Doxorubicin and ferrous ions could form acid-dissociable coordination complexes, which were incorporated into functionally tailored amorphous calcium carbonate nanoparticles and may confer severe ferroptotic damage to target tumor cells. 

The fast-growing cancer incidence and mortality worldwide have raised great challenges for the currently available anticancer options, which warrants the development of new therapeutic modalities based on novel antitumor mechanisms. Ferroptosis, a recently discovered form of non-apoptotic cell death, is one such candidate and has already demonstrated immense potential in clinical oncology as it provides alternative therapeutic opportunities for the management of treatment-resistant tumors. 

In a study published in Science Advances, Prof. YU Shuhong's group from the University of Science and Technology of China of the Chinese Academy of Sciences and Prof. LUO Zhong's group from Chongqing University reported a novel strategy to induce ferroptosis in target cancer cells and further amplify the ferroptotic damage for efficient tumor therapy. The nanoformulation was designed to be exclusively activated in the tumor microenvironment, which has demonstrated potent inhibition effect against multiple types of tumors while sparing healthy cells and tissues. 

“Overloading tumor cells with ferrous ions could readily initiate the ferroptotic death cascade, and the complexed doxorubicin may further amplify the ferroptotic damage by providing additional reactive oxygen species to sustain the lipid peroxidation,” said Prof. YU. 

“The benefit of coordinating Fe²⁺ ions with doxorubicin is manifold. It could not only enhance the stability of Fe²⁺ ions in biological environment, but also facilitate the subsequent lipid peroxidation process to promote ferroptosis. Moreover, doxorubicin is an FDA-approved anticancer drug capable of inhibiting the topoisomerase 2 in tumor cells to prevent DNA replication, leading to complementary ferroptosis/apoptosis effect against a broad spectrum of tumor indications,” said Prof. LUO. 

The underlying molecular mechanism for the synergy between Fe²⁺ and doxorubicin is that doxorubicin could generate high level of intracellular ROS by activating the intracellular NADPH oxidase 4 (NOX4) in tumor cells, which may supply H₂O₂ to sustain the Fe²⁺-catalyzed lipid peroxidation.  

Prof. YU Shuhong also commented on one of the major challenges for the tumor targeted delivery of the Fe²⁺-doxorubicin complex. “The coordination complex is rapidly dissociated into free Fe²⁺ and doxorubicin under acidic conditions. However, both species must interact with intracellular components to take effect, necessitating further refinement of the drug delivery process.” 

Inspired by the recent advances in self-assembly technology, the researchers developed an intricate self-assembly-based nanoplatform for the tumor-targeted cytosolic delivery of the Fe²⁺-doxorubicin complex.  

The Fe²⁺-doxorubicin complex were efficiently encapsulated into amorphous calcium carbonate nanospheres in a simple one-step co-condensation process, and the surface of the drug-loaded amorphous calcium nanoparticles was modified with polyamidoamine (PAMAM) dendrimer-based tumor-microenvironment-activatable multifunctional ligands, which would remain bioinert during circulation but switch to a tumor-affinitive state upon entering the matrix metalloproteinase-2 (MMP-2)-rich tumor microenvironment. 

Thanks to the acid sensitivity of the amorphous calcium carbonate contents, the nanoparticle could be readily degraded in the acidic tumor lysosomes to release the Fe²⁺-doxorubicin complex, which could be further reverted into free doxorubicin and Fe²⁺ through the protonation-induced dissociation. Meanwhile, the PAMAM dendrimers could disrupt the lysosomal membrane via “proton sponge” effect and release doxorubicin and Fe²⁺ to the cytosol, where the H₂O₂ produced during doxorubicin metabolism could stimulate the ferroptotic toxicity of Fe²⁺ ions to tumor cells. 

"We two groups have been collaborating closely on the synthesis and functionalization of biocompatible inorganic nanomaterials. Amorphous calcium carbonate nanoparticle is a very promising inorganic nanomaterial for biomedical applications. It’s easy to synthesize and has tunable drug loading capability. It could also be rapidly degraded in human body and the degradation products are all non-toxic," said Prof. LUO. 

"The nanoplatform is also a good example of repurposing old drugs for new applications. This would greatly benefit the clinical translation of the reported nanoformulation,” said Dr. LI Menghuan, a senior scientist at Prof. LUO Zhong’s group. In the future, the researchers hope to further simplify the synthesis procedures of the amorphous calcium carbonate-based nanoplatform and thoroughly investigate their efficacy and safety in a clinically relevant context.