kidney function . Patients at greatest risk for developing NSF after receiving GBCAs are those with impaired el imination of the d rug, including patients with acute kid ney i nj u ry (AKI ) or ch ron ic, severe kid ney disease (with a glomerular filtration rate or GFR < 30 mL/min/1.73 m 2 ).

Policies to minimize the risk of NSF include the use of the lowest possible diagnostic dose and risk-benefit analysis prior of the administration of the GBCA (Altun E . et al. Acad. Radiol, 2009, 16: 897-905). Even though a straightforward link between free Gd and NSF has stil l not been clearly established, transmetallation may be considered as a promising starting point for studies aimed at further increasing safety in the field of MRI CA. Divalent ions such as Zinc and Copper are essential trace elements and are the necessary components of many enzymes. Zn 2+ and Cu 2+ ions are widely spread in animal cells and in blood . Due to their physico-chemical affinity to the chelating moiety in GDBCA, these ions may promote transmetallation reactions with consequent release of free Gd 3+ in plasma.

The present invention and the new use of L-Histidine able to reduce the release of free gadolinium from GBDCA and its toxicity due to accumulation in target organs, might find an explanation by this mechanism.

L-Histidine is an amino acid : its use in human subjects has been described, even thoug h with controversial resu lts, for d ifferent purposes : as an anorectic agent i.e. in Schechter PJ and Prakash J. Am. J. Clin. Nutrition 1979, 32 : 1011-1014 or as a hypocholesterolemic agent in Gerber DA et al . J . Am . J . Clin . Nutrition 1971 , 1382-1383). Although a therapeutic effect has been observed in none of the cited references, after the administration of L-histidine up to 4g/day, serum parameters including protein bound zinc (namely albumin and a2-macroglobulin bound zinc), serum and urinary zinc levels, monitored d uring the 15-days treatment, were not shown to be grossly altered. It's presently marketed as a food integrator and, accordingly, it's use is considered safe since a long time in human subjects. Although histidine-rich proteins are known to chelate divalent metal ions like Zn 2+ and Cu 2+ , in vivo, this binding has been demonstrated to rely upon by their highly structured three-dimensional His-rich Zn-finger domains and in the cell, an extremely well compartmentalized environment.

Therefore the present use of L-Histidine and the therapeutic effects observed in vivo as shown in the present Application, should be considered quite unexpected.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1. Absorbance values as a function of time in the reaction of Gd(DTPA-BMA) (panel A) and Gd(DTPA) (panel B) with Cu(II) in the presence of citrate and L-Histidine in concentration of: 0.2 (black line, ♦), 0.4 (dark gray, ■), 0.6 (gray, A) and 0.8 (light gray, ·) mM ([GdL] = 2.0 mM, [Cu] = 0.1 mM, [cit] = 2.0 mM, [ H i s ] = 0.2-0.4-0.6-0.8 mM, [HEPES]=0.01 M, pH = 7.0, 25°C, 0.15 M NaCI).

Figure 2. Relaxivity values as a function of time for the decomplexation reaction of Gd(DTPA-BMA) (GdL) in artificial plasma ([GdL] = 1.0 mM, [His] = 0.5 or 1.0 mM, pH = 7.5, 37°C, 0.15 M NaCI). Artificial plasma (♦), comprising 0.5 mM His (■) or ImM His (A).

Figure 3. The UV spectrum of Cu(His) 2 in the presence of Gd(DTPA-BMA) excess (Panel A) and in the presence of citrate and Gd(DTPA-BMA) excess (Panel B). (Cu(His) 2 = 0.2 mM, Gd(DTPA-BMA)= 2.0 mM) (A). (Cu(His) 2 = 0.2 mM, Gd(DTPA-BMA)= 2.0 mM, Citrate=2.0 mM) (B). pH = 6.5, 25°C, 0.15 M NaCI.

SUMMARY OF THE INVENTION

It has been surprisingly found that L-histidine significantly reduces the release of free Gd 3+ from a Gadolinium Based Contrast Agent and the accumulation of free Gd 3+ into a patient's body, when administered together, or shortly before GDBCA administration.

Therefore according to a first aspect, the invention relates to L-Histidine or derivatives thereof for use in the prevention of a condition due to accumulation of Gadolinium (Gd 3+ ) in patient's target organs after a GDBCA (Gadolinium Based Contrast Agent) administration.

Said target organ is preferably selected from the group consisting of: bones, skin, blood, plasma and liver. The GDBCA is preferably selected from the group consisting of Gadoversetamide (OptiMark ® ), Gadodiamide (Omniscan ® ), Gadopentetate dimeglumine (Magnevist ® ), Gadoxetic acid and Gadoxetate disodium (Eovist ® ), Gadobutrol (Gadovist ® ), Gadofosfeset (Ablavar ® ), Gadobenate dimeglumine (MultiHance ® ), Gadoteridolo (ProHance ® ), Gadoteric acid (Dotarem ® ) . Particul arly preferred for co- administration with L-Histidine are linear ionic or non-ionic chelates, such as : Ga d od i a m i d e, Gadoversetamide, Gadopentetate dimeglumine, Gadobenate dimeglumine and Gadoxetic acid or Gadoxetate disodium.

Said cond ition is preferably a dermopathy, such as NSF (Nephrogenic systemic fibrosis), most preferably occurring in patients with renal function impairment, such as a chronic or acute renal disease, and with a reduced glomerular filtration rate, i.e. patients undergoing dialysis.

