One of the most challenging complications in orthopedic trauma surgery is the development of infection. Improved infection prophylaxis could be achieved by providing local delivery of antibiotics directly to the tissue-implant interface. Especially implant-associated bone infections caused by antibiotic-resistant pathogens pose significant clinical challenges to treating physicians. Prophylactic strategies that act against resistant organisms, such as methicillin-resistant Staphylococcus aureus (MRSA), are urgently required.

The objective of this experimental study was to determine the efficacy of a biodegradable Polymer-Lipid Encapsulation MatriX (PLEX) loaded with the antibiotic doxycycline as a local prophylactic strategy against implant-associated osteomyelitis in a humeral non-fracture rabbit model.

Activity of the PLEX-doxycycline-coating was tested against both a doxycycline susceptible (doxyS) methicillin-susceptible S. aureus (MSSA) as well as a doxycycline-resistant (doxyR) MRSA. In a rabbit intramedullary (IM) nail-related infection model, twelve rabbits received an inoculum of a doxyS MSSA direct into the medullary cavity of the humerus. After inoculation, animals received either a PLEX-doxycycline-coated nail, or an uncoated nail. The animals were observed for four weeks. Upon euthanasia, quantitative bacteriology was performed to determine bacterial load in tissues and biofilm formation on the implant. A second study was performed with sixteen rabbits receiving a DoxyR MRSA inoculum, again in coated and uncoated groups.

In vitro elution studies revealed that 25% of the doxycycline was released from the PLEX-coated implants within the first day, followed by a 3% release per day up to day 28. Quantitative bacteriology revealed the presence of osteomyelitis in all animals receiving an uncoated nail in both the MSSA and the DoxyR MRSA studies (figure). All rabbits receiving a PLEX-doxycycline-coated nail were culture negative in the doxyS MSSA-group and the surrounding bone displayed a normal physiological appearance in both histological sections and radiographs. In the doxyR MRSA inoculated rabbits, a statistically significant reduction in the number of culture-positive samples was observed for the PLEX-doxycycline-coated group when compared to the animals that had received an uncoated nail, although the reduction in bacterial burden did not reach statistical significance.

Improved prophylaxis against infection in trauma and orthopedic implant surgery is clearly required today. In this study, we investigated a PLEX-doxycycline-coated IM nail in a humeral non-fracture rabbit model. The PLEX-doxycycline coating on titanium alloy implants provided complete protection against implant-associated MSSA osteomyelitis, and resulted in a significant reduction in the number of culture positive samples when challenged with a doxycycline-resistant MRSA.

Bacterial infection of bone may result in bone destruction which is difficult to cure due to poor accessibility to bone of systemically-administrated antibiotic and poor performance of currently available local antibacterial treatments. PolyPid Ltd developed a novel local drug delivery system based on self-assembly of pharmaceutically approved lipids and polymers that encapsulate doxycycline (Doxy). The formulation is self-assembled lipid matrix via the interaction of the lipids (cholesterol and synthetic phospholipids) and biocompatible - biodegradable polymer (poly-lactic-co-glycolic).

The entrapped Doxy is located within the anhydrous environment and therefore fully protected from both enzymatic and long-term water-exposure-related degradation. The fine coating of the tri-calcium phosphate (TCP) bone filler by this Doxy-containing formulation (BonyPid™) is capable of releasing intact and active drug at zero-order kinetics for a predetermined period of up to 30 days. The coating of the TCP granules with the polymer-lipids-Doxy formula (BonyPid™) did not change the granules’ macroscopic shape, but altered its color from white to pale yellow, which resemble the color of the entrapped Doxy. The average sizes of the non-coated TCP granules and the coated granules BonyPid™ were similar, as determined by measuring the widest dimension of each granule (1135±241 µm and 1072±242 µm, respectively, P=0.16). The MIC for Doxy that was released from BonyPid™ at different time points was similar to the non-encapsulated Doxy, suggesting full bioavailability of the released drug. BonyPid™ formulation structure was characterised by different physical methods including wide angle X-ray analyses (WAXS), differential scanning calorimetric (DSC) and SEM. WAXS analyses of BonyPid™ samples show a strong signal in the range of 1.3–1.8 2θ°, suggesting that the polymer and lipid TCP coating is a highly organised nano-substructure.

The principle lipid in BonyPid™ formulation is phosphatidylcholine, which constitutes more than 85% of the overall lipid mass. It was found that the length of the acyl chains (14, 16 and 18 carbons, respectively) can significantly alter the release rate of Doxy during the prolonged (30 days), zero-order release phase, but did not alter the release profile. The anti-infection activity of BonyPid™ was tested in the rabbit tibia model contaminated with 5×10⁵S. aureus. Both acute and chronic infection models were tested. Only BonyPid™ treatment demonstrated a statistically significant reduced bone absorption over the infected group (P<0.04 for day 7, 14 and 21) and significantly lower bacterial bone concentration (p>0.05) on day 21 following the bone grafting and the bacterial inoculation. In addition it was found that BonyPid™ did not reduce the osteo-conductivity as compared to non-coated TCP bone-filler. The first-in-man study for the treatment of contaminated / infected severe open long-bone fractures of BonyPid™ completed its 6 months follow-up. The results demonstrated high safety profile and significant efficacy; early bone callus formation and 0% infections in the BonyPid™ target bone fracture.