Synthesis and biological evaluation of metallic compounds of Technetium-99m and Rhenium-188 for theranostic applications in Nuclear Medicine
Ref Dr. Cristina Bolzati

Nowadays 99mTc-based coordination complexes represent the great majority (> 80%) of the RF used in Single Photon Emission Tomography (SPECT). The success of this radioisotope is mainly due to the following properties: i) ideal chemical and physical characteristics (t1/2 = 6.02 h; Eγ = 140 keV); ii) easy availability and relatively low cost (production by a transportable 99Mo/99mTc generator) iii) chemical versatility.
The interest in the rhenium, congener of technetium in group 7, has grown since its isotope ¹⁸⁸Re [t1/2 = 17 h, Eβ- MeV = 2:12; E_g = 155 KeV (15%)], analogously to ^99mTc, has been produced in high specific activity, from a ¹⁸⁸W/¹⁸⁸Re generator system, which ensures daily doses for clinical studies. Thanks to its nuclear characteristics, 188Re is an excellent candidate for the development of therapeutic RP.
The remarkable chemical similarities between Tc and Re, which result, for stable complexes having the same molecular geometry, in a similar biological behavior (eg. same pharmacokinetics), combined with the favorable physical properties, and the equivalence of the production systems (generators ⁹⁹Mo/^99mTc and ¹⁸⁸W/¹⁸⁸Re), make these two radioisotopes, an ideal pair for the development of similar compounds ('matching pair') suitable for the diagnosis and therapy of tumors (theranostic radiopharmaceuticals).
In this context, the research is aimed at the design and synthesis of new inorganic compounds apt for the development of new technologies for the synthesis of radio-complexes useful in radiopharmaceutical and in nuclear medicine applications as potential theranostic agents.
In general, the applied strategies involve the synthesis of asymmetrical mononuclear complexes, characterized by the presence of different polydentate ligands, where only one of them is conjugated to a molecular vector selected within a class of proteins, peptides or pharmacophores.
The technologies developed are based on the use of bi- or tridentate phosphine ligands PNP (aminodiphosphine; Fig. 1) or PS (phosphinothiolate; Fig. 2) that upon coordination to the metal (M = Tc, Re) allow the formation of the reactive molecular fragments [M^V(N)PNP]²⁺ and [M^III(PS)2]⁺ respectively. The coordination sphere of the metal is saturated by bidentate ligand (XY) containing soft π-donors/s-donors coordinating atoms, as the pairs NH2˄S^-, O^-˄ S^-, S˄S^- o N˄S^-, to form stable asymmetrical complexes, in high yield. The chelator XY can be designed to carry a pharmacophore, a peptide or a protein to form a construct for molecular targeting applications.
Known advantages of these technologies are: i) high labeling efficiency obtained through a preparation easily transferable to a lyophilized cold-kit suitable for human administration; i) high in vivo stability of the resulting complexes; iii) easy conjugation of the XY chelate to molecular vectors.
The [M^V(N)PNP]²⁺ technology has also been used for the synthesis of a class of monocationic complexes of the type [^99mTc(N)(DTC)(PNP)]⁺ (DTC = dithiocarbamate). In vivo studies showed original imaging properties characterized by high and persistent heart uptake and a favourable heart-to-liver and heart-to-lung ratio. These results indicate that these complexes possess favorablebiologic properties and could be used in the development of new heart perfusion tracers.
In addition, studies addressed to elucidate the mechanisms of distribution, retention and elimination of some of these complexes, clearly demonstrated that the remarkable rapid efflux of these compounds from non-target tissues is strongly correlated to the action of P-gp transporters, suggesting that the [Tc(N)(PNP)]ⁿ⁺ building block can be viewed as a substrate of MDR P-gp and sister proteins (MRPn, BCRP). These findings open the possibility to extend the applicability of this new class of complexes in imaging and monitoring neoplastic forms as well as in assessing the activity, the expression and the function of P-gp and sister proteins in cancer and in neurodegenerative diseases such as AD, PD and epilepsy.

Copper (I) complexes as potential anticancer agents
Ref Dr. Francesco Tisato
Ref Dr. Marina Porchia

On these bases, we have synthesized and tested severalfamilies of Cu(I) derivatives and by varying the chemico-physical properties of the ligands, we correlated the biological activity of the resulting Cu(I) complexes with their charge, lipophilicity and dimension. In particular we focused on homoleptic Cu(I) compounds comprising water-solubletertiary phosphino ligands. Such "CuP₄"-type complexes are water soluble, stable to dismutation and, in cytotoxicity tests against a panel of human tumor cell lines, showed great antitumor efficacy with an average IC50 generally lower than that of cisplatin.
Among "CuP₄"-type complexes (Fig.3) [Cu(thp)₄][PF₆] (thp = tris hydroxymethylphosphine) (Fig.4)) distinguished itself for its remarkable in vitro antitumor activity against a wide range of solid tumors including platinum drug refractory/resistant tumors. Its efficacy was confirmed also by in vivo chemotherapy studies evaluated in a model of solid tumour, the syngeneic murine Lewis Lung Carcinoma (LLC) implanted i.m. in C57BL mice.
The overcoming of cross-resistance phenomena strongly supports the hypothesis of a different pathway of action of our copper(I) complexes from that of cisplatin. In fact studies on the mechanism of actions of [Cu(thp)4]+ have evidenced in most cases inhibition of 26S proteasome activity associated with endoplasmatic reticulum stress and activation of paraptosis, a mechanism of programmed celldeath alternative to apoptosis. Mass spectrometric studies on its interaction with model peptides containing methionine–rich sequence confirm the hypothesis that it utilizes the human copper transporter (hCTR1) for cellularinternalization mimicking the uptake of physiological Cu(I)ions (Fig.5).