Cofactors, potentially influencing the development of a Gd 3+ induced dermopathy or N S F can be found in hypercalcemia, co-therapy with phosphate chelating agents (such as sevelamer HCI), acidosis, co-anti- hypertensive therapies and iron therapies.

Accord ing to a further aspect the invention relates to a kit of parts for administration of a GDBCA, comprising a first container with a composition wherein the active ingredient is L-Histidine or derivatives thereof and a container comprising a GDBCA, said kit for use in a method for the prevention of a condition due to accumulation of Gadolinium after a GDBCA administration.

Said composition comprising L-Histidine is preferably an oral composition for multiple or single dosage unit(s) of L-Histidine, preferably comprising, as a said single unit dose, an amount of 0.2-20 g L-Histidine or derivatives thereof. DETAILED DESCRIPTION OF THE INVENTION

The i nventi o n is based on the effect of L-Histidine, preferably orally administered, on the release of free gadolinium after administration of a Gadolinium Based Contrast Agent (GDBCA).

By GDBCA are meant all contrast media based on Gadolinium and used generally for Contrast Enhanced Magnetic Resonance (CE-MR). Gadolinium, a paramagnetic metal in the lanthanide series, produces a large magnetic field that enhances the relaxation rates of water protons located close to the agent, thereby increasing signal intensity.

Since free Gd 3+ ions are toxic in vivo, in GDBCAs, the paramagnetic ion is chelated to create stable, inert, low molecular weight metal complexes. Macrocyclic Gd chelates, such as Gadobutrol (GadoVist ® , Bayer Shering Pharma AG), Gadoteridol (ProHance ® , Bracco) and Gadoterate meglumine (Dotarem ® , Guerbet) have a higher in vivo stability compared to linear Gd 3+ chelates, such as Gadoversetamide (OptiMark ® ), Gadodiamide (Gd-DTPA- BMA, OmniScan ® ), Gadopentetate dimeglumine (Gd-DTPA, Magnevist ® , Bayer Shering Pharma AG) and gadobenate dimeglumine (MultiHance ® , Bracco SpA). Gd based CA are rapidly cleared from intravascular spaces and eliminated from the human body mostly unchanged via (predominantly) renal excretion.

Ch el ated co m pl exes a re g e ne ra l l y sta b l e , a lth o u g h with d iffere nt thermodynamic stability constants. Thermodynamic stability costant affinities have been described for the following GDBCAs: Gadoversetamide, Gadodiamide (G d DT PA-BMA), Gadobutrol, Gd-DTPA, Gadofosveset- trisodium, Gadobenate dimeglumine (Gd-BOPTA), gadoxetic acid (EOB- DTPA), Gadoteridol (HP-D03A) a nd Gd-DOTA (Caravan P. et al. Inorg . Chem, 2001 : 40, 2170-2176).

One of the mechanisms which has been proposed to explain the somehow described toxicity of GDBCA, is their in vivo dissociation and/or metabolism of the complex itself, to yield free metal (Gd 3+ ) and ligand, through a process known as transmetallation (Morcos S K et al . , Eur J. Radiol ., 2008, 66 : 175-179). These authors postulate that incidence of NSF (Nephrogenic Systemic Fibrosis) correlate with the stability of the Gd 3+ - complexes being in fact more frequently associated with administration of Omniscan ® , a linear non-ionic CA than ProHance ® (a macrocyclic chelate).

Although transmetallation may, at least theoretically, affect a GDBCA in a way i nversely p ropo rtional to the thermodynamic stability of the paramagnetic metal/chelate complex, this parameter does not sufficiently describe the factors affecting the in vivo transmetallation process (Tweedle et al. Magn Reson Imaging 1991; 9 :409-415). Blood and, with the more reasons, living organisms are complex systems, where the presence of different ions and counter-ions in various concentrations and forms (free, bound to proteins or compartimentalized) influence the kinetics of the chelating reactions occurring in vivo. Indeed, also the kinetic stability of Gadolinium complexes is influenced by the presence of endogenous ions, counter-ions and other molecules in plasma . Citrate, i.e., a counter-ion physiologically present in the body, is known to form complexes with the Gd 3+ ions released in transmetallation reactions (G. E. Jackson, S. Wynchank, M . Woudenberg, Magn. Reson. Med. , 1990, 16, 57-66; L. Sarka, L. Burai, E. Brucher, Chem. Eur. J. , 2000, 6, 719-724) and, as described in more details in the experimental part, has been shown indeed to take part to the transmetallation reaction.

Transmetallation, where the free ligand may act as a chelator of endogenous positive ions (Ca 2+ , Zn 2+ , etc) with exchange of those ions in body tissues, generates in vivo free ligand and paramagnetic metal. Both species have been shown to possess a certain degree of acute toxicity, in terms of LD 50 , at least 20 ti mes h ig her tha n that of their respective complexes. However, while the newly formed complexes are excreted by the urinary system, Gd 3+ accumulates in tissues, with a well recognized toxicity, apparent in the skin and due to the triggering of an immune response to Gd 3+ .

Bones have been shown to behave as natural repository for unchelated gadolinium (Wedeking P. & Tweedle, Nucl . Med . Biol., 1988, 15 : 395-402), thus being a suitable indicator in the medium-long term time range of Gd- complex dissociation. It has been postulated that mainly Zn could be involved in displacing significant amounts of gadolinium because the concentration of the former in blood is relatively high (55-125 prnol/L) (Vander et al . , Invest Radiol 2001; 36: 115-122), but even less abundant species may influence the affinity and the kinetics of the displacement reaction as shown in the plasma model selected .

