platinum-based chemotherapeutics exhibit excellent antitumor properties. However, these drugs cause severe side effects including toxicity, drug resistance, and lack of tumor selectivity. Tumor-targeted drug delivery has demonstrated great potential to overcome these drawbacks. Herein, we aimed to design radioactive bisphosphonate-functionalized platinum (195m Pt-BP) complexes to confirm preferential accumulation of these pt-based drugs in metabolically active bone. In vitro nMR studies revealed that release of pt from pt Bp complexes increased with decreasing pH. Upon systemic administration to mice, Pt-BP exhibited a 4.5-fold higher affinity to bone compared to platinum complexes lacking the bone-seeking bisphosphonate moiety. these pt-Bp complexes formed less pt-DnA adducts compared to bisphosphonate-free platinum complexes, indicating that in vivo release of pt from pt-Bp complexes proceeded relatively slow. Subsequently, radioactive 195m pt-Bp complexes were synthesized using 195m pt(no 3) 2 (en) as precursor and injected intravenously into mice. Specific accumulation of 195m pt-Bp was observed at skeletal sites with high metabolic activity using micro-Spect/ct imaging. Furthermore, laser ablation-ICP-MS imaging of proximal tibia sections confirmed that 195m pt Bp colocalized with calcium in the trabeculae of mice tibia. Most types of tumors, i.e. breast, prostate, lung, kidney, and thyroid, metastasize to bone since its physiological environment facilitates the formation and growth of cancer cells 1,2. These metastases affect healthy bone, which then become the primary cause of mortality 3. Distant metastases are the leading cause of death for both breast and prostate cancer patients, with 65-75% and 90% of these patients developing bone metastases in advanced stages, respectively 4,5. Bone metastases are often associated with accelerated bone resorption leading to complications such as skeletal-related events (SREs), bone pain or hypercalcemia 6,7. Unfortunately, current treatments for bone metastases are limited. Bisphosphonates (BP) and denosumab are most commonly used for palliative treatment to prevent or limit SREs 8. Although such treatments inhibit osteoclast activity and prevent the progression of metastases, they do not kill cancer cells effectively and do not improve the quality of life of patients substantially 9. Consequently, the development of effective therapies to treat bone metastases remains a major clinical challenge.

Linker instability and impaired tumor targeting can affect the tolerability and efficacy of antibody-drug conjugates (ADCs). To improve these ADC characteristics, we recently described the use of a metal-organic linker, [ethylenediamineplatinum(II)], herein called Initial therapy studies in xenograft-bearing mice revealed that trastuzumab--auristatin F (AF) outperformed its maleimide benchmark trastuzumab-mal-AF and the Food and Drug Administration-approved ado-trastuzumab emtansine, both containing conventional linkers. In this study, we aimed to characterize -based ADCs for in vivo stability and tumor targeting usingPt and Zr. The γ-emitter Pt was used to produce the radiolabeled [Pt]Zr-Desferrioxamine (Zr-DFO) was conjugated to trastuzumab either via [Pt] (to histidine residues) or conventionally (to lysine residues) in order to monitor the biodistribution of antibody, payload, and linker separately. Linker stability was determined by evaluating the following ADCs for biodistribution in NCI-N87 xenograft-bearing nude mice 72 h after injection: trastuzumab-[Pt]-DFO-Zr, trastuzumab-[Pt]-AF, and Zr-DFO-(Lys)trastuzumab (control), all having drug-to-antibody ratios (DARs) of 2.2-2.5. To assess the influence of DAR on biodistribution,Zr-DFO-(Lys)trastuzumab--AF with an AF-to-antibody ratio of 0, 2.6, or 5.2 was evaluated 96 h after injection. Similar biodistributions were observed for trastuzumab-[Pt]-DFO-Zr, trastuzumab-[Pt]-AF, and Zr-DFO-(Lys)trastuzumab irrespective of the isotope used for biodistribution assessment. The fact that follows the antibody biodistribution indicates that the payload- bond is stable in vivo. Uptake of the 3 conjugates, as percentage injected dose (%ID) per gram of tissue, was about 30 %ID/g in tumor tissue but less than 10 %ID/g in most healthy tissues. Trastuzumab-[Pt]-AF (DAR 2.2) showed a tendency toward faster blood clearance and an elevated liver uptake, which increased significantly to 28.1 ± 4.2 %ID/g at a higher DAR of 5.2, as revealed from the biodistribution and PET imaging studies. As shown byPt/Zr labeling, ADCs containing the linker are stable in vivo. In the case of trastuzumab--AF (DARs 2.2 and 2.6), an unimpaired biodistribution was demonstrated.

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