## 1 Introduction  
Currently, surgical resection is a popular treatment method for tumor-induced bone defects [1; 2]. Nevertheless, the surgical resection cannot eliminate bone tumor cells completely. The residual bone tumor cells may lead to the relapse of the bone tumor [3; 4]. Therefore, it is important to develop antitumor implants to repair bone defects and prevent bone tumor relapse [5; 6]. [PERSON] et al. have reported that a bifunctional graphene oxide-modified \(\beta\)-tricalcium phosphate implant showed excellent photothermal effects in killing bone-tumor cells, which could be used for therapy of tumor-related bone defects [2]. [PERSON] et al. also reported that zoledronic acid-loaded magnesium-strontium alloys could inhibit giant bone cell tumors, which could be a potential implant in repairing the bone defects after tumor removal [7]. Compared to normal cells, hydrogen peroxide levels are greatly increased in many types of cancer cells [8]. It helps cancer cells to infiltrate and metastasize to other tissues, and hydrogen peroxide is also an important signal molecule regulating the entire process of tumor cell survival, proliferation, and apoptosis [9; 10]. As tumor cells have elevated reactive oxygen species levels and are under increased intrinsic oxidative stress, this provides an opportunity to kill the tumor cells based on their vulnerability to reactive oxygen species insults [11; 12; 13].  
Lanthanum (La), a rare earth element, has received increasing attention due to its antitumor property [14; 15]. The radius and valence of the La ion are 1.061 A and trivalent, respectively [16; 17; 18]. The La ion possesses a large ratio of the electric charge to ion radius, enabling a high ability to bind to the divalent metal binding sites on mitochondria. This binding leads to irreversible alteration of its structure and loss of biological activity, which results in disruption of tumor cell function and death [19; 20]. [PERSON] et al. reported that lanthanum compounds could selectively inhibit or kill tumor cells even in drug-resistant tumor cells [21]. [PERSON] et al. reported that lanthanum chloride could inhibit proliferation and induce the cervical tumor cells death [22]. Considering the antitumor property of La, it is reasonable to speculate that La could be used as an antitumor agent to grant implants the ability to kill bone tumor cells.  
As promising bone implant materials, magnesium (Mg) and its alloys possess degradability, good biocompatibility, and desirable mechanical properties [23; 24; 25; 26]. They can gradually degrade in the human body, thereby avoiding the second surgery for implant removal and decreasing the pain for patients [27; 28]. Mg as a necessary element in the human body can promote new bone formation, which is beneficial to bone defects repairing [29; 30]. Furthermore, Mg alloys possess similar elastic modulus and density to those of natural human bones, thus mitigating the stress shielding effect induced by serious mismatch in modulus between implants and natural bones [31]. To our knowledge, there are very few reports about the antitumor property of Mg alloy containing La [32; 33].  
In this study, La was alloyed to Mg-6 Zn-0.5 Zr (wt %, ZK60) through a selective laser melting technique to endow it with antitumor properties. Meanwhile, alloying is an effective approach to enhance the Mg degradation resistance by tailoring the phase morphology, distribution, potential, and size of the matrix [34; 35]. The antitumor property of ZK60-La was evaluated and the potential mechanisms were discussed. Furthermore, the degradation properties were investigated.