Endogenous ions likely to compete with Gd 3+ are Cu 2+ , Ca 2+ , and Zn 2+ . Among these, Cu 2+ is present in relatively small concentrations in the blood (1-10 prnol/L and [Cu 2+ ] tot bod y = 18 prnol/L: G. E. Jackson, S. Wynchank, M. Woudenberg, Magn. Reson. Med., 1990, 16, 57-66), whereas Ca 2+ has relatively low affinity for organic ligands like DTPA and DTPA-BMA. Free Gadolinium release from GDBCA is thought to be one of the causes of NSF (Nephrogenic Systemic fibrosis) ( Broome, 2008) in patients with renal failure (being the kidney, usually, the major excretory organ of GDBCA). Administration of histidine according to the present invention, which provides for the use of L-Histidine or derivatives thereof in the prevention of a condition due to accumulation of Gadolinium (Gd 3+ ) in a patient's target organs after a GDBCA (Gadolinium Based Contrast Agent) administration, allows a lower release of free gadolinium from GDBCA in vivo and its accumulation in tissues, p articularly into the bones and the skin, thus decreasing the risks associated to its toxicity.

Zinc and Copper are essential trace elements, required for the functionality of many enzymes. Thus, Zn 2+ and Cu 2+ ions are widely spread in animal cells and in blood . Reduction of transmetallation between divalent ions, such as Zn 2+ , Cu 2+ and Gd 3+ is believed to be at the basis of the reduction of g a d o l i n i u m a ccu m u l at i o n i n ti ss u es a fte r G D BCA a n d L-Histidine administration according to the present invention. In fact, it has been experimentally confirmed that an increase in Histidine concentration from 0.2 to 0.8 mM in the presence of citrate can decrease the amount and the rate of exchange ligand formed with Cu 2+ , for the Gd chelating moieties DTPA and DTPA-BMA, in a concentration dependent mode, in two different in vitro systems tested. The toxic effects of metal deposition is known and generally acknowledged : it is partly prevented by adding excess amount of chelating compounds in the CA formulation itself: with the exception of MultiHance ® and Dotarem ® , in fact, all other contrast agents comprise an excess chelating compounds (up to 28.4 mg/ml in OptiMark ® ). However, free Gd 3+ is still released within the body before excretion of the complexes.

I n th is reg a rd , the p resent invention which relates to pre- or coadministration of L-Histidine or derivatives thereof after GDBCA administration, in order to prevent Gd 3+ release and accumulation within the tissues, addresses the problem of toxicity due to paramagnetic metal accumulation after GDBCA administration.

The CA is preferably selected in the group consisting of ionic or non-ionic macrocyclic and linear Gd chelating agents, such as: Gadoversetamide (OptiMark ® ), Gadodiamide (Omniscan ® ), Gadopentetate dimeglumine (Magnevist ® ), Gadoxetic acid and Gadoxetate disodium (Eovist ® ), Gadobutrol (Gadovist ® ), Gadofosfeset (Ablavar ® ), Gadobenate dimeglumine (MultiHance ® ), Gadoteridolo (ProHance ® ), Gadoteric acid (Dotarem ® ). Particularly preferred for co-administration with L-Histidine are linear ionic or non-ionic chelates, such as: Gadodiamide, Gadoversetamide, Gadopentetate dimeglumine, Gadobenate dimeglumine and Gadoxetic acid or Gadoxetate disodium.

Preferably L-Histidine (or derivatives thereof) is ad ministered at least 1 hour, more preferably 55' (min), 50', 45', 40', 35', 30', 25', 20', 15', 10' or 5 min before administration of the CA. At the latest, L-Histidine is co- ad ministered together with the Contrast Agent. The optimal time for histidine administration, with respect to administration of the contrast agent, might more accurately be determined by calculating the relevant pharmacokinetic parameters, such as C max , (peak concentration) T max ,(time to achieve maximum concentration), ti /2 λ ζ (half-life) or AUC in the dose range of interest. In particular T max should be considered for determining the optimal time for GBDCA administration after histidine administration . This parameter is preferably to be calculated for histidine derivatives, such as sustained release forms or poly-histidine complexes, the pharmacokinetic parameters of which might differ from L-Histidine or in case an alternative administration route is envisaged . In these cases, the time required to reach peak levels (T max ) of L-Histidine in serum after a single administration by the alternative route, or alternative histidine form, would allow better adapting the time for GBDCA administration.

By the ora l route, it can be estimated that peak levels of L-Histidine administered i n the sa lt form ( HCI ) are reached 30-45 min after oral administration (Sitton N G, Ann Rheum Dis, 1988, 47 :48-52).

Since oral administration of L-Histidine in humans, u p to 4-6 g/day, for several days, has been reported to provide neither important adverse effects nor minor inconveniences, its use up to gram doses is foreseen for the present invention as wel l, preferably in a single oral administration during a time closely prior to the administration of the GBDCA.

The possibility of an oral administration, a well known adsorption kinetic in man, and the high doses tolerated, represent important advantages for a preventive therapy with L-Histidine in GBDCA induced conditions for which, heavy toxic effects have u ntil now only sporadically been reported . In general, i n fact, it's very important for a preventive therapy neither interfering with the patient's general well being nor inducing discomfort. In this regard, it should be mentioned that, although histidine can be administered also parenterally, a preferred administration is by os, before GDBCA, in order not to have the patient facing two injections within a short time from one another, instead of just one with the contrast agent. In fact, this may increase the patient's discomfort.

