Led by the University of Sydney, an international team has developed a new kind of bone implant that reduces the chance of infection, and therefore significantly decreases implant failure rates. In Australia, a fifth of conventional hip and knee replacements fail after 10 years.
New bone implants that simultaneously reduce inflammation and prevent biofilm (microorganism) build up, which can ordinarily lead to infections and implant failure, have been developed by an international team led by the University of Sydney.
Conventional orthopedic implants, which replace missing bone or support damaged bone, are becoming more common in Australia due to our aging population with osteoporosis, who have increasing access to surgery. In 2020 for example, over 1.7 million joint replacement procedures were performed.
Yet implants have a significant failure rate – the average failure rate for hip and knee replacements at 10 years is around 20 percent. This failure can result in a patient experiencing unnecessary pain, as well having to undergo revision surgery at a significant cost to them, their families, and the healthcare system.
They are enriched with gallium (a safe and highly antimicrobial chemical element), and defensin – a naturally occurring antimicrobial biomolecule in our body. The researchers added the gallium and defensin to poly-lactic acid (PLA), a commonly used implant material. PLA is a biodegradable plastic, derived from renewable resources such as corn starch, tapioca, or sugar cane.
The gallium was added using a megavolt particle accelerator at the Australian Nuclear Science and Technology Organisation (ANSTO)’s Centre for Accelerator Science – gallium ions were shot into the surface layers of the implant.
“It is fantastic to see accelerator physicists and biomaterials scientists working together to design materials of the future for this inspiring application,” said Dr. Ceri Brenner, Leader, ANSTO Centre for Accelerator Science.
An overview of the experiment has been published in ACS Applied Materials & Interfaces.
‘Undercover agent’ combats infection
“Gallium is an undercover agent that can be used to combat infection,” said lead author, Professor Wojciech Chrzanowski, from the University of Sydney Nano Institute and head of nanomedicine research at the Faculty of Medicine and Health.
“Gallium looks like iron and can exploit bacteria’s need for iron to trick them into taking it up. Once inside bacterial cells, gallium destroys them.
“So, in the case of bone implants, which are highly susceptible to infection as foreign substances are introduced into the body, gallium is a superhero substance. Defensin, a kind of protein, is also active against bacteria (as well as fungi and some viruses) and is a natural part of our immune systems,” he said.
Gallium and defensin together reduced the rate of surface bacteria on bone by almost 90 percent, compared to a control.
Additionally, both defensin and gallium reduced long-term inflammation by 45 percent, by decreasing immune cells’ reaction to the implant. The bone surfaces showed an increase in short-term inflammation, however, which is thought to initiate healing.
University of Sydney Ph.D. student and research co-lead Kamini Divakarla said: “Since we used biodegradable PLA, the coating will degrade when it has served its infection-prevention purpose.”
The researchers’ next step will be to test the new implant using antibiotic resistant bacteria strains and ensure that the implants promote bone formation and healing.
Metals have been used to fight a broad range of diseases from ancient civilizations to modern societies. Metallic ions addition to bioactive materials has been a subject of interest for the last few decades [, , , ].
The possibility of incorporating metallic ions dopants into bioactive materials has led to biomaterials with improved biological features to be tailored to specific clinical applications [1,, , , ]. For example, metallic ions like copper, strontium, zinc, silicon, boron, cerium, and gallium, usually incorporated in inorganic materials (e.g. in bioactive glasses and bioceramics), have emerged as potential therapeutic ions to enhance bone formation due to their ability to stimulate the expression of genes of osteoblast cells and to stimulate angiogenesis [1,2,9,10].
Furthermore, some therapeutic ions like silver, zinc, copper, cerium, and gallium have shown significant anti-bacterial and anti-inflammatory effects [, , , ]. This has also led to the development of antibiotic-free antibacterial agents exploiting antibacterial ion release .
Gallium is an important therapeutic ion for incorporation into bioactive materials. Gallium, a semi-metallic element in Group 13 of the periodic table, has shown a therapeutic effect for the treatment of numerous disorders. These could be categorized as follows; accelerated bone resorption, with or without elevated plasma calcium , autoimmune diseases and allograft rejection , hemostasis , and bacterial infections [, , ].
