Antimicrobials: Beyond Silver

Bob Michaels
Antimicrobial formulations based on ceragenins, zinc compounds, and long-chain, high-charge-density polymers are training their sights on microbes
Substrates innoculated with E. coli bacteria are shown untreated (top) and treated with QuickMed's Nimbus long-polymer-chain biocidal agent (bottom).

The high incidence of hospital-acquired infections (HAIs) is causing legions of doctors, hospital administrators, and insurers to break out in a cold sweat. Patients in hospitals and long-term care facilities, as well as recipients of implantable medical devices, are at particular risk of acquiring these life-threatening, but preventable, infections. In response to the millions of patients that have contracted HAIs, the Centers for Medicare and Medicaid Services have altered their related reimbursement policy in recent years, which, in turn, has sent medical device OEMs scrambling for effective antimicrobial solutions.

In an effort to thwart HAIs and device-related infections, medical device manufacturers and suppliers have developed an array of antimicrobial technologies based on ionic silver. While silver has emerged as a well-accepted and proven broad-spectrum material for combating Gram-positive, Gram-negative, and antibiotic-resistant bacteria, it is anything but risk-free. Silver ions exhibit cytotoxicity, which accumulates in body tissue, for example. And the tissue toxicity resulting from high concentrations of silver in wound beds can destroy fibroblasts and keratinocytes, which play a vital role in wound healing.

Some providers of antimicrobial technologies, however, are bucking the silver trend in the battle against ‘superbugs.’ By developing synthetic compounds that mimic the activity of the human immune system, exploring the use of elements other than silver, or designing permanently bonded antimicrobial agents that attack bacterial cell walls, a number of companies is expanding the range of possibilities for eradicating dangerous infections.

Ceragenins Grant Immunity
In contrast to antimicrobial systems based on ionic elements such as silver, a fundamentally different approach involves the development of synthetic compounds that simulate the body’s own immune response. On the cusp of product research and development in this field is N8 Medical LLC (Columbus, OH), which offers a synthetic class of materials called ceragenins that mimic the polyfunctional and broad-spectrum activity of a key component of the human innate immune system.

The human body has both an innate and an adaptive immune system, explains Carl Genberg, CTO of N8 Medical. “The innate immune system’s components display broad-spectrum activity against bacteria, fungi, and certain viruses; are rapidly bactericidal rather than bacteriostatic; and are able to kill pathogens on contact with no need to enter into the organism to effect lethality.” Ceragenins share these features in common with their natural counterparts.

Invented by Paul Sage at Brigham Young University, ceragenins are patented synthetic peptidomimetic compounds that display bactericidal, fungicidal, and virucidal properties against an array of pathogens. For example, they attack multidrug-resistant strains of Methicillin-resistant Staphylococcus aureus and Pseudomonas aeruginosa. To prevent the development of highly resistant microbes, ceragenins attack their outer membranes rather than their internal replication mechanisms, according to Genberg. The microbes cannot adapt to the antimicrobial agent because they are unlikely to evolve new membrane structures.

The lead ceragenin compound is CSA-13. A positively charged compound, CSA-13 is electrostatically attracted to negatively charged phospholipids found in high concentrations in the membranes of bacteria. Although its precise mechanism is the subject of ongoing research, CSA-13 appears to operate by ripping out patches of bacterial membrane, resulting in membrane depolarization, ion efflux, and rapid cell death. “In most cases, its minimum inhibitory concentration values are very close to the minimum bactericidal concentration values, indicating that it ‘inhibits’ bacterial growth by killing the bacteria rather than by merely suppressing its growth,” Genberg says.

CSA-13 is highly water-soluble, whereas silver is not, Genberg adds. In addition, it has been shown to both prevent and eradicate biofilms. “These factors,” Genberg notes, “may help explain why in head-to-head in vitro and in vivo preclinical testing, CSA-13-treated medical devices have been shown to offer weeks and months of antimicrobial protection—depending on the application—compared with several days of protection found in commercially available silver-treated medical devices.”