In contrast, oral administration of L-Histidine can be accomplished via a liquid solution comprising L-Histidine or derivatives thereof in a dose ranging from 10 to 200 mg/kg patient, not earlier than 60 min or preferably, from 45 to 15 min, or preferably 25-40 min before GBDCA administration, optionally together with or mixed within a liquid saline for hydration therapy.

Most common hydration mixtures come as powders to be mixed with water or an aqueous solution. Premixed solutions are also commercially available usually to be dissolved in 0.5-1 L of water to produce a solution containing (in mmol/L) Na 75-90, K 20, CI 65-80, and optionally comprising citrate 5- 15 and glucose 80-120. Histidine or derivatives thereof can be added to a hydration mix just before the addition of water.

As an alternative, histidine is administered as a solid composition, i.e. in tablets, together with water or hydration mixtures.

Target organs for free Gd accumulation based on experimental observations (S. Bussi et al., 2007), are generally considered the liver, spleen, bones and brain . According to the present invention a statistically significant decrease in free Gd 3+ deposition has been observed in bones (femur) and skin after administration of L-Histidine. These organs are indeed representative for an overall lower release of Gd 3+ from GBDCA in the presence of histidine or derivatives thereof. In particular, accumulation of Gd 3+ in the bones may be considered representative of the overall phenomenon, with a measure which is not affected by Gd 3+ fraction present in plasma, from which Gd it is very rapidly cleared, due to the scarce or null vascularisation in the bone tissue. Skin on its turn, represents the target organ for Gd 3+ deposition possibly leading to NSF, in kidney functionality impairment conditions.

A lower release of Gd 3+ from GBDCAs in the presence of L-Histidine has been confirmed by kinetic experimental data in a plasma model in vitro with either DTPA and DTPA-BMA, as better detailed in the experimental part.

Therefore the present invention relates to the still undisclosed use of L- Histidine or derivatives thereof for the prevention of metal accumulation, in particular Gd 3+ , after GBDCA administration ; according ly, the present invention relates to L-Histidine and derivatives thereof for use i n the preparation of a treatment preventing gadolinium deposition in target organs and, hence, for the preventing the toxic effect of this accumulation, in particular in the bones, spleen and skin.

Since metal accumulation, in particular free Gd 3+ has been accredited as a possible cause of NSF, in patients with impaired renal functions, the present invention also provides L-Histidine for use in a method aimed at the prevention of a co n d iti o n d u e to G d a cc u m u l at i o n after GDBCA administration, in patients with kidney functionality impairment. Said cond ition is preferably a dermopathy, such as NSF (Nephrogenic systemic fibrosis), most preferably occurring in patients with renal function impairment, such as a chronic or acute renal disease, and with a reduced glomerular filtration rate, i.e. in patients undergoing dialysis.

Cofactors, potentia l ly i nfl uencing the development of a Gd 3+ induced dermopathy can be found in hypercalcemia, co-therapy with phosphate chelating agents (such as sevelamer HCI), acidosis, co-anti-hypertensive therapies and iron therapies.

In particular animal studies carried out in rats, have confirmed that a single o ra l p re-treatment of L-Histidine (administered at the dose of 4.38 mmol/kg, before administration of Omniscan ® ) is able to decrease significantly the Gd 3+ levels in the bones (femur) and skin, measured as nmol/gram of fresh tissue.

In particular, L-Histidine administration 30 min before Omniscan ® administration induced a statistically significant decrease in Gd 3+ content in femur of about 40% (both as nmol/g fresh tissue and %ID) and in the skin. This data has been also confirmed by % Injected Dose at the bone level. It is known from the literature that bones and skin are the organs where the depositio n of free g ad o linium more frequently occurs (Bussi, 2007; Wedeking, 1988).

Histidine, is preferably L-Histidine, is administered as a powder of the di- hydrochloride or monohydrochloride salt either in the monohydrated as well as anhydrous form, preferably d issolved in an aqueous solution. The present invention also provides for L-Histidine as a poly-histidine complex or in any sustained or controlled release forms. Controlled release forms may comprise those described in US 2002/0004072.

When L-Histidine is administered orally, is preferably administered in a dose ra n g i ng from 10 to 200 mg/kg patient, not earl ier than 60 min or preferably, from 45 min to 15 min , or preferably from 40 min to 25 min before GBDCA administration, optionally together with, or mixed within, a liquid saline for hydration therapy. Even though oral administration is preferred because it's easier and usually well tolerated, L-Histidine can be ad ministered by any other route, i .e. parenterally.

According to the data in the preclinical model and to the well established safety in humans, L-Histidine can be administered in humans up to 200 mg/kg/day. Thus, accordingly for human use, single dosage units comprise L-Histidine or salts derivatives in a quantity comprised from 0.2-20 g.