In addition to these therapeutic effects, gallium ions show antineoplastic activity against certain types of cancers [, , ]. These features set the gallium ions apart from other commonly used therapeutic ions. Fig. 1 summarizes the biomedical areas of application of gallium containing materials.
The use of gallium in the biomedical field was initiated by the 1950s’ discovery that the radioactive isotope 67Ga, injected into rodents bearing implanted tumors, localized in high concentration within these tumors [24,25]. Due to this ability, the 67Ga isotope was used as a diagnostic tool for the detection of occult tumors or residual viable tumors following treatment in humans. Fluorodeoxyglucose – positron emission tomography (F-FDG- PET) scans have largely replaced the 67Ga scan in the last two decades , however, target-specific 68Ga labeled pharmaceuticals for molecular imaging are on clinical trials as advanced tools for PET studies [26,27]. 68Ga has a short half-life (t1/2 = 68 min) which enables rapid imaging .
Gallium was originally used only for imaging bone tumors, but in 1969, after the discovery of the ability of cumulation of 67Ga in soft tissue tumors, it became a useful tool for Hodgkin’s disease treatment . Initial studies suggested that all group 13 metals in the form of simple salts are capable of inhibiting tumor growth, but only gallium showed a therapeutic effect .
By the mid-1970s gallium nitrate had entered the clinical stages, and gallium became the second metal to show therapeutic activity in cancer patients after platinum . The oral administration of gallium in the form of simple salts like gallium chloride allows continuous delivery of gallium ions. On the other hand, a combination of gallium with biomaterials improves the delivery of gallium directly to the affected area while minimizing the negative effect on healthy cells, as will be reviewed in this paper.
Gallium ions are included in a wide range of bioactive materials to induce multiple therapeutic effects over time. Gallium has a positive effect on bone cell growth . Bone is a target organ for gallium; it accumulates within the bone and reduces calcium loss by inhibiting bone resorption without causing apparent damage to bone cells .
By observing the gallium distribution with synchrotron x-ray microscopy Bockman et al.  showed that gallium increased calcium and phosphorus content in bone, and also increased hydroxyapatite (HA) crystallites formation in maturing bone. Numerous studies report on the use of gallium salts for the treatment of bone loss such as osteoporosis, and bone metastases. However, when taken orally as a salt, the dose of Ga reaching the required bone site was low.
An alternative way to administer gallium nitrate is a continuous intravenous infusion for 5–7 days. However, this method is inconvenient. As a more convenient alternative, gallium can be delivered to the required site in a controlled manner from melt-derived and sol-gel derived bioactive glasses and bio-ceramic-based scaffolds designed for bone tissue regeneration . One of the most critical issues after surgery is bacterial infection associated to implanted biomaterials .
Gallium is also being tested in clinical trials to fight against infections [36,37]: gallium incorporation into coatings or scaffolds enhances the antibacterial properties of biomaterials . Recently, gallium’s hemostatic effects have been also examined [17,, , ].
The addition of gallium increased the capability of MBGs regarding platelet aggregation, thrombus formation, and blood coagulation activation . Gallium addition to bioactive materials addresses some major issues related to the aging population, as different gallium-containing bioactive materials show reliable results for the treatment of numerous diseases, and current successful research results could progress to translation to the clinic.
Mechanisms of biological activity of gallium
Mechanisms of therapeutic activity and biochemistry of gallium have been studied and reviewed in several articles [15,22,23,, , ]. These subjects will be therefore reviewed here only briefly.
Gallium is a Group 13 metal element of the periodic table and only exists in the oxidation state +3. Ga3+ does not have any known essential role in the body, but it shares certain similarities with Fe3+. For example, the octahedral ionic radius is 0.62 Å in Ga3+, and 0.645 Å for high spin Fe3+. Also, the tetrahedral ionic radius is 0.47 Å in Ga3+, and 0.49 Å for high spin Fe3+ . The ionization potential (4th ionization potential) values for Ga3+ and high spin Fe3+ are 64 eV and 54.8 eV, respectively.
Electron affinity (3rd ionization potential) value for Ga3+ is 30.71 eV and 30.65 eV for high spin Fe3+ . With these similarities, gallium can bond with iron-binding proteins. While the binding of iron to a protein promotes protein function, gallium, in contrast to iron, is not redox-active, so the substitution of gallium for iron in a protein usually disrupts its function and leads to negative downstream effects in cells [42,46,47].
