One-step, kit-based radiopharmaceuticals for molecular SPECT imaging: a versatile diphosphine chelator for ^99mTc...

Radiotracers labelled with technetium-99m (^99mTc) enable accessible diagnostic imaging of disease, provided that radiotracer preparation is simple. Whilst ^99mTc radiopharmaceuticals for imaging perfusion are routinely prepared from kits, and regularly used in healthcare, there are no ^99mTc-labelled receptor-targeted radiopharmaceuticals in widespread clinical use. This is in part due to the multistep radiosyntheses required for the latter. We demonstrate that the diphosphine, 2,3-bis(diphenylphosphino)maleic anhydride (BMA), is an excellent platform for preparation of kit-based, receptor-targeted ^99mTc-labelled radiotracers: its conjugates are simple to prepare and can be easily labelled with ^99mTc using one-step, kit-based protocols. Here, reaction of BMA with the α_vβ₃-integrin receptor targeted cyclic peptide, Arg-Gly-Asp-DPhe-Lys (RGD), provided the first diphosphine-peptide conjugate, DP-RGD. DP-RGD was incorporated into a "kit", and addition of a saline solution containing ^99mTcO₄^- to this kit, followed by heating, furnished the radiotracer [^99mTcO₂(DP-RGD)₂]⁺ in consistently high radiochemical yields (>90%). The analogous [ReO₂(DP-RGD)₂]⁺ compound was prepared and characterised, revealing that both [^99mTcO₂(DP-RGD)₂]⁺ and [ReO₂(DP-RGD)₂]⁺ consist of a mixture of cis and trans geometric isomers. Finally, [^99mTcO₂(DP-RGD)₂]⁺ exhibited high metabolic stability, and selectively targeted α_vβ₃-integrin receptors, enabling in vivo SPECT imaging of α_vβ₃-integrin receptor expression in mice.