Composition comprising L-Histid ine or derivatives thereof for use in the prevention of metal (Gd 3+ ) accumulation in target organs due to a GDBCA administration are comprised within the preferred embodiments of the invention. Particularly preferred compositions are oral composition either liquid or solid wherein histidine or derivatives thereof is present i n a quantity comprised from 0.2-20 g L-histidine hydrochloride per unit dose together with commonly used diluents (such as cellulose and derivatives thereof, polymethacrylates, starch and derivatives thereof, calcium carbonate or phosphate, alginates and derivatives), disintegrants (such as Na-croscarmellose), l u bricants, excipients, stabilizers, flavourings a n d monovalent cationic salt integrators (see as a general review: "The handbook of pharmaceutical excipients" 4 th ed, 2003, Pharmaceutical Press, editors Raimond C. Rowe, Paul J. Sheskey, Paul J. Weller).

According to a preferred embodiment, the present invention also refers to a kit of parts for administration of a GDBCA, comprising a container with a composition wherein the active ing red ient is L-Histidine or derivatives thereof and a container comprising a GDBCA.

The CA is preferably selected in the group consisting of ionic or non-ionic macrocyclic and linear Gd chelating agents, such as: Gadoversetamide (OptiMark ® ), Gadodiamide (Omniscan ® ), Gadopentetate dimeglumine (Magnevist ® ), Gadoxetic acid and Gadoxetate disodium (Eovist ® ), Gadobutrol (Gadovist ® ), Gadofosfeset (Ablavar ® ), Gadobenate dimeglumine (MultiHance ® ), Gadoteridolo (ProHance ® ), Gadoteric acid (Dotarem ® ). Particularly preferred for co-administration with L-Histidine are linear ionic or non-ionic chelates, such as : Gadodiamide, Gadoversetamide, Gadopentetate dimeglumine, Gadobenate dimeglumine and Gadoxetic acid or Gadoxetate disodium, and is optionally, but preferably present i n a prefilled syringe.

Histidine, or preferably L-Histidine, is in the form of a powder or a tablet, ready for administration or to be dissolved before administration. Liquid compositions comprising L-histidine ready to use or to be prepared at the moment of administration, are acceptable according to the presently disclosed use.

According to a further embodiment the invention refers to a therapeutic method for preventing the toxic effect of Gadolinium accumulation into a patient's tissue, wherein said tissue is preferably selected from the group consisting of: bones, skin, spleen and blood, and/or to prevent NSF, which may occur after GBDCA administration in subjects, preferably in subjects with renal function impairment, which consists essentially in administering L-Histidine or derivatives thereof, preferably in a single unit oral dose, not earlier than 60' before GDBCA administration.

It follows that the invention also comprises a diagnostic method consisting essentially of L-Histidine or derivatives thereof administration, preferably in a single unit oral dose, followed by GBDCA administration within one hour period, even more preferably within 55 min 50 min 45 min 40 min 35 min 30 min 25 min 20 min and 15 min. More preferably the time range is comprised from 45 and 15 min or even, more preferably, from 40 to 20 min. GBDCA are preferably selected in the group consisting of ionic or non- ionic macrocyclic and linear Gd chelating agents, such as: Gadoversetamide (OptiMark ® ), Gadodiamide (Omniscan ® ), Gadopentetate dimeglumine (Magnevist ® ), Gadoxetic acid and Gadoxetate disodium (Eovist ® ), Gadobutrol (Gadovist ® ), Gadofosfeset (Ablavar ® ), Gadobenate dimeglumine (MultiHance ® ), Gadoteridolo (ProHance ® ), Gadoteric acid (Dotarem ® ). Particularly preferred for co-administration with L-histidine are linear ionic or non-ionic chelates, such as : Gadodiamide, Gadoversetamide, Gadopentetate dimeglumine, Gadobenate dimeglumine and Gadoxetic acid or Gadoxetate disodium.

U n it d ose m ay co m p rise L-Histidine or salts and derivatives for administering Histidine at a dose comprised from 0.2 to 20 g L-Histidine. The following examples of the practice of the present invention are meant to be illustrative and are in no way limiting the scope of the invention.

EXPERIMENTAL PART

Example 1. Measurement of L-Histidine levels in plasma.

Preparation of the calibration curve and quality control samples.

Aliquots of L-Histidine dihydrochloride powder, accurately weighted, were transferred into 5 and 10 ml_ volumetric flasks. MilliQ-water was used to dissolve the powder. Standard solution was prepared to obtain CAL samples and CQ samples. The range of the stock standard solution was from 3.46 to 343 pg/mL. All standard solutions were stored at +4°C, in the dark: under these conditions they were considered stable for up to one month.

CAL samples were prepared by using the standard solution at the concentration of 3.46, 34.3, 68.6, 173 and 343 pg/mL and QC samples by using the standard solution at the concentration of 14.3, 67.1, 143 and 268 pg/mL.

CAL and QC samples were prepared by mixing 10 pL of Histidine standard solution with 90 pL of blank rat plasma.

To these samples 20 pL of trichloroacetic acid (16% w/v) were added to precipitate the plasma proteins. The tubes were vortexed for few seconds and centrifuged for 10 min at 2000 g at room temperature; to 90 pL of the clear supernatant 45 pL of a soaked solution of Sodium carbonate (200 mM) and 90 pL of derivatizing solution (4 mg/mL of Dansyl chloride) were added . The tubes were vortexed for few seconds and incubated under sti rri n g fo r 30 m i n utes at 60 °C . Five m i cro l ite rs of M eth yl a m i n e Hydrochloride were added and after cooling for 15 minutes, ten microliters of the sample were injected into the HPLC equipment.