Besides platinum, gallium is a metal ion with anticancer properties. Despite the presence of contradictory studies, the therapeutic activity of gallium is, to a large extent, associated with the competition of Fe3+ and Ga3+ for cellular uptake . The distribution of gallium is found to concentrate on proliferated tissues, including most tumors, due to a large amount of Fe3+ binding proteins .
The uptake system is thought to be associated with transferrin receptors which is illustrated in Fig. 2. In other words, highly proliferating tumor cells require more iron than normally dividing cells whereby, having a high concentration of receptors, they become an attractive target for gallium ions to bind to [15,48]. After gallium is taken into the cell, it binds to ribonucleotide reductase enzyme , which is responsible for DNA replication and repair, and prevents its activity, resulting in apoptosis through the mitochondrial pathway . Since gallium is taken up by cancer cells in larger amounts than by normal cells, the normal cells are not negatively affected, but the viability of cancer cells decreases .
Iron is a key element in metabolic and signaling functions of bacteria due to its involvement in major biological processes, including cellular respiration, DNA synthesis, oxygen transport, and defense mechanism towards reactive oxygen species (ROS) [, , ]. During infection, bacteria are faced with a shortage of iron since the host reduces iron availability as a part of the immune system response to prevent the proliferation of bacteria .
Therefore, bacteria develop high-affinity ferric iron uptake mechanisms (illustrated in Fig. 3). One of them is the production of low molecular mass compounds called siderophores. A siderophore receptor is a small secreted iron-binding molecule that is part of the bacteria iron uptake system, along with a siderophore receptor protein, which actively transfers iron into the cell, allowing its solubilization and extraction .
Considering the chemical similarities between Fe3+ and Ga3+ , microorganisms cannot easily distinguish between these two ions and it has been hypothesized that bacteria sequester Ga through their iron uptake system since Ga has been shown to bind to iron siderophores .
Hereby, Ga3+ competes with Fe3+ for incorporation into essential proteins and enzymes. Unlike Fe3+, Ga3+ cannot be reduced under physiological conditions, resulting in inhibition of several iron-dependent redox pathways. However, some mutant strains of P. aeruginosa were reported to develop resistance against gallium administrated in the form of a simple salt, such as Ga(NO3)3 [, , , ]. The mechanism of such gallium resistance is not yet fully understood. The available literature suggests that the outflow mechanism of gallium from the bacteria may be responsible for the development of pathogen’s resistance to gallium .
Besides its antibacterial and anticancer properties, many studies provide strong evidence of the osteogenic (anabolic) activity of gallium [15,60,61]. This activity is associated with a reduction in osteoclast activity and an increase in apoptosis-dependent cell death. Osteoclasts are multinucleated giant cells responsible for breaking down and resorbing bone tissue. They play an important role in liberating minerals and other molecules stored within the bone matrix.
On the other hand, osteoblasts are responsible for building new bone tissue. The reduction of osteoclast activity is thought to be associated with an increase in the amount of calcium and phosphorus in bone tissue . The postulated mechanism of Ga action is that it prevents the breakdown of the bone by blocking osteoclast activity, thus lowering the amount of free calcium in blood .
Gallium-treated bones showed an increased amount of calcium and phosphate content, which results in enhanced stability of bone associated with a larger size of HA crystals leading to higher resistance to bone resorption [63,64]. Bockman et al. proposed Ga uptake by HA, substituting it for calcium and altering the dissolution behavior of this phase .
Moreover, besides reducing the inflammation in the latter stage of wound healing, gallium also shows improvement during the very early stage of hemostasis – either coagulation, platelet activation, or clot formation [17,40,41,, , ]. This effect of gallium has been proven by comparing gallium containing composites with two different commercial products .
Although the exact mechanism is still unclear, studies indicate that gallium shows hemostasis capability with intrinsic coagulation pathway via activation of Factor XII in a similar manner to current commercial products [41,66,68,70].
reference link : https://www.sciencedirect.com/science/article/pii/S2452199X21006083?via%3Dihub#sec1
More information: Shiva Kamini Divakarla et al, Antimicrobial and Anti-inflammatory Gallium–Defensin Surface Coatings for Implantable Devices, ACS Applied Materials & Interfaces (2022). DOI: 10.1021/acsami.1c19579