Background <p>Radiopharmaceuticals targeting poly(ADP-ribose) polymerase (PARP), a nuclear protein that plays a role in DNA damage repair, are promising candidates for targeted radionuclide therapy to treat PARP-expressing cancers. Here, we evaluated the potential of a PARP inhibitor with an olaparib-like structure, labelled with the radiometals ¹⁶¹Tb or ⁶⁸Ga through a DOTA chelator, for targeted radionuclide therapy and PET imaging using in vitro models of cancer.</p> Methods <p>[¹⁶¹Tb]TbCl₃ and [⁶⁸Ga]GaCl₃ were used for radiolabelling a DOTA-conjugated derivative of the PARP inhibitor olaparib, yielding [¹⁶¹Tb]Tb-DOTA-olaparib and [⁶⁸Ga]Ga-DOTA-olaparib, respectively. Both compounds were evaluated in vitro for cell uptake, internalisation, selectivity, and effect on clonogenic survival across a panel of PARP-expressing human and murine cancer cell lines.</p> Results <p>[¹⁶¹Tb]Tb-DOTA-olaparib and [⁶⁸Ga]Ga-DOTA-olaparib were successfully produced with &gt; 95% labelling efficiency and apparent molar activities of up to 40 and 20&#xa0;MBq/nmol, respectively. Both radiolabelled compounds exhibited minimal uptake across all cancer cell lines tested. The compounds remained mainly associated with the cell membrane, with no significant internalisation detected. Neither cell association nor internalisation could be blocked using an excess of olaparib as a PARP-specific competitor ligand. The highly hydrophilic nature of both compounds, as indicated by a log D_7.4 value of − 3.1 ± 0.07 for [¹⁶¹Tb]Tb-DOTA-olaparib and − 2.8 ± 0.02 for [⁶⁸Ga]Ga-DOTA-olaparib, likely contributes to their limited ability to traverse cell membranes, thereby restricting intracellular access, necessary for targeting intranuclear PARP. Clonogenic survival of cells exposed to [¹⁶¹Tb]Tb-DOTA-olaparib was similar to that observed for cells exposed to [¹⁶¹Tb]Tb-DOTA.</p> Conclusion <p>[¹⁶¹Tb]Tb-DOTA-olaparib and [⁶⁸Ga]Ga-DOTA-olaparib exhibit negligible cellular uptake and consequently show insufficient specific binding to intranuclear PARP in cancer cells. Their limited ability to penetrate the cell membrane and interact with intracellular targets makes them unsuitable for effective PARP-targeted therapy or imaging applications.</p> Graphical abstract <p></p>