Test plasma samples (S) were prepared by adding 10 pL of Milli-Q water to 90 pL of undiluted plasma or 90 pL of plasma suitably diluted with blank plasma (if sample volume was not enough). To these samples 20 pL of trichloroacetic acid (16% w/v) were added to precipitate the plasma proteins. The tubes were vortexed for few seconds and centrifuged for 10 min at 2000 g at room temperature; to 90 pL of the clear supernatant 45 pL of a soaked solution of Sodium carbonate (200 mM) and 90 pL of derivatizing solution (4 mg/mL of Dansyl chloride) were added . The tubes were vortexed for few seconds and incubated under stirring for 30 minutes at 60°C. Five microliters of Methylamine Hydrochloride were added and after cooling for 15 minutes, ten microliters of the sample were injected into the H PLC equipment. All plasma samples collected during the study were stored at -20°C until analysis.

Results of HPLC analysis showed that it was possible to prepare a calibration curve for Histidine ranging from 3.46 to 343 pg/mL using a weighted linear regression. The goodness of the calibration curve and the performance of the chromatographic system complied to the defined acceptability criteria. The precision of the back calculated concentrations of Histidine CAL standards (CV%) was 1.19% at 10.4 pg/mL, 1.25% at 41.3 pg/mL, 2.40% at 75.6 pg/mL, 2.81% at 180 pg/mL and 2.29% at 350 pg/mL.

The accuracy of the back calculated concentrations of Histidine CAL standards (ACC%) was -1.52% at 10.4 pg/mL, -3.25% at 41.3 pg/mL, 0.34% at 75.6 pg/mL, -1.57% at 180 pg/mL and -3.20% at 350 pg/mL. The correlation coefficient value was 0.9993.

Histid ine content in CAL and QC samples was the result of endogenous

H istid i ne ( co n ce ntratio n i n bl a n k p l asm a pool ) a n d ad ded H isti dine (Calibration standards).

The precision of the back calculated concentrations of Histidine QC was between 85% and 115% for all the samples and considered acceptable.

The accuracy of the back calculated concentrations of Histidine QC was <

15% for all the samples.

Precision of test samples was always acceptable.

Example 2. Experimental design for histidine administration and pharmacokinetic studies.

Animals

- Species and strain : CD(SD)IGS BR rat

- sex of animals: males

- Weight and age at treatment: 306 - 376 g (10-14 weeks old)

- Supplier: Charles River Laboratories, Calco (LC), Italy All the procedures involving the animals were conducted according to the national and international laws on experimental animal (L. D. 116/92; C D. EEC 86/609; C. R. 2007/526/EC). No validated non animal alternatives are known to meet the objectives of the study.

Rats were used to determine the pharmacokinetic parameters after administration of three different histidine doses (100, 500 and 1000 mg/kg, three animals for each group).

Three animals were used for blank plasma sample collection for the calibration curve.

Statistical analysis

Plasma concentrations of Histidine as a function of time after injection C(t), were analyzed by non-parametric methods using the computer program WIN-NONLIN 4.0.1. For the tested doses, a non-compartmental analysis was performed using the average plasma concentrations from 3 animals for ea ch ti me po i nt (except whe n i n d i cated ) . Fo r Cm ax (peak plasma concentration) and the corresponding tmax (amount of time that a drug is present at Cmax in plasma) the observed values were reported . The terminal phase elimination rate constant (λζ) was estimated by log-linear regression of those data points visually assessed to be in the terminal phase of the profile. At least three terminal plasma concentrations were used for estimating λζ. The terminal elimination half life (t 1 /_Az, time required for a drug to decline to half of its original plasma value) was calculated as V/iKz = In 2/λζ. Area under the plasma concentration-ti me cu rve to the l ast observable plasma concentration (AUC(0 to t)) was calculated from observed data using the logarithmic trapezoidal method . Total area under the plasma concentration-time curve from zero to infinity (AUC(0 to ∞)) was estimated by AUC(0 to ∞) = AUC(0 to t) + ct /λζ, where ct is the predicted concentration at the last quantified time point. AUC defines extent of drug exposure.

Histidine basal values in rat plasma ranged from 4.60 to 8.39 pg/mL.

Cmax values of 9.0, 20.9 and 36.0 pg/rnL were obtained 30 min, 1 h and 30 m i n (tmax) after ora l a d m i n istratio n at 100 , 500 , 1000 mg/kg , respectively. The half life values associated with the terminal elimination phase Vhkz were 51, 47 and 13 minutes for animals treated at the three different doses, respectively (see Table A). These values suggest a slightly faster elimination from plasma for the highest dose.

The AUC(0 to∞) values were 0.55, 0.63 and 0.39 h*mg/ml_, respectively. The values were similar indicating a similar systemic exposure at the different doses. Moreover, they were in the range of the AUC(0 to∞) value (0.477 h*mg/ml_) fo u n d i n h ea l th y vo l u n tee rs a fte r a s i n g l e o ra l administration of Histidine at 100 mg/kg (Sitton N G, Ann Rheum Dis, 1988, 47 :48-52).

The total plasma clearance values were 180, 790 and 2562 mL/h/kg for the three doses, respectively. Clearance values increased with increasing the dose.

