Ferritin, a naturally expressed iron binding protein in human cells, plays crucial roles in the maintenance of iron balance and the resistance of cellular oxidation. With a 12 nm outer diameter shell and 8 nm diameter hollow cavity, composed of 24 self-assembly subunits, ferritin has emerged as a promising drug delivery vehicle due to its unique architecture, intrinsic tumor targeting property, and excellent biocompatibility. However, the mechanism of drug loading in ferritin nanocage is obscure.

A research group led by Prof. YAN Xiyun from the Institute of Biophysics of the Chinese Academy of Sciences (CAS) recently published the latest findings revealing the mechanism of drug loading for ferritin drug carrier (FDC) in Nano Today. The researchers reported the structure basis of drug entry channels on the surface of FDC and provided theoretical foundation to the clinical translation of ferritin drugs.

In their previous work, the researchers, also affiliated to the CAS Nano Engineering Laboratory, found that recombinant human ferritin heavy chains can identify transferrin receptors (TfR1), which is a well-known tumor biomarker, and developed a mild method to effectively loading drugs into ferritin nanocage. Moreover, they came up with the concept of FDC.

Similar to antibody-drug conjugates (ADC), FDC can specifically deliver drugs to tumor tissues via binding to receptors. In addition, FDC is superior to ADC in several aspects such as drug-loading capacity, thermal stability, and easier access to production. Nevertheless, the translation studies of FDC are impeded by the low efficiency and low yield of the drug loading process

Through crystal structure and protein mutation analysis, the researchers in this study identified a unique thermal-sensitive drug entry channel in HFn protein nanocage that could be utilized in efficiently loading small molecule chemotherapeutic drugs. The residues at the 43, 92, 91, 90, and 89 from one subunit, and residues at the 81, 79 from the neighboring subunit constitute the drug entry channel, of which the 'open' state is controlled by temperature.

Through drug loading by this process, the FDC possessed better stability, increased biosafety, and enhanced antitumor activity than those of HFn-Dox prepared by denaturation-based methods.

Intriguingly, the drug enter channel appears quite unique to HFn, and could be used for other small molecule drugs. It is conceivable that additional chemotherapeutic drugs, or combination of them might be loaded into the HFn cages for effective anti-cancer treatment, which will greatly facilitate the translational study of FDC in cancer-targeting therapies.