Structural variations across the lanthanide series of macrocyclic DOTA complexes: insights into the design of contrast agents for magnetic resonance imaging.

It was early shown that the macrocyclic Ln(DOTA) complexes (DOTA = 1,4,7,10-tetra-azacyclododecane-N,N',N' ',N' "-tetraacetic acid) exists in solution as a mixture of two enantiomeric pairs of diastereoisomers differing in the ligand conformation, namely, square antiprismatic (SA) and twisted square antiprismatic (TSA) geometries, respectively. Later, extensive (1)H NMR investigations suggested that a coordination change may be superimposed on this conformational equilibrium involving two additional structures in which the metal ion possesses a coordination number of eight (CN 8). It was predicted that these two species, lacking the apical coordinated water molecule, would maintain the SA and TSA coordination geometries, and therefore, they have been labeled as SA' and TSA', respectively. In this work we report the X-ray solid-state crystal structure determination of six Ln(DOTA) complexes representative of all four coordination geometry typologies deduced from NMR solution studies. A distinctive structural feature that discriminates SA (and SA') and TSA (and TSA') structures is represented by the twist angle between the two square planes of the antiprism, the basal four nitrogen, and the apical four oxygen planes. [Ce(DOTA)(H(2)O)](-) displays a TSA structural typology with a twist angle of 25 degrees and a Ce-O(water) distance of 2.59 A. The SA-type structure has been found in the case of complexes with Pr(III), Nd(III), and Dy(III), where the twist angle is 39, 39, and 38 degrees, respectively, and the metal-water oxygen distance varies significantly (Pr-O(w) 2.529 A; Nd-O(w) 2.508 A; and Dy-O(w) 2.474 A). [Tm(DOTA)](-) displays a TSA'-type structure with a twist angle of 24 degrees. As compared with the TSA structure of the corresponding Ce(III) complex, the Tm(III) complex shows an overall marked shrinkage of all metal-nitrogen and metal-oxygen distances (ca. 0.2 A), which reflects the contraction of the metal ionic radius across the series but also the effect associated with the decrease of the CN from 9 to 8. In [Sc(DOTA)](-), the even smaller ionic radius of Sc(III) shifts the geometry of the coordination cage to the more compact SA' typology with a twist angle of 41 degrees, a value very similar to that found in the SA structures of lanthanide(III) ions with CN 9. Finally, an investigation was made into the hydration spheres of the complexes with SA and TSA geometries to account for the experimental evidence of a markedly different rate of water exchange for the two isomeric structures. This is of fundamental importance to the understanding of the corresponding Gd(III) complexes as MRI contrast agents.

On Darwin's 'metaphysical notebooks'. II: "Metaphysics" and final cause.

The first part of this paper was published in Rivista di Biologia/Biology Forum 94 (2001). In the second part below an examination is made of the meaning of the term Metaphysics in some passages of the Darwinian Notebooks for the years 1836-1844. Metaphysics no longer defines a field of philosophical enquiries mainly concerning the being and the essence after the manner of Aristotle; it now refers to a kind of philosophy of mind after the manner of J. Locke's criticism of the Hypokeimenon. However Aristotle's Metaphysics also encompasses a treatment of the idea of causes, and of final cause particularly, in the explanation of events, and in the explanation of natural phenomena especially. The criticism of the idea of final cause in the interpretation of the world of life is one of Darwin's foundational acts in his early years. When conceiving his Système du monde, in the last years of the XVIII Century, Laplace could think that God is a hypothesis not really needed by science, as we are told. For the knowledge of organic nature to attain the status of science, it remained to be shown that since--certain of the exemplariness of Newton's Principles as much as cautious before the mystery of life--did not need the hypothesis of final ends in order to understand and explain the productions of the living nature: not only in the form of that final cause (the First Cause, the Vera Causa) in which Natural Theology still rested, but also in the form of nature's inner finality which still moulded Whewell's Kantian philosophy. Such demonstration is a very important subject in Darwin's early enquiries, where he criticises finalism as a projection of self-conceiving Man, likely inherited from a knowing of causality in nuce to be found also in animals.

On Darwin's 'metaphysical notebooks'. I: teleology and the project of a theory.