Table A: Pharmacokinetic values

Cmax values increased with increasing the dose. T max was calculated twice 30 min (or 1 h); AUC(0 to inf) were similar at the three doses. No clinical signs were observed up to 1000 mg/kg, indicating that the compound is well tolerated up to 5 times the highest dose foreseen for patients (200 mg/kg). Example 3. Administration of L-Histidine and measure of Gd 3+ target organ accumulation

Animals:

- Rat Sprague Dawley

- sex of animals: males

- Weight and age at treatment: 208 - 233 g, 7 weeks old

- Supplier: Charles River Laboratories, Calco (LC), Italy A period of at least 3 d ays was a l l owed between a n i ma l a rriva l a nd allocation to treatment groups.

Three animals were used in order to collect blank plasma samples for the calibration curve.

L-Histidine was orally administered by gavage, while Omniscan ® was intravenously administered .

Histidine was administered 30 min before Ominiscan ® administration at a dose of 4.38 mmol/kg; Omniscan ® (Gd(DTPA-BMA), at a dose of 1 mM/kg, was administered via the tail vein at an injection rate of 2 mL/min with an Harvard infusion pump. Histidine and Ominscan ® were administered at room temperature. Animals were sacrificed 24 h after Omniscan ® administration, and their organs were collected for ICP-MS gadolinium content determination.

Mortality and clinical signs

On the day of treatment animals were observed before treatment and at least once after dosing, to detect any clinical signs or reaction to treatment.

Severity, time of onset, duration of toxic signs and mortality were recorded, if any.

Gross pathology examination

On the day of scheduled sacrifice, animals were anesthetized with an inhalation agent, sevoflurane, at an espiratory concentration of 3% in induction and 1-2% in maintenance and bled after collection of blood from the abdominal aorta. After exsanguination, liver, spleen, femur and skin of all animals were excised, weighed and prepared for ICP-MS determination of gadolinium content. Sample digestion (spleen and femur) was performed by a microwave system (MARS-5 CEM Corporation).

Liver and skin samples were freeze-dried with a lyophilizer (Alpha-1-2LD Plus, CHRIST) after being frozen at -80°C for at least 1 h . After drying, dehydrated livers were weighed and then ground in a mortar. An amount of 0.3 g of the dried ground liver was accurately weighed and suspended in 1.0 ml_ of nitric acid (65% w/w). This solution was stored at 2-8 °C for at least 12 h. ICP-MS assay for gadolinium in biological samples (Bussi, 2007) was carried out on an ELAN 6100 Perkin El mer Spectrometer, operating with the following instrumental parameters:

Nebulizer flow: 0.95-0.98 L/min

Argon auxiliary gas flow: 0.2 L/min

Argon plasma gas flow: 15 L/min

Argon pressure (auxiliary, nebulizer, plasma) : 7.5 atm

RF power: 1250 W

Pump flow rate: 1.5 mL/min

Atomic Mass: 156.934 amu

Waiting start-up time: 45 min

Data analysis

Gadolinium content in organs was reported as pg of G d . Data were transformed into nmol/g tissue (Table B) and percent of injected dose (%ID, Table C) according to the following calculation :

nmol/g of fresh tissue = Gadolinium content ( U Q ) X 1000

157.25 x fresh organ weight (g)

%ID = 100 x Gadolinium content (μο)

Dose (μΓηοΙ/g) x animal weight(g) x 157.25

Statistical analysis was performed on nmol/g tissue and percent of injected dose (%ID) by applying the One-Way ANOVA test, followed by post-hoc Dunnett's test or the non-parametric Kruskal-Wallis test, followed by Mann- Whitney U test, when the data did not meet the assumptions of ANOVA. Results

No mortalities nor changes in clinical signs were observed both after histidine and Omniscan ® administration. No macroscopic changes were observed at necropsy.

In liver and spleen no significant changes in Gd 3+ content were noted in groups 2 and 3 (histidine pretreatment at 30 min and 1 h, respectively) in comparison to group 1 (no histidine pretreatment).

Gd 3+ content in skin showed a statistically significant decrease (p< 0.05) in group 2 (histidine pretreatment at 30 min), in comparison to group 1 (no histidine pretreatment). Values of 11.25 and 16.1 nmol/g tissue were assessed, respectively (Table B).

Gadolinium content in femur showed a statistically significant decrease (p< 0.01) in group 2 (histidine pretreatment at 30 min), in comparison to group 1 (no histidine pretreatment). The decrease was around 40% (6.90 nmol/g tissue versus 10.9 nmol/g tissue). This effect was also observed in %ID at the same significance level.

The %ID in liver, spleen and skin was not significantly different in group 2 (histidine pretreatment at 30 min) in comparison to group 1 (no histidine pretreatment) (Table C).

However the %ID in femour showed a statistically significant decrease (p< 0.01) in group 2 (histidine pretreatment at 30 min), in comparison to group 1 (no histidine pretreatment). A decrease of about 40% was observed (0.00166 %ID versus 0.00295 %ID).

Table B: Gadolinium content in spleen, femour, skin and liver (nmol/g of tissue, mean values and SD)

** p<0.01; * p<0.05

Table C: %ID in spleen, femour, skin and liver (mean values and SD).

** p<0.01 Histidine treatment 60 min before Omniscan ® administration didn't show, in this model, any effect on Gd 3+ accumulation. This result is consistent with the observed T max of L-Histidine (see Example 1 : T max at 100 and 1000 mg/kg = 0.5 h)

A statistically significant decrease in Gd 3+ content in femour of about 40% (both as nmol/g tissue and %ID) was observed in animals treated with histidine 30 min before Omniscan ® administration. A decrease in the total content of Gd was observed in the skin (Table B, p 0.05).

