Artificial metallopeptides as MRI contrast agents

Molinaro, Giulia (2024). Artificial metallopeptides as MRI contrast agents. University of Birmingham. Ph.D.

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Abstract

Concerns of potential Gd(III) toxicity, in particular related to the use of Gd(III)-based MRI contrast agents in patients with compromised renal function, has driven the development of MRI contrast agents with high efficiencies (defined as MRI relaxivities). The use of alternative paramagnetic metals than Gd(III), such as Mn(II), or different MRI techniques (chemical exchange saturation transfer, CEST) are currently being investigated. In this context, Cu(II) has been generally disregarded for use in MRI contrast agents because of its intrinsic low paramagnetism. This is despite it having other favourable physical properties, being less toxic than Gd(III), less expensive and its natural resources more easily exploited.
The work in this thesis is the first report of a Cu(II) coiled-coil with relaxivities comparable or even higher than its Gd(III) analogue and clinical Gd(III) agents, at a low, clinically relevant magnetic field (1.4 T). This is due to the inclusion of the metal ion within a macromolecule scaffold, which reduces molecular tumbling, and to the presence of two water molecules bound to Cu(II), compared to zero in the Gd(III) analogue.
The translation of the binding site along the coiled-coil and second-sphere mutations are explored to determine the impact of water accessibility to the binding site on Cu(II) affinity and MRI relaxivities. Again, Cu(II) complexes show different behaviour compared to the Gd(III) analogues. First-sphere mutations (such as introduction of His residues) are shown to lead to higher Cu(II) affinity constants. Moreover, this strategy provides a powerful tool to modulate relaxivities of the mutated complexes.
Mn(II) binding to the same class of coiled-coils is also explored, in terms of thermodynamic (Mn(II) affinity constants) and kinetic (Zn(II) transmetallation) stability. Such compounds show very high relaxivities, which can be attributed mainly to the presence of a large number of water molecules bound to Mn(II).
Coiled-coils with multiple binding sites are designed, aiming to introduce more than one metal (Gd or Cu) ion within the same scaffold. Here, a coiled-coil able to bind to three Ln(III) ions is reported. Similar conclusions as for the mono-binding metallopeptides are drawn: when a Gd(III) binding site is located at an extremity of the coil, the associated relaxivity is higher than for a complex with a buried binding site. In contrast, Cu(II) analogues present higher relaxivities in the presence of buried and hydrated binding sites. However, it is shown that two binding sites should be separated by at least on heptad to enable water entering in the hydrophobic core of the coiled-coil.
Finally, this work reports for the first time a Nd(III)-based coiled-coil able to generate CEST effect. This opens up further opportunities for the use of coiled-coils binding to a number of lanthanides, others than Gd(III), as paraCEST contrast agents.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):