Huxley's essay On the Reception of the 'Origin of Species' brings us close to the issue of cause and of why- and how-questions in the understanding of the living world. The present contribution, which is divided into two parts, reviews the problem of Teleology as conceived by Huxley and re-examines Darwin as the author who revealed the existence of a 'foundations problem' in the explanation of an entire realm of nature, i.e., the problem of explaining such realm in terms of its own, specific legality, or iuxta sua propria principia. In the first part the enquiry is mainly focused on the secularization of natural history after Paley; in the second part it is mainly focused on the desubjectivization of the inquiry into natural history after Erasmus Darwin and Lamarck. The second part will be published in the next issue of Rivista di Biologia/Biology Forum. In the first part below an analysis is made of Notebooks M and N. The author disputes the correctness of conceiving them only as the works where Darwin envisages the 'metaphysical' themes later to become the subject of The Expression of the Emotions. He suggests to conceive of them also as the works where Darwin defines the terms of the general project of his own, peculiar evolutionary theory. The author then outlines the intellectual progress of Darwin from the inosculation to the transmutation hypotheses. Darwin's reading of Malthus appears to be analytically decisive, because it offers him the vintage point to attack the metaphysical and theological citadels on the morphological side. Darwin is thus able to re-consider Erasmus' comprehensive zoonomic project, by displacing it, however, from the old idea of the scala naturae to the new one of the "coral of life", and by emphasising the distinction between "the fittest" and "the best" vs. the tradition of Natural Theology.

Gadobenate dimeglumine 0.5 M solution for injection (MultiHance) pharmaceutical formulation and physicochemical properties of a new magnetic resonance imaging contrast medium.

OBJECTIVE: To characterize the proton-magnetic relaxation properties and complexation equilibria of gadobenate dimeglumine and to develop a pharmaceutical formulation for injection. METHODS: Proton-magnetic relaxivities were determined at 20 MHz and 39 degrees C. Metal complexation was studied potentiometrically. Degradation pathways were identified through prolonged exposure to elevated temperature. RESULTS: Because of its size and very weak interaction with serum albumin, gadobenate dimeglumine has proton-magnetic relaxivities that are larger than those of gadopentetate dimeglumine in both water and biological fluids. With regard to metal complexation, the two products are indistinguishable. The metal complexation behavior and thermal stability of the product allowed a pharmaceutical formulation for injection containing 0.5 M gadobenate dimeglumine without excipients. The physicochemical properties of the formulated product were determined. CONCLUSION: Gadobenate dimeglumine has an elevated T1-relaxivity, especially in blood plasma. The high stability of the complex guarantees a negligible release of gadolinium ion. Gadobenate dimeglumine 0.5 M solution for injection has a shelf life of three years.

Hepatocyte-directed MR contrast agents. Can we take advantage of bile acids?

A series of gadolinium complexes conjugated to bile acids was prepared and investigated as possible hepatospecific MR imaging contrast agents. In the design of such compounds, features such as the nature of the bile acid, the site of conjugation on the bile acid skeleton, and the global charge of the conjugate were taken into account. Relaxivity measurements carried out in human serum indicate interaction of the conjugates with human serum proteins; even small structural variations significantly affect relaxivity in human serum. Pharmacokinetic data (biliary elimination in the range of 18.4-45.6%) show that bile acids can be used as address moieties to transport gadolinium complexes through hepatocytes. For a homogeneous series of compounds, differing only in the bile acid residue conjugated, it was unexpectedly found that cholic acid is twice as efficient an address moiety as cholylglycine or cholyltaurine. Preliminary results show that none of the conjugates is transported through the basolateral membrane of hepatocytes by the Na+/taurocholate carrier.

A new ytterbium chelate as contrast agent in chemical shift imaging and temperature sensitive probe for MR spectroscopy.

A paramagnetic Yb(III) complex that is the prototype of a novel class of probes for MRI and MRS has been developed. The complex displays highly shifted 1H resonances that are characterized by short relaxation times and, as such, may prove to be a valuable alternative in applications that currently require fluorine-containing probes. Selective excitation of the paramagnetically shifted resonances allows the spatial distribution of the complex to be mapped. This communication reports the images that were obtained by selectively exciting the most intense methyl group (-14.2 ppm at 27 degrees C) for complex concentrations ranging from 0.003-0.1 M. Spectroscopically, the complex may be used as a temperature probe since the proton chemical shifts exhibit a strong temperature dependence. In human serum the chemical shift difference of a selected pair of proton resonances was observed to follow a gradient of -0.42 +/- 0.01 ppm/degrees C. Furthermore, since the chemical shift of the methyl resonance displays a temperature coefficient of -0.04 +/- 0.01 ppm/degrees C, it should be possible to use the image phase for thermal mapping.