In conclusion, Gd 3+ levels in rat femur decreased (40% less) after a single oral pretreatment with histidine (at the dose of 4.38 mmol/kg) administered 30 min before the intravenous administration of Omniscan ® at the dose of 1 mmol/kg .

Example 4. Ki netics of the transmetal lation reactions of the Gd(DTPA), and Gd(DTPA-BMA) complexes with a divalent ion [Cu(II)] in the presence of citrate and histidine.

In order to have some clues on the kinetics of a possible transmetallation reactions in the presence of citrate (an important counter-ion present in pl asma a n d fou n d to be h ig h ly interactive with Gd 3+ ) and increasing amounts of histidine, the rates of the following reactions have been studied :

GdL + Cu 2+ + His + cit CuL + Gd(cit) + His (1 )

In the experiment, the concentration used for Gd 3+ complex (GdL), citrate and Cu 2+ were: 2.0, 2.0 and 0.1 mM, respectively, while the concentration of His was varied (0.2, 0.4, 0.6 and 0.8 mM).

The progress of the exchange reactions (1 ) has been studied by spectrophotometry (Cary IE spectrophotometer) at 300 nm, with the use of 1.0 cm quartz cell at 25°C in 0.15 M NaCI in the pH range 6 - 8. The experimental data (absorbance values) obtained for the reaction of Gd(DTPA-BMA) and Gd(DTPA) in the presence of Cu 2+ , histidine and citrate are shown in Figure 1.

The progress of the transmetallation reactions (1) results in the increase of the absorbance values because of the formation of CuL complexes. The data in Fig . 1 clearly show that the increase of histidine concentration has two effects: a) a decrease of the reaction rates (initial part of the curves) and b) more importantly, the extent of the conversions in the reactions (1) the saturation concentrations of CuL complexes significantly decreases. In other words, in the presence of histidine, both the rates and the extent of the transmetallation reactions occurring between the Gd-containing open-chain MRI contrast agents and the endogenous Cu 2+ significantly decrease, which in this model, decreases the amount of Gd 3+ , as Gd-cit specie in the reaction (1), corresponding to Gd available for exchange with body fluids. Example 5. Dissociation of the Gd(DTPA-BMA) complex in artificial plasma in the presence of histidine excess.

The following composition of artificial plasma according to a si mplified plasma model ( P. M . May, P. W . Linder, D. R. Wil l iams, J. Chem. Soc. Dalton Trans. , 1977, 588-595) was prepared :

0,025 M NaHCOs 2,5 mM CaCI 2 55 μΜ Asp

0,1 1 mM Na 3 Cit 16 μΜ ZnCI 2 48 μΜ Glu

1 ,0 mM Na 2 HPO 4 18 μΜ CuCI 2 178 μΜ Lys

0,15 M NaCI 85 μΜ L-His 0,7 mM HSA

0,01 M HEPES 23 μΜ Cys

1.0 mM Omniscan (Gd(DTPA-BMA)) was added to the solution and relaxivity values measured at 20 MHz with a Bruker Minispec MQ20 instrument at pH = 7.5 and 37°C.

As shown in Figure 2, relaxivity values increase as a function of time, indicating the dissociation of Gd(DTPA-BMA). As observed in Figure 2, the relaxivity values of Gd(DTPA-BMA) (Omniscan) in blood serum significantly increase with elapsing time, which was explained by the dissociation of the complex and interaction of „free" Gd 3+ with some components of the artificial plasma model. The addition of histidine to the sample (0.5 mM and 1.0 mM) resulted in a concentration dependent drop in the increase of the relaxivity values, probably due to the shift of the equilibria to the formation of higly stable Cu(His) 2 and Zn(His) 2 complexes and the absence of the transmetallation reactions between the Gd(DTPA-BMA) and Cu(His) 2 or Zn(His) 2 . Therefore the presence of histidine is beneficial for the stability of the Gd 3+ complex in this model.

It is possible to conclude that in body fluids transmetallation reactions may take place between the DTPA-derivative Gd 3+ complexes and endogenous metal ions, like Cu 2+ and Zn 2+ . The possibility of the transmetallation reactions can be pred icted on the basis of the stabil ity consta nts of complexes formed between the most important endogenous metals and ligands. The stability constants have been determined for the Gd 3+ , Cu 2+ and Zn 2+ complexes of the DTPA, DTPA-BMA, citrate and histidine ligands. The species distribution calculations based on the stability data indicate the importance of the citrate ion in transmetallation reactions, which may take place in the presence of citrate. The species distribution calculations have been experimentally verified by following the exchange reactions between Gd(DTPA-BMA) and Cu(His) 2 in the absence and presence of citrate. In Fig . 3A the spectrum of Cu(His) 2 shows no changes in the presence of Gd(DTPA- BMA) excess. However, when citrate is also present, the changes in the spectra in d icate the formation of Cu ( DTPA-BMA), indicating that the transmetallation reaction takes place (Fig . 3 B).

The addition of histidine (L-Histidine) generally reduces the extent of the conversion of the transmetal lation reactions. The results of the kinetic studies between the Gd 3+ complexes and Cu 2+ show that both the rates and the conversion of the transmetallation reactions decrease with increasing concentration of histidine. The stud ies on the rel axivity va l ues of the Gd(DTPA-BMA) in an artificial serum model also indicate that the extent of dissociation of the Gd 3+ complex significantly decreases in the presence of increasing concentration of histidine.