Capable of being coated onto, attached to, or incorporated into all commonly used polymers, metals, and other medical device materials, CSA-13 is being evaluated by OEMs for its potential use against bacterial colonization and biofilm formation. Such applications could include endotracheal tubes and urinary catheters. The NIH is also funding research at the University of Utah focusing on the chemical’s ability to serve as a coating for orthopedic implants. “While CSA-13 is still undergoing evaluation, the final product will be regulated as a combination medical device and reviewed by the Center for Devices and Radiological Health,” Genberg adds.

Antimicrobial Bonding
Like ceragenins, the Nimbus technology developed by Quick-Med Technologies Inc. (Gainesville, FL) kills bacteria from the outside rather than by directly entering the cell. A biocidal agent, Nimbus consists of a long polymer chain that concentrates a large number of charged sites, explains Bernd Liesenfeld, the company’s principal scientist. This concentration of active sites prevents bacterial growth and colonization on the treated article. The chain also has a high molecular weight, which acts to concentrate the charge groups, ensuring that treated segments have sufficient charge density to provide strong microbicidal functionality. Additionally, high molecular weight is important because for cationic biocides, this characteristic results in lower toxicity to eukaryotic cells.

The charges associated with cationic biocides, according to Liesenfeld, disrupt the bacterial cell wall by physically pulling out the individual molecules that compose the cell membrane, forming holes in the wall, and corrupting the cell’s integrity. In contrast to the action of other microbicides, this process does not require the cell to internalize the antimicrobial agent, a phenomenon that blocks the adaptive path through which bacteria can develop resistance to antimicrobials.

“Both silver and antibiotic agents kill bacterial cells by entering the cell and disrupting such functions as metabolic pathways,” Liesenfeld says. “Cells develop defenses to these agents by shifting their reliance to other metabolic pathways or through efflux pumps—mechanisms that pump the invaders out of the cell again, thereby increasing tolerance to the agent.” Polymeric biocides are unique in their ability to disassemble the structure of the cell wall from the outside, Liesenfeld adds. The cell cannot adapt by expelling the invaders more efficiently because they are not in the cell. Alteration of metabolic pathways is also not an option. “Because the structure of the cell wall is pretty much set in stone, bacterial cells have no ready mechanism to develop resistance to polymeric biocides that attack cell walls.”

While many antimicrobial agents are applied to medical devices in the form of coatings, the Nimbus is based on a cationic, or positively charged, polymeric biocidal agent that is permanently bonded to substrate materials. The bonded nature of this polymeric biocide is integral to its activity, according to Liesenfeld. “Having an antimicrobial agent bonded to the surface of a device means that the surface will resist microbicidal contamination. A bonded agent protects the surface effectively for a long time because there is no reservoir of active that can become depleted or washed out.”

But bonding is also integral to the agent’s safety profile, Liesenfeld adds. “Any agents that leach out of a dressing can impede wound healing because agents designed to kill bacterial cells often also have some toxicity toward the cells that make up human tissue.” In wound-care applications, for example, Nimbus was designed to avoid leaching while protecting the wound by excluding pathogens. Silver dressings, Liesenfeld asserts, follow the opposite principle: They work by leaching the active silver agent out of the dressing, and that active agent itself is known to be cytotoxic.

Although initially developed for wound-care applications, Nimbus technology is also suitable for a range of medical devices. “All the positive attributes of Nimbus—and cationic polymeric biocides in general—apply not only to wound care but to any application in which the safe, effective, and economical interruption of pathogen transfer is important for improving public health,” Liesenfeld notes.

For example, like wound-care dressings, catheters would benefit from an antimicrobial agent that is permanently affixed to the surface, eliminating the concern that extractable agents could be depleted from the device. While catheters have relatively smaller surface areas than wound dressings, the principles of bactericidal activity are the same. “In both wound-care and catheter applications, biofilm formation requires the deposition and growth of planktonic bacteria to initiate a new community,” Liesenfeld explains. “The suppression of bacterial colonization by antimicrobially effective surfaces can prevent the initial bacterial colonization that is the first step in biofilm formation.”

Zincing Microbes
Silver is the most widely used agent for killing bacteria on medical device surfaces. Yet, despite its prevalence, this noble metal is incompatible with some materials—such as acetal—that are commonly used to manufacture medical devices. Recognizing this deficiency, Microban International Ltd. (Huntersville, NC) is in the process of developing alternatives to silver-based antimicrobial agents that can be incorporated into acetal. One such candidate is zinc.

“In doing a lot of research, it appears that every single silver antimicrobial system we have looked at—and we have looked at a very wide range—has a destabilizing effect on acetal,” remarks Ivan Ong, vice president of research and development at Microban. “Acetals are prone to processing issues when combined with many additives, causing chemical degradation and formaldehyde offgassing in the final product.” Concluding that the silver cation itself seems to destabilize the acetal in some form, the company has determined that unless it is applied as a coating, silver is not compatible with this plastic material.

While other compounds can also destabilize acetal, including a zinc-based antimicrobial compound known as zinc pyrithione, the company has created what it believes is a fairly effective zinc-based antimicrobial system that does not have a negative impact on acetal when it is incorporated into the material. In addition, unlike traditional silver-zeolitic or coordinated metal-salt antimicrobials, Microban’s inorganic zinc-based antimicrobial systems can be tuned to cater to a range of performance and property requirements, Ong says. Adjustable properties include particle size, active contents, discharge rate, and resistance to radiation, solvents, and other destabilizing influences.

“Acetal may not be the most widespread material in the medical device industry,” Ong notes. “But it has its uses. It has some desirable polymeric properties that make it suitable for connectors, housings, and similar applications.” Compatible with acetal, Microban’s antimicrobial zinc compound does not cause destabilization or generate formaldehyde. The company's findings, Ong says, show that zinc performs effectively, resulting in greater than a 4-log reduction of many microbes.

Busting Tomorrow’s Superbugs
Looking to the future, Liesenfeld of Quick-Med believes that future FDA activities directed toward regulating antimicrobial technologies will focus increasingly on the consequences of leachable agents and components as well as the emergence of drug-resistant superbugs. More rare, but also of great concern, is resistance to silver, which researchers identify as having a genetic basis.

Another critical issue, according to Liesenfeld, will be biofilm formation. Composed of bacteria that grow on the device surface, biofilms eventually form a community by supporting each other, often by producing a protective structure around the community and often growing as a community of diverse species. The best strategy for preventing biofilms, Liesenfeld concludes, is to prevent initial bacterial colonization of a surface through use of an effective antimicrobial.

Returning to the alarming rise in HAIs, N8 Medical’s Genberg notes that recent changes in healthcare laws and state-mandated disclosures of HAI rates have made the adoption of cost-effective antimicrobial medical devices an important component of the national effort to reduce morbidity and mortality levels. “It is estimated that 70% of all HAIs are linked to the use of medical devices, since the abiotic surfaces of medical devices provide an ideal breeding ground for bacteria,” Genberg says. “We believe that increasing emphasis will be placed on comparative efficacy studies with well-defined clinically relevant endpoints, driving the rapid adoption of effective technologies.”

For example, claims based in part on subjective assessments that an antimicrobial-coated endotracheal tube is able to significantly reduce ventilator-associated pneumonia rates should be substantiated using several different criteria, Genberg comments. “Patients using such devices should experience shorter stays in the ICU, the duration of mechanical ventilation should be reduced, and mortality rates should fall.” And in the case of antimicrobial Foley catheters, the relevant clinical endpoint is clearly defined: “There must be evidence that instances of clinical infections—not just bacteremia—have declined.”

For a different take on non-silver-based antimicrobial strategies, see "Opinion: Silver-Alternative Antimicrobial Technologies Lack Versatility, Efficacy." Also, view the Web-exclusive gallery, "Five Novel Antimicrobial Technologies for Battling Bacteria."